Effects of ventral and dorsal CA1 subregional lesions on trace fear conditioning

Recent lines of research have focused on dissociating function between the dorsal and ventral hippocampus along space and anxiety dimensions. In the dorsal hippocampus, the CA1 subregion has been implicated in the acquisition of contextual fear as well as in the trace interval in trace fear conditioning. The present study was designed to test the relative contributions of dorsal (dCA1) and ventral CA1 (vCA1) in trace fear conditioning. Long-Evans rats received ibotenate lesions of the ventral CA1 (n=7), dorsal CA1 (n=9), or vehicle control lesions (n=8) prior to trace fear conditioning acquisition. Results suggest dCA1 and vCA1 groups show no significant deficits during acquisition when compared to control groups. dCA1 and vCA1 both show deficits in the retention of contextual fear when tested 24 h post-acquisition (P<.05 and P<.01, respectively), and vCA1 was impaired relative to dCA1 (P<.05). This is suggestive of a graded involvement in contextual retention between the dorsal and ventral aspects of CA1. dCA1 showed no deficit for retention of conditioned fear to the tone or the trace when tested 48 h post-acquisition, whereas vCA1 did show a significant deficit for the trace interval and a slight, non-significant reduction in freezing to the tone, when compared to the control group (p<.05). Overall the data are suggestive of a graded involvement in retention of fear conditioning between the dorsal and ventral aspects of CA1, but it is likely that vCA1 may be critically involved in retention of trace fear conditioning.     

Encoding, consolidation, and retrieval of contextual memory: differential involvement of dorsal CA3 and CA1 hippocampal subregions

Studies on human and animals shed light on the unique hippocampus contributions to relational memory. However, the particular role of each hippocampal subregion in memory processing is still not clear. Hippocampal computational models and theories have emphasized a unique function in memory for each hippocampal subregion, with the CA3 area acting as an autoassociative memory network and the CA1 area as a critical output structure. In order to understand the respective roles of the CA3- and CA1-hippocampal areas in the formation of contextual memory, we studied the effects of the reversible inactivation by lidocaine of the CA3 or CA1 areas of the dorsal hippocampus on acquisition, consolidation, and retrieval of a contextual fear conditioning. Whereas infusions of lidocaine never impaired elementary tone conditioning, their effects on contextual conditioning provided interesting clues about the role of these two hippocampal regions. They demonstrated first that the CA3 area is necessary for the rapid elaboration of a unified representation of the context. Secondly, they suggested that the CA1 area is rather involved in the consolidation process of contextual memory. Third, they showed that CA1 or CA3 inactivation during retention test has no effect on contextual fear retrieval when a recognition memory procedure is used. In conclusion, our findings point as evidence that CA1 and CA3 subregions of the dorsal hippocampus play important and different roles in the acquisition and consolidation of contextual fear memory, whereas they are not required for context recognition.     

Differential roles for hippocampal areas CA1 and CA3 in the contextual encoding and retrieval of extinguished fear

Recent studies demonstrate that context-specific memory retrieval after extinction requires the hippocampus. However, the contribution of hippocampal subfields to the context-dependent expression of extinction is not known. In the present experiments, we examined the roles of areas CA1 and CA3 of the dorsal hippocampus in the context specificity of extinction. After pairing an auditory conditional stimulus (CS) with an aversive footshock (unconditional stimulus or US), rats received extinction sessions in which the CS was presented without the US. In Experiment 1, pretraining neurotoxic lesions in either CA1 or CA3 eliminated the context dependence of extinguished fear. In Experiment 2, lesions of CA1 or CA3 were made after extinction training. In this case, only CA1 lesions impaired the context dependence of extinction. Collectively, these results reveal that both hippocampal areas CA1 and CA3 contribute to the acquisition of context-dependent extinction, but that only area CA1 is required for contextual memory retrieval.     

Cholinergic modulation of the hippocampus during encoding and retrieval of tone/shock-induced fear conditioning

We investigated the role of acetylcholine (ACh) during encoding and retrieval of tone/shock-induced fear conditioning with the aim of testing Hasselmo's cholinergic modulation model of encoding and retrieval using a task sensitive to hippocampal disruption. Lesions of the hippocampus impair acquisition and retention of contextual conditioning with no effect on tone conditioning. Cholinergic antagonists also impair acquisition of contextual conditioning. Saline, scopolamine, or physostigmine was administered directly into the CA3 subregion of the hippocampus 10 min before rats were trained on a tone/shock-induced fear conditioning paradigm. Freezing behavior was used as the measure of learning. The scopolamine group froze significantly less during acquisition to the context relative to controls. The scopolamine group also froze less to the context test administered 24 h posttraining. A finer analysis of the data revealed that scopolamine disrupted encoding but not retrieval. The physostigmine group initially froze less during acquisition to the context, although this was not significantly different from controls. During the context test, the physostigmine group froze less initially but quickly matched the freezing levels of controls. A finer analysis of the data indicated that physostigmine disrupted retrieval but not encoding. These results suggest that increased ACh levels are necessary for encoding new spatial contexts, whereas decreased ACh levels are necessary for retrieving previously learned spatial contexts.     

Differential contributions of dorsal vs. ventral hippocampus to auditory trace fear conditioning

The effect of excitotoxic lesions of dorsal vs. ventral hippocampus on the acquisition and expression of auditory trace fear conditioning was examined in two studies. In Experiment 1, animals received excitotoxic lesions of either the dorsal or ventral hippocampus or sham surgeries one week prior to conditioning, and were tested 24 h later. In Experiment 2, animals received excitotoxic lesions of either the dorsal or ventral hippocampus or sham surgeries 24 h after training, and were tested one week after surgery. Both pre- and post-training lesions of ventral hippocampus impaired the acquisition and expression, respectively, of auditory trace fear conditioning. Pre-training lesions of dorsal hippocampus had no effect on the acquisition of trace fear conditioning, while post-training lesions of dorsal hippocampus dramatically impaired expression during subsequent testing. Although in some cases animals with lesions of ventral hippocampus exhibited locomotor hyperactivity, it is unlikely that the pattern of observed deficits can be attributed to this effect. Collectively these data suggest that the dorsal and ventral hippocampus may contribute differentially to the mnemonic processes underlying fear trace conditioning.     

Intrahippocampal scopolamine impairs both acquisition and consolidation of contextual fear conditioning

Lesions of the dorsal hippocampus have been shown to disrupt both the acquisition and the consolidation of memories associated with contextual fear (fear of the place of conditioning), but do not affect fear conditioning to discrete cues (e.g., a tone). Blockade of central muscarinic cholinergic receptor activation results in selective acquisition deficits of contextual fear conditioning, but reportedly has little effect on consolidation. Here we show for the first time that direct infusion of the muscarinic cholinergic receptor antagonist, scopolamine, into the dorsal hippocampus produces a dose-dependent deficit in both acquisition and consolidation of contextual fear conditioning, while having no impact on simple tone conditioning.     

Kainic acid lesions disrupt fear-mediated memory processing

Previous research has shown that hippocampal lesions impair the expression of fear conditioning. This fear conditioning deficit may be due to memory impairment or a reduction in fear in lesioned animals. To address these possibilities, the authors examined unconditioned and conditioned fear in male Sprague-Dawley rats that had received intracerebroventricular (ICV) infusions of kainic acid (KA) 30 days prior to testing. Animals that had received bilateral ICV infusions of KA (1.0 microl of 0.8 mg/ml solution per side) exhibited cell loss that was primarily confined to the CA3 region of the dorsal hippocampus. Kainic acid lesions impaired contextual and cued fear conditioning but did not affect unconditioned fear in a light:dark test of anxiety. Moreover, animals with KA lesions did not habituate to the light:dark apparatus when tested over a 3-day period. These data suggest that decreases in fear conditioning produced by hippocampal lesions reflect a memory deficit and not a lack of fear.     

Disconnection analysis of CA3 and DG in mediating encoding but not retrieval in a spatial maze learning task

The dentate gyrus (DG) subregion of the hippocampus has been shown to be involved in encoding but not retrieval in a spatial maze task (modified Hebb-Williams maze). The first experiment in this study examined whether a lesion to the CA3 would contribute to a similar encoding deficit. A DG group was included in order to replicate previous results. Relative to controls, animals receiving CA3 lesions were impaired in encoding, not retrieval, on the modified Hebb-Williams maze--similar to a group that received DG lesions. This suggests the possibility that CA3 and DG are working together to mediate encoding processes. The second experiment in this study was designed to test the interaction between CA3 and DG using a disconnection paradigm. Animals with contralateral lesions (CA3 lesioned in one hemisphere, DG lesioned in the other hemisphere) showed a significant disruption effect on encoding, but not retrieval, when compared with animals with ipsilateral lesions (CA3 and DG lesioned in the same hemisphere, leaving the other hemisphere intact). This suggests an interaction between CA3 and DG in supporting encoding but not retrieval processes in a spatial maze learning task.     

Inverse temporal contributions of the dorsal hippocampus and medial prefrontal cortex to the expression of long-term fear memories

Retrograde amnesia following disruptions of hippocampal function is often temporally graded, with recent memories being more impaired. Evidence supports the existence of one or more neocortical long-term memory storage/retrieval site(s). Neurotoxic lesions of the medial prefrontal cortex (mPFC) or the dorsal hippocampus (DH) were made 1 day or 200 days following trace fear conditioning. Recently encoded trace fear memories were most disrupted by DH lesions, while remotely encoded trace and contextual memories were most disrupted by mPFC lesions. These data strongly support the consolidation theory of hippocampus function and implicate the mPFC as a site of long-term memory storage/retrieval.     

CB1 cannabinoid receptors modulate kinase and phosphatase activity during extinction of conditioned fear in mice

Cannabinoid receptors type 1 (CB1) play a central role in both short-term and long-term extinction of auditory-cued fear memory. The molecular mechanisms underlying this function remain to be clarified. Several studies indicated extracellular signal-regulated kinases (ERKs), the phosphatidylinositol 3-kinase with its downstream effector AKT, and the phosphatase calcineurin as potential molecular substrates of extinction behavior. To test the involvement of these kinase and phosphatase activities in CB1-dependent extinction of conditioned fear behavior, conditioned CB1-deficient mice (CB1(-/-)) and wild-type littermates (CB1(+/+)) were sacrificed 30 min after recall of fear memory, and activation of ERKs, AKT, and calcineurin was examined by Western blot analysis in different brain regions. As compared with CB1(+/+), the nonreinforced tone presentation 24 h after auditory-cued fear conditioning led to lower levels of phosphorylated ERKs and/or calcineurin in the basolateral amygdala complex, ventromedial prefrontal cortex, dorsal hippocampus, and ventral hippocampus of CB1(-/-). In contrast, higher levels of phosphorylated p44 ERK and calcineurin were observed in the central nucleus of the amygdala of CB1(-/-). Phosphorylation of AKT was more pronounced in the basolateral amygdala complex and the dorsal hippocampus of CB1(-/-). We propose that the endogenous cannabinoid system modulates extinction of aversive memories, at least in part via regulation of the activity of kinases and phosphatases in a brain structure-dependent manner.     

Electrolytic lesions of the dorsal hippocampus disrupt renewal of conditional fear after extinction

There is a growing body of evidence that the hippocampus is critical for context-dependent memory retrieval. In the present study, we used Pavlovian fear conditioning in rats to examine the role of the dorsal hippocampus (DH) in the context-specific expression of fear memory after extinction (i.e., renewal). Pre-training electrolytic lesions of the DH blunted the expression of conditional freezing to an auditory conditional stimulus (CS), but did not affect the acquisition of extinction to that CS. In contrast, DH lesions impaired the context-specific expression of extinction, eliminating the renewal of fear normally observed to a CS presented outside of the extinction context. Post-extinction DH lesions also eliminated the context dependence of fear extinction. These results are consistent with those using pharmacological inactivation of the DH and suggest that the DH is required for using contextual stimuli to regulate the expression of fear to a Pavlovian CS after extinction.     

Role of CA3 and CA1 subregions of the dorsal hippocampus on temporal processing of objects

Previous research in the dorsal CA1 and dorsal CA3 subregions of the hippocampus has been shown to play an important role in mediating temporal order memory for spatial location information. What is not known is whether the dorsal CA3 and dorsal CA1 subregions of the hippocampus are also involved in temporal order for visual object information. Rats with dorsal CA1, dorsal CA3 or control lesions were tested in a temporal order task for visual objects using an exploratory paradigm. The results indicated that the controls and the dorsal CA3 lesioned rats preferred the first rather then the last object they had explored previously, indicating good memory for temporal order of object presentation. In contrast, rats with dorsal CA1 lesions displayed a profound deficit in remembering the order of the visual object presentations in that they preferred the last object rather than the first. All three groups of rats preferred a novel object compared to a previously explored object suggesting normal detection of visual object novelty. The results suggest that only the dorsal CA1, but not dorsal CA3, region is critical for processing temporal information for visual objects without affecting the detection of new visual objects.     

Inhibition of PKA anchoring to A-kinase anchoring proteins impairs consolidation and facilitates extinction of contextual fear memories

Both genetic and pharmacological studies demonstrated that contextual fear conditioning is critically regulated by cyclic AMP-dependent protein kinase (PKA). Since PKA is a broad range protein kinase, a mechanism for confining its activity is required. It has been shown that intracellular spatial compartmentalization of PKA signaling is mediated by A-kinase anchoring proteins (AKAPs). Here, we investigated the role of PKA anchoring to AKAPs in different stages of the memory process (acquisition, consolidation, retrieval and extinction) using contextual fear conditioning, a hippocampus-dependent learning task. Mice were injected intracerebroventricularly or intrahippocampally with the membrane permeable PKA anchoring disrupting peptides St-Ht31 or St-superAKAP-IS at different time points during the memory process. Blocking PKA anchoring to AKAPs resulted in an impairment of fear memory consolidation. Moreover, disrupted PKA anchoring promoted contextual fear extinction in the mouse hippocampus. We conclude that the temporal and spatial compartmentalization of hippocampal PKA signaling pathways, as achieved by anchoring of PKA to AKAPs, is specifically instrumental in long-term contextual fear memory consolidation and extinction, but not in acquisition and retrieval.     

Nicotine Produces a Within-Subject Enhancement of Contextual Fear Conditioning in C57BL/6 Mice Independent of Sex

Nicotine enhances learning including contextual fear conditioning. The present study extends previous work on nicotine and conditioned fear to examine the nature of nicotine's enhancement of contextual fear conditioning and sex differences in contextual fear conditioning in C57BL/6 mice using a within-subjects design. Mice were trained by pairing of an auditory stimulus of 80 dB, 6 cps train of broad-band clicks conditioned stimulus (CS) with a 2 sec., 0.35 mA shock unconditioned stimulus (US). Twenty-four hours later mice were tested for freezing in the original context, and one hour later mice were retested in the same context. A 0.5 mg/kg dose of nicotine was given either for three conditions: (1) before training, testing, and retesting; (2) before training and retesting; and (3) before retesting only. The use of a within-subjects design allowed for testing if nicotine would produce state-dependent deficits in contextual fear conditioning. Nicotine did enhance contextual fear conditioning in the groups that received nicotine for both training and testing. Nicotine, however, did not alter freezing when given on training but not testing or testing but not training. No sex differences, however, existed for conditioning or for nicotine's effects on conditioning. These results suggest that nicotine enhanced acquisition and retrieval processes but did not produce state-dependent deficits when administered just for training or just for testing.     

Nicotine produces a within-subject enhancement of contextual fear conditioning in C57BL/6 mice independent of sex

Nicotine enhances learning including contextual fear conditioning. The present study extends previous work on nicotine and conditioned fear to examine the nature of nicotine’s enhancement of contextual fear conditioning and sex differences in contextual fear conditioning in C57BL/6 mice using a within-subjects design. Mice were trained by pairing of an auditory stimulus of 80 dB, 6 cps train of broad-band clicks conditioned stimulus (CS) with a 2 sec., 0.35 mA shock unconditioned stimulus (US). Twenty-four hours later mice were tested for freezing in the original context, and one hour later mice were retested in the same context. A 0.5 mg/kg dose of nicotine was given either for three conditions: (1) before training, testing, and retesting; (2) before training and retesting; and (3) before retesting only. The use of a within-subjects design allowed for testing if nicotine would produce state-dependent deficits in contextual fear conditioning. Nicotine did enhance contextual fear conditioning in the groups that received nicotine for both training and testing. Nicotine, however, did not alter freezing when given on training but not testing or testing but not training. No sex differences, however, existed for conditioning or for nicotine’s effects on conditioning. These results suggest that nicotine enhanced acquisition and retrieval processes but did not produc state-dependent deficits when administered just for training or just for testing.     

Behavioral characterization of a transection of dorsal CA3 subcortical efferents: comparison with scopolamine and physostigmine infusions into dorsal CA3

Cholinergic projections from the medial septum and diagonal band of Broca into the hippocampus have long been implicated in learning and memory. Projections from CA3 to neurons in the medial septum and the diagonal band of Broca have been anatomically characterized. The present experiments were designed to evaluate interactions between the dorsal CA3 subcortical efferents and the cholinergic efferents from the medial septum and diagonal band of Broca for spatial and nonspatial (visual object) novelty detection in the rat. In Experiment 1, physostigmine and scopolamine (both 0.4 microL at 30 microM) were infused into dorsal CA3 and animals were tested on a spatial and nonspatial (visual object) novelty detection paradigm. Scopolamine infusions into dorsal CA3 caused deficits for both spatial and nonspatial (visual object) novelty detection. Physostigmine infusions into dorsal CA3 enhanced both spatial and nonspatial (visual object) novelty detection. These data support models proposing that acetylcholine may control the dynamics for encoding, consolidation, and retrieval in the hippocampus. In Experiment 2, a selective transection of dorsal CA3 efferents in the fimbria resulted in deficits for spatial and nonspatial (visual object) novelty detection. These deficits were similar to the deficits caused by scopolamine infusions into dorsal CA3. These data demonstrate that dorsal CA3 and the medial septum/diagonal band of Broca interact, and that dorsal CA3 influences cholinergic inputs into the hippocampus to facilitate encoding.     

Differential effects of cannabinoid receptor agonist on social discrimination and contextual fear in amygdala and hippocampus

We examined whether the cannabinoid receptor agonist WIN55,212-2 (WIN; 5 μg/side) microinjected into the hippocampus or the amygdala would differentially affect memory processes in a neutral vs. an aversive task. In the aversive contextual fear task, WIN into the basolateral amygdala impaired fear acquisition/consolidation, but not retrieval. In the ventral subiculum (vSub), WIN impaired fear retrieval. In the neutral social discrimination task, WIN into the vSub impaired both acquisition/consolidation and retrieval, whereas in the medial amygdala WIN impaired acquisition. The results suggest that cannabinoid signaling differentially affects memory in a task-, region-, and memory stage-dependent manner.     

Effects of the selective M1 muscarinic receptor antagonist dicyclomine on emotional memory

The nonselective muscarinic antagonist scopolamine is known to impair the acquisition of some learning tasks such as inhibitory avoidance. There has been recent research into the effects of this drug in contextual fear conditioning and tone fear conditioning paradigms. The purpose of the present study was to assess the role of the selective M1 muscarinic antagonist dicyclomine in these paradigms and in the inhibitory avoidance test. Rats were administered different doses of dicyclomine or saline 30 min before acquisition training. The animals were tested 24 hr later, and it was observed that 16 mg/kg of dicyclomine impaired both contextual fear conditioning and inhibitory avoidance. However, dicyclomine (up to 64 mg/kg) did not affect tone fear conditioning. These results suggest that the selective M1 muscarinic antagonist dicyclomine differentially affects aversively motivated tasks known to be dependent on hippocampal integrity (such as contextual fear conditioning and inhibitory avoidance) but does not affect similar hippocampus-independent tasks.     

Post-training unilateral amygdala lesions selectively impair contextual fear memories

The basolateral amygdala (BLA) and the dorsal hippocampus (dHPC) are both structures with key roles in contextual fear conditioning. During fear conditioning, it is postulated that contextual representations of the environment are formed in the hippocampus, which are then associated with foot shock in the amygdala. However, it is not known to what extent a functional connection between these two structures is required. This study investigated the effect on contextual and cued fear conditioning of disconnecting the BLA and dHPC, using asymmetrically placed, excitotoxic unilateral lesions. Post-training lesions selectively impaired contextual, but not cued, fear, while pretraining lesions resulted in a similar but nonsignificant pattern of results. This effect was unexpectedly observed in both the contralateral disconnection group and the anticipated ipsilateral control, which prompted further examination of individual unilateral lesions of BLA and dHPC. Post-training unilateral dHPC lesions had no effect on contextual fear memories while bilateral dHPC lesions and unilateral BLA lesions resulted in a near total abolition of contextual fear but not cued conditioned fear. Again, pretraining unilateral BLA lesions resulted in a strong but nonsignificant trend to the impairment of contextual fear. Furthermore, an analysis of context test-induced Fos protein expression in the BLA contralateral to the lesion site revealed no differences between post-training SHAM and unilateral BLA lesioned animals. Therefore, post-training unilateral lesions of the BLA are sufficient to severely impair contextual, but not cued, fear memories.     

Transient activation of the CA3 Kappa opioid system in the dorsal hippocampus modulates complex memory processing in mice

The hippocampus plays a central role in various forms of complex learning and memory. Opioid peptides and receptors are abundant in the hippocampus. These peptides are co-released with glutamate from mossy fiber- and lateral perforant path-synapses. In this study, we evaluated the functional relevance of the CA3 Kappa opioid receptors (KOR) by transient pharmacological activation or inactivation using single bilateral intrahippocampal microinjections of a selective agonist (U50,488H, 1 or 2.5 nmol), a selective antagonist (nor-binaltorphimine, norBNI 5 nmol) or a mixture of both. C57Bl/6J mice were tested in a fear conditioning paradigm (FC) or in a modified version of the water maze task thought to reveal how flexibly animals can learn and manipulate spatial information (WM). In FC, the agonist (2.5 nmol) decreased context-induced (but not tone-induced) freezing whereas norBNI had no effect. The impairment caused by the agonist U50,488H was blocked by the injection of norBNI, suggesting that overstimulation of CA3-KOR impairs the acquisition and consolidation of contextual fear-related memory. In the WM task, mice were trained repeatedly each day to find a hidden platform. After having reached this goal, the platform position was changed the next day for a new task. U50,488H injection before the last task abolished the previously acquired ability to find rapidly a new platform location, whereas adding norBNI reversed this impairment. Thus, in the mouse, even partial and topographically restricted activation of CA3-KOR entails impairments in two different hippocampus-dependent tasks, indicating functional relevance of the kappa opioid system.