Convergent Coding of Recent and Remote Fear Memory in the Basolateral Amygdala

Effects of inhibiting the orexin system on nicotine reward memory and fear memory

The orexin system is associated with various brain functions, including reward processing and the stress response. Previous studies have suggested that orexin receptors (OXRs) antagonists could be beneficial in treating several mental disorders, such as drug addiction and post-traumatic stress disorder. However, the specific role of the orexin system in both appetitive (reward-related) and aversive (fear-related) memories remains unclear. In this study, we used nicotine-induced conditioned place preference (CPP) and fear conditioning models to evaluate the effects of suvorexant, a dual OXRs antagonist, and chemogenetic inhibition of orexin neurons in the lateral hypothalamus (LHA) on the retrieval, reconsolidation, and/or extinction of both nicotine reward memory and fear memory. Our findings indicated that suvorexant impaired the expression of nicotine CPP in a dose-dependent manner. These behavioral results aligned with neurochemical evidence from cFos staining, which showed activation of the LHA and orexin neurons in the LHA following the expression of nicotine CPP. Furthermore, using orexin-cre rats, we found that chemogenetic inhibition of orexin neurons in the LHA also reduced the expression of nicotine CPP. However, neither of these interventions affected the reconsolidation of nicotine reward memory. Moreover, neither suvorexant nor the chemogenetic inhibition of LHA orexin neurons affected the retrieval, reconsolidation, or extinction of fear memory, or anxiety-like behaviors, as evaluated by elevated plus maze and open field tests. In summary, our data suggest that the orexin system plays a crucial role in retrieving nicotine reward memory, and suggest that suvorexant may be a promising pharmacotherapy for treating nicotine addiction.

How Fear Memory is Updated: From Reconsolidation to Extinction?

Post-traumatic stress disorder (PTSD) is a psychiatric disorder caused by traumatic past experiences, rooted in the neurocircuits of fear memory formation. Memory processes include encoding, storing, and recalling to forgetting, suggesting the potential to erase fear memories through timely interventions. Conventional strategies such as medications or electroconvulsive therapy often fail to provide permanent relief and come with significant side-effects. This review explores how fear memory may be erased, particularly focusing on the mnemonic phases of reconsolidation and extinction. Reconsolidation strengthens memory, while extinction weakens it. Interfering with memory reconsolidation could diminish the fear response. Alternatively, the extinction of acquired memory could reduce the fear memory response. This review summarizes experimental animal models of PTSD, examines the nature and epidemiology of reconsolidation to extinction, and discusses current behavioral therapy aimed at transforming fear memories to treat PTSD. In sum, understanding how fear memory updates holds significant promise for PTSD treatment.

Functional synaptic connectivity of engrafted spinal cord neurons with locomotor circuitry in the injured spinal cord

Spinal cord injury (SCI) results in significant neurological deficits, with no currently available curative therapies. Neural progenitor cell (NPC) transplantation has emerged as a promising approach for neural repair, as graft-derived neurons (GDNs) can integrate into the host spinal cord and support axon regeneration. However, the mechanisms underlying functional recovery remain poorly understood. In this study, we investigate the synaptic integration of NPC-derived neurons into locomotor circuits, the projection patterns of distinct neuronal subtypes, and their potential to modulate motor circuit activity. Using transsynaptic tracing in a mouse thoracic contusion SCI model, we found that NPC-derived neurons form synaptic connections with host locomotor circuits, albeit at low frequencies. Furthermore, we mapped the axon projections of V0C and V2a interneurons, revealing distinct termination patterns within host spinal cord laminae. To assess functional integration, we employed chemogenetic activation of GDNs, which induced muscle activity in a subset of transplanted animals. However, NPC transplantation alone did not significantly improve locomotor recovery, highlighting a key challenge in the field. Our findings suggest that while GDNs can integrate into host circuits and modulate motor activity, synaptic connectivity remains a limiting factor in functional recovery. Future studies should focus on enhancing graft-host connectivity and optimizing transplantation strategies to maximize therapeutic benefits for SCI.

Integrating Aversive Memories in the Basolateral Amygdala

Decades of research have established a critical role of the basolateral complex of the amygdala (BLA) in the encoding and storage of aversive memories. Much of this work has utilized Pavlovian fear conditioning procedures in which animals experience a single aversive event. Although this effort has produced great insight into the neural mechanisms that support fear memories for an isolated aversive experience, much less is known about how amygdala circuits encode and integrate multiple emotional experiences. The emergence of methods to label and record neuronal ensembles over days allows a deeper understanding of how amygdala neurons encode and integrate distinct aversive episodes over time. Here, we review evidence that the BLA is an essential site for the persistent storage of long-term fear memory. As a long-term storage site for fear memory, a challenge for encoding multiple fear memories is the mechanisms by which BLA neurons allocate, integrate, and discriminate distinct experiences from one another. In this review, we discuss the historical evidence supporting the BLA as a critical site for long-term memory storage, as well as new evidence that stems from technological advances that allow researchers to simultaneously study the encoding and storage of multiple memory traces, including recent versus remote experiences. We explore the possibility that dysfunction in ensemble coding schemes contributes to the pathophysiology of posttraumatic stress disorder and argue that future studies should place increased emphasis on potential subregional differences in memory coding schemes in the amygdala to deepen our understanding of both normal and pathological emotional memory.

Auditory fear memory retrieval requires BLA-LS and LS-VMH circuitries via GABAergic and dopaminergic neurons

Fear and associated learning and memory are critical for developing defensive behavior. Excessive fear and anxiety are important components of post-traumatic stress disorder. However, the neurobiology of fear conditioning, especially tone-related fear memory retrieval, has not been clearly defined, which limits specific intervention development for patients with excessive fear and anxiety. Here, we show that auditory fear memory retrieval stimuli activate multiple brain regions including the lateral septum (LS). Inhibition of the LS and the connection between basolateral amygdala (BLA) and LS or between LS and ventromedial nucleus of the hypothalamus (VMH) attenuates tone-related fear conditioning and memory retrieval. Inhibiting GABAergic neurons or dopaminergic neurons in the LS also attenuates tone-related fear conditioning. Our data further show that fear conditioning is inhibited by blocking orexin B signaling in the LS. Our results indicate that the neural circuitries BLA-LS and LS-VMH are critical for tone-related fear conditioning and memory retrieval, and that GABAergic neurons, dopaminergic neurons and orexin signaling in the LS participate in this auditory fear conditioning.

Ethograms predict visual fear conditioning status in rats

Recognizing and responding to threat cues is essential to survival. Freezing is a predominant threat behavior in rats. We have recently shown that a threat cue can organize diverse behaviors beyond freezing, including locomotion (Chu et al., 2024). However, that experimental design was complex, required many sessions, and had rats receive many foot shock presentations. Moreover, the findings were descriptive. Here, we gave female and male Long Evans rats cue light illumination paired or unpaired with foot shock (eight total) in a conditioned suppression setting using a range of shock intensities (0.15, 0.25, 0.35, or 0.50 mA). We found that conditioned suppression was only observed at higher foot shock intensities (0.35 mA and 0.50 mA). We constructed comprehensive temporal ethograms by scoring 22,272 frames across 12 behavior categories in 200-ms intervals around cue light illumination. The 0.50 mA and 0.35 mA shock-paired visual cues suppressed reward seeking, rearing, and scaling, as well as light-directed rearing and light-directed scaling. These shock-paired visual cues further elicited locomotion and freezing. Linear discriminant analyses showed that ethogram data could accurately classify rats into paired and unpaired groups. Using complete ethogram data produced superior classification compared to behavior subsets, including an immobility subset featuring freezing. The results demonstrate diverse threat behaviors - in a short and simple procedure - containing sufficient information to distinguish the visual fear conditioning status of individual rats.

Combined loss of brevican, neurocan, tenascin-C and tenascin-R leads to impaired fear retrieval due to perineuronal net loss

In conditions such as neurodegenerative diseases, posttraumatic stress disorder (PTSD), addiction and spinal cord injuries, restricted synaptic plasticity hinders the formation of new neuronal connections, preventing the compensation and treatment of adverse behaviors. Perineuronal nets (PNNs) significantly restrict synaptic plasticity by inhibiting synapse formation. The digestion of PNNs has been associated with short-term cognitive improvements and reduced long-term memory, offering potential therapeutic benefits in PTSD. This study investigates the correlation between PNNs and fear memory processes in extracellular matrix (ECM) mutant mice, particularly focusing on the amygdala-medial prefrontal cortex (mPFC) circuit, which is crucial for fear memory generation and maintenance. Fear conditioning was conducted on mice lacking four key ECM-molecules: brevican, neurocan, tenascin-C and tenascin-R (4x KO). These mice exhibited severe impairments in memory consolidation, as evident by their inability to retrieve previously learned fear memories, coupled with reduced PNN density and disturbed synaptic integrity along their PNNs. Additionally, changes in neural activity in the basolateral amygdala (BL) and reductions in VGAT+ synaptic puncta in the amygdala-mPFC circuit were observed. In contrast, tenascin single KOs showed intact fear behavior and memory compared to their control groups. Impaired fear memory consolidation can be advantageous in certain conditions, such as PTSD, making the 4x KO mice an intriguing model for future fear conditioning studies and highlighting brevican, neurocan, Tnc, and Tnr as compelling targets for further investigation. This study underscores the significance of ECM regulation for synaptic organization and the potential of PNN modulation as a therapeutic target for fear memory-related conditions.

Multi-regional control of amygdalar dynamics reliably reflects fear memory age

The basolateral amygdala (BLA) is crucial for the encoding and expression of fear memory, yet it remains unexplored how neural activity in this region is dynamically influenced by distributed circuits across the brain to facilitate expression of fear memory of different ages. Using longitudinal multisite electrophysiological recordings in male mice, we find that the recall of older contextual fear memory is accompanied by weaker, yet more rhythmic, BLA gamma activity which is distally entrained by theta oscillations in both the hippocampal CA1 and the anterior cingulate cortex. Computational modeling with Light Gradient Boosting Machine using extracted oscillatory features from these three regions, as well as with Transformer using raw local field potentials, accurately classified remote from recent memory recall primarily based on BLA gamma and CA1 theta. These results demonstrate in a non-biased manner that multi-regional control of BLA activity serves as reliable neural signatures for memory age-dependent recall mechanisms.

Ventral Pallidum Neurons Are Necessary to Generalize and Express Fear-Related Responding in a Minimal Threat Setting

Fear generalization is a hallmark of anxiety disorders. Experimentally, fear generalization can be difficult to dissociate from its counterpart, fear discrimination. Here, we use minimal threat learning procedures to reveal such a dissociation. We show that in Long-Evans rats, an auditory threat cue predicting footshock on 10% of trials produces a discriminated fear response that does not generalize to a neutral auditory cue. In contrast, even slightly higher footshock probabilities (30 and 20%) produce fear generalization. AAV-mediated, caspase-3 deletion of ventral pallidum neurons abolishes fear generalization and reduces threat cue responding during extinction. The ventral pallidum's contribution to fear generalization and extinction threat responding does not depend on inputs from the nucleus accumbens. The results demonstrate a minimal threat learning approach to dissociate fear discrimination from fear generalization and a novel role for the ventral pallidum in generalizing and expressing fear.

Transcriptomic diversity of amygdalar subdivisions across humans and nonhuman primates

The amygdaloid complex mediates learning, memory, and emotions. Understanding the cellular and anatomical features that are specialized in the amygdala of primates versus other vertebrates requires a systematic, anatomically-resolved molecular analysis of constituent cell populations. We analyzed five nuclear subdivisions of the primate amygdala with single-nucleus RNA sequencing in macaques, baboons, and humans to examine gene expression profiles for excitatory and inhibitory neurons and confirmed our results with single-molecule FISH analysis. We identified distinct subtypes of FOXP2 + interneurons in the intercalated cell masses and protein-kinase C-δ interneurons in the central nucleus. We also establish that glutamatergic, pyramidal-like neurons are transcriptionally specialized within the basal, lateral, or accessory basal nuclei. Understanding the molecular heterogeneity of anatomically-resolved amygdalar neuron types provides a cellular framework for improving existing models of how amygdalar neural circuits contribute to cognition and mental health in humans by using nonhuman primates as a translational bridge.

Impacts of linseed oil diet on anxiety and memory extinction after early life stress: A sex-specific analysis of mitochondrial dysfunction, astrocytic markers, and inflammation in the amygdala

Early exposure to stressors affects how the organism reacts to stimuli, its emotional state throughout life, and how it deals with emotional memories. Consequently, it may affect susceptibility to psychopathology later in life. We used an animal model of early stress by maternal separation to study its potential impact on the extinction of aversive memories and anxiety-like behavior in adulthood, as well as its effects on mitochondrial functionality, inflammatory and astrocytic markers in the amygdala. We also assessed whether a diet enriched with linseed oil, known for its high content in omega-3 fats, could be used to attenuate the behavioral and neurochemical effects of early stress. Litters of Wistar rats were divided into controls (intact) or subjected to maternal separation (MS). They were subdivided into two groups receiving isocaloric diets enriched in soy or linseed oils at weaning. In adulthood, the animals were exposed to the open field and the elevated plus maze, to evaluate exploratory activity and anxiety-like behavior. They were also trained in a context of fear conditioning, and afterward subjected to an extinction session, followed by a test session to evaluate the extinction memory. Amygdalae were evaluated for inflammatory cytokines (interleukin (IL)-1beta, IL-6, and tumor-necrose factor (TNF)-alpha), mitochondrial functionality, and astrocyte markers (glial fibrillary acidic protein - GFAP, S100B, and glutamine synthetase activity). MS induced anxiety-like behavior in the elevated plus-maze, which was reversed by a diet enriched in linseed oil offered from weaning. When testing the memory of an extinction session of fear conditioning, MS animals showed more freezing behavior. MS males receiving a linseed oil-enriched diet had lower functional mitochondria in the amygdala. In addition, MS led to increased inflammatory cytokines, particularly IL-1beta, and the diet enriched in linseed oil further increased these levels in MS animals. MS also increased S100B levels. These results point to a higher emotionality presented by MS animals, with higher levels of inflammatory cytokines and S100B. While a diet enriched in linseed oil attenuated anxiety-like behavior, it further altered amygdala IL-1beta and reduced mitochondria functionality, particularly in males. MS also increased glutamine synthetase activity in the amygdala, and this effect was higher when the animals received a diet enriched in linseed oil, particularly in females. In conclusion, these results point to MS effects on emotional behavior, and neurochemical alterations in the amygdala, with sex-specific effects. Although a diet enriched in linseed oil appears to be able to reverse some of MS behavioral effects, these results must be considered with caution, since biochemical parameters could be worsened in MS animals receiving a linseed oil-enriched diet. This knowledge is important for the understanding of mechanisms of action of strategies aiming to reverse early stress effects, and future studies are warranted to determine possible interventions to promote resilience.

Activation of trace amine-associated receptor 1 ameliorates PTSD-like symptoms

Trace amine-associated receptor 1 (TAAR1) negatively modulates monoaminergic transmission in the mammalian brain and participates in many psychiatric disorders. Preclinical evidence indicate that selective TAAR1 agonists have anxiolytic effects and anti-stress properties. Post-traumatic stress disorder (PTSD) is an anxiety disorder triggered by experiencing or witnessing traumatic stressors. However, it remains unknown whether TAAR1 is involved in PTSD. Here, we investigated the role of TAAR1 in two PTSD animal models, including single prolonged stress (SPS)-induced impairment of fear extinction and stress-enhanced fear learning (SEFL). SPS decreased TAAR1 mRNA levels in the prefrontal cortex and ventral tegmental area. Acute treatment of the TAAR1 partial agonist RO5263397 attenuated SPS-induced anxiety-like behavior evaluated by the elevated-plus maze test. Compared to non-stressed animals, rats that experienced SPS showed higher freezing levels in the extinction retention test, indicating an impairment of fear extinction retention after SPS exposure. Acute and chronic treatment of RO5263397 ameliorated SPS-induced impairment of fear extinction retention. In the SEFL model, compared to the No-shock group, rats that experienced severe foot shock before fear conditioning showed higher freezing levels during the tests, indicating enhanced fear learning after stress exposure. Chronic treatment of RO5263397 partially attenuated the SEFL. Moreover, chronic treatment with the selective TAAR1 full agonist RO5166017 completely prevented the SEFL. Taken together, these data showed that pharmacological activation of TAAR1 could ameliorate PTSD-like symptoms. The present study thus provides the first evidence that TAAR1 might participate in the development of PTSD, and TAAR1 agonists could be potential pharmacological treatments for this disorder.

Behavioral outputs and overlapping circuits between conditional fear and active avoidance

Aversive learning can produce a wide variety of defensive behavioral responses depending on the circumstances, ranging from reactive responses like freezing to proactive avoidance responses. While most of this initial learning is behaviorally supported by an expectancy of an aversive outcome and neurally supported by activity within the basolateral amygdala, activity in other brain regions become necessary for the execution of defensive strategies that emerge in other aversive learning paradigms such as active avoidance. Here, we review the neural circuits that support both reactive and proactive defensive behaviors that are motivated by aversive learning, and identify commonalities between the neural substrates of these distinct (and often exclusive) behavioral strategies.

Neurotoxic lesions of the anterior claustrum influence cued fear memory in rats

Background

The claustrum (CLA), a subcortical area between the insular cortex and striatum, innervates almost all cortical regions of the mammalian brain. There is growing evidence that CLA participates in many brain functions, including memory, cognition, and stress response. It is proposed that dysfunction or malfunction of the CLA might be the pathology of some brain diseases, including stress-induced depression and anxiety. However, the role of the CLA in fear memory and anxiety disorders remains largely understudied.

Methods

We evaluated the influences of neurotoxic lesions of the CLA using auditory-cued fear memory and anxiety-like behaviors in rats.

Results

We found that lesions of anterior CLA (aCLA) but not posterior CLA (pCLA) before fear conditioning attenuated fear retrieval, facilitated extinction, and reduced freezing levels during the extinction retention test. Post-learning lesions of aCLA but not pCLA facilitated fear extinction and attenuated freezing behavior during the extinction retention test. Lesions of aCLA or pCLA did not affect anxiety-like behaviors evaluated by the open field test and elevated plus-maze test.

Conclusion

These data suggested that aCLA but not pCLA was involved in fear memory and extinction. Future studies are needed to further investigate the anatomical and functional connections of aCLA subareas that are involved in fear conditioning, which will deepen our understanding of CLA functions.

Elevated fear states facilitate ventral hippocampal engagement of basolateral amygdala neuronal activity

Fear memory formation and retention rely on the activation of distributed neural circuits. The basolateral amygdala (BLA) and ventral hippocampus (VH) in particular are two regions that support contextual fear memory processes and share reciprocal connections. The VH → BLA pathway is critical for increases in fear after initial learning, in both fear renewal following extinction learning and during fear generalization. This raises the possibility that functional changes in VH projections to the BLA support increases in learned fear. In line with this, fear can also be increased with alterations to the original content of the memory via reconsolidation, as in fear elevation procedures. However, very little is known about the functional changes in the VH → BLA pathway supporting reconsolidation-related increases in fear. In this study, we used in vivo extracellular electrophysiology to examine the functional neuronal changes within the BLA and in the VH → BLA pathway as a result of fear elevation and standard fear retrieval procedures. Elevated fear expression was accompanied by higher BLA spontaneous firing compared to a standard fear retrieval condition. Across a range of stimulation frequencies, we also found that VH stimulation evoked higher BLA firing following fear elevation compared to standard retrieval. These results suggest that fear elevation is associated with an increased capacity of the VH to drive neuronal activity in the BLA, highlighting a potential circuit involved in strengthening existing fear memories.

Neurobiology of Stress-Induced Nicotine Relapse

Tobacco smoking is the leading cause of preventable death and disease. Although there are some FAD-approved medicines for controlling smoking, the relapse rate remains very high. Among the factors that could induce nicotine relapse, stress might be the most important one. In the last decades, preclinical studies have generated many new findings that lead to a better understanding of stress-induced relapse of nicotine-seeking. Several molecules such as α3β4 nicotinic acetylcholine receptor, α2-adrenergic receptors, cannabinoid receptor 1, trace amine-associated receptor 1, and neuropeptide systems (corticotropin-releasing factor and its receptors, dynorphine and kappa opioid receptor) have been linked to stress-induced nicotine relapse. In this review, we discuss recent advances in the neurobiology, treatment targets, and potential therapeutics of stress-induced nicotine relapse. We also discuss some factors that may influence stress-induced nicotine relapse and that should be considered in future studies. In the final section, a perspective on some research directions is provided. Further investigation on the neurobiology of stress-induced nicotine relapse will shed light on the development of new medicines for controlling smoking and will help us understand the interactions between the stress and reward systems in the brain.

Pavlovian Fear Conditioning Is More than You Think It Is

A common neuroscience application of Pavlovian fear conditioning is to manipulate neuron-type activity, pair a cue with foot shock, then measure cue-elicited freezing in a novel context. If the manipulation reduces freezing, the neuron type is implicated in Pavlovian fear conditioning. This application reduces Pavlovian fear conditioning to a single concept. In this Viewpoint, I describe experiments supporting the view that Pavlovian fear conditioning refers to three distinct concepts: procedure, process, and behavior. An experimenter controls procedure, observes behavior, but infers process. Distinguishing these concepts is essential because: (1) a shock-paired cue can engage numerous processes and behaviors; (2) experimenter decisions about procedure influence the processes engaged and behaviors elicited; and (3) many processes are latent, imbuing the cue with properties that only manifest outside of the original conditioning setting. This means we could understand the complete neural basis of freezing, yet know little about the neural basis of fear. Neuroscientists can choose to use a variety of procedures to study a diversity of processes and behaviors. Manipulating neuron-type activity in multiple procedures can reveal specific, general, or complex neuron-type contributions to cue-elicited processes and behaviors. The results will be a broader and more detailed neural basis of fear with greater relevance to the spectrum of symptoms defining anxiety and stressor-related disorders.

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.

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.

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

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

Basolateral Amygdala Astrocytes Are Engaged by the Acquisition and Expression of a Contextual Fear Memory

Astrocytes are key cellular regulators within the brain. The basolateral amygdala (BLA) is implicated in fear memory processing, yet most research has entirely focused on neuronal mechanisms, despite a significant body of work implicating astrocytes in learning and memory. In the present study, we used in vivo fiber photometry in C57BL/6J male mice to record from amygdalar astrocytes across fear learning, recall, and three separate periods of extinction. We found that BLA astrocytes robustly responded to foot shock during acquisition, their activity remained remarkably elevated across days in comparison to unshocked control animals, and their increased activity persisted throughout extinction. Further, we found that astrocytes responded to the initiation and termination of freezing bouts during contextual fear conditioning and recall, and this behavior-locked pattern of activity did not persist throughout the extinction sessions. Importantly, astrocytes do not display these changes while exploring a novel context, suggesting that these observations are specific to the original fear-associated environment. Chemogenetic inhibition of fear ensembles in the BLA did not affect freezing behavior or astrocytic calcium dynamics. Overall, our work presents a real-time role for amygdalar astrocytes in fear processing and provides new insight into the emerging role of these cells in cognition and behavior.SIGNIFICANCE STATEMENT We show that basolateral amygdala astrocytes are robustly responsive to negative experiences, like shock, and display changed calcium activity patterns through fear learning and memory. Additionally, astrocytic calcium responses become time locked to the initiation and termination of freezing behavior during fear learning and recall. We find that astrocytes display calcium dynamics unique to a fear-conditioned context, and chemogenetic inhibition of BLA fear ensembles does not have an impact on freezing behavior or calcium dynamics. These findings show that astrocytes play a key real-time role in fear learning and memory.

Anterior basolateral amygdala neurons comprise a remote fear memory engram

Introduction

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.

Methods

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.

Results

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.

Discussion

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.

Insights into the encoding of memories through the circuitry of fear

Associative learning induces physical changes to a network of cells, known as the memory engram. Fear is widely used as a model to understand the circuit motifs that underpin associative memories. Recent advances suggest that the distinct circuitry engaged by different conditioned stimuli (e.g. tone vs. context) can provide insights into what information is being encoded in the fear engram. Moreover, as the fear memory matures, the circuitry engaged indicates how information is remodelled after learning and hints at potential mechanisms for consolidation. Finally, we propose that the consolidation of fear memories involves plasticity of engram cells through coordinated activity between brain regions, and the inherent characteristics of the circuitry may mediate this process.

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.

(2R,6R)-hydroxynorketamine targeting the basolateral amygdala regulates fear memory

(2R,6R)-Hydroxynorketamine (HNK), a ketamine metabolite, has been proposed as an ideal next-generation antidepressant due to its rapid-acting and long-lasting antidepression-relevant actions. Interestingly, recent studies have shown that (2R,6R)-HNK may have diverse impacts on memory formation. However, its effect on fear memory extinction is still unknown. In the present study, we assessed the effects of (2R,6R)-HNK on synaptic transmission and plasticity in the basolateral amygdala (BLA) and explored its actions on auditory fear memory extinction. Adult male C57BL/6J mice were used in this study. The extracellular electrophysiological recording was conducted to assay synaptic transmission and plasticity. The auditory fear conditioning paradigm was performed to test fear extinction. The results showed that (2R,6R)-HNK at 30 mg/kg increased the number of c-fos-positive cells in the BLA. Moreover, (2R,6R)-HNK enhanced the induction and maintenance of long-term potentiation (LTP) in the BLA in a dose-dependent manner (at 1, 10, and 30 mg/kg). In addition, (2R,6R)-HNK at 30 mg/kg and directly slice perfusion of (2R,6R)-HNK enhanced BLA synaptic transmission. Furthermore, intra-BLA application and systemic administration of (2R,6R)-HNK reduced the retrieval of recent fear memory and decreased the retrieval of remote fear memory. Both local and systemic (2R,6R)-HNK also inhibited the spontaneous recovery of remote fear memory. Taken together, these results indicated that (2R,6R)-HNK could regulate BLA synaptic transmission and plasticity and act through the BLA to modulate fear memory. The results revealed that (2R,6R)-HNK may be a potential drug to treat posttraumatic stress disorder (PTSD) patients.

Negative valence systems: sustained threat and the predatory imminence continuum

This review describes the relationship between the National Institute of Mental Health (U.S.A.) Research Domain Criteria (RDoC) Negative Valence System related to responses to threat and the Predatory Imminence Continuum model of antipredator defensive behavior. While the original RDoC constructs of Potential Threat (anxiety) and Acute Threat (fear) fit well with the pre-encounter and post-encounter defense modes of the predatory imminence model, the Sustained Threat construct does not. Early research on the bed nuclei of the stria terminalis (BST) suggested that when fear responding needed to be sustained for a prolonged duration this region was important. However, follow-up studies indicated that the BST becomes critical not because responses needed to be sustained but rather when the stimuli triggering fear were more difficult to learn about, particularly when aversive stimuli were difficult to accurately predict. Instead, it is argued that the BST and the hippocampus act to expand the range of conditions that can trigger post-encounter defense (Acute Threat). It is further suggested that sustained threat refers to situations where the predatory imminence continuum becomes distorted causing defensive behavior to intrude into times when organisms should be engaging in other adaptive behaviors. Stress is seen as something that can cause a long-term disturbance of the continuum and this disturbance is a state of sustained threat.

Astrocyte secretes IL-6 to modulate PSD-95 palmitoylation in basolateral amygdala and depression-like behaviors induced by peripheral nerve injury

Dysfunction of glutamatergic synaptic plasticity in basolateral amygdala (BLA) constitutes a critical pathogenic mechanism underlying the depression-like behaviors induced by chronic pain. Astrocytes serve as an important supporting cell modulating glutamatergic synaptic transmission. Here, we found that peripheral spared nerve injury (SNI) induced astrocyte activation to release IL-6 in BLA. Inhibition of astrocyte activity attenuated SNI-induced IL-6 overexpression and depression-like behaviors. Moreover, SNI enhanced the abundance of DHHC2 in synaptosome and DHHC3 in Golgi apparatus, promoted PSD-95 palmitoylation, and increased the recruitment of GluR1 and NR2B at synapses. Suppression of IL-6 or PSD-95 palmitoylation attenuated the synaptic accumulation of GluR1 and NR2B in BLA and improved depression-like behaviors induced by SNI. Furthermore, IL-6 downstream PI3K increased the expression of DHHC3 in Golgi apparatus and facilitated the interaction of palmitoylated PSD-95 with GluR1 and NR2B at synapses. These findings collectively suggested that SNI activated astrocyte to release IL-6 in BLA, which promoted PSD-95 palmitoylation and enhanced the synaptic trafficking of GluR1 and NR2B, and subsequently mediated the depression-like behaviors induced by nerve injury.

Neural Oscillations in Aversively Motivated Behavior

Fear and anxiety-based disorders are highly debilitating and among the most prevalent psychiatric disorders. These disorders are associated with abnormal network oscillations in the brain, yet a comprehensive understanding of the role of network oscillations in the regulation of aversively motivated behavior is lacking. In this review, we examine the oscillatory correlates of fear and anxiety with a particular focus on rhythms in the theta and gamma-range. First, we describe neural oscillations and their link to neural function by detailing the role of well-studied theta and gamma rhythms to spatial and memory functions of the hippocampus. We then describe how theta and gamma oscillations act to synchronize brain structures to guide adaptive fear and anxiety-like behavior. In short, that hippocampal network oscillations act to integrate spatial information with motivationally salient information from the amygdala during states of anxiety before routing this information via theta oscillations to appropriate target regions, such as the prefrontal cortex. Moreover, theta and gamma oscillations develop in the amygdala and neocortical areas during the encoding of fear memories, and interregional synchronization reflects the retrieval of both recent and remotely encoded fear memories. Finally, we argue that the thalamic nucleus reuniens represents a key node synchronizing prefrontal-hippocampal theta dynamics for the retrieval of episodic extinction memories in the hippocampus.

Temporally and anatomically specific contributions of the human amygdala to threat and safety learning

Neural plasticity in subareas of the rodent amygdala is widely known to be essential for Pavlovian threat conditioning and safety learning. However, less consistent results have been observed in human neuroimaging studies. Here, we identify and test three important factors that may contribute to these discrepancies: the temporal profile of amygdala response in threat conditioning, the anatomical specificity of amygdala responses during threat conditioning and safety learning, and insufficient power to identify these responses. We combined data across multiple studies using a well-validated human threat conditioning paradigm to examine amygdala involvement during threat conditioning and safety learning. In 601 humans, we show that two amygdala subregions tracked the conditioned stimulus with aversive shock during early conditioning while only one demonstrated delayed responding to a stimulus not paired with shock. Our findings identify cross-species similarities in temporal- and anatomical-specific amygdala contributions to threat and safety learning, affirm human amygdala involvement in associative learning and highlight important factors for future associative learning research in humans.

Adaptation of Threat Responses Within the Negative Valence Framework

External threats are a major source of our experience of negatively valanced emotion. As a threat becomes closer and more real, our specific behavior patterns and our experiences of negative affect change in response to the perceived imminence of threat. Recognizing this, the National Institute of Mental Health's Research Domain Criteria (RDoC) Negative Valence system is largely based around different levels of threat imminence. This perspective describes the correspondence between the RDoC Negative Valence System and a particular neurobiological/neuroecological model of reactions to threat, the Predatory Imminence Continuum (PIC) Theory. Using the COVID-19 pandemic as an illustration, we describe both adaptive and maladaptive behavior patterns from this perspective to illustrate how behavior in response to a crisis may get shaped. We end with suggestions on how further consideration of the PIC suggests potential modifications of the negative valence systems RDoC.