Remote Fear Research Articles

Microglial activation in the medial prefrontal cortex after remote fear recall participates in the regulation of auditory fear extinction

Excessive or inappropriate fear responses can lead to anxiety-related disorders, such as post-traumatic stress disorder (PTSD). Studies have shown that microglial activation occurs after fear conditioning and that microglial inhibition impacts fear memory. However, the role of microglia in fear memory recall remains unclear. In this study, we investigated the activated profiles of microglia after the recall of remote-cued fear memory and the role of activated microglia in the extinction of remote-cued fear in adult male C57BL/6 mice. The results revealed that the expression of the microglia marker Iba1 increased in the medial prefrontal cortex (mPFC) at 10 min and 1 h following remote-cued fear recall, which was accompanied by amoeboid morphology. Inhibiting microglial activation through PLX3397 treatment before remote fear recall did not affect recall, reconsolidation, or regular extinction but facilitated recall-extinction and mitigated spontaneous recovery. Moreover, our results demonstrated reduced co-expression of Iba1 and postsynaptic density protein 95 (PSD95) in the mPFC, along with decreases in the p-PI3K/PI3K ratio, p-Akt/Akt ratio, and KLF4 expression after PLX3397 treatment. Our results suggest that microglial activation after remote fear recall impedes fear extinction through the pruning of synapses in the mPFC, accompanied by alterations in the expression of the PI3K/AKT/KLF4 pathway. This finding can help elucidate the mechanism involved in remote fear extinction, contributing to the theoretical foundation for the intervention and treatment of PTSD.

CK2 negatively regulates the extinction of remote fear memory

Cognitive behavioral therapy, rooted in exposure therapy, is currently the primary approach employed in the treatment of anxiety-related conditions, including post-traumatic stress disorder (PTSD). In laboratory settings, fear extinction in animals is a commonly employed technique to investigate exposure therapy; however, the precise mechanisms underlying fear extinction remain elusive. Casein kinase 2 (CK2), which regulates neuroplasticity via phosphorylation of its substrates, has a significant influence in various neurological disorders, such as Alzheimer’s disease and Parkinson’s disease, as well as in the process of learning and memory. In this study, we adopted a classical Pavlovian fear conditioning model to investigate the involvement of CK2 in remote fear memory extinction and its underlying mechanisms. The results indicated that the activity of CK2 in the medial prefrontal cortex (mPFC) of mice was significantly upregulated after extinction training of remote cued fear memory. Notably, administration of the CK2 inhibitor CX-4945 prior to extinction training facilitated the extinction of remote fear memory. In addition, CX-4945 significantly upregulated the expression of p-ERK1/2 and p-CREB in the mPFC. Our results suggest that CK2 negatively regulates remote fear memory extinction, at least in part, by inhibiting the ERK-CREB pathway. These findings contribute to our understanding of the underlying mechanisms of remote cued fear extinction, thereby offering a theoretical foundation and identifying potential targets for the intervention and treatment of PTSD.

Remote memory in a Bayesian model of context fear conditioning (BaconREM).

Here, we propose a model of remote memory (BaconREM), which is an extension of a previously published Bayesian model of context fear learning (BACON) that accounts for many aspects of recently learned context fear. BaconREM simulates most known phenomenology of remote context fear as studied in rodents and makes new predictions. In particular, it predicts the well-known observation that fear that was conditioned to a recently encoded context becomes hippocampus-independent and shows much-enhanced generalization ("hyper-generalization") when systems consolidation occurs (i.e., when memory becomes remote). However, the model also predicts that there should be circumstances under which the generalizability of remote fear may not increase or even decrease. It also predicts the established finding that a "reminder" exposure to a feared context can abolish hyper-generalization while at the same time making remote fear again hippocampus-dependent. This observation has in the past been taken to suggest that reminders facilitate access to detail memory that remains permanently in the hippocampus even after systems consolidation is complete. However, the present model simulates this result even though it totally moves all the contextual memory that it retains to the neo-cortex when context fear becomes remote.

Selective enhancement of fear extinction by inhibiting neuronal adenylyl cyclase 1 (AC1) in aged mice

Adenylyl cyclase 1 (AC1) is a selective subtype of ACs, which is selectively expressed in neurons. The activation of AC1 is activity-dependent, and AC1 plays an important role in cortical excitation that contributes to chronic pain and related emotional disorders. Previous studies have reported that human-used NB001 (hNB001, a selective AC1 inhibitor) produced analgesic effects in different animal models of chronic pain. However, the potential effects of hNB001 on learning and memory have been less investigated. In the present study, we found that hNB001 affected neither the induction nor the expression of trace fear, but selectively enhanced the relearning ability during the extinction in aged mice. By contrast, the same application of hNB001 did not affect recent, remote auditory fear memory, or remote fear extinction in either adult or aged mice. Furthermore, a single or consecutive 30-day oral administration of hNB001 did not affect acute nociceptive response, motor function, or anxiety-like behavior in either adult or aged mice. Our results are consistent with previous findings that inhibition of AC1 did not affect general sensory, emotional, and motor functions in adult mice, and provide strong evidence that inhibiting the activity of AC1 may be beneficial for certain forms of learning and memory in aged mice.

Abstract WP323: Late-Stage Degeneration Due to Middle-Cerebral Artery Occlusion Observed in White Matter Tracts and Specific Forebrain Regions in Middle-Aged Female Rats is Attenuated by mir20a-3p Treatment: A Magnetic Resonance-Diffusion Tensor Imaging and Conventional Morphometry Study

Introduction: Our earlier studies show that the small non-coding RNA, mir20a-3p, is neuroprotective after stroke, and reduces sensory-motor impairments in the acute phase and cognitive decline in the long-term in female rats. Cognitive decline due to vascular diseases, such as stroke, is associated with volume loss in the cortex, the limbic structures and white matter tracts. In this study, we examined the volume of forebrain white matter tracts, cortical and limbic structures in animals with characterized cognitive impairment due to stroke, and the efficacy of microRNA treatment. Methodology: Middle-aged females were subjected to ischemic stroke using endothelin 1, injected adjacent to the left middle cerebral artery (MCA). Mir-20a-3p mimic (n=10) or scrambled oligo (n=8) was administered i.v. 4h, 24h and 70d post stroke. Animals were assessed periodically for cognitive performance up to 100d after stroke using both the cued fear conditioning test and the novel object recognition test (NORT). After perfusion fixation, T2 and diffusion tensor imaging (DTI) magnetic resonance imaging of the brain was performed. Thereafter, brains were processed for Weil myelin staining, followed by quantification of the corpus callosum and internal capsule. Results: Stroke resulted in impairment in both the cued fear conditioning test and the NORT, which was attenuated by mir-20a-3p treatment. Compared to sham (no-stroke) animals, volumetric analysis of DTI scans showed a significant reduction in the volume of substantia nigra (p<0.02) and the entorhinal cortex (P=0.0402) in stroke +scrambled group, but not in Stroke+ Mir20 treated group. White matter tracts showed a significant reduction in the corpus callosum and internal capsule volume in the ischemic hemisphere as compared to non-ischemic hemisphere in MCAo animals treated with the scrambled oligo. In contrast, sham and MCAo+Mir-20a-3p animals displayed no hemispheric differences in volume in either tract. Conclusion: Cognitive changes measured with remote fear memory retrieval and NORT were associated with volume reductions in the substantia nigra, the entorhinal cortex, and the major forebrain white matter tracts due to stroke. These stroke-induced deficits were attenuated in Mir-20-3p treated animals.

Hippocampal Trauma Memory Processing Conveying Susceptibility to Traumatic Stress

While the majority of the population is ever exposed to a traumatic event during their lifetime, only a fraction develops posttraumatic stress disorder (PTSD). Disrupted trauma memory processing has been proposed as a core factor underlying PTSD symptomatology. We used transgenic Targeted-Recombination-in-Active-Populations (TRAP) mice to investigate potential alterations in trauma-related hippocampal memory engrams associated with the development of PTSD-like symptomatology. Mice were exposed to a stress-enhanced fear learning paradigm, in which prior exposure to a stressor affects the learning of a subsequent fearful event (contextual fear conditioning using foot shocks), during which neuronal activity was labeled. One week later, mice were behaviorally phenotyped to identify mice resilient and susceptible to developing PTSD-like symptomatology. Three weeks post-learning, mice were re-exposed to the conditioning context to induce remote fear memory recall, and associated hippocampal neuronal activity was assessed. While no differences in the size of the hippocampal neuronal ensemble activated during fear learning were observed between groups, susceptible mice displayed a smaller ensemble activated upon remote fear memory recall in the ventral CA1, higher regional hippocampal parvalbuminneuronal density and a relatively lower activity of parvalbumininterneurons upon recall. Investigation of potential epigenetic regulators of the engram revealed rather generic (rather than engram-specific) differences between groups, with susceptible mice displaying lower hippocampal histone deacetylase 2 expression, and higher methylation and hydroxymethylation levels. These finding implicate variation in epigenetic regulation within the hippocampus, as well as reduced regional hippocampal activity during remote fear memory recall in interindividual differences in susceptibility to traumatic stress.

Histone acetylation increase rescues a weak remote fear memory in rats

Histone acetylation increase rescues a weak remote fear memory in rats

Emotional Value of Fear Memory and the Role of the Ventral Hippocampus in Systems Consolidation

Recent studies have explored the circuitry involving the ventral hippocampus (vHPC), the amygdala, and the prefrontal cortex, a pathway mainly activated to store contextual information efficiently. Lesions in the vHPC impair remote memory, but not in the short term. However, how the vHPC is affected by distinct memory strength or its role in systems consolidation has not yet been elucidated. Here, we investigated how distinct training intensities, with strong or weak contextual fear conditioning, affect activation of the dorsal hippocampus (dHPC) and the vHPC. We found that the time course of memory consolidation differs in fear memories of different training intensities in both the dHPC and vHPC. Our results also indicate that memory generalization happens alongside greater activation of the vHPC, and these processes occur faster with stronger fear memories. The vHPC is required for the expression of remote fear memory and may control contextual fear generalization, a view corroborated by the fact that inactivation of the vHPC suppresses generalized fear expression, making memory more precise again. Systems consolidation occurs concomitantly with greater activation of the vHPC, which is accelerated in stronger fear memories. These findings lead us to propose that greater activation of the vHPC could be used as a marker for memory generalization.

Increase in histone acetylation rescues a weak remote fear memory in rats

There is a growing body of evidence of memory-enhancing effects of histone deacetylase (HDAC) inhibitors in different species and models. Less clearly is understood whether the increased histone acetylation is able to facilitate the remote fear memory. Thus, the aim of the current study was to examine the ability of HDAC inhibitor sodium butyrate (SB) to ameliorate weakening of the remote fear memory in rats. To assess the ability of HDAC inhibitor SB to improve remote fear memory we compared the performance of two laboratory strains of rats, Wistar and Long-Evans, in context fear conditioning task six months after training before and after the SB administration. We found that the rats showed a strong fear response to the context 24 h after the end of conditioned fear training, full absence of fear after 6 months, and high fear response after the SB administration without additional learning. In control experiments, we found that time-dependent decrease in conditioned fear response to the context was similar in rats under vehicle administration. Moreover, the data obtained showed that both rats’ strains showed a similar decrease in freezing response over time, and HDAC inhibition improved the weak remote fear memory in both of them. In addition, the decrease in freezing and memory reinstatement by males matched completely to the female rats’ performance. These results indicate that HDAC inhibition appears to have the same “rescue” effects on remote fear memory reinstatement regardless of the strain and gender of rats.

Presence of a remote fear memory engram in the central amygdala.

Fear memory formation and recall are highly regulated processes, with the central amygdala (CeA) contributing to fear memory-related behaviors. We recently reported that a remote fear memory engram is resident in the anterior basolateral amygdala (aBLA). However, the extent to which downstream neurons in the CeA participate in this engram is unknown. We tested the hypothesis that CeA neurons activated during fear memory formation are reactivated during remote memory retrieval such that a CeA engram participates in remote fear memory recall and its associated behavior. Using contextual fear conditioning in TRAP2;Ai14 mice, we identified, by persistent Cre-dependent tdTomato expression (i.e., "TRAPing"), CeA neurons that were c-fos-activated during memory formation. Twenty-one days later, we quantified neurons activated during remote memory recall using Fos immunohistochemistry. Dual labeling was used to identify the subpopulation of CeA neurons that was both activated during memory formation and reactivated during recall. Compared with their context-conditioned (no shock) controls, fear-conditioned (electric shock) mice (n = 5/group) exhibited more robust fear memory-related behavior (freezing) as well as larger populations of activated (tdTomato+) and reactivated (dual-labeled) CeA neurons. Most neurons in both groups were mainly located in the capsular CeA subdivision (CeAC). Notably, however, only the size of the TRAPed population distributed throughout the CeA was significantly correlated with time spent freezing during remote fear memory recall. Our findings indicate that fear memory formation robustly activates CeA neurons and that a subset located mainly in the CeAC may contribute to both remote fear memory storage/retrieval and the resulting fear-like behavior.

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

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

A prolonged stress rat model recapitulates some PTSD-like changes in sleep and neuronal connectivity

Chronic post-traumatic stress disorder (PTSD) exhibits psychological abnormalities during fear memory processing in rodent models. To simulate long-term impaired fear extinction in PTSD patients, we constructed a seven-day model with multiple prolonged stress (MPS) by modifying manipulation repetitions, intensity, and unpredictability of stressors. Behavioral and neural changes following MPS conveyed longitudinal PTSD-like effects in rats for 6 weeks. Extended fear memory was estimated through fear retrieval induced-freezing behavior and increased long-term serum corticosterone concentrations after MPS manipulation. Additionally, memory retrieval and behavioral anxiety tasks continued enhancing theta oscillation activity in the prefrontal cortex-basal lateral amygdala-ventral hippocampus pathway for an extended period. Moreover, MPS and remote fear retrieval stimuli disrupted sleep-wake activities to consolidate fear memory. Our prolonged fear memory, neuronal connectivity, anxiety, and sleep alteration results demonstrated integrated chronic PTSD symptoms in an MPS-induced rodent model.

Elevated corticosterone after fear learning impairs remote auditory memory retrieval and alters brain network connectivity.

Glucocorticoids are potent memory modulators that can modify behavior in an adaptive or maladaptive manner. Elevated glucocorticoid levels after learning promote memory consolidation at recent time points, but their effects on remote time points are not well established. Here we set out to assess whether corticosterone (CORT) given after learning modifies remote fear memory. To that end, mice were exposed to a mild auditory fear conditioning paradigm followed by a single 2 mg/kg CORT injection, and after 28 d, auditory memory was assessed. Neuronal activation was investigated using immunohistochemistry for the immediate early gene c-Fos, and coactivation of brain regions was determined using a correlation matrix analysis. CORT-treated mice displayed significantly less remote auditory memory retrieval. While the net activity of studied brain regions was similar compared with the control condition, CORT-induced remote memory impairment was associated with altered correlated activity between brain regions. Specifically, connectivity of the lateral amygdala with the basal amygdala and the dorsal dentate gyrus was significantly reduced in CORT-treated mice, suggesting disrupted network connectivity that may underlie diminished remote memory retrieval. Elucidating the pathways underlying these effects could help provide mechanistic insight into the effects of stress on memory and possibly provide therapeutic targets for psychopathology.

Altered regulation of oligodendrocytes associated with parvalbumin neurons in the ventral hippocampus underlies fear generalization in male mice.

Fear generalization is a neurobiological process by which an organism interprets a novel stimulus as threatening because of its similarity to previously learned fear-inducing stimuli. Because recent studies have suggested that the communication between oligodendrocyte precursor cells (OPCs) and parvalbumin (PV)-expressing GABAergic neurons (PV neurons) may play critical roles in stress-related disorders, we examined the involvement of these cells in fear generalization. We first tested the behavioral characteristics of mouse models for conventional fear conditioning (cFC) and modified FC (mFC) with severe electric foot shocks and found that fear generalization was observed in mice treated with mFC but not in mice treated with cFC. The expression levels of genes related to OPCs, oligodendrocytes (OLs), and myelin in the ventral hippocampus were lower in mFC mice than in cFC mice. The densities of OPCs and OLs were decreased in the ventral hippocampus of mFC mice compared to cFC mice. The myelination ratios of PV neurons in the ventral hippocampus were lower in mFC mice than in cFC mice. The chemogenetic activation of PV neurons in the ventral hippocampus of mFC mice reduced fear generalization. The expression levels of genes related to OPCs, OLs, and myelin were recovered following the activation of PV neurons. Finally, the myelination ratios of PV neurons were increased after the activation of PV neurons. Our results suggest that altered regulation of OLs specifically associated with axons of PV neurons in the ventral hippocampus may underlie the generalization of remote fear memory following severe stress exposure.

Anterior basolateral amygdala neurons comprise a remote fear memory engram.

Threatening environmental cues often generate enduring fear memories, but how these are formed and stored remains actively investigated. Recall of a recent fear memory is thought to reflect reactivation of neurons, in multiple brain regions, activated during memory formation, indicating that anatomically distributed and interconnected neuronal ensembles comprise fear memory engrams. The extent to which anatomically specific activation-reactivation engrams persist during long-term fear memory recall, however, remains largely unexplored. We hypothesized that principal neurons in the anterior basolateral amygdala (aBLA), which encode negative valence, acutely reactivate during remote fear memory recall to drive fear behavior. Using adult offspring of TRAP2 and Ai14 mice, persistent tdTomato expression was used to "TRAP" aBLA neurons that underwent Fos-activation during contextual fear conditioning (electric shocks) or context only conditioning (no shocks) (n = 5/group). Three weeks later, mice were re-exposed to the same context cues for remote memory recall, then sacrificed for Fos immunohistochemistry. TRAPed (tdTomato +), Fos +, and reactivated (double-labeled) neuronal ensembles were larger in fear- than context-conditioned mice, with the middle sub-region and middle/caudal dorsomedial quadrants of aBLA displaying the greatest densities of all three ensemble populations. Whereas tdTomato + ensembles were dominantly glutamatergic in context and fear groups, freezing behavior during remote memory recall was not correlated with ensemble sizes in either group. We conclude that although an aBLA-inclusive fear memory engram forms and persists at a remote time point, plasticity impacting electrophysiological responses of engram neurons, not their population size, encodes fear memory and drives behavioral manifestations of long-term fear memory recall.

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

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

Standardized extract of Ginkgo biloba enhances memory persistence over time

BackgroundPreviously, our group has shown that treatment with a standardized extract of Ginkgo biloba leaves (EGb) before acquisition/encoding of the conditioned suppression and object recognition task produced recent memory persistence by upregulating CREB-1 and BDNF in the hippocampal formation in a dose-dependent manner. Moreover, memory durability is influenced by processes that occur after encoding. Since the effects of EGb on consolidation processes and memory persistence are unknown, the characterization of these effects will help elucidate both the beneficial and harmful effects of “cognitive enhancers” treatments. MethodsWe investigated the effects of EGb treatment (250, 500 and 1000 mg Kg−1) on consolidation of conditioned lick suppression memory and its persistence, i.e., assessing fear memory retrieval and extinction, 2–4 days (recent long-term memory) or 40–42 days (remote long-term memory) after its acquisition. We also analyzed the compounds in the EGb by UHPLC-HRMS/MS and molecular networking (MN) for dereplication. ResultsWe show, for the first time, that EGb-induced persistence of conditioned suppression. Interestingly, while a higher dose promoted both recent and remote long-term memory persistence without preventing the acquisition of fear extinction memory, the lower dose induced the persistence of original fear remote memory and prevented extinction fear memory formation. The intermediate dose seems to be related to anti-anxiety effects. Furthermore, dereplication of EGb assisted by GNPS detected, besides ginkgolides A, B and D, distinct flavonoids, including quercetin, rutin and nicotiflorin. ConclusionThis study is the first to show that EGb treatment modulates recent and remote fear memory consolidation and its persistence. Additionally, our phytochemical analysis of the EGb corroborates the presence of flavonoids and terpenoids, which may account for the biological effects of EGb on the duration of memory and shed light on its potential as an adjunct to the treatment of memory decline or anxiety disorders.

Abstract WP233: Sex Differences In The Impact Of Focal Ischemia On Cognitive Impairment In Middle-aged Rats And The Effect Of Iv Mir20a-3p Treatment

Introduction: Our previous studies show that the small non-coding RNA, mir20a-3p, is neuroprotective for stroke in the acute phase (Branyan et al., 2021). Here we used a battery of tests to assess the integrated functioning of affective and cognitive circuits in the chronic phase of stroke and the impact of mir20a-3p. Methods: Middle-aged (12-month-old) female and male Sprague-Dawley rats were subject to middle cerebral artery occlusion (MCAo) using endothelin-1. Mir20a-3p mimic or scrambled oligo was administered i.v. 4h, 24h, and 60d after stroke. Cognitive changes (30-100d after stroke) were assessed by contextual fear conditioning (CFC) and the novel object recognition test (NORT). Blood samples were collected at 60 and 100d to assess cytokines and short chain fatty acids. Results: MCAo impaired sensory motor performance in the acute phase, which was ameliorated by Mir20a-3p treatment. At 30, 60 and 100d after stroke, retrieval of fear memory was significantly reduced over time in sham and stroke males and females. However, the rate of decline of fear memory was significantly accelerated in the scramble treated female stroke group, but not in the scramble treated male stroke group. Similarly, declarative memory (NORT) declined in all female groups at 30 days post stroke and continued to worsen only in the stroke+ scrambled treated group. Male rats did not show any change in performance due to stroke or its treatment at comparable time points of testing. Hyperintensities in the forebrain of T2 weighted MRI images were decreased in mir20a-3p treated stroke females compared to males. Correlation analysis showed that in females at 60d post stroke, the extent of Remote Fear memory recall was highly positively correlated with the levels of the SCFA butyric acid (+0.81) and valeric acid (+0.61), as well as the pro-inflammatory cytokine IL-12p70 (+0.64), and the serine hydrolase, butyrylcholinesterase (+0.71). Conclusion: Although acute sensory-motor impairment is seen to a similar extent in males and females, stroke significantly affected cognitive function in females but not males, and this was associated with alterations in inflammatory mediators. Mir20a-3p, which is predicted to repress inflammatory analytes, abrogated stroke effects in females.

The effectiveness of extinction training in male rats: Temporal considerations and brain mechanisms

The extinction of conditioned fear is frequently used in laboratories as a model for human exposure therapy and is crucial for studies of posttraumatic stress disorder (PTSD). However, the efficacy of specific protocols can vary greatly, and the underlying brain mechanisms are not sufficiently clarified. To address this issue, variable starting time (one or twenty-eight days after fear conditioning) and extinction protocols were used, and the efficacy and durability of fear extinction were also studied. Changes in the behavior, stress hormone levels and neuronal activation patterns of stressed rats were analyzed. Conditioned fear was rapidly and efficiently extinguished by all the protocols investigated. However, when these extinction protocols were initiated one day after fear training, conditioned fear relapsed spontaneously four weeks later. In contrast, when extinction trials were started 28 days after conditioning, no relapse occurred. Hormone measurements taken by the end of extinction trials indicated that adrenocorticotropin, but not corticosterone responses reflected behavioral extinction without any sign of relapse. The last extinction training increased the activation of the medial prefrontal cortex and decreased the activation of the central and medial amygdala when extinction began one day after fear conditioning. By contrast, the activation of the basolateral amygdala and the entire hippocampus decreased by the last training session when extinction started 28 days after fear conditioning. Our findings show that extinction training can extinguish remote fear memories more effectively than recent ones, and that the brain mechanisms underlying remote and recent fear memory extinction differ. Laboratory models should also focus on a later time point to increase their translational value.

Blockade of D-serine signaling and adult hippocampal neurogenesis attenuates remote contextual fear memory following multiple memory retrievals in male mice.

The retrieval of fear memories induces two opposing processes, reconsolidation, and extinction. The memory reconsolidation is an active process that involves gene expression and updates an existing memory. It is hypothesized that blockade of reconsolidation by manipulating the neurobiological factors, which are mechanistically involved in the process, could weaken or disrupt the original fear memory. The N-methyl-D-aspartate (NMDA) receptor and hippocampal neurogenesis play crucial roles in hippocampus-dependent memory processes, including reconsolidation. Using contextual fear conditioning paradigm with multiple retrievals, we attempted to weaken the original contextual fear memory by repeatedly disrupting retrieval-induced reconsolidation via downregulation of NMDA receptor signaling and inhibition of neurogenesis. In the first experiment, prior to fear conditioning, NMDA receptor signaling was downregulated by the genetic reduction of its co-agonist, D-serine, and the neurogenesis was dampened by focal X-ray irradiation on the hippocampus. We found that simultaneous D-serine reduction and neurogenesis dampening resulted in a progressive decrease in freezing following each retrieval, leading to an attenuation of remote contextual fear memory on day 28. In the second experiment using the same behavioral protocols, after conditioning, pharmacological approaches were conducted to simultaneously block D-serine signaling and neurogenesis, resulting in a similar suppressive effect on the remote fear memory. The present findings provide insights for understanding the role of D-serine-mediated NMDA receptor signaling and neurogenesis in memory retrieval and the maintenance of remote fear memory, and improving the efficacy of exposure-based therapy for the treatment of post-traumatic stress disorder (PTSD).