Dynamic remodeling of a basolateral-to-central amygdala glutamatergic circuit across fear states

The representational geometry of emotional states in basolateral amygdala

Sensory stimuli associated with aversive outcomes cause multiple behavioral responses related to an animal's evolving emotional state, but neural mechanisms underlying these processes remain unclear. Here aversive stimuli were presented to mice, eliciting two responses reflecting fear and flight to safety: tremble and ingress into a virtual burrow. Inactivation of basolateral amygdala (BLA) eliminated differential responses to aversive and neutral stimuli without eliminating responses themselves, suggesting BLA signals valence, not motor commands. However, two-photon imaging revealed that neurons typically exhibited mixed selectivity for stimulus identity, valence, tremble and/or ingress. Despite heterogeneous selectivity, BLA representational geometry was lower-dimensional when encoding valence, tremble and safety, enabling generalization of emotions across conditions. Further, tremble and valence coding directions were orthogonal, allowing linear readouts to specialize. Thus BLA representational geometry confers two computational properties that identify specialized neural circuits encoding variables describing emotional states: generalization across conditions, and readouts lacking interference from other readouts.

BLA DBS improves anxiety and fear by correcting weakened synaptic transmission from BLA to adBNST and CeL in a mouse model of foot shock

Deep brain stimulation (DBS) in the basal lateral amygdala (BLA) has been established to correct symptoms of refractory post-traumatic stress disorder (PTSD). However, how BLA DBS operates in correcting PTSD symptoms and how the BLA elicits pathological fear and anxiety in PTSD remain unclear. Here, we discover that excitatory synaptic transmission from the BLA projection neurons (PNs) to the adBNST, and lateral central amygdala (CeL) is greatly suppressed in a mouse PTSD model induced by foot shock (FS). BLA DBS revises the weakened inputs from the BLA to these two areas to improve fear and anxiety. Optogenetic manipulation of the BLA-adBNST and BLA-CeL circuits shows that both circuits are responsible for anxiety but the BLA-CeL for fear in FS mice. Our results reveal that synaptic transmission dysregulation of the BLA-adBNST or BLA-CeL circuits is reversed by BLA DBS, which improves anxiety and fear in the FS mouse model.

Fear Learning: An Evolving Picture for Plasticity at Synaptic Afferents to the Amygdala

Unraveling the neuronal mechanisms of fear learning might allow neuroscientists to make links between a learned behavior and the underlying plasticity at specific synaptic connections. In fear learning, an innocuous sensory event such as a tone (called the conditioned stimulus, CS) acquires an emotional value when paired with an aversive outcome (unconditioned stimulus, US). Here, we review earlier studies that have shown that synaptic plasticity at thalamic and cortical afferents to the lateral amygdala (LA) is critical for the formation of auditory-cued fear memories. Despite the early progress, it has remained unclear whether there are separate synaptic inputs that carry US information to the LA to act as a teaching signal for plasticity at CS-coding synapses. Recent findings have begun to fill this gap by showing, first, that thalamic and cortical auditory afferents can also carry US information; second, that the release of neuromodulators contributes to US-driven teaching signals; and third, that synaptic plasticity additionally happens at connections up- and downstream of the LA. Together, a picture emerges in which coordinated synaptic plasticity in serial and parallel circuits enables the formation of a finely regulated fear memory.

MPP2 interacts with SK2 to rescue the excitability of glutamatergic neurons in the BLA and facilitate the extinction of conditioned fear in mice

AIMS

The basolateral amygdala (BLA) plays an integral role in anxiety disorders (such as post traumatic stress disorder) stem from dysregulated fear memory. The excitability of glutamatergic neurons in the BLA correlates with fear memory, and the afterhyperpolarization current (IAHP ) mediated by small-conductance calcium-activated potassium channel subtype 2 (SK2) dominates the excitability of glutamatergicneurons. This study aimed to explore the effect of MPP2 interacts with SK2 in the excitability of glutamatergic neurons in the BLA and the extinction of conditioned fear in mice.

METHODS

Fear memory was analyzed via freezing percentage. Western blotting and fluorescence quantitative PCR were used to determine the expression of protein and mRNA respectively. Electrophysiology was employed to measure the excitability of glutamatergic neurons and IAHP .

RESULTS

Fear conditioning decreased the levels of synaptic SK2 channels in the BLA, which were restored following fear extinction. Notably, reduced expression of synaptic SK2 channels in the BLA during fear conditioning was caused by the increased activity of protein kinase A (PKA), while increased levels of synaptic SK2 channels in the BLA during fear extinction were mediated by interactions with membrane-palmitoylated protein 2 (MPP2).

CONCLUSIONS

Our results revealed that MPP2 interacts with the SK2 channels and rescues the excitability of glutamatergic neurons by increasing the expression of synaptic SK2 channels in the BLA to promote the normalization of anxiety disorders and provide a new direction for the treatment.

Distinct Circuits From the Central Lateral Amygdala to the Ventral Part of the Bed Nucleus of Stria Terminalis Regulate Different Fear Memory

BACKGROUND

The ability to differentiate stimuli that predict fear is critical for survival; however, the underlying molecular and circuit mechanisms remain poorly understood.

METHODS

We combined transgenic mice, in vivo transsynaptic circuit-dissecting anatomical approaches, optogenetics, pharmacological methods, and electrophysiological recording to investigate the involvement of specific extended amygdala circuits in different fear memory.

RESULTS

We identified the projections from central lateral amygdala (CeL) protein kinase C δ (PKCδ)-positive neurons and somatostatin (SST)-positive neurons to GABAergic (gamma-aminobutyric acidergic) and glutamatergic neurons in the ventral part of the bed nucleus of stria terminalis (vBNST). Prolonged optogenetic activation or inhibition of the PKCδCeL-vBNST pathway specifically reduced context fear memory, whereas the SSTCeL-vBNST pathway mainly reduced tone fear memory. Intriguingly, optogenetic manipulation of vBNST neurons that received the projection from PKCδCeL neurons exerted bidirectional regulation of context fear, whereas manipulation of vBNST neurons that received the projection from SSTCeL neurons could bidirectionally regulate both context and tone fear memory. We subsequently demonstrated the presence of δ and κ opioid receptor protein expression within the CeL-vBNST circuits, potentially accounting for the discrepancy between prolonged activation of GABAergic circuits and inhibition of downstream vBNST neurons. Finally, administration of an opioid receptor antagonist cocktail on the PKCδCeL-vBNST or SSTCeL-vBNST pathway successfully restored context or tone fear memory reduction induced by prolonged activation of the circuits.

CONCLUSIONS

Together, these findings establish a functional role for distinct CeL-vBNST circuits in the differential regulation and appropriate maintenance of fear.

From aversive associations to defensive programs: experience-dependent synaptic modifications in the central amygdala

Plasticity elicited by fear conditioning (FC) is thought to support the storage of aversive associative memories. Although work over the past decade has revealed FC-induced plasticity beyond canonical sites in the basolateral complex of the amygdala (BLA), it is not known whether modifications across distributed circuits make equivalent or distinct contributions to aversive memory. Here, we review evidence demonstrating that experience-dependent synaptic plasticity in the central nucleus of the amygdala (CeA) has a circumscribed role in memory expression per se, guiding the selection of defensive programs in response to acquired threats. We argue that the CeA may be a key example of a broader phenomenon by which synaptic plasticity at specific nodes of a distributed network makes a complementary contribution to distinct memory processes.

Cannabidiol Differentially Modulates Synaptic Release and Cellular Excitability in Amygdala Subnuclei

Cannabidiol (CBD) is a non-psychoactive constituent of the Cannabis plant that has purported effectiveness in treating an array of stress-related neuropsychiatric disorders. The amygdala is a subcortical brain structure that regulates emotional behavior, and its dysfunction has been linked to numerous disorders including anxiety and posttraumatic stress disorder. Despite this, the direct effects of CBD on synaptic and cellular function in the amygdala are not known. Using electrophysiology and pharmacology, we report that CBD reduces presynaptic neurotransmitter release in the amygdala, and these effects are dependent on subnucleus and cell type. Furthermore, CBD broadly decreases cellular excitability across amygdala subnuclei. These data reveal physiological mechanisms by which CBD modulates amygdala activity and could provide insights into how CBD could affect emotional and stress-related behavioral responses.

Distinct populations of corticotropin-releasing factor (CRF) neurons mediate divergent yet complementary defensive behaviors in response to a threat

Defensive behaviors in response to a threat are shared across the animal kingdom. Active (fleeing, sheltering) or passive (freezing, avoiding) defensive responses are adaptive and facilitate survival. Selecting appropriate defensive strategy depends on intensity, proximity, temporal threat threshold, and past experiences. Hypothalamic corticotropin-releasing factor (CRF) is a major driver of an acute stress response, whereas extrahypothalamic CRF mediates stress-related affective behaviors. In this review, we shift the focus from a monolithic role of CRF as an anxiogenic peptide to comprehensively dissecting contributions of distinct populations of CRF neurons in mediating defensive behaviors. Direct interrogation of CRF neurons of the central amygdala (CeA) or the bed nucleus of the stria terminalis (BNST) show they drive unconditioned defensive responses, such as vigilance and avoidance of open spaces. Although both populations also contribute to learned fear responses in familiar, threatening contexts, CeA-CRF neurons are particularly attuned to the ever-changing environment. Depending on threat intensities, they facilitate discrimination of salient stimuli predicting manageable threats, and prevent their generalization. Finally, hypothalamic CRF neurons mediate initial threat assessment and active defense such as escape to shelter. Overall, these three major populations of CRF neurons demonstrate divergent, yet complementary contributions to the versatile defense system: heightened vigilance, discriminating salient threats, and active escape, representing three legs of the defense tripod. Despite the 'CRF exhaustion' in the field of affective neuroscience, understanding contributions of specific CRF neurons during adaptive defensive behaviors is needed in order to understand the implications of their dysregulation in fear- and anxiety-related psychiatric disorders. This article is part of the Special Issue on "Fear, Anxiety and PTSD".

Plastic and stimulus-specific coding of salient events in the central amygdala

The central amygdala (CeA) is implicated in a range of mental processes including attention, motivation, memory formation and extinction and in behaviours driven by either aversive or appetitive stimuli1-7. How it participates in these divergent functions remains elusive. Here we show that somatostatin-expressing (Sst+) CeA neurons, which mediate much of CeA functions3,6,8-10, generate experience-dependent and stimulus-specific evaluative signals essential for learning. The population responses of these neurons in mice encode the identities of a wide range of salient stimuli, with the responses of separate subpopulations selectively representing the stimuli that have contrasting valences, sensory modalities or physical properties (for example, shock and water reward). These signals scale with stimulus intensity, undergo pronounced amplification and transformation during learning, and are required for both reward and aversive learning. Notably, these signals contribute to the responses of dopamine neurons to reward and reward prediction error, but not to their responses to aversive stimuli. In line with this, Sst+ CeA neuron outputs to dopamine areas are required for reward learning, but are dispensable for aversive learning. Our results suggest that Sst+ CeA neurons selectively process information about differing salient events for evaluation during learning, supporting the diverse roles of the CeA. In particular, the information for dopamine neurons facilitates reward evaluation.

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.

Basolateral amygdala astrocytes modulate diabetic neuropathic pain and may be a potential therapeutic target for koumine

BACKGROUND AND PURPOSE

New remedies are required for the treatment of diabetic neuropathic pain (DNP) due to insufficient efficacy of available therapies. Here, we used chemogenetic approaches combined with in vivo pharmacology to elucidate the role of basolateral amygdala (BLA) astrocytes in DNP pathogenesis and provide new insights into therapeutic strategies for DNP.

EXPERIMENTAL APPROACH

A streptozotocin-induced DNP model was established. Designer receptors exclusively activated by designer drugs (DREADDs) were used to regulate astrocyte activity. Mechanical hyperalgesia was assessed using the electronic von Frey test. Anxiety-like behaviours were detected using open field and elevated plus maze tests. Astrocytic activity was detected by immunofluorescence, and cytokine content was determined by ELISA.

KEY RESULTS

BLA astrocytes were regulated by DREADDs, and inhibition of BLA astrocytes attenuated mechanical allodynia and pain-related negative emotions in DNP rats. In contrast, temporary activation of BLA astrocytes induced allodynia without anxious behaviours in naive rats. In addition, koumine (KM) alleviated mechanical allodynia and anxiety-like behaviours in DNP rats, inhibited the activation of BLA astrocytes and suppressed the inflammatory response. Furthermore, persistent activation of BLA astrocytes through chemogenetics mimicked chronic pain, and KM alleviated the pain hypersensitivity and anxiety-like behaviours.

CONCLUSION AND IMPLICATIONS

DREADDs bidirectionally regulate the activity of BLA astrocytes, which proves for the first time the role of BLA astrocyte activation in the pathogenesis of DNP and represents a novel therapeutic strategy for DNP. KM ameliorates DNP, perhaps by inhibiting the activation of BLA astrocytes and reveal KM as a potential candidate for treating DNP.

The basolateral amygdala to posterior insular cortex tract is necessary for social interaction with stressed juvenile rats

Vocalizations, chemosignals, and behaviors are influenced by one's internal affective state and are used by others to shape social behaviors. A network of interconnected brain structures, often called the social behavior network or social decision-making network, integrates these stimuli and coordinates social behaviors, and in-network connectivity deficits underlie several psychiatric disorders such as schizophrenia and autism spectrum disorders. Here, we investigated the role of the basolateral amygdala (BLA) and its projections to the posterior insular cortex, regions independently implicated in a range of sociocognitive processes, in a social affective preference (SAP) test. Viral vectors containing the gene coding for inhibitory chemogenetic receptors (AAV5-hSyn-hM4Di-mCherry) were injected into the BLA. SAP tests, which allow for the observation of unconditioned behavioral responses to the affective states of others, were conducted after inhibition of the BLA by systemic administration of the hM4Di agonist clozapine-n-oxide (CNO), or inhibition of BLA-insula terminals by direct infusion of CNO to the insula. After vehicle infusions, rats displayed preference for interactions with stressed juvenile conspecifics. However, CNO treatment eliminated preference behavior. The current results suggest that social decision making involves the transfer of emotional information from the BLA to the insula which represents a previously unrecognized anatomical substrate for social cognition.

Hippocampus and amygdala fear memory engrams re-emerge after contextual fear relapse

The formation and extinction of fear memories represent two forms of learning that each engage the hippocampus and amygdala. How cell populations in these areas contribute to fear relapse, however, remains unclear. Here, we demonstrate that, in male mice, cells active during fear conditioning in the dentate gyrus of hippocampus exhibit decreased activity during extinction and are re-engaged after contextual fear relapse. In vivo calcium imaging reveals that relapse drives population dynamics in the basolateral amygdala to revert to a network state similar to the state present during fear conditioning. Finally, we find that optogenetic inactivation of neuronal ensembles active during fear conditioning in either the hippocampus or amygdala is sufficient to disrupt fear expression after relapse, while optogenetic stimulation of these same ensembles after extinction is insufficient to artificially mimic fear relapse. These results suggest that fear relapse triggers a partial re-emergence of the original fear memory representation, providing new insight into the neural substrates of fear relapse.

BDNF Val66Met genotype and adolescent glucocorticoid treatment induce sex-specific disruptions to fear extinction and amygdala GABAergic interneuron expression in mice

BACKGROUND

The BDNF Val66Met single nucleotide polymorphism has been implicated in stress sensitivity and Post-Traumatic Stress Disorder (PTSD) risk. We previously reported that chronic young-adult stress hormone treatment enhanced fear memory in adult BDNF^Val66Met mice with the Met/Met genotype. This study aimed to extend this work to fear extinction learning, spontaneous recovery of fear, and neurobiological correlates in the amygdala.

METHODS

Male and female Val/Val and Met/Met mice received corticosterone in their drinking water during late adolescence to model chronic stress. Following a 2-week recovery period, the mice underwent fear conditioning and extinction training. Immunofluorescent labelling was used to assess density of three interneuron subtypes; somatostatin, parvalbumin and calretinin, within distinct amygdala nuclei.

RESULTS

No significant effects of genotype, treatment or sex were found for fear learning. However, adolescent CORT treatment selectively abolished fear extinction of female Met/Met mice. No effect of genotype, sex, or treatment was observed for spontaneous recovery of fear. Significant main effects of genotype and CORT emerged for somatostatin and calretinin cell density, again in females only, further supporting sex-specific effects of the Met/Met genotype and chronic CORT exposure.

CONCLUSION

BDNF Val66Met genotype interacts with chronic adolescent stress hormone exposure to abolish fear extinction in female Met/Met mice in adulthood. This effect was associated with female-specific interneuron dysfunction induced by either genotype or stress hormone exposure, depending on the interneuron subtype. These data provide biological insight into the role of BDNF in sex differences in sensitivity to stress and vulnerability to stress-related disorders in adulthood.

The central nucleus of the amygdala and the construction of defensive modes across the threat-imminence continuum

In nature, animals display defensive behaviors that reflect the spatiotemporal distance of threats. Laboratory-based paradigms that elicit specific defensive responses in rodents have provided valuable insight into the brain mechanisms that mediate the construction of defensive modes with varying degrees of threat imminence. In this Review, we discuss accumulating evidence that the central nucleus of the amygdala (CeA) plays a key role in this process. Specifically, we propose that the mutually inhibitory circuits of the CeA use a winner-takes-all strategy that supports transitioning across defensive modes and the execution of specific defensive behaviors to previously formed threat associations. Our proposal provides a conceptual framework in which seemingly divergent observations regarding CeA function can be interpreted and identifies various areas of priority for future research.

Topographic organization underlies intrinsic and morphological heterogeneity of central amygdala neurons expressing corticotropin-releasing hormone

The central nucleus of the amygdala (CeA) network consists of a heterogeneous population of inhibitory GABAergic neurons distributed across distinct subregions. While the specific roles for molecularly defined CeA neurons have been extensively studied, our understanding of functional heterogeneity within classes of molecularly distinct CeA neurons remains incomplete. In addition, manipulation of genetically defined CeA neurons has produced inconsistent behavioral results potentially due to broad targeting across CeA subregions. Therefore, elucidating heterogeneity within molecularly defined neurons in subdivisions of the CeA is pivotal for gaining a complete understanding of how CeA circuits function. Here, we used a multifaceted approach involving transgenic reporter mice, brain slice electrophysiology, and neuronal morphology to dissect the heterogeneity of corticotropin‐releasing hormone (CRH) neurons in topographically distinct subregions of the CeA. Our results revealed that intrinsic and morphological properties of CRH‐expressing (CRH+) neurons in the lateral (CeL) and medial (CeM) subdivisions of the CeA were significantly different. We found that CeL‐CRH+ neurons are relatively homogeneous in morphology and firing profile. Conversely, CeM‐CRH+ neurons displayed heterogeneous electrophysiological and morphological phenotypes. Overall, these results show phenotypic differences between CRH+ neurons in CeL and CeM.

Single-Nucleus RNA Sequencing Reveals that Decorin Expression in the Amygdala Regulates Perineuronal Nets Expression and Fear Conditioning Response after Traumatic Brain Injury

Traumatic brain injury (TBI) is a risk factor for posttraumatic stress disorder (PTSD). Augmented fear is a defining characteristic of PTSD, and the amygdala is considered the main brain region to process fear. The mechanism by which the amygdala is involved in fear conditioning after TBI is still unclear. Using single-nucleus RNA sequencing (snRNA-seq), transcriptional changes in cells in the amygdala after TBI are investigated. In total, 72 328 nuclei are obtained from the sham and TBI groups. 7 cell types, and analysis of differentially expressed genes (DEGs) reveals widespread transcriptional changes in each cell type after TBI are identified. In in vivo experiments, it is demonstrated that Decorin (Dcn) expression in the excitatory neurons of the amygdala significantly increased after TBI, and Dcn knockout in the amygdala mitigates TBI-associated fear conditioning. Of note, this effect is caused by a Dcn-mediated decrease in the expression of perineuronal nets (PNNs), which affect the glutamate-γ-aminobutyric acid balance in the amygdala. Finally, the results suggest that Dcn functions by interacting with collagen VI α3 (Col6a3). Consequently, the findings reveal transcriptional changes in different cell types of the amygdala after TBI and provide direct evidence that Dcn relieves fear conditioning by regulating PNNs.

Neuronal Circuits Associated with Fear Memory: Potential Therapeutic Targets for Posttraumatic Stress Disorder

Posttraumatic stress disorder (PTSD) is a psychiatric disorder that is associated with long-lasting memories of traumatic experiences. Extinction and discrimination of fear memory have become therapeutic targets for PTSD. Newly developed optogenetics and advanced in vivo imaging techniques have provided unprecedented spatiotemporal tools to characterize the activity, connectivity, and functionality of specific cell types in complicated neuronal circuits. The use of such tools has offered mechanistic insights into the exquisite organization of the circuitry underlying the extinction and discrimination of fear memory. This review focuses on the acquisition of more detailed, comprehensive, and integrated neural circuits to understand how the brain regulates the extinction and discrimination of fear memory. A future challenge is to translate these researches into effective therapeutic treatment for PTSD from the perspective of precise regulation of the neural circuits associated with the extinction and discrimination of fear memories.

Evidence in primates supporting the use of chemogenetics for the treatment of human refractory neuropsychiatric disorders

Non-human primate (NHP) models are essential for developing and translating new treatments that target neural circuit dysfunction underlying human psychopathology. As a proof-of-concept for treating neuropsychiatric disorders, we used a NHP model of pathological anxiety to investigate the feasibility of decreasing anxiety by chemogenetically (DREADDs [designer receptors exclusively activated by designer drugs]) reducing amygdala neuronal activity. Intraoperative MRI surgery was used to infect dorsal amygdala neurons with AAV5-hSyn-HA-hM4Di in young rhesus monkeys. In vivo microPET studies with [11C]-deschloroclozapine and postmortem autoradiography with [3H]-clozapine demonstrated selective hM4Di binding in the amygdala, and neuronal expression of hM4Di was confirmed with immunohistochemistry. Additionally, because of its high affinity for DREADDs, and its approved use in humans, we developed an individualized, low-dose clozapine administration strategy to induce DREADD-mediated amygdala inhibition. Compared to controls, clozapine selectively decreased anxiety-related freezing behavior in the human intruder paradigm in hM4Di-expressing monkeys, while coo vocalizations and locomotion were unaffected. These results are an important step in establishing chemogenetic strategies for patients with refractory neuropsychiatric disorders in which amygdala alterations are central to disease pathophysiology.

Neurochemically distinct populations of the bed nucleus of stria terminalis modulate innate fear response to weak threat evoked by predator odor stimuli

Anxiety and trauma-related disorders are characterized by significant alterations in threat detection, resulting in inadequate fear responses evoked by weak threats or safety stimuli. Recent research pointed out the important role of the bed nucleus of stria terminalis (BNST) in threat anticipation and fear modulation under ambiguous threats, hence, exaggerated fear may be traced back to altered BNST function. To test this hypothesis, we chemogenetically inhibited specific BNST neuronal populations (corticotropin-releasing hormone - BNST^CRH and somatostatin - BNST^SST expressing neurons) in a predator odor-evoked innate fear paradigm. The rationale for this paradigm was threefold: (1) predatory cues are particularly strong danger signals for all vertebrate species evoking defensive responses on the flight-avoidance-freezing dimension (conservative mechanisms), (2) predator odor can be presented in a scalable manner (from weak to strong), and (3) higher-order processing of olfactory information including predatory odor stimuli is integrated by the BNST. Accordingly, we exposed adult male mice to low and high predatory threats presented by means of cat urine, or low- and high-dose of 2-methyl-2-thiazoline (2MT), a synthetic derivate of a fox anogenital product, which evoked low and high fear response, respectively. Then, we tested the impact of chemogenetic inhibition of BNST^CRH and BNST^SST neurons on innate fear responses using crh- and sst-ires-cre mouse lines. We observed that BNST^SST inhibition was effective only under low threat conditions, resulting in reduced avoidance and increased exploration of the odor source. In contrast, BNST^CRH inhibition had no impact on 2MT-evoked responses, but enhanced fear responses to cat odor, representing an even weaker threat stimulus. These findings support the notion that BNST is recruited by uncertain or remote, potential threats, and CRH and SST neurons orchestrate innate fear responses in complementary ways.

Translational relevance of behavioral, neural, and electroencephalographic profiles in a mouse model of post-traumatic stress disorder

Post-traumatic stress disorder (PTSD) is a severe, long-term psychological disorder triggered by distressing events. The neural basis and underlying mechanisms of PTSD are not completely understood. Therefore, it is important to determine the pathology of PTSD using reliable animal models that mimic the symptoms of patients. However, the lack of evidence on the clinical relevance of PTSD animal models makes it difficult to interpret preclinical studies from a translational perspective. In this study, we performed a comprehensive screening of the behavioral, neuronal, glial, and electroencephalographic (EEG) profiles in the single prolonged stress and electric foot shock (SPS&S) mouse model. Based on the clinical features of PTSD, we observed fearful and excessive responses to trauma-related environments in the SPS&S mouse model that lasted longer than 14 days. The mice exhibited a defective and strong resistance to the extinction of fear memories caused by auditory cues and also showed enhanced innate fear induced by visual stimuli with concomitant phobias and anxiety. Furthermore, neurons, astrocytes, and microglia in PTSD-related brain regions were activated, supporting abnormal brain activation and neuroimmune changes. EEG assessment also revealed decreased power and impaired coupling strength between cortical regions. These results demonstrated that the SPS&S mouse model recapitulates the behavioral symptoms as well as neural and EEG profiles of PTSD patients, justifying the preclinical use of this mouse model.

Distinctive Regulation of Emotional Behaviors and Fear-Related Gene Expression Responses in Two Extended Amygdala Subnuclei With Similar Molecular Profiles

The central nucleus of the amygdala (CeA) and the lateral division of the bed nucleus of the stria terminalis (BNST) are the two major nuclei of the central extended amygdala that plays essential roles in threat processing, responsible for emotional states such as fear and anxiety. While some studies suggested functional differences between these nuclei, others showed anatomical and neurochemical similarities. Despite their complex subnuclear organization, subnuclei-specific functional impact on behavior and their underlying molecular profiles remain obscure. We here constitutively inhibited neurotransmission of protein kinase C-δ-positive (PKCδ+) neurons-a major cell type of the lateral subdivision of the CeA (CeL) and the oval nucleus of the BNST (BNSTov)-and found striking subnuclei-specific effects on fear- and anxiety-related behaviors, respectively. To obtain molecular clues for this dissociation, we conducted RNA sequencing in subnuclei-targeted micropunch samples. The CeL and the BNSTov displayed similar gene expression profiles at the basal level; however, both displayed differential gene expression when animals were exposed to fear-related stimuli, with a more robust expression change in the CeL. These findings provide novel insights into the molecular makeup and differential engagement of distinct subnuclei of the extended amygdala, critical for regulation of threat processing.

Central amygdala micro-circuits mediate fear extinction

Fear extinction is an adaptive process whereby defensive responses are attenuated following repeated experience of prior fear-related stimuli without harm. The formation of extinction memories involves interactions between various corticolimbic structures, resulting in reduced central amygdala (CEA) output. Recent studies show, however, the CEA is not merely an output relay of fear responses but contains multiple neuronal subpopulations that interact to calibrate levels of fear responding. Here, by integrating behavioural, in vivo electrophysiological, anatomical and optogenetic approaches in mice we demonstrate that fear extinction produces reversible, stimulus- and context-specific changes in neuronal responses to conditioned stimuli in functionally and genetically defined cell types in the lateral (CEl) and medial (CEm) CEA. Moreover, we show these alterations are absent when extinction is deficient and that selective silencing of protein kinase C delta-expressing (PKCδ) CEl neurons impairs fear extinction. Our findings identify CEA inhibitory microcircuits that act as critical elements within the brain networks mediating fear extinction.

The amygdala modulates prepulse inhibition of the auditory startle reflex through excitatory inputs to the caudal pontine reticular nucleus

Background

Sensorimotor gating is a fundamental pre-attentive process that is defined as the inhibition of a motor response by a sensory event. Sensorimotor gating, commonly measured using the prepulse inhibition (PPI) of the auditory startle reflex task, is impaired in patients suffering from various neurological and psychiatric disorders. PPI deficits are a hallmark of schizophrenia, and they are often associated with attention and other cognitive impairments. Although the reversal of PPI deficits in animal models is widely used in pre-clinical research for antipsychotic drug screening, the neurotransmitter systems and synaptic mechanisms underlying PPI are still not resolved, even under physiological conditions. Recent evidence ruled out the longstanding hypothesis that PPI is mediated by midbrain cholinergic inputs to the caudal pontine reticular nucleus (PnC). Instead, glutamatergic, glycinergic, and GABAergic inhibitory mechanisms are now suggested to be crucial for PPI, at the PnC level. Since amygdalar dysfunctions alter PPI and are common to pathologies displaying sensorimotor gating deficits, the present study was designed to test that direct projections to the PnC originating from the amygdala contribute to PPI.

Results

Using wild type and transgenic mice expressing eGFP under the control of the glycine transporter type 2 promoter (GlyT2-eGFP mice), we first employed tract-tracing, morphological reconstructions, and immunohistochemical analyses to demonstrate that the central nucleus of the amygdala (CeA) sends glutamatergic inputs lateroventrally to PnC neurons, including GlyT2⁺ cells. Then, we showed the contribution of the CeA-PnC excitatory synapses to PPI in vivo by demonstrating that optogenetic inhibition of this connection decreases PPI, and optogenetic activation induces partial PPI. Finally, in GlyT2-Cre mice, whole-cell recordings of GlyT2⁺ PnC neurons in vitro paired with optogenetic stimulation of CeA fibers, as well as photo-inhibition of GlyT2⁺ PnC neurons in vivo, allowed us to implicate GlyT2⁺ neurons in the PPI pathway.

Conclusions

Our results uncover a feedforward inhibitory mechanism within the brainstem startle circuit by which amygdalar glutamatergic inputs and GlyT2⁺ PnC neurons contribute to PPI. We are providing new insights to the clinically relevant theoretical construct of PPI, which is disrupted in various neuropsychiatric and neurological diseases.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-01050-z.

Sex-Specific Stress-Related Behavioral Phenotypes and Central Amygdala Dysfunction in a Mouse Model of 16p11.2 Microdeletion

Background

Substantial evidence indicates that a microdeletion on human chromosome 16p11.2 is linked to neurodevelopmental disorders, including autism spectrum disorder (ASD). Carriers of this deletion show divergent symptoms besides the core features of autism spectrum disorder, such as anxiety and emotional symptoms. The neural mechanisms underlying these symptoms are poorly understood.

Methods

We used mice heterozygous for a deletion allele of the genomic region corresponding to the human 16p11.2 microdeletion locus (i.e., 16p11.2 del/+ mice) and their sex-matched wild-type littermates for the study and examined their anxiety-related behaviors, auditory perception, and central amygdala circuit function using behavioral, circuit tracing, and electrophysiological techniques.

Results

Mice heterozygous for a deletion allele of the genomic region corresponding to the human 16p11.2 microdeletion locus (i.e., 16p11.2 del/+ mice) had sex-specific anxiety-related behavioral and neural circuit changes. Specifically, we found that female, but not male, 16p11.2 del/+ mice showed enhanced fear generalization-a hallmark of anxiety disorders-after auditory fear conditioning and displayed increased anxiety-like behaviors after physical restraint stress. Notably, such sex-specific behavioral changes were paralleled by an increase in activity in central amygdala neurons projecting to the globus pallidus in female, but not male, 16p11.2 del/+ mice.

Conclusions

Together, these results reveal female-specific anxiety phenotypes related to 16p11.2 microdeletion syndrome and a potential underlying neural circuit mechanism. Our study therefore identifies previously underappreciated sex-specific behavioral and neural changes in a genetic model of 16p11.2 microdeletion syndrome and highlights the importance of investigating female-specific aspects of this syndrome for targeted treatment strategies.

Central amygdala circuits in valence and salience processing

Behavioral responses to environmental stimuli are dictated by the affective valence of the stimulus, good (positive valence) or bad (negative valence). These stimuli can innately elicit an affective response that promotes approach or avoidance behavior. In addition to innately valenced stimuli, valence can also be assigned to initially neutral stimuli through associative learning. A stimulus of a given valence can vary in salience depending on the strength of the stimulus, the underlying state of the animal, and the context of the stimulus presentation. Salience endows the stimulus with the ability to direct attention and elicit preparatory responses to mount an incentive-based motivated behavior. The central nucleus of the amygdala (CeA) has emerged as an early integration point for valence and salience detection to engage preparatory autonomic responses and behavioral posturing in response to both aversive and appetitive stimuli. There are numerous cell types in the CeA that are involved in valence and salience processing through a variety of connections, and we will review the recent progress that has been made in identifying these circuit elements and their roles in these processes.

Distinct circuits in rat central amygdala for defensive behaviors evoked by socially signaled imminent versus remote danger

Animals display a rich repertoire of defensive responses adequate to the threat proximity. In social species, these reactions can be additionally influenced by the behavior of fearful conspecifics. However, the majority of neuroscientific studies on socially triggered defensive responses focuses on one type of behavior, freezing. To study a broader range of socially triggered reactions and underlying mechanisms, we directly compared two experimental paradigms, mimicking occurrence of the imminent versus remote threat. Observation of a partner currently experiencing aversive stimulation evokes passive defensive responses in the observer rats. Similar interaction with a partner that has just undergone the aversive stimulation prompts animals to increase active exploration. Although the observers display behaviors similar to those of the aversively stimulated demonstrators, their reactions are not synchronized in time, suggesting that observers' responses are caused by the change in their affective state rather than mimicry. Using opsins targeted to behaviorally activated neurons, we tagged central amygdala (CeA) cells implicated in observers' responses to either imminent or remote threat and reactivated them during the exploration of a novel environment. The manipulation revealed that the two populations of CeA cells promote passive or active defensive responses, respectively. Further experiments confirmed that the two populations of cells at least partially differ in expression of molecular markers (protein kinase C-δ [PKC-δ] and corticotropin-releasing factor [CRF]) and connectivity patterns (receiving input from the basolateral amygdala or from the anterior insula). The results are consistent with the literature on single subjects' fear conditioning, suggesting that similar neuronal circuits control defensive responses in social and non-social contexts.

Somatostatin Neurons of the Bed Nucleus of Stria Terminalis Enhance Associative Fear Memory Consolidation in Mice

Excessive fear learning and generalized, extinction-resistant fear memories are core symptoms of anxiety and trauma-related disorders. Despite significant evidence from clinical studies reporting hyperactivity of the bed nucleus of stria terminalis (BNST) under these conditions, the role of BNST in fear learning and expression is still not clarified. Here, we tested how BNST modulates fear learning in male mice using a chemogenetic approach. Activation of GABAergic neurons of BNST during fear conditioning or memory consolidation resulted in enhanced cue-related fear recall. Importantly, BNST activation had no acute impact on fear expression during conditioning or recalls, but it enhanced cue-related fear recall subsequently, potentially via altered activity of downstream regions. Enhanced fear memory consolidation could be replicated by selectively activating somatostatin (SOM), but not corticotropin-releasing factor (CRF), neurons of the BNST, which was accompanied by increased fear generalization. Our findings suggest the significant modulation of fear memory strength by specific circuits of the BNST.SIGNIFICANCE STATEMENT The bed nucleus of stria terminalis (BNST) mediates different defensive behaviors, and its connections implicate its integrative modulatory role in fear memory formation; however, the involvement of BNST in fear learning has yet to be elucidated in detail. Our data highlight that BNST stimulation enhances fear memory formation without direct effects on fear expression. Our study identified somatostatin (SOM) cells within the extended amygdala as specific neurons promoting fear memory formation. These data underline the importance of anxiety circuits in maladaptive fear memory formation, indicating elevated BNST activity as a potential vulnerability factor to anxiety and trauma-related disorders.

An endocannabinoid-regulated basolateral amygdala-nucleus accumbens circuit modulates sociability

Deficits in social interaction (SI) are a core symptom of autism spectrum disorders (ASDs); however, treatments for social deficits are notably lacking. Elucidating brain circuits and neuromodulatory signaling systems that regulate sociability could facilitate a deeper understanding of ASD pathophysiology and reveal novel treatments for ASDs. Here we found that in vivo optogenetic activation of the basolateral amygdala-nucleus accumbens (BLA-NAc) glutamatergic circuit reduced SI and increased social avoidance in mice. Furthermore, we found that 2-arachidonoylglycerol (2-AG) endocannabinoid signaling reduced BLA-NAc glutamatergic activity and that pharmacological 2-AG augmentation via administration of JZL184, a monoacylglycerol lipase inhibitor, blocked SI deficits associated with in vivo BLA-NAc stimulation. Additionally, optogenetic inhibition of the BLA-NAc circuit markedly increased SI in the Shank3B-/- mouse, an ASD model with substantial SI impairment, without affecting SI in WT mice. Finally, we demonstrated that JZL184 delivered systemically or directly to the NAc also normalized SI deficits in Shank3B-/- mice, while ex vivo JZL184 application corrected aberrant NAc excitatory and inhibitory neurotransmission and reduced BLA-NAc-elicited feed-forward inhibition of NAc neurons in Shank3B-/- mice. These data reveal circuit-level and neuromodulatory mechanisms regulating social function relevant to ASDs and suggest 2-AG augmentation could reduce social deficits via modulation of excitatory and inhibitory neurotransmission in the NAc.

Learning dynamics of electrophysiological brain signals during human fear conditioning

Electrophysiological studies in rodents allow recording neural activity during threats with high temporal and spatial precision. Although fMRI has helped translate insights about the anatomy of underlying brain circuits to humans, the temporal dynamics of neural fear processes remain opaque and require EEG. To date, studies on electrophysiological brain signals in humans have helped to elucidate underlying perceptual and attentional processes, but have widely ignored how fear memory traces evolve over time. The low signal-to-noise ratio of EEG demands aggregations across high numbers of trials, which will wash out transient neurobiological processes that are induced by learning and prone to habituation. Here, our goal was to unravel the plasticity and temporal emergence of EEG responses during fear conditioning. To this end, we developed a new sequential-set fear conditioning paradigm that comprises three successive acquisition and extinction phases, each with a novel CS+/CS- set. Each set consists of two different neutral faces on different background colors which serve as CS+ and CS-, respectively. Thereby, this design provides sufficient trials for EEG analyses while tripling the relative amount of trials that tap into more transient neurobiological processes. Consistent with prior studies on ERP components, data-driven topographic EEG analyses revealed that ERP amplitudes were potentiated during time periods from 33-60 ms, 108-200 ms, and 468-820 ms indicating that fear conditioning prioritizes early sensory processing in the brain, but also facilitates neural responding during later attentional and evaluative stages. Importantly, averaging across the three CS+/CS- sets allowed us to probe the temporal evolution of neural processes: Responses during each of the three time windows gradually increased from early to late fear conditioning, while long-latency (460-730 ms) electrocortical responses diminished throughout fear extinction. Our novel paradigm demonstrates how short-, mid-, and long-latency EEG responses change during fear conditioning and extinction, findings that enlighten the learning curve of neurophysiological responses to threat in humans.

Projections from Infralimbic Cortex to Paraventricular Thalamus Mediate Fear Extinction Retrieval

The paraventricular nucleus of the thalamus (PVT), which serves as a hub, receives dense projections from the medial prefrontal cortex (mPFC) and projects to the lateral division of central amygdala (CeL). The infralimbic (IL) cortex plays a crucial role in encoding and recalling fear extinction memory. Here, we found that neurons in the PVT and IL were strongly activated during fear extinction retrieval. Silencing PVT neurons inhibited extinction retrieval at recent time point (24 h after extinction), while activating them promoted extinction retrieval at remote time point (7 d after extinction), suggesting a critical role of the PVT in extinction retrieval. In the mPFC-PVT circuit, projections from IL rather than prelimbic cortex to the PVT were dominant, and disrupting the IL-PVT projection suppressed extinction retrieval. Moreover, the axons of PVT neurons preferentially projected to the CeL. Silencing the PVT-CeL circuit also suppressed extinction retrieval. Together, our findings reveal a new neural circuit for fear extinction retrieval outside the classical IL-amygdala circuit.

A Central Amygdala-Globus Pallidus Circuit Conveys Unconditioned Stimulus-Related Information and Controls Fear Learning

The central amygdala (CeA) is critically involved in a range of adaptive behaviors, including defensive behaviors. Neurons in the CeA send long-range projections to a number of extra-amygdala targets, but the functions of these projections remain elusive. Here, we report that a previously neglected CeA-to-globus pallidus external segment (GPe) circuit plays an essential role in classical fear conditioning. By anatomic tracing, in situ hybridization and channelrhodopsin (ChR2)-assisted circuit mapping in both male and female mice, we found that a subset of CeA neurons send projections to the GPe, and the majority of these GPe-projecting CeA neurons express the neuropeptide somatostatin. Notably, chronic inhibition of GPe-projecting CeA neurons with the tetanus toxin light chain (TeLC) completely blocks auditory fear conditioning. In vivo fiber photometry revealed that these neurons are selectively excited by the unconditioned stimulus (US) during fear conditioning. Furthermore, transient optogenetic inactivation or activation of these neurons selectively during US presentation impairs or promotes, respectively, fear learning. Our results suggest that a major function of GPe-projecting CeA neurons is to represent and convey US-related information through the CeA-GPe circuit, thereby regulating learning in fear conditioning.SIGNIFICANCE STATEMENT The central amygdala (CeA) has been implicated in the establishment of defensive behaviors toward threats, but the underlying circuit mechanisms remain unclear. Here, we found that a subpopulation of neurons in the CeA, which are mainly those that express the neuropeptide somatostatin, send projections to the globus pallidus external segment (GPe), and this CeA-GPe circuit conveys unconditioned stimulus (US)-related information during classical fear conditioning, thereby having an indispensable role in learning. Our results reveal a previously unknown circuit mechanism for fear learning.

Modulation of expression of fear by oxytocin signaling in the central amygdala: From reduction of fear to regulation of defensive behavior style

Many studies in preclinical animal models have described fear-reducing effects of the neuropeptide oxytocin in the central nucleus of the amygdala. However, recent studies have refined the role of oxytocin in the central amygdala, which may extend to the selection of an active defensive coping style in the face of immediate threat, and also fear-enhancing effects have been reported. On top of this, oxytocin enables the discrimination of unfamiliar conspecifics on the basis of their emotional state, which could allow for the selection of an appropriate coping style. This is in line with many observations that support the hypothesis that the precise outcome of oxytocin signaling in the central amygdala or other brain regions depends on the emotional or physiological state of an animal. In this review, we highlight a number of studies to exemplify the diverse effects oxytocin exerts on fear in the central amygdala of rodents. These are discussed in the context of the organization of the neural network within the central amygdala and in relation to the oxytocin-synthesizing neurons in the hypothalamus.