Disruption of fear memory consolidation and reconsolidation by actin filament arrest in the basolateral amygdala

Methionine oxidation of actin cytoskeleton attenuates traumatic memory retention via reactivating dendritic spine morphogenesis

Post-traumatic stress disorder (PTSD) is characterized by hypermnesia of the trauma and a persistent fear response. The molecular mechanisms underlying the retention of traumatic memories remain largely unknown, which hinders the development of more effective treatments. Utilizing auditory fear conditioning, we demonstrate that a redox-dependent dynamic pathway for dendritic spine morphogenesis in the basolateral amygdala (BLA) is crucial for traumatic memory retention. Exposure to a fear-induced event markedly increased the reduction of oxidized filamentous actin (F-actin) and decreased the expression of the molecule interacting with CasL 1 (MICAL1), a methionine-oxidizing enzyme that directly oxidizes and depolymerizes F-actin, leading to cytoskeletal dynamic abnormalities in the BLA, which impairs dendritic spine morphogenesis and contributes to the persistence of fearful memories. Following fear conditioning, overexpression of MICAL1 in the BLA inhibited freezing behavior during fear memory retrieval via reactivating cytokinesis, whereas overexpression of methionine sulfoxide reductase B 1, a key enzyme that reduces oxidized F-actin monomer, increased freezing behavior during retrieval. Notably, intra-BLA injection of semaphorin 3A, an endogenous activator of MICAL1, rapidly disrupted fear memory within a short time window after conditioning. Collectively, our results indicate that redox modulation of actin cytoskeleton in the BLA is functionally linked to fear memory retention and PTSD-like memory.

Mitigation of synaptic and memory impairments via F-actin stabilization in Alzheimer's disease

BACKGROUND

Synaptic dysfunction, characterized by synapse loss and structural alterations, emerges as a prominent correlate of cognitive decline in Alzheimer's disease (AD). Actin cytoskeleton, which serves as the structural backbone of synaptic architecture, is observed to be lost from synapses in AD. Actin cytoskeleton loss compromises synaptic integrity, affecting glutamatergic receptor levels, neurotransmission, and synaptic strength. Understanding these molecular changes is crucial for developing interventions targeting synaptic dysfunction, potentially mitigating cognitive decline in AD.

METHODS

In this study, we investigated the synaptic actin interactome using mass spectrometry in a mouse model of AD, APP/PS1. Our objective was to explore how alterations in synaptic actin dynamics, particularly the interaction between PSD-95 and actin, contribute to synaptic and cognitive impairment in AD. To assess the impact of restoring F-actin levels on synaptic and cognitive functions in APP/PS1 mice, we administered F-actin stabilizing agent, jasplakinolide. Behavioral deficits in the mice were evaluated using the contextual fear conditioning paradigm. We utilized primary neuronal cultures to study the synaptic levels of AMPA and NMDA receptors and the dynamics of PSD-95 actin association. Furthermore, we analyzed postmortem brain tissue samples from subjects with no cognitive impairment (NCI), mild cognitive impairment (MCI), and Alzheimer's dementia (AD) to determine the association between PSD-95 and actin.

RESULTS

We found a significant reduction in PSD-95-actin association in synaptosomes from middle-aged APP/PS1 mice compared to wild-type (WT) mice. Treatment with jasplakinolide, an actin stabilizer, reversed deficits in memory recall, restored PSD-95-actin association, and increased synaptic F-actin levels in APP/PS1 mice. Additionally, actin stabilization led to elevated synaptic levels of AMPA and NMDA receptors, enhanced dendritic spine density, suggesting improved neurotransmission and synaptic strength in primary cortical neurons from APP/PS1 mice. Furthermore, analysis of postmortem human tissue with NCI, MCI and AD subjects revealed disrupted PSD-95-actin interactions, underscoring the clinical relevance of our preclinical studies.

CONCLUSION

Our study elucidates disrupted PSD-95 actin interactions across different models, highlighting potential therapeutic targets for AD. Stabilizing F-actin restores synaptic integrity and ameliorates cognitive deficits in APP/PS1 mice, suggesting that targeting synaptic actin regulation could be a promising therapeutic strategy to mitigate cognitive decline in AD.

NCK1 Modulates Neuronal Actin Dynamics and Promotes Dendritic Spine, Synapse, and Memory Formation

Memory formation and maintenance is a dynamic process involving the modulation of the actin cytoskeleton at synapses. Understanding the signaling pathways that contribute to actin modulation is important for our understanding of synapse formation and function, as well as learning and memory. Here, we focused on the importance of the actin regulator, noncatalytic region of tyrosine kinase adaptor protein 1 (NCK1), in hippocampal dependent behaviors and development. We report that male mice lacking NCK1 have impairments in both short-term and working memory, as well as spatial learning. Additionally, we report sex differences in memory impairment showing that female mice deficient in NCK1 fail at reversal learning in a spatial learning task. We find that NCK1 is expressed in postmitotic neurons but is dispensable for neuronal proliferation and migration in the developing hippocampus. Morphologically, NCK1 is not necessary for overall neuronal dendrite development. However, neurons lacking NCK1 have lower dendritic spine and synapse densities in vitro and in vivo EM analysis reveal increased postsynaptic density (PSD) thickness in the hippocampal CA1 region of NCK1-deficient mice. Mechanistically, we find the turnover of actin-filaments in dendritic spines is accelerated in neurons that lack NCK1. Together, these findings suggest that NCK1 contributes to hippocampal-dependent memory by stabilizing actin dynamics and dendritic spine formation.SIGNIFICANCE STATEMENT Understanding the molecular signaling pathways that contribute to memory formation, maintenance, and elimination will lead to a better understanding of the genetic influences on cognition and cognitive disorders and will direct future therapeutics. Here, we report that the noncatalytic region of tyrosine kinase adaptor protein 1 (NCK1) adaptor protein modulates actin-filament turnover in hippocampal dendritic spines. Mice lacking NCK1 show sex-dependent deficits in hippocampal memory formation tasks, have altered postsynaptic densities, and reduced synaptic density. Together, our work implicates NCK1 in the regulation of actin cytoskeleton dynamics and normal synapse development which is essential for memory formation.

EphrinA4 mimetic peptide impairs fear conditioning memory reconsolidation in lateral amygdala

Fear memory may undergo a process after memory reactivation called reconsolidation. To examine the roles of ephrinA4 in fear memory reconsolidation an inhibitory ephrinA4 mimetic peptide (pep-ephrinA4), that targets the EphA binding site and inhibits EphA activation, was used. Pep-ephrinA4 was microinjected into the lateral amygdala (LA) of fear-conditioned rats 24 h after training and 30 min before tone CS memory retrieval. Memory retrieval was unaffected by pep-ephrinA4. However, the animals were impaired in fear memory tested 1 h or 24 h afterward when compared to controls. Fear-conditioned animals injected with pep-ephrinA4 into LA immediately after long-term memory retrieval were unaffected when tested 24 h afterward. Microinjection into LA of a peptide originated from an ephrinA4 site that does not interact with EphA did not affect fear memory reconsolidation. Rats that were administrated with pep-ephrinA4 systemically 24 h after fear conditioning and 30 min before CS memory retrieval were impaired in long-term fear conditioning memory tested 24 h afterward when compared to the control peptide. These results show that ephrinA4 binding sites are needed for long-term fear memory reconsolidation in LA and may serve as a target for the treatment of fear-related disorders by blocking reconsolidation.

Transgenic modeling of Ndr2 gene amplification reveals disturbance of hippocampus circuitry and function

Duplications and deletions of short chromosomal fragments are increasingly recognized as the cause for rare neurodevelopmental conditions and disorders. The NDR2 gene encodes a protein kinase important for neuronal development and is part of a microduplication region on chromosome 12 that is associated with intellectual disabilities, autism, and epilepsy. We developed a conditional transgenic mouse with increased Ndr2 expression in postmigratory forebrain neurons to study the consequences of an increased gene dosage of this Hippo pathway kinase on brain circuitry and cognitive functions. Our analysis reveals reduced terminal fields and synaptic transmission of hippocampal mossy fibers, altered hippocampal network activity, and deficits in mossy fiber-dependent behaviors. Reduced doublecortin expression and protein interactome analysis indicate that transgenic Ndr2 disturbs the maturation of granule cells in the dentate gyrus. Together, our data suggest that increased expression of Ndr2 may critically contribute to the development of intellectual disabilities upon gene amplification.

Impairments to Consolidation, Reconsolidation, and Long-Term Memory Maintenance Lead to Memory Erasure

An enduring problem in neuroscience is determining whether cases of amnesia result from eradication of the memory trace (storage impairment) or if the trace is present but inaccessible (retrieval impairment). The most direct approach to resolving this question is to quantify changes in the brain mechanisms of long-term memory (BM-LTM). This approach argues that if the amnesia is due to a retrieval failure, BM-LTM should remain at levels comparable to trained, unimpaired animals. Conversely, if memories are erased, BM-LTM should be reduced to resemble untrained levels. Here we review the use of BM-LTM in a number of studies that induced amnesia by targeting memory maintenance or reconsolidation. The literature strongly suggests that such amnesia is due to storage rather than retrieval impairments. We also describe the shortcomings of the purely behavioral protocol that purports to show recovery from amnesia as a method of understanding the nature of amnesia.

Methamphetamine Learning Induces Persistent and Selective Nonmuscle Myosin II-Dependent Spine Motility in the Basolateral Amygdala

Nonmuscle myosin II inhibition (NMIIi) in the basolateral amygdala (BLA), but not dorsal hippocampus (CA1), selectively disrupts memories associated with methamphetamine (METH) days after learning, without retrieval. However, the molecular mechanisms underlying this selective vulnerability remain poorly understood. A known function of NMII is to transiently activate synaptic actin dynamics with learning. Therefore, we hypothesized that METH-associated learning perpetuates NMII-driven actin dynamics in synapses, leading to an extended window of vulnerability for memory disruption. We used time-lapse two-photon imaging of dendritic spine motility in acutely prepared brain slices from female and male mice following METH-associated learning as a readout of actin-myosin dynamics. Spine motility was persistently increased in the BLA, but not in CA1. Consistent with the memory disrupting effect of intra-BLA NMII inhibition, METH-induced changes to BLA spine dynamics were reversed by a single systemic injection of an NMII inhibitor. Intra-CA1 NMII inhibition, on the other hand, did not disrupt METH-associated memory. Thus, we report identification of a previously unknown ability for spine actin dynamics to persist days after stimulation and that this is under the control of NMII. Further, these perpetual NMII-driven spine actin dynamics in BLA neurons may contribute to the unique susceptibility of METH-associated memories.SIGNIFICANCE STATEMENT There are no Food and Drug Administration-approved pharmacotherapies to prevent relapse to the use of stimulants, such as methamphetamine (METH). Environmental cues become associated with drug use, such that the memories can elicit strong motivation to seek the drug during abstinence. We previously reported that the storage of METH-associated memories is uniquely vulnerable to immediate, retrieval-independent, and lasting disruption by direct actin depolymerization or by inhibiting the actin driver nonmuscle myosin II (NMII) in the BLA or systemically. Here we report a potential structural mechanism responsible for the unique vulnerability of METH-associated memories and METH-seeking behavior to NMII inhibition within the BLA.

Long-term memory is maintained by continuous activity of Arp2/3 in lateral amygdala

Evidence indicates that long-term memory formation involves alterations in synaptic efficacy produced by modifications in neural transmission and morphology. However, it is not clear how such changes induced by learning, that encode memory, are maintained over long period of time to preserve long-term memory. It has been shown that the actin nucleating protein Arp2/3 is essential for supporting neuronal morphology and synaptic transmission. We therefore hypothesized that continuous Arp2/3 activity is needed to maintain long-term memory over time. To test this hypothesis we microinjected into lateral amygdala (LA) of rats CK-666, a specific inhibitor of Arp2/3, two days after fear conditioning and tested the effect on long-term fear memory maintenance a day afterward. We found that injection of CK-666 two days after training abolished fear conditioning memory. Fear conditioning could be formed when a control compound CK-689 was applied two days after training. Microinjection of CK-666 a day before fear conditioning training had no effect on fear conditioning learning and long-term memory formation. We revealed that Arp2/3 is also needed to maintain long-term conditioned taste aversion (CTA) memory in LA. Microinjection of CK-666 two days after CTA training impaired long-term memory tested a day afterwards. We conclude that continuous activity of Arp2/3 in LA is essential for the maintenance of long-term memory.

Resilience and Vulnerability to Trauma: Early Life Interventions Modulate Aversive Memory Reconsolidation in the Dorsal Hippocampus

Early life experiences program lifelong responses to stress. In agreement, resilience and vulnerability to psychopathologies, such as posttraumatic stress disorder (PTSD), have been suggested to depend on the early background. New therapies have targeted memory reconsolidation as a strategy to modify the emotional valence of traumatic memories. Here, we used animal models to study the molecular mechanism through which early experiences may later affect aversive memory reconsolidation. Handling (H)—separation of pups from dams for 10 min—or maternal separation (MS) — 3-h separation—were performed from PDN1–10, using non-handled (NH) litters as controls. Adult males were trained in a contextual fear conditioning (CFC) task; 24 h later, a short reactivation session was conducted in the conditioned or in a novel context, followed by administration of midazolam 3 mg/kg i.p. (mdz), known to disturb reconsolidation, or vehicle; a test session was performed 24 h after. The immunocontent of relevant proteins was studied 15 and 60 min after memory reactivation in the dorsal hippocampus (dHc) and basolateral amygdala complex (BLA). Mdz-treated controls (NH) showed decreased freezing to the conditioned context, consistent with reconsolidation impairment, but H and MS were resistant to labilization. Additionally, MS males showed increased freezing to the novel context, suggesting fear generalization; H rats showed lower freezing than the other groups, in accordance with previous suggestions of reduced emotionality facing adversities. Increased levels of Zif268, GluN2B, β-actin and polyubiquitination found in the BLA of all groups suggest that memory reconsolidation was triggered. In the dHc, only NH showed increased Zif268 levels after memory retrieval; also, a delay in ERK1/2 activation was found in H and MS animals. We showed here that reconsolidation of a contextual fear memory is insensitive to interference by a GABAergic drug in adult male rats exposed to different neonatal experiences; surprisingly, we found no differences in the reconsolidation process in the BLA, but the dHc appears to suffer temporal desynchronization in the engagement of reconsolidation. Our results support a hippocampal-dependent mechanism for reconsolidation resistance in models of early experiences, which aligns with current hypotheses for the etiology of PTSD.

The role of nonmuscle myosin II in polydrug memories and memory reconsolidation

Using pharmacologic and genetic approaches targeting actin or the actin-driving molecular motor, nonmuscle myosin II (NMII), we previously discovered an immediate, retrieval-independent, and long-lasting disruption of methamphetamine- (METH-) and amphetamine-associated memories. A single intrabasolateral amygdala complex infusion or systemic administration of the NMII inhibitor Blebbistatin (Blebb) is sufficient to produce this disruption, which is selective, having no retrieval-independent effect on memories for fear, food reward, cocaine, or morphine. However, it was unclear if Blebb treatment would disrupt memories of other stimulants and amphetamine class drugs, such as nicotine (NIC) or mephedrone (MEPH; bath salts). Moreover, many individuals abuse multiple drugs, but it was unknown if Blebb could disrupt polydrug memories, or if the inclusion of another substance would render Blebb no longer able to disrupt METH-associated memories. Therefore, the present study had two primary goals: (1) to determine the ability of Blebb to disrupt NIC- or MEPH-associated memories, and (2) to determine the ability of METH to modify other unconditioned stimulus (US) associations’ susceptibility to Blebb. To this end, using the conditional place preference model, mice were conditioned to NIC and MEPH alone or METH in combination with NIC, morphine, or foot shock. We report that, unlike METH, there was no retrieval-independent effect of Blebb on NIC- or MEPH-associated memories. However, similar to cocaine, reconsolidation of the memory for both drugs was disrupted. Further, when combined with METH administration, NIC- and morphine-, but not fear-, associated memories were rendered susceptible to disruption by Blebb. Given the high rate of polydrug use and the resurgence of METH use, these results have important implications for the treatment of substance use disorder.

The Role of Actin Cytoskeleton in Dendritic Spines in the Maintenance of Long-Term Memory

Evidence indicates that long-term memory formation involves alterations in synaptic efficacy produced by modifications in neural transmission and morphology. However, it is not clear how such alterations induced by learning, that encode memory, are maintained over long period of time to preserve long-term memory. This is especially intriguing as the half-life of most of the proteins that underlie such changes is usually in the range of hours to days and these proteins may change their location over time. In this review we describe studies that indicate the involvement of dendritic spines in memory formation and its maintenance. These studies show that learning leads to changes in the number and morphology of spines. Disruption in spines morphology or manipulations that lead to alteration in their number after consolidation are associated with impairment in memory maintenance. We further ask how changes in dendritic spines morphology, induced by learning and reputed to encode memory, are maintained to preserve long-term memory. We propose a mechanism, based on studies described in the review, whereby the actin cytoskeleton and its regulatory proteins involved in the initial alteration in spine morphology induced by learning are also essential for spine structural stabilization that maintains long-term memory. In this model glutamate receptors and other synaptic receptors activation during learning leads to the creation of new actin cytoskeletal scaffold leading to changes in spines morphology and memory formation. This new actin cytoskeletal scaffold is preserved beyond actin and its regulatory proteins turnover and dynamics by active stabilization of the level and activity of actin regulatory proteins within these memory spines.

Memory Reconsolidation

Scientific advances in the last decades uncovered that memory is not a stable, fixed entity. Apparently stable memories may become transiently labile and susceptible to modifications when retrieved due to the process of reconsolidation. Here, we review the initial evidence and the logic on which reconsolidation theory is based, the wide range of conditions in which it has been reported and recent findings further revealing the fascinating nature of this process. Special focus is given to conceptual issues of when and why reconsolidation happen and its possible outcomes. Last, we discuss the potential clinical implications of memory modifications by reconsolidation.

Affecting long-term fear memory formation through optical control of Rac1 GTPase and PAK activity in lateral amygdala

Fear conditioning, a behavioral model for studying fear-related disorders, is believed to be formed by alterations of synaptic efficacy mediated by changes in synaptic transmission and neuronal morphology in lateral amygdala (LA). Rac GTPase and its downstream effector p21-activated kinase (PAK) are involved in such key neuronal functions. Here we show that optical activation of Rac1 GTPase using photoactivatable form of Rac1 (PA-Rac1) in amygdala led to phosphorylation of PAK and inhibition of long-term but not short-term auditory fear conditioning memory formation. Activation of PA-Rac1 in LA one day after fear conditioning had no effect on long-term fear memory tested 24 hrs after PA-Rac1 activation. Inhibition of PAK in LA by microinjection of the PAK inhibitor IPA-3 30 minutes before fear conditioning enhanced long-term but not short-term fear memory formation. Our results demonstrate that photoactivation of Rac1 GTPase in lateral amygdala impairs fear memory formation. Moreover, Rac1 effector PAK activity during fear conditioning constrains the formation of fear memory in LA. Thus, Rac GTPase and PAK proteins may serve as targets for treatment of fear and anxiety disorders.

MicroRNA-mediated disruption of dendritogenesis during a critical period of development influences cognitive capacity later in life

The prenatal period of cortical development is important for the establishment of neural circuitry and functional connectivity of the brain; however, the molecular mechanisms underlying this process remain unclear. Here we report that disruption of the actin-cytoskeletal network in the developing mouse prefrontal cortex alters dendritic morphogenesis and synapse formation, leading to enhanced formation of fear-related memory in adulthood. These effects are mediated by a brain-enriched microRNA, miR-9, through its negative regulation of diaphanous homologous protein 1 (Diap1), a key organizer of the actin cytoskeletal assembly. Our findings not only revealed important regulation of dendritogenesis and synaptogenesis during early brain development but also demonstrated a tight link between these early developmental events and cognitive functions later in life.

Try to Remember: Interplay between Memory and Substance Use Disorder

Memories associated with substance use disorders, or substance-associated cues increase the likelihood of craving and relapse during abstinence. There is a growing consensus that manipulation of synaptic plasticity may reduce the strength of substance abuse-related memories. On the biological front, there are new insights that suggest memories associated with substance use disorder may follow unique neurobiological pathways that render them more accessible to pharmacological intervention. In parallel to this, research in neurochemistry has identified several potential candidate molecules that could influence the formation and maintenance of long-term memory. Drugs that target these molecules (blebbistatin, isradipine and zeta inhibitory peptide) have shown promise at the preclinical stage. In this review, we shall provide an overview of the evolving understanding on the biochemical mechanisms involved in memory formation and expound on the premise that substance use disorder is a learning disorder.

Memory disrupting effects of nonmuscle myosin II inhibition depend on the class of abused drug and brain region

Depolymerizing actin in the amygdala through nonmuscle myosin II inhibition (NMIIi) produces a selective, lasting, and retrieval-independent disruption of the storage of methamphetamine-associated memories. Here we report a similar disruption of memories associated with amphetamine, but not cocaine or morphine, by NMIIi. Reconsolidation appeared to be disrupted with cocaine. Unlike in the amygdala, methamphetamine-associated memory storage was not disrupted by NMIIi in the hippocampus, nucleus accumbens, or orbitofrontal cortex. NMIIi in the hippocampus did appear to disrupt reconsolidation. Identification of the unique mechanisms responsible for NMII-mediated, amygdala-dependent disruption of memory storage associated with the amphetamine class may enable induction of retrieval-independent vulnerability to other pathological memories.

Arp2/3 and VASP Are Essential for Fear Memory Formation in Lateral Amygdala

The actin cytoskeleton is involved in key neuronal functions such as synaptic transmission and morphogenesis. However, the roles and regulation of actin cytoskeleton in memory formation remain to be clarified. In this study, we unveil the mechanism whereby actin cytoskeleton is regulated to form memory by exploring the roles of the major actin-regulatory proteins Arp2/3, VASP, and formins in long-term memory formation. Inhibition of Arp2/3, involved in actin filament branching and neuronal morphogenesis, in lateral amygdala (LA) with the specific inhibitor CK-666 during fear conditioning impaired long-term, but not short-term, fear memory. The inactive isomer CK-689 had no effect on memory formation. We observed that Arp2/3 is colocalized with the actin-regulatory protein profilin in LA neurons of fear-conditioned rats. VASP binding to profilin is needed for profilin-mediated stabilization of actin cytoskeleton and dendritic spine morphology. Microinjection of poly-proline peptide [G(GP₅)₃] into LA, to interfere with VASP binding to profilin, impaired long-term but not short-term fear memory formation. Control peptide [G(GA₅)₃] had no effect. Inhibiting formins, which regulate linear actin elongation, in LA during fear conditioning by microinjecting the formin-specific inhibitor SMIFH2 into LA had no effect on long-term fear memory formation. We conclude that Arp2/3 and VASP, through the profilin binding site, are essential for the formation of long-term fear memory in LA and propose a model whereby these proteins subserve cellular events, leading to memory consolidation.

Nonmuscle myosin IIB as a therapeutic target for the prevention of relapse to methamphetamine use

Memories associated with drug use increase vulnerability to relapse in substance use disorder (SUD), and there are no pharmacotherapies for the prevention of relapse. Previously, we reported a promising finding that storage of memories associated with methamphetamine (METH), but not memories for fear or food reward, is vulnerable to disruption by actin depolymerization in the basolateral amygdala complex (BLC). However, actin is not a viable therapeutic target because of its numerous functions throughout the body. Here we report the discovery of a viable therapeutic target, nonmuscle myosin IIB (NMIIB), a molecular motor that supports memory by directly driving synaptic actin polymerization. A single intra-BLC treatment with Blebbistatin (Blebb), a small-molecule inhibitor of class II myosin isoforms, including NMIIB, produced a long-lasting disruption of context-induced drug seeking (at least 30 days). Further, postconsolidation genetic knockdown of Myh10, the heavy chain of the most highly expressed NMII in the BLC, was sufficient to produce METH-associated memory loss. Blebb was found to be highly brain penetrant. A single systemic injection of the compound selectively disrupted the storage of METH-associated memory and reversed the accompanying increase in BLC spine density. This effect was specific to METH-associated memory, as it had no effect on an auditory fear memory. The effect was also independent of retrieval, as METH-associated memory was disrupted 24 h after a single systemic injection of Blebb delivered in the home cage. Together, these results argue for the further development of small-molecule inhibitors of NMII as potential therapeutics for the prevention of SUD relapse triggered by drug associations.

The Role of Actin Cytoskeleton in Memory Formation in Amygdala

The central, lateral and basolateral amygdala (BLA) nuclei are essential for the formation of long-term memories including emotional and drug-related memories. Studying cellular and molecular mechanisms of memory in amygdala may lead to better understanding of how memory is formed and of fear and addiction-related disorders. A challenge is to identify molecules activated by learning that subserve cellular changes needed for memory formation and maintenance in amygdala. Recent studies show that activation of synaptic receptors during fear and drug-related learning leads to alteration in actin cytoskeleton dynamics and structure in amygdala. Such changes in actin cytoskeleton in amygdala are essential for fear and drug-related memories formation. Moreover, the actin cytoskeleton subserves, after learning, changes in neuronal morphogenesis and glutamate receptors trafficking in amygdala. These cellular events are involved in fear and drug-related memories formation. Actin polymerization is also needed for the maintenance of drug-associated memories in amygdala. Thus, the actin cytoskeleton is a key mediator between receptor activation during learning and cellular changes subserving long-term memory (LTM) in amygdala. The actin cytoskeleton may serve as a target for pharmacological treatment of fear memory associated with fear and anxiety disorders and drug addiction to prevent the debilitating consequences of these diseases.

Inhibition of actin polymerization in the NAc shell inhibits morphine-induced CPP by disrupting its reconsolidation

Drug-associated contextual cues contribute to drug craving and relapse after abstinence, which is a major challenge to drug addiction treatment. Previous studies showed that disrupting memory reconsolidation impairs drug reward memory. However, the underlying mechanisms remain elusive. Although actin polymerization is involved in memory formation, its role in the reconsolidation of drug reward memory is unknown. In addition, the specific brain areas responsible for drug memory have not been fully identified. In the present study, we found that inhibiting actin polymerization in the nucleus accumbens (NAc) shell, but not the NAc core, abolishes morphine-induced conditioned place preference (CPP) by disrupting its reconsolidation in rats. Moreover, this effect persists for more than 2 weeks by a single injection of the actin polymerization inhibitor, which is not reversed by a morphine-priming injection. Furthermore, the application of actin polymerization inhibitor outside the reconsolidation window has no effect on morphine-associated contextual memory. Taken together, our findings first demonstrate that inhibiting actin polymerization erases morphine-induced CPP by disrupting its reconsolidation. Our study suggests that inhibition of actin polymerization during drug memory reconsolidation may be a potential approach to prevent drug relapse.

Post-training activation of Rac1 in the basolateral amygdala is required for the formation of both short-term and long-term auditory fear memory

Rac1, a member of the Rho family of small GTPases, is crucial for morphological changes of the mature neuronal synapse including spine formation and activity-dependent spine enlargement, while its role in the formation of associated memories, such as conditioned fear memory, is not clear. Here, we report that selective deletion of Rac1 in excitatory neurons, but not in parvalbumin inhibitory neurons, impaired short- and long-term memories (STM and LTM) of fear conditioning. Conditional knockout of Rac1 before associative fear training in the basolateral amygdala (BLA), a key area for fear memory acquisition and storage, impaired fear memory. The expression of dominant-negative mutant of Rac1, or infusion of Rac1 inhibitor NSC23766 into BLA blocked both STM and LTM of fear conditioning. Furthermore, selective inhibition of Rac1 activation in BLA immediately following fear conditioning impaired STM and LTM, demonstrating that fear conditioning-induced Rac1 activation in BLA plays a critical role in the formation of both STM and LTM of conditioned fear.

Neuroepigenetic Regulation of Pathogenic Memories

Our unique collection of memories determines our individuality and shapes our future interactions with the world. Remarkable advances into the neurobiological basis of memory have identified key epigenetic mechanisms that support the stability of memory. Various forms of epigenetic regulation at the levels of DNA methylation, histone modification, and non-coding RNAs (ncRNAs) can modulate transcriptional and translational events required for memory processes. By changing the cellular profile in the brain's emotional, reward, and memory circuits, these epigenetic modifications have also been linked to perseverant, pathogenic memories. In this review, we will delve into the relevance of epigenetic dysregulation to pathogenic memory mechanisms by focusing on two neuropsychiatric disorders perpetuated by aberrant memory associations: substance use disorder (SUD) and post-traumatic stress disorder (PTSD). As our understanding improves, neuroepigenetic mechanisms may someday be harnessed to develop novel therapeutic targets for the treatment of these chronic, relapsing disorders.

Protein synthesis and consolidation of memory-related synaptic changes

Although sometimes disputed, it has been assumed for several decades that new proteins synthesized following a learning event are required for consolidation of subsequent memory. Published findings and new results described here challenge this idea. Protein synthesis inhibitors did not prevent Theta Bust Stimulation (TBS) from producing extremely stable long-term potentiation (LTP) in experiments using standard hippocampal slice protocols. However, the inhibitors were effective under conditions that likely depleted protein levels prior to attempts to induce the potentiation effect. Experiments showed that induction of LTP at one input, and thus a prior episode of protein synthesis, eliminated the effects of inhibitors on potentiation of a second input even in depleted slices. These observations suggest that a primary role of translation and transcription processes initiated by learning events is to prepare neurons to support future learning. Other work has provided support for an alternative theory of consolidation. Specifically, if the synaptic changes that support memory are to endure, learning events/TBS must engage a complex set of signaling processes that reorganize and re-stabilize the spine actin cytoskeleton. This is accomplished in fast (10 min) and slow (50 min) stages with the first requiring integrin activation and the second a recovery of integrin functioning. These results align with, and provide mechanisms for, the long-held view that memories are established and consolidated over a set of temporally distinct phases. This article is part of a Special Issue entitled SI: Brain and Memory.

EphrinA4 mimetic peptide targeted to EphA binding site impairs the formation of long-term fear memory in lateral amygdala

Fear conditioning leads to long-term fear memory formation and is a model for studying fear-related psychopathologies conditions such as phobias and posttraumatic stress disorder. Long-term fear memory formation is believed to involve alterations of synaptic efficacy mediated by changes in synaptic transmission and morphology in lateral amygdala (LA). EphrinA4 and its cognate Eph receptors are intimately involved in regulating neuronal morphogenesis, synaptic transmission and plasticity. To assess possible roles of ephrinA4 in fear memory formation we designed and used a specific inhibitory ephrinA4 mimetic peptide (pep-ephrinA4) targeted to EphA binding site. We show that this peptide, composed of the ephrinA4 binding domain, interacts with EphA4 and inhibits ephrinA4-induced phosphorylation of EphA4. Microinjection of the pep-ephrinA4 into rat LA 30 min before training impaired long- but not short-term fear conditioning memory. Microinjection of a control peptide derived from a nonbinding E helix site of ephrinA4, that does not interact with EphA, had no effect on fear memory formation. Microinjection of pep-ephrinA4 into areas adjacent to the amygdala had no effect on fear memory. Acute systemic administration of pep-ephrinA4 1 h after training also impaired long-term fear conditioning memory formation. These results demonstrate that ephrinA4 binding sites in LA are essential for long-term fear memory formation. Moreover, our research shows that ephrinA4 binding sites may serve as a target for pharmacological treatment of fear and anxiety disorders.

GAD65 haplodeficiency conveys resilience in animal models of stress-induced psychopathology

GABAergic mechanisms are critically involved in the control of fear and anxiety, but their role in the development of stress-induced psychopathologies, including post-traumatic stress disorder (PTSD) and mood disorders is not sufficiently understood. We studied these functions in two established mouse models of risk factors for stress-induced psychopathologies employing variable juvenile stress and/or social isolation. A battery of emotional tests in adulthood revealed the induction of contextually generalized fear, anxiety, hyperarousal and depression-like symptoms in these paradigms. These reflect the multitude and complexity of stress effects in human PTSD patients. With factor analysis we were able to identify parameters that reflect these different behavioral domains in stressed animals and thus provide a basis for an integrated scoring of affectedness more closely resembling the clinical situation than isolated parameters. To test the applicability of these models to genetic approaches we further tested the role of GABA using heterozygous mice with targeted mutation of the GABA synthesizing enzyme GAD65 [GAD65(+/−) mice], which show a delayed postnatal increase in tissue GABA content in limbic and cortical brain areas. Unexpectedly, GAD65(+/−) mice did not show changes in exploratory activity regardless of the stressor type and were after the variable juvenile stress procedure protected from the development of contextual generalization in an auditory fear conditioning experiment. Our data demonstrate the complex nature of behavioral alterations in rodent models of stress-related psychopathologies and suggest that GAD65 haplodeficiency, likely through its effect on the postnatal maturation of GABAergic transmission, conveys resilience to some of these stress-induced effects.

Selective, retrieval-independent disruption of methamphetamine-associated memory by actin depolymerization

BACKGROUND

Memories associated with drugs of abuse, such as methamphetamine (METH), increase relapse vulnerability to substance use disorder. There is a growing consensus that memory is supported by structural and functional plasticity driven by F-actin polymerization in postsynaptic dendritic spines at excitatory synapses. However, the mechanisms responsible for the long-term maintenance of memories, after consolidation has occurred, are largely unknown.

METHODS

Conditioned place preference (n = 112) and context-induced reinstatement of self-administration (n = 19) were used to assess the role of F-actin polymerization and myosin II, a molecular motor that drives memory-promoting dendritic spine actin polymerization, in the maintenance of METH-associated memories and related structural plasticity.

RESULTS

Memories formed through association with METH but not associations with foot shock or food reward were disrupted by a highly-specific actin cycling inhibitor when infused into the amygdala during the postconsolidation maintenance phase. This selective effect of depolymerization on METH-associated memory was immediate, persistent, and did not depend upon retrieval or strength of the association. Inhibition of non-muscle myosin II also resulted in a disruption of METH-associated memory.

CONCLUSIONS

Thus, drug-associated memories seem to be actively maintained by a unique form of cycling F-actin driven by myosin II. This finding provides a potential therapeutic approach for the selective treatment of unwanted memories associated with psychiatric disorders that is both selective and does not rely on retrieval of the memory. The results further suggest that memory maintenance depends upon the preservation of polymerized actin.

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.

Long-lasting increase of corticosterone after fear memory reactivation: anxiolytic effects and network activity modulation in the ventral hippocampus

Pathological fear and anxiety can be studied, in rodents, with fear conditioning and exposure to reminder cues. These paradigms are thought to critically involve the ventral hippocampus, which also serves as key site of glucocorticoid action in the brain. Here, we demonstrate a long-lasting reduction of kainate-induced gamma oscillations in slice preparations of the ventral hippocampal area CA3, 30 days after a single fear conditioning training. Reduction of gamma power was sensitive to corticosterone application and associated with a decrease in glucocorticoid and mineralocorticoid receptor mRNA expression across strata of the ventral hippocampal CA3. A fear reactivation session 24 h after the initial conditioning normalized receptor expression levels and attenuated the corticosterone-mediated recovery of gamma oscillations. It moreover increased both baseline and stimulus-induced corticosterone plasma levels and evoked a generalization of fear memory to the background context. Reduced ventral hippocampal gamma oscillation in both fear reactivated and non-reactivated mice were associated with a decrease of anxiety-like behavior in an elevated plus maze. Taking advantage of the circadian fluctuation in corticosterone, we demonstrated the association of high endogenous basal corticosterone plasma concentrations during morning hours with reduced anxiety-like behavior in fear reactivated mice. The anxiolytic effect of the hormone was verified with local applications to the ventral hippocampus. Our data suggest that corticosterone acting on ventral hippocampal network activity has anxiolytic-like effects following fear exposure, highlighting its potential therapeutic value for anxiety disorders.

MicroRNA-182 regulates amygdala-dependent memory formation

De novo protein synthesis supports long-lasting functional and structural plasticity and is a molecular requirement for new memory formation. Recent evidence has suggested that microRNAs may be involved in regulating the molecular mechanisms underlying neural plasticity. MicroRNAs are endogenous, noncoding RNAs capable of post-transcriptional repression of their mRNA targets. To explore the potential for microRNA-mediated regulation of amygdala-dependent memory formation, we performed expression profiling of microRNAs in the lateral amygdala of rats 1 h after auditory fear conditioning. Microarray analysis revealed that over half of all known microRNAs are endogenously expressed in the lateral amygdala, with 7 microRNAs upregulated and 32 downregulated by auditory fear training. Bioinformatic analysis identified several of the downregulated microRNAs as potential repressors of actin-regulating proteins known to be involved in plasticity and memory. Downregulation of one of these microRNAs by auditory fear conditioning, miR-182, was confirmed by quantitative real-time PCR. Overexpression of miR-182 within the lateral amygdala resulted in decreased expression of the protein but not mRNA of two synapse-enriched regulators of actin known to modulate structural plasticity, cortactin and Rac1. The overexpression of miR-182 also disrupted long-term but not short-term auditory fear memory. These data indicate that learning-induced suppression of miR-182, a microRNA previously uncharacterized in the brain, supports long-term memory formation in the amygdala and suggests it does so, at least in part, through the derepression of key actin-regulating proteins. These findings further indicate that microRNAs may represent a previously underappreciated mechanism for regulating protein synthesis during memory consolidation.

D-cycloserine in the basolateral amygdala prevents extinction and enhances reconsolidation of odor-reward associative learning in rats

It is well established that D-cycloserine (DCS), a partial agonist of the NMDA receptor glycine site, enhances learning and memory processes. Although the effects of DCS have been especially elucidated in the extinction and reconsolidation of aversive behavioral paradigms or drug-related behaviors, they have not been clearly determined in appetitive tasks using natural reinforcers. The current study examined the effects of pre-retrieval intra-basolateral amygdala (BLA) infusions of DCS on the extinction and reconsolidation of an appetitive odor discrimination task. Rats were trained to discriminate between three odors, one of which was associated with a palatable food reward, and, 20 min prior to extinction learning (experiment 1) or reactivation (experiment 2), they received bilateral intra-BLA infusions of DCS or vehicle. In experiment 1, DCS infusion reduced the rate of extinction learning, weakened extinction retention in a post-extinction test and enhanced reacquisition of the ODT task. In experiment 2, DCS improved subsequent memory expression in the reconsolidation test performed one day after the reactivation session. Such results indicate the involvement of BLA NMDA receptors in odor-food reward associative memory and suggest that DCS may potentiate the persistence or strength of the original memory trace.

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.

Modulation of fear memory by retrieval and extinction: a clue for memory deconsolidation

Memories are fragile and easily forgotten at first, but after a consolidation period of hours to weeks, are inscribed in our brains as stable traces, no longer vulnerable to conventional amnesic treatments. Retrieval of a memory renders it labile, akin to the early stages of consolidation. This phenomenon has been explored as memory reactivation, in the sense that the memory is temporarily 'deconsolidated', allowing a short time window for amnesic intervention. This window closes again after reconsolidation, which restores the stability of the memory. In contrast to this 'transient deconsolidation' and the short-spanned amnesic effects of consolidation blockers, some specific treatments can disrupt even consolidated memory, leading to apparent amnesia. We propose the term 'amnesic deconsolidation' to describe such processes that lead to disruption of consolidated memory and/or consolidated memory traces. We review studies of these 'amnesic deconsolidation' treatments that enhance memory extinction, alleviate relapse, and reverse learning-induced plasticity. The transient deconsolidation that memory retrieval induces and the amnesic deconsolidation that these regimes induce both seem to dislodge a component that stabilizes consolidated memory. Characterizing this component, at both molecular and network levels, will provide a key to developing clinical treatments for memory-related disorders and to defining the consolidated memory trace.

The roles of the actin cytoskeleton in fear memory formation

The formation and storage of fear memory is needed to adapt behavior and avoid danger during subsequent fearful events. However, fear memory may also play a significant role in stress and anxiety disorders. When fear becomes disproportionate to that necessary to cope with a given stimulus, or begins to occur in inappropriate situations, a fear or anxiety disorder exists. Thus, the study of cellular and molecular mechanisms underpinning fear memory may shed light on the formation of memory and on anxiety and stress related disorders. Evidence indicates that fear learning leads to changes in neuronal synaptic transmission and morphology in brain areas underlying fear memory formation including the amygdala and hippocampus. The actin cytoskeleton has been shown to participate in these key neuronal processes. Recent findings show that the actin cytoskeleton is needed for fear memory formation and extinction. Moreover, the actin cytoskeleton is involved in synaptic plasticity and in neuronal morphogenesis in brain areas that mediate fear memory. The actin cytoskeleton may therefore mediate between synaptic transmission during fear learning and long-term cellular alterations mandatory for fear memory formation.