Cellular imaging of zif268 expression in the hippocampus and amygdala during contextual and cued fear memory retrieval: selective activation of hippocampal CA1 neurons during the recall of contextual memories

The marginal division of the striatum and hippocampus has different role and mechanism in learning and memory

The memory function of the hippocampal formation (Hip) and the marginal division (MrD) of neostriatum was compared. Rats with bilateral lesions of the MrD either immediate or 24 h after training in Y-maze were found to have decrease in correct runs in both groups. However, animals with transected afferent and efferent nerve bundles to isolate the Hip immediately or 24 h after training in Y-maze were found to show a decrease in correct runs only in the group injured immediately after Y-maze training but not in the 24 h group suggesting that MrD is likely involved in the entire process of long-term memory consolidation whereas the Hip only contributes to memory in the early stage. In addition, animals treated with a NMDA receptor (NMDAR) blocker, e.g. MK-801, showed decreased correct runs in Y-maze test and in expression level of phosphorylated CREB (pCREB) in neurons of the MrD but not in the Hip. Furthermore, animals treated with okadaic acid (OA), a potent protein phosphatase 1 inhibitor, showed increased correct runs in the Y-maze test. The expression level of pCREB and c-Fos and c-Jun was found increased in neurons of the MrD and the Hip in response to OA treatment. In conclusion, NMDAR and pCREB are involved in memory functions of both the Hip and the MrD. NMDAR might regulate pCREB level in neurons of the MrD but not in the Hip. Hence, the processes and mechanism of learning and memory involved in the MrD and the Hip may be different.

Egr-1 mRNA expression patterns in the prefrontal cortex, hippocampus, and amygdala during variants of contextual fear conditioning in adolescent rats

We report activation of the immediate-early gene Egr-1 in the lateral amygdala (LA), hippocampus (CA1), and medial prefrontal cortex (mPFC) 30-min following the training phase in the context pre-exposure facilitation effect (CPFE) and standard context fear conditioning (180 s context exposure→shock). On day one of the CPFE paradigm, postnatal day (PD) 31 rats (±1) were pre-exposed to Context A (Pre) or Context B (Alt-Pre) for 5 min followed by five additional 1-min exposures. A day later, Pre and Alt-Pre rats received a 2-s, 1.5 mA footshock immediately upon placement in Context A. Animals included in in situ hybridization were then sacrificed 30 (±3) min later. On day three, the behaviorally-tested Pre rats showed significantly more fear-conditioned freezing in Context A than Alt-Pre rats. Standard context fear conditioning groups showed much greater freezing than the Pre group, as well as no shock and immediate-shock controls. Thirty minutes after immediate shock training, Pre rats showed increased Egr-1 mRNA in the prelimbic mPFC relative to Alt-Pre rats. Standard context conditioning selectively increased Egr-1 in CA1. In the LA and mPFC, Egr-1 increased to a similar extent in no shock, immediate shock, and standard context conditioning relative to homecage controls. The present study demonstrates that Egr-1 mRNA expression has a complex relationship to fear learning in different brain regions and variants of context conditioning.

Differential role of Rac in the basolateral amygdala and cornu ammonis 1 in the reconsolidation of auditory and contextual Pavlovian fear memory in rats

RATIONALE AND OBJECTIVES

A conditioned stimulus (CS) is associated with a fearful unconditioned stimulus (US) in the traditional fear conditioning model. After fear conditioning, the CS-US association memory undergoes the consolidation process to become stable. Consolidated memory enters an unstable state after retrieval and requires the reconsolidation process to stabilize again. Evidence indicates the important role of Rac (Ras-related C3 botulinum toxin substrate) in the acquisition and extinction of fear memory. In the present study, we hypothesized that Rac in the amygdala is crucial for the reconsolidation of auditory and contextual Pavlovian fear memory.

METHODS

Auditory and contextual fear conditioning and microinjections of the Rac inhibitor NSC23766 were used to explore the role of Rac in the reconsolidation of auditory and contextual Pavlovian fear memory in rats.

RESULTS

A microinjection of NSC23766 into the basolateral amygdala (BLA) but not central amygdala (CeA) or cornu ammonis 1 (CA1) immediately after memory retrieval disrupted the reconsolidation of auditory Pavlovian fear memory. A microinjection of NSC23766 into the CA1 but not BLA or CeA after memory retrieval disrupted the reconsolidation of contextual Pavlovian fear memory.

CONCLUSIONS

Our experiments demonstrate that Rac in the BLA is crucial for the reconsolidation of auditory Pavlovian fear memory, whereas Rac in the CA1 is critical for the reconsolidation of contextual Pavlovian fear memory.

Repression of contexual fear memory induced by isoflurane is accompanied by reduction in histone acetylation and rescued by sodium butyrate

BACKGROUND

Isoflurane produces amnesia in mice during contextual fear conditioning (CFC) trials. Histone acetylation is a form of chromatin modification involved in the transcriptional regulation underlying memory formation. We investigated whether isoflurane-induced repression of contextual fear memory is related to altered histone acetylation in the hippocampus, and whether it can be rescued by the histone deacetylases inhibitor sodium butyrate (SB).

METHODS

Adult C57BL/6 mice were chronically given intraperitoneal injections of SB or vehicle for 28 days. Immediately before CFC training, the mice were exposed to isoflurane or air for 30 min and CFC testing was performed the next day. Hippocampal histone acetylation was analysed 1 h after CFC training. c-Fos, an immediate early gene (IEG) suggested to participate in learning and memory formation, was also investigated at the same timepoint.

RESULTS

Mice exposed to isoflurane showed a reduction in freezing time during the CFC test. These mice also exhibited reduced hippocampal H3K14, H4K5, and H4K12 acetylation 1 h after CFC training, and also decreased c-Fos expression. All of these changes were attenuated in isoflurane-exposed mice that were chronically treated with SB.

CONCLUSIONS

Isoflurane suppresses histone acetylation and down-regulates c-Fos gene expression in CA1 of the hippocampus after CFC training. These changes are associated with isoflurane-induced amnesia. The HDAC inhibitor SB prevented repressed contextual fear memory, presumably by promoting histone acetylation and histone acetylation-mediated gene expression in response to CFC training.

Inhibition of projections from the basolateral amygdala to the entorhinal cortex disrupts the acquisition of contextual fear

The development of excessive fear and/or stress responses to environmental cues such as contexts associated with a traumatic event is a hallmark of post-traumatic stress disorder (PTSD). The basolateral amygdala (BLA) has been implicated as a key structure mediating contextual fear conditioning. In addition, the hippocampus has an integral role in the encoding and processing of contexts associated with strong, salient stimuli such as fear. Given that both the BLA and hippocampus play an important role in the regulation of contextual fear conditioning, examining the functional connectivity between these two structures may elucidate a role for this pathway in the development of PTSD. Here, we used optogenetic strategies to demonstrate that the BLA sends a strong glutamatergic projection to the hippocampal formation through the entorhinal cortex (EC). Next, we photoinhibited glutamatergic fibers from the BLA terminating in the EC during the acquisition or expression of contextual fear conditioning. In mice that received optical inhibition of the BLA-to-EC pathway during the acquisition session, we observed a significant decrease in freezing behavior in a context re-exposure session. In contrast, we observed no differences in freezing behavior in mice that were only photoinhibited during the context re-exposure session. These data demonstrate an important role for the BLA-to-EC glutamatergic pathway in the acquisition of contextual fear conditioning.

The transcription factor Zif268/Egr1, brain plasticity, and memory

The capacity to remember our past experiences and organize our future draws on a number of cognitive processes that allow our brain to form and store neural representations that can be recalled and updated at will. In the brain, these processes require mechanisms of neural plasticity in the activated circuits, brought about by cellular and molecular changes within the neurons activated during learning. At the cellular level, a wealth of experimental data accumulated in recent years provides evidence that signaling from synapses to nucleus and the rapid regulation of the expression of immediate early genes encoding inducible, regulatory transcription factors is a key step in the mechanisms underlying synaptic plasticity and the modification of neural networks required for the laying down of memories. In the activated neurons, these transcriptional events are thought to mediate the activation of selective gene programs and subsequent synthesis of proteins, leading to stable functional and structural remodeling of the activated networks, so that the memory can later be reactivated upon recall. Over the past few decades, novel insights have been gained in identifying key transcriptional regulators that can control the genomic response of synaptically activated neurons. Here, as an example of this approach, we focus on one such activity-dependent transcription factor, Zif268, known to be implicated in neuronal plasticity and memory formation. We summarize current knowledge about the regulation and function of Zif268 in different types of brain plasticity and memory processes.

Impaired extinction of learned contextual fear memory in early growth response 1 knockout mice

Inductive expression of early growth response 1 (Egr-1) in neurons is associated with many forms of neuronal activity. However, only a few Egr-1 target genes are known in the brain. The results of this study demonstrate that Egr-1 knockout (KO) mice display impaired contextual extinction learning and normal fear acquisition relative to wild-type (WT) control animals. Genome-wide microarray experiments revealed 368 differentially expressed genes in the hippocampus of Egr-1 WT exposed to different phases of a fear conditioning paradigm compared to gene expression profiles in the hippocampus of KO mice. Some of genes, such as serotonin receptor 2C (Htr2c), neuropeptide B (Npb), neuronal PAS domain protein 4 (Npas4), NPY receptor Y1 (Npy1r), fatty acid binding protein 7 (Fabp7), and neuropeptide Y (Npy) are known to regulate processing of fearful memories, and promoter analyses demonstrated that several of these genes contained Egr-1 binding sites. This study provides a useful list of potential Egr-1 target genes which may be regulated during fear memory processing.

Activity in prelimbic cortex subserves fear memory reconsolidation over time

The prelimbic cortex has been implicated in the consolidation of previously learned fear. Herein, we report that temporarily inactivating this medial prefrontal cortex subregion with the GABA_A agonist muscimol (4.0 nmol in 0.2 μL per hemisphere) was able to equally disrupt 1-, 7-, and 21-d-old contextual fear memories after their brief retrieval in rats. In all cases, this effect was prevented when memory reactivation was omitted. These results indicate that recent and remote fear memories are susceptible to reconsolidation blockade induced by prelimbic cortex inactivation. It was also demonstrated that the disrupting effect of prelimbic cortex inactivation on fear memory persisted over 11 d, and did not show extinction-related features, such as reinstatement. Infusing the same dose and volume of muscimol bilaterally into the infralimbic cortex after brief retrieval/reactivation of the fear memory did not disrupt it, as seen in prelimbic cortex-inactivated animals. The expression of Zif268/Egr1, the product of an immediate early gene related to memory reconsolidation, was also less pronounced in the infralimbic cortex than in prelimbic cortex following memory retrieval/reactivation. Altogether, the present findings highlight that activity in the prelimbic cortex may reestablish reactivated aversive memories and, therefore, contribute to their maintenance over time.

Learned together, extinguished apart: reducing fear to complex stimuli

Pairing a previously neutral conditioned stimulus (CS; e.g., a tone) to an aversive unconditioned stimulus (US; e.g., a footshock) leads to associative learning such that the tone alone comes to elicit a conditioned response (e.g., freezing). We have previously shown that an extinction session that occurs within the reconsolidation window attenuates fear responding and prevents the return of fear in pure tone Pavlovian fear conditioning. Here we sought to examine whether this effect also applies to a more complex fear memory. First, we show that after fear conditioning to the simultaneous presentation of a tone and a light (T+L) coterminating with a shock, the compound memory that ensues is more resistant to fear extinction than simple tone-shock pairings. Next, we demonstrate that the compound memory can be disrupted by interrupting the reconsolidation of the two individual components using a sequential retrieval+extinction paradigm, provided the stronger compound component is retrieved first. These findings provide insight into how compound memories are encoded, and could have important implications for PTSD treatment.

Insights into food preference in hybrid F1 of Siniperca chuatsi (♀) × Siniperca scherzeri (♂) mandarin fish through transcriptome analysis

BACKGROUND

As economically relevant traits, feeding behavior and food preference domestication determine production cost and profitability. Although there are intensive research efforts on feeding behavior and food intake, little is known about food preference. Mandarin fish accept only live prey fish and refuse dead prey fish or artificial diets. Very little is currently known about the genes regulating this unique food preference.

RESULTS

Using transcriptome sequencing and digital gene expression profiling, we identified 1,986 and 4,526 differentially expressed genes in feeders and nonfeeders of dead prey fish, respectively. Up-regulation of Crbp, Rgr and Rdh8, and down-regulation of Gc expression, consistent with greater visual ability in feeders, could promote positive phototaxis. Altered expressions of period, casein kinase and Rev-erbα might reset circadian phase. Down-regulation of orexigenic and up-regulation of anorexigenic genes in feeders were associated with lower appetite. The mRNA levels of Creb, c-fos, C/EBP, zif268, Bdnf and Syt were dramatically decreased in feeders, which might result in significant deficiency in memory retention of its natural food preference (live prey fish). There were roughly 100 times more potential SNPs in feeders than in nonfeeders.

CONCLUSIONS

In summary, differential expression in the genes identified shed new light on why mandarin fish only feed on live prey fish, with pathways regulating retinal photosensitivity, circadian rhythm, appetite control, learning and memory involved. We also found dramatic difference in SNP abundance in feeders vs nonfeeders. These differences together might account for the different food preferences. Elucidating the genes regulating the unique food preference (live prey fish) in mandarin fish could lead to a better understanding of mechanisms controlling food preference in animals, including mammals.

Egr-1 increases in the prefrontal cortex following training in the context preexposure facilitation effect (CPFE) paradigm

The context pre-exposure facilitation effect (CPFE) is a modified form of standard contextual fear conditioning that dissociates learning about the context during a preexposure phase from learning the context-shock association during an immediate shock training phase conducted on separate days. Fear conditioning in the CPFE is an associative process in which only animals that are preexposed to the same context they are later given an immediate shock in demonstrate freezing when tested for conditioned fear memory. Previous research has shown that the hippocampus and amygdala are necessary for different phases of the CPFE, but whether other brain regions are also involved is unknown. The present study examined expression of the immediate-early gene early growth response gene 1 (Egr-1; also called Zif268, Ngfi-a, Krox-24) in the dorsal hippocampus, lateral nucleus of the amygdala, retrosplenial cortex, and several prefrontal cortex regions (infralimbic and prelimbic medial prefrontal cortex, anterior cingulate, and orbitofrontal cortex) following each phase of the CPFE in juvenile rats. Animals preexposed to the conditioning context displayed fear conditioned freezing during a retention test whereas rats preexposed to an alternate context did not. Following context preexposure, Egr-1 mRNA was elevated in context and alternate context exposed animals compared to home-cage control rats in almost all regions analyzed. Following the context-shock training phase, fear conditioned rats displayed significantly more Egr-1 mRNA expression in the infralimbic, prelimbic, and orbitofrontal cortices compared to the alternate context preexposed control rats. These differences in Egr-1 expression were not found in amygdala between the preexposed context and alternate context rats. No sex differences were observed following preexposure or training in any regions analyzed. The findings suggest that increased expression of Egr-1 within the prefrontal cortex is associated with contextual fear conditioning in the CPFE paradigm.

Recall and reconsolidation of contextual fear memory: differential control by ERK and Zif268 expression dosage

Compelling evidence points to the existence of independent cellular processes involved in the consolidation and reconsolidation of memory. For instance, a double dissociation has been reported between hippocampal Extracellular-Regulated Kinase-1/2 (ERK1/2) activity being necessary for contextual fear conditioning (CFC) consolidation but not reconsolidation. Conversely, hippocampal expression of the immediate early gene Zif268 is necessary for CFC reconsolidation but not consolidation. Since we previously reported that ERK1/2 controls the transcription of Zif268 in the hippocampus, we examined the precise role of ERK1/2 activity and Zif268 gene expression dosage in CFC memory processing. For this, we first assessed performance of Zif268 homozygous and heterozygous mutant mice in a CFC paradigm. Whereas Zif268−/− mice displayed a deficit of both consolidation and reconsolidation, Zif268+/− mice displayed a selective deficit of reconsolidation only, therefore pointing to the relationship between Zif268 gene expression dosage and CFC memory processing. Zif268 gene expression dosage interfered with the reconsolidation process if and only if CFC memory was relatively recently encoded and directly reactivated. Furthermore, CFC memory strengthening previously reported to involve Zif268 expression in the hippocampus was spared in Zif268+/− mice. Finally, blocking ERK1/2 activity prior to CFC retrieval prevented the deficit of reconsolidation observed in Zif268+/− mice. Collectively, these results highlight a tight relationship between Zif268 gene expression dosage and CFC memory processing. They also suggest that ERK1/2 activity upon CFC memory recall is necessary for its retrieval, a prerequisite for its reactivation and subsequent reconsolidation.

A BDNF sensitive mechanism is involved in the fear memory resulting from the interaction between stress and the retrieval of an established trace

The present study investigates the fear memory resulting from the interaction of a stressful experience and the retrieval of an established fear memory trace. Such a combination enhanced both fear expression and fear retention in adult Wistar rats. Likewise, midazolam intra-basolateral amygdala (BLA) infusion prior to stress attenuated the enhancement of fear memory thus suggesting the involvement of a stress-induced reduction of the GABAergic transmission in BLA in the stress-induced enhancing effect. It has been suggested that, unlike the immediate-early gene Zif268 which is related to the reconsolidation process, the expression of hippocampal brain-derived neurotrophic factor (BDNF) is highly correlated with consolidation. We therefore evaluate the relative contribution of these two neurobiological processes to the fear memory resulting from the above-mentioned interaction. Intra-dorsal hippocampus (DH) infusions of either the antisense Zif268 or the inhibitor of the protein degradation (Clasto-Lactacystin β-Lactone), suggested to be involved in the retrieval-dependent destabilization process, did not affect the resulting contextual memory. In contrast, the knockdown of hippocampal BDNF mitigated the stress-induced facilitating influence on fear retention. In addition, the retrieval experience elevated BDNF level in DH at 60 min after recall exclusively in stressed animals. These findings suggest the involvement of a hippocampal BDNF sensitive mechanism in the stress-promoting influence on the fear memory following retrieval.

Remembering to attend: the anterior cingulate cortex and remote memory

Damage to the hippocampus, as first demonstrated with patient HM, results in a profound anterograde and temporally-graded retrograde amnesia. The observation that older memories could still be consciously recollected led to the proposal that, over time, information initially processed in the hippocampus is stored in a distributed cortical network. The anterior cingulate cortex (ACC) has recently been implicated in this process. Studies in rodents have demonstrated that the ACC is necessary for recalling behaviors learned a month or more in the past, but not for the same behaviors learned the previous day. Precisely how the ACC contributes to the recall of remote memories is unknown. Is this role distinct from myriad others proposed for the ACC, or has the approach taken in these studies of assessing function at different points after learning provided a new window through which to view established processes? The present review seeks to address this question. First, the data will be presented implicating the ACC in recall of remote memory. This will be followed by a discussion of studies describing two other primary roles of the ACC, mediating attention and premotor planning, with an emphasis on data collected in rodents, as these will be most directly comparable to the memory studies presented. The available evidence supports a connection among these roles, and suggests a possible synthesis for otherwise seemingly disparate functions reported for the ACC.

Reactivation of neural ensembles during the retrieval of recent and remote memory

BACKGROUND

Episodic memories are encoded within hippocampal and neocortical circuits. Retrieving these memories is assumed to involve reactivation of neural ensembles that were established during learning. Although it has been possible to follow the activity of individual neurons shortly after learning, it has not been possible to examine their activity weeks later during retrieval. We addressed this issue by using a stable form of GFP (H2B-GFP) to permanently tag neurons that are active during contextual fear conditioning.

RESULTS

H2B-GFP expression in transgenic mice was increased by learning and could be regulated by doxycycline (DOX). Using this system, we found a large network of neurons in the hippocampus, amygdala, and neocortex that were active during context fear conditioning and subsequent memory retrieval 2 days later. Reactivation was contingent on memory retrieval and was not observed when animals were trained and tested in different environments. When memory was retrieved several weeks after learning, reactivation was altered in the hippocampus and amygdala but remained unchanged in the cortex.

CONCLUSIONS

Retrieving a recently formed context fear memory reactivates neurons in the hippocampus, amygdala, and cortex. Several weeks after learning, the degree of reactivation is altered in hippocampal and amygdala networks but remains stable in the cortex.

Dynamic histone marks in the hippocampus and cortex facilitate memory consolidation

Memory consolidation requires a timely controlled interplay between the hippocampus, a brain region important for memory formation, and the cortex, a region recruited for memory storage. Here we show that memory consolidation is associated with specific epigenetic modifications on histone proteins that have a distinct dynamic in these brain areas. While in the hippocampus, histone post-translational modifications (PTMs) are rapidly and transiently activated after learning, in the cortex they are induced with delay but persist over time. When these histone PTMs are increased in vivo by transgenic intervention or intense training, they facilitate memory consolidation. Conversely, when they are pharmacologically blocked, memory consolidation is impaired. These histone PTMs are further associated with the expression of the immediate early gene zif268, a transcription factor that favours memory consolidation. These findings reveal the spatiotemporal dynamics of histone marks during memory consolidation, and demonstrate their inherent 'mnemonic' property.

Artificial theta stimulation impairs encoding of contextual fear memory

Several experiments have demonstrated an intimate relationship between hippocampal theta rhythm (4-12 Hz) and memory. Lesioning the medial septum or fimbria-fornix, a fiber track connecting the hippocampus and the medial septum, abolishes the theta rhythm and results in a severe impairment in declarative memory. To assess whether there is a causal relationship between hippocampal theta and memory formation we investigated whether restoration of hippocampal theta by electrical stimulation during the encoding phase also restores fimbria-fornix lesion induced memory deficit in rats in the fear conditioning paradigm. Male Wistar rats underwent sham or fimbria-fornix lesion operation. Stimulation electrodes were implanted in the ventral hippocampal commissure and recording electrodes in the septal hippocampus. Artificial theta stimulation of 8 Hz was delivered during 3-min free exploration of the test cage in half of the rats before aversive conditioning with three foot shocks during 2 min. Memory was assessed by total freezing time in the same environment 24 h and 28 h after fear conditioning, and in an intervening test session in a different context. As expected, fimbria-fornix lesion impaired fear memory and dramatically attenuated hippocampal theta power. Artificial theta stimulation produced continuous theta oscillations that were almost similar to endogenous theta rhythm in amplitude and frequency. However, contrary to our predictions, artificial theta stimulation impaired conditioned fear response in both sham and fimbria-fornix lesioned animals. These data suggest that restoration of theta oscillation per se is not sufficient to support memory encoding after fimbria-fornix lesion and that universal theta oscillation in the hippocampus with a fixed frequency may actually impair memory.

Altered expression of synaptotagmin 13 mRNA in adult mouse brain after contextual fear conditioning

Contextual fear memory processing requires coordinated changes in neuronal activity and molecular networks within brain. A large number of fear memory-related genes, however, still remain to be identified. Synaptotagmin 13 (Syt13), an atypical member of synaptotagmin family, is highly expressed in brain, but its functional roles within brain have not yet been clarified. Here, we report that the expression of Syt13 mRNA in adult mouse brain was altered following contextual fear conditioning. C57BL/6 mice were exposed to a novel context and stimulated by strong electrical footshock according to a contextual fear conditioning protocol. After 24 h, the mice were re-exposed to the context without electrical footshock for the retrieval of contextual fear memory. To investigate the relationship between Syt13 and contextual fear memory, we carried out in situ hybridization and analyzed gene expression patterns for Syt13 at four groups representing temporal changes in brain activity during contextual fear memory formation. Contextual fear conditioning test induced significant changes in mRNA levels for Syt13 within various brain regions, including lateral amygdala, somatosensory cortex, piriform cortex, habenula, thalamus, and hypothalamus, during both acquisition and retrieval sessions. Our data suggest that Syt13 may be involved in the process of contextual fear memory.

A role for the insular cortex in long-term memory for context-evoked drug craving in rats

Drug craving critically depends on the function of the interoceptive insular cortex, and may be triggered by contextual cues. However, the role of the insula in the long-term memory linking context with drug craving remains unknown. Such a memory trace probably resides in some neocortical region, much like other declarative memories. Studies in humans and rats suggest that the insula may include such a region. Rats chronically implanted with bilateral injection cannulae into the high-order rostral agranular insular cortex (RAIC) or the primary interoceptive posterior insula (pIC) were conditioned to prefer the initially aversive compartment of a 2-compartment place preference apparatus by repeatedly pairing it to amphetamine. We found a reversible but long-lasting loss (ca. 24 days) of amphetamine-conditioned place preference (CPP) and a decreased expression in the insula of zif268, a crucial protein in memory reconsolidation, when anisomycin (ANI) was microinjected into the RAIC immediately after the reactivation of the conditioned amphetamine/context memory. ANI infusion into the RAIC without reactivation did not change CPP, whereas ANI infusion into pIC plus caused a 15 days loss of CPP. We also found a 24 days loss of CPP when we reversibly inactivated pIC during extinction trials. We interpret these findings as evidence that the insular cortex, including the RAIC, is involved in a context/drug effect association. These results add a drug-related memory function to the insular cortex to the previously found role of the pIC in the perception of craving or malaise.

Neural and cellular mechanisms of fear and extinction memory formation

Over the course of natural history, countless animal species have evolved adaptive behavioral systems to cope with dangerous situations and promote survival. Emotional memories are central to these defense systems because they are rapidly acquired and prepare organisms for future threat. Unfortunately, the persistence and intrusion of memories of fearful experiences are quite common and can lead to pathogenic conditions, such as anxiety and phobias. Over the course of the last 30 years, neuroscientists and psychologists alike have attempted to understand the mechanisms by which the brain encodes and maintains these aversive memories. Of equal interest, though, is the neurobiology of extinction memory formation as this may shape current therapeutic techniques. Here we review the extant literature on the neurobiology of fear and extinction memory formation, with a strong focus on the cellular and molecular mechanisms underlying these processes.

Egr-1 antisense oligodeoxynucleotide administration into the olfactory bulb impairs olfactory learning in the greater short-nosed fruit bat Cynopterus sphinx

Postsynaptic densities (PSDs) contain proteins that regulate synaptic transmission. We examined two important examples of these, calcium/calmodulin-dependent protein kinase II (CaMKII) and PSD-95, in regard to the functional role of early growth response gene-1 (egr-1) in regulation of olfactory learning in the greater short-nosed fruit bat Cynopterus sphinx (family Pteropodidae). To test whether activation of egr-1 in the olfactory bulb (OB) is required for olfactory memory of these bats, bilaterally canulated individuals were infused with antisense (AS) or non-sense (NS)-oligodeoxynucleotides (ODN) of egr-1, or with phosphate buffer saline (PBS), 2h before the olfactory training. Our results showed that behavioral training significantly up-regulates immediate early gene (IEG) EGR-1 and key synaptic proteins Synaptotagmin-1(SYT-1), CaMKII and PSD-95, and phosphorylation of CaMKII in the OB at the protein level per se. Subsequently, we observed that egr-1 antisense-ODN infusion in the OB impaired olfactory memory and down regulates the expression of CaMKII and PSD-95, and the phosphorylation of CaMKII but not SYT-1. In contrast, NS-ODN or PBS had no effect on the expression of the PSDs CaMKII or PSD-95, or on the phosphorylation of CaMKII. When the egr-1 NS-ODN was infused in the OB after training for the novel odor there was no effect on olfactory memory. These findings suggest that egr-1 control the activation of CaMKII and PSD-95 during the process of olfactory memory formation.

The timing of multiple retrieval events can alter GluR1 phosphorylation and the requirement for protein synthesis in fear memory reconsolidation

Numerous studies have indicated that maintaining a fear memory after retrieval requires de novo protein synthesis. However, no study to date has examined how the temporal dynamics of repeated retrieval events affect this protein synthesis requirement. The present study varied the timing of a second retrieval of an established auditory fear memory and followed this second retrieval with infusions of the protein synthesis inhibitor anisomycin (ANI) into the basolateral amygdala. Results indicated that the memory-impairing effects of ANI were not observed when the second retrieval occurred soon after the first (within 1 h), and that the inhibitor gradually regained effectiveness as the retrieval episodes were spaced further apart. Additionally, if the second of the closely timed retrievals was omitted prior to ANI infusions, long-term memory deficits were observed, suggesting that the altered effectiveness of ANI was due specifically to the second retrieval event. Further experiments revealed that the second retrieval was not associated with a change in Zif268 protein expression but did produce a rapid and persistent dephosphorylation of GluR1 receptors at Ser845, an AMPAR trafficking site known to regulate the stability of GluR2 lacking AMPARs, which have been shown to be important in memory updating. This suggests that the precise timing of multiple CS presentations during the reconsolidation window may affect the destabilization state of the memory trace.

Revisiting the hippocampal-amygdala pathway in primates: association with immature-appearing neurons

Elucidation of the 'fear circuit' has opened exciting avenues for understanding and treating human anxiety disorders. However, the translation of rodent to human studies, and vice versa, depends on understanding the homology in relevant circuits across species. Although abundant evidence indicates that the hippocampal-amygdala circuit mediates contextual fear learning, previous studies indicate that this pathway is more restricted in primates than in rodents. Moreover, cellular components of the amygdala differ across species. The paralaminar nucleus (PL) of the amygdala, a structure that is closely associated with the basal nucleus, is one example, having no clear homologue in rodents. In both human and nonhuman primates, the PL contains a subpopulation of immature-appearing neurons, which merge into the corticoamygdaloid transition area (CTA). To understand whether immature-appearing neurons are positioned to participate in fear circuitry, we first mapped the hippocampal-amygdala projection in the monkey. We then determined whether immature-appearing neurons were targets of this path. Retrograde results show that the hippocampal inputs to the amygdala originate in uncal region (CA1') and the rostral prosubiculum, consistent with earlier studies. The amygdalohippocampal area, ventral basal nucleus, the medial paralaminar nucleus, and its confluence with the CTA are the main targets of this projection. Immature neurons are prominent in the PL and CTA, and are overlapped by anterogradely labeled fibers from CA1', particularly in the medial PL and CTA. Hippocampal inputs to the amygdala are more focused in higher primates compared to rodents, supporting previous anatomic studies and recent data from human functional imaging studies of contextual fear. At the cellular level, a hippocampal interaction with immature neurons in the amygdala suggests a novel substrate for cellular plasticity, with implications for mechanisms underlying contextual learning and emotional memory processes.

Social isolation stress down-regulates cortical early growth response 1 (Egr-1) expression in mice

Social isolation stress induces behavioral disturbances such as aggression, cognitive impairments, and deficits in prepulse inhibition in mice. Social isolation mice have, therefore, been studied as an animal model of neuropsychiatric disorders such as schizophrenia. Recently, the decrease in early growth response (Egr) gene expression levels were reported in the post-mortem brains of schizophrenia patients. In this study, we investigate the effects of social isolation stress on the expression levels of Egr mRNA and protein in the frontal cortex. Social isolation stress exposure significantly down-regulated the expression of Egr-1 protein and Egr-1 gene transcript in nucleus of cortical neurons in a manner dependent on a social isolation period. This stress had no effect on the expression level of Egr-1 in the striatum or the expression levels of other Egr family members (Egr-2, -3, and -4) in the frontal cortex. These results suggest that the decrease in Egr-1 expression in the frontal cortex may be involved in social isolation stress-induced behavioral abnormalities.

Hippocampal extracellular signal-regulated kinase signaling has a role in passive avoidance memory retrieval induced by GABAA Receptor modulation in mice

Available evidence strongly suggests that the γ-aminobutyric acid type A (GABA(A)) receptor has a crucial role in memory retrieval. However, the signaling mechanisms underlying the role of GABA(A) receptor modulation in memory retrieval are unclear. We conducted one-trial passive avoidance task with pre-retention trial drug administration in the hippocampus to test the effects of GABA(A) receptor modulation on memory retrieval. We further tested the co-involvement of signaling molecules: extracellular signal-regulated kinase (ERK), Ca(2+)/calmodulin-dependent protein kinase II (CaMKII), and cAMP responsive element-binding protein (CREB). First, we observed that the phosphorylation of hippocampal ERK was required for memory retrieval during the task. Accordingly, to investigate whether GABA(A) receptor activation or inhibition induces ERK phosphorylation during memory retrieval, drugs that target the GABA(A) receptor were administered into the hippocampus before the retention trial. Muscimol, a GABA(A) receptor agonist, and diazepam, an agonist to benzodiazepine-binding site of GABA(A) receptor, blocked retention trial-induced ERK phosphorylation and impaired memory retrieval. Furthermore, co-treatment with sub-effective dose of U0126, a mitogen-activated protein kinase inhibitor, blocked the upregulation of ERK phosphorylation and impaired memory retrieval, and bicuculline methiodide (BMI), a GABA(A) receptor antagonist, increased ERK phosphorylation induced by the retention trial and facilitated memory retrieval. Finally, the effects of BMI were blocked by the co-application of a sub-effective dose of U0126. These results suggest that GABA(A) receptor-mediated memory retrieval is closely related to ERK activity.

The persistence of maladaptive memory: addiction, drug memories and anti-relapse treatments

Addiction is a chronic, relapsing disorder, characterised by the long-term propensity of addicted individuals to relapse. A major factor that obstructs the attainment of abstinence is the persistence of maladaptive drug-associated memories, which can maintain drug-seeking and taking behaviour and promote unconscious relapse of these habits. Thus, addiction can be conceptualised as a disorder of aberrant learning of the formation of strong instrumental memories linking actions to drug-seeking and taking outcomes that ultimately are expressed as persistent stimulus-response habits; of previously neutral environmental stimuli that become associated with drug highs (and/or withdrawal states) through pavlovian conditioning, and of the subsequent interactions between pavlovian and instrumental memories to influence relapse behaviour. Understanding the psychological, neurobiological and molecular basis of these drug memories may produce new methods of pro-abstinence, anti-relapse treatments for addiction.

Peripheral bacterial endotoxin administration triggers both memory consolidation and reconsolidation deficits in mice

Peripherally administered inflammatory stimuli, such as lipopolysaccharide (LPS), induce the synthesis and release of proinflammatory cytokines and chemokines in the periphery and the central nervous system, and trigger a variety of neurobiological responses. Indeed, prior reports indicate that peripheral LPS administration in rats disrupts contextual fear memory consolidation processes, potentially due to elevated cytokine expression. We used a similar, but partially olfaction-based, contextual fear conditioning paradigm to examine the effects of LPS on memory consolidation and reconsolidation in mice. Additionally, interleukin-1β (IL-1β), brain-derived neurotrophic factor (BDNF), and zinc finger (Zif)-268 mRNA expression in the hippocampus and the cortex, along with peripheral cytokines and chemokines, were assessed. As hypothesized, LPS administered immediately or 2 h, but not 12 h, post-training impaired memory consolidation processes that support the storage of the conditioned contextual fear memory. Additionally, as hypothesized, LPS administered immediately following the fear memory trace reactivation session impaired memory reconsolidation processes. Four hours post-injection, both central cytokine and peripheral cytokine and chemokine levels were heightened in LPS-treated animals, with a simultaneous decrease in BDNF, but not Zif-268, mRNA. Collectively, these data reinforce prior work showing LPS- and cytokine-related effects on memory consolidation, and extend this work to memory reconsolidation.

Molecular mechanisms of fear learning and memory

Pavlovian fear conditioning is a particularly useful behavioral paradigm for exploring the molecular mechanisms of learning and memory because a well-defined response to a specific environmental stimulus is produced through associative learning processes. Synaptic plasticity in the lateral nucleus of the amygdala (LA) underlies this form of associative learning. Here, we summarize the molecular mechanisms that contribute to this synaptic plasticity in the context of auditory fear conditioning, the form of fear conditioning best understood at the molecular level. We discuss the neurotransmitter systems and signaling cascades that contribute to three phases of auditory fear conditioning: acquisition, consolidation, and reconsolidation. These studies suggest that multiple intracellular signaling pathways, including those triggered by activation of Hebbian processes and neuromodulatory receptors, interact to produce neural plasticity in the LA and behavioral fear conditioning. Collectively, this body of research illustrates the power of fear conditioning as a model system for characterizing the mechanisms of learning and memory in mammals and potentially for understanding fear-related disorders, such as PTSD and phobias.

Early postnatal stress alters extracellular signal-regulated kinase signaling in the corticolimbic system modulating emotional circuitry in adult rats

The present study elucidated whether early life stress alters the extracellular signal-regulated kinase (ERK) pathway that underlies fear retrieval and fear extinction based on a contextual fear conditioning paradigm, using a juvenile stress model. Levels of phospho-ERK (pERK), the active form of ERK, increased after fear retrieval in the hippocampal CA1 region but not in the medial prefrontal cortex (mPFC). ERK activation in the CA1 following fear retrieval was not observed in adult rats who received aversive footshock (FS) stimuli during the second postnatal period (2wFS), which exhibited low levels of freezing. In fear extinction, pERK levels in the CA1 were increased by repeated extinction trials, but they were not altered after extinction retrieval. In contrast, pERK levels in the mPFC did not change during extinction training, but were enhanced after extinction retrieval. These findings were compatible in part with electrophysiological data showing that synaptic transmission in the CA1 field and mPFC was enhanced during extinction training and extinction retrieval, respectively. ERK activation in the CA1 and mPFC associated with extinction processes did not occur in rats that received FS stimuli during the third postnatal period (3wFS), which exhibited sustained freezing behavior. The repressed ERK signaling and extinction deficit observed in the 3wFS group were ameliorated by treatment with the partial N-methyl-D-aspartate receptor agonist D-cycloserine. These findings suggest that early postnatal stress induced the downregulation of ERK signaling in distinct brain regions through region-specific regulation, which may lead to increased behavioral abnormalities or emotional vulnerabilities in adulthood.

The neurobiology of fear memory reconsolidation and psychoanalytic theory

Advances in both experimental neuroscience and psychoanalytic theory and technique have made it possible to consider mechanisms by which psychodynamic psychotherapies might have an impact at the cellular and molecular level. Here potential analogies are drawn between (1) the mechanisms and results of blocking the reconsolidation of conditioned fear memories in the laboratory and (2) several key aspects of psychoanalytic process. A review of the biology of conditioned fear memory, including differences between extinction and inhibition of reconsolidation, indicates that this biology may have relevance to various ways in which psychoanalytic therapy is effective. The ideas proposed here might lead to further experimental attempts to understand the molecular biology of psychoanalysis.

Juvenile and adult rats differ in cocaine reward and expression of zif268 in the forebrain

Adolescents are more likely to experiment with and become addicted to drugs of abuse. A number of studies indicate that the developmental forebrain may be responsible for making adolescents vulnerable to the addictive properties of such drugs. The aim of this study was to first compare behavioral responses to novelty and cocaine between juvenile and adult rats and then compare levels of the immediate-early gene zif268 activation in several forebrain areas via in situ hybridization. We found that juveniles demonstrated higher locomotion scores and required a higher dose of cocaine than adults to establish a conditioned place preference. Additionally, at this higher dose, juvenile rats exhibited higher levels of zif268 mRNA in the prefrontal cortex compared with adults. A developmental effect for increased zif268 mRNA was also observed in the striatum and nucleus accumbens, but there was no interaction with the cocaine dose. These findings hold interesting implications for the study of the molecular mechanisms underlying juvenile drug addiction.

Song exposure regulates known and novel microRNAs in the zebra finch auditory forebrain

BACKGROUND

In an important model for neuroscience, songbirds learn to discriminate songs they hear during tape-recorded playbacks, as demonstrated by song-specific habituation of both behavioral and neurogenomic responses in the auditory forebrain. We hypothesized that microRNAs (miRNAs or miRs) may participate in the changing pattern of gene expression induced by song exposure. To test this, we used massively parallel Illumina sequencing to analyse small RNAs from auditory forebrain of adult zebra finches exposed to tape-recorded birdsong or silence.

RESULTS

In the auditory forebrain, we identified 121 known miRNAs conserved in other vertebrates. We also identified 34 novel miRNAs that do not align to human or chicken genomes. Five conserved miRNAs showed significant and consistent changes in copy number after song exposure across three biological replications of the song-silence comparison, with two increasing (tgu-miR-25, tgu-miR-192) and three decreasing (tgu-miR-92, tgu-miR-124, tgu-miR-129-5p). We also detected a locus on the Z sex chromosome that produces three different novel miRNAs, with supporting evidence from Northern blot and TaqMan qPCR assays for differential expression in males and females and in response to song playbacks. One of these, tgu-miR-2954-3p, is predicted (by TargetScan) to regulate eight song-responsive mRNAs that all have functions in cellular proliferation and neuronal differentiation.

CONCLUSIONS

The experience of hearing another bird singing alters the profile of miRNAs in the auditory forebrain of zebra finches. The response involves both known conserved miRNAs and novel miRNAs described so far only in the zebra finch, including a novel sex-linked, song-responsive miRNA. These results indicate that miRNAs are likely to contribute to the unique behavioural biology of learned song communication in songbirds.

Consolidation and reconsolidation of contextual fear memory requires mammalian target of rapamycin-dependent translation in the dorsal hippocampus

The mammalian target of rapamycin (mTOR) pathway is important for regulating protein translation. The present study characterized the role of mTOR-dependent translation in the dorsal hippocampus (DH) during the consolidation and reconsolidation of contextual fear memory. We first showed that fear conditioning resulted in increased phosphorylation of p70s6 kinase (p70s6K) in the DH and that infusion of the mTOR inhibitor rapamycin (RAP) into the DH immediately after training disrupted formation of long-term contextual fear memory. Additionally we showed that p70s6K was activated after retrieval of a previously stored fear memory, and inhibition of mTOR by DH infusion of RAP blocked the reconsolidation of contextual fear memory. Together these results demonstrate that within the DH translational control through the mTOR pathway is important for consolidation as well as the stability of fear memory after retrieval.

Distinctive features of Egr transcription factor regulation and DNA binding activity in CA1 of the hippocampus in synaptic plasticity and consolidation and reconsolidation of fear memory

Activity-dependent regulation of Egr1/Zif268, a transcription factor (TF) of the Egr family, is essential for stabilization of dentate gyrus synaptic plasticity and consolidation and reconsolidation of several forms of memory. The gene can be rapidly induced in selective brain circuits after certain types of learning or after recall. Here, we focused on area CA1 and examined regulation of Egr1, Egr2, and Egr3 mRNA and protein, and their DNA binding activity to the Egr response element (ERE) at different times after LTP in vivo and after learning and recall of a fear memory. We found LTP in CA1 leads to rapid induction of the three Egrs, however only Egr1 protein was overexpressed without a co-ordinated change in binding activity, indicating a fundamental difference between CA1 and dentate gyrus LTP. Our investigations in fear memory reveal that both learning and retrieval lead to an increase in binding of constitutively expressed Egr1 and Egr3 to the ERE, but not Egr2. Memory recall was also associated with increased Egr1 protein translation. The nature and temporal dynamics of these changes and tests for interactions between TFs suggest that in addition to ERE-mediated transcription, Egr1 in CA1 may interact with the TF c-Fos to regulate genes via other DNA response elements.

Norepinephrine and ß₁-adrenergic signaling facilitate activation of hippocampal CA1 pyramidal neurons during contextual memory retrieval

We previously described a role for adrenergic signaling in the hippocampus to promote contextual and spatial memory retrieval. A subsequent study performing expression analysis of the immediate-early gene (IEG) Arc suggested that activation of CA1 but not CA3 pyramidal neurons during memory retrieval is impaired in the absence of NE. The current study sought to confirm and extend those observations by performing expression analysis of a second IEG product, Fos, following a much greater variety of testing conditions. In mutant mice lacking NE, induction of Fos was normal in all regions of the hippocampus and amygdala shortly after fear conditioning. In contrast, when testing contextual fear 1 day after training, induction of Fos in CA1 and the central nucleus of the amygdala (CeA), but not CA3, the dentate gyrus or other amygdaloid nuclei, was impaired in the mutant mice. This pattern corresponded to the memory retrieval deficit exhibited by these mice. On the other hand, induction was normal in CA1 and CeA when testing cued fear 1 day after training, or contextual fear 1 week or 1 month after training, conditions in which retrieval are normal in the absence of NE. Acute restoration of NE in the mutant mice before testing but not before training rescued retrieval of contextual fear and restored Fos induction in CA1 and CeA. Because NE facilitates retrieval through the activation of β(1)-adrenergic receptors, β(1) knockout mice were also examined and found to exhibit reduced induction of Fos in CA1 and CeA following retrieval. Based on these and previous results, we hypothesize that adrenergic signaling is critical for the full activation of CA1 pyramidal neurons in response to excitatory input from CA3 pyramidal neurons conveying retrieved contextual information.

Spatial learning-induced egr-1 expression in telencephalon of gold fish Carassius auratus

The immediate-early gene (egr-1) expression was used to examine the neuron's response in telencephalon of goldfish during spatial learning in small space. Fishes were pre-exposed in the experimental apparatus and trained to pick food from the tray in a rectangular-shaped arena. The apparatus was divided into identical compartments comprising three gates to provide different spatial tasks. After the fish learned to pass through the gate one, two more gates were introduced one by one. Fish made more number of attempts and took longer time (P < 0.05) to pass through the first gate than the gate two or three. This active learning induces the expression of egr-1 in telencephalon as established by western blot analysis. Subsequently, the fish learn quickly to cross the similar type of second and third gate and make fewer errors with a corresponding decline in the level of egr-1 expression. As the fish learned to pass through all the three gates, third gate was replaced by modified gate three. Interestingly, the level of egr-1 expression increased again, when the fish exhibit a high exploratory behavior to cross the modified gate three. The present study shows that egr-1 expression is induced in the telencephalon of goldfish while intensively acquiring geometric spatial information to pass through the gates.

Fatty-acid binding proteins modulate sleep and enhance long-term memory consolidation in Drosophila

Sleep is thought to be important for memory consolidation, since sleep deprivation has been shown to interfere with memory processing. However, the effects of augmenting sleep on memory formation are not well known, and testing the role of sleep in memory enhancement has been limited to pharmacological and behavioral approaches. Here we test the effect of overexpressing the brain-type fatty acid binding protein (Fabp7) on sleep and long-term memory (LTM) formation in Drosophila melanogaster. Transgenic flies carrying the murine Fabp7 or the Drosophila homologue dFabp had reduced baseline sleep but normal LTM, while Fabp induction produced increases in both net sleep and LTM. We also define a post-training consolidation “window” that is sufficient for the observed Fabp-mediated memory enhancement. Since Fabp overexpression increases consolidated daytime sleep bouts, these data support a role for longer naps in improving memory and provide a novel role for lipid-binding proteins in regulating memory consolidation concurrently with changes in behavioral state.

Differential involvement of protein synthesis and actin rearrangement in the reacquisition of contextual fear conditioning

Extinction learning is associated with a decline of the conditioned fear response (CR). However, re-exposure to the unconditioned stimulus (US, shock) is associated with the return of the fear response. This study aimed to study the role of protein synthesis and actin rearrangement in the CA1 hippocampal subregion and the basolateral amygdala (BLA) in acquisition and reacquisition of contextual fear conditioning. To that end, we trained rats on contextual fear conditioning and extinction, and on the last extinction training session we reconditioned the animals by re-exposure to the US. Immediately after, rats were microinfused with the protein synthesis inhibitor anisomycin or the actin rearrangement inhibitor cytochalasin D into either the BLA or the CA1. The results of this study show differential involvement of anisomycin and cytochalasin D in the acquisition and reacquisition of contextual fear conditioning. Specifically, while the microinfusion of anisomycin into the BLA or the CA1 immediately after reconditioning of fear did not inhibit the return of fear, the microinfusion of cytochalsin D into either the BLA or the CA1 attenuated fear responses. Interestingly, the initial acquisition of contextual fear memory is dependent on intra-BLA and CA1 protein synthesis and cytoskeletal rearrangement, since the microinfusion of these drugs blocked the formation of long-term fear memory. The results suggest that the two processes of acquisition and reacquisition of fear are not identical and they engage different mechanisms.

Early growth response gene 1 (Egr-1) is required for new and reactivated fear memories in the lateral amygdala

The immediate-early gene early growth response gene-1 (EGR-1, zif-268) has been extensively studied in synaptic plasticity and memory formation in a variety of memory systems. However, a convincing role for EGR-1 in amygdala-dependent memory consolidation processes has yet to emerge. In the present study, we have examined the role of EGR-1 in the consolidation and reconsolidation of amygdala-dependent auditory Pavlovian fear conditioning. In our first series of experiments, we show that EGR-1 is regulated following auditory fear conditioning in the lateral nucleus of the amygdala (LA). Next, we use antisense oligodeoxynucleotide (ODN) knockdown of EGR-1 in the LA to show that training-induced expression of EGR-1 is required for memory consolidation of auditory fear conditioning; that is, long-term memory (LTM) is significantly impaired while acquisition and short-term memory (STM) are intact. In a second set of experiments, we show that EGR-1 is regulated in the LA by retrieval of an auditory fear memory. We then show that retrieval-induced expression of EGR-1 in the LA is required for memory reconsolidation of auditory fear conditioning; that is, post-retrieval (PR)-LTM is significantly impaired while memory retrieval and PR-STM are intact. Additional experiments show these effects to be restricted to the LA, to be temporally graded, and unlikely to be due to nonspecific toxicity within the LA. Collectively, our findings strongly implicate a role for EGR-1 in both the initial consolidation and in the reconsolidation of auditory fear memories in the LA.

The exclusive induction of extinction is gated by BDNF

We have previously reported that the reconsolidation and extinction of hippocampal-dependent contextual fear memory can be initiated by a single context conditioned stimulus (CS) presentation of either short or long duration, and that both processes require protein synthesis in this brain region. Furthermore, reconsolidation depends on Zif268 activity in this region. Here we show that by infusing a recombinant brain-derived neurotrophic factor (rBDNF) directly into the brain of rats, that high levels of mature BDNF in the hippocampus at retrieval constrain the extinction of the fear memory after prolonged memory recall. We also show after a short CS exposure that reconsolidation was impaired using antisense oligonucleotides targeting Zif268, and that, similarly, reductions in conditioned behavior were observed after prolonged CS presentation when extinction is constrained by high levels of BDNF. This is direct evidence that in the mammalian brain extinction proceeds exclusively after prolonged CS exposure. In addition, that BDNF activity in the hippocampus contributes to a molecular switch for the extinction of hippocampal-dependent memory.

The effects of atorvastatin on memory deficit and seizure susceptibility in pentylentetrazole-kindled rats

Deficits in memory function have been observed in pentylentetrazole (PTZ)-kindled rats. In the present study we examined the effects of atorvastatin ((3-hydroxy-3-methylglutaryl-coenzyme A [HMG-CoA] reductase inhibitor) on PTZ kindling and related memory deficits in rats trained with the passive avoidance test. Subconvulsive PTZ doses rendered a gradual increase in seizure activity. PTZ kindling caused long-term memory to deteriorate. Atorvastatin per se and in PTZ-kindled rats improved learning and memory functions. It also prolonged latency (time to appearance of spike potentials) and diminished the amplitude and frequency of spike potentials, which indicate epileptic discharges. These novel findings suggest that the favorable effect of the atorvastatin on memory deficits provoked by PTZ kindling might be of clinical utility.

Amygdalar roles during exposure to a live predator and to a predator-associated context

The amygdala plays a critical role in determining the emotional significance of sensory stimuli and the production of fear-related responses. Large amygdalar lesions have been shown to practically abolish innate defensiveness to a predator; however, it is not clear how the different amygdalar systems participate in the defensive response to a live predator. Our first aim was to provide a comprehensive analysis of the amygdalar activation pattern during exposure to a live cat and to a predator-associated context. Accordingly, exposure to a live predator up-regulated Fos expression in the medial amygdalar nucleus (MEA) and in the lateral and posterior basomedial nuclei, the former responding to predator-related pheromonal information and the latter two nuclei likely to integrate a wider array of predatory sensory information, ranging from olfactory to non-olfactory ones, such as visual and auditory sensory inputs. Next, we tested how the amygdalar nuclei most responsive to predator exposure (i.e. the medial, posterior basomedial and lateral amygdalar nuclei) and the central amygdalar nucleus (CEA) influence both unconditioned and contextual conditioned anti-predatory defensive behavior. Medial amygdalar nucleus lesions practically abolished defensive responses during cat exposure, whereas lesions of the posterior basomedial or lateral amygdalar nuclei reduced freezing and increased risk assessment displays (i.e. crouch sniff and stretch postures), a pattern of responses compatible with decreased defensiveness to predator stimuli. Moreover, the present findings suggest a role for the posterior basomedial and lateral amygdalar nuclei in the conditioning responses to a predator-related context. We have further shown that the CEA does not seem to be involved in either unconditioned or contextual conditioned anti-predatory responses. Overall, the present results help to clarify the amygdalar systems involved in processing predator-related sensory stimuli and how they influence the expression of unconditioned and contextual conditioned anti-predatory responses.