Mechanisms of fear extinction

Critically Assessing the Unanswered Questions of How, Where, and When to Induce Plasticity in the Posttraumatic Stress Disorder Network with Transcranial Magnetic Stimulation

Extinction of traumatic memory, a primary treatment approach (termed exposure therapy) in post-traumatic stress disorder (PTSD), occurs through relearning and may be subserved at the molecular level by long-term potentiation (LTP) of relevant circuits. In parallel, repetitive transcranial magnetic stimulation (rTMS) is thought to work through LTP-like mechanisms and may provide a novel, safe, and effective treatment for PTSD. Our recent failed randomized controlled trial (1) emphasizes the necessity of correctly identifying cortical targets, directionality of TMS protocol, and role of memory activation. Here we provide a systematic review of TMS for PTSD to further identify how, where, and when TMS treatment should be delivered to alleviate PTSD symptoms. We conducted a systematic review of the literature searching for rTMS clinical trials involving PTSD patients and outcomes. We searched MEDLINE through October 25th, 2023 for "TMS and PTSD" and "transcranial magnetic stimulation and posttraumatic stress disorder." Thirty-one publications met our inclusion criteria (k=17 randomized controlled trials (RCTs), k=14 open label). RCT protocols were varied in TMS protocols, cortical TMS targets, and memory activation protocols. There was no clear superiority across protocols of low-frequency (k=5) vs. high-frequency protocols (k=6), or by stimulation location. Memory provocation or exposure protocols (k=7) appear to enhance response. Overall, TMS appears to be effective in treating PTSD symptoms across a variety of TMS frequencies, hemispheric target differences, and exposure protocols. Disparate protocols may be conceptually harmonized when viewed as potentiating proposed anxiolytic networks or suppressing anxiogenic networks.

Attenuating conditioned fear using imagery-based interventions: An overview

A growing literature has sought to include mental imagery in fear conditioning studies. Imaginal extinction and imagery rescripting are mental imagery-based interventions that reduce conditioned fear. In the current study, we reviewed the recent findings on the efficacy of imaginal extinction and imagery rescripting as interventions to attenuate conditioned fear responses among healthy individuals. In accordance with the PRISMA guidelines, we conducted a literature search in four databases, PubMed, Scopus, Science Direct, and Web of Science to find published original empirical articles involving imagery-based interventions using a fear conditioning paradigm. The inclusion criteria were (i) use of an imagery-based intervention (either imaginal extinction or imagery rescripting), and (ii) use of a differential fear conditioning paradigm. 13 original articles reporting 15 experimental studies were included in the review. The review revealed that imagery-based interventions are effective in reducing conditioned fear. Although studies have shown that imaginal extinction and standard extinction have comparable effects in fear extinction, many studies have not been conducted to confirm the findings, or explore the underlying mechanisms. We also found the need for a standardized intervention protocol to enhance experimental control in intervention-based fear conditioning studies.

Memory Trace for Fear Extinction: Fragile yet Reinforceable

Fear extinction is a biological process in which learned fear behavior diminishes without anticipated reinforcement, allowing the organism to re-adapt to ever-changing situations. Based on the behavioral hypothesis that extinction is new learning and forms an extinction memory, this new memory is more readily forgettable than the original fear memory. The brain's cellular and synaptic traces underpinning this inherently fragile yet reinforceable extinction memory remain unclear. Intriguing questions are about the whereabouts of the engram neurons that emerged during extinction learning and how they constitute a dynamically evolving functional construct that works in concert to store and express the extinction memory. In this review, we discuss recent advances in the engram circuits and their neural connectivity plasticity for fear extinction, aiming to establish a conceptual framework for understanding the dynamic competition between fear and extinction memories in adaptive control of conditioned fear responses.

Corticosterone after early adolescent stress prevents social avoidance, aversive behavior, and morphine-conditioned place preference in adulthood

RATIONALE

Stress during childhood or adolescence increases vulnerability to psychiatric disorders in adults. In adult rodents, the delayed effects of stress can increase anxiety-like behavior. These effects, however, can be prevented with post-stress administration of corticosterone (CORT). The effectiveness of CORT in preventing adolescent stress-induced emotional behavior alterations in adulthood has yet to be investigated.

OBJECTIVES

Here, we investigated the interactions between early adolescent stress and exogenous corticosterone on adult social, aversive, and drug-seeking behavior in mice, which are translationally related to symptoms associated with psychiatric and substance abuse disorders.

METHODS AND RESULTS

A single administration of CORT in drinking water (400ug/mL) for 24 h after social defeat or context fear conditioning prevents defeat-induced social avoidance, alters fear processing, prevents adolescent stress-induced anhedonia, and prevents stress-potentiated morphine place preference in adulthood. Exogenous CORT did not immediately prevent stress-induced potentiation of morphine conditioned-place preference in adolescents but did so in adult mice. However, when administered to adolescent mice, CORT also prevented the incubation of morphine-conditioned place preference into adulthood. Lastly, exogenous CORT administration blunted endogenous corticosterone but was unrelated to freezing behavior during a fear test.

CONCLUSIONS

This is the first demonstration of adolescent post-stress CORT promoting socio-emotional resilience and preventing drug-seeking behavior. Our data suggest elevated corticosterone after a stress experience promotes resilience for at least 40 days across the developmental transition from adolescence to adulthood and is effective for socio-emotional and drug-seeking behavior. These results are critical for understanding how adolescent stress impacts emotional and drug-seeking behavior into adulthood.

Feeling more confident to encounter negative emotions: The mediating role of distress tolerance on the relationship between self-efficacy and outcomes of exposure and response prevention for OCD

BACKGROUND

While exposure and response prevention (ERP) is the first-line treatment for obsessive-compulsive disorder (OCD), up to half of patients do not effectively respond. In an effort to better understand the mechanisms behind ERP, the inhibitory learning model emphasizes the roles of increasing perceived self-efficacy and distress tolerance. While self-efficacy and distress tolerance have separately been shown to predict OCD symptoms and treatment outcomes, no studies have assessed their joint effects in ERP. The current study examined distress tolerance as a mediator of the relationship between self-efficacy and ERP outcomes.

METHODS

Patients in an intensive ERP-based treatment program (N = 116) completed weekly self-report measures.

RESULTS

Over the course of treatment, as OCD symptoms reduced, self-efficacy and distress tolerance both significantly increased. Importantly, increases in self-efficacy and distress tolerance mediated each other in explaining symptom reduction, suggesting a possible bi-directional effect.

LIMITATIONS

The temporal relationship between changes in self-efficacy and distress tolerance is worthy of further investigation. In addition, the current sample had limited racial diversity and might not be representative of patients receiving lower levels of care. Findings merit replication to be ascertained of their reliability.

CONCLUSIONS

Findings suggest that during ERP, patients gain confidence in their abilities both to cope with general challenges and to withstand distress, potentially helping them engage with exposures and overcome initial fears. These findings provide support for the inhibitory learning model and highlight the mechanistic roles of self-efficacy and distress tolerance in ERP. Clinical implications to target both in treatment are discussed.

Structural covariance, topological organization, and volumetric features of amygdala subnuclei in posttraumatic stress disorder

The amygdala is divided into functional subnuclei which have been challenging to investigate due to functional magnetic resonance imaging (MRI) limitations in mapping small neural structures. Hence their role in the neurobiology of posttraumatic stress disorder (PTSD) remains poorly understood. Examination of covariance of structural MRI measures could be an alternate approach to circumvent this issue. T1-weighted anatomical scans from a 3 T scanner from non-trauma-exposed controls (NEC; n = 71, 75 % female) and PTSD participants (n = 67, 69 % female) were parcellated into 105 brain regions. Pearson's r partial correlations were computed for three and nine bilateral amygdala subnuclei and every other brain region, corrected for age, sex, and total brain volume. Pairwise correlation comparisons were performed to examine subnuclei covariance profiles between-groups. Graph theory was employed to investigate subnuclei network topology. Volumetric measures were compared to investigate structural changes. We found differences between amygdala subnuclei in covariance with the hippocampus for both groups, and additionally with temporal brain regions for the PTSD group. Network topology demonstrated the importance of the right basal nucleus in facilitating network communication only in PTSD. There were no between-group differences for any of the three structural metrics. These findings are in line with previous work that has failed to find structural differences for amygdala subnuclei between PTSD and controls. However, differences between amygdala subnuclei covariance profiles observed in our study highlight the need to investigate amygdala subnuclei functional connectivity in PTSD using higher field strength fMRI for better spatial resolution.

Minimizing Variability in Developmental Fear Studies in Mice: Toward Improved Replicability in the Field

In rodents, the first weeks of postnatal life feature remarkable changes in fear memory acquisition, retention, extinction, and discrimination. Early development is also marked by profound changes in brain circuits underlying fear memory processing, with heightened sensitivity to environmental influences and stress, providing a powerful model to study the intersection between brain structure, function, and the impacts of stress. Nevertheless, difficulties related to breeding and housing young rodents, preweaning manipulations, and potential increased variability within that population pose considerable challenges to developmental fear research. Here we discuss several factors that may promote variability in studies examining fear conditioning in young rodents and provide recommendations to increase replicability. We focus primarily on experimental conditions, design, and analysis of rodent fear data, with an emphasis on mouse studies. The convergence of anatomical, synaptic, physiological, and behavioral changes during early life may increase variability, but careful practice and transparency in reporting may improve rigor and consensus in the field. © 2024 The Authors. Current Protocols published by Wiley Periodicals LLC.

Healing wounded trees: clinicians' perspectives on treatment of complex posttraumatic stress disorder

While treatment guidelines agree on the first-line interventions for the treatment of posttraumatic stress disorder (PTSD), there is an ongoing debate between experts regarding the treatment of complex posttraumatic stress disorder (C-PTSD). As scientific research is slowly emerging, different treatment approaches are used in clinical practice This article aims to provide a set of treatment options for C-PTSD in adult survivors of repeated exposure to severe violence and abuse, both in childhood and later on in life. The developmental-contextual perspective on mental health forms the basis of this approach. This perspective is elaborated using the tree metaphor. Then, several treatment strategies are suggested. The presented strategies are a combination of the existing evidence-based approaches for the treatment of PTSD and personality disorders. They target psychological damage in survivors while taking their developmental trajectories and ecological environments into consideration. The treatment model presented is based on longstanding clinical practice and it may be a promising framework for treating C-PTSD. However, it still needs to be scientifically examined for acceptability and effectiveness.

Loss of mGlu5 receptors in somatostatin-expressing neurons alters negative emotional states

Subtype 5 metabotropic glutamate receptors (mGlu5) are known to play an important role in regulating cognitive, social and valence systems. However, it remains largely unknown at which circuits and neuronal types mGlu5 act to influence these behavioral domains. Altered tissue- or cell-specific expression or function of mGlu5 has been proposed to contribute to the exacerbation of neuropsychiatric disorders. Here, we examined how these receptors regulate the activity of somatostatin-expressing (SST+) neurons, as well as their influence on behavior and brain rhythmic activity. Loss of mGlu5 in SST+ neurons elicited excitatory synaptic dysfunction in a region and sex-specific manner together with a range of emotional imbalances including diminished social novelty preference, reduced anxiety-like behavior and decreased freezing during retrieval of fear memories. In addition, the absence of mGlu5 in SST+ neurons during fear processing impaired theta frequency oscillatory activity in the medial prefrontal cortex and ventral hippocampus. These findings reveal a critical role of mGlu5 in controlling SST+ neurons excitability necessary for regulating negative emotional states.

Effects of cannabidiol on fear conditioning in anxiety disorders: decreased threat expectation during retention, but no enhanced fear re-extinction

RATIONALE

Preclinical research suggests that pharmacologically elevating cannabinoid levels may attenuate fear memory expression and enhance fear extinction.

OBJECTIVES

We studied the effects of cannabidiol (CBD) on fear memory expression and fear re-extinction in 69 patients with panic disorder with agoraphobia or with social anxiety disorder. Moderation by sex, diagnosis, and serotonergic antidepressant (AD) use was explored.

METHODS

A cued fear conditioning paradigm was applied before the first treatment session with 300 mg CBD/placebo augmented exposure therapy. Study medication was administered orally preceding 8 weekly sessions. Fear acquisition and suboptimal extinction took place prior to the first medication ingestion (T0). After the first medication ingestion (T1), we investigated effects on fear memory expression at retention and fear re-extinction. Subjective fear, shock expectancy, skin conductance, and startle responses to conditioned (CS+) and safety stimulus (CS-) were measured.

RESULTS

Across the sample, CBD reduced shock expectancy at retention under low and ambiguous threat of shock, but fear re-extinction at T1 was unaffected by CBD. However, in AD users, re-extinction of subjective fear was impaired in the CBD condition compared to placebo. In female AD users, CBD interfered with safety learning measured with fear-potentiated startle.

CONCLUSIONS

The current findings provide no evidence for enhanced fear re-extinction by CBD. However, CBD acutely decreased threat expectation at retention, without affecting other indices of fear. More studies are needed to elucidate possible interactions with AD use and sex, as well as potential effects of CBD on threat expectancies.

Fear in action: Fear conditioning and alleviation through body movements

Fear memories enhance survival especially when the memories guide defensive movements to minimize harm. Accordingly, fear memories and body movements have tight relationships in animals: Fear memory acquisition results in adapting reactive defense movements, while training active defense movements reduces fear memory. However, evidence in humans is scarce because their movements are typically suppressed in experiments. Here, we tracked adult participants' body motions while they underwent ecologically valid fear conditioning in a 3D virtual space. First, with body motion tracking, we revealed that distinct spatiotemporal body movement patterns emerge through fear conditioning. Second, subsequent training to actively avoid threats with naturalistic defensive actions led to a long-term (24 h) reduction of physiological and embodied conditioned responses, while extinction or vicarious training only transiently reduced the responses. Together, our results highlight the role of body movements in human fear memory and its intervention.

Dorsal hippocampus represents locations to avoid as well as locations to approach during approach-avoidance conflict

Worrying about perceived threats is a hallmark of multiple psychological disorders including anxiety. This concern about future events is particularly important when an individual is faced with an approach-avoidance conflict. Potential goals to approach are known to be represented in the dorsal hippocampus during theta sweeps. Similarly, important non-local information is represented during hippocampal high synchrony events (HSEs), which are correlated with sharp-wave ripples (SWRs). It is likely that potential future threats may be similarly represented. We examined how threats and rewards were represented within the hippocampus during approach-avoidance conflicts in rats faced with a predator-like robot guarding a food reward. We found representations of the pseudo-predator during HSEs when hesitating in the nest, and during theta prior to retreating as the rats approached the pseudo-predator. After the first attack, we observed new place fields appearing at the location of the robot (not the location the rat was when attacked). The anxiolytic diazepam reduced anxiety-like behavior and altered hippocampal local field potentials, including reducing SWRs, suggesting that one potential mechanism of diazepam's actions may be through altered representations of imagined threat. These results suggest that hippocampal representation of potential threats could be an important mechanism that underlies worry and a potential target for anxiolytics.

Is disgust more resistant to extinction than fear? A meta-analytic review of laboratory paradigms

Disgust can be acquired via evaluative conditioning; a process by which a neutral stimulus (conditioned stimulus; CS) comes to be evaluated as disgusting due to its pairing with an inherently disgusting stimulus (unconditioned stimulus; US). Research has shown that conditioned disgust responses are resistant to extinction which may have implications for disorders (i.e., contamination-based obsessive-compulsive disorder, specific phobias, and post-traumatic stress disorder) in which heightened disgust has been implicated. Importantly, extinction is the primary mechanism by which exposure therapies are thought to achieve symptom reduction for these disorders. Exposure therapies were originally modeled on fear extinction, whereas disgust extinction was largely overlooked until recently. Accordingly, differences in the degree to which learned disgust and fear can be attenuated via extinction learning remains unclear. The present investigation was a meta-analysis directly comparing the degree of extinction of conditioned disgust (n = 14) and conditioned fear (n = 14) in laboratory paradigms. Extinction was operationalized as the standardized mean difference (SMD) in evaluative ratings between the CS+ (the CS paired with the US) and CS- (the unpaired CS) after extinction training. Results of a subgroup analysis indicated that disgust (SMD = 0.52) was significantly more resistant to extinction than fear (SMD = 0.37). Additionally, a series of meta-regression analyses indicated that extinction was not influenced by important study characteristics (e.g., sex, age, number of conditioning and extinction trials). The findings suggest that extinction-based approaches may be less effective at attenuating learned disgust and research is needed to better optimize treatments for disgust-related disorders.

Restoring the Function of Thalamocortical Circuit Through Correcting Thalamic Kv3.2 Channelopathy Normalizes Fear Extinction Impairments in a PTSD Mouse Model

Impaired extinction of fear memory is one of the most common symptoms in post-traumatic stress disorder (PTSD), with limited therapeutic strategies due to the poor understanding of its underlying neural substrates. In this study, functional screening is performed and identified hyperactivity in the mediodorsal thalamic nucleus (MD) during fear extinction. Furthermore, the encoding patterns of the hyperactivated MD is investigated during persistent fear responses using multiple machine learning algorithms. The anterior cingulate cortex (ACC) is also identified as a functional downstream region of the MD that mediates the extinction of fear memory. The thalamocortical circuit is comprehensively analyzed and found that the MD-ACC parvalbumin interneurons circuit is preferentially enhanced in PTSD mice, disrupting the local excitatory and inhibitory balance. It is found that decreased phosphorylation of the Kv3.2 channel contributed to the hyperactivated MD, primarily to the malfunctioning thalamocortical circuit. Using a lipid nanoparticle-based RNA therapy strategy, channelopathy is corrected via a methoxylated siRNA targeting the protein phosphatase 6 catalytic subunit and restored fear memory extinction in PTSD mice. These findings highlight the function of the thalamocortical circuit in PTSD-related impaired extinction of fear memory and provide therapeutic insights into Kv3.2-targeted RNA therapy for PTSD.

Unconditioned response to a naturally aversive stimulus is associated with sensitized defensive responding and self-reported fearful traits in a PTSD sample

Unconditioned responding (UCR) to a naturally aversive stimulus is associated with defensive responding to a conditioned threat cue (CS+) and a conditioned safety cue (CS-) in trauma-exposed individuals during fear acquisition. However, the relationships of UCR with defensive responses during extinction training, posttraumatic stress disorder (PTSD) symptom severity, and fearful traits in trauma-exposed individuals are not known. In a sample of 100 trauma-exposed adults with a continuum of PTSD severity, we recorded startle responses and skin conductance responses (SCR) during fear acquisition and extinction training using a 140 psi, 250-ms air blast to the larynx as the unconditioned stimulus. We explored dimensional associations of two different measures of UCR (unconditioned startle and unconditioned SCR) with conditioned defensive responding to CS+ and CS-, conditioned fear (CS+ minus CS-), PTSD symptom severity, and a measure of fearful traits (composite of fear survey schedule, anxiety sensitivity index, and Connor-Davidson resilience scale). Unconditioned startle was positively associated with startle potentiation to the threat cue and the safety cue across both learning phases (CS+ Acquisition, CS- Acquisition, CS+ Extinction Training, CS- Extinction Training) and with fearful traits. Unconditioned SCR was positively associated with SCR to the CS+ and CS- and SCR difference score during Acquisition. Neither type of UCR was associated with PTSD symptom severity. Our findings suggest that UCR, particularly unconditioned startle to a naturally aversive stimulus, may inform research on biomarkers and treatment targets for symptoms of pervasive and persistent fear in trauma-exposed individuals.

Opposing Motor Memories in the Direct and Indirect Pathways of the Basal Ganglia

Loss of dopamine neurons causes motor deterioration in Parkinson's disease patients. We have previously reported that in addition to acute motor impairment, the impaired motor behavior is encoded into long-term memory in an experience-dependent and task-specific manner, a phenomenon we refer to as aberrant inhibitory motor learning. Although normal motor learning and aberrant inhibitory learning oppose each other and this is manifested in apparent motor performance, in the present study, we found that normal motor memory acquired prior to aberrant inhibitory learning remains preserved in the brain, suggesting the existence of independent storage. To investigate the neuronal circuits underlying these two opposing memories, we took advantage of the RNA-binding protein YTHDF1, an m 6 A RNA methylation reader involved in the regulation of protein synthesis and learning/memory. Conditional deletion of Ythdf1 in either D1 or D2 receptor-expressing neurons revealed that normal motor memory is stored in the D1 (direct) pathway of the basal ganglia, while inhibitory memory is stored in the D2 (indirect) pathway. Furthermore, fiber photometry recordings of GCaMP signals from striatal D1 (dSPN) and D2 (iSPN) receptor-expressing neurons support the preservation of normal memory in the direct pathway after aberrant inhibitory learning, with activities of dSPN predictive of motor performance. Finally, a computational model based on activities of motor cortical neurons, dSPN and iSPN neurons, and their interactions through the basal ganglia loops supports the above observations. These findings have important implications for novel approaches in treating Parkinson's disease by reactivating preserved normal memory, and in treating hyperkinetic movement disorders such as chorea or tics by erasing aberrant motor memories.

Differential recruitment of brain circuits during fear extinction in non-stressed compared to stress resilient animals

Dysfunctional fear responses in post-traumatic stress disorder (PTSD) may be partly explained by an inability to effectively extinguish fear responses elicited by trauma-related cues. However, only a subset of individuals exposed to traumatic stress develop PTSD. Therefore, studying fear extinction deficits in animal models of individual differences could help identify neural substrates underlying vulnerability or resilience to the effects of stress. We used a rat model of social defeat in which rats segregate into passively and actively coping rats. In previous work, we showed that passively coping rats exhibit disruptions in social interaction whereas actively coping rats do not display behaviors differently from controls, indicating their resilience. Here, adult male rats exposed to 7 days of social defeat were tested for fear extinction, retention of extinction, and persistence of retention using contextual fear and ethologically-relevant fear tests. Passively coping rats exhibited elevated freezing in response to the previously extinguished context. Analyses of cFos expressing cells across select brain regions showed high correlations within dorsal hippocampal subregions, while passively coping rats had high correlations between the dorsal hippocampus CA1 and the central and basolateral subregions of the amygdala. Importantly, although control and actively coping rats showed similar levels of behavioral extinction, there was little similarity between activated structures, suggesting stress resilience in response to chronic social defeat involves an adaptive differential recruitment of brain circuits to successfully extinguish fear memories.

The sexually divergent cFos activation map of fear extinction

Objective

Post-traumatic stress disorder (PTSD) is a neuropsychiatric disorder that can develop after experiencing or witnessing a traumatic event. Exposure therapy is a common treatment for PTSD, but it has varying levels of efficacy depending on sex. In this study, we aimed to compare the sexual dimorphism in brain activation during the extinction of fear conditioning in male and female rats by detecting the c-fos levels in the whole brain.

Methods

Thirty-two rats (Male: n = 16; Female: n = 16) were randomly separated into the extinction group as well as the non-extinction group, and fear conditioning was followed by extinction and non-extinction, respectively. Subsequently, brain sections from the sacrificed animal were performed immunofluorescence and the collected data were analyzed by repeated two-way ANOVAs as well as Pearson Correlation Coefficient.

Results

Our findings showed that most brain areas activated during extinction were similar in both male and female rats, except for the reuniens thalamic nucleus and ventral hippocampi. Furthermore, we found differences in the correlation between c-fos activation levels and freezing behavior during extinction between male and female rats. Specifically, in male rats, c-fos activation in the anterior cingulate cortex was negatively correlated with the freezing level, while c-fos activation in the retrosplenial granular cortex was positively correlated with the freezing level; but in female rats did not exhibit any correlation between c-fos activation and freezing level. Finally, the functional connectivity analysis revealed differences in the neural networks involved in extinction learning between male and female rats. In male rats, the infralimbic cortex and insular cortex, anterior cingulate cortex and retrosplenial granular cortex, and dorsal dentate gyrus and dCA3 were strongly correlated after extinction. In female rats, prelimbic cortex and basolateral amygdala, insular cortex and dCA3, and anterior cingulate cortex and dCA1 were significantly correlated.

Conclusion

These results suggest divergent neural networks involved in extinction learning in male and female rats and provide a clue for improving the clinical treatment of exposure therapy based on the sexual difference.

Targeting Human Glucocorticoid Receptors in Fear Learning: A Multiscale Integrated Approach to Study Functional Connectivity

Fear extinction is a phenomenon that involves a gradual reduction in conditioned fear responses through repeated exposure to fear-inducing cues. Functional brain connectivity assessments, such as functional magnetic resonance imaging (fMRI), provide valuable insights into how brain regions communicate during these processes. Stress, a ubiquitous aspect of life, influences fear learning and extinction by changing the activity of the amygdala, prefrontal cortex, and hippocampus, leading to enhanced fear responses and/or impaired extinction. Glucocorticoid receptors (GRs) are key to the stress response and show a dual function in fear regulation: while they enhance the consolidation of fear memories, they also facilitate extinction. Accordingly, GR dysregulation is associated with anxiety and mood disorders. Recent advancements in cognitive neuroscience underscore the need for a comprehensive understanding that integrates perspectives from the molecular, cellular, and systems levels. In particular, neuropharmacology provides valuable insights into neurotransmitter and receptor systems, aiding the investigation of mechanisms underlying fear regulation and potential therapeutic targets. A notable player in this context is cortisol, a key stress hormone, which significantly influences both fear memory reconsolidation and extinction processes. Gaining a thorough understanding of these intricate interactions has implications in terms of addressing psychiatric disorders related to stress. This review sheds light on the complex interactions between cognitive processes, emotions, and their neural bases. In this endeavor, our aim is to reshape the comprehension of fear, stress, and their implications for emotional well-being, ultimately aiding in the development of therapeutic interventions.

MDMA-assisted psychotherapy for PTSD: Growing evidence for memory effects mediating treatment efficacy

The application of MDMA in conjunction with psychotherapy has in recent years seen a resurgence of clinical, scientific, and public interest in the treatment of posttraumatic stress disorder (PTSD). Clinical trials have shown promising safety and efficacy, but the mechanisms underlying this treatment form remain largely unestablished. This article explores recent preclinical and clinical evidence suggesting that the treatment's efficacy may be influenced by the mnemonic effects of MDMA. We review data on the effects of MDMA on fear extinction and fear reconsolidation and the utility of these processes for PTSD treatment. We corroborate our findings by incorporating research from cognitive psychology and psychopharmacology and offer recommendations for future research.

Hypothalamic CRH neurons represent physiological memory of positive and negative experience

Recalling a salient experience provokes specific behaviors and changes in the physiology or internal state. Relatively little is known about how physiological memories are encoded. We examined the neural substrates of physiological memory by probing CRHPVN neurons of mice, which control the endocrine response to stress. Here we show these cells exhibit contextual memory following exposure to a stimulus with negative or positive valence. Specifically, a negative stimulus invokes a two-factor learning rule that favors an increase in the activity of weak cells during recall. In contrast, the contextual memory of positive valence relies on a one-factor rule to decrease activity of CRHPVN neurons. Finally, the aversive memory in CRHPVN neurons outlasts the behavioral response. These observations provide information about how specific physiological memories of aversive and appetitive experience are represented and demonstrate that behavioral readouts may not accurately reflect physiological changes invoked by the memory of salient experiences.

Functional architecture of dopamine neurons driving fear extinction learning

The ability to extinguish fear responses to stimuli that no longer predict danger is critical for adaptive behavior and increases the likelihood of survival. During fear extinction, dopamine (DA) neurons signal the absence of the expected aversive outcome, and this extinction prediction error (EPE) signal is crucial for initiating and driving extinction learning. However, the neural circuits underlying the EPE signal have remained elusive. Here, we investigate the input-output circuitry of EPE-encoding DA neurons in male mice. By employing projection-specific fiber photometry and optogenetics, we demonstrate that these neurons project to a restricted subregion of the nucleus accumbens. Comprehensive anatomical analyses, as well as projection-specific chemogenetic manipulations combined with recordings of DA biosensors, further uncover the dorsal raphe as one key input structure critical for generating the EPE signal. Together, our results reveal for the first time the functional architecture of EPE-encoding DA neurons crucial for driving fear extinction learning.

Dopaminergic D2-like receptor stimulation affects attention on contextual information and modulates BOLD activation of extinction-related brain areas

Contextual information is essential for learning and memory processes and plays a crucial role during the recall of extinction memory, and in the renewal effect, which is the context-dependent recovery of an extinguished response. The dopaminergic system is known to be involved in regulating attentional processes by shifting attention to novel and salient contextual cues. Higher dopamine levels are associated with a better recall of previously learned stimulus-outcome associations and enhanced encoding, as well as retrieval of contextual information which promotes renewal. In this fMRI study, we aimed to investigate the impact of processing contextual information and the influence of dopaminergic D2-like receptor activation on attention to contextual information during a predictive learning task as well as upon extinction learning, memory performance, and activity of extinction-related brain areas. A single oral dose of 1.25 mg bromocriptine or an identical-looking placebo was administered to the participants. We modified a predictive learning task that in previous studies reliably evoked a renewal effect, by increasing the complexity of contextual information. We analysed fixations and dwell on contextual cues by use of eye-tracking and correlated these with behavioural performance and BOLD activation of extinction-related brain areas. Our results indicate that the group with dopaminergic D2-like receptor stimulation had higher attention to task-relevant contextual information and greater/lower BOLD activation of brain regions associated with cognitive control during extinction learning and recall. Moreover, renewal responses were almost completely absent. Since this behavioural effect was observed for both treatment groups, we assume that this was due to the complexity of the altered task design.

The Neurotransmission Basis of Post-Traumatic Stress Disorders by the Fear Conditioning Paradigm

Fear conditioning constitutes the best and most reproducible paradigm to study the neurobiological mechanisms underlying emotions. On the other hand, studies on the synaptic plasticity phenomena underlying fear conditioning present neural circuits enforcing this learning pattern related to post-traumatic stress disorder (PTSD). Notably, in both humans and the rodent model, fear conditioning and context rely on dependent neurocircuitry in the amygdala and prefrontal cortex, cingulate gyrus, and hippocampus. In this review, an overview of the role that classical neurotransmitters play in the contextual conditioning model of fear, and therefore in PTSD, was reported.

Frontopolar multifocal transcranial direct current stimulation reduces conditioned fear reactivity during extinction training: A pilot randomized controlled trial

Exposure-based therapies for anxiety and related disorders are believed to depend on fear extinction learning and corresponding changes in extinction circuitry. Frontopolar multifocal transcranial direct current stimulation (tDCS) has been shown to improve therapeutic safety learning during in vivo exposure and may modulate functional connectivity of networks implicated in fear processing and inhibition. A pilot randomized controlled trial was completed to determine the effects of frontopolar tDCS on extinction learning and memory. Community volunteers (n = 35) completed a 3-day fear extinction paradigm with measurement of electrodermal activity. Participants were randomized (single-blind) to 20-min of sham (n = 17, 30 s. ramp in/out) or active (n = 18) frontopolar (anode over Fpz, 10-10 EEG) multifocal tDCS (20-min, 1.5 mA) prior to extinction training. Mixed ANOVAs revealed a significant group*trial effect on skin conductance response (SCR) to the conditioned stimulus (CS + ) during extinction training (p = 0.007, Cohen's d = 0.55). The effects of frontopolar tDCS were greatest during the first two extinction trials, suggesting that tDCS may have promoted fear inhibition prior to safety learning. Return of fear to the CS + during tests were comparable across conditions (ps > 0.50). These findings suggest that frontopolar tDCS may modulate the processing of threat cues and associated circuitry or promote the inhibition of fear. This has clear implications for the treatment of anxiety and related disorders with therapeutic exposure.

Cortisol decreases activation in extinction related brain areas resulting in an impaired recall of context-dependent extinction memory

Conditioned responding gradually stops during successful extinction learning. The renewal effect is defined as the recovery of a extinguished conditioned response when the context of extinction is different from acquisition. The stress hormone cortisol is known to have an influence on extinction memory and associative learning. Different effects of cortisol on behaviour and brain activity have been observed with respect to stress timing, duration, and intensity. However, the influence of cortisol prior to the initial encoding of stimulus-outcome associations on extinction learning, renewal and its behavioural and neurobiological correlates is still largely unknown. In our study, 60 human participants received 20 mg cortisol or placebo and then learned, extinguished, and recalled the associations between food stimuli presented in distinct contexts and different outcomes in three subsequent task phases. Learning performance during acquisition and extinction phases was equally good for both treatment groups. In the cortisol group, significantly more participants showed renewal compared to placebo. In the subgroup of participants with renewal, cortisol treated participants showed significantly better extinction learning performance compared to placebo. Participants showing renewal had in general difficulties with recalling extinction memory, but in contrast to placebo, the cortisol group exhibited a context-dependent impairment of extinction memory recall. Imaging analyses revealed that cortisol decreased activation in the hippocampus during acquisition. The cortisol group also showed reduced dorsolateral prefrontal cortex activation when extinction learning took place in a different context, but enhanced activation in inferior frontal gyrus during extinction learning without context change. During recall, cortisol decreased ventromedial prefrontal cortex activation. Taken together, our findings illustrate cortisol as a potent modulator of extinction learning and recall of extinction memory which also promotes renewal.

Manipulating critical memory periods to treat psychiatry disorders

The persistence of pathological memory is the basis of several psychiatric disorders. Memory retrieval induces "reconsolidation", a time interval during which the original memory becomes labile and destabilized. Time- and retrieval-dependent processes and memory reconsolidation are critical periods for memory interference. Modulating memory reconsolidation has received considerable research attention as a treatment protocol for several psychiatric conditions such as posttraumatic stress disorder, addiction, anxiety, and trauma-related disorders. This specific time window provides an opportunity for intervention regarding mental diseases. This article reviews the effect of modulating memory reconsolidation using behavioral-, brain stimulation-, and pharmacological-based interventions, which may help bridge the gap between intervention in laboratories and application in clinical practice. The potential advantages, limitations, challenges, and opportunities for memory reconsolidation manipulations were discussed.

Thalamic nucleus reuniens coordinates prefrontal-hippocampal synchrony to suppress extinguished fear

Traumatic events result in vivid and enduring fear memories. Suppressing the retrieval of these memories is central to behavioral therapies for pathological fear. The medial prefrontal cortex (mPFC) and hippocampus (HPC) have been implicated in retrieval suppression, but how mPFC-HPC activity is coordinated during extinction retrieval is unclear. Here we show that after extinction training, coherent theta oscillations (6-9 Hz) in the HPC and mPFC are correlated with the suppression of conditioned freezing in male and female rats. Inactivation of the nucleus reuniens (RE), a thalamic hub interconnecting the mPFC and HPC, reduces extinction-related Fos expression in both the mPFC and HPC, dampens mPFC-HPC theta coherence, and impairs extinction retrieval. Conversely, theta-paced optogenetic stimulation of RE augments fear suppression and reduces relapse of extinguished fear. Collectively, these results demonstrate a role for RE in coordinating mPFC-HPC interactions to suppress fear memories after extinction.

Inactivation of medial or lateral orbitofrontal cortex during fear extinction did not interfere with fear renewal

Hyperactive orbitofrontal cortical activation is commonly seen in patients of obsessive-compulsive disorder (OCD). Previous studies from our laboratory showed that for rats with aberrant activation of the orbitofrontal cortex (OFC) during the extinction phase, they were unable to use contexts as the reference for proper retrieval of fear memory during renewal test. This result supported the phenomenon that many OCD patients show poor regulation of fear-related behavior. Since there are robust anatomical connections of the OFC with the fear-circuit, we aim to further examine whether the OFC is actively engaged in fear regulation under normal circumstances. In this study, the lateral or medial OFC was inactivated during the extinction phase using the ABA fear renewal procedure. We found that these animals showed intact fear renewal during retrieval test with their freezing levels equivalent to the control rats, revealing that the OFC did not have decisive roles in extinction acquisition. Together with our previous study, we suggest that the OFC only interferes with fear regulation when it becomes pathophysiologically hyperactive.

Transcranial magnetic stimulation and transcranial direct current stimulation affect explicit but not implicit emotion regulation: a meta-analysis

Emotion regulation (ER) refers to the process through which people influence the occurrence, experience, and expression of emotions. It can be established in an explicit (voluntary) or implicit (automatic) way, both of which are essential for mental and physical well-being. Recent evidence has highlighted the potential of transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS) targeting the prefrontal cortex (PFC) to modulate ER. However, previous studies have only evaluated the effects of TMS and tDCS on explicit ER, leaving implicit ER relatively unexplored. In this review and meta-analysis, we systematically evaluated the effects of TMS and tDCS over the PFC on the two forms of ER, using both subjective and physiological response as outcome indicators. Twenty-seven studies were included in our study. Both subjective (Hedges' g = - 0.20) and physiological (Hedges' g = - 0.65) results indicated a significant effect of TMS and tDCS targeting PFC on down-regulation of explicit ER, but not implicit ER (Hedges' g = - 0.04). Moreover, moderation analysis indicated that the effect of TMS and tDCS on the down-regulating of subjective experience was moderated by several factors, including stimulation method, target area, target hemisphere, and stimulation timing. Specifically, our results showed that applying TMS or targeting the right PFC, particularly the right ventrolateral prefrontal cortex, or using offline TMS and tDCS produced a larger stimulation effect on ER. In summary, these findings suggest that TMS and tDCS has a positive effect on explicit, but not implicit ER. The distinct TMS and tDCS effect on the two forms of ER help deepen our understanding of TMS and tDCS use and provide valuable insights for the development of tailored TMS and tDCS protocols for explicit and implicit regulation.

Neurobeachin haploinsufficient mice display sex-independent alterations in cued and contextual fear conditioning

Neurobeachin ( NBEA ) is a cytoplasmic protein that regulates receptor trafficking, neurotransmitter and hormone secretion, as well as synaptic connectivity. Recently, hippocampus-dependent contextual extinction, the gradual decrease of a conditioned fear response to a context, was suggested to be specifically impaired in male mice with Nbea deficiency ( Nbea+/- ). The current study examines the role of sex in this effect and whether Nbea also influences cued fear conditioning. We included both female and male mice and used a phased contextual and cued fear acquisition protocol that consists of different phases allowing us to assess fear acquisition, cued and contextual fear memory and within-phase extinction. Performance of Nbea+/- mice during assessment of both contextual and cued fear memory was significantly altered compared to controls, independent of sex. Follow-up analyses revealed that this altered performance could be indicative of impaired within-phase extinction. Altered within-phase extinction was not exclusively attributable to hippocampus, and independent of sex. Our results rather suggest that Nbea influences complex learning more broadly across different brain structures.

Imaginal extinction without imagery: Dissociating the effects of visual imagery and propositional thought by contrasting participants with aphantasia, simulated aphantasia, and controls

Imaginal exposure is a standard procedure of cognitive behavioral therapy for the treatment of anxiety and panic disorders. It is often used when in vivo exposure is not possible, too stressful for patients, or would be too expensive. Peter Lang's Bio-Informational Theory implies that imaginal exposure is effective because of the perceptual proximity of mental imagery to real events, whereas empirical findings suggest that propositional thought of fear stimuli (i.e., thinking about the stimuli without seeing them in the mind's eye) could be sufficient in therapeutical contexts. Exposure to propositional thought, instead of vivid mental imagery, would be more tolerable for patients since vivid imagery is associated with high emotional distress. To investigate whether mental imagery or propositional thought is crucial for the success of imaginal exposure, participants with the rare state of aphantasia (= absence of sensory mental imagery but with intact propositional thought) and two control groups were subjected to a fear conditioning paradigm followed by imaginal exposure and a reinstatement procedure. During imaginal exposure, control group 1 (N = 30) stared at a bright screen to disrupt visual imagery by incoming luminance (= simulated aphantasia), whereas control group 2 (N = 30) and participants with actual aphantasia (N = 30) kept their eyes closed. The results show […].

Post-traumatic Stress Disorder: A Narrative Review of Pharmacological and Psychotherapeutic Interventions

Post-traumatic stress disorder (PTSD) is a complex mental health condition affecting individuals exposed to traumatic events. This paper is a narrative review of the existing literature on pharmacological and psychotherapeutic interventions for PTSD. Treatment includes selective serotonin reuptake inhibitors (SSRIs), serotonin-norepinephrine reuptake inhibitors (SNRIs), and alpha-1 adrenergic receptor antagonists. By exploring the outcomes of these interventions, the review seeks to provide valuable insights into their potential as PTSD treatment options. The paper also highlights the importance of tailoring treatment plans to individual needs and discusses emerging treatments, such as mindfulness-based therapies, virtual reality therapy, and neurostimulation techniques. By integrating findings from various studies, it aims to offer valuable information to optimize treatment strategies and enhance outcomes for individuals suffering from PTSD. The goal is to support informed decision-making, ultimately leading to more effective and tailored approaches to address the challenges posed by this debilitating condition.

How engram mediates learning, extinction, and relapse

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

Trait anxiety is associated with hidden state inference during aversive reversal learning

Updating beliefs in changing environments can be driven by gradually adapting expectations or by relying on inferred hidden states (i.e. contexts), and changes therein. Previous work suggests that increased reliance on context could underly fear relapse phenomena that hinder clinical treatment of anxiety disorders. We test whether trait anxiety variations in a healthy population influence how much individuals rely on hidden-state inference. In a Pavlovian learning task, participants observed cues that predicted an upcoming electrical shock with repeatedly changing probability, and were asked to provide expectancy ratings on every trial. We show that trait anxiety is associated with steeper expectation switches after contingency reversals and reduced oddball learning. Furthermore, trait anxiety is related to better fit of a state inference, compared to a gradual learning, model when contingency changes are large. Our findings support previous work suggesting hidden-state inference as a mechanism behind anxiety-related to fear relapse phenomena.

Pre-treatment amygdala activation and habituation predict symptom change in post-traumatic stress disorder

Trauma-focused psychotherapy approaches are the first-line treatment option for post-traumatic stress disorder (PTSD); however, up to a third of patients remain symptomatic even after completion of the treatment. Predicting which patients will respond to a given treatment option would support personalized treatments and improve the efficiency of healthcare systems. Although previous neuroimaging studies have examined possible pre-treatment predictors of response to treatment, the findings have been somewhat inconsistent, and no other study has examined habituation to stimuli as a predictor. In this study, 16 treatment-seeking adults (M_Age = 43.63, n = 10 women) with a primary diagnosis of PTSD passively viewed pictures of emotional facial expressions during functional magnetic resonance imaging (fMRI). After scanning, participants rated facial expressions on both valence and arousal. Participants then completed eight weekly sessions of prolonged exposure (PE) therapy. PTSD symptom severity was measured before and after treatment. Overall, participants showed symptomatic improvement with PE. Consistent with hypotheses, lesser activation in the amygdala and greater activation in the ventromedial prefrontal cortex during the presentation of fearful vs. happy facial expressions, as well as a greater decline in amygdala activation across blocks of fearful facial expressions at baseline, were associated with greater reduction of PTSD symptoms. Given that the repeated presentation of emotional material underlies PE, changes in brain responses with repeated stimulus presentations warrant further studies as potential predictors of response to exposure therapies.

Acan downregulation in parvalbumin GABAergic cells reduces spontaneous recovery of fear memories

While persistence of fear memories is essential for survival, a failure to inhibit fear in response to harmless stimuli is a feature of anxiety disorders. Extinction training only temporarily suppresses fear memory recovery in adults, but it is highly effective in juvenile rodents. Maturation of GABAergic circuits, in particular of parvalbumin-positive (PV+) cells, restricts plasticity in the adult brain, thus reducing PV+ cell maturation could promote the suppression of fear memories following extinction training in adults. Epigenetic modifications such as histone acetylation control gene accessibility for transcription and help couple synaptic activity to changes in gene expression. Histone deacetylase 2 (Hdac2), in particular, restrains both structural and functional synaptic plasticity. However, whether and how Hdac2 controls the maturation of postnatal PV+ cells is not well understood. Here, we show that PV+- cell specific Hdac2 deletion limits spontaneous fear memory recovery in adult mice, while enhancing PV+ cell bouton remodeling and reducing perineuronal net aggregation around PV+ cells in prefrontal cortex and basolateral amygdala. Prefrontal cortex PV+ cells lacking Hdac2, show reduced expression of Acan, a critical perineuronal net component, which is rescued by Hdac2 re-expression. Pharmacological inhibition of Hdac2 before extinction training is sufficient to reduce both spontaneous fear memory recovery and Acan expression in wild-type adult mice, while these effects are occluded in PV+-cell specific Hdac2 conditional knockout mice. Finally, a brief knock-down of Acan expression mediated by intravenous siRNA delivery before extinction training but after fear memory acquisition is sufficient to reduce spontaneous fear recovery in wild-type mice. Altogether, these data suggest that controlled manipulation of PV+ cells by targeting Hdac2 activity, or the expression of its downstream effector Acan, promotes the long-term efficacy of extinction training in adults.

Peroxisome proliferator-activated receptor-α activation facilitates contextual fear extinction and modulates intrinsic excitability of dentate gyrus neurons

The dentate gyrus (DG) of the hippocampus encodes contextual information associated with fear, and cell activity in the DG is required for acquisition and extinction of contextual fear. However, the underlying molecular mechanisms are not fully understood. Here we show that mice deficient for peroxisome proliferator-activated receptor-α (PPARα) exhibited a slower rate of contextual fear extinction. Furthermore, selective deletion of PPARα in the DG attenuated, while activation of PPARα in the DG by local infusion of aspirin facilitated extinction of contextual fear. The intrinsic excitability of DG granule neurons was reduced by PPARα deficiency but increased by activation of PPARα with aspirin. Using RNA-Seq transcriptome we found that the transcription level of neuropeptide S receptor 1 (Npsr1) was tightly correlated with PPARα activation. Our results provide evidence that PPARα plays an important role in regulating DG neuronal excitability and contextual fear extinction.

Altered fear engram encoding underlying conditioned versus unconditioned stimulus-initiated memory updating

It is known that post-retrieval extinction but not extinction alone could erase fear memory. However, whether the coding pattern of original fear engrams is remodeled or inhibited remains largely unclear. We found increased reactivation of engram cells in the prelimbic cortex and basolateral amygdala during memory updating. Moreover, conditioned stimulus- and unconditioned stimulus-initiated memory updating depends on the engram cell reactivation in the prelimbic cortex and basolateral amygdala, respectively. Last, we found that memory updating causes increased overlapping between fear and extinction cells, and the original fear engram encoding was altered during memory updating. Our data provide the first evidence to show the overlapping ensembles between fear and extinction cells and the functional reorganization of original engrams underlying conditioned stimulus- and unconditioned stimulus-initiated memory updating.

2MDR, a Microcomputer-Controlled Visual Stimulation Device for Psychotherapy-Like Treatments of Mice

Post-traumatic stress disorder and other mental disorders can be treated by an established psychotherapy called Eye Movement Desensitization and Reprocessing (EMDR). In EMDR, patients are confronted with traumatic memories while they are stimulated with alternating bilateral stimuli (ABS). How ABS affects the brain and whether ABS could be adapted to different patients or mental disorders is unknown. Interestingly, ABS reduced conditioned fear in mice. Yet, an approach to systematically test complex visual stimuli and compare respective differences in emotional processing based on semiautomated/automated behavioral analysis is lacking. We developed 2MDR (MultiModal Visual Stimulation to Desensitize Rodents), a novel, open-source, low-cost, customizable device that can be integrated in and transistor-transistor logic (TTL) controlled by commercial rodent behavioral setups. 2MDR allows the design and precise steering of multimodal visual stimuli in the head direction of freely moving mice. Optimized videography allows semiautomatic analysis of rodent behavior during visual stimulation. Detailed building, integration, and treatment instructions along with open-source software provide easy access for inexperienced users. Using 2MDR, we confirmed that EMDR-like ABS persistently improves fear extinction in mice and showed for the first time that ABS-mediated anxiolytic effects strongly depend on physical stimulus properties such as ABS brightness. 2MDR not only enables researchers to interfere with mouse behavior in an EMDR-like setting, but also demonstrates that visual stimuli can be used as a noninvasive brain stimulation to differentially alter emotional processing in mice.

Sleep efficiency predicts improvements in fear extinction and PTSD symptoms during prolonged exposure for veterans with comorbid insomnia

Prolonged exposure (PE) is an evidenced-based psychotherapy for PTSD, but many Veterans fail to achieve a clinically meaningful response. Sleep issues are prevalent in Veterans and may interfere with PE by disrupting the learning and consolidation of fear extinction memories during PE exposures. Here, we examined whether changes in fear extinction across imaginal exposures and PTSD symptoms during PE were predicted by diary-assessed levels of nightly sleep efficiency (SE; i.e., percent of time in bed spent sleeping), which may indirectly index sleep fragmentation and sleep-facilitated memory processes. Participants were Veterans with PTSD and comorbid insomnia (N = 40) participating in a clinical trial of cognitive-behavioral therapy for insomnia plus PE. SE was measured via nightly sleep diaries, fear extinction was operationalized as a reduction in peak distress between weekly imaginal exposures, and PTSD symptoms were assessed bi-weekly. Cross-lagged panel models revealed that higher sleep efficiency during the week predicted lower peak distress at the subsequent imaginal exposure and lower PTSD symptoms at the subsequent assessment, whereas PTSD symptoms and peak distress did not predict subsequent sleep efficiency. Efficient sleep may facilitate fear extinction and PTSD reduction during PE. Targeting sleep efficiency could improve PE effectiveness for Veterans with comorbid insomnia.

Stressor control and regional inflammatory responses in the brain: regulation by the basolateral amygdala

Increasing evidence has connected the development of certain neuropsychiatric disorders, as well as neurodegenerative diseases, to stress-induced dysregulation of the immune system. We have shown that escapable (ES) and inescapable (IS) footshock stress, and memories associated with ES or IS, can differentially alter inflammatory-related gene expression in brain in a region dependent manner. We have also demonstrated that the basolateral amygdala (BLA) regulates stress- and fear memory-induced alterations in sleep, and that differential sleep and immune responses in the brain to ES and IS appear to be integrated during fear conditioning and then reproduced by fear memory recall. In this study, we investigated the role of BLA in influencing regional inflammatory responses within the hippocampus (HPC) and medial prefrontal cortex (mPFC) by optogenetically stimulating or inhibiting BLA in male C57BL/6 mice during footshock stress in our yoked shuttlebox paradigm based on ES and IS. Then, mice were immediately euthanized and RNA extracted from brain regions of interest and loaded into NanoString® Mouse Neuroinflammation Panels for compilation of gene expression profiles. Results showed differential regional effects in gene expression and activated pathways involved in inflammatory-related signaling following ES and IS, and these differences were altered depending on amygdalar excitation or inhibition. These findings demonstrate that the stress-induced immune response, or "parainflammation", is affected by stressor controllability and that BLA influences regional parainflammation to ES or IS in HPC and mPFC. The study illustrates how stress-induced parainflammation can be regulated at the neurocircuit level and suggests that this approach can be useful for uncovering circuit and immune interactions in mediating differential stress outcomes.

Modulation of synapse-related gene expression in the cerebellum and prefrontal cortex of rats subjected to the contextual fear conditioning paradigm

The contextual fear conditioning (CFC) paradigm is the most productive approach for understanding the neurobiology of learning and memory as it allows to follow the evolution of memory traces of a conditioned stimulus and a specific context. The formation of long-term memory involves alterations in synaptic efficacy and neural transmission. It is known that the prefrontal cortex (PFC) exerts top-down control over subcortical structures to regulate behavioural responses. Moreover, cerebellar structures are involved in storing conditioned responses. The purpose of this research was to determine if the response to conditioning and stressful challenge is associated with alterations in synapse-related genes mRNA levels in the PFC, cerebellar vermis (V), and hemispheres (H) of young adult male rats. Four groups of Wistar rats were examined: naïve, CFC, shock only (SO), and exploration (EXPL). The behavioural response was evaluated by measuring the total freezing duration. Real-Time PCR was employed to quantify mRNA levels of some genes involved in synaptic plasticity. The results obtained from this study showed alterations in gene expression in different synapse-related genes after exposure to stressful stimuli and positioning to new environment. In conclusion, conditioning behavioural stimuli change the expression profile of molecules involved in neural transmission.

Cerebellar nuclei neurons projecting to the lateral parabrachial nucleus modulate classical fear conditioning

Multiple brain regions are engaged in classical fear conditioning. Despite evidence for cerebellar involvement in fear conditioning, the mechanisms by which cerebellar outputs modulate fear learning and memory remain unclear. We identify a population of deep cerebellar nucleus (DCN) neurons with monosynaptic glutamatergic projections to the lateral parabrachial nucleus (lPBN) (DCN→lPBN neurons) in mice. While optogenetic suppression of DCN→lPBN neurons impairs auditory fear memory, activation of DCN→lPBN neurons elicits freezing behavior only after auditory fear conditioning. Moreover, auditory fear conditioning potentiates DCN-lPBN synapses, and subsequently, auditory cue activates lPBN neurons after fear conditioning. Furthermore, DCN→lPBN neuron activation can replace the auditory cue but not footshock in fear conditioning. These findings demonstrate that cerebellar nuclei modulate auditory fear conditioning via transmitting conditioned stimuli signals to the lPBN. Collectively, our findings suggest that the DCN-lPBN circuit is a part of neuronal substrates within interconnected brain regions underscoring auditory fear memory.

Selective Disruption of Perineuronal Nets in Mice Lacking Crtl1 is Sufficient to Make Fear Memories Susceptible to Erasure

The ability to store, retrieve, and extinguish memories of adverse experiences is an essential skill for animals' survival. The cellular and molecular factors that underlie such processes are only partially known. Using chondroitinase ABC treatment targeting chondroitin sulfate proteoglycans (CSPGs), previous studies showed that the maturation of the extracellular matrix makes fear memory resistant to deletion. Mice lacking the cartilage link protein Crtl1 (Crtl1-KO mice) display normal CSPG levels but impaired CSPG condensation in perineuronal nets (PNNs). Thus, we asked whether the presence of PNNs in the adult brain is responsible for the appearance of persistent fear memories by investigating fear extinction in Crtl1-KO mice. We found that mutant mice displayed fear memory erasure after an extinction protocol as revealed by analysis of freezing and pupil dynamics. Fear memory erasure did not depend on passive loss of retention; moreover, we demonstrated that, after extinction training, conditioned Crtl1-KO mice display no neural activation in the amygdala (Zif268 staining) in comparison to control animals. Taken together, our findings suggest that the aggregation of CSPGs into PNNs regulates the boundaries of the critical period for fear extinction.

Neuropharmacological Modulation of N-methyl-D-aspartate, Noradrenaline and Endocannabinoid Receptors in Fear Extinction Learning: Synaptic Transmission and Plasticity

Learning to recognize and respond to potential threats is crucial for survival. Pavlovian threat conditioning represents a key paradigm for investigating the neurobiological mechanisms of fear learning. In this review, we address the role of specific neuropharmacological adjuvants that act on neurochemical synaptic transmission, as well as on brain plasticity processes implicated in fear memory. We focus on novel neuropharmacological manipulations targeting glutamatergic, noradrenergic, and endocannabinoid systems, and address how the modulation of these neurobiological systems affects fear extinction learning in humans. We show that the administration of N-methyl-D-aspartate (NMDA) agonists and modulation of the endocannabinoid system by fatty acid amide hydrolase (FAAH) inhibition can boost extinction learning through the stabilization and regulation of the receptor concentration. On the other hand, elevated noradrenaline levels dynamically modulate fear learning, hindering long-term extinction processes. These pharmacological interventions could provide novel targeted treatments and prevention strategies for fear-based and anxiety-related disorders.

A novel probe of attentional bias for threat in specific phobia: Application of the "MouseView.js" approach

Although attentional bias for threat has been implicated in anxiety disorders, traditional attentional bias measures have been criticized for lack of reliability and validity, and eye tracking technologies can be cost-prohibitive. MouseView.js was recently developed to mimic eye tracking online by using the computer cursor as a proxy for gaze, and although it is equally reliable, MouseView.js' utility for capturing attentional bias for threat in anxiety-related disorders remains unclear. To fill this knowledge gap, snake phobic and non-phobic participants (N = 62) completed a behavioral avoidance task (BAT) and the MouseView.js task which consisted of 10-second exposures to blurred, side-by-side images of either pleasant-neutral or threat-neutral pairings and were instructed to freely move the mouse to reveal the images. Results demonstrated that snake phobic participants had significantly shorter average mouse dwell time on threat images than non-phobic individuals and showed a significant reduction in average dwell time on threat images following the first presentation of the threat-neutral pairing. Additionally, dwell time on threat images significantly mediated the group differences in steps completed on the BAT. Results highlight the utility of MouseView.js in capturing avoidant patterns of attentional bias for threat that may also partially drive avoidance in snake phobia. Implications for capturing attentional bias for threat in anxiety disorders more broadly are discussed.

Basal forebrain cholinergic signaling in the basolateral amygdala promotes strength and durability of fear memories

The basolateral amygdala (BLA) complex receives dense cholinergic projections from the nucleus basalis of Meynert (NBM) and the horizontal limb of the diagonal band of Broca (HDB). The present experiments examined whether these projections regulate the formation, extinction, and renewal of fear memories. This was achieved by employing a Pavlovian fear conditioning protocol and optogenetics in transgenic rats. Silencing NBM projections during fear conditioning weakened the fear memory produced by that conditioning and abolished its renewal after extinction. By contrast, silencing HDB projections during fear conditioning had no effect. Silencing NBM or HDB projections during extinction enhanced the loss of fear produced by extinction, but only HDB silencing prevented renewal. Next, we found that systemic blockade of nicotinic acetylcholine receptors during fear conditioning mimicked the effects produced by silencing NBM projections during fear conditioning. However, this blockade had no effect when given during extinction. These findings indicate that basal forebrain cholinergic signaling in the BLA plays a critical role in fear regulation by promoting strength and durability of fear memories. We concluded that cholinergic compounds may improve treatments for post-traumatic stress disorder by durably stripping fear memories from their fear-eliciting capacity.

NMDAR-dependent synaptic potentiation via APPL1 signaling is required for the accessibility of a prefrontal neuronal assembly in retrieving fear extinction

BACKGROUND

The ventromedial prefrontal cortex (vmPFC) has been viewed as a locus to store and recall extinction memory. However, the synaptic and cellular mechanisms underlying this process remain elusive.

METHODS

We combined transgenic mice, electrophysiological recording, activity-dependent cell labeling, and chemogenetic manipulation to analyze the role of adaptor protein APPL1 in the vmPFC for fear extinction retrieval.

RESULTS

We found that both constitutive and conditional APPL1 knockout decreases NMDA receptor (NMDAR) function in the vmPFC and impairs fear extinction retrieval. Moreover, APPL1 undergoes nuclear translocation during extinction retrieval. Blocking APPL1 nucleocytoplasmic translocation reduces NMDAR currents and disrupts extinction retrieval. We further identified a prefrontal neuronal ensemble that is both necessary and sufficient for the storage of extinction memory. Inducible APPL1 knockout in this ensemble abolishes NMDAR-dependent synaptic potentiation and disrupts extinction retrieval, while simultaneously chemogenetic activation of this ensemble rescues the impaired behaviors.

CONCLUSIONS

Therefore, our results indicate that a prefrontal neuronal ensemble stores extinction memory, and APPL1 signaling supports these neurons to retrieve extinction memory via controlling NMDAR-dependent potentiation.

The lifetime impact of stress on fear regulation and cortical function

A variety of stressful experiences can influence the ability to form and subsequently inhibit fear memory. While nonsocial stress can impact fear learning and memory throughout the lifespan, psychosocial stressors that involve negative social experiences or changes to the social environment have a disproportionately high impact during adolescence. Here, we review converging lines of evidence that suggest that development of prefrontal cortical circuitry necessary for both social experiences and fear learning is altered by stress exposure in a way that impacts both social and fear behaviors throughout the lifespan. Further, we suggest that psychosocial stress, through its impact on the prefrontal cortex, may be especially detrimental during early developmental periods characterized by higher sociability. This article is part of the Special Issue on 'Fear, Anxiety and PTSD'.