The labile nature of consolidation theory

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.

Updating appetitive memory during reconsolidation window: critical role of cue-directed behavior and amygdala central nucleus

When presented with a light cue followed by food, some rats simply approach the foodcup (Nonorienters), while others first orient to the light in addition to displaying the food-cup approach behavior (Orienters). Cue-directed orienting may reflect enhanced attentional and/or emotional processing of the cue, suggesting divergent natures of cue-information processing in Orienters and Nonorienters. The current studies investigate how differences in cue processing might manifest in appetitive memory retrieval and updating using a paradigm developed to persistently attenuate fear responses (Retrieval-extinction paradigm; Monfils et al., 2009). First, we examined whether the retrieval-extinction paradigm could attenuate appetitive responses in Orienters and Nonorienters. Next, we investigated if the appetitive memory could be updated using reversal learning (fear conditioning) during the reconsolidation window (as opposed to repeated unreinforced trials, i.e., extinction). Both extinction and new fear learning given within the reconsolidation window were effective at persistently updating the initial appetitive memory in the Orienters, but not the Nonorienters. Since conditioned orienting is mediated by the amygdala central nucleus (CeA), our final experiment examined the CeA's role in the retrieval-extinction process. Bilateral CeA lesions interfered with the retrieval-extinction paradigm-did not prevent spontaneous recovery of food-cup approach. Together, our studies demonstrate the critical role of conditioned orienting behavior and the CeA in updating appetitive memory during the reconsolidation window.

New technologies for examining the role of neuronal ensembles in drug addiction and fear

Correlational data suggest that learned associations are encoded within neuronal ensembles. However, it has been difficult to prove that neuronal ensembles mediate learned behaviours because traditional pharmacological and lesion methods, and even newer cell type-specific methods, affect both activated and non-activated neurons. In addition, previous studies on synaptic and molecular alterations induced by learning did not distinguish between behaviourally activated and non-activated neurons. Here, we describe three new approaches--Daun02 inactivation, FACS sorting of activated neurons and Fos-GFP transgenic rats--that have been used to selectively target and study activated neuronal ensembles in models of conditioned drug effects and relapse. We also describe two new tools--Fos-tTA transgenic mice and inactivation of CREB-overexpressing neurons--that have been used to study the role of neuronal ensembles in conditioned fear.

The reinstatement model of drug relapse: recent neurobiological findings, emerging research topics, and translational research

BACKGROUND AND RATIONALE

Results from many clinical studies suggest that drug relapse and craving are often provoked by acute exposure to the self-administered drug or related drugs, drug-associated cues or contexts, or certain stressors. During the last two decades, this clinical scenario has been studied in laboratory animals by using the reinstatement model. In this model, reinstatement of drug seeking by drug priming, drug cues or contexts, or certain stressors is assessed following drug self-administration training and subsequent extinction of the drug-reinforced responding.

OBJECTIVE

In this review, we first summarize recent (2009-present) neurobiological findings from studies using the reinstatement model. We then discuss emerging research topics, including the impact of interfering with putative reconsolidation processes on cue- and context-induced reinstatement of drug seeking, and similarities and differences in mechanisms of reinstatement across drug classes. We conclude by discussing results from recent human studies that were inspired by results from rat studies using the reinstatement model.

CONCLUSIONS

Main conclusions from the studies reviewed highlight: (1) the ventral subiculum and lateral hypothalamus as emerging brain areas important for reinstatement of drug seeking, (2) the existence of differences in brain mechanisms controlling reinstatement of drug seeking across drug classes, (3) the utility of the reinstatement model for assessing the effect of reconsolidation-related manipulations on cue-induced drug seeking, and (4) the encouraging pharmacological concordance between results from rat studies using the reinstatement model and human laboratory studies on cue- and stress-induced drug craving.

Memory reconsolidation in aversive and appetitive settings

Memory reconsolidation has been observed across species and in a number of behavioral paradigms. The majority of memory reconsolidation studies have been carried out in Pavlovian fear conditioning and other aversive memory settings, with potential implications for the treatment of post-traumatic stress disorder. However, there is a growing literature on memory reconsolidation in appetitive reward-related memory paradigms, including translational models of drug addiction. While there appears to be substantial similarity in the basic phenomenon and underlying mechanisms of memory reconsolidation across unconditioned stimulus valence, there are also notable discrepancies. These arise both when comparing aversive to appetitive paradigms and also across different paradigms within the same valence of memory. We review the demonstration of memory reconsolidation across different aversive and appetitive memory paradigms, the commonalities and differences in underlying mechanisms and the conditions under which each memory undergoes reconsolidation. We focus particularly on whether principles derived from the aversive literature are applicable to appetitive settings, and also whether the expanding literature in appetitive paradigms is informative for fear memory reconsolidation.

Modulatory mechanisms of cortisol effects on emotional learning and memory: novel perspectives

It has long been known that cortisol affects learning and memory processes. Despite a wealth of research dedicated to cortisol effects on learning and memory, the strength or even directionality of the effects often vary. A number of the factors that alter cortisol's effects on learning and memory are well-known. For instance, effects of cortisol can be modulated by emotional arousal and the memory phase under study. Despite great advances in understanding factors that explain variability in cortisol's effects, additional modulators of cortisol effects on memory exist that are less widely acknowledged in current basic experimental research. The goal of the current review is to disseminate knowledge regarding less well-known modulators of cortisol effects on learning and memory. Since several models for the etiology of anxiety, such as post-traumatic stress disorder (PTSD), incorporate stress and the concomitant release of cortisol as important vulnerability factors, enhanced understanding of mechanisms by which cortisol exerts beneficial as opposed to detrimental effects on memory is very important. Further elucidation of the factors that modulate (or alter) cortisol's effects on memory will allow reconciliation of seemingly inconsistent findings in the basic and clinical literatures. The present review is based on a symposium as part of the 42nd International Society of Psychoneuroendocrinology Conference, New York, USA, that highlighted some of those modulators and their underlying mechanisms.

Competitive Trace Theory: A Role for the Hippocampus in Contextual Interference during Retrieval

Much controversy exists regarding the role of the hippocampus in retrieval. The two dominant and competing accounts have been the Standard Model of Systems Consolidation (SMSC) and Multiple Trace Theory (MTT), which specifically make opposing predictions as to the necessity of the hippocampus for retrieval of remote memories. Under SMSC, memories eventually become independent of the hippocampus as they become more reliant on cortical connectivity, and thus the hippocampus is not required for retrieval of remote memories, only recent ones. MTT on the other hand claims that the hippocampus is always required no matter the age of the memory. We argue that this dissociation may be too simplistic, and a continuum model may be better suited to address the role of the hippocampus in retrieval of remote memories. Such a model is presented here with the main function of the hippocampus during retrieval being "recontextualization," or the reconstruction of memory using overlapping traces. As memories get older, they are decontextualized due to competition among partially overlapping traces and become more semantic and reliant on neocortical storage. In this framework dubbed the Competitive Trace Theory (CTT), consolidation events that lead to the strengthening of memories enhance conceptual knowledge (semantic memory) at the expense of contextual details (episodic memory). As a result, remote memories are more likely to have a stronger semantic representation. At the same time, remote memories are also more likely to include illusory details. The CTT is a novel candidate model that may provide some resolution to the memory consolidation debate.

Genome-wide analysis of H4K5 acetylation associated with fear memory in mice

BACKGROUND

Histone acetylation has been implicated in learning and memory in the brain, however, its function at the level of the genome and at individual genetic loci remains poorly investigated. This study examines a key acetylation mark, histone H4 lysine 5 acetylation (H4K5ac), genome-wide and its role in activity-dependent gene transcription in the adult mouse hippocampus following contextual fear conditioning.

RESULTS

Using ChIP-Seq, we identified 23,235 genes in which H4K5ac correlates with absolute gene expression in the hippocampus. However, in the absence of transcription factor binding sites 150 bp upstream of the transcription start site, genes were associated with higher H4K5ac and expression levels. We further establish H4K5ac as a ubiquitous modification across the genome. Approximately one-third of all genes have above average H4K5ac, of which ~15% are specific to memory formation and ~65% are co-acetylated for H4K12. Although H4K5ac is prevalent across the genome, enrichment of H4K5ac at specific regions in the promoter and coding region are associated with different levels of gene expression. Additionally, unbiased peak calling for genes differentially acetylated for H4K5ac identified 114 unique genes specific to fear memory, over half of which have not previously been associated with memory processes.

CONCLUSIONS

Our data provide novel insights into potential mechanisms of gene priming and bookmarking by histone acetylation following hippocampal memory activation. Specifically, we propose that hyperacetylation of H4K5 may prime genes for rapid expression following activity. More broadly, this study strengthens the importance of histone posttranslational modifications for the differential regulation of transcriptional programs in cognitive processes.

Initiation, labile, and stabilization phases of experience-dependent plasticity at neocortical synapses

Alteration of sensory input can change the strength of neocortical synapses. Selective activation of a subset of whiskers is sufficient to potentiate layer 4-layer 2/3 excitatory synapses in the mouse somatosensory (barrel) cortex, a process that is NMDAR dependent. By analyzing the time course of sensory-induced synaptic change, we have identified three distinct phases for synaptic strengthening in vivo. After an early, NMDAR-dependent phase where selective whisker activation is rapidly translated into increased synaptic strength, we identify a second phase where this potentiation is profoundly reduced by an input-specific, NMDAR-dependent depression. This labile phase is transient, lasting only a few hours, and may require ongoing sensory input for synaptic weakening. Residual synaptic strength is maintained in a third phase, the stabilization phase, which requires mGluR5 signaling. Identification of these three phases will facilitate a molecular dissection of the pathways that regulate synaptic lability and stabilization, and suggest potential approaches to modulate learning.

Recognition memory and synaptic plasticity in the perirhinal and prefrontal cortices

Work is reviewed that relates recognition memory to studies of synaptic plasticity mechanisms in perirhinal and prefrontal cortices. The aim is to consider evidence that perirhinal cortex and medial prefrontal cortex store rather than merely transmit information necessary for recognition memory and, if so, to consider what mechanisms are potentially available within these cortices for producing such storage through synaptic change. Interventions with known actions on plasticity mechanisms are reviewed in relation to their effects on recognition memory processes. These interventions importantly include those involving antagonism of glutamatergic and cholinergic receptors but also inhibition of plasticity consolidation and expression mechanisms. It is concluded that there is strong evidence that perirhinal cortex is involved in information storage necessary for object recognition memory and, moreover, that such storage involves synaptic weakening mechanisms including the removal of AMPA glutamate receptors from synapses. There is good evidence that medial prefrontal cortex is necessary for associative and temporal order recognition memory and that this cortex expresses plasticity mechanisms that potentially allow the storage of information. However, the case for medial prefrontal cortex acting as a store requires further support.

The neural basis of flashback formation: the impact of viewing trauma

BACKGROUND

Psychological traumatic events, such as war or road traffic accidents, are widespread. A small but significant proportion of survivors develop post-traumatic stress disorder (PTSD). Distressing, sensory-based involuntary memories of trauma (henceforth 'flashbacks') are the hallmark symptom of PTSD. Understanding the development of flashbacks may aid their prevention. This work is the first to combine the trauma film paradigm (as an experimental analogue for flashback development) with neuroimaging to investigate the neural basis of flashback aetiology. We investigated the hypothesis that involuntary recall of trauma (flashback) is determined during the original event encoding. Method A total of 22 healthy volunteers viewed a traumatic film whilst undergoing functional magnetic resonance imaging (fMRI). They kept a 1-week diary to record flashbacks to specific film scenes. Using a novel prospective fMRI design, we compared brain activation for those film scenes that subsequently induced flashbacks with both non-traumatic control scenes and scenes with traumatic content that did not elicit flashbacks ('potentials').

RESULTS

Encoding of scenes that later caused flashbacks was associated with widespread increases in activation, including in the amygdala, striatum, rostral anterior cingulate cortex, thalamus and ventral occipital cortex. The left inferior frontal gyrus and bilateral middle temporal gyrus also exhibited increased activation but only relative to 'potentials'. Thus, these latter regions appeared to distinguish between traumatic content that subsequently flashed back and comparable content that did not.

CONCLUSIONS

Results provide the first prospective evidence that the brain behaves differently whilst experiencing emotional events that will subsequently become involuntary memories - flashbacks. Understanding the neural basis of analogue flashback memory formation may aid the development of treatment interventions for this PTSD feature.

Vestibular benefits to task savings in motor adaptation

In everyday life, we seamlessly adapt our movements and consolidate them to multiple behavioral contexts. This natural flexibility seems to be contingent on the presence of movement-related sensorimotor cues and cannot be reproduced when static visual or haptic cues are given to signify different behavioral contexts. So far, only sensorimotor cues that dissociate the sensorimotor plans prior to force field exposure have been successful in learning two opposing perturbations. Here we show that vestibular cues, which are only available during the perturbation, improve the formation and recall of multiple control strategies. We exposed subjects to inertial forces by accelerating them laterally on a vestibular platform. The coupling between reaching movement (forward-backward) and acceleration direction (leftward-rightward) switched every 160 trials, resulting in two opposite force environments. When exposed for a second time to the same environment, with the opposite environment in between, subjects showed retention resulting in an ∼3 times faster adaptation rate compared with the first exposure. Our results suggest that vestibular cues provide contextual information throughout the reach, which is used to facilitate independent learning and recall of multiple motor memories. Vestibular cues provide feedback about the underlying cause of reach errors, thereby disambiguating the various task environments and reducing interference of motor memories.

Metabotropic glutamate receptor I (mGluR1) antagonism impairs cocaine-induced conditioned place preference via inhibition of protein synthesis

Antagonism of group I metabotropic glutamate receptors (mGluR1 and mGluR5) reduces behavioral effects of drugs of abuse, including cocaine. However, the underlying mechanisms remain poorly understood. Activation of mGluR5 increases protein synthesis at synapses. Although mGluR5-induced excessive protein synthesis has been implicated in the pathology of fragile X syndrome, it remains unknown whether group I mGluR-mediated protein synthesis is involved in any behavioral effects of drugs of abuse. We report that group I mGluR agonist DHPG induced more pronounced initial depression of inhibitory postsynaptic currents (IPSCs) followed by modest long-term depression (I-LTD) in dopamine neurons of rat ventral tegmental area (VTA) through the activation of mGluR1. The early component of DHPG-induced depression of IPSCs was mediated by the cannabinoid CB1 receptors, while DHPG-induced I-LTD was dependent on protein synthesis. Western blotting analysis indicates that mGluR1 was coupled to extracellular signal-regulated kinase (ERK) and mammalian target of rapamycin (mTOR) signaling pathways to increase translation. We also show that cocaine conditioning activated translation machinery in the VTA via an mGluR1-dependent mechanism. Furthermore, intra-VTA microinjections of mGluR1 antagonist JNJ16259685 and protein synthesis inhibitor cycloheximide significantly attenuated or blocked the acquisition of cocaine-induced conditioned place preference (CPP) and activation of translation elongation factors. Taken together, these results suggest that mGluR1 antagonism inhibits de novo protein synthesis; this effect may block the formation of cocaine-cue associations and thus provide a mechanism for the reduction in CPP to cocaine.

Extracellular signal-regulated kinase in the basolateral amygdala, but not the nucleus accumbens core, is critical for context-response-cocaine memory reconsolidation in rats

The reconsolidation of cocaine memories following retrieval is necessary for the sustained ability of a cocaine-paired environmental context to elicit cocaine seeking. Extracellular signal-regulated kinase (ERK) is an intracellular signaling molecule involved in nucleus accumbens core (NACc)-mediated reconsolidation of Pavlovian cocaine memories. Here, we used a rodent model of drug context-elicited relapse to test the hypothesis that ERK would be similarly required for the reconsolidation of context-response-cocaine memories that underlie drug context-induced reinstatement of instrumental cocaine-seeking behavior, with a focus on the NACc and on the basolateral amygdala (BLA), another important locus for the reconsolidation of cocaine memories. We show that the mitogen-activated protein kinase (MEK)/ERK1/2 inhibitor, U0126 (1.0 μg/0.5 μl/hemisphere), microinfused bilaterally into the BLA--but not the NACc--immediately after brief re-exposure to a previously cocaine-paired context (that is, cocaine-memory reactivation), significantly attenuated subsequent drug context-induced cocaine seeking relative to vehicle (VEH). This effect in the BLA was associated with a transient inhibition of ERK1/2 phosphorylation, and it depended on memory reactivation given that U0126 administered following exposure to a novel context did not alter subsequent cocaine seeking. Furthermore, similar to U0126, baclofen+muscimol-induced (B+M; 106.8/5.7 ng/0.5 μl/hemisphere) neural inactivation of the NACc, following cocaine-memory reactivation, failed to alter subsequent cocaine seeking. These findings demonstrate that ERK activation in the BLA, but not the NACc, is required for the reconsolidation of context-response-cocaine associative memories. Together with prior research, these results suggest that contextual drug-memory reconsolidation in Pavlovian and instrumental settings involves distinct neuroanatomical mechanisms.

Over-expectation generated in a complex appetitive goal-tracking task is capable of inducing memory reconsolidation

RATIONALE

Discrepancies in an expected outcome have been demonstrated to result in modification of behaviour in both appetitive and aversive conditioning settings.

OBJECTIVES

In this study, we sought to establish whether overexpectation generated from compound conditioning with two previously rewarded stimuli was able to induce memory destabilisation and subsequent reconsolidation in a Pavlovian conditioned approach setting.

RESULTS

It was shown that 4 days, but not 1 day, of overexpectation training was required to induce memory reconsolidation, and this was disrupted by application of the NMDA subtype of glutamate receptor antagonist MK-801 prior to overexpectation training, but not by MK-801 application 6 h post-training.

CONCLUSIONS

These data provide evidence that the memories underlying Pavlovian conditioned approach do undergo reconsolidation and that such reconsolidation can be triggered by overexpectation. Therefore, the updating of appetitive conditioned stimulus and unconditioned stimulus associations underpinning conditioned responding in manners other than extinction training is likely achieved through memory reconsolidation.

About sleep's role in memory

Over more than a century of research has established the fact that sleep benefits the retention of memory. In this review we aim to comprehensively cover the field of "sleep and memory" research by providing a historical perspective on concepts and a discussion of more recent key findings. Whereas initial theories posed a passive role for sleep enhancing memories by protecting them from interfering stimuli, current theories highlight an active role for sleep in which memories undergo a process of system consolidation during sleep. Whereas older research concentrated on the role of rapid-eye-movement (REM) sleep, recent work has revealed the importance of slow-wave sleep (SWS) for memory consolidation and also enlightened some of the underlying electrophysiological, neurochemical, and genetic mechanisms, as well as developmental aspects in these processes. Specifically, newer findings characterize sleep as a brain state optimizing memory consolidation, in opposition to the waking brain being optimized for encoding of memories. Consolidation originates from reactivation of recently encoded neuronal memory representations, which occur during SWS and transform respective representations for integration into long-term memory. Ensuing REM sleep may stabilize transformed memories. While elaborated with respect to hippocampus-dependent memories, the concept of an active redistribution of memory representations from networks serving as temporary store into long-term stores might hold also for non-hippocampus-dependent memory, and even for nonneuronal, i.e., immunological memories, giving rise to the idea that the offline consolidation of memory during sleep represents a principle of long-term memory formation established in quite different physiological systems.

Stress modulation of reconsolidation

Memories are consolidated and are inscribed as stable traces in the brain; however, once they are retrieved, they are rendered labile and can be modified in a process termed reconsolidation. Studies illustrate the power of behavioral stress and stress hormones to modulate memory processes while focusing on consolidation. However, sparse evidence indicates a critical role of stress in modulating reconsolidation. In this review, we discuss the effects of stress and stress-related neurotransmitter systems on reconsolidation of emotional and non-emotional types of memories. We show that although some general features underlie consolidation and reconsolidation, there is a possible dissimilarity between the two processes that may be dependent on factors such as the cognitive task employed, specific type of stressor, and the arousal state of the animal. The ability to disrupt or facilitate the reconsolidation of emotional and drug-related memories by stress exposure has important implications for the treatment of anxiety disorders linked to traumatic memories, such as post-traumatic stress disorder and of drug-of-abuse memories.

Synaptic plasticity by antidromic firing during hippocampal network oscillations

Learning and other cognitive tasks require integrating new experiences into context. In contrast to sensory-evoked synaptic plasticity, comparatively little is known of how synaptic plasticity may be regulated by intrinsic activity in the brain, much of which can involve nonclassical modes of neuronal firing and integration. Coherent high-frequency oscillations of electrical activity in CA1 hippocampal neurons [sharp-wave ripple complexes (SPW-Rs)] functionally couple neurons into transient ensembles. These oscillations occur during slow-wave sleep or at rest. Neurons that participate in SPW-Rs are distinguished from adjacent nonparticipating neurons by firing action potentials that are initiated ectopically in the distal region of axons and propagate antidromically to the cell body. This activity is facilitated by GABA(A)-mediated depolarization of axons and electrotonic coupling. The possible effects of antidromic firing on synaptic strength are unknown. We find that facilitation of spontaneous SPW-Rs in hippocampal slices by increasing gap-junction coupling or by GABA(A)-mediated axon depolarization resulted in a reduction of synaptic strength, and electrical stimulation of axons evoked a widespread, long-lasting synaptic depression. Unlike other forms of synaptic plasticity, this synaptic depression is not dependent upon synaptic input or glutamate receptor activation, but rather requires L-type calcium channel activation and functional gap junctions. Synaptic stimulation delivered after antidromic firing, which was otherwise too weak to induce synaptic potentiation, triggered a long-lasting increase in synaptic strength. Rescaling synaptic weights in subsets of neurons firing antidromically during SPW-Rs might contribute to memory consolidation by sharpening specificity of subsequent synaptic input and promoting incorporation of novel information.

Augmentation of exposure therapy with post-session administration of D-cycloserine

BACKGROUND

Pre-session administration of d-cycloserine (DCS) has been found to augment exposure therapy outcomes in a variety of anxiety disorders. To be able to enhance learning only for successful exposure sessions, it would be beneficial to have the option of administering DCS after rather than before the session, a strategy encouraged by pre-clinical work. We believe the present study is the first published report on the efficacy of post-session administration of DCS in humans.

METHOD

Adults (N = 29) with a DSM-IV diagnosis of acrophobia were randomized to receive two sessions of virtual reality exposure therapy (VRE) in combination with placebo or 50 mg of DCS. Instead of administering the pill prior to each of the sessions, as has been done in extant work, we administered the pill immediately following each session. Measures of acrophobia severity were collected at baseline, at each treatment session, 1-week post-treatment, and at 1-month follow-up.

RESULTS

Mixed-effects repeated-measures ANOVAs and GLMMs revealed significant improvement in all outcome measures over time, but no between-group differences were observed. At post-treatment, 63.5% of patients in the placebo condition vs. 60.0% of those in the DCS condition were in remission. At 1-month follow up, 63.4% of those in the placebo condition vs. 66.6% of those in the DCS condition were in remission.

CONCLUSIONS

These findings do not support the application of post-session DCS administration for augmenting the efficacy of exposure-based treatments. Possible reasons for these findings are discussed.

TRIAL REGISTRY

The Trial is registered at ClinicalTrials.gov (NCT01102803).

The anaesthetic combination of ketamine/midazolam does not alter the acquisition of spatial and motor tasks in adult mice

The ketamine/midazolam association of a dissociative with a sedative agent is used for the induction and maintenance of anaesthesia in laboratory animals. Anaesthesia may interfere with research results through side-effects on the nervous system, such as memory impairment. It is known that ketamine and midazolam affect cognition; however, their effects have not been clarified when used in a context of balanced anaesthesia. Thus, this study evaluated the effects of ketamine/midazolam on the acquisition of motor and of a spatial memory task in adult mice. Twenty-eight C57BL/6 adult male mice were divided into three groups: untreated control, treated with ketamine/midazolam (75 mg/kg / 10 mg/kg) and treated with midazolam (10 mg/kg) groups. Respiratory rate, heart rate and systolic pressure were measured every 5 min in the animals treated with ketamine/midazolam, as this was the only group that exhibited loss of the righting reflex. One day after treatment, animals were tested in the open field, rotarod and radial arm maze. There were no differences between treatments regarding open-field activity, rotarod performance or number of working and reference memory errors in the radial arm maze task. In conclusion, the learning process of spatial and motor tasks was not disrupted by the anaesthetic combination of ketamine/midazolam. These results suggest its safe use in adult mice in projects where acquisition of a spatial and motor task is necessary.

Self-awareness, self-regulation, and self-transcendence (S-ART): a framework for understanding the neurobiological mechanisms of mindfulness

Mindfulness-as a state, trait, process, type of meditation, and intervention has proven to be beneficial across a diverse group of psychological disorders as well as for general stress reduction. Yet, there remains a lack of clarity in the operationalization of this construct, and underlying mechanisms. Here, we provide an integrative theoretical framework and systems-based neurobiological model that explains the mechanisms by which mindfulness reduces biases related to self-processing and creates a sustainable healthy mind. Mindfulness is described through systematic mental training that develops meta-awareness (self-awareness), an ability to effectively modulate one's behavior (self-regulation), and a positive relationship between self and other that transcends self-focused needs and increases prosocial characteristics (self-transcendence). This framework of self-awareness, -regulation, and -transcendence (S-ART) illustrates a method for becoming aware of the conditions that cause (and remove) distortions or biases. The development of S-ART through meditation is proposed to modulate self-specifying and narrative self-networks through an integrative fronto-parietal control network. Relevant perceptual, cognitive, emotional, and behavioral neuropsychological processes are highlighted as supporting mechanisms for S-ART, including intention and motivation, attention regulation, emotion regulation, extinction and reconsolidation, prosociality, non-attachment, and decentering. The S-ART framework and neurobiological model is based on our growing understanding of the mechanisms for neurocognition, empirical literature, and through dismantling the specific meditation practices thought to cultivate mindfulness. The proposed framework will inform future research in the contemplative sciences and target specific areas for development in the treatment of psychological disorders.

Self-awareness, self-regulation, and self-transcendence (S-ART): a framework for understanding the neurobiological mechanisms of mindfulness

Mindfulness—as a state, trait, process, type of meditation, and intervention has proven to be beneficial across a diverse group of psychological disorders as well as for general stress reduction. Yet, there remains a lack of clarity in the operationalization of this construct, and underlying mechanisms. Here, we provide an integrative theoretical framework and systems-based neurobiological model that explains the mechanisms by which mindfulness reduces biases related to self-processing and creates a sustainable healthy mind. Mindfulness is described through systematic mental training that develops meta-awareness (self-awareness), an ability to effectively modulate one's behavior (self-regulation), and a positive relationship between self and other that transcends self-focused needs and increases prosocial characteristics (self-transcendence). This framework of self-awareness, -regulation, and -transcendence (S-ART) illustrates a method for becoming aware of the conditions that cause (and remove) distortions or biases. The development of S-ART through meditation is proposed to modulate self-specifying and narrative self-networks through an integrative fronto-parietal control network. Relevant perceptual, cognitive, emotional, and behavioral neuropsychological processes are highlighted as supporting mechanisms for S-ART, including intention and motivation, attention regulation, emotion regulation, extinction and reconsolidation, prosociality, non-attachment, and decentering. The S-ART framework and neurobiological model is based on our growing understanding of the mechanisms for neurocognition, empirical literature, and through dismantling the specific meditation practices thought to cultivate mindfulness. The proposed framework will inform future research in the contemplative sciences and target specific areas for development in the treatment of psychological disorders.

Memory maintenance by PKMζ--an evolutionary perspective

Long-term memory is believed to be maintained by persistent modifications of synaptic transmission within the neural circuits that mediate behavior. Thus, long-term potentiation (LTP) is widely studied as a potential physiological basis for the persistent enhancement of synaptic strength that might sustain memory. Whereas the molecular mechanisms that initially induce LTP have been extensively characterized, the mechanisms that persistently maintain the potentiation have not. Recently, however, a candidate molecular mechanism linking the maintenance of LTP and the storage of long-term memory has been identified. The persistent activity of the autonomously active, atypical protein kinase C (aPKC) isoform, PKMζ, is both necessary and sufficient for maintaining LTP. Furthermore, blocking PKMζ activity by pharmacological or dominant negative inhibitors disrupts previously stored long-term memories in a variety of neural circuits, including spatial and trace memories in the hippocampus, aversive memories in the basolateral amygdala, appetitive memories in the nucleus accumbens, habit memory in the dorsal lateral striatum, and elementary associations, extinction, and skilled sensorimotor memories in the neocortex. During LTP and memory formation, PKMζ is synthesized de novo as a constitutively active kinase. This molecular mechanism for memory storage is evolutionarily conserved. PKMζ formation through new protein synthesis likely originated in early vertebrates ~500 million years ago during the Cambrian period. Other mechanisms for forming persistently active PKM from aPKC are found in invertebrates, and inhibiting this atypical PKM disrupts long-term memory in the invertebrate model systems Drosophila melanogaster and Aplysia californica. Conversely, overexpressing PKMζ enhances memory in flies and rodents. PKMζ persistently enhances synaptic strength by maintaining increased numbers of AMPA receptors at postsynaptic sites, a mechanism that might have evolved from the general function of aPKC in trafficking membrane proteins to the apical compartment of polarized cells. This mechanism of memory may have had adaptive advantages because it is both stable and reversible, as demonstrated by the downregulation of experience-dependent, long-term increases in PKMζ after extinction and reconsolidation blockade that attenuate learned behavior. Thus, PKMζ, the “working end” of LTP, is a component of an evolutionarily conserved molecular mechanism for the persistent, yet flexible storage of long-term memory.

Off-line Arc transcription in active ensembles during fear memory retrieval

Neural activity and de novo protein synthesis during a rest period following memory retrieval in the amygdala is necessary for stabilization of reactivated fear memory. Arc/Arg3.1 (Arc) expression is regulated by neural activity and is a critical protein for memory reconsolidation. However, it remains unclear whether memory retrieval alters Arc transcription during subsequent rest. In this study, the populations of mouse lateral amygdala neurons that transcribe Arc during memory retrieval and at rest were detected using Arc cellular compartment analysis of temporal activity by fluorescence in situ hybridization (Arc catFISH). Results demonstrated that memory retrieval alters the composition of neuronal populations, which activate Arc transcription during subsequent rest. Approximately 50% of neurons that transcribe Arc at subsequent rest, transcribed Arc during memory retrieval, whereas only approximately 10% of neurons that transcribed Arc during a rest period prior to memory retrieval transcribe Arc during memory retrieval. In contrast, re-exposure to the chamber induced less preferential Arc transcription in latent inhibited mice that received shocks but recalled less conditioned fear. Taken together, these findings indicate that neuronal subpopulations activated during fear memory retrieval preferentially transcribe Arc during subsequent rest in the lateral amygdala. This preferential Arc transcription may contribute to memory reconsolidation.

Reconsolidation of long-term memory in Aplysia

When an animal is reminded of a prior experience and shortly afterward treated with a protein synthesis inhibitor, the consolidated memory for the experience can be disrupted; by contrast, protein synthesis inhibition without prior reminding commonly does not disrupt long-term memory [1-3]. Such results imply that the reminding triggers reconsolidation of the memory. Here, we asked whether the behavioral and synaptic changes associated with the memory for long-term sensitization (LTS) of the siphon-withdrawal reflex in the marine snail Aplysia californica [4, 5] could undergo reconsolidation. In support of this idea, we found that when sensitized animals were given abbreviated reminder sensitization training 48-96 hr after the original sensitization training, followed by treatment with the protein synthesis inhibitor anisomycin, LTS was disrupted. We also found that long-term (≥ 24 hr) facilitation (LTF) [6], which can be induced in the monosynaptic connection between Aplysia sensory and motor neurons in dissociated cell culture by multiple spaced pulses of the endogenous facilitatory transmitter serotonin (5-HT) [7, 8], could be eliminated by treating the synapses with one reminder pulse of 5-HT, followed by anisomycin, at 48 hr after the original training. Our results provide a simple model system for understanding the synaptic basis of reconsolidation.

What pharmacological interventions indicate concerning the role of the perirhinal cortex in recognition memory

Findings of pharmacological studies that have investigated the involvement of specific regions of the brain in recognition memory are reviewed. The particular emphasis of the review concerns what such studies indicate concerning the role of the perirhinal cortex in recognition memory. Most of the studies involve rats and most have investigated recognition memory for objects. Pharmacological studies provide a large body of evidence supporting the essential role of the perirhinal cortex in the acquisition, consolidation and retrieval of object recognition memory. Such studies provide increasingly detailed evidence concerning both the neurotransmitter systems and the underlying intracellular mechanisms involved in recognition memory processes. They have provided evidence in support of synaptic weakening as a major synaptic plastic process within perirhinal cortex underlying object recognition memory. They have also supplied confirmatory evidence that that there is more than one synaptic plastic process involved. The demonstrated necessity to long-term recognition memory of intracellular signalling mechanisms related to synaptic modification within perirhinal cortex establishes a central role for the region in the information storage underlying such memory. Perirhinal cortex is thereby established as an information storage site rather than solely a processing station. Pharmacological studies have also supplied new evidence concerning the detailed roles of other regions, including the hippocampus and the medial prefrontal cortex in different types of recognition memory tasks that include a spatial or temporal component. In so doing, they have also further defined the contribution of perirhinal cortex to such tasks. To date it appears that the contribution of perirhinal cortex to associative and temporal order memory reflects that in simple object recognition memory, namely that perirhinal cortex provides information concerning objects and their prior occurrence (novelty/familiarity).

Enhanced extinction of aversive memories by high-frequency stimulation of the rat infralimbic cortex

Electrical stimulation of the rodent medial prefrontal cortex (mPFC), including the infralimbic cortex (IL), immediately prior to or during fear extinction training facilitates extinction memory. Here we examined the effects of high-frequency stimulation (HFS) of the rat IL either prior to conditioning or following retrieval of the conditioned memory, on extinction of Pavlovian fear and conditioned taste aversion (CTA). IL-HFS applied immediately after fear memory retrieval, but not three hours after retrieval or prior to conditioning, subsequently reduced freezing during fear extinction. Similarly, IL-HFS given immediately, but not three hours after, retrieval of a CTA memory reduced aversion during extinction. These data indicate that HFS of the IL may be an effective method for reducing both learned fear and learned aversion.

Reinstatement of extinguished fear by an unextinguished conditional stimulus

Anxiety disorders are often treated using extinction-based exposure therapy, but relapse is common and can occur as a result of reinstatement, whereby an aversive "trigger" can reinstate extinguished fear. Animal models of reinstatement commonly utilize a Pavlovian fear conditioning procedure, in which subjects are first trained to fear a conditional stimulus (CS) by pairing it with an aversive unconditional stimulus (US), and then extinguished by repeated presentations of the CS alone. Reinstatement is typically induced by exposing subjects to an aversive US after extinction, but here we show that exposure to a non-extinguished CS can reinstate conditional fear responding to an extinguished CS, a phenomenon we refer to as "conditional reinstatement" (CRI). Rats were trained to fear two CSs (light and tone) and subsequently underwent extinction training to only one CS (counterbalanced). Presenting the unextinguished CS (but not a novel cue) immediately after extinction reinstated conditional fear responding to the extinguished CS in a test session given 24 h later. These findings indicate that reinstatement of extinguished fear can be triggered by exposure to conditional as well as unconditional aversive stimuli, and this may help to explain why relapse is common following clinical extinction therapy in humans. Further study of CRI using animal models may prove useful for developing refined extinction therapies that are more resistant to reinstatement.

Reconsolidation of drug memories

Persistent, unwanted memories are believed to be key contributors to drug addiction and the chronic relapse problem over the lifetime of the addict. Contrary to the long-held idea that memories are static and fixed, new studies in the last decade have shown that memories are dynamic and changeable. However, they are changeable only under specific conditions. When a memory is retrieved (reactivated), it becomes labile for a period of minutes to hours and then is reconsolidated to maintain long-term memory. Recent findings indicate that even well-established long-term memories may be susceptible to disruption by interfering with reconsolidation through delivery of certain amnestic agents during memory retrieval. Here I review the growing literature on memory reconsolidation in animal models of addiction, including sensitization, conditioned place preference and self-administration. I also discuss (a) several issues that need to be considered in interpreting the findings from reconsolidation studies and (b) future challenges and directions for memory reconsolidation studies in the field of addiction. The findings indicate promise for using this approach as a therapy for disrupting the long-lasting memories that can trigger relapse.

Contextual fear memories formed in the absence of the dorsal hippocampus decay across time

Mammals suffering damage to the hippocampus display a dramatic loss of explicit, recently formed memories (retrograde amnesia). In contrast, deficits in the ability to form new memories following hippocampal damage (anterograde amnesia) can be overcome with sufficient training. By combining contextual fear conditioning with lesions of the dorsal hippocampus in rats, we discovered that while animals can form long-term contextual fear memories in the absence of the hippocampus, these memories decay with time, lacking the permanence that is a hallmark characteristic of normal fear memories. These findings indicate that while it is initially possible to acquire explicit memories when the hippocampus is compromised, these memories cannot transfer from a recent to remote state. This suggests that memories formed outside the hippocampus may nevertheless require the hippocampus to undergo systems consolidation, which has important clinical implications for the treatment of memory disorders.

GSK-3β and memory formation

In Alzheimer’s disease (AD), tau hyperphosphorylation and neurofibrillary tangle (NFT) formation are strongly associated with dementia, a characteristic and early feature of this disease. Glycogen synthase kinase 3β (GSK-3β) is a pivotal kinase in both the normal and pathological phosphorylation of tau. In the diseased state, hyperphosphorylated tau is deposited in NFTs, the formation of which, drive the disease process. GSK-3β which is also involved in long-term depression induction, interacts with tau to inhibit synaptic long-term potentiation. Strong lines of evidence suggest that the activation of GSK-3β is responsible for the memory deficits seen in both advanced age and AD. In this review, we will focus on the role of GSK-3β in brain function, particularly in memory maintenance. We will examine human and mouse studies which suggest a role for GSK-3β in memory maintenance and the eventual development of memory deficits.

A memory retrieval-extinction procedure to prevent drug craving and relapse

Drug use and relapse involve learned associations between drug-associated environmental cues and drug effects. Extinction procedures in the clinic can suppress conditioned responses to drug cues, but the extinguished responses typically reemerge after exposure to the drug itself (reinstatement), the drug-associated environment (renewal), or the passage of time (spontaneous recovery). We describe a memory retrieval-extinction procedure that decreases conditioned drug effects and drug seeking in rat models of relapse, and drug craving in abstinent heroin addicts. In rats, daily retrieval of drug-associated memories 10 minutes or 1 hour but not 6 hours before extinction sessions attenuated drug-induced reinstatement, spontaneous recovery, and renewal of conditioned drug effects and drug seeking. In heroin addicts, retrieval of drug-associated memories 10 minutes before extinction sessions attenuated cue-induced heroin craving 1, 30, and 180 days later. The memory retrieval-extinction procedure is a promising nonpharmacological method for decreasing drug craving and relapse during abstinence.

Posttraining transcranial magnetic stimulation of striate cortex disrupts consolidation early in visual skill learning

Practice-induced improvements in skilled performance reflect "offline " consolidation processes extending beyond daily training sessions. According to visual learning theories, an early, fast learning phase driven by high-level areas is followed by a late, asymptotic learning phase driven by low-level, retinotopic areas when higher resolution is required. Thus, low-level areas would not contribute to learning and offline consolidation until late learning. Recent studies have challenged this notion, demonstrating modified responses to trained stimuli in primary visual cortex (V1) and offline activity after very limited training. However, the behavioral relevance of modified V1 activity for offline consolidation of visual skill memory in V1 after early training sessions remains unclear. Here, we used neuronavigated transcranial magnetic stimulation (TMS) directed to a trained retinotopic V1 location to test for behaviorally relevant consolidation in human low-level visual cortex. Applying TMS to the trained V1 location within 45 min of the first or second training session strongly interfered with learning, as measured by impaired performance the next day. The interference was conditional on task context and occurred only when training in the location targeted by TMS was followed by training in a second location before TMS. In this condition, high-level areas may become coupled to the second location and uncoupled from the previously trained low-level representation, thereby rendering consolidation vulnerable to interference. Our data show that, during the earliest phases of skill learning in the lowest-level visual areas, a behaviorally relevant form of consolidation exists of which the robustness is controlled by high-level, contextual factors.

Activation of PKCzeta and PKMzeta in the nucleus accumbens core is necessary for the retrieval, consolidation and reconsolidation of drug memory

One of the greatest challenges in the treatment of substance dependence is to reverse the control that drug-associated stimuli have gained over the addict's behavior, as these drug-associated memories increase the risk of relapse even after long periods of abstinence. We report here that inhibition of the atypical protein kinase C isoform PKCzeta and its constitutively active isoform PKMzeta with the pseudosubstrate inhibitor ZIP administered locally into the nucleus accumbens core reversibly inhibited the retrieval of drug-associated memory and drug (remifentanil) seeking, whereas a scrambled ZIP peptide or staurosporine, an effective inhibitor of c/nPKC-, CaMKII-, and PKA kinases that does not affect PKCzeta/PKMzeta activity, was without effect on these memory processes. Acquisition or extinction of drug-associated memory remained unaffected by PKCzeta- and PKMzeta inhibition.

Post-retrieval propranolol treatment does not modulate reconsolidation or extinction of ethanol-induced conditioned place preference

The reconsolidation hypothesis posits that established emotional memories, when reactivated, become labile and susceptible to disruption. Post-retrieval injection of propranolol (PRO), a nonspecific β-adrenergic receptor antagonist, impairs subsequent retention performance of a cocaine- and a morphine-induced conditioned place preference (CPP), implicating the noradrenergic system in the reconsolidation processes of drug-seeking behavior. An important question is whether post-retrieval PRO disrupts memory for the drug-cue associations, or instead interferes with extinction. In the present study, we evaluated the role of the β-adrenergic system on the reconsolidation and extinction of ethanol-induced CPP. Male DBA/2J mice were trained using a weak or a strong conditioning procedure, achieved by varying the ethanol conditioning dose (1 or 2 g/kg) and the number of ethanol trials (2 or 4). After acquisition of ethanol CPP, animals were given a single post-retrieval injection of PRO (0, 10 or 30 mg/kg) and tested for memory reconsolidation 24 h later. Also, after the first reconsolidation test, mice received 18 additional 15-min choice extinction tests in which PRO was injected immediately after every test. Contrary to the prediction of the reconsolidation hypothesis, a single PRO injection after the retrieval test did not modify subsequent memory retention. In addition, repeated post-retrieval administration of PRO did not interfere with extinction of CPP in mice. Overall, our data suggest that the β-adrenergic receptor does not modulate the associative processes underlying ethanol CPP.

Disruption of memory reconsolidation impairs storage of other, non-reactivated memory

Two hypotheses were tested in this study. First, blockade of neural activity by lidocaine immediately following the retrieval of a memory may impair the reconsolidation and subsequent expression of that memory. Second, a non-retrieved memory would not be affected by this lidocaine treatment. Since the basolateral nucleus of the amygdala (BLA) is involved in emotion-related memory, an intra-BLA lidocaine infusion was used immediately after the retrieval of two emotion-related memories, the step-through passive avoidance response (PA) and cocaine-induced conditioned place preference (CPP). Intra-BLA lidocaine infusion immediately after cocaine-induced CPP retrieval diminished CPP magnitude in retests. However, intra-BLA lidocaine infusion alone did not affect cocaine-induced CPP performance. Intra-BLA lidocaine infusion immediately after PA retrieval decreased PA performance in retests. Omission of PA retrieval procedure, intra-BLA lidocaine infusion did not affect subsequent PA performance. Surprisingly, intra-BLA lidocaine infusion immediately following the retrieval of PA or cocaine-induced CPP diminished both PA and cocaine-induced CPP performance in the retests. Finally, Fos-staining results revealed that a number of BLA neurons were activated by the retrieval of both cocaine-induced CPP and PA. We conclude that inactivation of neural activity in BLA immediately following retrieval of a fear or cocaine-conditioned memory can impair subsequent expression of both memories. More importantly, retrieval of a memory does not seem to be an absolute condition for rapidly changing the memory.

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.

Kappa Opioid Receptor-Mediated Disruption of Novel Object Recognition: Relevance for Psychostimulant Treatment

Kappa opioid receptor (KOR) agonists are potentially valuable as therapeutics for the treatment of psychostimulant reward as they suppress dopamine signaling in reward circuitry to repress drug seeking behavior. However, KOR agonists are also associated with sedation and cognitive dysfunction. The extent to which learning and memory disruption or hypolocomotion underlie KOR agonists' role in counteracting the rewarding effects of psychostimulants is of interest. C57BL/6J mice were pretreated with vehicle (saline, 0.9%), the KOR agonist (trans)-3,4-dichloro-N-methyl-N-[2-(1- pyrrolidinyl)-cyclohexyl] benzeneacetamide (U50,488), or the peripherally-restricted agonist D-Phe-D-Phe-D-lle-D-Arg- NH(2) (ffir-NH(2)), through central (i.c.v.) or peripheral (i.p.) routes of administration. Locomotor activity was assessed via activity monitoring chambers and rotorod. Cognitive performance was assessed in a novel object recognition task. Prolonged hypolocomotion was observed following administration of 1.0 and 10.0, but not 0.3 mg/kg U50,488. Central, but not peripheral, administration of ffir-NH(2) (a KOR agonist that does not cross the blood-brain barrier) also reduced motor behavior. Systemic pretreatment with the low dose of U50,488 (0.3 mg/kg, i.p.) significantly impaired performance in the novel object recognition task. Likewise, ffir-NH(2) significantly reduced novel object recognition after central (i.c.v.), but not peripheral (i.p.), administration. U50,488- and ffir-NH(2)-mediated deficits in novel object recognition were prevented by pretreatment with KOR antagonists. Cocaine-induced conditioned place preference was subsequently assessed and was reduced by pretreatment with U50,488 (0.3 mg/kg, i.p.). Together, these results suggest that the activation of centrally-located kappa opioid receptors may induce cognitive and mnemonic disruption independent of hypolocomotor effects which may contribute to the KOR-mediated suppression of psychostimulant reward.

Memory coding in plastic neuronal subpopulations within the amygdala

Specific neuronal subpopulations within specific brain areas are responsible for learning and memory. A fear memory engages a subset of lateral amygdala neurons, but whether multiple contextual fear memories engage the same or different subsets of lateral amygdala neurons remains unclear. Here, we demonstrate the representation of multiple contextual fear memories in the amygdala with cellular and temporal resolution using a large-scale imaging method. Mice were conditioned with a footshock in 2 separate chambers. They were then re-exposed to either the same conditioning chamber twice or 2 different conditioning chambers. The activities of individual neurons related to the re-exposures were determined by the subcellular distribution of Arc/Arg3.1 RNA. Reactivation of different memories activated partially (about 50%) overlapping neurons, whereas reactivation of the same memory activated more overlapping (about 65%) neurons. These findings indicate that lateral amygdala neurons related to different fear memories are partly common, and that a small but significant neuronal population (2.7% of total lateral amygdala neurons) encodes differences in individual fear memories. Moreover, memory retrieval increased the size of the neuronal subpopulation activated during subsequent retrieval. Taken together, our findings indicate that small plastic subsets of neurons encode fear memories from individual contexts.