Brain region-specific gene expression activation required for reconsolidation and extinction of contextual fear memory

Paeoniflorin exerts anti-PTSD effects in adult rats by modulating hippocampus and amygdala histone acetylation modifications in response to early life stress

Early life stress (ELS) can cause long-term changes by epigenetic factors, especially histone acetylation modification, playing a crucial role, affect normal cognition, mood, and behavior, and increase susceptibility to post-traumatic stress disorder (PTSD) in adulthood. It has been found that paeoniflorin (PF) can cross the blood-brain barrier to exert anti-PTSD effects on adult PTSD rats. However, whether PF can alleviate the harmful effects caused by ELS in adulthood has not yet been reported. Therefore, to explore the relationship between ELS and PTSD susceptibility in adulthood and its mechanism, in this study, SPS was used as a stressor of ELS, and the mathematical tool Z-normalization was employed as an evaluation criterion of behavioral resilience susceptibility. To investigate the regulatory mechanism of PF on histone acetylation in the hippocampus and amygdala of ELS rats in adulthood, using changes in HATs/HDACs as the entry point, meanwhile, the epigenetic marks (H3K9 and H4K12) in the key brain regions of ELS (hippocampus and amygdala) were evaluated, and the effects of PF on behavioral representation and PTSD susceptibility were observed. This study found that ELS lead to a series of PTSD-like behaviors in adulthood and caused imbalance of HATs/HDACs ratio in the hippocampus and amygdala, which confirms that ELS is an important risk factor for the development of PTSD in adulthood. In addition, paeoniflorin may improve ELS-induced PTSD-like behaviors and reduce the susceptibility of ELS rats to develop PTSD in adulthood by modulating the HATs/HDACs ratio in the hippocampus and amygdala.

Inhibition of ERK1/2 or CRMP2 Disrupts Alcohol Memory Reconsolidation and Prevents Relapse in Rats

Relapse to alcohol abuse, often caused by cue-induced alcohol craving, is a major challenge in alcohol addiction treatment. Therefore, disrupting the cue-alcohol memories can suppress relapse. Upon retrieval, memories transiently destabilize before they reconsolidate in a process that requires protein synthesis. Evidence suggests that the mammalian target of rapamycin complex 1 (mTORC1), governing the translation of a subset of dendritic proteins, is crucial for memory reconsolidation. Here, we explored the involvement of two regulatory pathways of mTORC1, phosphoinositide 3-kinase (PI3K)-AKT and extracellular regulated kinase 1/2 (ERK1/2), in the reconsolidation process in a rat (Wistar) model of alcohol self-administration. We found that retrieval of alcohol memories using an odor-taste cue increased ERK1/2 activation in the amygdala, while the PI3K-AKT pathway remained unaffected. Importantly, ERK1/2 inhibition after alcohol memory retrieval impaired alcohol-memory reconsolidation and led to long-lasting relapse suppression. Attenuation of relapse was also induced by post-retrieval administration of lacosamide, an inhibitor of collapsin response mediator protein-2 (CRMP2)-a translational product of mTORC1. Together, our findings indicate the crucial role of ERK1/2 and CRMP2 in the reconsolidation of alcohol memories, with their inhibition as potential treatment targets for relapse prevention.

Distinct roles of excitatory and inhibitory neurons in the medial prefrontal cortex in the expression and reconsolidation of methamphetamine-associated memory in male mice

Methamphetamine, a commonly abused drug, is known for its high relapse rate. The persistence of addictive memories associated with methamphetamine poses a significant challenge in preventing relapse. Memory retrieval and subsequent reconsolidation provide an opportunity to disrupt addictive memories. However, the key node in the brain network involved in methamphetamine-associated memory retrieval has not been clearly defined. In this study, using the conditioned place preference in male mice, whole brain c-FOS mapping and functional connectivity analysis, together with chemogenetic manipulations of neural circuits, we identified the medial prefrontal cortex (mPFC) as a critical hub that integrates inputs from the retrosplenial cortex and the ventral tegmental area to support both the expression and reconsolidation of methamphetamine-associated memory during its retrieval. Surprisingly, with further cell-type specific analysis and manipulation, we also observed that methamphetamine-associated memory retrieval activated inhibitory neurons in the mPFC to facilitate memory reconsolidation, while suppressing excitatory neurons to aid memory expression. These findings provide novel insights into the neural circuits and cellular mechanisms involved in the retrieval process of addictive memories. They suggest that targeting the balance between excitation and inhibition in the mPFC during memory retrieval could be a promising treatment strategy to prevent relapse in methamphetamine addiction.

Exercise induction at expression immediate early gene (c-Fos, ARC, EGR-1) in the hippocampus: a systematic review

The immediate early gene exhibits activation markers in the nervous system consisting of ARC, EGR-1, and c-Fos and is related to synaptic plasticity, especially in the hippocampus. Immediate early gene expression is affected by physical exercise, which induces direct ARC, EGR-1, and c-Fos expression.

Objective

To assess the impact of exercise, we conducted a literature study to determine the expression levels of immediate early genes (ARC, c-Fos, and EGR-1).

Methods

The databases accessed for online literature included PubMed-Medline, Scopus, and ScienceDirect. The original English articles were selected using the following keywords in the title: (Exercise OR physical activity) AND (c-Fos) AND (Hippocampus), (Exercise OR physical activity) AND (ARC) AND (Hippocampus), (Exercise OR physical activity) AND (EGR-1 OR zif268) AND (Hippocampus).

Results

Physical exercise can affect the expression of EGR-1, c-Fos, and ARC in the hippocampus, an important part of the brain involved in learning and memory. High-intensity physical exercise can increase c-Fos expression, indicating neural activation. Furthermore, the expression of the ARC gene also increases due to physical exercise. ARC is a gene that plays a role in synaptic plasticity and regulation of learning and memory, changes in synaptic structure and increased synaptic connections, while EGR-1 also plays a role in synaptic plasticity, a genetic change that affects learning and memory. Overall, exercise or regular physical exercise can increase the expression of ARC, c-Fos, and EGR-1 in the hippocampus. This reflects the changes in neuroplasticity and synaptic plasticity that occur in response to physical activity. These changes can improve cognitive function, learning, and memory.

Conclusion

c-Fos, EGR-1, and ARC expression increases in hippocampal neurons after exercise, enhancing synaptic plasticity and neurogenesis associated with learning and memory.

Aging as a Consequence of the Adaptation-Maladaptation Dilemma

In aging, the organism is unable to counteract certain harmful influences over its lifetime which leads to progressive dysfunction and eventually death, thus delineating aging as one failed process of adaptation to a set of aging stimuli. A central problem in understanding aging is hence to explain why the organism cannot adapt to these aging stimuli. The adaptation-maladaptation theory of aging proposes that in aging adaptation processes such as adaptive transcription, epigenetic remodeling, and metabolic plasticity drive dysfunction themselves over time (maladaptation) and thereby cause aging-related disorders such as cancer and metabolic dysregulation. The central dilemma of aging is thus that the set of adaptation mechanisms that the body uses to deal with internal and external stressors acts as a stressor itself and cannot be effectively counteracted. The only available option for the organism to decrease maladaptation may be a program to progressively reduce the output of adaptive cascades (e.g., via genomic methylation) which then leads to reduced physiological adaptation capacity and syndromes like frailty, immunosenescence, and cognitive decline. The adaptation-maladaptation dilemma of aging entails that certain biological mechanisms can simultaneously protect against aging as well as drive aging. The key to longevity may lie in uncoupling adaptation from maladaptation.

Fear memory regulation by the cAMP signaling pathway as an index of reexperiencing symptoms in posttraumatic stress disorder

Posttraumatic stress disorder (PTSD) is a psychiatric disorder associated with traumatic memory, yet its etiology remains unclear. Reexperiencing symptoms are specific to PTSD compared to other anxiety-related disorders. Importantly, reexperiencing can be mimicked by retrieval-related events of fear memory in animal models of traumatic memory. Recent studies revealed candidate PTSD-associated genes that were related to the cyclic adenosine monophosphate (cAMP) signaling pathway. Here, we demonstrate the tight linkage between facilitated cAMP signaling and PTSD by analyzing loss- and gain-of-cAMP signaling effects on fear memory in mice and the transcriptomes of fear memory-activated mice and female PTSD patients with reexperiencing symptoms. Pharmacological and optogenetic upregulation or downregulation of cAMP signaling transduction enhanced or impaired, respectively, the retrieval and subsequent maintenance of fear memory in mice. In line with these observations, integrative mouse and human transcriptome analysis revealed the reduced mRNA expression of phosphodiesterase 4B (PDE4B), an enzyme that degrades cAMP, in the peripheral blood of PTSD patients showing more severe reexperiencing symptoms and the mouse hippocampus after fear memory retrieval. Importantly, more severe reexperiencing symptoms and lower PDE4B mRNA levels were correlated with decreased DNA methylation of a locus within PDE4B, suggesting the involvement of methylation in the mechanism of PTSD. These findings raise the possibility that the facilitation of cAMP signaling mediating the downregulation of PDE4B expression enhances traumatic memory, thereby playing a key role in the reexperiencing symptoms of PTSD patients as a functional index of these symptoms.

Engrams of Fear Memory Attenuation

Fear attenuation is an etiologically relevant process for animal survival, since once acquired information needs to be continuously updated in the face of changing environmental contingencies. Thus, when situations are encountered that were originally perceived as fearful but are no longer so, fear must be attenuated, otherwise, it risks becoming maladaptive. But what happens to the original memory trace of fear during fear attenuation? In this chapter, we review the studies that have started to approach this question from an engram perspective. We find evidence pointing to both the original memory trace of fear being suppressed, as well as it being updated towards safety. These seemingly conflicting results reflect a well-established dichotomy in the field of fear memory attenuation, namely whether fear attenuation is mediated by an inhibitory mechanism that suppresses fear expression, called extinction, or by an updating mechanism that allows the fear memory to reconsolidate in a different form, called reconsolidation-updating. Which of these scenarios takes the upper hand is ultimately influenced by the behavioral paradigms used to induce fear attenuation, but is an important area for further study as the precise cell populations underlying fear attenuation and the molecular mechanisms therein can now be understood at unprecedented resolution.

Unveiling Transcriptional and Epigenetic Mechanisms Within Engram Cells: Insights into Memory Formation and Stability

Memory traces for behavioral experiences, such as fear conditioning or taste aversion, are believed to be stored through biophysical and molecular changes in distributed neuronal ensembles across various brain regions. These ensembles are known as engrams, and the cells that constitute them are referred to as engram cells. Recent advancements in techniques for labeling and manipulating neural activity have facilitated the study of engram cells throughout different memory phases, including acquisition, allocation, long-term storage, retrieval, and erasure. In this chapter, we will explore the application of next-generation sequencing methods to engram research, shedding new light on the contribution of transcriptional and epigenetic mechanisms to engram formation and stability.

Cellular mechanisms of contextual fear memory reconsolidation: Role of hippocampal SFKs, TrkB receptors and GluN2B-containing NMDA receptors

Memories are stored into long-term representations through a process that depends on protein synthesis. However, a consolidated memory is not static and inflexible and can be reactivated under certain circumstances, the retrieval is able to reactivate memories and destabilize them engaging a process of restabilization known as reconsolidation. Although the molecular mechanisms that mediate fear memory reconsolidation are not entirely known, so here we investigated the molecular mechanisms in the hippocampus involved in contextual fear conditioning memory (CFC) reconsolidation in male Wistar rats. We demonstrated that the blockade of Src family kinases (SFKs), GluN2B-containing NMDA receptors and TrkB receptors (TrkBR) in the CA1 region of the hippocampus immediately after the reactivation session impaired contextual fear memory reconsolidation. These impairments were blocked by the neurotrophin BDNF and the NMDAR agonist, D-Serine. Considering that the study of the link between synaptic proteins is crucial for understanding memory processes, targeting the reconsolidation process may provide new ways of disrupting maladaptive memories, such as those seen in post-traumatic stress disorder. Here we provide new insights into the cellular mechanisms involved in contextual fear memory reconsolidation, demonstrating that SFKs, GluN2B-containing NMDAR, and TrkBR are necessary for the reconsolidation process. Our findings suggest a link between BDNF and SFKs and GluN2B-containing NMDAR as well as a link between NMDAR and SFKs and TrkBR in fear memory reconsolidation. These preliminary pharmacological findings provide new evidence of the mechanisms involved in the reconsolidation of fear memory and have the potential to contribute to the development of treatments for psychiatric disorders involving maladaptive memories.

Drug memory reconsolidation: from molecular mechanisms to the clinical context

Since its rediscovery at the beginning of the 21st Century, memory reconsolidation has been proposed to be a therapeutic target for reducing the impact of emotional memories that can go awry in mental health disorders such as drug addiction (substance use disorder, SUD). Addiction can be conceptualised as a disorder of learning and memory, in which both pavlovian and instrumental learning systems become hijacked into supporting drug-seeking and drug-taking behaviours. The past two decades of research have characterised the details of the molecular pathways supporting the reconsolidation of pavlovian cue-drug memories, with more recent work indicating that the reconsolidation of instrumental drug-seeking memories also relies upon similar mechanisms. This narrative review considers what is known about the mechanisms underlying the reconsolidation of pavlovian and instrumental memories associated with drug use, how these approaches have translated to experimental medicine studies, and the challenges and opportunities for the clinical use of reconsolidation-based therapies.

Phosphodiesterase 4 inhibition after retrieval switches the memory fate favoring extinction instead of reconsolidation

Phosphodiesterase 4 (PDE4), an enzyme expressed in the dorsal hippocampus (DH), hydrolyzes the cAMP, limiting the PKA-induced CREB phosphorylation (pCREB) and BDNF expression. Depending on the brain region, PKA and pCREB mediate reconsolidation or extinction, whereas BDNF is mainly related to extinction facilitation. The mechanisms underpinning the switch between reconsolidation and extinction are relatively unknown. Here, we tested the hypothesis that PDE4 might control these processes. We showed in Wistar rats submitted to contextual fear conditioning that PDE4 inhibition with roflumilast (ROF) within the DH, after a short retrieval, did not change freezing behavior after one day (TestA1). After 10 days, the ROF-treated group significantly reduced the expression of freezing behavior. This effect depended on retrieval, Test A1 exposure, and reinstated after a remainder foot shock, suggesting an extinction facilitation. The ROF effect depended on PKA after retrieval or, protein synthesis after Test A1. After retrieval, ROF treatment did not change the pCREB/CREB ratio in the DH. It enhanced proBDNF expression without changing pre-proBDNF or mature BDNF in the DH after Test A1. The results suggest that the inhibition of PDE4 in the DH after a short retrieval changes the memory sensibility from reconsolidation to extinction via regulating proBDNF expression.

A mouse model of ATRX deficiency with cognitive deficits and autistic traits

BACKGROUND

ATRX is an ATP-dependent chromatin remodeling protein with essential roles in safeguarding genome integrity and modulating gene expression. Deficiencies in this protein cause ATR-X syndrome, a condition characterized by intellectual disability and an array of developmental abnormalities, including features of autism. Previous studies demonstrated that deleting ATRX in mouse forebrain excitatory neurons postnatally resulted in male-specific memory deficits, but no apparent autistic-like behaviours.

METHODS

We generated mice with an earlier embryonic deletion of ATRX in forebrain excitatory neurons and characterized their behaviour using a series of memory and autistic-related paradigms.

RESULTS

We found that mutant mice displayed a broader spectrum of impairments, including fear memory, decreased anxiety-like behaviour, hyperactivity, as well as self-injurious and repetitive grooming. Sex-specific alterations were also observed, including male-specific aggression, sensory gating impairments, and decreased social memory.

CONCLUSIONS

Collectively, the findings indicate that early developmental abnormalities arising from ATRX deficiency in forebrain excitatory neurons contribute to the presentation of fear memory deficits as well as autistic-like behaviours.

Time-course analysis of frontal gene expression profiles in the rat model of posttraumatic stress disorder and a comparison with the conditioned fear model

Posttraumatic stress disorder (PTSD) is a complex disorder that involves physiological, emotional, and cognitive dysregulation that may occur after exposure to a life-threatening event. In contrast with the condition of learned fear with resilience to extinction, abnormal fear with impaired fear extinction and exaggeration are considered crucial factors for the pathological development of PTSD. The prefrontal cortex (mPFC) is considered a critical region of top-down control in fear regulation, which involves the modulation of fear expression and extinction. The pathological course of PTSD is usually chronic and persistent; a number of studies have indicated temporal progression in gene expression and phenotypes may be involved in PTSD pathology. In the current study, we use a well-established modified single-prolonged stress (SPS&FS) rat model to feature PTSD-like phenotypes and compared it with a footshock fear conditioning model (FS model); we collected the frontal tissue after extreme stress exposure or fear conditioning and extracted RNA for transcriptome-level gene sequencing. We compared the genetic profiling of the mPFC at early (<2 h after solely FS or SPS&FS exposure) and late (7 days after solely FS or SPS&FS exposure) stages in these two models. First, we identified temporal differences in the expressional patterns between these two models and found pathways such as protein synthesis factor eukaryotic initiation factor 2 (EIF2), transcription factor NF-E2-related factor 2 (NRF2)-mediated oxidative stress response, and acute phase responding signaling enriched in the early stage in both models with significant p-values. Furthermore, in the late stage, the sirtuin signaling pathway was enriched in both models; other pathways such as STAT3, cAMP, lipid metabolism, Gα signaling, and increased fear were especially enriched in the late stage of the SPS&FS model. However, pathways such as VDR/RXR, GP6, and PPAR signaling were activated significantly in the FS model's late stage. Last, the network analysis revealed the temporal dynamics of psychological disorder, the endocrine system, and also genes related to increased fear in the two models. This study could help elucidate the genetic temporal alteration and stage-specific pathways in these two models, as well as a better understanding of the transcriptome-level differences between them.

Increased hippocampal CREB phosphorylation after retrieval of remote contextual fear memories in Carioca high-conditioned freezing rats

The participation of the hippocampal formation in consolidation and reconsolidation of contextual fear memories has been widely recognized and known to be dependent on the activation of the cAMP response element (CRE) binding protein (CREB) pathway. Recent findings have challenged the prevailing view that over time contextual fear memories migrate to neocortical circuits and no longer require the hippocampus for retrieval of remote fearful memories. It has also recently been found that this brain structure is important for the maintenance and recall of remote fear memories associated with aversive events, a common trait in stress-related disorders such as generalized anxiety disorder (GAD), major depression, and post-traumatic stress disorder. In view of these findings, here we examined the putative role of CREB in the hippocampus of an animal model of GAD during the retrieval of remote contextual fear memories. Specifically, we evaluated CREB phosphorylation in the hippocampus of male Carioca High- and Low-conditioned Freezing rats (CHF and CLF, respectively) upon re-exposure of animals to contextual cues associated to footshocks weeks after fear conditioning. Age-matched male rats from a randomized crossbreeding population served as controls (CTL). Adrenal catecholamine levels were also measured as a biological marker of stress response. Seven weeks after contextual fear conditioning, half of the sample of CHF (n = 9), CLF (n = 10) and CTL (n = 10) rats were randomly assigned to return to the same context chamber where footshocks were previously administrated (Context condition), while the remaining animals were individually placed in standard housing cages (Control condition). Western blot results indicated that pCREB levels were significantly increased in the hippocampus of CHF rats for both Context and Control conditions when compared to the other experimental groups. CHF rats in the Context condition also exhibited significant more freezing than that observed for both CLF and CTL rats. Lastly, CHF animals in the Context condition displayed significantly higher adrenal catecholamine levels than those in the Control condition, whereas no differences in catecholamine levels were observed between Context and Control conditions for CLF and CTL rats. These findings are discussed from a perspective in which the hippocampus plays a role in the maintenance and recall of remote contextual fear memories via the CREB pathway.

The Lysine Demethylase KDM7A Regulates Immediate Early Genes in Neurons

Lysine demethylase KDM7A removes histone modifications H3K9me1/2 and H3K27me1/2. KDM7A plays critical roles in gene expression and contribute to biological processes including tumorigenesis, metabolism, and embryonic development. However, the functions of KDM7A in mammalian nervous system are still poorly explored. In this study, functional roles of KDM7A are comprehensively investigated in neuronal cells by applying CUT&Tag-seq, RNA-seq and mice models. Knockdown of Kdm7a in N2A cells result in the alteration of histone modifications near transcription start sites (TSSs) and the expression changes of a large number of genes. In particular, the expression of immediate early genes (IEGs), a series of genes maintaining the function of the nervous system and associating with neurological disorders, are significantly decreased upon Kdm7a knockdown. Furthermore, in vivo knockdown of Kdm7a in dentate gyrus (DG) neuron of mice hippocampus, via Adeno-associated virus (AAV)-based stereotaxic microinjection, led to a significant decrease of the expression of c-Fos, a marker of neuron activity. Behavior assays in mice further revealed that Kdm7a knockdown in hippocampus repress neuron activity, which leading to impairment of emotion and memory. Collectively, the study reveals that KDM7A affects neuron functions by regulating IEGs, which may provide new clues for understanding epigenetic mechanisms in neurological disorders.

Consolidation, reconsolidation, and extinction of contextual fear memory depend on de novo protein synthesis in the locus coeruleus

Memory consolidation is the process underlying the stabilization of labile short-term memory and the generation of long-term memory for persistent memory storage. The retrieval of contextual fear memory induces two distinct and opposite memory processes: reconsolidation and extinction. Reconsolidation re-stabilizes retrieved memory for re-storage, whereas memory extinction weakens fear memory and generates a new inhibitory memory. Importantly, the requirement for new gene expression is a critical biochemical feature of the consolidation, reconsolidation, and long-term extinction of memory. The locus coeruleus (LC) is a small nucleus in the brain stem that is composed predominantly of noradrenergic neurons that project to many brain regions. Recent studies have shown that the LC plays modulatory roles in the consolidation and extinction of auditory fear memory through its projections to brain regions contributing to memory storage. Here, we show that the LC is required for the consolidation, reconsolidation, and long-term extinction of contextual fear memory. We first observed that c-fos expression was induced in the LC following contextual fear conditioning to induce consolidation and following short and long re-exposure to the conditioning context to induce reconsolidation and long-term extinction, respectively. More importantly, inhibition of protein synthesis in the LC by a micro-infusion of anisomycin blocked the consolidation, reconsolidation, and long-term extinction of contextual fear memory. Our findings suggest that consolidation, reconsolidation, and long-term extinction occur in the LC and that the LC plays an essential role in memory storage and maintenance.

The Basolateral Amygdala: The Core of a Network for Threat Conditioning, Extinction, and Second-Order Threat Conditioning

Threat conditioning, extinction, and second-order threat conditioning studied in animal models provide insight into the brain-based mechanisms of fear- and anxiety-related disorders and their treatment. Much attention has been paid to the role of the basolateral amygdala (BLA) in such processes, an overview of which is presented in this review. More recent evidence suggests that the BLA serves as the core of a greater network of structures in these forms of learning, including associative and sensory cortices. The BLA is importantly regulated by hippocampal and prefrontal inputs, as well as by the catecholaminergic neuromodulators, norepinephrine and dopamine, that may provide important prediction-error or learning signals for these forms of learning. The sensory cortices may be required for the long-term storage of threat memories. As such, future research may further investigate the potential of the sensory cortices for the long-term storage of extinction and second-order conditioning memories.

The ubiquitin-proteasome system and learning-dependent synaptic plasticity - A 10 year update

Over 25 years ago, a seminal paper demonstrated that the ubiquitin-proteasome system (UPS) was involved in activity-dependent synaptic plasticity. Interest in this topic began to expand around 2008 following another seminal paper showing that UPS-mediated protein degradation controlled the "destabilization" of memories following retrieval, though we remained with only a basic understanding of how the UPS regulated activity- and learning-dependent synaptic plasticity. However, over the last 10 years there has been an explosion of papers on this topic that has significantly changed our understanding of how ubiquitin-proteasome signaling regulates synaptic plasticity and memory formation. Importantly, we now know that the UPS controls much more than protein degradation, is involved in plasticity underlying drugs of abuse and that there are significant sex differences in how ubiquitin-proteasome signaling is used for memory storage processes. Here, we aim to provide a critical 10-year update on the role of ubiquitin-proteasome signaling in synaptic plasticity and memory formation, including updated cellular models of how ubiquitin-proteasome activity could be regulating learning-dependent synaptic plasticity in the brain.

nNOS and Neurological, Neuropsychiatric Disorders: A 20-Year Story

In the central nervous system, nitric oxide (NO), a free gas with multitudinous bioactivities, is mainly produced from the oxidation of L-arginine by neuronal nitric oxide synthase (nNOS). In the past 20 years, the studies in our group and other laboratories have suggested a significant involvement of nNOS in a variety of neurological and neuropsychiatric disorders. In particular, the interactions between the PDZ domain of nNOS and its adaptor proteins, including post-synaptic density 95, the carboxy-terminal PDZ ligand of nNOS, and the serotonin transporter, significantly influence the subcellular localization and functions of nNOS in the brain. The nNOS-mediated protein-protein interactions provide new attractive targets and guide the discovery of therapeutic drugs for neurological and neuropsychiatric disorders. Here, we summarize the work on the roles of nNOS and its association with multiple adaptor proteins on neurological and neuropsychiatric disorders.

How engram mediates learning, extinction, and relapse

Fear learning ensures survival through an expression of certain behavior as a conditioned fear response. Fear memory is processed and stored in a fear memory circuit, including the amygdala, hippocampus, and prefrontal cortex. A gradual decrease in conditioned fear response can be induced by fear extinction, which is mediated through the weakening of the original fear memory traces and the newly formed inhibition of those traces. Fear memory can also recover after extinction, which shows flexible control of the fear memory state. Here, we demonstrate how fear engram, which is a physical substrate of fear memory, changes during fear extinction and relapse by reviewing recent studies regarding engram.

Role of hippocampal Wnt signaling pathways on contextual fear memory reconsolidation

Memories already consolidated when reactivated return to a labile state and can be modified, this process is known as reconsolidation. It is known the Wnt signaling pathways can modulate hippocampal synaptic plasticity as well as learning and memory. Yet, Wnt signaling pathways interact with NMDA (N-methyl-D-aspartate) receptors. However, whether canonical Wnt/β-catenin and non-canonical Wnt/Ca2+ signaling pathways are required in the CA1 region of hippocampus for contextual fear memory reconsolidation remains unclear. So, here we verified that the inhibition of canonical Wnt/β-catenin pathway with DKK1 (Dickkopf-1) into CA1 impaired the reconsolidation of contextual fear conditioning (CFC) memory when administered immediately and 2h after reactivation session but not 6h later, while the inhibition of non-canonical Wnt/Ca²⁺ signaling pathway with SFRP1 (Secreted frizzled-related protein-1) into CA1 immediately after reactivation session had no effect. Moreover, the impairment induced by DKK1 was blocked by the administration of the agonist of the NMDA receptors glycine site, D-Serine, immediately and 2h after reactivation session. We found that hippocampal canonical Wnt/β-catenin is necessary to the reconsolidation of CFC memory at least two hours after reactivation, while non-canonical Wnt/Ca²⁺ signaling pathway is not involved in this process and, that there is a link between Wnt/β-catenin signaling pathway and NMDA receptors. In view of this, this study provides new evidence regarding the neural mechanisms underlying contextual fear memory reconsolidation and contributes to provide a new possible target for the treatment of fear related disorders.

Phosphorylation of PSD-95 at serine 73 in dCA1 is required for extinction of contextual fear

The updating of contextual memories is essential for survival in a changing environment. Accumulating data indicate that the dorsal CA1 area (dCA1) contributes to this process. However, the cellular and molecular mechanisms of contextual fear memory updating remain poorly understood. Postsynaptic density protein 95 (PSD-95) regulates the structure and function of glutamatergic synapses. Here, using dCA1-targeted genetic manipulations in vivo, combined with ex vivo 3D electron microscopy and electrophysiology, we identify a novel, synaptic mechanism that is induced during attenuation of contextual fear memories and involves phosphorylation of PSD-95 at Serine 73 in dCA1. Our data provide the proof that PSD-95-dependent synaptic plasticity in dCA1 is required for updating of contextual fear memory.

Interaction between reconsolidation and extinction of fear memory

Memory retrieval is not a passive process. When a memory is retrieved, it returns to a labile state and undergoes reconsolidation to be re-stored. The discovery of this memory reconsolidation has had a major impact on memory consolidation theory. In other words, it suggested that memory is more dynamic than expected and can be modified through reconsolidation. Conversely, a conditioned fear memory undergoes memory extinction after retrieval, and it is thought that extinction does not reflect its erasure, but rather new inhibitory learning of the original conditioned memory. We have investigated the relationship between memory reconsolidation and extinction by comparing their behavioral, cellular, and molecular mechanisms. Memory reconsolidation and extinction have opposite functions on contextual fear and inhibitory avoidance memories; reconsolidation maintains or strengthens fear memory, whereas extinction weakens it. Importantly, reconsolidation and extinction are contrasting memory processes not only at the behavioral level but also at cellular and molecular levels. Furthermore, our analysis revealed that reconsolidation and extinction are not independent processes, but interact with each other. Interestingly, we also found a "memory transition process" that switches the fear memory process from reconsolidation to extinction after retrieval. Investigating the mechanisms of reconsolidation and extinction will contribute to our understanding of the dynamic nature of memory.

Face your fears: attenuating remote fear memories by reconsolidation-updating

Traumatic events generate some of the most enduring memories, yet little is known about how long-lasting fear memories can be attenuated. In this review, we collect the surprisingly sparse evidence on remote fear memory attenuation from both animal and human research. What is becoming apparent is twofold: although remote fear memories are more resistant to change compared with recent ones, they can nevertheless be attenuated when interventions are targeted toward the period of memory malleability instigated by memory recall, the reconsolidation window. We describe the physiological mechanisms underlying remote reconsolidation-updating approaches and highlight how they can be enhanced through interventions promoting synaptic plasticity. By capitalizing on an intrinsically relevant phase of memory, reconsolidation-updating harbors the potential to permanently alter remote fear memories.

Effects of delta-9 tetrahydrocannabinol on fear memory labilization and reconsolidation: A putative role of GluN2B-NMDA receptor within the dorsal hippocampus

Cannabis preparations could be an effective reconsolidation-based treatment for post-traumatic stress disorder. However, the effects of Δ⁹-tetrahydrocannabinol (THC) in fear memory labilization, a critical condition for retrieval-induced reconsolidation, are undetermined. We sought to investigate the effect of a conventional and an ultra-low dose of THC in memory labilization of adult male Wistar rats submitted to contextual fear conditioning. Pretreatment with THC 0.002, but not THC 0.3 mg/kg, i. p., before memory retrieval, did not change memory expression during the retrieval but impaired reconsolidation. No treatment changed freezing expression in an unpaired context. Before retrieval, THC 0.3, but not THC 0.002, decreased GluN2A-NMDA expression and the GluN2A/GluN2B ratio in the dorsal hippocampus (DH) 24 h later. No changes were observed immediately after retrieval. Pretreatment with THC 0.3 abolished the reconsolidation-impairing effect of anisomycin injected into the DH, suggesting an impairment in memory labilization. This effect was associated with an increased freezing expression in the unpaired context and was not observed with the THC ultra-low dose. The GluN2B-NMDA antagonism increased fear generalization in the anisomycin-treated group but restored its reconsolidation-impairing effect and reduced fear generalization when animals were pretreated with THC 0.3. GluN2A-NMDA antagonism or inhibition of the ubiquitin-proteasome system in the DH did not interfere with the effects of THC 0.3. Our findings indicate that THC causes a bidirectional effect on fear memory labilization that depends on hippocampal GluN2B-NMDA receptors' involvement in fear memory generalization.

Hippocampal cholinergic receptors and the mTOR participation in fear-motivated inhibitory avoidance extinction memory

Evidence has demonstrated the hippocampal cholinergic system and the mammalian target of rapamycin (mTOR) participation during the memory formation of aversive events. This study assessed the role of these systems in the hippocampus for the extinction memory process by submitting male Wistar rats to fear-motivated step-down inhibitory avoidance (IA). The post-extinction session administration of the nicotinic and muscarinic cholinergic receptor antagonists, mecamylamine and scopolamine, respectively, both at doses of 2 µg/µl/side, and rapamycin, an mTOR inhibitor (0.02 µg/µl/side), into the CA1 region of the dorsal hippocampus, impaired the IA extinction memory. Furthermore, the nicotinic and muscarinic cholinergic receptor agonists, nicotine and muscarine, respectively, had a dose-dependent effect on the IA extinction memory when administered intra-CA1, immediately after the extinction session. Nicotine (0.6 µg/µl/side) and muscarine (0.02 µg/µl/side), respectively, had no effect, while the higher doses (6 and 2 µg/µl/side, respectively) impaired the IA extinction memory. Interestingly, the co-administration of muscarine at the lower dose blocked the impairment that was induced by rapamycin. This effect was not observed when nicotine at the lower dose was co-administered. These results have demonstrated the participation of the cholinergic receptors and mTOR in the hippocampus for IA extinction, and that the cholinergic agonists had a dose-dependent effect on the IA extinction memory. This study provides insights related to the behavioural aspects and the neurobiological properties underlying the early stage of fear-motivated IA extinction memory consolidation and suggests that there is hippocampal muscarinic receptor participation independent of mTOR in this memory process.

Long-term memory, synaptic plasticity and dopamine in rodent medial prefrontal cortex: Role in executive functions

Mnemonic functions, supporting rodent behavior in complex tasks, include both long-term and (short-term) working memory components. While working memory is thought to rely on persistent activity states in an active neural network, long-term memory and synaptic plasticity contribute to the formation of the underlying synaptic structure, determining the range of possible states. Whereas, the implication of working memory in executive functions, mediated by the prefrontal cortex (PFC) in primates and rodents, has been extensively studied, the contribution of long-term memory component to these tasks received little attention. This review summarizes available experimental data and theoretical work concerning cellular mechanisms of synaptic plasticity in the medial region of rodent PFC and the link between plasticity, memory and behavior in PFC-dependent tasks. A special attention is devoted to unique properties of dopaminergic modulation of prefrontal synaptic plasticity and its contribution to executive functions.

Cannabidiol impairs fear memory reconsolidation in female rats through dorsal hippocampus CB1 but not CB2 receptor interaction

Women present increased susceptibility to anxiety- and stress-related disorders compared to men. A potentially promising pharmacological-based strategy to regulate abnormal aversive memories disrupts their reconsolidation stage after reactivation and destabilization. Male rodent findings indicate that cannabidiol (CBD), a relatively safe and effective treatment for several mental health conditions, can impair the reconsolidation of aversive memories. However, whether and how CBD influences it in females is still unknown. The present study addressed this question in contextually fear-conditioned female rats. We report that systemically administered CBD impaired their reconsolidation, reducing freezing expression for over a week. This action was restricted to a time when the reconsolidation presumably lasted (< six hours post-retrieval) and depended on memory reactivation/destabilization. Moreover, the impairing effects of CBD on memory reconsolidation relied on the activation of cannabinoid type-1 but not type-2 receptors located in the CA1 subregion of the dorsal hippocampus. CBD applied directly to this brain area was sufficient to reproduce the effects of systemic CBD treatment. Contextual fear memories attenuated by CBD did not show reinstatement, an extinction-related feature. By demonstrating that destabilized fear memories are sensitive to CBD and how it hinders mechanisms in the DH CA1 that may restabilize them in female rats, the present findings concur that reconsolidation blockers are viable and could be effective in disrupting abnormally persistent and distressing aversive memories such as those related to posttraumatic stress disorder.

Structural brain abnormalities in endothelial nitric oxide synthase-deficient mice revealed by high-resolution magnetic resonance imaging

INTRODUCTION

Endothelial nitric oxide synthase (eNOS) produces nitric oxide, which is essential for a variety of physiological functions in the brain. Previous work has demonstrated the detrimental effects of eNOS deficiency on brain function in male eNOS knockout (eNOS KO) mice. However, the effect of eNOS deficiency on brain structure and any association between these effects and sex is unknown.

METHODS

This study used three-dimensional high-resolution ex vivo magnetic resonance imaging and behavioral tests of anxiety and cognitive performance to investigate structure-function relationships in the brain of female and male eNOS KO mice in young adulthood.

RESULTS

While there were no differences in anxiety-like behavior or locomotion, there was a sex-specific deficit in contextual fear memory retention in male, but not in female, eNOS mice compared to wild-type controls. Moreover, we found that eNOS deficiency induced changes in multiple brain regions that are involved in learning and fear memory including the hippocampus, amygdala, hypothalamus, and areas of the cortex. Several of these MRI-detectable neuroanatomical changes were dependent on sex.

CONCLUSION

The observation that eNOS deficiency impacts brain structure at an early age demonstrates the importance of eNOS for healthy brain development.

Effects of 5.8 GHz microwave on hippocampal synaptic plasticity of rats

OBJECTIVE

5.8 GHz spectrum is gaining more attention in wireless technology. To explore the potential hazards, we investigated the effect of exposure to 5.8 GHz microwave on learning and memory ability of rats. Methods: Morris Water maze (MWM), Novel object recognition (NOR) and Fear conditioning test (FCT) were used to evaluate the ability of spatial and non-spatial memory of rats. The hippocampal morphology, the level of brain injury factors in serum and the mitochondrial membrane potential of hippocampal neurons was examined to evaluate the damage of hippocampal neurons. The density of dendritic spines, the ultrastructure of synapses and the level of PSD95, Synaptophysin, p-CREB and CREB were detected to evaluate the hippocampal synaptic plasticity.

RESULTS

Compared with Sham group, there was no significant difference in the performance of ethology (in MWM, NOR, FCT) in Microwave 2 h group or Microwave 4 h group. The hippocampal morphology, the serum level of brain injury factors and the content of mitochondrial JC-1 monomer in Microwave 2 h group or Microwave 4 h group did not change obviously, compared with Sham group. The density of dendritic spines, the ultrastructure of synapse and the level of PSD95, Synaptophysin, p-CREB and CREB in hippocampus in Microwave 2 h group or Microwave 4 h group did not significantly change, compared with Sham group.

CONCLUSION

Under this experimental condition, exposure to 5.8 GHz microwave could not affect the hippocampal synaptic plasticity of rats.

Reactivation of cocaine contextual memory engages mechanistic target of rapamycin/S6 kinase 1 signaling

Mechanistic target of rapamycin (mTOR) C1 and its downstream effectors have been implicated in synaptic plasticity and memory. Our prior work demonstrated that reactivation of cocaine memory engages a signaling pathway consisting of Akt, glycogen synthase kinase-3β (GSK3β), and mTORC1. The present study sought to identify other components of mTORC1 signaling involved in the reconsolidation of cocaine contextual memory, including eukaryotic translation initiation factor 4E (eIF4E)-eIF4G interactions, p70 S6 kinase polypeptide 1 (p70S6K, S6K1) activity, and activity-regulated cytoskeleton (Arc) expression. Cocaine contextual memory was established in adult CD-1 mice using conditioned place preference. After cocaine place preference was established, mice were briefly re-exposed to the cocaine-paired context to reactivate the cocaine memory and brains examined. Western blot analysis showed that phosphorylation of the mTORC1 target, p70S6K, in nucleus accumbens and hippocampus was enhanced 60 min following reactivation of cocaine memories. Inhibition of mTORC1 with systemic administration of rapamycin or inhibition of p70S6K with systemic PF-4708671 after reactivation of cocaine contextual memory abolished the established cocaine place preference. Immunoprecipitation assays showed that reactivation of cocaine memory did not affect eIF4E-eIF4G interactions in nucleus accumbens or hippocampus. Levels of Arc mRNA were significantly elevated 60 and 120 min after cocaine memory reactivation and returned to baseline 24 h later. These findings demonstrate that mTORC1 and p70S6K are required for reconsolidation of cocaine contextual memory.

Disrupting reconsolidation by PKA inhibitor in BLA reduces heroin-seeking behavior

Drug abuse is considered a maladaptive pathology of emotional memory and is associated with craving and relapse induced by drug-associated stimuli or drugs. Reconsolidation is an independent memory process with a strict time window followed by the reactivation of drug-associated stimulus depending on the basolateral amygdala (BLA). Pharmacology or behavior treatment that disrupts the reconsolidation can effectively attenuate drug-seeking in addicts. Here, we hypothesized that heroin-memory reconsolidation requires cAMP-dependent protein kinase A (PKA) of BLA based on the fundamental effect of PKA in synaptic plasticity and memory process. After 10 days of acquisition, the rats underwent 11 days of extinction training and then received the intra-BLA infusions of the PKA inhibitor Rp-cAMPS at different time windows with/without a reactivation session. The results show that PKA inhibitor treatment in the reconsolidation time window disrupts the reconsolidation and consequently reduces cue-induced reinstatement, heroin-induced reinstatement, and spontaneous recovery of heroin-seeking behavior in the rats. In contrast, there was no effect on cue-induced reinstatement in the intra-BLA infusion of PKA inhibitor 6 h after reactivation or without reactivation. These data suggest that PKA inhibition disrupts the reconsolidation of heroin-associated memory, reduces subsequent drug seeking, and prevents relapse, which is retrieval-dependent, time-limited, and BLA-dependent.

Contextual fear conditioning regulates synapse-related gene transcription in mouse microglia

Microglia have been suggested to be involved in the underlying mechanism of conditional fear memory formation by regulating inflammatory cytokines. However, the mechanism linking microglia and neuronal activity related to fear conditioning remains unclear. This study characterized the transcription profile of microglia in a fear memory conditional mouse model. Compared with those in control mice microglia, the most significantly induced genes were synapse-related, whereas immune-related genes were reduced due to fear memory consolidation. Whilst the increased expression of synapse-related genes was reversed after fear memory extinction, that of immunological genes was not, strongly suggesting a connection between microglia, neurons, and a dysregulated immune response following contextual fear conditioning. Furthermore, in the hippocampal microglia, we found that the expression of neurotransmitter release regulators, γ-aminobutyric acid (GABA) receptor GABRB3 and synapsin 1/2, increased under fear memory consolidation and restored (decreased) after extinction. In addition, compared with the transcription profile in peripheral monocytes, few overlapping genes were not enriched in biological processes. Taken together, the identified conditional fear stress-induced changes in mouse microglial transcription profiles suggest that microglia-neuron communication mediates contextual fear conditioning.

The evidence for and against reactivation-induced memory updating in humans and nonhuman animals

Systematic investigation of reactivation-induced memory updating began in the 1960s, and a wave of research in this area followed the seminal articulation of "reconsolidation" theory in the early 2000s. Myriad studies indicate that memory reactivation can cause previously consolidated memories to become labile and sensitive to weakening, strengthening, or other forms of modification. However, from its nascent period to the present, the field has been beset by inconsistencies in researchers' abilities to replicate seemingly established effects. Here we review these many studies, synthesizing the human and nonhuman animal literature, and suggest that these failures-to-replicate reflect a highly complex and delicately balanced memory modification system, the substrates of which must be finely tuned to enable adaptive memory updating while limiting maladaptive, inaccurate modifications. A systematic approach to the entire body of evidence, integrating positive and null findings, will yield a comprehensive understanding of the complex and dynamic nature of long-term memory storage and the potential for harnessing modification processes to treat mental disorders driven by pervasive maladaptive memories.

Selective δ-Opioid Receptor Agonist, KNT-127, Facilitates Contextual Fear Extinction via Infralimbic Cortex and Amygdala in Mice

Facilitation of fear extinction is a desirable action for the drugs to treat fear-related diseases, such as posttraumatic stress disorder (PTSD). We previously reported that a selective agonist of the δ-opioid receptor (DOP), KNT-127, facilitates contextual fear extinction in mice. However, its site of action in the brain and the underlying molecular mechanism remains unknown. Here, we investigated brain regions and cellular signaling pathways that may mediate the action of KNT-127 on fear extinction. Twenty-four hours after the fear conditioning, mice were reexposed to the conditioning chamber for 6 min as extinction training (reexposure 1). KNT-127 was microinjected into either the basolateral nucleus of the amygdala (BLA), hippocampus (HPC), prelimbic (PL), or infralimbic (IL) subregions of the medial prefrontal cortex, 30 min before reexposure 1. Next day, mice were reexposed to the chamber for 6 min as memory testing (reexposure 2). KNT-127 that infused into the BLA and IL, but not HPC or PL, significantly reduced the freezing response in reexposure 2 compared with those of control. The effect of KNT-127 administered into the BLA and IL was antagonized by pretreatment with a selective DOP antagonist. Further, the effect of KNT-127 was abolished by local administration of MEK/ERK inhibitor into the BLA, and PI3K/Akt inhibitor into the IL, respectively. These results suggested that the effect of KNT-127 was mediated by MEK/ERK signaling in the BLA, PI3K/Akt signaling in the IL, and DOPs in both brain regions. Here, we propose that DOPs play a role in fear extinction via distinct signaling pathways in the BLA and IL.

A role for deficits in GABAergic neurosteroids and their metabolites with NMDA receptor antagonist activity in the pathophysiology of posttraumatic stress disorder

Trauma-focused psychotherapies show general efficacy in post-traumatic stress disorder (PTSD), although outcomes vary substantially among individuals with PTSD and many patients do not achieve clinically meaningful symptom improvement. Several factors may contribute to poor treatment response, including genetic or environmental (e.g., stress) effects on neurobiological factors involved in learning and memory processes critical to PTSD recovery. In this review, we discuss the relationship between deficient GABAergic neurosteroid metabolites of progesterone, allopregnanolone (Allo) and pregnanolone (PA), and PTSD symptoms in men and women or PTSD-like behavioral abnormalities observed in male rodent models of PTSD. We also review the role and molecular underpinnings of learning and memory processes relevant to PTSD recovery, including extinction, extinction retention, reconsolidation of reactivated aversive memories and episodic non-aversive memory. We then discuss preclinical and clinical research that supports a role in these learning and memory processes for GABAergic neurosteroids and sulfated metabolites of Allo and PA that allosterically antagonize NMDA receptor function. Studies supporting the possible therapeutic impact of appropriately timed, acutely administered Allo or Allo analogs to facilitate extinction retention and/or block reconsolidation of aversive memories are also reviewed. Finally, we discuss important future directions for research in this area. Examining the varied and composite effects in PTSD of these metabolites of progesterone, as well as neuroactive derivatives of other parent steroids produced in the brain and the periphery, will likely enable a broadening of targets for treatment development. Defining contributions of these neuroactive steroids to common PTSD-comorbid psychiatric and medical conditions, as well as subpopulation-specific underlying dysfunctional physiological processes such as hypothalamic-pituitary-adrenal axis and immune system dysregulation, may also enable development of more effective multisystem precision medicines to prevent and treat the broader, polymorbid sequelae of extreme and chronic stress.

Pten haploinsufficiency causes desynchronized growth of brain areas involved in sensory processing

How changes in brain scaling relate to altered behavior is an important question in neurodevelopmental disorder research. Mice with germline Pten haploinsufficiency (Pten ^+/-) closely mirror the abnormal brain scaling and behavioral deficits seen in humans with macrocephaly/autism syndrome, which is caused by PTEN mutations. We explored whether deviation from normal patterns of growth can predict behavioral abnormalities. Brain regions associated with sensory processing (e.g., pons and inferior colliculus) had the biggest deviations from expected volume. While Pten ^+/- mice showed little or no abnormal behavior on most assays, both sexes showed sensory deficits, including impaired sensorimotor gating and hyporeactivity to high-intensity stimuli. Developmental analysis of this phenotype showed sexual dimorphism for hyporeactivity. Mapping behavioral phenotypes of Pten ^+/- mice onto relevant brain regions suggested abnormal behavior is likely when associated with relatively enlarged brain regions, while unchanged or relatively decreased brain regions have little predictive value.

Effects of hippocampal IP3R inhibition on contextual fear memory consolidation, retrieval, reconsolidation and extinction

Intracellular calcium stores (ICS) play a dynamic role in neuronal calcium (Ca²⁺) homeostasis both by buffering Ca²⁺ excess in the cytoplasm or providing an additional source of Ca²⁺ when concentration increase is needed. However, in spite of the large body of evidence showing Ca²⁺ as an essential second messenger in many signaling cascades underlying synaptic plasticity, the direct involvement of the intracellular Ca²⁺-release channels (ICRCs) in memory processing has been highly overlooked. Here we investigated the role of the ICRC inositol 1,4,5-trisphosphate receptor (IP₃R) activity during different memory phases using pharmacological inhibition in the dorsal hippocampus during contextual fear conditioning. We first found that post-training administration of the IP₃R antagonist 2-aminoethyl diphenylborinate (2-APB) impaired memory consolidation in a dose and time-dependent manner. Inhibiting IP₃Rs also disrupted memory retrieval. Contextual fear memory reconsolidation or extinction, however, were not sensitive to IP₃R blockade. Taken together, our results indicate that hippocampal IP₃Rs play an important role in contextual fear memory consolidation and retrieval.

Molecular motor KIF3B in the prelimbic cortex constrains the consolidation of contextual fear memory

Molecular and cellular mechanisms underlying the role of the prelimbic cortex in contextual fear memory remain elusive. Here we examined the kinesin family of molecular motor proteins (KIFs) in the prelimbic cortex for their role in mediating contextual fear, a form of associative memory. KIFs function as critical mediators of synaptic transmission and plasticity by their ability to modulate microtubule function and transport of gene products. However, the regulation and function of KIFs in the prelimbic cortex insofar as mediating memory consolidation is not known. We find that within one hour of contextual fear conditioning, the expression of KIF3B is upregulated in the prelimbic but not the infralimbic cortex. Importantly, lentiviral-mediated knockdown of KIF3B in the prelimbic cortex produces deficits in consolidation while reducing freezing behavior during extinction of contextual fear. We also find that the depletion of KIF3B increases spine density within prelimbic neurons. Taken together, these results illuminate a key role for KIF3B in the prelimbic cortex as far as mediating contextual fear memory.

Regulation of learned fear expression through the MgN-amygdala pathway

Heightened fear responding is characteristic of fear- and anxiety-related disorders, including post-traumatic stress disorder. Neural plasticity in the amygdala is essential for both initial fear learning and fear expression, and strengthening of synaptic connections between the medial geniculate nucleus (MgN) and amygdala is critical for auditory fear learning. However, very little is known about what happens in the MgN-amygdala pathway during fear recall and extinction, in which conditional fear decreases with repeated presentations of the auditory stimulus alone. In the present study, we found that optogenetic inhibition of activity in the MgN-amygdala pathway during fear retrieval and extinction reduced expression of conditional fear. While this effect persisted for at least two weeks following pathway inhibition, it was specific to the context in which optogenetic inhibition occurred, linking MgN-BLA inhibition to facilitation of extinction-like processes. Reduced fear expression through inhibition of the MgN-amygdala pathway was further characterized by similar synaptic expression of GluA1 and GluA2 AMPA receptor subunits compared to what was seen in controls. Inhibition also decreased CREB phosphorylation in the amygdala, similar to what has been reported following auditory fear extinction. We then demonstrated that this effect was reduced by inhibition of GluN2B-containing NMDA receptors. These results demonstrate a new and important role for the MgN-amygdala pathway in extinction-like processes, and show that suppressing activity in this pathway results in a persistent decrease in fear behavior.

Memory destabilization during reconsolidation: a consequence of homeostatic plasticity?

Remembering is not a static process: When retrieved, a memory can be destabilized and become prone to modifications. This phenomenon has been demonstrated in a number of brain regions, but the neuronal mechanisms that rule memory destabilization and its boundary conditions remain elusive. Using two distinct computational models that combine Hebbian plasticity and synaptic downscaling, we show that homeostatic plasticity can function as a destabilization mechanism, accounting for behavioral results of protein synthesis inhibition upon reactivation with different re-exposure times. Furthermore, by performing systematic reviews, we identify a series of overlapping molecular mechanisms between memory destabilization and synaptic downscaling, although direct experimental links between both phenomena remain scarce. In light of these results, we propose a theoretical framework where memory destabilization can emerge as an epiphenomenon of homeostatic adaptations prompted by memory retrieval.

Cannabidiol and the corticoraphe circuit in post-traumatic stress disorder

Post-Traumatic Stress Disorder (PTSD), characterized by re-experiencing, avoidance, negative affect, and impaired memory processing, may develop after traumatic events. PTSD is complicated by impaired plasticity and medial prefrontal cortex (mPFC) activity, hyperactivity of the amygdala, and impaired fear extinction. Cannabidiol (CBD) is a promising candidate for treatment due to its multimodal action that enhances plasticity and calms hyperexcitability. CBD’s mechanism in the mPFC of PTSD patients has been explored extensively, but literature on the mechanism in the dorsal raphe nucleus (DRN) is lacking. Following the PRISMA guidelines, we examined current literature regarding CBD in PTSD and overlapping symptomologies to propose a mechanism by which CBD treats PTSD via corticoraphe circuit. Acute CBD inhibits excess 5-HT release from DRN to amygdala and releases anandamide (AEA) onto amygdala inputs. By first reducing amygdala and DRN hyperactivity, CBD begins to ameliorate activity disparity between mPFC and amygdala. Chronic CBD recruits the mPFC, creating harmonious corticoraphe signaling. DRN releases enough 5-HT to ameliorate mPFC hypoactivity, while the mPFC continuously excites DRN 5-HT neurons via glutamate. Meanwhile, AEA regulates corticoraphe activity to stabilize signaling. AEA prevents DRN GABAergic interneurons from inhibiting 5-HT release so the DRN can assist the mPFC in overcoming its hypoactivity. DRN-mediated restoration of mPFC activity underlies CBD’s mechanism on fear extinction and learning of stress coping.

•

CBD reduces PTSD symptoms via the DRN and corticoraphe circuit.

•

Acute effects of CBD reduce DRN-amygdala excitatory signaling to lessen the activity disparity between amygdala and mPFC.

•

Chronic CBD officially resolves mPFC hypoactivity by facilitating 5-HT release from DRN to mPFC.

•

CBD-facilitated endocannabinoid signaling stabilizes DRN activity and restores mPFC inhibitory control.

•

Chronically administered CBD acts via the corticoraphe circuit to favor fear extinction over fear memory reconsolidation.

Transcriptomic signature of extinction learning in the brain of the fire-bellied toad, Bombina orientalis

Insight into the molecular and cellular mechanisms of learning and memory from a diverse array of taxa contributes to our understanding of the evolution of these processes. The fire-bellied toad, Bombina orientalis, is a basal anuran amphibian model species who could help us describe shared and divergent characteristics of learning and memory mechanisms between amphibians and other vertebrates, and hence answer questions about the evolution of learning. Utilizing next generation sequencing techniques, we profiled gene expression patterns associated with the extinction of prey-catching conditioning in the brain of the fire-bellied toad. For this purpose, gene expression was at first compared between toads sacrificed after acquisition and extinction of the conditioned response. A second comparison was done between toads submitted to extinction following either short or long acquisition training, which results in toads displaying response extinction or resistance to extinction, respectively. We analyzed brain tissue transcription profiles common to both acquisition and extinction learning, or unique to extinction learning and resistance to extinction, and found significant overlap in gene expression related to molecular pathways involving neuronal plasticity (e.g. structural modification, transcription). However, extinction learning induced a unique GABAergic transcriptomic signal, which may be responsible for suppression of the original response memory. Further, when comparing extinction learning in short- and long-trained groups, short training engaged many pathways related to neuronal plasticity, as expected, but long training engaged molecular pathways related to the suppression of learning through epigenetic mediated transcriptional suppression and inhibitory neurotransmission. Overall, gene expression patterns associated with extinction learning in the fire-bellied toad were similar to those found in mammals submitted to extinction, although some divergent profiles highlighted potential differences in the mechanisms of learning and memory among tetrapods.

c-Abl regulates a synaptic plasticity-related transcriptional program involved in memory and learning

Memory consolidation requires activation of a gene expression program that allows de novo protein synthesis. But the molecular mechanisms that favour or restrict that program are poorly understood. The kinase c-Abl can modulate gene expression through transcription factors and chromatin modifiers. Here, we show that c-Abl ablation in the brain improves learning acquisition and memory consolidation in mice. Its absence also affects gene expression profiles in the mouse hippocampus. We found that genes involved in synaptic plasticity and actin cytoskeleton dynamics, such as Arp2 and Thorase, are up-regulated at the mRNA and protein levels in trained c-Abl KO mice and by a chemical-LTP stimulus. Trained c-Abl KO mice also show that dendritic spines are larger than in wild-type mice and present at a higher density. These results indicate that c-Abl kinase is an important part of the mechanism that limits or restricts signalling of relevant gene programs involved in morphological and functional spine changes upon neuronal stimulation.

Tumor necrosis factor α negatively regulates the retrieval and reconsolidation of hippocampus-dependent memory

Neural inflammation is associated with cognitive decline, especially learning and memory. Tumor necrosis factor α (TNFα) is a major cytokine generated during neuroinflammation. Previous studies indicated that TNFα impairs hippocampus-dependent memory including contextual fear and spatial memories. However, it is unknown which memory processes are impaired by TNFα. Here, we show that TNFα blocked the retrieval and reconsolidation of contextual fear and spatial memories. Micro-infusion of TNFα into the dorsal hippocampus at 6-18 h before retrieval impaired the retrieval of contextual fear memory, although micro-infusion before contextual fear conditioning had no effect on memory formation. Interestingly, hippocampal TNFα micro-infusion before memory retrieval decreased freezing responses, even at 24 h after retrieval, suggesting that TNFα impairs the reconsolidation of contextual fear memory. Similarly, hippocampal TNFα micro-infusion impaired the retrieval and reconsolidation of spatial memory in the Morris water maze. Consistent with these observations, hippocampal TNFα micro-infusion before retrieval blocked the induction of c-fos expression in the hippocampus, which is a marker of neural activation, in response to the retrieval of contextual fear memory. Collectively, our findings indicate that TNFα negatively regulates the retrieval and reconsolidation of hippocampus-dependent memory.

Engram Size Varies with Learning and Reflects Memory Content and Precision

Memories are rarely acquired under ideal conditions, rendering them vulnerable to profound omissions, errors, and ambiguities. Consistent with this, recent work using context fear conditioning has shown that memories formed after inadequate learning time display a variety of maladaptive properties, including overgeneralization to similar contexts. However, the neuronal basis of such poor learning and memory imprecision remains unknown. Using c-fos to track neuronal activity in male mice, we examined how these learning-dependent changes in context fear memory precision are encoded in hippocampal ensembles. We found that the total number of c-fos-encoding cells did not correspond with learning history but instead more closely reflected the length of the session immediately preceding c-fos measurement. However, using a c-fos-driven tagging method (TRAP2 mouse line), we found that the degree of learning and memory specificity corresponded with neuronal activity in a subset of dentate gyrus cells that were active during both learning and recall. Comprehensive memories acquired after longer learning intervals were associated with more double-labeled cells. These were preferentially reactivated in the conditioning context compared with a similar context, paralleling behavioral discrimination. Conversely, impoverished memories acquired after shorter learning intervals were associated with fewer double-labeled cells. These were reactivated equally in both contexts, corresponding with overgeneralization. Together, these findings provide two surprising conclusions. First, engram size varies with learning. Second, larger engrams support better neuronal and behavioral discrimination. These findings are incorporated into a model that describes how neuronal activity is influenced by previous learning and present experience, thus driving behavior.

SIGNIFICANCE STATEMENT Memories are not always formed under ideal circumstances. This is especially true in traumatic situations, such as car accidents, where individuals have insufficient time to process what happened around them. Such memories have the potential to overgeneralize to irrelevant situations, producing inappropriate fear and contributing to disorders, such as post-traumatic stress disorder. However, it is unknown how such poorly formed fear memories are encoded within the brain. We find that restricting learning time results in fear memories that are encoded by fewer hippocampal cells. Moreover, these fewer cells are inappropriately reactivated in both dangerous and safe contexts. These findings suggest that fear memories formed at brief periods overgeneralize because they lack the detail-rich information necessary to support neuronal discrimination.

GluN2B and GluN2A-containing NMDAR are differentially involved in extinction memory destabilization and restabilization during reconsolidation

Extinction memory destabilized by recall is restabilized through mTOR-dependent reconsolidation in the hippocampus, but the upstream pathways controlling these processes remain unknown. Hippocampal NMDARs drive local protein synthesis via mTOR signaling and may control active memory maintenance. We found that in adult male Wistar rats, intra dorsal-CA1 administration of the non-subunit selective NMDAR antagonist AP5 or of the GluN2A subunit-containing NMDAR antagonist TCN201 after step down inhibitory avoidance (SDIA) extinction memory recall impaired extinction memory retention and caused SDIA memory recovery. On the contrary, pre-recall administration of AP5 or of the GluN2B subunit-containing NMDAR antagonist RO25-6981 had no effect on extinction memory recall or retention per se but hindered the recovery of the avoidance response induced by post-recall intra-CA1 infusion of the mTOR inhibitor rapamycin. Our results indicate that GluN2B-containing NMDARs are necessary for extinction memory destabilization whereas GluN2A-containing NMDARs are involved in its restabilization, and suggest that pharmacological modulation of the relative activation state of these receptor subtypes around the moment of extinction memory recall may regulate the dominance of extinction memory over the original memory trace.

Active Transition of Fear Memory Phase from Reconsolidation to Extinction through ERK-Mediated Prevention of Reconsolidation

The retrieval of fear memory induces two opposite memory process, i.e., reconsolidation and extinction. Brief retrieval induces reconsolidation to maintain or enhance fear memory, while prolonged retrieval extinguishes this memory. Although the mechanisms of reconsolidation and extinction have been investigated, it remains unknown how fear memory phases are switched from reconsolidation to extinction during memory retrieval. Here, we show that an extracellular signal-regulated kinase (ERK)-dependent memory transition process after retrieval regulates the switch of memory phases from reconsolidation to extinction by preventing induction of reconsolidation in an inhibitory avoidance (IA) task in male mice. First, the transition memory phase, which cancels the induction of reconsolidation, but is insufficient for the acquisition of extinction, was identified after reconsolidation, but before extinction phases. Second, the reconsolidation, transition, and extinction phases after memory retrieval showed distinct molecular and cellular signatures through cAMP responsive element binding protein (CREB) and ERK phosphorylation in the amygdala, hippocampus, and medial prefrontal cortex (mPFC). The reconsolidation phase showed increased CREB phosphorylation, while the extinction phase displayed several neural populations with various combinations of CREB and/or ERK phosphorylation, in these brain regions. Interestingly, the three memory phases, including the transition phase, showed transient ERK activation immediately after retrieval. Most importantly, the blockade of ERK in the amygdala, hippocampus, or mPFC at the transition memory phase disinhibited reconsolidation-induced enhancement of IA memory. These observations suggest that the ERK-signaling pathway actively regulates the transition of memory phase from reconsolidation to extinction and this process functions as a switch that cancels reconsolidation of fear memory.SIGNIFICANCE STATEMENT Retrieval of fear memory induces two opposite memory process; reconsolidation and extinction. Reconsolidation maintains/enhances fear memory, while extinction weakens fear memory. It remains unknown how memory phases are switched from reconsolidation to extinction during retrieval. Here, we identified an active memory transition process functioning as a switch that inhibits reconsolidation. This memory transition phase showed a transient increase of extracellular signal-regulated kinase (ERK) phosphorylation in the amygdala, hippocampus and medial prefrontal cortex (mPFC). Interestingly, inhibition of ERK in these regions at the transition phase disinhibited the reconsolidation-mediated enhancement of inhibitory avoidance (IA) memory. These findings suggest that the transition memory process actively regulates the switch of fear memory phases of fear memory by preventing induction of reconsolidation through the activation of the ERK-signaling pathway.

DNA Double-Strand Breaks Are a Critical Regulator of Fear Memory Reconsolidation

Numerous studies have shown that following retrieval, a previously consolidated memory requires increased transcriptional regulation in order to be reconsolidated. Previously, it was reported that histone H3 lysine-4 trimethylation (H3K4me3), a marker of active transcription, is increased in the hippocampus after the retrieval of contextual fear memory. However, it is currently unknown how this epigenetic mark is regulated during the reconsolidation process. Furthermore, though recent evidence suggests that neuronal activity triggers DNA double-strand breaks (DSBs) in some early-response genes, it is currently unknown if DSBs contribute to the reconsolidation of a memory following retrieval. Here, using chromatin immunoprecipitation (ChIP) analyses, we report a significant overlap between DSBs and H3K4me3 in area CA1 of the hippocampus during the reconsolidation process. We found an increase in phosphorylation of histone H2A.X at serine 139 (H2A.XpS139), a marker of DSB, in the Npas4, but not c-fos, promoter region 5 min after retrieval, which correlated with increased H3K4me3 levels, suggesting that the two epigenetic marks may work in concert during the reconsolidation process. Consistent with this, in vivo siRNA-mediated knockdown of topoisomerase II β, the enzyme responsible for DSB, prior to retrieval, reduced Npas4 promoter-specific H2A.XpS139 and H3K4me3 levels and impaired long-term memory, indicating an indispensable role of DSBs in the memory reconsolidation process. Collectively, our data propose a novel mechanism for memory reconsolidation through increases in epigenetic-mediated transcriptional control via DNA double-strand breaks.