Extinction of remotely acquired fear depends on an inhibitory NR2B/PKA pathway in the retrosplenial cortex

Modulation of contextual fear acquisition and extinction by acute and chronic relaxin-3 receptor (RXFP3) activation in the rat retrosplenial cortex

The retrosplenial cortex (RSC) plays a central role in processing contextual fear conditioning. In addition to corticocortical and thalamocortical projections, the RSC receives subcortical inputs, including a substantial projection from the nucleus incertus in the pontine tegmentum. This GABAergic projection contains the neuropeptide, relaxin-3 (RLN3), which inhibits target neurons via its Gi/o-protein-coupled receptor, RXFP3. To assess this peptidergic system role in contextual fear conditioning, we bilaterally injected the RSC of adult rats with an adeno-associated-virus (AAV), expressing the chimeric RXFP3 agonist R3/I5 or a control AAV, and subjected them to contextual fear conditioning. The R3/I5 injected rats did not display any major differences to control-injected and naïve rats but displayed a significantly delayed extinction. Subsequently, we employed acute bilateral injections of the specific RXFP3 agonist peptide, RXFP3-Analogue 2 (A2), into RSC. While the administration of A2 before each extinction trial had no impact on the extinction process, treatment with A2 before each acquisition trial resulted in delayed extinction. In related anatomical studies, we detected an enrichment of RLN3-immunoreactive nerve fibers in deep layers of the RSC, and a higher level of co-localization of RXFP3 mRNA with vesicular GABA transporter (vGAT) mRNA than with vesicular glutamate transporter-1 (vGLUT1) mRNA across the RSC, consistent with an effect of RLN3/RXFP3 signalling on the intrinsic, inhibitory circuits within the RSC. These findings suggest that contextual conditioning processes in the RSC involve, in part, RLN3 afferent modulation of local inhibitory neurons that provides a stronger memory acquisition which, in turn, retards the extinction process.

Fear extinction is impaired in aged rats

Normal aging is accompanied by broad loss of cognitive function in humans and rodents, including declines in cognitive flexibility. In extinction, a conditional stimulus (CS) that was previously paired with a footshock is presented alone. This procedure reliably reduces conditional freezing behavior in young adult rats. Here, we aimed to investigate how normal aging affects extinction learning. Using young (3 months) and aged (20 months) male and female Long Evans rats, we compared extinction (using 20 CS-alone presentations) to a no extinction control (equal exposure to the conditioning chamber without CS presentations) following delay fear conditioning. We found that young animals in the extinction group showed a decrease in freezing following extinction; aged animals did not. We next examined changes in neural activity using expression of the immediate early gene zif268. In young animals, extinction corresponded with decreased expression of zif268 in the basolateral amygdala and anterior retrosplenial cortex; this was not observed in aged animals. Further, aged animals showed increased zif268 expression in each region examined, suggesting that dysfunction in neural activity precedes cognitive deficits. These results demonstrate that aging impacts both extinction learning and neural activity.

Update on neurobiological mechanisms of fear: illuminating the direction of mechanism exploration and treatment development of trauma and fear-related disorders

Fear refers to an adaptive response in the face of danger, and the formed fear memory acts as a warning when the individual faces a dangerous situation again, which is of great significance to the survival of humans and animals. Excessive fear response caused by abnormal fear memory can lead to neuropsychiatric disorders. Fear memory has been studied for a long time, which is of a certain guiding effect on the treatment of fear-related disorders. With continuous technological innovations, the study of fear has gradually shifted from the level of brain regions to deeper neural (micro) circuits between brain regions and even within single brain regions, as well as molecular mechanisms. This article briefly outlines the basic knowledge of fear memory and reviews the neurobiological mechanisms of fear extinction and relapse, which aims to provide new insights for future basic research on fear emotions and new ideas for treating trauma and fear-related disorders.

Acute stress persistently alters instrumental motivation without affecting appetitive Pavlovian conditioning, extinction, or contextual renewal

In two experiments, we adapted the stress-enhanced fear learning approach to evaluate the persistent effects of acute stress on appetitive learning and motivation in adult male Long Evans rats. In Experiment 1, we found that exposure to a battery of footshocks in one context had no effect on the acquisition, extinction, or contextual renewal of an appetitive Pavlovian discrimination in different contexts. However, when rats were subsequently trained to respond on a progressive ratio instrumental schedule, rats with a history of shock showed lower response rates and progressive ratio break points. Extinction of the shock-associated context had little effect on progressive ratio responding. In Experiment 2, we replicated this instrumental responding deficit with a continuous reinforcement schedule when the Pavlovian phases did not intervene in the time between shock and instrumental testing. Our findings here demonstrate that highly stressful acute experiences produce long-lasting deficits in instrumental motivation for food in male rats.

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.

Stress-induced generalization of negative memories is mediated by an extended hippocampal circuit

Memories of negative experiences exert important control of behavior in the face of actual or anticipated threat. Sometimes, however, this control extends to non-threatening situations, a phenomenon known as overgeneralization of negative memories. Overgeneralization is a reliable cognitive phenotype of major depressive disorder, generalized anxiety disorder, and post-traumatic stress disorder. We therefore sought to develop an animal model to study stress-induced generalization of negative memories (SIG) and determine its dependence on the episodic-like memory circuit. We found that male and female mice, which were trained to differentiate a threatening from neutral context, exhibited robust SIG in response to subsequent social stress. Using chemogenetic circuit manipulations during memory retrieval, we demonstrated that both excitatory afferents to the dorsal hippocampus (DH) from the ventral tegmental area (VTA), and excitatory efferents from the DH to the retrosplenial cortex (RSC) contribute to SIG. Based on the known roles of these projections, we suggest that (1) by targeting subcortical VTA circuits that provide valence signals to the DH, stress prioritizes the retrieval of negative over neutral memories, and (2) by forwarding such information to the RSC, stress engages cortical mechanisms that support the retrieval of general relative to specific memory features. Altogether, these results suggest that various components of the extended hippocampal circuit can serve as treatment targets for memory overgeneralization.

Contributions of the rodent cingulate-retrosplenial cortical axis to associative learning and memory: A proposed circuit for persistent memory maintenance

While the anterior cingulate (ACC) and retrosplenial (RSC) cortices have been extensively studied for their role in spatial navigation, less is known about how they contribute to associative learning and later memory recall. The limited work that has been conducted on this topic suggests that each of these cortical regions makes distinct, but similar contributions to associative learning and memory. Here, we review evidence from the rodent literature demonstrating that while ACC activity seems to be necessary at remote time points associated with imprecise or generalized memories, the role of the RSC seems to be uniform over time. Together, the lines of evidence reviewed here suggest that the ACC and RSC likely function together to support memory formation and maintenance following associative learning.

Switching From Fear to No Fear by Different Neural Ensembles in Mouse Retrosplenial Cortex

Fear extinction is generally considered a form of new learning that inhibits previously acquired fear memories. Here, by tracking immediate early gene expression in vivo, we found that contextual fear extinction training evoked distinct neural ensembles in mouse retrosplenial cortex (RSC). The optogenetic reactivation of these extinction-activated neurons in the RSC was sufficient to suppress a fear response, while the reactivation of conditioning-activated neurons in the same area promoted a fear response. The generation of such an extinction-memory-related neural ensemble was associated with adult neurogenesis, as abolishing newborn neurons in the adult hippocampus via X-ray irradiation eliminated both the extinction-activated neurons in the RSC and the optogenetic-reactivation-induced suppression of contextual fear memory. Therefore, switching from fear to no fear in response to the same context is modulated by the RSC through an extinction-activated neural ensemble, the generation of which might require adult neurogenesis in the hippocampus.

Anterior retrosplenial cortex is required for long-term object recognition memory

The retrosplenial cortex (RSC) is implicated on navigation and contextual memory. Lesions studies showed that the RSC shares functional similarities with the hippocampus (HP). Here we evaluated the role of the anterior RSC (aRSC) in the “what” and “where” components of recognition memory and contrasted it with that of the dorsal HP (dHP). Our behavioral and molecular findings show functional differences between the aRSC and the dHP in recognition memory. The inactivation of the aRSC, but not the dHP, impairs the consolidation and expression of the “what” memory component. In addition, object recognition task is accompanied by c-Fos levels increase in the aRSC. Interestingly, we found that the aRSC is recruited to process the “what” memory component only if it is active during acquisition. In contrast, both the aRSC and dHP are required for encoding the “where” component, which correlates with c-Fos levels increase. Our findings introduce a novel role of the aRSC in recognition memory, processing not only the “where”, but also the “what” memory component.

Know safety, no fear

Every day we are bombarded by stimuli that must be assessed for their potential for harm or benefit. Once a stimulus is learned to predict harm, it can elicit fear responses. Such learning can last a lifetime but is not always beneficial for an organism. For an organism to thrive in its environment, it must know when to engage in defensive, avoidance behaviors and when to engage in non-defensive, approach behaviors. Fear should be suppressed in situations that are not dangerous: when a novel, innocuous stimulus resembles a feared stimulus, when a feared stimulus no longer predicts harm, or when there is an option to avoid harm. A cardinal feature of anxiety disorders is the inability to suppress fear adaptively. In PTSD, for instance, learned fear is expressed inappropriately in safe situations and is resistant to extinction. In this review, we discuss mechanisms of suppressing fear responses during stimulus discrimination, fear extinction, and active avoidance, focusing on the well-studied tripartite circuit consisting of the amygdala, medial prefrontal cortex and hippocampus.

Sleep Deprivation Disrupts Acquisition of Contextual Fear Extinction by Affecting Circadian Oscillation of Hippocampal-Infralimbic proBDNF

Extensive evidence showed that mature brain-derived neurotrophic factor (mBDNF) levels displayed a circadian pattern. Circadian disruption, for example, sleep deprivation (SD), induced functional and behavioral deficits. However, compared with that of mature form, the biological role of the pro-peptide, proBDNF, was poorly understood. Here, we found that proBDNF was expressed under circadian rhythm in the ventral hippocampus (vHPC). SD rats exhibited deficits in acquisition of conditioned extinction and damped rhythmicity in vHPC proBDNF activity that were accompanied by SD between zeitgeber time (ZT)0 and ZT4, but not the late stage of sleep period. Furthermore, SD affected fear extinction through vHPC-IL proBDNF signaling, which was associated with NR2B subunits of NMDA receptors. More importantly, infusion of proBDNF could mitigate SD-induced abnormal neural activity, by suppressing the enhanced basal firing rate of IL-RS and elevating the depressed neural response that evoked by acquisition of conditioned extinction. Therefore, this finding provided the first evidence that circadian oscillation of vHPC proBDNF activity contributed to the effects of SD on acquisition of conditioned fear extinction, and suggested a new therapeutic target to reverse the cognitive deficits in sleep-related mental disorder, such as post-traumatic stress disorder (PTSD).

Fear-context association during memory retrieval requires input from granular to dysgranular retrosplenial cortex

The contribution of the granular (area 29, A29) and dysgranular (area 30, A30) subdivisions of the retrosplenial cortex (RSC) to contextual fear memory (CFM) retrieval remains elusive. Here, intact and orchiectomized (ORC) male rats received an intraperitoneal (I.P.) injection of saline (control) or 5 mg/Kg MK801 after training and memory formation. In ORC, but not in intact males, this MK801 treatment selectively induces overt loss of neurons in layers IV-Va of A29 (A29^MK801 neurons) (Sigwald et al., 2016). Compared to ORC-saline, ORC-MK801 rats showed impaired CFM retrieval in an A-B-A design for contextual fear conditioning (CFC), however context recognition was not affected. In ORC-MK801 rats, neither novel object recognition nor object-in-context discrimination were impaired, further indicating that A29^MK801 neurons are not required for contextual recognition. Elevated plus maze test showed that anxiety-like behavior was not affected in ORC-MK801 animals, suggesting that loss of A29^MK801 neurons does not affect the emotional state that could impair freezing during test. Importantly, in a sensory preconditioning test, higher order CFM retrieval was abolished in ORC-MK801, but not in male-MK801. Collectively, these observations indicate that A29^MK801 neurons are critically required for retrieving fear-context association. For dissecting the anatomofunctional contribution of A29^MK801 neurons to CFM retrieval, expression of c-Fos and Egr-1 was used to map brain-wide neuronal activity. In control male rats CFC and CFM retrieval was associated with significant enhancement of both proteins in limbic structures and A30, but not in A29, suggesting that neurons in A30 and limbic structures encode and store the associative experience. Notably, in ORC but not in intact males, MK801 impairs CFM retrieval and expression of c-Fos and Egr-1 proteins in A30, without affecting their expression in limbic structures. Thus, the loss of A29^MK801 neurons after CFM formation precludes activation of associative neurons in A30, impairing CFM recall. FluoroGold retrograde track-tracing confirmed that A29^MK801 neurons project to A30. Silver staining provide evidence that MK801 in ORC rats induces axonal deafferentation of A29^MK801 neuron in A30. Collectively, our experiments provide the first evidence that A30 neurons participate in encoding and storing CFM while A29 is required for their activation during recall.

Distinct hippocampal engrams control extinction and relapse of fear memory

Learned fear often relapses after extinction, suggesting that extinction training generates a new memory that coexists with the original fear memory; however, the mechanisms governing expression of competing fear and extinction memories remain unclear. We used activity-dependent neural tagging to investigate representations of fear and extinction memories in the dentate gyrus (DG). We demonstrate that extinction training suppresses reactivation of context fear engram cells, while activating a second ensemble, a putative extinction engram. Optogenetic inhibition of neurons that were active during extinction training increased fear after extinction training, whereas silencing neurons that were active during fear training reduced spontaneous recovery of fear. Optogenetic stimulation of fear acquisition neurons increased fear, while stimulation of extinction neurons suppressed fear and prevented spontaneous recovery. Our results indicate the hippocampus generates a fear extinction representation and that interactions between hippocampal fear and extinction representations govern suppression and relapse of fear after extinction.

A cFos activation map of remote fear memory attenuation

RATIONALE

The experience of strong traumata leads to the formation of enduring fear memories that may degenerate into post-traumatic stress disorder. One of the most successful treatments for this condition consists of extinction training during which the repeated exposure to trauma-inducing stimuli in a safe environment results in an attenuation of the fearful component of trauma-related memories. While numerous studies have investigated the neural substrates of recent (e.g., 1-day-old) fear memory attenuation, much less is known about the neural networks mediating the attenuation of remote (e.g., 30-day-old) fear memories. Since extinction training becomes less effective when applied long after the original encoding of the traumatic memory, this represents an important gap in memory research.

OBJECTIVES

Here, we aimed to generate a comprehensive map of brain activation upon effective remote fear memory attenuation in the mouse.

METHODS

We developed an efficient extinction training paradigm for 1-month-old contextual fear memory attenuation and performed cFos immunohistochemistry and network connectivity analyses on a set of cortical, amygdalar, thalamic, and hippocampal regions.

RESULTS

Remote fear memory attenuation induced cFos in the prelimbic cortex, the basolateral amygdala, the nucleus reuniens of the thalamus, and the ventral fields of the hippocampal CA1 and CA3. All these structures were equally recruited by remote fear memory recall, but not by the recall of a familiar neutral context.

CONCLUSION

These results suggest that progressive fear attenuation mediated by repetitive exposure is accompanied by sustained neuronal activation and not reverted to a pre-conditioning brain state. These findings contribute to the identification of brain areas as targets for therapeutic approaches against traumatic memories.

N-Methyl D-aspartate receptor subunit signaling in fear extinction

N-Methyl D-aspartate receptors (NMDAR) are central mediators of glutamate actions underlying learning and memory processes including those required for extinction of fear and fear-related behaviors. Consistent with this view, in animal models, antagonists of NMDAR typically impair fear extinction, whereas partial agonists have facilitating effects. Promoting NMDAR function has thus been recognized as a promising strategy towards reduction of fear symptoms in patients suffering from anxiety disorders and post-traumatic disorder (PTSD). Nevertheless, application of these drugs in clinical trials has proved of limited utility. Here we summarize recent advances in our knowledge of NMDAR pharmacology relevant for fear extinction, focusing on molecular, cellular, and circuit aspects of NMDAR function as they relate to fear extinction at the level of behavior and cognition. We also discuss how these advances from animal models might help to understand and overcome the limitations of existing approaches in human anxiety disorders and how novel, more specific, and personalized approaches might help advance future therapeutic strategies.

Role of retrosplenial cortex in processing stress-related context memories

This work summarizes evidence for the role of RSC in processing fear-inducing context memories. Specifically, we discuss molecular, cellular, and network mechanisms by which RSC might contribute the processing of contextual fear memories. We focus on glutamatergic and cholinergic mechanisms underlying encoding, retrieval, and extinction of context-dependent fear. RSC mechanisms underlying retrieval of recently and remotely acquired memories are compared to memory mechanisms of anterior cortices. Due to the strong connectivity between hippocampus and RSC, we also compare the extent to which their mechanisms of encoding, retrieval, and extinction show overlap. At a theoretical level, we discuss the role of RSC in the framework of systems consolidation as well as retrieval-induced memory modulation. Lastly, we emphasize the implication of these findings for psychopathologies associated with neurological and psychiatric disorders. (PsycINFO Database Record (c) 2018 APA, all rights reserved).

Orexin A Differentially Influences the Extinction Retention of Recent and Remote Fear Memory

Recently the role of the orexin system in the learning and memory, especially orexin A, which could enhance fear memory through regulating the activity of amygdala, has drawn considerable attention. However, the relationship between orexin A and extinction memory remains unclear. To investigate the effect of orexin A on extinction memory in humans, we recruited 43 male subjects and divided them into a recent group and remote group. After acquiring Pavlovian fear conditioning, individuals in recent group experienced fear extinction 24 h after acquisition, and remote group underwent extinction 2 weeks later. Meanwhile, plasma orexin A levels before extinction were measured by enzyme-linked immunosorbent assay. Both groups received memory test 24 h after fear extinction. The results showed that both recent and remote groups successfully acquired fear conditioning and had spontaneous recovery at test. In particular, the correlational analysis indicated that orexin A levels before extinction were negatively associated with fear responses during test only in recent group, but not in remote group. Moreover, individuals with high orexin A levels still kept low fear responses after extinction in recent group by subgroup analyses. The results suggest that orexin A could influence the retention of recent fear memory extinction, without affecting remote fear extinction. These findings remind us the orexin system can be a potential treatment target for fear-related disorders, and the mechanisms of recent and remote fear extinction may be different.

Requirement of an Early Activation of BDNF/c-Fos Cascade in the Retrosplenial Cortex for the Persistence of a Long-Lasting Aversive Memory

During the past few years, there has been growing interest in the role of the retrosplenial cortex (RSC) in memory processing. However, little is known about the molecular changes that take place in this brain region during memory formation. In the present work, we studied the early post-training participation of RSC in the formation of a long-lasting memory in rats. We found an increase in c-Fos levels in the anterior part of the RSC (aRSC) after inhibitory avoidance (IA) training. Interestingly, this increase was associated with memory durability, since blocking c-Fos expression using specific antisense oligonucleotides (ASO) impaired long-lasting retention 7 days after training without affecting memory expression 2 days after training. In addition, we showed that BDNF is one of the upstream signals for c-Fos expression required for memory persistence, since blocking BDNF synthesis prevents IA training-induced increase in c-Fos levels in aRSC and affects memory persistence. In addition, we found that injection of BDNF into aRSC around training was sufficient to establish a persistent memory and that this effect was prevented by c-fos ASO infusion into the same structure. These findings reveal an early post-training involvement of aRSC in the processing of a long-lasting aversive memory.

Network oscillatory activity driven by context memory processing is differently regulated by glutamatergic and cholinergic neurotransmission

Memory retrieval requires coordinated intra- and inter-regional activity in networks of brain structures. Dysfunction of these networks and memory impairment are seen in many psychiatric disorders, but relatively little is known about how memory retrieval and memory failure are represented at the level of local and regional oscillatory activity. To address this question, we measured local field potentials (LFPs) from mice as they explored a novel context, retrieved memories for contextual fear conditioning, and after administration of two amnestic agents: the NMDA receptor antagonist MK-801 and muscarinic acetylcholine receptor antagonist scopolamine (SCOP). LFPs were simultaneously recorded from retrosplenial cortex (RSC), dorsal hippocampus (DH), and anterior cingulate cortex (ACC), which are involved in processing contextual memories, and analyzed for changes in intra-regional power and inter-regional peak coherence of oscillations across multiple frequency bands. Context encoding and memory retrieval sessions yielded similar patterns of changes across all three structures, including decreased delta power and increased theta peak coherence. Baseline effects of MK-801 and SCOP were primarily targeted to gamma oscillations, but in opposite directions. Both drugs also blocked memory retrieval, as indicated by reduced freezing when mice were returned to the conditioning context, but this common behavioral impairment was only associated with power and peak coherence disruptions after MK-801 treatment. These findings point to neural signatures for memory impairment, whose underlying mechanisms may serve as therapeutic targets for related psychiatric disorders.

Distinct retrosplenial cortex cell populations and their spike dynamics during ketamine-induced unconscious state

Ketamine is known to induce psychotic-like symptoms, including delirium and visual hallucinations. It also causes neuronal damage and cell death in the retrosplenial cortex (RSC), an area that is thought to be a part of high visual cortical pathways and at least partially responsible for ketamine's psychotomimetic activities. However, the basic physiological properties of RSC cells as well as their response to ketamine in vivo remained largely unexplored. Here, we combine a computational method, the Inter-Spike Interval Classification Analysis (ISICA), and in vivo recordings to uncover and profile excitatory cell subtypes within layers 2&3 and 5&6 of the RSC in mice within both conscious, sleep, and ketamine-induced unconscious states. We demonstrate two distinct excitatory principal cell sub-populations, namely, high-bursting excitatory principal cells and low-bursting excitatory principal cells, within layers 2&3, and show that this classification is robust over the conscious states, namely quiet awake, and natural unconscious sleep periods. Similarly, we provide evidence of high-bursting and low-bursting excitatory principal cell sub-populations within layers 5&6 that remained distinct during quiet awake and sleep states. We further examined how these subtypes are dynamically altered by ketamine. During ketamine-induced unconscious state, these distinct excitatory principal cell subtypes in both layer 2&3 and layer 5&6 exhibited distinct dynamics. We also uncovered different dynamics of local field potential under various brain states in layer 2&3 and layer 5&6. Interestingly, ketamine administration induced high gamma oscillations in layer 2&3 of the RSC, but not layer 5&6. Our results show that excitatory principal cells within RSC layers 2&3 and 5&6 contain multiple physiologically distinct sub-populations, and they are differentially affected by ketamine.

A GluN2B-Selective NMDAR Antagonist Reverses Synapse Loss and Cognitive Impairment Produced by the HIV-1 Protein Tat

HIV-associated neurocognitive disorder (HAND) affects approximately half of HIV-infected patients. Loss of synaptic connections is a hallmark of many neurocognitive disorders, including HAND. The HIV-1 protein transactivator of transcription (Tat) disrupts synaptic connections both in vitro and in vivo and has been linked to impaired neurocognitive function in humans. In vitro studies have shown that ifenprodil, an antagonist selective for GluN2B-containing NMDARs, reverses synapse loss when applied after Tat. Here, we tested the hypothesis that Tat-induced loss and ifenprodil-mediated rescue of synaptic spines in vivo would predict impairment and rescue of cognitive function. Using intracranial multiphoton imaging, we found that infusion of 100 ng of HIV-1 Tat into the lateral ventricle of yellow fluorescent protein-expressing transgenic mice produced a 17 ± 1% loss of dendritic spines in layer 1 of retrosplenial cortex. Repeated imaging of the same dendrites over 3 weeks enabled longitudinal experiments that demonstrated sustained spine loss after Tat infusion and transient rescue after ifenprodil administration (10 mg/kg, i.p.). Parallel trace fear conditioning experiments showed that spine loss predicted learning deficits and that the time course of ifenprodil-induced rescue of spine density correlated with restoration of cognitive function. These results show for the first time that, during exposure to an HIV-1 neurotoxin in vivo, alteration of GluN2B-containing NMDAR signaling suppresses spine density and impairs learning. Pharmacological inhibition of these NMDARs rescued spines and restored cognitive function. Drugs that rescue synapses may improve neurocognitive function in HAND.SIGNIFICANCE STATEMENT Synaptodendritic damage correlates with cognitive decline in HIV-associated neurocognitive disorder (HAND) patients. We developed an in vivo imaging approach for longitudinal tracking of spine density that enabled correlation of synaptic changes with behavioral outcomes in a model of HAND. We show for the first time that spine loss after exposure to an HIV-1 protein can be reversed pharmacologically and that loss and recovery of dendritic spines predict impairment and restoration of cognitive function, respectively. Therefore, synapse loss, the hallmark of cognitive decline in HAND, is reversible. Drugs that restore spine density may have broad application for improving cognitive function during the early phases of neurodegenerative diseases.

Muscarinic acetylcholine receptors act in synergy to facilitate learning and memory

Understanding how episodic memories are formed and retrieved is necessary if we are to treat disorders in which they malfunction. Muscarinic acetylcholine receptors (mAChR) in the hippocampus and cortex underlie memory formation, but there is conflicting evidence regarding their role in memory retrieval. Additionally, there is no consensus on which mAChR subtypes are critical for memory processing. Using pharmacological and genetic approaches, we found that (1) encoding and retrieval of contextual memory requires mAChR in the dorsal hippocampus (DH) and retrosplenial cortex (RSC), (2) memory formation requires hippocampal M₃ and cooperative activity of RSC M₁ and M_3, and (3) memory retrieval is more impaired by inactivation of multiple M₁–M₄ mAChR in DH or RSC than inactivation of individual receptor subtypes. Contrary to the view that acetylcholine supports learning but is detrimental to memory retrieval, we found that coactivation of multiple mAChR is required for retrieval of both recently and remotely acquired context memories. Manipulations with higher receptor specificity were generally less potent than manipulations targeting multiple receptor subtypes, suggesting that mAChR act in synergy to regulate memory processes. These findings provide unique insight into the development of therapies for amnestic symptoms, suggesting that broadly acting, rather than receptor-specific, mAchR agonists and positive allosteric modulators may be the most effective therapeutic approach.

The neurocircuitry of remote cued fear memory

Memories of threatening, fear-evoking events can persist even over a lifetime. While fear memory is widely considered to be a highly persistent and durable form of memory, its circuits are not. This article reviews the dynamic temporal representation of remote fear memory in the brain, at the level of local circuits and distributed networks. Data from the study of Pavlovian cued fear conditioning suggests memory retrieval remains amygdala-dependent, even over protracted time scales, all the while interconnected cortical and subcortical circuits are newly recruited and progressively reorganized. A deeper understanding into how the neurocircuitry of cued fear memory reorganizes with the passage of time will advance our ongoing search for the elusive physical changes representing fear memories in the brain. Considering that persistent, pathological fear memories are a hallmark feature of post-traumatic stress disorder (PTSD), the behavioral and circuit-level study of remote cued fear memory retrieval adds a key element towards a systems understanding of PTSD.

Conditioning- and time-dependent increases in context fear and generalization

A prominent feature of fear memories and anxiety disorders is that they endure across extended periods of time. Here, we examine how the severity of the initial fear experience influences incubation, generalization, and sensitization of contextual fear memories across time. Adult rats were presented with either five, two, one, or zero shocks (1.2 mA, 2 sec) during contextual fear conditioning. Following a recent (1 d) or remote (28 d) retention interval all subjects were returned to the original training context to measure fear memory and/or to a novel context to measure the specificity of fear conditioning. Our results indicate rats that received two or five shocks show an “incubation”-like enhancement of fear between recent and remote retention intervals, while single-shocked animals show stable levels of context fear memory. Moreover, when fear was tested in a novel context, 1 and 2 shocked groups failed to freeze, whereas five shocked rats showed a time-dependent generalization of context memory. Stress enhancement of fear learning to a second round of conditioning was evident in all previously shocked animals. Based on these results, we conclude that the severity or number of foot shocks determines not only the level of fear memory, but also the time-dependent incubation of fear and its generalization across distinct contexts.

Fear Memory

Fear memory is the best-studied form of memory. It was thoroughly investigated in the past 60 years mostly using two classical conditioning procedures (contextual fear conditioning and fear conditioning to a tone) and one instrumental procedure (one-trial inhibitory avoidance). Fear memory is formed in the hippocampus (contextual conditioning and inhibitory avoidance), in the basolateral amygdala (inhibitory avoidance), and in the lateral amygdala (conditioning to a tone). The circuitry involves, in addition, the pre- and infralimbic ventromedial prefrontal cortex, the central amygdala subnuclei, and the dentate gyrus. Fear learning models, notably inhibitory avoidance, have also been very useful for the analysis of the biochemical mechanisms of memory consolidation as a whole. These studies have capitalized on in vitro observations on long-term potentiation and other kinds of plasticity. The effect of a very large number of drugs on fear learning has been intensively studied, often as a prelude to the investigation of effects on anxiety. The extinction of fear learning involves to an extent a reversal of the flow of information in the mentioned structures and is used in the therapy of posttraumatic stress disorder and fear memories in general.

Analysis of coherent activity between retrosplenial cortex, hippocampus, thalamus, and anterior cingulate cortex during retrieval of recent and remote context fear memory

Memory for contextual fear conditioning relies upon the retrosplenial cortex (RSC) regardless of how long ago conditioning occurred, whereas areas connected to the RSC, such as the dorsal hippocampus (DH) and anterior cingulate cortex (ACC) appear to play time-limited roles. To better understand whether these brain regions functionally interact during memory processing and how the passage of time affects these interactions, we simultaneously recorded local field potentials (LFPs) from these three regions as well as anterior dorsal thalamus (ADT), which provides one of the strongest inputs to RSC, and measured coherence of oscillatory activity within the theta (4-12Hz) and gamma (30-80Hz) frequency bands. We identified changes of theta coherence related to encoding, retrieval, and extinction of context fear, whereas changes in gamma coherence were restricted to fear extinction. Specifically, exposure to a novel context and retrieval of recently acquired fear conditioning memory were associated with increased theta coherence between RSC and all three other structures. In contrast, RSC-DH and RSC-ADT theta coherence were decreased in mice that successfully retrieved, relative to mice that failed to retrieve, remote memory. Greater RSC-ADT theta and gamma coherence were observed during recent, compared to remote, extinction of freezing responses. Thus, the degree of coherence between RSC and connected brain areas may predict and contribute to context memory retrieval and retrieval-related phenomena such as fear extinction. Importantly, although theta coherence in this circuit increases during memory encoding and retrieval of recent memory, failure to decrease RSC-DH theta coherence might be linked to retrieval deficit in the long term, and possibly contribute to aberrant memory processing characteristic of neuropsychiatric disorders.

The cannabinoid system in the retrosplenial cortex modulates fear memory consolidation, reconsolidation, and extinction

Despite the fact that the cannabinoid receptor type 1 (CB1R) plays a pivotal role in emotional memory processing in different regions of the brain, its function in the retrosplenial cortex (RSC) remains unknown. Here, using contextual fear conditioning in rats, we showed that a post-training intra-RSC infusion of the CB1R antagonist AM251 impaired, and the agonist CP55940 improved, long-term memory consolidation. Additionally, a post-reactivation infusion of AM251 enhanced memory reconsolidation, while CP55940 had the opposite effect. Finally, AM251 blocked extinction, whereas CP55940 facilitated it and maintained memory extinguished over time. Altogether, our data strongly suggest that the cannabinoid system of the RSC modulates emotional memory.

Dissociation of the Role of Infralimbic Cortex in Learning and Consolidation of Extinction of Recent and Remote Aversion Memory

Medial prefrontal circuits have been reported to undergo a major reorganization over time and gradually take a more important role for remote emotional memories such as contextual fear memory or food aversion memory. The medial prefrontal cortex, and specifically its ventral subregion, the infralimbic cortex (IL), was also reported to be critical for recent memory extinction of contextual fear conditioning and conditioned odor aversion. However, its exact role in the extinction of remotely acquired information is still not clear. Using postretrieval blockade of protein synthesis or inactivation of the IL, we showed that the IL is similarly required for extinction consolidation of recent and remote fear memory. However, in odor aversion memory, the IL was only involved in extinction consolidation of recent, but not remote, memory. In contrast, only remote retrieval of aversion memory induced c-Fos activation in the IL and preretrieval inactivation of the IL with lidocaine impaired subsequent extinction of remote but not recent memory, indicating IL is necessary for extinction learning of remote aversion memory. In contrast to the effects in odor aversion, our data show that the involvement of the IL in the consolidation of fear extinction does not depend on the memory age. More importantly, our data indicate that the IL is implicated in the extinction of fear and nonfear-based associations and suggest dissociation in the engagement of the IL in the learning and consolidation of food aversion extinction over time.

Regulation of fear extinction versus other affective behaviors by discrete cortical scaffolding complexes associated with NR2B and PKA signaling

In patients suffering from post-traumatic stress disorder (PTSD), fear evoked by trauma-related memories lasts long past the traumatic event and it is often complicated by general anxiety and depressed mood. This poses a treatment challenge, as drugs beneficial for some symptoms might exacerbate others. For example, in preclinical studies, antagonists of the NR2B subunit of N-methyl-d-aspartate receptors and activators of cAMP-dependent protein kinase (PKA) act as potent antidepressants and anxiolytics, but they block fear extinction. Using mice, we attempted to overcome this problem by interfering with individual NR2B and PKA signaling complexes organized by scaffolding proteins. We infused cell-permeable Tat peptides that displaced either NR2B from receptor for activated C kinase 1 (RACK1), or PKA from A-kinase anchor proteins (AKAPs) or microtubule-associated proteins (MAPs). The infusions were targeted to the retrosplenial cortex, an area involved in both fear extinction of remotely acquired memories and in mood regulation. Tat-RACK1 and Tat-AKAP enhanced fear extinction, all peptides reduced anxiety and none affected baseline depression-like behavior. However, disruption of PKA complexes distinctively interfered with the rapid antidepressant actions of the N-methyl-D-aspartate receptors antagonist MK-801 in that Tat-MAP2 blocked, whereas Tat-AKAP completely inverted the effect of MK-801 from antidepressant to depressant. These effects were unrelated to the MK-801-induced changes of brain-derived neurotrophic factor messenger RNA levels. Together, the findings suggest that NR2B-RACK1 complexes specifically contribute to fear extinction, and may provide a target for the treatment of PTSD. AKAP-PKA, on the other hand, appears to modulate fear extinction and antidepressant responses in opposite directions.

Retrosplenial Cortex and Long-Term Memory: Molecules to Behavior

The retrosplenial cortex (RSC) is reciprocally connected with the hippocampus and various parahippocampal cortical regions, suggesting that RSC is well-positioned to contribute to hippocampal-dependent memory. Consistent with this, substantial behavioral evidence indicates that RSC is essential for consolidating and/or retrieving contextual and spatial memories. In addition, there is growing evidence that RSC neurons undergo activity-dependent plastic changes during memory formation and retrieval. In this paper we review both the behavioral and cellular/molecular data and posit that the RSC has a particularly important role in the storage and retrieval of spatial and contextual memories perhaps due its involvement in binding together multiple cues in the environment. We identify remaining questions and avenues for future research that take advantage of emerging methods to selectively manipulate RSC neurons both spatially and temporally and to image the RSC in awake, behaving animals.

The learning of fear extinction

Recent work on the extinction of fear-motivated learning places emphasis on its putative circuitry and on its modulation. Extinction is the learned inhibition of retrieval of previously acquired responses. Fear extinction is used as a major component of exposure therapy in the treatment of fear memories such as those of the posttraumatic stress disorder (PTSD). It is initiated and maintained by interactions between the hippocampus, basolateral amygdala and ventromedial prefrontal cortex, which involve feedback regulation of the latter by the other two areas. Fear extinction depends on NMDA receptor activation. It is positively modulated by d-serine acting on the glycine site of NMDA receptors and blocked by AP5 (2-amino-5-phosphono propionate) in the three structures. In addition, histamine acting on H2 receptors and endocannabinoids acting on CB1 receptors in the three brain areas mentioned, and muscarinic cholinergic fibers from the medial septum to hippocampal CA1 positively modulate fear extinction. Importantly, fear extinction can be made state-dependent on circulating epinephrine, which may play a role in situations of stress. Exposure to a novel experience can strongly enhance the consolidation of fear extinction through a synaptic tagging and capture mechanism; this may be useful in the therapy of states caused by fear memory like PTSD.

NMDA Receptors and Oxidative Stress Induced by the Major Metabolites Accumulating in HMG Lyase Deficiency Mediate Hypophosphorylation of Cytoskeletal Proteins in Brain From Adolescent Rats: Potential Mechanisms Contributing to the Neuropathology of This Disease

Neurological symptoms and cerebral abnormalities are commonly observed in patients with 3-hydroxy-3-methylglutaryl-CoA lyase (HMG lyase) deficiency, which is biochemically characterized by predominant tissue accumulation of 3-hydroxy-3-methylglutaric (HMG), 3-methylglutaric (MGA), and 3-methylglutaconic (MGT) acids. Since the pathogenesis of this disease is poorly known, the present study evaluated the effects of these compounds on the cytoskeleton phosphorylating system in rat brain. HMG, MGA, and MGT caused hypophosphorylation of glial fibrillary acidic protein (GFAP) and of the neurofilament subunits NFL, NFM, and NFH. HMG-induced hypophosphorylation was mediated by inhibiting the cAMP-dependent protein kinase (PKA) on Ser55 residue of NFL and c-Jun kinase (JNK) by acting on KSP repeats of NFM and NFH subunits. We also evidenced that the subunit NR2B of NMDA receptor and Ca(2+) was involved in HMG-elicited hypophosphorylation of cytoskeletal proteins. Furthermore, the antioxidants L-NAME and TROLOX fully prevented both the hypophosphorylation and the inhibition of PKA and JNK caused by HMG, suggesting that oxidative damage may underlie these effects. These findings indicate that the main metabolites accumulating in HMG lyase deficiency provoke hypophosphorylation of cytoskeleton neural proteins with the involvement of NMDA receptors, Ca(2+), and reactive species. It is presumed that these alterations may contribute to the neuropathology of this disease.

Selective neuronal degeneration in the retrosplenial cortex impairs the recall of contextual fear memory

The retrosplenial cortex (RSC) is one of the largest cortical areas in rodents, and is subdivided in two main regions, A29 and A30, according to their cytoarchitectural organization and connectivities. However, very little is known about the functional activity of each RSC subdivision during the execution of complex cognitive tasks. Here, we used a well-established fear learning protocol that induced long-lasting contextual fear memory and showed that during evocation of the fear memory, the expression of early growth response gene 1 was up-regulated in A30, and in other brain areas implicated in fear and spatial memory, however, was down-regulated in A29, including layers IV and V. To search for the participation of A29 on fear memory, we triggered selective degeneration of neurons within cortical layers IV and V of A29 by using a non-invasive protocol that takes advantage of the vulnerability that these neurons have MK801-toxicity and the modulation of this neurodegeneration by testosterone. Application of 5 mg/kg MK801 in intact males induced negligible neuronal degeneration of A29 neurons and had no impact on fear memory retrieval. However, in orchiectomized rats, 5 mg/kg MK801 induced overt degeneration of layers IV-V neurons of A29, significantly impairing fear memory recall. Degeneration of A29 neurons did not affect exploratory or anxiety-related behavior nor altered unconditioned freezing. Importantly, protecting A29 neurons from MK801-toxicity by testosterone preserved fear memory recall in orchiectomized rats. Thus, neurons within cortical layers IV-V of A29 are critically required for efficient retrieval of contextual fear memory.