Acute pre-learning stress selectively impairs hippocampus-dependent fear memory consolidation: Behavioral and molecular evidence.

Despite compelling evidence that stress or stress-related hormones influence fear memory consolidation processes, the understanding of molecular mechanisms underlying the effects of stress is still fragmentary. The release of corticosterone in response to pre-learning stress exposure has been demonstrated to modulate positively or negatively memory encoding and/or consolidation according to many variables such as stress intensity, the emotional valence of the learned material or the interval between stressful episode and learning experience. Here, we report that contextual but not cued fear memory consolidation was selectively impaired in male mice exposed to a 50 min-period of restraint stress just before the unpaired fear conditioning session. In addition to behavioral impairment, acute stress down-regulated activated/phosphorylated ERK1/2 (pERK1/2) in dorsal hippocampal area CA1 in mice sacrificed 60 min and 9 h after unpaired conditioning. In lateral amygdala, although acute stress by itself increased the level of pERK1/2 it nevertheless blocked the peak of pERK1/2 that was normally observed 15 min after unpaired conditioning. To examine whether stress-induced corticosterone overflow was responsible of these detrimental effects, the corticosterone synthesis inhibitor, metyrapone, was administered 30 min before stress exposure. Metyrapone abrogated the stress-induced contextual fear memory deficits but did not alleviate the effects of stress on pERK1/2 and its downstream target phosphorylated CREB (pCREB) in hippocampus CA1 and lateral amygdala. Collectively, our observations suggest that consolidation of hippocampus-dependent memory and the associated signaling pathway are particularly sensitive to stress. However, behavioral normalization by preventive metyrapone treatment was not accompanied by renormalization of the canonical signaling pathway. A new avenue would be to consider surrogate mechanisms involving proper metyrapone influence on both nongenomic and genomic actions of glucocorticoid receptors.

Post-training, intrahippocampal HDAC inhibition differentially impacts neural circuits underlying spatial memory in adult and aged mice.

Converging evidence indicates that pharmacologically elevating histone acetylation using post-training, systemic or intrahippocampal, administration of histone deacetylase inhibitor (HDACi) can enhance memory consolidation processes in young rodents but it is not yet clear, whether such treatment is sufficient to prevent memory impairments associated with aging. To address this question, we used a 1-day massed spatial learning task in the water maze to investigate the effects of immediate post-training injection of the HDACi trichostatin A (TSA) into the dorsal hippocampus on long-term memory consolidation in 3-4 and 18-20 month-old mice. We show that TSA improved the 24 h-memory retention for the hidden platform location in young-adults, but failed to rescue memory impairments in older mice. The results further indicate that Young-TSA mice sacrificed 1 h after training had a robust increase in histone H4 acetylation in the dorsal hippocampal CA1 region (dCA1) and the dorsomedial part of the striatum (DMS), a structure important for spatial information processing. Importantly, TSA infusion in aged mice completely rescued altered H4 acetylation in the dCA1 but failed to alleviate age-associated decreased H4 acetylation in the DMS. Moreover, intrahippocampal TSA infusion produced concomitant decreases (in adults) or increases (in older mice) of acetylated histone levels in the ventral hippocampus (vCA1 and vCA3) and the lateral amygdala, two structures critically involved in stress and emotional responses. These data suggest that the failure of post-training, intrahippocampal TSA injection to reverse age-associated memory impairments may be related to an inability to recruit appropriate circuit-specific epigenetic patterns during early consolidation processes.

Hippocampal Injections of Oligomeric Amyloid ß-peptide (1-42) Induce Selective Working Memory Deficits and Long-lasting Alterations of ERK Signaling Pathway.

Increasing evidence suggests that abnormal brain accumulation of soluble rather than aggregated amyloid-ß1-42 oligomers (Aßo(1-42)) plays a causal role in Alzheimer's disease (AD). However, as yet, animal's models of AD based on oligomeric amyloid-ß1-42 injections in the brain have not investigated their long-lasting impacts on molecular and cognitive functions. In addition, the injections have been most often performed in ventricles, but not in the hippocampus, in spite of the fact that the hippocampus is importantly involved in memory processes and is strongly and precociously affected during the early stages of AD. Thus, in the present study, we investigated the long-lasting impacts of intra-hippocampal injections of oligomeric forms of Aßo(1-42) on working and spatial memory and on the related activation of ERK1/2. Indeed, the extracellular signal-regulated kinase (ERK) which is involved in memory function had been found to be activated by amyloid peptides. We found that repeated bilateral injections (1injection/day over 4 successive days) of oligomeric forms of Aßo(1-42) into the dorsal hippocampus lead to long-lasting impairments in two working memory tasks, these deficits being observed 7 days after the last injection, while spatial memory remained unaffected. Moreover, the working memory deficits were correlated with sustained impairments of ERK1/2 activation in the medial prefrontal cortex (mPFC) and the septum, two brain areas tightly connected with the hippocampus and involved in working memory. Thus, our study is first to evidence that sub-chronic injections of oligomeric forms of Aßo(1-42) into the dorsal hippocampus produces the main sign of cognitive impairments corresponding to the early stages of AD, via long-lasting alterations of an ERK/MAPK pathway in an interconnected brain networks.

Impaired hippocampus-dependent spatial flexibility and sociability represent autism-like phenotypes in GluK2 mice.

Autism is a complex neurodevelopmental disorder with high heritability. grik2 (which encodes the GluK2 subunit of kainate receptors) has been identified as a susceptibility gene in Autism Spectrum Disorders (ASD), but its role in the core and associated symptoms of ASD still remains elusive. We used mice lacking GluK2 (GluK2 KO) to examine their endophenotype with a view to modeling aspects of autism, including social deficits, stereotyped and repetitive behavior and decreased cognitive abilities. Anxiety was recorded in the elevated plus maze, social behavior in a three-chamber apparatus, and cognition in different water maze protocols. Deletion of the GluK2 gene reduced locomotor activity and sociability as indicated by the social interaction task. In addition, GluK2 KO mice learnt to locate a hidden platform in a water maze surrounded by a curtain with hanging cues faster than wild-type mice. They maintained a bias toward the target quadrant when some of these cues were removed, at which point wild-types orthogonalized the behavior and showed no memory. However, GluK2 KO mice were impaired in spatial reversal learning. These behavioral data together with previously published electrophysiology showing severe anomalies in CA3 network activity, suggest a computational shift in this network for enhanced propensity of pattern completion that would explain the loss of behavioral flexibility in GluK2 KO mice. Although a single mutation cannot recapitulate the entire core symptoms of ASD, our data provide evidence for glutamatergic dysfunction underlying a number of social- and cognition-related phenotypes relevant to ASD.

Partial loss in septo-hippocampal cholinergic neurons alters memory-dependent measures of brain connectivity without overt memory deficits.

The functional relevance of septo-hippocampal cholinergic (SHC) degeneration to the degradation of hippocampus-dependent declarative memory (DM) in aging and Alzheimer's disease (AD) remains ill-defined. Specifically, selective SHC lesions often fail to induce overt memory impairments in animal models. In spite of apparent normal performance, however, neuronal activity within relevant brain structures might be altered by SHC disruption. We hypothesized that partial SHC degeneration may contribute to functional alterations within memory circuits occurring in aging before DM decline. In young adult mice, we studied the effects of behaviorally ineffective (saporin-induced) SHC lesions - similar in extent to that seen in aged animals - on activity patterns and functional connectivity between three main neural memory systems: the septo-hippocampal system, the striatum and the amygdala that sustain declarative, procedural and emotional memory, respectively. Animals were trained in a radial maze procedure dissociating the human equivalents of relational/DM and non-R/DM expressions in animals. Test-induced Fos activation pattern revealed that the partial SHC lesion significantly altered the brain's functional activities and connectivity (co-activation pattern) despite the absence of overt behavioral deficit. Specifically, hippocampal CA3 hyperactivity and abnormal septo-hippocampo-amygdalar inter-connectivity resemble those observed in aging and prodromal AD. Hence, SHC neurons critically coordinate hippocampal function in concert with extra-hippocampal structures in accordance with specific mnemonic demand. Although partial SHC degeneration is not sufficient to impact DM performance by itself, the connectivity change might predispose the emergence of subsequent DM loss when, due to additional age-related insults, the brain can no longer compensate the holistic imbalance caused by cholinergic loss.

Detection of age-dependent working memory deterioration in APP751SL mice.

Despite huge advances on Alzheimer's disease (AD) etiology, the clinical diagnosis remains the unique commonly used tool to detect the onset of the disease. For instance, epidemiological studies report that the combination of episodic and working memory disorders represents the most consistent sign of progression from mild cognitive impairment to AD. However, such working memory disorders failed to be observed early in transgenic mouse models of AD because the behavioral procedures used do not tackle properly crucial components of working memory. The aim of the present work was to assess early occurrence of working memory impairments in APP(751SL) mice. Therefore, we designed a new behavioral task in the water-maze, based on the principle of a delayed matching to place task, where spatial recognition was assessed for four different platform locations within a single session. First, we showed that dorsal hippocampal but not medial prefrontal cortex lesions in C57Bl6 mice induced a time-dependent impairment of spatial recognition. Then, the hippocampal-like memory alterations were reproduced in 7-8-month-old APP(751SL) mice but not in younger animals (5-6-month-old). We also demonstrated that these working memory deficits are related to progressive Aß accumulation in the hippocampus, but not in the other selected brain structures.

Acetylcholine and memory: a long, complex and chaotic but still living relationship.

Even though "procholinergic" drugs are almost the sole kind of treatments currently used as cognitive enhancers in patients with Alzheimer's disease, the role of acetylcholine (ACh) in learning and memory is still poorly understood. In this short review, we focus on the septo-hippocampal cholinergic system and try to demonstrate that understanding ACh-memory relationships requires taking into account two characteristics of memory function. First, this function is polymorphic and relies on multiple neural systems. It appears that hippocampal ACh may not only modulate specific computational function of the hippocampus but also contributes to the functional coordination of multiple memory systems in a task-dependent manner. Second, memorization implies different phases which are differentially regulated by ACh. Namely, several lines of evidence suggest a "biphasic" involvement with hippocampal ACh facilitating memory encoding but hampering memory consolidation and retrieval, and low hippocampal ACh promoting consolidation of declarative memory. By spotting major determinants of memory modulation by hippocampal ACh, we hope that the present non exhaustive review will help to improve our understanding of the complexity of ACh-memory relationships.

The schizophrenia susceptibility gene neuregulin 1 modulates tolerance to the effects of cannabinoids.

Cannabis increases the risk of schizophrenia in genetically vulnerable individuals. In this study we aim to show that the schizophrenia susceptibility gene neuregulin 1 (Nrg1) modulates the development of tolerance to cannabinoids in mice. Nrg1 heterozygous (HET) and wild-type (WT) mice were treated daily for 15 d with the synthetic analogue of Δ9-tetrahydrocannabinol, CP55,940 (0.4 mg/kg). We measured the impact of this exposure on locomotor activity, anxiety, prepulse inhibition (PPI), body temperature and FosB/ΔFosB immunohistochemistry. Tolerance to CP55,940-induced hypothermia and locomotor suppression developed more rapidly in Nrg1 HET mice than WT mice. Conversely in the light-dark test, while tolerance to the anxiogenic effect of CP55,940 developed in WT mice over days of testing, Nrg1 hypomorphs maintained marked anxiety even after 15 d of treatment. Repeated cannabinoid exposure selectively increased FosB/ΔFosB expression in the lateral septum, ventral part (LSV) of Nrg1 HET but not WT mice. On day 1 of exposure opposite effects of CP55,940 treatment were observed on PPI, i.e. it was facilitated in Nrg1 hypomorphs and impaired in WT mice, despite the drug significantly impairing the acoustic startle reflex equally in both genotypes. These effects of CP55,940 on PPI were not maintained as both genotypes became tolerant to cannabinoid action with repeated exposure. Our results highlight that Nrg1 modulates the development of cannabinoid tolerance dependent on the parameter being measured. Furthermore, these data reinforce the notion that the VLS is an important brain region involved in Nrg1-cannabinoid interactions.

Activating a memory system focuses connectivity toward its central structure.

This report investigates in what way functional connectivity may explain how two memory systems that share almost all their structures, can function as separate systems. The first series of experiments was aimed at demonstrating the reliability of our experimental design by showing that acquisition of the spatial version of a water cross-maze task (stimulus-stimulus associations) was impaired by dorsal hippocampal lesions whereas the cue version (stimulus-reinforcement association) was altered by amygdala lesion. Then, we evaluated how these two tasks induce different patterns of connectivity. The connectivity was evaluated by calculating the correlations between the zif-268 immunoreactivity of 22 structures composing the hippocampus and the amygdala systems. We designed a new statistical procedure to demonstrate double dissociations on the basis of brain regional intercorrelations. Our data show that the correlations between the hippocampus and the other structures of the memory system are higher in the place-learning group compared to the cue-learning group, whereas they are enhanced with the amygdala in the latter group compared to the former. This demonstrates that the activation of a memory system consists in the focusing of functional connectivity toward the central structure of the system. This may explain how several memory systems can share the same structures while remaining independent.

Characterization of cognition alteration across the course of the disease in APP751SL mice with parallel estimation of cerebral Abeta deposition.

Current transgenic mouse models of Alzheimer disease constitute a relevant tool to examine the relationships between neuropathological lesions, neurodegeneration and clinical syndromes. Nevertheless, addressing the relation between Abeta deposition and cognition deterioration requires careful adjustment for age to decipher underlying mechanisms of impairments and identify potential therapeutic targets. In the present work we have carried out a detailed behavioral analysis of the APP(751SL) transgenic mouse model testing 6 age-points (from 2 to 19-20 months) and estimating in parallel the cerebral Abeta deposition. The immunohistochemistry study indicated a fast progression of Abeta(17-24) staining in several brain structures that reached for most of them, a maximal level at 7-8 months of age. Behavioral experiments showed that APP(751SL) mice displayed alterations in some general functions (muscular strength, motor activity) whereas other functions are preserved (anxiety, exploration). Acquisition and extinction of an appetitive operant conditioning were used to assess early learning deficits. Hippocampal but not dorso-lateral striatal lesion was shown to delay extinction. Although some learning deficits were detected at 5-6 months in the acquisition of the operant conditioning task, more robust impairments in extinction were observed in 7-8-month-old mice. Indeed, spatial memory deficit was associated to a selective hippocampal CA1 impairment of learning-induced Zif268 activation. Because this mouse model displayed gradual memory deficits it gives the opportunity to investigate the temporal progression of molecular and cellular mechanisms that induce cognitive decline.

Endocytosis controls glutamate-induced nuclear accumulation of ERK.

Nuclear translocation of activated extracellular signal-regulated kinases (ERK) in neurons is critical for gene regulations underlying long-term neuronal adaptation and memory formation. However, it is unknown how activated ERK travel from the post-synaptic elements where their activation occurs, to the nucleus where they translocate to exert their transcriptional roles. In cultured neurons, we identified endocytosis as a prime event in glutamate-induced nuclear trafficking of ERK2. We show that glutamate triggers a rapid recruitment of ERK2 to a protein complex comprising markers of the clathrin-dependent endocytotic and AMPA/glutamate receptor subtype. Inhibition of endocytosis results in a neuritic withholding of activated ERK2 without modification of ERK2 activity. As a consequence, endocytosis blockade alters ERK-dependent nuclear events, such as mitogen and stressed-activated kinase-1 (MSK-1) activation, histone H3 phosphorylation and gene regulations. Our data provide the first evidence that the endocytic pathway controls ERK nuclear translocation and ERK-dependent gene regulations induced by glutamate.

Chronic treatment with Delta(9)-tetrahydrocannabinol impairs spatial memory and reduces zif268 expression in the mouse forebrain.

Few studies have investigated the effects of chronic cannabinoid exposure on memory performance and whether tolerance occurs to cannabinoid-induced memory impairment. Here, we studied the effects of repeated exposure to Delta-tetrahydrocannabinol (THC: 1 mg/kg) on spatial memory and zif268 expression in mice. One group of animals was not pretreated with THC, whereas another group was injected with 13 daily injections of THC before memory testing in the Morris water maze. Both groups were administered with THC throughout the memory-testing phase of the experiment. THC decreased spatial memory and reversal learning, even in animals that received the THC pretreatment and were tolerant to the locomotor suppressant effects of the drug. Zif268 immunoreactivity was reduced in the CA3 of the hippocampus and in the prefrontal cortex only in non-pretreated animals, indicating that although tolerance to the effects of THC on neuronal activity was evident, cannabinoid-induced memory impairment in these animals persisted even after 24 days of exposure. This study shows that after extended administration of THC, its spatial memory-impairing effects are resistant to tolerance.

T3 administration in adult hypothyroid mice modulates expression of proteins involved in striatal synaptic plasticity and improves motor behavior.

Adult-onset hypothyroidism is associated with neurological changes such as cognitive dysfunction and impaired learning, which may be related to alterations of synaptic plasticity. We investigate the consequence of adult-onset hypothyroidism on thyroid-mediated transcription events in striatal synaptic plasticity, and the effect of triiodothyronine (T3) replacement. We used hypothyroid mice, treated with propylthiouracil (PTU) and methimazole (MMI), with or without subsequent administration of T3. We evaluated the amount of T3 nuclear receptors (TRalpha1, TRbeta) and striatal plasticity indicators: neurogranin (RC3), Ras homolog enriched in striatum (Rhes), Ca2+/calmodulin-dependent protein kinase (CaMKII), and dopamine- and cAMP-regulated phosphoprotein (DARPP-32). In addition, we assessed hypothyroid mice motor behavior as related to striatum synaptic functions. Hypothyroid mice exhibited significantly reduced TRbeta, RC3 and Rhes expression. T3 administration reversed the expression of TRbeta, RC3, and up-regulated CaMKII levels as well as motor behavior, and decreased DARPP-32 protein phosphorylation. We suggest that thyroid hormone modulation had a major impact on striatal synaptic plasticity of adult mice which produced in turn motor behavior modifications.

Biphasic ERK1/2 activation in both the hippocampus and amygdala may reveal a system consolidation of contextual fear memory.

There is accumulating evidences to suggest that memory consolidation in some conditions involves two waves of neuronal plastic change. Using two fear conditioning procedures in male C57BL/6J mice, we have recently shown that consolidation of the foreground contextual fear memory required two waves of ERK1/2 activation in hippocampal CA1, while consolidation of cue conditioning was only associated with the early phase of activation. The present experiment further showed that this bi-phasic pattern of ERK1/2 activation was not restricted to hippocampal CA1, but could also be observed in other fear memory-related brain areas. The unpaired conditioning procedure (context in foreground) induced two waves of ERK1/2 activation in hippocampal CA1 and CA3, as well as in the LA and BLA nuclei of the amygdala. In contrast, the paired conditioning procedure (context in background) led to a transient early phase only in hippocampal CA1 and LA. In addition, ERK1/2 phosphorylation in the hippocampus was found to correlate with that in the amygdala nuclei specifically after the unpaired procedure. Taken together, our data suggest that the observed biphasic pattern of neuronal plastic events may reflect the interplay between hippocampal and amygdala activity-dependent plasticity critical for the system consolidation of contextual fear memory.

Extracellular hippocampal acetylcholine level controls amygdala function and promotes adaptive conditioned emotional response.

Ample data indicate that tone and contextual fear conditioning differentially require the amygdala and the hippocampus. However, mechanisms subserving the adaptive selection among environmental stimuli (discrete tone vs context) of those that best predict an aversive event are still elusive. Because the hippocampal cholinergic neurotransmission is thought to play a critical role in the coordination between different memory systems leading to the selection of appropriate behavioral strategies, we hypothesized that this cholinergic signal may control the competing acquisition of amygdala-mediated tone and contextual conditioning. Using pavlovian fear conditioning in mice, we first show a higher level of hippocampal acetylcholine release and a specific pattern of extracellular signal-regulated kinase 1/2 (ERK1/2) activation within the lateral (LA) and basolateral (BLA) amygdala under conditions in which the context is a better predictor than a discrete tone stimulus. Second, we demonstrate that levels of hippocampal cholinergic neurotransmission are causally related to the patterns of ERK1/2 activation in amygdala nuclei and actually determine the selection among the context or the simple tone the stimulus that best predicts the aversive event. Specifically, decreasing the hippocampal cholinergic signal not only impaired contextual conditioning but also mimicked conditioning to the discrete tone, both in terms of the behavioral outcome and the LA/BLA ERK1/2 activation pattern. Conversely, increasing this cholinergic signal not only disrupted tone conditioning but also promoted contextual fear conditioning. Hence, these findings highlight that hippocampal cholinergic neurotransmission controls amygdala function, thereby leading to the selection of relevant emotional information.

Extinction of auditory fear conditioning requires MAPK/ERK activation in the basolateral amygdala.

Whereas the neuronal substrates underlying the acquisition of auditory fear conditioning have been widely studied, the substrates and mechanisms mediating the acquisition of fear extinction remain largely elusive. Previous reports indicate that consolidation of fear extinction depends on the mitogen-activated protein kinase/extracellular-signal regulated kinase (MAPK/ERK) signalling pathway and on protein synthesis in the medial prefrontal cortex (mPFC). Based on experiments using the fear-potentiated startle paradigm suggesting a role for neuronal plasticity in the basolateral amygdala (BLA) during fear extinction, we directly addressed whether MAPK/ERK signalling in the basolateral amygdala is necessary for the acquisition of fear extinction using conditioned freezing as a read-out. First, we investigated the regional and temporal pattern of MAPK/ERK activation in the BLA following extinction learning in C57Bl/6J mice. Our results indicate that acquisition of extinction is associated with an increase of phosphorylated MAPK/ERK in the BLA. Moreover, we found that inhibition of the MAPK/ERK signalling pathway by intrabasolateral amygdala infusion of the MEK inhibitor, U0126, completely blocks acquisition of extinction. Thus, our results indicate that the MAPK/ERK signalling pathway is required for extinction of auditory fear conditioning in the BLA, and support a role for neuronal plasticity in the BLA during the acquisition of fear extinction.

Foreground contextual fear memory consolidation requires two independent phases of hippocampal ERK/CREB activation.

Fear conditioning is a popular model for investigating physiological and cellular mechanisms of memory formation. In this paradigm, a footshock is either systematically associated to a tone (paired conditioning) or is pseudorandomly distributed (unpaired conditioning). In the former procedure, the tone/shock association is acquired, whereas in the latter procedure, the context/shock association will prevail. Animals with chronically implanted recording electrodes show enhanced amplitude of the extracellularly recorded field EPSP in CA1 pyramidal cells for up to 24 h after unpaired, but not paired, fear conditioning. This is paralleled by a differential activation of the ERK/CREB pathway in CA1, which is monophasic in paired conditioning (0-15 min post-conditioning), but biphasic (0-1 h and 9-12 h post-conditioning) in unpaired conditioning as revealed by immunocytochemistry and Western blotting. Intrahippocampal injection of the MEK inhibitor U0126 prior to each phase prevents the activation of both ERK1/2 and CREB after unpaired conditioning. Block of any activation phase leads to memory impairment. We finally reveal that the biphasic activation of ERK/CREB activity is independently regulated, yet both phases are critically required for the consolidation of long-term memories following unpaired fear conditioning. These data provide compelling evidence that CA1 serves different forms of memory by expressing differential cellular mechanisms that are dependent on the training regime.

Glutamate receptor function in learning and memory.

The contribution of glutamate to synaptic transmission, plasticity and development is well established; current evidence is based on diverse approaches to decipher function and malfunction of this principal transmitter. With respect to learning and memory, we are now able to identify more specifically the role played by the three main glutamate receptor classes in learning and memory: centre stage is clearly the NMDA receptor, with overwhelming evidence proving its involvement in the actual learning process (encoding), throughout the animal kingdom. This is discussed with respect to many different types of learning. Evidence for the contribution of the AMPA receptors (AMPARs) is less clear-cut due to the general problem of specificity: block of AMPARs will shutdown neuronal communication, and this will affect various components essential for learning. Therefore, the role of AMPARs cannot be established in isolation. Problems of interpretation are outlined and a specific involvement of AMPARs in the regulation of neuronal excitation related to learning is proposed. Metabotropic glutamate receptors (mGluRs) may contribute very little to the actual acquisition of new information. However, memory formation appears to require mGluRs, through the modulation of consolidation and/or recall. Overall, mGluR functions seem variable and dependent on brain structure and learning task.

Effect of chronic inhibition of calpains in the hippocampus on spatial discrimination learning and protein kinase C.

Several behavioral and electrophysiological studies have suggested that a sustained activation of protein kinase C would be required to underlie persistent changes associated with memory formation. Limited proteolysis of PKCs by calpains, calcium-activated proteases, cleaves the catalytic and the regulatory domains, generating a free catalytic fragment termed PKM, constitutively active. In order to investigate the potential physiological importance of this limited proteolysis as a mechanism of PKC activation, we have studied the effect of the calpastatin peptide, a specific calpain inhibitor, on the learning of a spatial discrimination task in a radial maze. Thus, using osmotic micro-pumps, the calpastatin peptide was infused bilaterally into the dorsal hippocampus during the six sessions of training and the probe test. The treatment was shown to facilitate the performance of the mice on the two last training sessions and on the probe test. This behavioral effect was shown to correspond to the reduced calpain activity observed in the hippocampus at the very end of the 7-day infusion of the calpastatin peptide, suggesting a relation between both events. In addition, PKC activity measured immediately after the probe test was notably decreased in the membrane fraction of the hippocampus. Although protein levels of PKCs and calpains quantified by western blot were not affected by calpastatin infusion, we found a noticeable correlation between mu-calpain and PKCgamma levels confirming the particular relationship between both proteins. These results suggest that calpains influence on PKCs activity may affect cellular mechanisms during memory processes.