Time-dependent modulation of inhibitory avoidance memory by spermidine in rats

The polyamines, spermine, spermidine, and putrescine, are a group of aliphatic amines that may act as physiological modulators of N-methyl-D-aspartate (NMDA) receptors. Although the modulatory role of polyamines in NMDA receptor function has long been known, the effects of polyamines on learning and memory only recently began to be unraveled. In the present study, we investigated the effect of bilateral infusions of spermidine (0.02-2 nmol), a polyamine agonist, into the CA1 region of the rat dorsal hippocampus on inhibitory avoidance learning 30 min pre-training, immediately post-training, 6 h post-training, or 10 min pre-test. Bilateral microinjections of 0.2 nmol spermidine prolonged step-down latencies compared to the respective control group when administered 30 min pre-training or immediately post-training. These results provide evidence that the modulatory effects of spermidine on the acquisition and/or early consolidation of memory of inhibitory avoidance tasks in the hippocampus occur within a limited time window.     

Agmatine facilitates memory of an inhibitory avoidance task in adult rats

Agmatine is a new putative neurotransmitter; however, the physiological role(s) of this endogenous released polyamine is still to be determined. We investigated its cognitive effect in an inhibitory avoidance task in adult rats. Agmatine (0.1, 1, 10, and 20 mg/kg) or saline was administered ip immediately after training or 1 h before testing. Posttraining injection of agmatine facilitated (p < 0.05) memory consolidation in this task; however pretest treatment showed no effect on retrieval (p > 0.05). We suggest that the facilitatory effect of agmatine on memory consolidation in inhibitory avoidance task might be mediated through the activation of the locus coeruleus.     

Effects of posttraining treatments in the posterior cingulate cortex on short- and long-term memory for inhibitory avoidance in rats

Adult male Wistar rats were bilaterally implanted with indwelling cannulae in the caudal region of the posterior cingulate cortex. After recovery, animals were trained in a step-down inhibitory avoidance task (3.0-s, 0.4-mA foot shock) and received, right after training, a 0.5-microl infusion of vehicle (phosphate-buffered saline, pH 7.4), of the GABA(A) receptor agonist muscimol (0.1 or 0.5 microg), of the cAMP-dependent protein kinase (PKA) stimulant Sp-cAMPS (0.1 or 0.5 microg), or of the PKA inhibitor Rp-cAMPS (0.1 or 0.5 microg). Animals were tested twice, 1.5 h and, again, 24 h after training, in order to examine the effects of these agents on short- and long-term memory, respectively. Muscimol (0.5 but not 0.1 microg) hindered retention for both short- and long-term memory (p <.05). Rp-cAMPS (0.1 or 0.5 microg) hindered retention for short-term memory (p <.05). In addition, these animals showed lower, but not significantly lower, latencies than controls in the test session for long-term memory (p >.10). A trend toward an amnesic effect on long-term memory was also observed after Sp-cAMPS infusion at 0.1 microg (p <.10). These results show that strong stimulation of GABAergic synapses in the caudal region of the rat posterior cingulate cortex right after training impairs short- and long-term memory (the latter less dramatically). The same occurs by inhibiting PKA activity with regard to STM and possibly to LTM.     

Retrieval does not induce reconsolidation of inhibitory avoidance memory

It has been suggested that retrieval during a nonreinforced test induces reconsolidation instead of extinction of the mnemonic trace. Reconsolidation would preserve the original memory from the labilization induced by its nonreinforced recall through a hitherto uncharacterized mechanism requiring protein synthesis. Given the importance that such a process would have in terms of maintaining, as part of the animal behavioral repertoire, a learned response that has been devalued by experience, we analyzed its existence for the memory associated with a one-trial, step-down inhibitory avoidance task (IA), a memory whose consolidation and extinction require protein synthesis in the CA1 region of the dorsal hippocampus (CA1) and involve the participation of the basolateral amygdala (BLA) and entorhinal cortex (ENT). Rats were trained in IA, and 24 h later they were submitted either to a pure reactivation session (retrieval without stepping down), which was unable by itself to initiate extinction of the avoidance response, or to a second training session. Fifteen minutes before or 3 h after either the reactivation or the retraining sessions, animals were infused with the protein synthesis inhibitor anisomycin (ANI) into CA1, BLA, or ENT. Contrary to the prediction of the reconsolidation hypothesis, none of these treatments affected subsequent memory retention. Because reconsolidation is regarded to be a direct consequence of retrieval, one would expect that, when given before a retention test or a pure reactivation session, enhancers of memory expression should permanently improve retention and, therefore, facilitate retrieval both in that and in subsequent sessions. Using two well-known retrieval enhancers, noradrenaline and adrenocorticotropin(1-24), we could not find any evidence suggestive of reconsolidation. Hence, our results indicate that there is no retrieval-induced, protein synthesis-dependent process that would cause reconsolidation of IA memory.     

Differential roles of hippocampal metabotropic glutamate receptors 1 and 5 in inhibitory avoidance learning

Group I metabotropic glutamate receptors (mGlu1 and 5) have been implicated in synaptic plasticity and learning and memory. However, much of our understanding of how these receptors in different brain regions contribute to distinct memory stages in different learning tasks remains incomplete. The present study investigated the effects of the mGlu5 receptor antagonist, 2-methyl-6-(phenylethynyl)-pyridine (MPEP), and mGlu1 receptor antagonist, (S)-(+)-alpha-amino-4-carboxy-2-methylbenzene-acetic acid (LY 367385) in the dorsal hippocampus on the consolidation and extinction of memory for inhibitory avoidance learning. Male, Sprague-Dawley rats were trained in a single-trial step-down inhibitory avoidance task. MPEP, LY 367385 or saline were infused bilaterally into the CA1 region immediately after training or immediately after the first retention test which was given 24h after training. Rats receiving MPEP (1.5 or 5.0 microg/side) or LY 367385 (0.7 or 2.0 microg/side) infusion exhibited a dose-dependent decrease in retention when tested 24h later. MPEP was ineffective while LY 367385 significantly attenuated extinction when injected after the first retention test using an extinction procedure. These findings indicate a selective participation of hippocampal group I mGlu receptors in memory processing in this task.     

The effects of cholinergic drugs upon recognition memory in rats

This study measured the effects of the muscarinic blocker, scopolamine, upon object recognition. In order to test object recognition, rats were trained to choose between two distinctive goal boxes, one of which was familiar, and the other was novel. Selection of the unfamiliar goal box was always rewarded (nonmatching-to-sample), and new pairs of start/goal boxes were used on every trial.

In the first experiment it was found that injections of 0.05 mg/kg scopolamine hydrochloride and above produced significant impairments on this nonspatial test of working memory. A second experiment examined whether scopolamine caused a loss of retention by comparing the effects of the drug when the interval between stimulus presentation and choice test was increased from just over 0 sec to 60 sec. While the highest dose of scopolamine hydrobromide (0.06 mg/kg) was sufficient to produce a significant impairment on the longer retention interval, there was no evidence that this dose produced faster forgetting of the stimuli. This result suggests that the drug caused a general depression in performance, which may or may not reflect amnesic properties. In contrast, simultaneous tests with the anticholinesterase, physostigmine, indicated that increasing available acetyl choline might attenuate the effects of the retention intervals. A final series of control tests revealed that the rats relied on cues from a variety of sensory modalities in order to perform the nonmatching task.     

On the requirement of nitric oxide signaling in the amygdala for consolidation of inhibitory avoidance memory

Evidence suggests that the NO/sGC/PKG pathway plays a key role in memory processing but the actual participation of this signaling cascade in the amygdala during memory consolidation remains unknown. Here, we show that when infused in the amygdala immediately after inhibitory avoidance training, but not later, the NO synthase inhibitor L-NNA hindered long-term memory retention without affecting locomotion, exploratory behavior, anxiety state or retrieval of the avoidance response. The amnesic effect of L-NNA was not state-dependent and was mimicked by the soluble guanylyl cyclase inhibitor LY83583 and the PKG inhibitor KT-5823. On the contrary, post-training intra-amygdala infusion of the NOS substrate L-Arg, the NO-releasing compound SNAP or the non-hydrolysable analog of cGMP 8Br-cGMP increased memory retention in a dose-dependent manner. Co-infusion of 8Br-cGMP reversed the amnesic effect of L-NNA and LY83583 but not that of KT-5823. Our data indicate that the NO-induced activation of PKG in the amygdala is a necessary step for consolidation of inhibitory avoidance memory.     

Amitriptyline administered after consolidation of inhibitory avoidance does not affect memory retrieval

In Experiment 1, the effect of the administration of the antidepressant amitriptyline (30 mg/kg) for 21 days on the acquisition and consolidation of the inhibitory avoidance task was studied in male and female mice. In Experiment 2, it was evaluated whether amitriptyline administered after the consolidation of this task would block the memory retrieval. Anxiety and spontaneous activity in the elevated plus maze were also assessed. When amitriptyline was given before the training phase of inhibitory avoidance it blocked learning in males and there was a tendency in the same direction in females. However, the drug administered between training and test phases did not affect conditioning. These effects of amitriptyline seem to be independent of its actions on anxiety and locomotor activity. It may be that the effects observed are related to the therapeutic effects of antidepressants.     

Involvement of basolateral amygdala alpha2-adrenoceptors in modulating consolidation of inhibitory avoidance memory

These experiments investigated the role of the alpha(2)-adrenoceptors of the basolateral nucleus of the amygdala (BLA) in modulating the retention of inhibitory avoidance (IA). In Experiment 1, male Sprague Dawley rats implanted with bilateral cannulae in the BLA received microinfusions of a selective alpha(2)-adrenoceptor antagonist idazoxan 20 min either before or immediately after training. Retention was tested 48 h later. Idazoxan induced a dose-dependent enhancement of retention performance and was more effective when administered post-training. In Experiment 2, animals received pre- or post-training intra-BLA infusions of a selective alpha(2)-adrenoceptor agonist UK 14,304. The agonist induced a dose-dependent impairment of retention performance and, as with the antagonist treatments, post-training infusions were more effective. These results provide additional evidence that consolidation of inhibitory avoidance memory depends critically on prolonged activation of the noradrenergic system in the BLA and indicate that this modulatory influence is mediated, in part, by pre-synaptic alpha(2)-adrenoceptors.     

Critical role of the cholinergic system for object-in-place associative recognition memory

Object-in-place memory, which relies on the formation of associations between an object and the place in which it was encountered, depends upon a neural circuit comprising the perirhinal (PRH) and medial prefrontal (mPFC) cortices. This study examined the contribution of muscarinic cholinergic neurotransmission within this circuit to such object-in-place associative memory. Intracerebral administration of scopolamine in the PRH or mPFC impaired memory acquisition, but not retrieval and importantly we showed that unilateral blockade of muscarinic receptors simultaneously in both regions in opposite hemispheres, significantly impaired performance. Thus, object-in-place associative memory depends upon cholinergic modulation of neurones within the PRH-PFC circuit.Recognition memory enables individuals to judge whether stimuli have been encountered before. In its most basic form such judgments may be made on the basis of simply whether a stimulus is familiar or novel (familiarity discrimination). However, these judgments may also be made using associations formed between a stimulus and the location or environmental setting in which it was previously encountered. Such object-in-place associative memory in animals is of particular interest as it is acquired rapidly and it requires the integration of object and spatial information and thus has been described as an analog of human episodic memory (Wilson et al. 2008).The perirhinal cortex (PRH) in the medial temporal lobe is a critical neural structure for object recognition and object-in-place associative memory (Bussey et al. 2000; Barker et al. 2007), but unlike object recognition, this memory process is also dependent on the medial prefrontal cortex (mPFC) (Kesner and Ragozzino 2003; Browning et al. 2005; Barker et al. 2007) and crucially it has been shown to depend upon a functional interaction between the PRH and mPFC, with each region making a distinct cognitive contribution to the memory formation (Barker et al. 2007; Barker and Warburton 2008).Having identified two neural regions critical for object-in-place associative memory, we now extend our investigations to explore the underlying cellular mechanisms mediating acquisition or retrieval of this memory process. The present study focused on the neurotransmitter acetylcholine as cholinergic innervation of the PRH is crucial for familiarity discrimination (Tang et al. 1997; Easton and Gaffan 2001; Warburton et al. 2003; Abe et al. 2004; Winters and Bussey 2005). In contrast, the role of muscarinic receptor neurotransmission in the PRH or mPFC in object-in-place associative memory is unknown. Further, while it might appear that object recognition memory and object-in-place memory are likely to share common neural substrates, recent data from our laboratory suggest that this may not be the case (Griffiths et al. 2008).To explore the importance of muscarinic cholinergic neurotransmission within the PRH-mPFC circuit for object-in-place memory, rats were implanted with bilateral cannulae aimed at the PRH or mPFC or both regions to allow direct intracerebral administration of scopolamine during distinct stages of an object-in-place task. Memory performance was tested following either a short (5 min) or long (1 h) retention delay. All animal procedures were performed in accordance with the United Kingdom Animals Scientific Procedures Act (1986) and associated guidelines. Details of the surgery, infusion procedures, behavioral testing, and histology have been published previously (Barker and Warburton 2008). Briefly, male DA rats (230–250 g, Bantin and Kingman, UK) housed under a 12-h/12-h light/dark cycle (light phase 18:00–6:00 h), were anesthetized with isoflurane (induction 4%, maintenance 2%–3%) and surgically implanted with bilateral cannulae aimed at either the PRH or mPFC or both regions. After a two-week recovery period all rats were handled, habituated, and then tested in the object-in-place memory task.Sample phase: Each rat was placed in a black open-topped wooden arena (50 × 90 × 100 cm) containing four different objects (A, B, C, D) constructed from “Duplo” (Lego UK Ltd.). The walls of the arena were surrounded with a black cloth to a height of 1.5 m, and the floor covered with sawdust. The objects were placed 15 cm from the walls (see Fig. 1A) and each rat was allowed to explore the objects for 5 min, after which it was removed for the delay (5 min or 1 h). Exploratory behavior was defined as the animal directing its nose toward the object at a distance of <2 cm. Any other behavior, such as looking around while sitting on or resting against the object, was not recorded. Subjects that failed to complete a minimum of 15-s exploration in the sample phase or 10 s of exploration in the test phase were excluded from the analysis.Open in a separate windowFigure 1.Diagrammatic representations of the individual infusion sites in each animal. (A) Bilateral medial prefrontal (mPFC) group. (B) Bilateral perirhinal (PRH) group. (C) The mPFC infusion sites of the PRH+mPFC group. (D) The PRH infusion sites of the PRH+mPFC group. All of the infusion sites were within the PRH or mPFC.Test phase: Two of the objects, e.g., B and D, exchanged positions and the subjects were replaced in the arena for 3 min. The time spent exploring the two objects that had changed position was compared to the time spent exploring the two objects that had remained in the same position. If object-in-place memory is intact, subjects spend more time exploring the “moved” objects, compared to the “unmoved” objects. Scopolamine hydrobromide (Sigma-Aldrich) dissolved in sterile 0.9% saline solution was administered at a dose of 10 μg/μL per hemisphere (Schroeder and Packard 2002; Warburton et al. 2003; Winters et al. 2006); control infusions consisted of saline. The infusions were given either 15 min before the sample phase or 15 min before the test phase. At the end of the experiment, each rat was anesthetized and perfused transcardially. Coronal brain sections (40 μm) were stained with cresyl-violet to verify the cannulae locations. All the rats in the PRH group had the tip of the bilateral cannulae in the PRH and all the rats in the mPFC group had tips in the ventral portion of the prelimbic or dorsal portion of the infralimbic region of the prefrontal cortex (Fig. 1B). From unpublished observations, using Indian ink and radiolabeled scopolamine, the region infused is estimated to be 1–1.5 mm³, and largely confined to perirhinal cortex or the prelimbic/infralimbic regions of the prefrontal cortex. This spread is consistent with previously quoted results in other brain regions (Martin 1991; Izquierdo et al. 2000; Attwell et al. 2001). Figure 2, A and B show the performance of the rats receiving bilateral infusions of scopolamine or vehicle into either the PRH (n = 12) or mPFC (n = 12) 15 min prior to the sample phase. After a minimum of 48 h, vehicle or scopolamine was infused in a cross-over design and the animal retested using different objects. A three-way ANOVA (drug × region × delay) showed that scopolamine infusion into either region significantly impaired the acquisition of object-in-place memory (main effect of drug F _(1,35) = 63.87, P < 0.001). The magnitude of the deficit was similar irrespective of the region into which scopolamine was infused (region F _(1,35) < 1.0) or the delay employed (delay F _(1,35) < 1.0). Further analyses to examine whether individual groups discriminated between the objects, using a within subjects t-test (two-tailed), confirmed that vehicle-treated animals in the PRH and mPFC groups showed a significant preference for the moved objects over the objects that had remained in the same position, irrespective of the retention delay (PRH 5 min t ₍₉₎ = 2.96, P < 0.02; 1 h t ₍₁₀₎ = 5.71, P < 0.001: mPFC 5 min t ₍₅₎ = 5.47, P < 0.005; 1 h t ₍₁₁₎ = 9.89, P < 0.001), while scopolamine infusion into the PRH or mPFC significantly disrupted the animal''s ability to discriminate (PRH 5min t ₍₉₎ = 0.13, P = 0.9; 1 h t ₍₁₀₎ = 0.92, P = 0.38: mPFC 5 min t ₍₅₎ = 0.051, P = 0.961; 1 h t ₍₁₁₎ = 0.68, P = 0.51). Scopolamine was without effect on the total amount of exploration completed in the sample or test phases (all Fs < 1.0).Open in a separate windowFigure 2.Discrimination between the objects was calculated using a discrimination ratio, which takes into account individual differences in the total amount of exploration. The discrimination ratio is calculated as follows: the difference in time spent by each animal exploring objects that changed position compared to the objects that remained in the same position divided by the total time spent exploring all objects. (A) Infusion of scopolamine (Scop) into the perirhinal cortex (PRH) significantly impaired performance in the object-in-place task following a 5 min and a 1 h delay. (B) Infusion of scopolamine (Scop) into the medial prefrontal cortex (mPFC) significantly impaired performance in the object-in-place task following a 5 min and a 1 h delay. Illustrated for each group is the mean (+ SEM) discrimination ratio. * P < 0.05; ** P < 0.01; and *** P < 0.001 difference between groups.It could be argued that the impairment produced by intracortical infusions of scopolamine following a short delay, reflects an effect on retrieval as well as acquisition. Therefore, we examined the effect of pretest administration of scopolamine (infusion 15 min before the start of the test phase) in the mPFC or PRH following a 1 h delay. No significant impairments were found (mean discrimination ratio ± SEM: PRH vehicle 0.38 ± 0.07, scopolamine 0.46 ± 0.11; mPFC vehicle 0.37 ± 0.08, scopolamine 0.44 ± 0.05), confirmed by a nonsignificant drug effect (F _(1,14) < 1.0, P > 0.1) and nonsignificant drug × area interaction (F _(1,14) = <  1.0, P > 0.1). In addition all groups significantly discriminated between the moved objects compared to objects in the same location (PRH vehicle t ₍₇₎ = 4.95, P < 0.01; PRH scopolamine t ₍₇₎ = 3.45, P < 0.05; mPFC vehicle t ₍₇₎ = 4.26, P < 0.01; mPFC scopolamine t ₍₇₎ = 8.37, P < 0.01). Scopolamine was without effect on the total amount of exploration completed in the test phase (drug × region F _(1,14) < 1.0, P > 0.05).To evaluate the importance of intrahemispheric interactions between these cortical regions and the cholinergic system, a third group of animals had cannulae implanted into both the PRH and mPFC (n = 12). In this experiment the behavioral effects of unilateral scopolamine infusions into the PRH and mPFC in the same hemisphere (Scop Ipsi) were compared with the effects of unilateral scopolamine infusions into opposite hemispheres (Scop Contra). The animals assigned to the Scop Ipsi group on day one, received infusions into opposite hemispheres (Scop Contra) on day two (minimum of 48 h later). Likewise, the animals in the Scop Contra group on day one, received ipsilateral infusions on day two. Figure 3 shows discrimination performance following a 5 min or 1 h delay. A two-way within-subject ANOVA revealed that the Scop Contra group was significantly impaired (infusion F _(1,20) = 44.35, P < 0.001) irrespective of the delay (infusion × delay F _(1,20) < 1.0, P < 0.05). Further analysis confirmed that the Scop Contra group failed to discriminate between the moved and unmoved objects (5 min t ₍₁₀₎ = 0.70, P > 0.1; 1 h t ₍₁₀₎ = 1.03, P > 0.1), while the Scop Ipsi group preferentially explored the moved objects (5 min t ₍₁₀₎ = 9.99, P < 0.0001; 1 h t ₍₁₀₎ = 4.34, P = 0.001).Open in a separate windowFigure 3.Unilateral scopolamine infusions into the PRH and mPFC in opposite hemispheres (Scop Contra) impaired object-in-place performance following both a 5 min and a 1 h delay. Scopolamine infusions into both the PRH and mPFC in the same hemisphere (Scop Ipsi) had no effect on performance following either delay. ** P < 0.01 and *** P < 0.001 difference between groups.Scopolamine was without effect on overall exploration levels during the sample (infusion × delay F _(1,20) < 1.0, P > 0.05; infusion F _(1,20) < 1.0, P > 0.05; delay F _(1,20) < 1.0, P > 0.05) or test phases (infusion × delay F _(1,20) < 1.0, P  >  0.05; infusion F _(1,20)  <  1.0, P > 0.05). There was a significant main effect of delay (F _(1,20) = 10.67, P < 0.01), as the Scop Ipsi and Scop Contra groups completed a greater amount of exploration in the test phase following a 1 h delay compared to a 5 min delay.These results demonstrate that acquisition, but not retrieval of object-in-place memory, is dependent upon muscarinic cholinergic neurotransmission in both the mPFC and PRH. Thus, acute bilateral administration of scopolamine directly into the mPFC or PRH before the sample phase impaired both short- and long-term memory performances. In contrast administration of scopolamine into either the mPFC or PRH prior to the test phase had no effect. Significantly, co-administration of scopolamine into the PRH and mPFC in opposite hemispheres produced a significant impairment in both short-term and long-term object-in-place memory compared to performance following co-administration of scopolamine into the PRH and mPFC in the same hemisphere. Thus, concomitant activation of cholinergic muscarinic receptors is necessary in both regions for the formation of object-in-place associative recognition memory.Our previous studies investigating the role of the mPFC and PRH in object-in-place associative memory suggest that these regions make different cognitive contributions to this mnemonic process. Thus, the PRH appears to be primarily involved in the acquisition of “object” information, while we have hypothesized that the role of the mPFC is to integrate object and place information (Barker et al. 2007). As administration of scopolamine into either region disrupted performance following a long- or short-retention delay, the present data suggest that the neural mechanisms underlying both these different cognitive processes must be dependent upon cholinergic neurotransmission.The results demonstrate that muscarinic receptor neurotransmission is clearly critical for acquisition of the object-in-place task as no impairment was produced when scopolamine was administered only prior to the test phase. While the current study is the first to investigate the importance of cholinergic neurotransmission in object-in-place associative memory, a number of previous studies have shown that intra-PRH infusions of scopolamine block discrimination of novel and familiar objects when administered prior to the sample phase, but not when administered immediately after the sample phase or prior to the test phase (Aigner and Mishkin 1986; Aigner et al. 1991; Warburton et al. 2003; Winters et al. 2006). Thus, together these results support the hypothesis that muscarinic cholinergic neurotransmission within the PRH is necessary for encoding representations of new visual stimuli for subsequent recognition (Turchi et al. 2005), but not for the retrieval of such information. The present results also show for the first time that muscarinic receptor neurotransmission within the mPFC is crucial for the encoding, but not the retrieval of object-in-place memory.It may be argued that the disruptions in performance following administration of scopolamine reflect disruptions in attentional processing. Indeed muscarinic cholinergic neurotransmission in the prefrontal cortex has been implicated in both mnemonic and attentional processes (Voytko et al. 1994; Everitt and Robbins 1997; Chudasama et al. 2004; Dalley et al. 2004). However, deficits in attentional processing are typically observed when the attentional demands of the tasks are high, for example, when very short (millisecond) stimulus exposure times are used (Chudasama et al. 2004; Dalley et al. 2004). In the present study, the exposure time to the stimuli is relatively long (minutes); further there was no evidence of a drug-associated change in explorative behavior following either an infusion into the mPFC or PRH or simultaneously into both regions. Thus, it seems unlikely that the impairments in memory observed can be attributed purely to an attentional deficit, although it is possible that during the encoding of the object-in-place task attentional processes are also recruited involving the cholinergic afferents to the mPFC or PRH.The results showing that simultaneous muscarinic cholinergic blockade in the PRH and mPFC produces a significant impairment in performance support our previous findings of a neural system for object-in-place memory and extend these findings to show that cholinergic neurotransmission is a key component within the system. Our results also support those studies in primates demonstrating a circuit involving the basal forebrain, frontal cortex, and inferior temporal cortex is necessary for object memory encoding (Easton et al. 2002; Easton and Parker 2003).Results from our laboratory have shown that the maintenance of long-term, but not short-term, object-in-place memory is critically dependent upon concurrent NMDA receptor activation in the PRH and mPFC (Barker and Warburton 2008), while short-term object-in-place performance is dependent upon kainate receptor activation in the PRH. Hence, we have argued that there may be multiple cellular mechanisms underlying encoding of information for the short or long term. The present study contrasts with these findings as it demonstrates the necessity for muscarinic receptor activation for both short- and long-term object-in-place memory. Primate studies have indicated that a synergistic interaction between the cholinergic and glutamatergic systems plays an important role in the regulation of visual recognition memory (Matsuoka and Aigner 1996). Hence, further investigations are warranted to explore such interactions in the rat; for example, an interaction between NMDA and muscarinic receptor neurotransmission may mediate long-term recognition memory, while a kainate–muscarinic receptor interaction may mediate short-term recognition memory. Further, the extent to which the contribution of the cholinergic system to encoding of object-in-place memory within the PRH-mPFC system is the same for both short- or long-term memory is unknown.Our results have demonstrated that when a subject is required to use information concerning an association between an object and a place to produce a behavioral response, muscarinic cholinergic receptors in the mPFC are involved. Further, the object-in-place task requires the subject to acquire and remember the topographical relationship between the objects, a process that is known to depend upon the parietal cortex (Goodrich-Hunsaker et al. 2005). The precise contribution of object and spatial information processing in the parietal cortex to the operation of the PRH-mPFC circuit has yet to be determined.In conclusion, the cholinergic projections to the PRH and mPFC originating in the basal forebrain (Wenk et al. 1980) are an important component of the neural mechanisms underlying short- and long-term object-in-place associative memory.     

Histamine enhances inhibitory avoidance memory consolidation through a H2 receptor-dependent mechanism

Several evidences suggest that brain histamine is involved in memory consolidation but the actual contribution of the hippocampal histaminergic system to this process remains controversial. Here, we show that when infused into the CA1 region of the dorsal hippocampus immediately after training in an inhibitory avoidance task, but not later, histamine induced a dose-dependent promnesic effect without altering locomotor activity, exploratory behavior, anxiety state or retrieval of the avoidance response. The facilitatory effect of intra-CA1 histamine was mimicked by the histamine N-methyltransferase inhibitor SKF-91844 as well as by the H2 receptor agonist dimaprit and it was blocked completely by the H2 receptor antagonist ranitidine. Conversely, the promnesic action of histamine was unaffected by the H1 receptor antagonist pyrilamine, the H3 receptor antagonist, thioperamide, and the NMDAr polyamine-binding site antagonist ifenprodil. By themselves, ranitidine, pyrilamine, thioperamide, and ifenprodil did not affect IA memory consolidation. Our data indicate that, when given into CA1, histamine enhances memory consolidation through a mechanism that involves activation of H2 receptors; however, endogenous CA1 histamine does not seem to participate in the consolidation of IA memory at least at the post-training times analyzed.     

Oral administration of guanosine impairs inhibitory avoidance performance in rats and mice

Extracellular guanine-based purines, mainly the nucleoside guanosine, have recently been shown to exert neuroprotective effects, which seem to be related to antagonism of the glutamatergic system. In this study, we investigated the effects of acute oral administration of guanosine on inhibitory avoidance task in rats and mice. We also studied its effects on locomotor activity, anxiety-related behaviors and mechanisms of action involving the purinergic system. Guanosine (2.0 and 7.5mg/kg, per os), administered 75min pretraining, dose-dependently impaired retention of the inhibitory avoidance task in rats and mice, an effect not prevented by the adenosine receptor antagonist caffeine. Guanosine presented no effects on locomotor activity and anxiety-related behaviors. This amnesic effect of guanosine may be compatible with inhibition of glutamatergic system and seems to be not mediated by adenosine.     

Posttraining infusion of cholinergic drugs into the ventral subiculum modulated memory in an inhibitory avoidance task: Interaction with the bed nucleus of the stria terminalis

The ventral subiculum (vSUB), a hippocampal efferent target implicated in learning and stress coping, receives cholinergic input and sends glutamatergic output to the bed nucleus of the stria terminalis (BNST). This study examined the roles of vSUB muscarinic activation and its interaction with BNST N-methyl-d-aspartate and noradrenergic receptors in formation of aversive memory. Male Wistar rats with cannulae implanted into the vSUB or BNST were trained on a step-through inhibitory avoidance task. Shortly after training, they received cholinergic drugs infused into the vSUB and/or glutamatergic or noradrenergic drugs infused into the BNST. Results of the 1-day retention tests showed that intra-vSUB infusion of oxotremorine (0.01 μg) or scopolamine (0.3 or 3.0 μg) enhanced or impaired retention, respectively. Both effects were dose- and time-dependent, and 0.001 μg oxotremorine attenuated the amnesia induced by 3.0 μg scopolamine. The oxotremorine-induced memory enhancement was blocked by intra-BNST infusion of dl-2-amino-5-phosphonovaleric acid or propranolol at a dose not affecting retention; the amnesia induced by scopolamine was blunted by intra-BNST infusion of glutamate or norepinephrine at a dose with a negligible effect on retention. These data suggest that in an inhibitory avoidance task muscarinic activation of the vSUB modulated memory formation by interacting with the BNST glutamatergic and noradrenergic functions.     

Post-training disruption of Arc protein expression in the anterior cingulate cortex impairs long-term memory for inhibitory avoidance training

The activity-regulated-cytoskeletal-associated protein (Arc) has a well established role in memory consolidation and synaptic plasticity in the hippocampus and amygdala. However the role of Arc within the anterior cingulate cortex (ACC), an area of the brain involved in processing memory for pain, has yet to be examined. Here we sought to determine if Arc protein within neurons of the rat ACC is necessary for the consolidation of a single-trial, contextual inhibitory avoidance (IA) task. Immunohistochemistry and western blotting revealed an increase in Arc protein within the ACC following IA training in a shock-specific manner, suggesting that ACC Arc expression may play a critical role in the consolidation of the aversive task. To directly test this hypothesis, male Sprague-Dawley rats were trained on the IA task and given post-training intra-ACC infusions of Arc antisense oligodeoxynucleotides (ODNs), designed to suppress Arc translation, or control scrambled ODNs that do not suppress Arc translation. Memory retention was tested 48h after training. Arc antisense-induced disruption of Arc protein expression in the ACC impaired long-term memory for the IA task as compared to rats given intra-ACC infusions of the scrambled control ODNS, suggesting that Arc expression in the ACC is important for the consolidation of emotional memory. Results further indicate that knock down of Arc 6h after training impairs IA memory. This is consistent with time course findings indicating elevated Arc expression at 3 and 6h after IA training but not 12 or 48h. Taken together, these findings support the hypothesis that Arc expression in the ACC participates in synaptic plasticity that underlies long-term memory.     

Dissociation of data-based and expectancy-based memory following hippocampal lesions in rats

Sham-operated and nonoperated animals or animals with hippocampal lesions were presented with sets of trials to test both expectancy-based and data-based memory within the same task. During the study phase of each trial the animals were presented with a constant sequence of five arms on an eight-arm radial maze followed by a test phase in which a recognition test requiring a win-stay rule was used. Expectancy-based memory was measured during the study phase of the trials as a pattern of correct or incorrect orienting responses in anticipation of the ensuing doors in the constant sequence. Both groups of animals acquired correct orienting responses at the same rate, emitted the same pattern of correct orienting responses, and made the same number and pattern of intralist and extralist intrusion errors. Data-based memory was measured during the test phase of the trial as correct recognition test performance. During the test phase the animals with hippocampal lesions were impaired relative to controls on both immediate and 24-h recognition tests. These results suggest that the hippocampus might mediate only data-based, but not expectancy-based, memory and imply a possible dissociation between expectancy-based and data-based memory systems.