Posterior parietal cortex and episodic retrieval: Convergent and divergent effects of attention and memory

Functional neuroimaging studies of humans engaged in retrieval from episodic memory have revealed a surprisingly consistent pattern of retrieval-related activity in lateral posterior parietal cortex (PPC). Given the well-established role of lateral PPC in subserving goal-directed and reflexive attention, it has been hypothesized that PPC activation during retrieval reflects the recruitment of parietal attention mechanisms during remembering. Here, we evaluate this hypothesis by considering the anatomical overlap of retrieval and attention effects in lateral PPC. We begin by briefly reviewing the literature implicating dorsal PPC in goal-directed attention and ventral PPC in reflexive attention. We then discuss the pattern of dorsal and ventral PPC activation during episodic retrieval, and conclude with consideration of the degree of anatomical convergence across the two domains. This assessment revealed that predominantly divergent subregions of lateral PPC are engaged during acts of episodic retrieval and during goal-directed and reflexive attention, suggesting that PPC retrieval effects reflect functionally distinct mechanisms from these forms of attention. Although attention must play a role in aspects of retrieval, the data reviewed here suggest that further investigation into the relationship between processes of attention and memory, as well as alternative accounts of PPC contributions to retrieval, is warranted.Episodic memory—declarative memory for events—has long been known to depend on the medial temporal lobe and, to a lesser extent, the prefrontal cortex (Squire 1992; Shimamura 1995; Wheeler et al. 1995; Gabrieli 1998; Eichenbaum and Cohen 2001; Squire et al. 2004). Recently, an explosion of functional neuroimaging studies has revealed that episodic retrieval is also consistently associated with activity in lateral posterior parietal cortex (PPC), including in the intraparietal sulcus and inferior parietal lobule (Figs. 1, ​,2;2; for detailed review, see Wagner et al. 2005; Cabeza 2008; Cabeza et al. 2008; Ciaramelli et al. 2008; Vilberg and Rugg 2008b; Olson and Berryhill 2009). This unexpected finding raises the possibility that parietal mechanisms may be more central to episodic retrieval than previously thought.Open in a separate windowFigure 1.Anatomy of posterior parietal cortex (PPC). A posterior-lateral view of human PPC is depicted, with PPC separated into dorsal and ventral portions by the intraparietal sulcus (IPS). Dorsal PPC includes the superior parietal lobule (SPL) and IPS. Ventral PPC includes inferior parietal lobule (IPL) and its subregions: supramarginal gyrus (SMG), temporoparietal junction (TPJ), and angular gyrus (AnG).Open in a separate windowFigure 2.Left lateral PPC activity during episodic retrieval. (A) A comparison of hits relative to correct rejections reported by Kahn et al. (2004) revealed “old/new” effects in dorsal PPC, inclusive of IPS. Average signal change within IPS was greater for items perceived as old (hits and false alarms) vs. those believed to be new (misses and correct rejections). (B) A comparison of successful, relative to unsuccessful, cued recall by Kuhl et al. (2007) revealed greater activity in AnG, compatible with the broader literature on recollection success effects (see Fig. 4). In addition, effects were observed in more anterior aspects of ventral PPC (SMG), as well as in dorsal PPC (principally SPL) (see Discussion). (C) Orienting to memory in attempts to recollect, independent of recollection success, is often associated with activity in dorsal PPC. For example, comparison of temporal recency judgments to novelty-based decisions elicited greater IPS activity (Dudukovic and Wagner 2007).At the neuropsychological level, human lesion evidence regarding the necessity of lateral PPC mechanisms for episodic retrieval is limited and mixed (Berryhill et al. 2007; Davidson et al. 2008; Haramati et al. 2008; Simons et al. 2008). By contrast, other neuropsychological data indicate that lateral PPC is unambiguously associated with another cognitive domain—attention (Posner et al. 1984; Mesulam 1999; Parton et al. 2004). This latter lesion literature is further complemented by rich functional neuroimaging evidence implicating dorsal and ventral PPC in goal-directed and reflexive attention, respectively (for review, see Corbetta and Shulman 2002; Corbetta et al. 2008).Drawing from the rich literature linking attention to lateral PPC, memory researchers have recently proposed that lateral PPC activity during episodic retrieval tasks reflects the engagement of attention mechanisms during remembering (Cabeza 2008; Cabeza et al. 2008; Ciaramelli et al. 2008; Olson and Berryhill 2009). Specifically, it has been hypothesized that: (1) Dorsal PPC activity during retrieval may reflect the recruitment of goal-directed attention in service of performing retrieval tasks and (2) ventral PPC engagement during retrieval may mark the reflexive capture of attention by mnemonic representations. While prior comprehensive reviews of the neuroimaging literature on parietal correlates of episodic retrieval have documented functional dissociations along the dorsal/ventral axis of lateral PPC, which qualitatively parallel those seen in the attention literature, evaluation of the hypothesis that PPC retrieval activity reflects attention mechanisms further requires an assessment of the degree to which attention and retrieval effects co-localize. Here we review lateral PPC correlates of both episodic retrieval and attention, with the goal of directly assessing to the degree of anatomic overlap.It should be noted from the outset that the aim of the present review is to evaluate the hypothesis that lateral PPC episodic retrieval effects can be explained in terms of goal-directed and reflexive attention mechanisms. As such, we a priori imposed three constraints that served to focus our treatment of these two substantial literatures. First, while both the dual-attention and memory retrieval literatures focus on effects on the lateral parietal surface, retrieval effects are predominantly left lateralized. Thus, we constrained our analysis of attention and retrieval findings to left lateral PPC.⁵ Second, because prior retrieval reviews focused theoretical discussion on dual-attention accounts, here we similarly constrained our treatment of the extensive attention literature to include only those effects relevant to dual-attention theory. Finally, because the preponderance of evidence offered in support of dual-attention theory''s proposed dorsal attention network derives from studies of visual attention, the present review of the dorsal network is also confined to visual attention. As such, the present review should not be viewed as a comprehensive review of the entire attention literature.We first survey the functional neuroimaging literature on parietal correlates of goal-directed and reflexive attention, and then discuss how these correlates converge and diverge with the patterns of lateral PPC activity present during episodic retrieval. We conclude by considering theoretical frameworks that focus on the role of attention in episodic retrieval, as well as nonattention-based accounts of PPC activity during retrieval, and we highlight open questions that await further investigation.     

Post-training reversible inactivation of the hippocampus enhances novel object recognition memory

Research on the role of the hippocampus in object recognition memory has produced conflicting results. Previous studies have used permanent hippocampal lesions to assess the requirement for the hippocampus in the object recognition task. However, permanent hippocampal lesions may impact performance through effects on processes besides memory consolidation including acquisition, retrieval, and performance. To overcome this limitation, we used an intrahippocampal injection of the GABA agonist muscimol to reversibly inactivate the hippocampus immediately after training mice in two versions of an object recognition task. We found that the inactivation of the dorsal hippocampus after training impairs object-place recognition memory but enhances novel object recognition (NOR) memory. However, inactivation of the dorsal hippocampus after repeated exposure to the training context did not affect object recognition memory. Our findings suggest that object recognition memory formation does not require the hippocampus and, moreover, that activity in the hippocampus can interfere with the consolidation of object recognition memory when object information encoding occurs in an unfamiliar environment.The medial temporal lobe plays an important role in recognition memory formation, as damage to this brain structure in humans, monkeys, and rodents impairs performance in recognition memory tasks (for review, see Squire et al. 2007). Within the medial temporal lobe, studies have consistently demonstrated that the perirhinal cortex is involved in this form of memory (Brown and Aggleton 2001; Winters and Bussey 2005; Winters et al. 2007, 2008; Balderas et al. 2008). In contrast, the role of the hippocampus in object recognition memory remains a source of debate. Some studies have reported novel object recognition (NOR) impairments in animals with hippocampal lesions (Clark et al. 2000; Broadbent et al. 2004, 2010), yet others have reported no impairments (Winters et al. 2004; Good et al. 2007). Differences in hippocampal lesion size and behavioral procedures among the different studies have been implicated as the source of discrepancy in these findings (Ainge et al. 2006), but previous studies have not examined the consequences of environment familiarity on the hippocampus dependence of object recognition memory.Previous studies addressing the role of the hippocampus in recognition memory relied on permanent, pre-training lesions (Clark et al. 2000; Broadbent et al. 2004; Winters et al. 2004; Good et al. 2007). Permanent lesions inactivate the hippocampus not only during the consolidation phase, but also during habituation, acquisition, and memory retrieval, potentially confounding interpretation of the results. Furthermore, permanent lesion studies require long surgery recovery times during which extrahippocampal changes may emerge to mask or compensate for the loss of hippocampal function. To overcome these problems, we reversibly inactivated the dorsal hippocampus after training mice in two versions of the object recognition task. We infused muscimol, a γ-aminobutyric acid (GABA) receptor type A agonist, into the dorsal hippocampus immediately after training in an object-place recognition task or immediately following training in a NOR task. Consistent with previous studies (Save et al. 1992; Galani et al. 1998; Mumby et al. 2002; Stupien et al. 2003; Aggleton and Brown 2005), we observed that hippocampal inactivation impairs object-place recognition memory. Interestingly, we observed that the degree of contextual familiarity can influence NOR memory formation. We found that when shorter periods of habituation to the experimental environment were used, hippocampal inactivation enhances long-term NOR memory. In contrast, after extended periods of contextual habituation, long-term recognition memory was unaltered by hippocampal inactivation. Together these results suggest that if familiarization with objects occurs at a stage in which the contextual environment is relatively novel, the hippocampus plays an inhibitory role on the consolidation of object recognition memory. Supporting this view, we observed that object recognition memory is unaffected by hippocampal inactivation when initial exploration of the objects occurred in a familiar environment.     

Theta bursts in the olfactory nerve paired with β-adrenoceptor activation induce calcium elevation in mitral cells: A mechanism for odor preference learning in the neonate rat

Odor preference learning in the neonate rat follows pairing of odor input and noradrenergic activation of β-adrenoceptors. Odor learning is hypothesized to be supported by enhanced mitral cell activation. Here a mechanism for enhanced mitral cell signaling is described. Theta bursts in the olfactory nerve (ON) produce long-term potentiation (LTP) of glomerular excitatory postsynaptic potentials (EPSPs) and of excitatory postsynaptic currents (EPSCs) in the periglomerular (PG) and external tufted (ET) cells. Theta bursts paired with β-adrenoceptor activation significantly elevate mitral cell (MC) calcium. Juxtaglomerular inhibitory network depression by β-adrenoceptor activation appears to increase calcium in MCs in response to theta burst stimulation.Early odor preference learning provides us with a model in which the necessary and sufficient inputs for learning can be localized to a relatively simple cortical structure, the olfactory bulb (Sullivan et al. 2000). The critical changes for this natural form of learning occur in the olfactory bulb network (Coopersmith and Leon 1986, 1987, 1995; Wilson et al. 1987; Woo et al. 1987; Wilson and Leon 1988; Wilson and Leon 1991; Guthrie et al. 1993; Johnson et al. 1995; Yuan et al. 2002). Odor preference learning is induced by the pairing of odor with activation of the locus coeruleus noradrenergic system, a component of our arousal circuitry (Harley 1987; Berridge and Waterhouse 2003; Berridge 2008), which is critically involved in other forms of memory (Harley 1987; Berridge and Waterhouse 2003) and compromised in diseases of memory, such as Alzheimer''s (Palmer and DeKosky 1993; Weinshenker 2008). The unconditioned stimulus for early odor preference learning is mediated by β-adrenoceptor activation in the olfactory bulb (Sullivan et al. 1991, 1992; Wilson and Sullivan 1991, 1994; Wilson et al. 1994; Harley et al. 2006). β-Adrenoceptor agonists and antagonists infused in the olfactory bulb can induce or block learning, respectively (Sullivan et al. 1989, 2000).Despite the fact that the behavioral model for early odor preference learning has been established for decades (Sullivan et al. 1988, 1989), and despite the fact that the olfactory bulb circuit contains synapses that are essential for the formation of a localizable long-term memory that is easily indexed in behavior (Coopersmith and Leon 1986; Wilson et al. 1987; Woo et al. 1987; Wilson and Leon 1988; Woo and Leon 1991; Johnson et al. 1995; McLean et al. 1999; Yuan et al. 2002), the circuitry and the synapses in the olfactory bulb that mediate learning are not well understood. Long-term potentiation (LTP), the putative synaptic model for associative learning in other brain regions (Bliss and Lomo 1973; Brown et al. 1988; Barnes 1995; Malenka 1994), has not been demonstrated compellingly in the rat olfactory bulb. The lack of evidence for a synaptic locus and a mechanism to support odor preference learning is partly due to the dissociation of the neural changes previously observed following early odor learning (seen at the glomerular input level) (Coopersmith and Leon 1986; Wilson et al. 1987; Woo et al. 1987; Wilson and Leon 1988; Woo and Leon 1991; Johnson et al. 1995; Yuan et al. 2002) and the innervation pattern of noradrenergic fibers in the olfactory bulb (seen mostly in the deep layers of the olfactory bulb, but sparse in the glomerular layer) (McLean et al. 1989; McLean and Shipley 1991).McLean and colleagues have proposed, based on physiological and anatomical evidence, that early odor preference learning leads to a long-term facilitation of the olfactory nerve (ON) inputs to mitral cells (MCs, the main output cell of the olfactory bulb) (McLean et al. 1999; Yuan et al. 2003b; McLean and Harley 2004). In the present study, odor input is mimicked in vitro by theta burst stimulation (TBS) of the ON, and the modulation of glomerular and MC responses to theta bursts alone and in conjunction with bath application of the β-adrenoceptor agonist, isoproterenol, is assessed. The results support the McLean glomerular/MC hypothesis of early odor preference learning.In the first set of experiments (Fig. 1A–D), the effects of theta burst ON input on the field glomerular excitatory postsynaptic potential (EPSP) were tested. The ON was stimulated by a single test stimulus (20–100 μA) every 20 sec in horizontal olfactory bulb slices from postnatal 6–14-d-old Sprague–Dawley rats (Fig. 1A). TBS (10 bursts of high frequency stimulation at 5 Hz, each burst containing five pulses at 100 Hz, same stimulation intensity as test stimuli) that mimics the sniffing cycles in the ON (Kepecs et al. 2006) was given after a baseline was taken. All the experiments were done in aCSF containing (in millimolars) 119 NaCl, 2.5 KCl, 1.3 MgSO₄, 1 NaH₂PO₄, 26.2 NaHCO₃, 22 mM glucose, and 2.5 CaCl₂, equilibrated with 95% O₂/5% CO₂. Field recording pipettes were filled with aCSF. All recordings were acquired at 30°C–32°C. Data are presented as mean ± SEM. Student''s t-test was used to determine statistical significance.Open in a separate windowFigure 1.Theta LTP can be induced in the olfactory bulb first synapses. (A) Stimulation and recording configuration. A bipolar stimulation pipette was placed on a bundle of ON fibers that innervated the recorded glomerulus. ON, olfactory nerve; GL, glomerular layer; MC, mitral cell. (B–D) LTP of glomerular field EPSPs induced by ON theta burst stimulation (TBS). (B) Time course of glomerular field EPSPs (upper: single example; lower: average traces from N = 34 recordings). Note field EPSP was potentiated following TBS. (C) Paired-pulse ratio (PPR, N = 7) of ON field EPSPs (interval 50 msec) was depressed following TBS. (D) LTP of glomerular field EPSPs was D-APV independent (N = 5). (E–G) TBS of ON induced LTP in postsynaptic external tufted (ET) cells. (E) Time course of EPSCs recorded from ET cells (N = 6). (F) PPR of EPSCs (N = 4). (G) LTP of ET cell EPSCs are D-APV independent (N = 3). (H,I) TBS of ON induced potentiation of periglomerular (PG) cell EPSCs. (H) Time course of EPSCs (N = 8). (I) PPR of EPSCs (N = 4).There was on average a 14.5 ± 2.5% increase in the field EPSP peak amplitude at 30 min post TBS induction (N = 34, t = 5.82, P < 0.001, Fig. 1B). Bath application of D-APV (50 μM), an NMDAR antagonist, did not eliminate TBS potentiation; the EPSP peak is 115.6 ± 5.4% of baseline, 30 min post-induction (N = 5, t = 2.90, P = 0.022, Fig. 1D). This result suggests that plasticity occurs at the first step of odor processing (Ennis et al. 1998; Mutoh et al. 2005; Tyler et al. 2007; Dong et al. 2008; Jones et al. 2008). Hence, the synapses between the ON and its postsynaptic neurons are potential targets for learning-dependent plasticity as suggested by the long-lasting metabolic and anatomical changes observed at the olfactory bulb glomerular level following early odor preference training (Coopersmith and Leon 1986; Wilson et al. 1987; Woo et al. 1987; Wilson and Leon 1988; Woo and Leon 1991; Johnson et al. 1995; Yuan et al. 2002). Paired stimuli given to the ON using a 50-msec interval were used to test presynaptic changes to TBS. Paired-pulse ratios, an indicator of changes in presynaptic release (Murphy et al. 2004), decreased following TBS (91.5 ± 2.8% of control, N = 7, t = 3.05, P = 0.011, Fig. 1C). The decrease in paired-pulse ratio suggests TBS potentiation is presynaptically mediated. The glomerular potentiation seen here is consistent with a recent adult mouse model showing an increase in odor-specific glomeruli and olfactory sensory neurons following odor learning (Jones et al. 2008).In the second set of experiments, glomerular excitatory postsynaptic currents (EPSCs) in postsynaptic juxtaglomerular (JG) cells were recorded in voltage-clamp mode (membrane potential held at −60 mV). The effects of TBS on JG cell EPSCs were tested. Patch pipettes were filled with an internal solution containing (in millimolars) 114 K-gluconate, 17.5 KCl, 4 NaCl, 4 MgCl₂, 10 HEPES, 0.2 EGTA, 3 Mg₂ATP, and 0.3 Na₂GTP. There are two main populations of JG cells in the glomeruli, periglomerular (PG) cells, and external tufted (ET) cells. PG cells are inhibitory on MCs (Murphy et al. 2005), while ET cells are excitatory neurons, which receive monosynaptic ON input and then excite PG interneurons (Hayar et al. 2004). PG cells form two functionally distinct populations: ∼30% are driven by monosynaptic ON input, while the remaining population receive their input mainly through ET cells (ON–ET–PG circuit) (Shao et al. 2009). These JG cells form a glomerular inhibitory network in which inhibitory PG cells (activated by either ON input or ET cells) provide both feed-forward and feedback inhibition to MCs of the olfactory bulb through dendrodendritic synapses (Hayar et al. 2004; Murphy et al. 2005). ET cells can be distinguished from PG cells by their morphology, location, and characteristic spontaneous rhythmic spike bursting pattern recorded extracellularly before switching to whole-cell voltage-clamp mode (Hayar et al. 2004; Dong et al. 2007). ET cells were significantly potentiated by TBS while the TBS effect on PG cells was more moderate and more variable (ET: 124.1 ± 11.4% of control at 30 min post-induction, N = 6, t = 2.12, P = 0.043, Fig. 1E; PG: 115.7 ± 15.5%, N = 8, t = 1.01, P = 0.172, Fig. 1H). It should be noted that there is a stronger dialysis effect in small cells, such as the PG cells, which is suggested to account for the weaker and more variable potentiation in this subgroup. Three PG cells showed compelling potentiation (164 ± 13.5% of control at 30 min post-induction, t = 4.73, P = 0.021). The paired-pulse ratio of the EPSCs was reduced following TBS induction (ET: 81.6 ± 4.6% of control, N = 4, t = 4.01, P = 0.014, Fig. 1F; PG: 80.1 ± 10.5%, t = 1.90, P = 0.070, Fig. 1I), consistent with a presynaptic expression mechanism. Furthermore, ET cell LTP was NMDA-receptor independent when tested with APV (161.9 ± 23.3% of control, 15 min post-induction, N = 3, t = 2.66, P = 0.028, Fig. 1G).The role of theta LTP in learning is of particular interest because this pattern of activation is likely to occur in the ON. Since theta is the frequency associated with sniffing in the olfactory bulb (Kepecs et al. 2006), it is straightforward to hypothesize that it has a role in the learning of odor associations. Yet odor preference learning does not occur with odor exposure per se. An interaction between a theta paced odor input and the arousal modulator, norepinephrine (NE), is critical.In the third set of experiments, calcium imaging was used to examine the network of MC responses in each slice. Conventionally, it has been assumed that the glomerular field EPSP mostly reflects MC activity (Mori 1987; Shipley et al. 1996). But recent studies demonstrate the contribution of other cell types to the glomerular field EPSP such as the JG cells (ET and PG cells) (Karnup et al. 2006) monitored here. Therefore, the glomerular field EPSP is not a simple summation of synchronized MC activity. Moreover, synaptic activities at MC dendritic tufts in glomeruli may not propagate efficiently enough to the soma to change MC spiking rate due to the significant length of MC apical dendrites (Karnup et al. 2006). Therefore, this set of experiments used direct monitoring of MC activity through calcium imaging to characterize activity-dependent changes in the olfactory bulb output map. Calcium imaging permits the examination of a large population of neurons with single-cell resolution (Egger 2007).A calcium indicator dye, Oregon Green BAPTA-1, was pressure-injected into the MC layer to stain populations of MCs (Fig. 2A,B). Calcium transients were imaged at 494 nm excitation (15–20 Hz, 2 × 2 binning). Regions of interest (∼20 μm diameter) centered over MC somata were used for kinetic analysis. By selecting 8–12 cells per slice (including both strongly and weakly activated cells), and measuring single MC somatic calcium transients (ΔF/F, average over six to eight trials, background subtracted from an immediate adjacent area next to the somata), changes were observed in the MC responses to ON stimulation (40–100 μA) 30 min following TBS (the same time point at which the magnitude of theta LTP in the glomerular layer was measured). The overall MC calcium responses to ON stimulation were not significantly affected by TBS (113.0 ± 10.5% of control, N = 74 cells from nine slices, t = 1.24, P = 0.220, Fig. 2 panel C1). The olfactory bulb network is, as noted, functionally complex with both feed-forward and feedback inhibition modifying the MC responses to ON/odor stimulation (Murphy et al. 2005). Given the increase in coupling to inhibitory interneurons seen in the second set of experiments, it may be that enhanced feed-forward inhibition from the inhibitory JG network prevented an increase in MC responding. However, it should be noted that increases in MC responses related to TBS could be masked due to the limitation of the in vitro methodology (e.g., selected cells may include those projecting to glomeruli further away from the stimulation site and those deeper in the tissue which have weaker signal-to-noise ratios). However, as subsequent experiments, described below, reveal significant MC calcium responses with the same in vitro methodology, it is likely that the response to TBS alone is relatively minor.Open in a separate windowFigure 2.Pairing of TBS and isoproterenol (ISO) increased MC calcium responses. (A) ΔF/F calcium image (20× objective) of a slice stained with a calcium indicator dye, Oregon Green BAPTA-1 AM. The MC layer is labeled by white dashed lines. Scale bar, 50 μm. (B) An example of ΔF/F calcium imaging showing enhanced calcium responses following TBS+ISO in one MC (white asterisk). Lower traces are calcium transients recorded from the soma of the MC. (C) Averaged calcium transient changes in MCs following TBS, ISO, and TBS+ISO (normalized to control), measured at 30 min post-manipulations. **P < 0.01. (C1) Single-cell calcium transient changes before and after TBS. Dashed black line indicates no change. Cells above the dashed line show increased calcium responses, whereas those below the dashed line show decreased calcium responses. (C2) MC calcium responses to paired TBS and ISO. Note that the majority of MCs showed increased calcium responses following TBS+ISO. (C3) MC calcium responses to ISO only.Since TBS itself was not sufficient to produce significant MC calcium responses, the combined effect of TBS and the β-adrenoceptor agonist, isoproterenol, were examined. Isoproterenol was applied to the bath solution 5–10 min before the TBS and washed out after the TBS induction. The pairing of TBS and isoproterenol (10 μM) increased MC calcium responses in most of the cells measured (137.0 ± 11.0% of control, N = 55 from five slices, t = 3.27, P < 0.001, Fig. 2 panel C2). Five to ten min application of isoproterenol alone to the bath solution did not alter the MC responses observed 30 min after isoproterenol washout (102.5 ± 8.8% of control, N = 47 from four slices, t = 0.28, P = 0.781, Fig. 2 panel C3). Thus, the potentiated MC calcium response was only seen with paired isoproterenol and TBS. As a caveat it should be noted that MCs fire action potentials spontaneously in vivo at ∼3 Hz (Cang and Isaacson 2003) and in vitro at ∼15 Hz (up to 75 Hz) (Griff et al. 2008). A change in ΔF/F calcium response reflects a change in the ratio of the evoked response over the baseline spontaneous response. Changes in MC spontaneous firing, therefore, can affect the ΔF/F calcium signal measured from the MC. However, if pairing TBS with isoproterenol increased spontaneous firing, the increase in the evoked firing rate of MCs had to occur to a greater degree.This result, that only the pairing of TBS with isoproterenol enhanced MC calcium responses, correlates with the behavioral studies (Langdon et al. 1997; Sullivan and Leon 1987; Sullivan et al. 2000) showing that only the pairing of an odor and isoproterenol produces associative learning, while either odor alone or isoproterenol alone does not lead to associative learning. It supports the view that at the level of physiological mechanism, classical conditioning is the interaction of arousal modulation and theta modulation while either alone does not create the necessary associative conditions. Consistent with previous work showing enhanced CREB phosphorylation in MCs following learning (McLean et al. 1999; Yuan et al. 2000, 2003a), the present calcium imaging results support the hypothesis that odor learning results in increased firing in odor encoding MCs. Increased firing in MCs is also consistent with recent work by Gire and Schoppa showing that the pairing of NE with TBS induces an enhancement of MC long-lasting depolarization and gamma frequency oscillation (Gire and Schoppa 2008). Interestingly, MC firing is mainly suppressed to a familiar odor in neonatal rats (Wilson et al. 1985). TBS potentiation of the inhibitory JG circuitry may contribute to the odor habituation observed behaviorally.In the fourth set of experiments, the effects of isoproterenol on JG cell activity were examined. Previous research using slice physiology to identify a synaptic site of NE action was constrained by the known distribution of noradrenergic input to the olfactory bulb. Noradrenergic fibers project heavily to the subglomerular layers; they terminate densely in the internal plexiform and the granule cell (GC) layers, and moderately in the external plexiform and the MC layers (McLean et al. 1989). Based on this anatomical evidence, it was assumed that either GCs or MCs were the potential targets for β-adrenoceptor action. However, the Ennis group showed that the β-adrenoceptor agonist isoproterenol has no direct effect on MC excitability in acute rat olfactory bulb slices (Hayar et al. 2001). Although isoproterenol caused an inward current in MCs in voltage-clamp mode, this inward current was blocked by synaptic transmission blockers, suggesting an indirect effect of isoproterenol, possibly through interneurons. NE could disinhibit MCs through suppressing GC activity as reported in the turtle and dissociated rat olfactory bulb cultures (Jahr and Nicoll 1982; Trombley and Shepherd 1992; Trombley 1994). However, this effect was attributed to an α2-, but not β-adrenoceptor, mediated presynaptic inhibition of GC and/or MC dendrites (Trombley 1992, 1994; Trombley and Shepherd 1992). Since isoproterenol here altered MC calcium responses to ON theta burst input, it was possible that JG cells were potential targets for NE action. Studies using immunocytochemistry (Yuan et al. 2003b) and receptor autoradiography (Woo and Leon 1995) show that β-adrenoceptors are expressed in JG cells (Woo and Leon 1995; Yuan et al. 2003b), as well as in MCs (Yuan et al. 2003b) and GCs (Woo and Leon 1995).With the application of isoproterenol the JG cell EPSCs to ON stimulation were suppressed (ET: 86.6 ± 6.0% of control, N = 6, t = 2.25, P = 0.037, Fig. 3A; PG: 87.3 ± 3.6%, t = 3.55, P = 0.006, Fig. 3A). Isoproterenol was applied for 5 min in the bath solution and then washed out. The effect of isoproterenol was reversed after washout (94.8 ± 6.7%, N = 3, t = 0.78, P = 0.259, Fig. 3A). Similar results were found with calcium imaging using the averaged cell calcium transients from 8–20 cells per slice (Fig. 3B). The averaged response from each slice was counted as one experiment. JG cell calcium transients were suppressed in the presence of isoproterenol (83.7 ± 3.0% of control, N = 7, t = 5.40, P = 0.002, Fig. 3B). The effect of isoproterenol was reversed 30 min following washout (100.6 ± 1.8% of control, N = 3, t = 0.36, P = 0.754). In two experiments, recordings were made from slices that had a cut between the MC and the glomerular layer to isolate the potential secondary effect from MCs onto JG cells. In this case, isoproterenol still suppressed JG cell calcium transients (Fig. 3B, dashed circles).Open in a separate windowFigure 3.The effects of ISO on JG cells (PG+ET cells). (A) ISO (10 μM) application suppressed EPSCs of ET cells (N = 6) and PG cells (N = 7), which was reversed following ISO washout (N = 3). *P < 0.05, **P < 0.01. (B) ISO application suppressed ON-evoked calcium transients in JG cells (N = 7 slices). The ISO effect was reversed 30 min after washout (N = 3). N = 2 experiments were recorded from slices of a cut that was made between the glomerular layer and the MCs layer. **P < 0.01Given the results of both whole-cell recording and calcium imaging, it was reasonable to hypothesize that isoproterenol would decrease JG cell activity, reducing GABA release onto MCs and causing MC disinhibition. Indeed, it has been shown NE application caused a reduction of inhibitory postsynaptic currents (IPSCs) in MCs (Gire and Schoppa 2008). Taken together these findings support the enhanced MC excitation model for early odor preference learning. While the present experiments were conducted with the β_1/2 agonist, isoproterenol, associative odor preference learning depends on β₁, not β₂, receptors (Harley et al. 2006). β₁ adrenoceptors are also expressed in JG cells (Yuan et al. 2003b). The role of specific β-adrenoceptor subtypes in mediating MC disinhibition will be tested in the future studies.The present results argue that potentiation of MC calcium responses occurs only when theta frequency activity is paired with β-adrenoceptor activation. They also suggest that one critical role of NE activation via β-adrenoceptors in the olfactory bulb is to suppress the inhibitory JG network, subsequently transiently disinhibiting MCs and providing the conditions for strengthening ON–MC connections in selected glomeruli. This mechanism likely operates in concert with changes promoted by patterned cAMP waves in MCs (Cui et al. 2007).     

Perirhinal cortex is necessary for acquiring,but not for retrieving object–place paired association

Remembering events frequently involves associating objects and their associated locations in space, and it has been implicated that the areas associated with the hippocampus are important in this function. The current study examined the role of the perirhinal cortex in retrieving familiar object–place paired associates, as well as in acquiring novel ones. Rats were required to visit one of two locations of a radial-arm maze and choose one of the objects (from a pair of different toy objects) exclusively associated with a given arm. Excitotoxic lesions of the perirhinal cortex initially impaired the normal retrieval of object–place paired-associative memories that had been learned presurgically, but the animals relearned gradually to the level of controls. In contrast, when required to associate a novel pair of objects with the same locations of the maze, the same lesioned rats were severely impaired with minimal learning, if any, taking place throughout an extensive testing period. However, the lesioned rats were normal in discriminating two different objects presented in a fixed arm in the maze. The results suggest that the perirhinal cortex is indispensable to forming discrete representations for object–place paired associates. Its role, however, may be compensated for by other structures when familiar object–place paired associative memories need to be retrieved.Remembering an event in space often requires associating objects and their locations. Associating object and place information into a unitary event representation is believed to be a foundation of episodic memory (Cahusac et al. 1989; Gaffan 1994; Davachi 2006). It has been suggested that the hippocampus and its associated regions in the medial temporal lobe (MTL) are essential in this cognitive process, and amnesic patients with damage in the MTL structures exhibit severe deficits in associating object and place information (Smith and Milner 1981; Vargha-Khadem et al. 1997; Stepankova et al. 2004). Animal models produced by localized lesions in the hippocampus and other MTL structures also support the idea by showing that the lesioned animals are impaired in associating objects and places (Parkinson et al. 1988; Gaffan and Parker 1996; Sziklas et al. 1998; Bussey et al. 2001; Gilbert and Kesner 2003, 2004; Malkova and Mishkin 2003; Lee et al. 2005; Bachevalier and Nemanic 2008; Kesner et al. 2008; Lee and Solivan 2008). Although the theoretical importance of the MTL structures in object–place association has been well acknowledged, specific contributions of the MTL structures in object–place associative memory are poorly understood. The current study examined the role of the perirhinal cortex, one of the extra hippocampal regions in the MTL, using a behavioral paradigm previously shown to be dependent on the intact hippocampus (Lee and Solivan 2008).The literature suggests that the role of the hippocampus in the object–place paired-associate task is to put together object and place information into a unified and distinct event representation. It has been suggested that spatial information and nonspatial information (such as object information) may be streamed into the hippocampus in a relatively segregated fashion, the former information mostly fed through the medial entorhinal cortex to the hippocampus via the postrhinal cortex and the latter being fed through the lateral entorhinal cortex via the perirhinal cortex (Mishkin et al. 1997; Suzuki et al. 1997; Burwell 2000; Fyhn et al. 2004; Witter and Amaral 2004; Hafting et al. 2005; Hargreaves et al. 2005; Furtak et al. 2007; Kerr et al. 2007). In our previous study (Lee and Solivan 2008) in which rats were required to discriminate rewarding versus nonrewarding pairs of similar object–place paired associates, the hippocampal lesioned rats demonstrated severe and irrecoverable deficits. The results from the study not only corroborate the long-held view that the hippocampus associates object and place information, but also demonstrate that the hippocampus is critical for disambiguating similar object–place paired associates. However, it requires examining functions of other upstream structures of the hippocampus to conclusively assign the role of associating object and place information to the hippocampus. If, for example, lesions produced in the perirhinal cortex produce similar deficits, it would be premature to conclude that the association between object and place information uniquely occurs in the hippocampus.To elucidate the relative contributions of the MTL structures in the hippocampal-dependent object–place paired-associate task (Fig. 1), we manipulated the perirhinal cortex in the current study, one of the regions implicated as an object-information provider to the hippocampus (Knierim et al. 2006; Eichenbaum and Lipton 2008). Here we tested whether the perirhinal cortex was involved in the acquisition of new object–place paired associations. Importantly, we also tested the perirhinal cortical contributions to retrieving learned paired associates between objects and places. In the current study, the rats needed to pay attention to both object and place information. Therefore, if the perirhinal cortex is unique in its function for providing object information to the hippocampus, it is predicted that lesions in the perirhinal cortex will produce severe deficits as seen in the hippocampal lesioned animals in our previous study. A simple object-discrimination task that did not require spatial information was also employed to further examine the role of the perirhinal cortex only in specific conditions.Open in a separate windowFigure 1.Illustration of the radial arm maze and behavioral paradigms. (A) Phase 1: Two objects (Spider-Man and LEGO block) were presented on arms 3 and 5 in gray color. Only one of the objects was rewarded in arm 3 (Spider-Man) and arm 5 (LEGO block) irrespective of its locations in the choice platform. Possible configuration of objects and appropriate choices are provided for both arms. In each trial, only one arm was open in the maze and objects were available in that open arm. (B) Phase 2: For acquisition of novel object–place paired associations, a pair of new objects (Barney and Girl) was presented on arms 3 and 5. Possible locations of the objects are shown as in A. Each object was rewarded only in a particular arm (Barney in arm 3 and Girl in arm 5) irrespective of its location in the choice platform. (C) Phase 3: Illustration of the task using only one arm (arm 4) in the maze. Two new objects (Mr. Potatohead and Cylinder) were used and the Mr. Potatohead choice was rewarded regardless of its location in the choice platform.     

Making context memories independent of the hippocampus

We present evidence that certain learning parameters can make a memory, even a very recent one, become independent of the hippocampus. We confirm earlier findings that damage to the hippocampus causes severe retrograde amnesia for context memories, but we show that repeated learning sessions create a context memory that is not vulnerable to the damage. The findings demonstrate that memories normally dependent on the hippocampus are incrementally strengthened in other memory networks with additional learning. The latter provides a new account for patterns of hippocampal retrograde amnesia and how memories may become independent of the hippocampus.Contextual fear conditioning can be supported by two neural systems, one that contains the hippocampus (HPC), and one that does not. Evidence for this assertion comes from studies in which the HPC, in rats, is damaged either before or after the contextual fear conditioning. Extensive damage to the HPC before conditioning has little effect on contextual fear conditioning (Maren et al. 1997; Frankland et al. 1998; Wiltgen et al. 2006). This result can only mean that there is a non-HPC memory system that can support fear of context. In contrast, there is unequivocal evidence that moderate to extensive damage to the HPC soon after learning severely impairs the ability of the conditioning context to evoke fear, suggesting that the HPC normally makes a major contribution to this type of memory (Kim and Fanselow 1992; Maren et al. 1997; Frankland et al. 1998; Anagnostaras et al. 1999; Debiec et al. 2002; Lehmann et al. 2007b; Sutherland et al. 2008; Wang et al. 2009).The dissociable effects of pre- and post-training HPC damage on contextual fear conditioning have been interpreted as suggesting that: (1) When the HPC is intact during learning it interferes with other systems and prevents them from acquiring an independent contextual fear conditioning memory, and (2) when the HPC is absent, these other systems are released from this interference and are able to rapidly acquire an independent memory (Maren et al. 1997; Frankland et al. 1998; Fanselow and Poulos 2004; Driscoll et al. 2005; Lehmann et al. 2006; Sutherland et al. 2006). The latter interference from the HPC on the other memory systems has been termed overshadowing. Supplemental Figure S1 depicts data from our laboratory demonstrating the overshadowing phenomenon and the dissociable effects of HPC damage induced before and after contextual fear conditioning.Very little, however, is known about the parameters determining the extent to which the HPC system interferes with the non-HPC system for control over contextual fear. The purpose of the current study is to provide some insight into this issue. Typically, contextual fear conditioning in rats is conducted in a single conditioning session in which a configuration of static background cues is paired with several footshocks. When returned to the conditioning context, rats display several species-specific defensive responses including freezing (i.e., absence of movement except for breathing). Several theorists have proposed that non-HPC systems are more likely to be recruited when there are multiple experiences with similar events, which, in turn, would mitigate the necessity of the HPC for memory expression (O''Keefe and Nadel 1978; Sherry and Schacter 1987; McClelland et al. 1995; O''Reilly and Rudy 2001; White and McDonald 2002). Accordingly, we hypothesized that repeated contextual fear conditioning sessions separated by hours and days would overcome the HPC interference or overshadowing effect. In other words, with repeated learning sessions, enough information would be incrementally captured by the non-HPC system to support a contextual fear memory that would survive complete damage to the HPC.Adult male rats received 11 fear-conditioning sessions across 6 d. In each session, they were placed in a context and received mild footshocks (Shock Context). Concurrently, the rats were exposed 10 times to another context in which they never received shock (No-Shock Context). The No-Shock Context served as a control condition to measure whether the rats simply showed generalized fear or could show context-specific memory. Within 72 h following the last conditioning session, rats either received sham surgery or complete lesions of the HPC using the neurotoxin N-methyl-d-aspartic acid (NMDA) (Lehmann et al. 2007a). Rats were then tested for retention in both the Shock and No-Shock Contexts in a counterbalanced order. In addition, in a single learning episode, another group of rats received a matching number of shocks (i.e., 12 shocks) and context exposure (i.e., 17 min), and then received surgery 7–10 d after conditioning. The latter interval is identical to the interval between the initial conditioning session and surgery in the repeated learning condition. Figure 1 illustrates and describes the design of the experiments.Open in a separate windowFigure 1.Illustration of the experimental design used in (A) the single conditioning session and (B) repeated conditioning session experiments. In A the rats were initially placed in the conditioning chamber for 17 min and received the first of 12 footshocks (1 mA/2 sec) at the 300-sec mark, and then one every following 58 sec after shock offset. Seven to 10 d later, the rats either received sham or HPC damage (Sx). Approximately 10 d after, the rats were returned to the chamber to assess freezing over a 5-min retention test. In B the rats were placed initially in the conditioning chamber for 1 min and received a shock at the 45-sec mark (Shock Context). Approximately 45 min later, the rats were placed in a different chamber for 1 min and did not receive shock (No-Shock Context). The procedure was repeated twice daily for five consecutive days, and the Shock and No-Shock chamber order was counterbalanced according to the principles of a Latin Square design. The rats then received sham or HPC damage 1–3 d later. The rats'' retention was assessed in both contexts ∼10 d after surgery in both the Shock and No-Shock Context in a counterbalanced order with a 24-h span between tests. Importantly, the number of shocks, context exposure time, and interval between initial learning and surgery were matched between both experiments.When all shocks were delivered in a single session, HPC damage caused profound retrograde amnesia. As illustrated in Figure 2A, the HPC rats displayed significantly less freezing than control rats during the retention test (t₍₈₎ = 23.895, P < 0.001). This result replicates all previous studies in which the HPC was damaged days after a single contextual fear conditioning training session (Kim and Fanselow 1992; Maren et al. 1997; Frankland et al. 1998; Anagnostaras et al. 1999; Debiec et al. 2002; Lehmann et al. 2007b; Sutherland et al. 2008).Open in a separate windowFigure 2.Mean (± SEM) percent time freezing by Sham and HPC rats during the retention test of the (A) single conditioning (12 shocks) experiment and (B) repeated conditioning session experiment. In A the HPC rats showed significantly less freezing (P < 0.001) than the Sham rats, suggesting that the damage caused profound retrograde amnesia for contextual fear conditioning learned in a single session 7–10 d before surgery. In B the performance of the HPC rats did not significantly differ from the Sham rats, and they exhibited significantly more freezing in the Shock Context than the No-Shock Context (P < 0.001). Consequently, repeated conditioning sessions prevented the retrograde amnesic effects normally observed in contextual fear conditioning following HPC damage, suggesting that other neural networks were now able to support the memory.In striking contrast, memory for contextual fear conditioning was spared when the HPC was damaged after repeated conditioning sessions. Figure 2B shows the percent time spent freezing during the retention test in the Shock and No-Shock Contexts. An ANOVA with between-group factor (Lesion: Sham and HPC) and within-group factor (Context: Shock and No-Shock) revealed a significant main effect of Context (F_(1,14) = 84.731, P < 0.001), indicating that the rats displayed higher levels of freezing in the Shock than in the No-Shock Context. The effect of Lesion (F_(1,14) = 4.280, P = 0.058) was not significant, nor was the Lesion × Context interaction (F_(1,14) = 0.877, P = 0.369), suggesting that extensive HPC damage did not impair memory. The tendency for an effect of Lesion is due to the HPC rats freezing less than the Sham rats in the No-Shock Context (P = 0.06) rather than freezing less in the Shock Context (P = 0.457).The repeated conditioning sessions clearly enabled a contextual fear representation to be established in non-HPC memory systems. However, it is surprising that the HPC damage did not impair the ability to discriminate between the Shock and No-Shock Context, because evidence suggests that context discrimination is dependent on the HPC (see Moscovitch et al. 2006). Indeed, studies of rats with HPC damage induced before learning have shown that contextual fear conditioning is acquired quickly by non-HPC systems in a single session, but the ability to discriminate between the training context and a new context is lost (Frankland et al. 1998; Antoniadis and McDonald 2000; Winocur et al. 2007). Hence, it is significant in the present study that the HPC damage did not impair context discrimination abilities in the rats that received repeated learning episodes. The latter appear to have established a context representation, outside of the HPC, that was not bereft of details. Yet, one should consider that the rats in the repeated sessions experiment received experience in both the Shock and No-Shock Contexts prior to surgery, and this discrimination training procedure may have established two different non-HPC representations. It remains possible that HPC damage would impair the ability to discriminate the Shock Context from a new context, which is what is found in anterograde amnesia studies (Frankland et al. 1998; Antoniadis and McDonald 2000; Winocur et al. 2007). To address this possibility, a new experiment examined whether HPC-damaged rats could discriminate the Shock Context from a Novel Context. Rats were trained with the same repeated learning protocol as described earlier, with the exception that the rats were never placed in the No-Shock Context prior to surgery. One to 3 d following learning, the rats either received Sham or complete HPC damage. They were then tested for retention in the Shock and the Novel (i.e., No-Shock) Context in a counterbalanced order. Figure 3 shows the percent time spent freezing during the retention test in the Shock and Novel Contexts. An ANOVA with between-group factor (Lesion: Sham and HPC) and within-group factor (Context: Shock and Novel) revealed that the rats froze significantly more in the Shock than the Novel Context (F_(1,10) = 57.393, P < 0.001). However, no significant difference was found between the HPC and Sham groups (F_(1,10) = 0.597, P = 0.458) and the Lesion × Context interaction did not reach significance (F_(1,10) = 0.123, P = 0.733). Thus, as in the previous repeated sessions experiment, the HPC damage did not cause retrograde amnesia for contextual fear conditioning and, more importantly, the HPC damage did not impair the ability to discriminate between the original context and new context.Open in a separate windowFigure 3.Mean (± SEM) percent time freezing by Sham and HPC rats in the Shock and Novel Contexts during the retention tests of the discrimination experiment. The rats exhibited significantly more freezing in the Shock than the Novel Context (P < 0.001), and the HPC rats did not significantly differ from the Sham rats, suggesting that the HPC-damaged rats remembered the specific meaning of the Shock Context as well as control rats. Hence, repeated conditioning sessions established a context-rich representation in non-HPC systems, which supports successful context discriminations.The absence of amnesia for contextual fear conditioning in the current study is not due to insufficient damage to the HPC. We calculated (see Lehmann et al. 2007b) that an average of 83% of the HPC was damaged across rats (smallest: 64%; largest: 90%) in the repeated learning experiments (see Supplemental material for more histological details). The amount of HPC damage is substantially more than that found in most studies reporting impairments for contextual fear conditioning following HPC damage (Kim and Fanselow 1992; Maren et al. 1997; Frankland et al. 1998; Anagnostaras et al. 1999; Debiec et al. 2002) and more than for the single-session experiment (average 76%) in which we currently report amnesia. Therefore, the amount of HPC damage inflicted in the rats in this study is certainly sufficient to disrupt HPC-dependent memories.Like others (Kim and Fanselow 1992; Maren et al. 1997; Frankland et al. 1998; Anagnostaras et al. 1999; Debiec et al. 2002; Lehmann et al. 2007b; Sutherland et al. 2008), we found that damage to the HPC after a single contextual fear-conditioning session involving multiple shocks produces profound retrograde amnesia for contextual fear conditioning. However, in two separate experiments, distributing shock across multiple conditioning sessions prevented this amnesia. In one case, the rats experienced Context–Shock pairings in one context and no shock in another context. Following this training, rats with damage to the HPC did not differ from control rats in the absolute amount of freezing in the training context nor in their ability to discriminate between the two contexts. In the second case, rats only received the multiple Context–Shock sessions. Rats with damage to the HPC could not be distinguished from control rats during the test in the training context or in their responses to a novel context. These findings provide new support for the general idea that contextual fear conditioning can be supported by both HPC and non-HPC systems. This conclusion is supported by (1) the finding that damage to the HPC following a single conditioning session virtually eliminates freezing during the test, implying the importance of the HPC system, and (2) that following multiple conditioning sessions, damage to the HPC has no effect on either contextual fear displayed in the training context or their ability to discriminate the training context from other contexts, suggesting the existence of non-HPC systems that can support contextual fear. The findings also reveal that the overshadowing or interference by the HPC over the non-HPC memory systems for control over contextual fear is not absolute. Following a single conditioning session, removal of the HPC produced a devastating retrograde amnesia, illustrating substantial overshadowing. However, distributing conditioning across several sessions completely attenuated the effects of damage to the HPC, revealing that non-HPC systems can support contextual fear conditioning despite the HPC, and revealed the importance of multiple sessions for this to occur.The overshadowing by the HPC is based on the familiar idea in associative learning at the behavioral level, where through a competitive process some of the cues that redundantly predict a reinforcer acquire the ability to generate strong conditioned responding, while other equally predictive, but less salient cues do not (Stout et al. 2003). Conditioning to the less potent cues proceeds more effectively if the more potent competitors are absent. Following the same principle, if the HPC representation is active, then learning in the non-HPC systems suffers strong interference. In contrast, in the absence of the HPC representation, learning in non-HPC systems is released from this interfering effect of the HPC. Thus, the learning rate in non-HPC networks is potently lowered by the activity of the HPC. However, with repeated learning, other structures, which are overshadowed by the HPC, may cumulatively build a representation that achieves HPC independence. The current findings clearly support this hypothesis, whereby repeated learning episodes incrementally established a contextual fear-conditioning representation outside of the HPC that mitigated the usual retrograde amnesic effects of HPC damage.One important question is where does the HPC interference occur? Biedenkapp and Rudy (2009) recently reported that the HPC competes with the basolateral region of the amygdala during fear conditioning. Previously, Guarraci et al. (1999) found that the amount of conditioned fear produced by training could be increased if the dopamine D1 receptor agonist {"type":"entrez-protein","attrs":{"text":"SKF82958","term_id":"1156217255","term_text":"SKF82958"}}SKF82958 was injected into the basolateral region. Biedenkapp and Rudy (2009) reasoned that if this is the area where the HPC interferes with non-HPC systems for the association with shock, then a local infusion of {"type":"entrez-protein","attrs":{"text":"SKF82958","term_id":"1156217255","term_text":"SKF82958"}}SKF82958 before a single session of contextual fear conditioning should attenuate the interference and allow the non-HPC system to gain more control over contextual fear. Their data supported this hypothesis, which leads to the possibility that with multiple conditioning sessions, the non-HPC system gradually gains association with these fear-supporting neurons in this region of the brain.Patients with bilateral damage to the HPC often exhibit temporally graded retrograde amnesia, such that recently acquired memories are lost, whereas remote memories, especially those acquired years before the damage, are more likely to be spared (Scoville and Milner 1957; Rempel-Clower et al. 1996). This pattern of amnesia is taken as evidence for temporally based systems consolidation, whereby over time the essential support for memories is “switched” from dependence on the HPC to neocortical networks (McClelland et al. 1995; Squire and Alvarez 1995; Anagnostaras et al. 2001; Meeter and Murre 2004; Squire et al. 2004; Wiltgen et al. 2004; Frankland and Bontempi 2005). Our research, however, points to another process for becoming independent of the HPC, a change in the strength of the representation in non-HPC systems during learning rather than a consolidation process linked to the passage of time since the learning episode. A study of a former London taxi driver with bilateral HPC damage alludes to this possibility (Maguire et al. 2006). This amnesic patient showed greater retrograde amnesia for roads that he used less commonly than the major arteries that he used regularly. Hence, greater exposure to the major arteries established memories in non-HPC systems, whereas roads with less exposure remained dependent on the HPC regardless of the age of the memory. Our findings add support to this view, because studies examining the effects of complete HPC damage after a single conditioning episode suggest that the HPC is permanently involved in contextual fear conditioning (Lehmann et al. 2007b; Sutherland et al. 2008); yet, with repeated learning episodes we clearly demonstrated that the memory rapidly becomes independent of the HPC. The latter is important because the process for memories becoming independent of the HPC need not require systems consolidation.In conclusion, this is the first example of intact contextual fear memories following complete HPC damage induced soon after learning. Importantly, repetition of the learning episode underlies the change in memory from HPC dependent to HPC independent. We argue that each learning episode incrementally establishes a representation in non-HPC memory systems—a representation that ultimately becomes sufficiently strong to support memory expression without the HPC. The current findings also demonstrate the critical need to consider learning parameters when discussing patterns of retrograde amnesia and the role of the HPC in memory.     

New behavioral protocols to extend our knowledge of rodent object recognition memory

Animals often show an innate preference for novelty. This preference facilitates spontaneous exploration tasks of novelty discrimination (recognition memory). In response to limitations with standard spontaneous object recognition procedures for rodents, a new task (“bow-tie maze”) was devised. This task combines features of delayed nonmatching-to-sample with spontaneous exploration. The present study explored aspects of object recognition in the bow-tie maze not amenable to standard procedures. Two rat strains (Lister Hooded, Dark Agouti) displayed very reliable object recognition in both the light and dark, with the Lister Hooded strain showing superior performance (Experiment 1). These findings reveal the potential contribution of tactile and odor cues in object recognition. As the bow-tie maze task permits multiple trials within a session, it was possible to derive forgetting curves both within-session and between-sessions (Experiment 1). In Experiment 2, rats with hippocampal or fornix lesions performed at normal levels on the basic version of the recognition task, contrasting with the marked deficits previously seen after perirhinal cortex lesions. Next, the training protocol was adapted (Experiment 3), and this modified version was used successfully with mice (Experiment 4). The overall findings demonstrate the efficacy of this new behavioral task and advance our understanding of object recognition.Understanding the neural basis of recognition memory, the ability to discriminate whether a stimulus is novel or familiar, is heavily reliant on animal research. Here, advances have been closely tied to the introduction of new behavioral tests. The preeminent example concerns one-trial tests of recognition memory for monkeys using delayed nonmatching-to-sample (Mishkin and Delacour 1975). These tasks reward the natural preference that monkeys have for selecting novel items and permit multiple recognition trials within a single session. These features make the task relatively easy to train and then maximize findings from small group sizes. Although rat tasks closely based on delayed nonmatching-to-sample have been devised (Aggleton 1985; Mumby et al. 1990; Steckler et al. 1998; Prusky et al. 2004), they are very rarely employed as they are difficult to train and performance levels are unreliable.Almost all studies of rodent recognition memory now employ the spontaneous object recognition test and its direct variants (Ennaceur and Delacour 1988; Dix and Aggleton 1999; Winters et al. 2008). These tasks again take advantage of an innate preference for novel items, but this preference is displayed by spending more time exploring novel than familiar stimuli. In the standard version of the task, a rodent (rat or mouse) is placed in an arena containing two identical objects and then freely allowed to explore these objects for several minutes (“sample” phase). After a delay, the rodent is placed back in the arena (“test” phase), which now contains one familiar object (a copy of the sample phase objects) and a novel object. Recognition is signified by greater exploration of the novel object. Because the task measures spontaneous behavior, it requires minimal pretraining, but for the same reason it is prone to considerable variance. Unlike delayed nonmatching-to-sample, each trial (sample plus test phase) takes many minutes, and so only one recognition trial is normally given per session. Advantages are that proactive interference between objects is minimized, and the one-trial design lends itself to episodic-like tests of memory (Dere et al. 2005; Good et al. 2007). Disadvantages include the fact that data accumulation, with appropriate counterbalancing, is slow.To address these limitations, a new object recognition test using a “bow-tie maze” (Fig. 1A) has been developed for rats (Albasser et al. 2010). This test combines features of delayed nonmatching-to-sample with spontaneous object preference: It permits multiple trials per session, but the measure of recognition comes from the preferential exploration of novelty. The rat is first placed in one end of a bow-tie–shaped maze that contains a single object (object A; Fig. 1B). After a minute, the rat is allowed to run to the other end of the maze where there are two dissimilar objects (A and B; Fig. 1B). Object A is familiar as it is identical to the object previously explored, while object B is novel. Consequently, a rat will typically prefer to explore object B. On the next trial, a minute later, the rat shuttles back to the initial start point, but this time encounters objects B and C. Object B is now familiar, while object C is novel. The next trial, 1 min later, is between object C (now familiar) and object D (novel), and so on. A food reward placed under every object promotes shuttling back and forth within the maze, and encourages interaction with the objects. Unlike delayed nonmatching-to-sample, the food reward is not contingent on first selecting the novel object.Open in a separate windowFigure 1.(A) Schematic of the bow-tie maze. A sliding door separates the two ends of the maze in which two objects are placed. (B) General procedure showing the presentation order of the objects in the standard object recognition task. All objects are rewarded (+). (Arrow) Rat movements. (Black print) Novel objects, (gray print) familiar objects.The present study used the bow-tie maze to explore recognition memory on four fronts. In Experiment 1, recognition in the light and recognition in the dark were compared to help determine the cues available to detect object familiarity. It is known that rats can perform recognition tasks when solely reliant on visual cues (Aggleton 1985; Bartko et al. 2007; Winters and Reid 2010), but object recognition based on other modalities remains largely unexplored (Winters and Reid 2010). Potential cues for recognition include odor differences and tactile information. It is known that rats can discriminate novel from familiar olfactory cues (Otto and Eichenbaum 1992; Kesner et al. 2002; Fortin et al. 2004; Wolff et al. 2006), while tactile (e.g., vibrissae) cues can be used to distinguish surfaces (e.g., Birrell and Brown 2000). The bow-tie maze is ideal for studying object recognition in the dark as the rats are rewarded for visiting items in set locations (to receive food rewards), and so should readily approach the objects. In contrast, running the standard spontaneous object recognition test (in an arena) in the dark would be problematic as it is not clear how the rats would first appreciate the presence of the to-be-discriminated objects.An additional goal of Experiment 1 was to determine how readily the bow-tie maze could be used to compile within-session and between-session forgetting curves. A limitation with the standard spontaneous object task is that with only one trial per session it can be very time consuming to create forgetting curves, while within-session forgetting curves for individual animals are not feasible. These shortcomings create limitations when examining manipulations thought to affect memory. The possibility of deriving within-session forgetting curves is particularly appealing as: (1) it minimized the impact of those factors that introduce variance when performance is compared across sessions, and (2) the animal need not be removed from the maze, which could additionally disrupt performance, e.g., by increasing stress. A further component of Experiment 1 manipulated object memory strength by presenting objects either once (“single”) or six times (“repeated”). Recognition was tested after a 3-h delay with the twin goals of determining whether repeated presentation would aid performance and whether these performance levels would be sufficiently above chance so that they could be used to examine factors involved in longer term memory.At the same time, Experiment 1 provided the opportunity to compare two rat strains, Dark Agouti (DA) and Lister Hooded (LH). Previous studies suggest that the Dark Agouti strain might be particularly good at visual recognition tasks (Aggleton 1996), though others have argued that this strain has aberrant behavioral properties, including higher anxiety and higher levels of inappropriate nonspatial behaviors in spatial learning tasks (Mechan et al. 2002; Harker and Whishaw 2004; but see Aggleton and Vann 2004).Experiment 2 examined the ability of rats with either hippocampal or fornix lesions to perform object recognition in the bow-tie maze. There is a longstanding debate over the impact of hippocampal damage on recognition memory, with mixed findings coming from spontaneous object recognition tests (Clark et al. 2000; Mumby 2001; Gaskin et al. 2003; Winters et al. 2008). The bow-tie maze should prove informative as numerous trials can be run to assess the impact of selective brain lesions. Although in this initial study only short retention delays were examined, these same delays and conditions are highly sensitive to perirhinal cortex lesions (Aggleton et al. 2010; Horne et al. 2010), a brain region regarded as vital for recognition memory (Brown and Aggleton 2001). In Experiment 3 the training protocol changed so that objects did not cover food rewards. Rather, a single food reward was always placed between the test objects. This modification was examined because: (1) it would preclude any exploration scores that were simply derived from attempts to move the test objects in order to uncover the food reward, and (2) it would introduce a task variant that might be amenable to small rodents not able to move objects. Accordingly, Experiment 4 examined the performance of mice (strain C57Bl/6) on a test of object recognition based on the modified version of the bow-tie maze from Experiment 3.     

The basolateral amygdala and nucleus accumbens core mediate dissociable aspects of drug memory reconsolidation

A distributed limbic-corticostriatal circuitry is implicated in cue-induced drug craving and relapse. Exposure to drug-paired cues not only precipitates relapse, but also triggers the reactivation and reconsolidation of the cue-drug memory. However, the limbic cortical-striatal circuitry underlying drug memory reconsolidation is unclear. The aim of this study was to investigate the involvement of the nucleus accumbens core and the basolateral amygdala in the reconsolidation of a cocaine-conditioned stimulus-evoked memory. Antisense oligodeoxynucleotides (ASO) were infused into each structure to knock down the expression of the immediate-early gene zif268, which is known to be required for memory reconsolidation. Control infusions used missense oligodeoxynucleotides (MSO). The effects of zif268 knockdown were measured in two complementary paradigms widely used to assess the impact of drug-paired CSs upon drug seeking: the acquisition of a new instrumental response with conditioned reinforcement and conditioned place preference. The results show that both intranucleus accumbens core and intrabasolateral amygdala zif268 ASO infusions at memory reactivation impaired the reconsolidation of the memory underlying a cocaine-conditioned place preference. However, knockdown of zif268 in the nucleus accumbens at memory reactivation had no effect on the memory underlying the conditioned reinforcing properties of the cocaine-paired CS measured subsequently, and this is in contrast to the marked impairment observed previously following intrabasolateral amygdala zif268 ASO infusions. These results suggest that both the basolateral amygdala and nucleus accumbens core are key structures within limbic cortical-striatal circuitry where reconsolidation of a cue-drug memory occurs. However reconsolidation of memory representations formed during Pavlovian conditioning are differentially localized in each site.Through Pavlovian association with the effects of addictive drugs, a conditioned stimulus (CS) acquires both general motivational and sensory-specific conditioned reinforcing properties (Everitt et al. 2000). These associations contribute to the high likelihood of relapse in addicted individuals, yet the extinction of drug CSs by nonreinforced exposure has proved to be of limited therapeutic utility (Conklin and Tiffany 2002). In abstinent humans, drug CSs evoke salient and persistent memories of drug-taking experiences, inducing craving and relapse (Childress et al. 1988; O''Brien et al. 1992), while in animals they also precipitate relapse to, or reinstatement of, drug-seeking behavior (de Wit and Stewart 1981; Meil and See 1996; Fuchs et al. 1998; Weiss 2000). Thus, disrupting drug-related memories might significantly diminish relapse propensity on subsequent exposure to drug-paired CSs, and thereby promote abstinence.Exposure to a drug-associated CS also triggers a process of memory reconsolidation, which restabilizes the reactivated and labile memory (Nader 2003). While reconsolidation may adaptively update memories (Dudai 2006; Hupbach et al. 2007; Rossato et al. 2007; Lee 2009), its disruption may reduce the impact of intrusive or aberrant memories on behavior subsequently (Lee et al. 2005, 2006; Brunet et al. 2008; Kindt et al. 2009; Taubenfeld et al. 2009). The reconsolidation of CS–cocaine memories has been shown to depend upon protein synthesis and expression of the plasticity-associated immediate-early gene, zif268, in the basolateral amygdala (BLA), since zif268 knockdown at memory reactivation disrupted the acquired conditioned reinforcing properties of the CS measured in drug-seeking tasks days or weeks later (Lee et al. 2005, 2006).Although the BLA has an established role in CS-drug memory reconsolidation, it remains unclear whether other sites within limbic cortical-ventral striatal circuitry participate in this process. The nucleus accumbens core (AcbC) is a primary candidate, as zif268 is up-regulated in the AcbC as well as in the BLA following exposure to cocaine CSs (Thomas et al. 2003). Furthermore, the AcbC, which is strongly implicated in Pavlovian influences on drug seeking and relapse (Cardinal et al. 2002; Kalivas and McFarland 2003), has been shown to be a site where the reconsolidation of a drug conditioned place preference (CPP) memory can be disrupted (Miller and Marshall 2005).Given the evidence of increased zif268 expression in the AcbC following CS-drug memory reactivation, we investigated its requirement in the reconsolidation of cocaine-associated memories. To address this issue, we employed two different but complementary paradigms widely used to measure the conditioned effects of CSs associated with drugs of abuse: the acquisition of a new instrumental response with conditioned reinforcement (ANR) and CPP. These procedures have been used successfully to investigate the mechanisms underlying the reconsolidation of appetitive Pavlovian memories, but it is likely that they depend upon different associative mechanisms (Everitt et al. 1991; White and McDonald 1993) that in turn depend upon different neural loci within limbic cortical-striatal circuitry (Cardinal et al. 2002). Therefore, to enable a full comparison with the functional involvement of the BLA, we investigated the necessity for BLA zif268 expression in drug memory reconsolidation as assessed in the CPP paradigm.     

Recognition memory: Adding a response deadline eliminates recollection but spares familiarity

A current controversy in memory research concerns whether recognition is supported by distinct processes of familiarity and recollection, or instead by a single process wherein familiarity and recollection reflect weak and strong memories, respectively. Recent studies using receiver operating characteristic (ROC) analyses in an animal model have shown that manipulations of the memory demands can eliminate the contribution of familiarity while sparing recollection. Here it is shown that a different manipulation, specifically the addition of a response deadline in recognition testing, results in the opposite performance pattern, eliminating the contribution of recollection while sparing that of familiarity. This dissociation, combined with the earlier findings, demonstrates that familiarity and recollection are differentially sensitive to specific memory demands, strongly supporting the dual process view.Receiver operating characteristic (ROC) analysis holds the promise of dissecting the contributions to recognition memory of episodic recollection and familiarity (Yonelinas 2001), and this method can be applied equally well to examine these memory processes in animals as well as humans (Fortin et al. 2004; Sauvage et al. 2008). According to the dual process model, recollection is indexed by the asymmetry of the ROC function whereas familiarity is measured by the degree of curvilinearity of that function, and correspondingly, these two parameters can vary independently (Yonelinas 2001). However, there is controversy about this interpretation of ROC components. Some have argued that the asymmetry and curvilinearity of the ROC function both reflect the strength of memories mediated by a single process (Wixted 2007), and correspondingly, these components of the ROC increase or decrease together in stronger or weaker memories, respectively (Squire et al. 2007).A resolution of this controversy can be advanced by examining whether the ROC asymmetry and curvilinearity are independently influenced by task manipulations that favor either recollection or familiarity, consistent with dual process theory, or instead are similarly influenced by conditions that affect memory strength. Recent data from an animal model of recognition have shown that adding a demand for remembering associations between independent stimuli eliminates the ROC curvilinearity without affecting the asymmetry, consistent with the dual process view (Sauvage et al. 2008; for discussion of associative recognition, see Mayes et al. 2007). However, in order to provide compelling evidence of independence of the two ROC components, it is also critical to show that other memory demands that favor familiarity produce the opposite pattern, elimination of the ROC asymmetry while sparing its curvilinearity. Together these findings would constitute a double dissociation between the two parameters of the ROC function that cannot be explained by a single process theory.As originally conceived in models proposed in the 1970s, familiarity is characterized as a perceptually driven, pattern matching process that is completed rapidly, whereas recollection is characterized as a conceptually driven, organizational process that requires more time (Mandler 1972; Atkinson and Juola 1973, 1974; for reviews, see Yonelinas 2002; Mandler 2008). Consistent with this view, the results of several studies that employ response deadlines in the test phase report that familiarity is more rapid than recollection. For example, forcing people to make speeded recognition responses has little effect on simple yes–no recognition but strikingly reduces performance when subjects must remember where or when an item was studied (Yonelinas and Jacoby 1994; Gronlund et al. 1997; Hintzman et al. 1998). Other studies that require subjects to oppose familiarity and recollection reveal a two-component temporal function that includes a rapidly available familiarity process and a slower recollective process (Dosher 1984; Gronlund and Ratcliff 1989; Hintzman and Curran 1994; McElree et al. 1999). In addition, studies that measure brain evoked response potentials (ERPs) have revealed two distinct ERP modulations commonly observed during recognition: a mid-frontal negativity onsetting about 400 msec after stimulus onset that is associated with familiarity, and a parietally distributed positivity beginning about 500 msec after stimulus onset that is associated with recollection (Smith 1993; Duzel et al. 1997; Curran 2004; Duarte et al. 2006; Woodruff et al. 2006; but see Voss and Paller 2009).Dual process theory predicts that applying an appropriate early response deadline should allow sufficient time for contribution of familiarity but not that of recollection, and so should reduce the ROC asymmetry while sparing its curvilinearity, opposite to the already observed effects of associative memory demands that favor recollection (Sauvage et al. 2008). Confirmation of this prediction combined with the previous findings of the opposite effects in associative recognition would constitute a double dissociation between the features of recollection and familiarity. This result would therefore strongly support the conclusion that the asymmetry and curvilinearity are independent parameters of the ROC function that are differentially linked to features of recollection and familiarity, respectively.     

The NMDA antagonist MK-801 disrupts reconsolidation of a cocaine-associated memory for conditioned place preference but not for self-administration in rats

Recent research suggests that drug-related memories are reactivated after exposure to environmental cues and may undergo reconsolidation, a process that can strengthen memories. Conversely, reconsolidation may be disrupted by certain pharmacological agents such that the drug-associated memory is weakened. Several studies have demonstrated disruption of memory reconsolidation using a drug-induced conditioned place preference (CPP) task, but no studies have explored whether cocaine-associated memories can be similarly disrupted in cocaine self-administering animals after a cocaine priming injection, which powerfully reinstates drug-seeking behavior. Here we used cocaine-induced CPP and cocaine self-administration to investigate whether the N-methyl-D-aspartate receptor antagonist (+)-5methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate (MK-801) given just prior to reactivation sessions would suppress subsequent cocaine-primed reinstatement (disruption of reconsolidation). Systemic injection of MK-801 (0.05 or 0.20 mg/kg administered intraperitoneally) in rats just prior to reactivation of the cocaine-associated memory in the CPP context attenuated subsequent cocaine-primed reinstatement, while no disruption occurred in rats that did not receive reactivation in the CPP context. However, in rats trained to self-administer cocaine, systemic administration of MK-801 just prior to either of two different types of reactivation sessions had no effect on subsequent cocaine-primed reinstatement of lever-pressing behavior. Thus, systemic administration of MK-801 disrupted the reconsolidation of a cocaine-associated memory for CPP but not for self-administration. These findings suggest that cocaine-CPP and self-administration do not use similar neurochemical processes to disrupt reconsolidation or that cocaine-associated memories in self-administering rats do not undergo reconsolidation, as assessed by lever-pressing behavior under cocaine reinstatement conditions.The ability to disrupt previously consolidated memories in a reactivation-dependent manner is thought to be due to the disruption of a memory reconsolidation process. This disruption of reconsolidation has been observed in a wide variety of tasks and species (Nader et al. 2000b; Sara 2000; Alberini 2005; Riccio et al. 2006). Early reconsolidation experiments primarily focused on aversive learning paradigms, with an emphasis on disruption of reconsolidation as a potential treatment for posttraumatic stress disorder (Misanin et al. 1968; Nader et al. 2000a; Debiec and Ledoux 2004; Brunet et al. 2008). Only more recently have investigators demonstrated that appetitive memories also undergo reconsolidation; most, but not all (Yim et al. 2006), studies found a disruption of expression for the drug-associated memory, suggesting the potential to target the reconsolidation process as a treatment for drug addiction (Lee et al. 2005; Miller and Marshall 2005; Milekic et al. 2006; Valjent et al. 2006; Brown et al. 2007; Kelley et al. 2007; Sadler et al. 2007; Fricks-Gleason and Marshall 2008; Milton et al. 2008a, b).Miller and Marshall (2005) showed that reconsolidation of cocaine conditioned place preference (CPP) in the rat could be disrupted by either pre- or post-treatment of a phosphorylation inhibitor of extracellular signal-regulated kinase (1/2) (ERK) in a reactivation-dependent manner. Other studies have shown that protein synthesis inhibitors (Milekic et al. 2006), a matrix metalloproteinase (MMP) inhibitor (Brown et al. 2007), a β-noradrenergic receptor antagonist (Bernardi et al. 2006; Robinson and Franklin 2007a; Fricks-Gleason and Marshall 2008), and an N-methyl-D-aspartate (NMDA) receptor antagonist (Kelley et al. 2007; Sadler et al. 2007) can also disrupt the reconsolidation of drug-associated CPP memories. Studies by Lee and colleagues have shown that Zif268 antisense oligodeoxynucleotide infused into the basolateral amygdala prior to reactivation of memory for a cocaine-associated cue (the conditioned stimulus or CS) disrupts the ability of cocaine-associated cues to establish subsequent acquisition of a new instrumental response (Lee et al. 2005), and the ability of a drug-associated cue to induce relapse under a second-order schedule (Lee et al. 2006a). Thus, cocaine-associated memories appear to undergo reconsolidation in both Pavlovian and operant conditioning paradigms.Relapse to drug-seeking or drug-taking behavior can occur after re-exposure to three types of stimuli: the drug itself, drug-associated contextual and discrete cues, and stress; and all of these may promote relapse in humans (for review, see Epstein et al. 2006). Only a few CPP studies (Valjent et al. 2006; Brown et al. 2007) and no self-administration studies to our knowledge have tested whether the drug-associated memory can be rendered susceptible to disruption by pharmacological agents such that subsequent cocaine-primed reinstatement is suppressed. This drug-primed effect is observed in humans, producing relapse (Ludwig et al. 1974; Jaffe et al. 1989), and in rats, producing robust reinstatement of drug-seeking behavior in both CPP and self-administration tasks (McFarland and Ettenberg 1997; McFarland and Kalivas 2001; Sanchez and Sorg 2001; Kalivas and McFarland 2003). The development of a treatment strategy that makes use of the reconsolidation process will ultimately need to be powerful enough to diminish drug-seeking behavior in the presence of sizable doses of the drug itself. Therefore, the primary goal of this study was to determine whether drug-primed reinstatement could be suppressed in rats that have the memory reactivated in the presence of a pharmacological agent in cocaine self-administering rats. Since we previously have demonstrated the ability to disrupt cocaine-primed reinstatement only in animals in which the memory was reactivated using cocaine-induced CPP, we also tested the extent to which the same parameters used to disrupt reconsolidation in a cocaine-induced CPP task would disrupt reconsolidation in a cocaine self-administration task under conditions of drug-induced reinstatement.To examine this question, we chose the noncompetitive NMDA receptor antagonist (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate (MK-801). MK-801 has been shown to disrupt reconsolidation of spatial tasks (Przybyslawski and Sara 1997), fear tasks (Lee et al. 2006b), amphetamine-induced CPP (Sadler et al. 2007), cocaine-induced CPP (Kelley et al. 2007), and sucrose self-administration (Lee and Everitt 2008). Importantly, the two studies examining CPP using MK-801 did not explore whether MK-801 suppressed drug-seeking behavior in a manner that was dependent on whether the memory was reactivated, leaving open the possibility that it was not a reconsolidation process that was disrupted by MK-801.Here we demonstrate that MK-801 injected prior to cocaine-primed reinstatement of CPP disrupted subsequent cocaine-primed reinstatement of CPP, and this disruption was dependent on CPP contextual reactivation since injection of MK-801 and cocaine in the home cage did not disrupt subsequent cocaine-primed reinstatement of CPP. However, drug-seeking behavior in animals trained for cocaine self-administration was not disrupted when rats were reactivated under the same parameters that disrupted cocaine-induced CPP or when rats were given a reactivation session identical to their self-administration sessions. We thus demonstrate for the first time that memories associated with cocaine-induced CPP and cocaine self-administration are not similarly susceptible to disruption by MK-801.     

A cognitive map for object memory in the hippocampus

The hippocampus has been proposed to support a cognitive map, a mental representation of the spatial layout of an environment as well as the nonspatial items encountered in that environment. In the present study, we recorded simultaneously from 43 to 61 hippocampal pyramidal cells as rats performed an object recognition memory task in which novel and repeated objects were encountered in different locations on a circular track. Multivariate analyses of the neural data indicated that information about object identity was represented secondarily to the primary information dimension of object location. In addition, the neural data related to performance on the recognition memory task. The results suggested that objects were represented as points of interest on the hippocampal cognitive map and that this map was useful in remembering encounters with particular objects in specific locations.The hippocampus plays an important role in spatial memory for both humans and rodents (O''Keefe 1999; Burgess et al. 2002). Findings from many studies in rodents indicate that the hippocampus supports memory for locations referenced to external landmarks, a capacity that O''Keefe and Nadel (1978) described over 30 yr ago as a “cognitive map” (using a term they borrowed from Tolman 1948). In the time since that pioneering thesis, it has become clear that the rodent hippocampus is also important for nonspatial memory (Eichenbaum et al. 1999). Damage to the rat hippocampus (defined here as CA fields, dentate gyrus, and subiculum) leads to impairments on nonspatial tasks, including object recognition memory (Clark et al. 2000; Fortin et al. 2004), transitive odor associations (Bunsey and Eichenbaum 1996), memory for temporal order (Fortin et al. 2002; Kesner et al. 2002), and social transmission of food preference (Alvarez et al. 2001; Clark et al. 2002).The circuitry by which information arrives at and exits from the hippocampus is consistent with the idea that the hippocampus is important for both spatial and nonspatial memory. In both rats and macaques, detailed anatomical studies have indicated that spatial information arrives at the hippocampus via the postrhinal cortex (parahippocampal cortex in primates) and the medial entorhinal cortex, whereas nonspatial information takes a path largely through the perirhinal cortex and lateral entorhinal cortex (Witter and Amaral 1991; Suzuki and Amaral 1994; Witter et al. 2000). Thus, the hippocampus is ideally situated to combine spatial and nonspatial information in the service of remembering item–location associations (Manns and Eichenbaum 2006).Single-unit recording studies in the rat hippocampus have largely focused on the spatial correlates of hippocampal pyramidal neuron firing rates. Fewer studies have investigated nonspatial correlates of hippocampal activity during memory tasks for nonspatial items. However, in one such study, Wood et al. (1999) found that some individual hippocampal pyramidal neurons responded to particular odors and that others responded to particular odors in specific locations during an odor recognition memory task. Thus, activity of individual cells appeared to contain information about nonspatial items as well as spatial locations.An important question is how the activity of individual hippocampal neurons combine to represent item–location associations as a neural ensemble. In particular, how is an encounter with an object in a particular location represented in the pattern of spiking among many hippocampal pyramidal neurons? How might this representation relate to memory for the object or for the location? In the present study, we recorded simultaneously from 43 to 61 hippocampal pyramidal cells as rats performed an object recognition memory task in which novel and repeated objects were encountered in different locations on a circular track. Multivariate analyses of the neural data indicated that information about object identity was represented secondarily to the primary information dimension of object location. In addition, the analyses indicated that the neural data related to performance on the recognition memory task. The results suggest that objects were represented as points of interest on the hippocampal cognitive map and that this map was useful in remembering encounters with particular objects in specific locations.     

Recognition memory and the hippocampus: A test of the hippocampal contribution to recollection and familiarity

It has been suggested that the hippocampus selectively supports recollection and that adjacent cortex in the medial temporal lobe can support familiarity. Alternatively, it has been suggested that the hippocampus supports both recollection and familiarity. We tested these suggestions by assessing the performance of patients with hippocampal lesions on recognition memory tests that differ in the extent to which recollection and familiarity contribute to the recognition decision. When targets and foils are highly similar, prior evidence suggests that, on a forced-choice test in which targets are presented together with highly similar, corresponding foils (the FC-C format), performance is supported primarily by familiarity. By contrast, when targets are presented together with foils that are similar to other targets (the FC-NC format) or when memory is tested in a yes/no (Y/N) format, performance is based much more strongly on recollection. Accordingly, a finding that hippocampal damage impaired both Y/N recognition and FC-NC recognition but spared FC-C recognition would suggest that the hippocampus selectively supports recollection. We administered Y/N, FC-C, and FC-NC tests to five memory-impaired patients with circumscribed hippocampal lesions and 14 controls. The patients were impaired on all three types of recognition test, and there was no indication that the patients were disproportionately benefited or disproportionately impaired on any test. This pattern of performance suggests that the hippocampus supports both recollection and familiarity.The medial temporal lobe (the hippocampus plus the entorhinal, perirhinal, and parahippocampal cortices) is essential for the recognition of past experience. The capacity for recognition is widely thought to depend on two distinct processes, recollection and familiarity (Mandler 1980; Yonelinas et al. 2002; Wixted 2007). Recollection involves remembering specific details about the episode in which an item was encountered. Familiarity involves simply knowing that an item was presented, even when no additional information can be retrieved about the learning episode itself. This distinction has been prominent in recent discussions about memory, particularly in relation to its possible anatomical basis. One proposal is that recollection depends on the hippocampus and that familiarity depends on the adjacent medial temporal lobe cortex (Brown and Aggleton 2001; Mayes et al. 2002; Yonelinas et al. 2002). Alternatively, it has been suggested that the hippocampus is important for both recollection and familiarity and that the distinction between these two processes does not illuminate the functional differences between the hippocampus and adjacent cortex (Rutishauser et al. 2006; Wais et al. 2006; Squire et al. 2007; Wixted 2007).To clarify the role of the hippocampus in recognition memory, the performance of memory-impaired patients with hippocampal damage has often been compared to that of controls on tests that differ in the extent to which recollection and familiarity contribute to the recognition memory decision. One such approach involves the use of highly similar targets and foils tested using a yes/no (Y/N) format and a forced-choice corresponding (FC-C) format (Holdstock et al. 2002; Westerberg et al. 2006; Bayley et al. 2008). In the case of the Y/N format, participants see a list of targets intermixed with foils (each similar to one of the targets) and are asked to respond “yes” to the targets and “no” to the foils. In the FC-C format, participants see a target presented together with its corresponding, similar foil and are asked to identify the target. According to an idea advanced by Norman and O''Reilly (2003), familiarity can support FC-C recognition because one can make effective use of small but consistent differences between the familiarity signals triggered by the target and foil items. That is, when targets and foils are highly similar and are presented together in the FC-C format, familiarity for the target is likely to slightly and reliably exceed that of the similar foil, allowing the target to be correctly chosen on the basis of familiarity. By contrast, in the Y/N format, slight differences between the familiarity signals of target items and their corresponding foils no longer provide useful information, because the targets and their corresponding foils are not presented together at test. Accordingly, good performance with the Y/N format is more dependent on recollection than with the FC-C format.The same explanation may account for why performance on FC-C tests sometimes exceeds performance on forced-choice noncorresponding (FC-NC) tests (e.g., Hintzman 1988). In the FC-NC format, target items are presented together with a noncorresponding foil that is similar to another target item from the study list (Fig. 1). Thus, the FC-C and FC-NC tests differ only in that the similar targets and foils are presented together in the former but not in the latter. When the corresponding targets and foils are presented together (in the FC-C format), participants can make effective use of consistent differences in familiarity values (as discussed above). When the corresponding targets and foils are not presented together, participants cannot make use of the small differences that they might detect through a side-by-side comparison.Open in a separate windowFigure 1.Test format and materials for the three kinds of recognition test. For each test, 12 images, either objects (A) or silhouettes (B), were presented at study, and memory was tested in one of three ways. For the forced-choice corresponding test (FC-C), each target (a studied item) was presented together with a highly similar foil (a new item). For the forced-choice noncorresponding test (FC-NC), each target was presented together with a foil that was highly similar to a different target from the study list. Participants were asked to point to exactly the same image they had seen during study. For the yes/no test (Y/N), the 12 targets and the 12 foils (each similar to one of the targets) were intermixed and presented one at a time. Participants were asked to respond “yes” if they had seen exactly the same image before and “no” if they had not. Asterisks identify the target items.A recent study (Migo et al. 2009) provides empirical support for the suggestion that familiarity primarily contributes to the recognition decision in the FC-C format, whereas recollection contributes more strongly in the Y/N format and in the FC-NC format. In the Migo et al. (2009) study, healthy participants received FC-C, FC-NC, and Y/N tests after receiving either standard instructions to make their decisions based on familiarity or recollection or modified instructions to base their decisions on familiarity only. On the FC-NC test and on the Y/N test, performance using familiarity alone was significantly worse than under standard instructions. On the FC-C test, performance using familiarity alone was nearly as good as under standard instructions. This result supports the earlier suggestion that FC-C is primarily supported by familiarity, whereas recollection plays a larger role in FC-NC and Y/N recognition (Norman and O''Reilly 2003).If the hippocampus is critical for recollection but not for familiarity, and if good performance on the FC-C format can be achieved using familiarity alone, then patients with focal hippocampal lesions should be differentially impaired on the Y/N and FC-NC recognition test formats compared with the FC-C format. Three recent studies investigated this issue by assessing the effects of hippocampal damage (or presumed hippocampal damage) on Y/N and FC-C tests that used highly similar targets and foils (the FC-NC format was not used in these earlier studies). All three studies used black-and-white silhouette images of objects, and the FC-C test involved the target item and three similar foils (i.e., multiple-choice with four alternatives). In one study, a single patient with bilateral hippocampal damage (patient Y.R.) was impaired on the test of Y/N recognition but was unimpaired on the test of FC-C recognition (Holdstock et al. 2002). A similar result was reported in patients with a diagnosis of mild cognitive impairment (MCI) (Westerberg et al. 2006). These findings have been taken to support the suggestion that the hippocampus selectively supports recollection. However, the study by Holdstock and colleagues (2002) involved a single patient (Y.R.), and findings from a single patient need not agree with findings obtained from a group of patients (see Discussion). In addition, the Westerberg et al. (2006) study involved individuals with a diagnosis of MCI for whom anatomical data were not available. It is therefore not clear what the status of the hippocampus was in these patients. Moreover, a standard Y/N procedure involving 12 study items and 24 test items (as opposed to the 12 study test items and 60 test items used in previous studies [Holdstock et al. 2002; Westerberg et al. 2006; Bayley et al. 2008]) might be better suited to address the question of whether Y/N recognition is selectively impaired in hippocampal patients. Bayley et al. (2008) tested five patients with circumscribed hippocampal damage using the same test materials and procedure as in Holdstock et al. (2002). When all 60 test items of the Y/N test were scored, the patients were found to be more impaired on the Y/N test than on the FC-C test. However, when only the first 24 test items of the Y/N test were scored (according to the standard Y/N procedure), the patients were found to be impaired on both the Y/N and FC-C tests to a similar degree.The earlier studies (Holdstock et al. 2002; Westerberg et al. 2006; Bayley et al. 2008) compared Y/N recognition to four-alternative FC-C recognition in which each target was presented along with three corresponding foils. None of the earlier studies investigated how patients with hippocampal lesions performed on FC-NC recognition. As Migo et al. (2009) point out, FC-C and FC-NC tests are better matched than the FC-C and Y/N tests because they are both forced-choice tests, and both use the same number of test trials. As such, they argue, it would be more useful to compare FC-C recognition performance to FC-NC recognition performance to determine whether patients with hippocampal lesions are intact or impaired on familiarity-based tests.The present study assessed Y/N, two-alternative FC-C, and two-alternative FC-NC recognition, using highly similar targets and foils. The tests were given to five patients with circumscribed hippocampal damage and 14 matched controls. The Y/N test involved an equal number of targets and foils (unlike the design used in the prior studies), and a two-alternative format was used for the forced-choice tests to make them comparable to the Y/N test (i.e., all three tests involved the same number of targets and foils). The FC-C and FC-NC tests were identical except with respect to how the targets and their corresponding foils were paired on the recognition test.To extend our findings beyond the stimuli used in prior studies, we also used color photographs of objects in addition to black-and-white silhouettes. Using these three test formats, we asked the following questions: (1) Does hippocampal damage impair Y/N recognition memory but spare, or disproportionately benefit, FC-C recognition (as suggested by the view that the hippocampus supports recollection and not familiarity)? Or does hippocampal damage impair Y/N and FC-C recognition similarly (as suggested by the view that the hippocampus supports both recollection and familiarity)? (2) Does hippocampal damage disproportionately benefit FC-C recognition relative to FC-NC recognition, or does hippocampal damage impair FC-C and FC-NC recognition similarly?     

Beta-adrenergic receptor activation during distinct patterns of stimulation critically modulates the PKA-dependence of LTP in the mouse hippocampus

Activation of β-adrenergic receptors (β-ARs) enhances hippocampal memory consolidation and long-term potentiation (LTP), a likely mechanism for memory storage. One signaling pathway linked to β-AR activation is the cAMP-PKA pathway. PKA is critical for the consolidation of hippocampal long-term memory and for the expression of some forms of long-lasting hippocampal LTP. How does β-AR activation affect the PKA-dependence, and persistence, of LTP elicited by distinct stimulation frequencies? Here, we use in vitro electrophysiology to show that patterns of stimulation determine the temporal phase of LTP affected by β-AR activation. In addition, only specific patterns of stimulation recruit PKA-dependent LTP following β-AR activation. Impairments of PKA-dependent LTP maintenance generated by pharmacologic or genetic deficiency of PKA activity are also abolished by concurrent activation of β-ARs. Taken together, our data show that, depending on patterns of synaptic stimulation, activation of β-ARs can gate the PKA-dependence and persistence of synaptic plasticity. We suggest that this may allow neuromodulatory receptors to fine-tune neural information processing to meet the demands imposed by numerous synaptic activity profiles. This is a form of “metaplasticity” that could control the efficacy of consolidation of hippocampal long-term memories.The hippocampus importantly contributes to memory function in the mammalian brain (Zola-Morgan et al. 1986; Eichenbaum et al. 1990; Otto and Eichenbaum 1992; Phillips and LeDoux 1992; Remondes and Schuman 2004). It has reciprocal connections with numerous cortical areas, including those responsible for high-level integration of spatial and contextual data from the external environment (Lavenex and Amaral 2000). As such, the hippocampus is well positioned to receive and survey a broad range of information and select behaviorally salient data for long-term storage. Activity-dependent enhancement of hippocampal synaptic strength can store information carried in patterns of afferent neural activity (Bliss and Collingridge 1993; Moser et al. 1998; Nathe and Frank 2003; Whitlock et al. 2006). Substantial evidence suggests that long-term potentiation (LTP) of synaptic strength plays important roles in the formation of long-term memory (LTM) (Doyere and Laroche 1992; Bourtchuladze et al. 1994; Abel and Lattal 2001; Genoux et al. 2002). As such, mechanistic studies of LTP have shed valuable light on how the mammalian brain stores new information.The hippocampus receives dense noradrenergic projections from the locus coeruleus, a brain structure that can influence many vital brain functions, including attention, sleep, arousal, mood regulation, learning, and memory (Berridge and Waterhouse 2003). Both α- and β-adrenergic receptor subtypes are present on hippocampal neurons (Morrison and Foote 1986; Berridge and Waterhouse 2003), and noradrenaline (NA) acts on hippocampal β-adrenergic receptors (β-ARs) to facilitate the retention and recall of memory (Izquierdo et al. 1998; Ji et al. 2003; Murchison et al. 2004). In humans, stimulation of the noradrenergic neuromodulatory system enhances memory for emotional stimuli, and inhibition of β-ARs prevents this memory enhancement (Cahill et al. 1994; van Stegeren et al. 1998; O’Carroll et al. 1999).Consistent with the notion that selective enhancement of LTM may occur following β-AR activation, stimulation of β-ARs can also facilitate the persistence of LTP. In areas CA3 and CA1, β-AR activation facilitates the induction of long-lasting LTP when paired with certain patterns of electrical stimulation (Huang and Kandel 1996; Gelinas and Nguyen 2005). However, the mechanisms by which different patterns of stimulation control synaptic responsiveness to β-AR activation are unclear.β-ARs couple to guanine-nucleotide-binding regulatory Gs proteins to stimulate adenylyl cyclase activity and increase intracellular cAMP (Seeds and Gilman 1971; Maguire et al. 1977). A main target of cAMP signaling is activation of cAMP-dependent protein kinase (PKA), a kinase that is required for some forms of long-lasting LTP and for consolidation of hippocampal LTM (Frey et al. 1993; Abel et al. 1997; Nguyen and Woo 2003). Interestingly, the PKA-dependence of hippocampal LTP displays plasticity: Specific temporal patterns of synaptic stimulation, such as repeated and temporally spaced 100-Hz stimulation, elicit LTP that requires PKA for its expression (Woo et al. 2003). Also, spatial “enrichment” can increase the PKA-dependence of LTP in mice, and this is correlated with improved hippocampal memory function (Duffy et al. 2001). However, it is unclear whether activation of β-ARs can critically gate the PKA-dependence of LTP. In this study, we examine the effects of β-AR activation on LTP generated by various patterns of afferent stimulation in area CA1 of the hippocampus, and we determine the role of PKA in these β-AR-modulated forms of LTP.     

Intrahippocampal infusions of anisomycin produce amnesia: Contribution of increased release of norepinephrine,dopamine, and acetylcholine

Intra-amygdala injections of anisomycin produce large increases in the release of norepinephrine (NE), dopamine (DA), and serotonin in the amygdala. Pretreatment with intra-amygdala injections of the β-adrenergic receptor antagonist propranolol attenuates anisomycin-induced amnesia without reversing the inhibition of protein synthesis, and injections of NE alone produce amnesia. These findings suggest that abnormal neurotransmitter responses may be the basis for amnesia produced by inhibition of protein synthesis. The present experiment extends these findings to the hippocampus and adds acetylcholine (ACh) to the list of neurotransmitters affected by anisomycin. Using in vivo microdialysis at the site of injection, release of NE, DA, and ACh was measured before and after injections of anisomycin into the hippocampus. Anisomycin impaired inhibitory avoidance memory when rats were tested 48 h after training and also produced substantial increases in local release of NE, DA, and ACh. In an additional experiment, pretreatment with intrahippocampal injections of propranolol prior to anisomycin and training significantly attenuated anisomycin-induced amnesia. The disruption of neurotransmitter release patterns at the site of injection appears to contribute significantly to the mechanisms underlying amnesia produced by protein synthesis inhibitors, calling into question the dominant interpretation that the amnesia reflects loss of training-initiated protein synthesis necessary for memory formation. Instead, the findings suggest that proteins needed for memory formation are available prior to an experience, and that post-translational modifications of these proteins may be sufficient to enable the formation of new memories.A dominant view of the molecular basis for memory is that the formation of long-term memory for an experience depends on de novo protein synthesis initiated by that experience (Davis and Squire 1984; Frey and Morris 1998; Kandel 2001; Dudai 2002; Nader 2003; Alberini 2008). This view is supported by numerous studies showing that drugs that interfere with protein synthesis by inhibiting translational processes near the time of training produce later amnesia.Despite the centrality of experience-induced protein synthesis in contemporary models of memory formation, the necessity of protein synthesis for memory consolidation and long-term potentiation (LTP) stabilization has been questioned since the beginning of experiments of this type (e.g., Flexner and Goodman 1975; Barraco and Stettner 1976; Flood et al. 1978; Martinez et al. 1981), and continues to be questioned in several recent reviews (Routtenberg and Rekart 2005; Gold 2006, 2008; Radulovic and Tronson 2008; Routtenberg 2008; Rudy 2008). There are many instances of intact memories formed in the presence of extensive inhibition of protein synthesis, and a wide range of behavioral and pharmacological manipulations can rescue memory impaired by protein synthesis inhibitors. For example, amnesia is attenuated in a graded manner by increasing the training trials and foot shock intensity in avoidance tasks (Flood et al. 1975, 1978). Moreover, a wide range of stimulants, such as amphetamine, strychnine, corticosteroids, and caffeine, block amnesia induced by anisomycin (Flood et al. 1978). Like memory, LTP is sometimes insensitive to protein synthesis inhibitors. Simultaneous inhibition of both protein synthesis and degradation does not interfere with induction and maintenance of LTP (Fonseca et al. 2006a). Also, the specific schedule and frequency of test pulses after induction of LTP determine the vulnerability of LTP to anisomycin-induced impairment; anisomycin treatment does not impair LTP unless test pulses at a rate of 1/10 sec were administered during the anisomycin exposure (Fonseca et al. 2006b).Findings that memory and LTP can survive the inhibition of protein synthesis challenge the necessity of specific training- or stimulation-initiated protein synthesis for memory formation and synaptic plasticity. Several actions of protein synthesis inhibitors offer alternative accounts for amnesia produced by these drugs. These include cell sickness (Rudy et al. 2006; Rudy 2008), activation of protein kinases and superinduction of immediate early genes (Radulovic and Tronson 2008), abnormal neural electrical activity (Agnihotri et al. 2004; Xu et al. 2005), and intrusion of neural “noise” that masks the primary changes representing memory formation (Gold 2006). Neural responses to inhibition of protein synthesis such as these may impair memory either secondary to or independent of interference with protein synthesis.Another example of the mechanisms by which inhibition of protein synthesis might impair memory is by altering neurotransmitter functions. This possibility was suggested in early studies (e.g., Flexner and Goodman 1975; Quartermain et al. 1977) and has recently been supported by studies of neurotransmitter release at the site of intra-amygdala injections of anisomycin (Canal et al. 2007). In addition to impairing later memory after inhibitory avoidance training, pretraining injections of anisomycin into the amygdala produced rapid and dramatic increases in release of norepinephrine (NE), dopamine (DA), and serotonin (5-HT) at the sites of injection. The release of NE and DA then plummeted below baselines from 2 to 6 h after anisomycin injections, recovering within 48 h after anisomycin injection. The possibility that these neurochemical changes contribute to anisomycin-induced amnesia was supported by studies showing attenuation of amnesia in rats pretreated with intra-amygdala injections of the β-adrenergic receptor antagonist propranolol, apparently acting to blunt the effects of the large increases in release of NE after anisomycin injection. In addition, amnesia was produced by injections of high doses of norepinephrine into the amygdala.In addition to amnesias produced by anisomycin injections into the amygdala, as above, anisomycin also impairs memory when administered to other memory systems, including the hippocampus, where anisomycin impairs inhibitory avoidance memory (Quevedo et al. 1999; Debiec et al. 2002; Milekic et al. 2006). The present study extends the prior findings (Canal et al. 2007) in several respects. Experiments presented here determine whether anisomycin injections into the hippocampus result in changes in release of the catecholamines, NE and DA, at the site of injection, as seen previously in the amygdala. Additionally, the present experiments determine whether intrahippocampal injections of anisomycin result in increased release of acetylcholine, a neurotransmitter not examined in the previous study. To examine parallels with earlier amygdala findings, a further experiment determines whether intrahippocampal pretreatment with propranolol is effective in attenuating anisomycin-induced amnesia.     

Trace and contextual fear conditioning require neural activity and NMDA receptor-dependent transmission in the medial prefrontal cortex

The contribution of the medial prefrontal cortex (mPFC) to the formation of memory is a subject of considerable recent interest. Notably, the mechanisms supporting memory acquisition in this structure are poorly understood. The mPFC has been implicated in the acquisition of trace fear conditioning, a task that requires the association of a conditional stimulus (CS) and an aversive unconditional stimulus (UCS) across a temporal gap. In both rat and human subjects, frontal regions show increased activity during the trace interval separating the CS and UCS. We investigated the contribution of prefrontal neural activity in the rat to the acquisition of trace fear conditioning using microinfusions of the γ-aminobutyric acid type A (GABA_A) receptor agonist muscimol. We also investigated the role of prefrontal N-methyl-d-aspartate (NMDA) receptor-mediated signaling in trace fear conditioning using the NMDA receptor antagonist 2-amino-5-phosphonovaleric acid (APV). Temporary inactivation of prefrontal activity with muscimol or blockade of NMDA receptor-dependent transmission in mPFC impaired the acquisition of trace, but not delay, conditional fear responses. Simultaneously acquired contextual fear responses were also impaired in drug-treated rats exposed to trace or delay, but not unpaired, training protocols. Our results support the idea that synaptic plasticity within the mPFC is critical for the long-term storage of memory in trace fear conditioning.The prefrontal cortex participates in a wide range of complex cognitive functions including working memory, attention, and behavioral inhibition (Fuster 2001). In recent years, the known functions of the prefrontal cortex have been extended to include a role in long-term memory encoding and retrieval (Blumenfeld and Ranganath 2006; Jung et al. 2008). The prefrontal cortex may be involved in the acquisition, expression, extinction, and systems consolidation of memory (Frankland et al. 2004; Santini et al. 2004; Takehara-Nishiuchi et al. 2005; Corcoran and Quirk 2007; Jung et al. 2008). Of these processes, the mechanisms supporting the acquisition of memory may be the least understood. Recently, the medial prefrontal cortex (mPFC) has been shown to be important for trace fear conditioning (Runyan et al. 2004; Gilmartin and McEchron 2005), which provides a powerful model system for studying the neurobiological basis of prefrontal contributions to memory. Trace fear conditioning is a variant of standard “delay” fear conditioning in which a neutral conditional stimulus (CS) is paired with an aversive unconditional stimulus (UCS). Trace conditioning differs from delay conditioning by the addition of a stimulus-free “trace” interval of several seconds separating the CS and UCS. Learning the CS–UCS association across this interval requires forebrain structures such as the hippocampus and mPFC. Importantly, the mPFC and hippocampus are only necessary for learning when a trace interval separates the stimuli (Solomon et al. 1986; Kronforst-Collins and Disterhoft 1998; McEchron et al. 1998; Takehara-Nishiuchi et al. 2005). This forebrain dependence has led to the hypothesis that neural activity in these structures is necessary to bridge the CS–UCS temporal gap. In support of this hypothesis, single neurons recorded from the prelimbic area of the rat mPFC exhibit sustained increases in firing during the CS and trace interval in trace fear conditioning (Baeg et al. 2001; Gilmartin and McEchron 2005). Similar sustained responses are not observed following the CS in delay conditioned animals or unpaired control animals. This pattern of activity is consistent with a working memory or “bridging” role for mPFC in trace fear conditioning, but it is not clear whether this activity is actually necessary for learning. We address this issue here using the γ-aminobutyric acid type A (GABA_A) receptor agonist muscimol to temporarily inactivate cellular activity in the prelimbic mPFC during the acquisition of trace fear conditioning.The contribution of mPFC to the long-term storage (i.e., 24 h or more) of trace fear conditioning, as opposed to a strictly working memory role (i.e., seconds to minutes), is a matter of some debate. Recent reports suggest that intact prefrontal activity at the time of testing is required for the recall of trace fear conditioning 2 d after training (Blum et al. 2006a), while mPFC lesions performed 1 d after training fail to disrupt the memory (Quinn et al. 2008). The findings from the former study may reflect a role for prelimbic mPFC in the expression of conditional fear rather than memory storage per se (Corcoran and Quirk 2007). However, blockade of the intracellular mitogen-activated protein kinase (MAPK) cascade during training impairs the subsequent retention of trace fear conditioning 48 h later (Runyan et al. 2004). Activation of the MAPK signaling cascade can result in the synthesis of proteins necessary for synaptic strengthening, providing a potential mechanism by which mPFC may participate in memory storage. To better understand the nature of the prefrontal contribution to long-term memory, more information is needed about fundamental plasticity mechanisms in this structure. Dependence on N-methyl-d-aspartate receptors (NMDAR) is a key feature of many forms of long-term memory, both in vitro and in vivo. The induction of long-term potentiation (LTP) in the hippocampus, a cellular model of long-term plasticity and information storage, requires NMDAR activation (Reymann et al. 1989). Genetic knockdown or pharmacological blockade of NMDAR-mediated neurotransmission in the hippocampus impairs several forms of hippocampus-dependent memory, including trace fear conditioning (Tonegawa et al. 1996; Huerta et al. 2000; Quinn et al. 2005), but it is unknown if activation of these receptors is necessary in the mPFC for the acquisition of trace fear conditioning. Data from in vivo electrophysiology studies have shown that stimulation of ventral hippocampal inputs to prelimbic neurons in mPFC produces LTP, and the induction of prefrontal LTP depends upon functional NMDARs (Laroche et al. 1990; Jay et al. 1995). If the role of mPFC in trace fear conditioning goes beyond simply maintaining CS information in working memory, then activation of NMDAR may be critical to memory formation. We test this hypothesis by reversibly blocking NMDAR neurotransmission with 2-amino-5-phosphonovaleric acid (APV) during training to examine the role of prefrontal NMDAR to the acquisition of trace fear conditioning.Another important question is whether mPFC contributes to the formation of contextual fear memories. Fear to the training context is acquired simultaneously with fear to the auditory CS in both trace and delay fear conditioning. Conflicting reports in the literature suggest the role of mPFC in contextual fear conditioning is unclear. Damage to ventral areas of mPFC prior to delay fear conditioning has failed to impair context fear acquisition (Morgan et al. 1993). Prefrontal lesions incorporating dorsal mPFC have in some cases been reported to augment fear responses to the context (Morgan and LeDoux 1995), while blockade of NMDAR transmission has impaired contextual fear conditioning (Zhao et al. 2005). Post-training lesions of mPFC impair context fear retention (Quinn et al. 2008) in trace and delay conditioning. Contextual fear responses were assessed in this study to determine the contribution of neuronal activity and NMDAR-mediated signaling in mPFC to the acquisition of contextual fear conditioning.     

Cannabinoid and cholinergic systems interact during performance of a short-term memory task in the rat

It is now well established that cannabinoid agonists such as Δ⁹–tetrahydrocannabinol (THC), anandamide, and WIN 55,212-2 (WIN-2) produce potent and specific deficits in working memory (WM)/short-term memory (STM) tasks in rodents. Although mediated through activation of CB1 receptors located in memory-related brain regions such as the hippocampus and prefrontal cortex, these may, in part, be due to a reduction in acetylcholine release (i.e., cholinergic hypofunction). To determine the interaction between cannabinoid and cholinergic systems, we exposed rats treated with WIN-2 or cholinergic drugs to a hippocampal-dependent delayed nonmatch to sample (DNMS) task to study STM, and recorded hippocampal single-unit activity in vivo. WIN-2 induced significant deficits in DNMS performance and reduced the average firing and bursting rates of hippocampal principal cells through a CB1 receptor-mediated mechanism. Rivastigmine, an acetylcholinesterase inhibitor, reversed these STM deficits and normalized hippocampal discharge rates. Effects were specific to 1 mg/kg WIN-2 as rivastigmine failed to reverse the behavioral and physiological deficits that were observed in the presence of MK-801, an NMDA receptor antagonist. This supports the notion that cannabinoid-modulated cholinergic activity is a mechanism underlying the performance deficits in DNMS. Whether deficits are due to reduced nicotinic or muscarinic receptor activation, or both, awaits further analysis.Administration of both synthetic and phytocannabinoids, including Δ⁹–tetrahydrocannabinol (Δ⁹–THC), WIN 55,212-2 (WIN-2), and CP 55,940, impair working memory (WM) and short-term memory (STM) through a CB1 receptor-mediated mechanism in rats (Lichtman et al. 1995; Lichtman and Martin 1996; Hampson and Deadwyler 1998, 1999, 2000; Braida and Sala 2000; Egashira et al. 2002). This suggestive evidence for endocannabinoid involvement in memory formation was confirmed by Terranova and coworkers (1996), who demonstrated that the CB1 receptor antagonist rimonabant facilitated short-term olfactory memory, and this was partially reversed by the muscarinic receptor antagonist scopolamine. This suggests an interaction between cannabinoid and cholinergic systems such that endocannabinoid tone suppresses cholinergic transmission. Consequently, rats pretreated with eptastigmine, a second-generation cholinesterase inhibitor remained unaffected by the full CB1 receptor agonist CP 55,940 when tested in an eight arm radial maze (Braida and Sala 2000). And more recent evidence from Mishima and coworkers (2002) suggests that a block of cholinesterase with physostigmine and tetrahydroaminoacridine protects against WM impairments induced by Δ⁹–THC. These findings further support a potential role of the cholinergic system in cannabinoid-induced memory impairments.The exact mechanisms for this interaction still remain elusive, although cholinergic projection neurons from medial septum to hippocampus are likely to play an important role (Harkany et al. 2003, 2005; Fitz et al. 2008). However, the neuromodulatory action of pharmacologically active cannabinoids on septo-hippocampal cholinergic activity in vivo remains unexplored. Within the hippocampus, cannabinoids presynaptically inhibit the release of acetylcholine, possibly through the activation of CB1 receptors located on cholinergic nerve terminals given that these effects were blocked by rimonabant (Gifford and Ashby Jr. 1996; Gifford et al. 1997a, 2000; Kathmann et al. 2001a). Direct in vivo microdialysis studies in awake rats also showed cannabinoid-induced decreases in acetylcholine release in the hippocampus through a CB1 receptor-mediated mechanism (Gessa et al. 1997; Carta et al. 1998). High doses of rimonabant alone increase the amount of acetylcholine release in the hippocampus (Gessa et al. 1997, 1998) either by blocking the tonic inhibitory influence of endocannabinoids and/or through its inverse agonism at CB1 receptors. Such actions are in agreement with a 100% greater increase in electrically evoked hippocampal acetylcholine release in CB1^−/− mice (Kathmann et al. 2001b).In contrast, low doses of Δ⁹–THC (0.01–0.15 mg/kg), WIN-2 (0.01–0.5 mg/kg), and HU-210 (0.001–0.004 mg/kg) have been shown to enhance acetylcholine release (Acquas et al. 2000, 2001), indicating that cannabinoid modulation of acetylcholine release in the hippocampus is “biphasic.” This has been further supported by the work carried out by Tzavara and coworkers (2003), who demonstrated that low (0.5 mg/kg, intraperitoneally [i.p.]) and high (5 mg/kg, i.p.) doses of WIN-2 induce transient stimulation and prolonged inhibition of hippocampal acetylcholine efflux, respectively. This demonstrates that the dose of cannabinoids plays a key role in determining how much acetylcholine is released in the hippocampus.Such an interaction is likely to play an important role during the performance of a delayed nonmatch to sample (DNMS) task but has not been explored. Hence, a comprehensive pharmacological assessment was carried out here to (1) reveal the existence of such an interaction in terms of DNMS performance and (2) assess a possible cannabinoid-acetylcholine cross-talk on burst characteristics of hippocampal principal cells in CA3 and CA1.     

The amygdala is not necessary for unconditioned stimulus inflation after Pavlovian fear conditioning in rats

The basolateral complex (BLA) and central nucleus (CEA) of the amygdala play critical roles in associative learning, including Pavlovian conditioning. However, the precise role for these structures in Pavlovian conditioning is not clear. Recent work in appetitive conditioning paradigms suggests that the amygdala, particularly the BLA, has an important role in representing the value of the unconditioned stimulus (US). It is not known whether the amygdala performs such a function in aversive paradigms, such as Pavlovian fear conditioning in rats. To address this issue, Experiments 1 and 2 used temporary pharmacological inactivation of the amygdala prior to a US inflation procedure to assess its role in revaluing shock USs after either overtraining (Experiment 1) or limited training (Experiment 2), respectively. Inactivation of the BLA or CEA during the inflation session did not affect subsequent increases in conditioned freezing observed to either the tone conditioned stimulus (CS) or the conditioning context in either experiment. In Experiment 3, NBQX infusions into the BLA impaired the acquisition of auditory fear conditioning with an inflation-magnitude US, indicating that the amygdala is required for associative learning with intense USs. Together, these results suggest that the amygdala is not required for revaluing an aversive US despite being required for the acquisition of fear to that US.Pavlovian fear conditioning in rats is a behavioral model used to investigate the neurobiology underlying the development and maintenance of fear learning and memory (Grillon et al. 1996; LeDoux 1998, 2000; Bouton et al. 2001; Maren 2001b, 2005; Kim and Jung 2006). In this model, an innocuous conditioned stimulus (CS), such as a tone, is paired with an aversive unconditioned stimulus (US), such as a footshock. After one or more pairings, the rat learns that the CS predicts the US. As a consequence, CS presentations alone elicit a conditioned fear response (CR), which includes increases in heart rate, arterial blood pressure, hypoalgesia, potentiated acoustic startle, stress hormone release, and freezing (somatomotor immobility).The amygdala has been identified as one of the major regions in which fear memories are encoded and stored. Within the amygdala, the basolateral complex of the amygdala (BLA; consisting of the lateral, basolateral, and basomedial nuclei) and the central nucleus of the amygdala (CEA) receive convergent CS and US information and are involved in the acquisition of fear memories (LeDoux 1998, 2000; Fendt and Fanselow 1999; Davis and Whalen 2001; Maren 2001b; Schafe et al. 2001; Fanselow and Gale 2003; Wilensky et al. 2006; Zimmerman et al. 2007). In addition, the CEA has an important role in the expression of fear CRs (Fendt and Fanselow 1999; LeDoux 2000; Davis and Whalen 2001; Maren 2001b; Fanselow and Gale 2003). In support of this, many studies have shown that either permanent or temporary lesions of the BLA or CEA prevent the acquisition and/or expression of fear memories (Helmstetter 1992; Helmstetter and Bellgowan 1994; Campeau and Davis 1995; Maren et al. 1996a,b; Killcross et al. 1997; Muller et al. 1997; Walker and Davis 1997; Cousens and Otto 1998; Maren 1998, 1999, 2001a,b; Wilensky et al. 1999, 2000, 2006; Goosens and Maren 2001, 2003; Nader et al. 2001; Fanselow and Gale 2003; Gale et al. 2004; Koo et al. 2004; Zimmerman et al. 2007).In addition to its role in encoding CS–US associations during conditioning, recent work suggests that the amygdala is also involved in representing properties of the US itself. For example, temporary or permanent lesions of the BLA reduce both decrements in conditioned responding after devaluation of a food US (Hatfield et al. 1996; Killcross et al. 1997; Blundell et al. 2001; Balleine et al. 2003; Everitt et al. 2003; Pickens et al. 2003; Holland 2004) and increments in conditional responding after inflation of a shock US (Fanselow and Gale 2003). Moreover, recent electrophysiological studies in primates indicate that amygdala neurons represent the value of both aversive and appetitive outcomes (Paton et al. 2006; Belova et al. 2007, 2008; Salzman et al. 2007). These studies suggest that one function of the BLA is to represent specific properties of biologically significant events, such as the food or shock USs that are typically used in Pavlovian conditioning paradigms. By this view, the BLA may represent specific sensory properties of USs that shape the nature of learned behavioral responses to the US (Balleine and Killcross 2006) and allow CSs to gain access to the incentive value of the US (Everitt et al. 2003).In contrast to this view, we recently reported that rats with neurotoxic BLA lesions exhibit normal US revaluation after Pavlovian fear conditioning (Rabinak and Maren 2008). In this study, auditory fear conditioning (75 CS–US trials) with a moderate footshock (1 mA) was followed by several exposures (five US-alone trials) to an intense footshock (3 mA) during an inflation session. Both intact rats and rats with BLA lesions exhibit a robust increase in conditional freezing to the auditory CS during a subsequent retention test (Rabinak and Maren 2008). Control experiments suggested that this was due to a revaluation of the US with which the CS was associated, rather than nonassociative sensitization of fear engendered by exposure to intense shock. These data reveal that the BLA may not be necessary for representing properties of shock USs during Pavlovian fear conditioning. To address these issues further, we have examined the consequence of reversible pharmacological manipulations of the amygdala during US inflation on conditional fear responses established with either extensive or limited training.