Regulating anxiety with extrasynaptic inhibition

Confronting the anxiety of Generation Z: electroacupuncture therapy regulates oxidative stress and microglia activity in amygdala-basolateral of socially isolated mice

Introduction

Anxiety disorders are prevalent mental health conditions characterized by significant impairments in daily functioning and social interactions. Despite the effectiveness of pharmacological treatments, challenges such as medication resistance, adverse side effects, and the high rate of relapse necessitate the exploration of alternative therapies. Recently, electroacupuncture (EA) has garnered attention as a promising non-pharmacological intervention for anxiety disorders; however, the mechanisms by which EA exerts its anxiolytic effects remain poorly understood. This study aims to elucidate the role of microglial cells in anxiety, specifically examining how EA modulates microglial morphology and function within the basolateral amygdala (BLA) in the context of anxiety induced by social isolation.

Methods

Utilizing a mouse model of social isolation-induced anxiety, we evaluated anxiety-like behaviors through the Elevated Plus Maze (EPM) and Open Field Test (OFT). Additionally, biochemical analyses and immunofluorescence imaging were performed to assess the expression of NADPH oxidase 2 (NOX2), microglial activation markers, and levels of oxidative stress.

Results

Our findings reveal that EA treatment significantly mitigates anxiety-like behaviors in mice, correlating with a reduction in NOX2 expression within BLA microglia and decreased levels of reactive oxygen species (ROS). Furthermore, EA was observed to restore normal microglial morphology, indicating its potential role in modulating microglial activity.

Discussion

The results of this study suggest that EA exerts its anxiolytic effects through the modulation of oxidative stress and the activity of microglia in the BLA. These findings provide new insights into the cellular mechanisms underlying the therapeutic effects of EA, highlighting the potential for non-pharmacological strategies in the management of anxiety disorders and paving the way for future research aimed at improving clinical outcomes for individuals suffering from anxiety.

Anxiolytic Efficacy of Indirubin: In Vivo Approach Along with Receptor Binding Profiling and Molecular Interaction with GABAergic Pathways

Anxiety is a natural response to stress, characterized by feelings of worry, fear, or unease. The current research was conducted to investigate the anxiolytic effect of indirubin (IND) in different behavioral paradigms in Swiss albino mice. To observe the animal's behavioural response to assess anxiolytic activity, different tests were performed, such as the open-field (square cross, grooming, and rearing), swing, dark-light, and hole cross tests. The experimental mice were administered IND (5 and 10 mg/kg, p.o.), where diazepam (DZP) and vehicle were used as positive and negative controls, respectively. In addition, a combination treatment (DZP+IND-10) was provided to the animals to determine the modulatory effect of IND on DZP. Molecular docking approach was also conducted to determine the binding energy of IND with the GABAA receptor (α2 and α3 subunits) and pharmacokinetics were also estimated. The findings revealed that IND dose-dependently significantly (p<0.05) reduced the animal's movement exerting calming behavior like DZP. IND also demonstrated the highest docking score (-7.7 kcal/mol) against the α3 subunit, while DZP showed a lower docking value (-6.4 kcal/mol) than IND. The ADMET analysis revealed that IND has proper drug-likeness and pharmacokinetic characteristics. In conclusion, IND exerted anxiolytic effects through GABAergic Pathways.

Gene Deficiency of δ Subunit-Containing GABAA Receptor in mPFC Lead Learning and Memory Impairment in Mice

Maintaining GABAergic inhibition within physiological limits in the medial prefrontal cortex (mPFC) is critical for working memory. While synaptic GABAAR typically mediate the primary component of mPFC inhibition, the role of extrasynaptic δ-GABAAR in working memory remains unclear. To investigate this, we used fiber photometry to examine the effects of δ-GABAAR in freely moving mice. Our results indicate that the loss of δ-GABAAR expression leads to learning and memory impairment. Specifically, activation of δ-GABAAR impaired learning and memory in WT mice but enhanced learning and memory in δ+/- knockout mice. Furthermore, δ-GABAAR activation increased calcium activity in the mPFC pyramidal neurons, an effect not observed in δ-Cas9-sgRNA virus-infected mice. Collectively, these findings suggest that δ-GABAAR deficiency impairs learning and memory by modulating the excitability of pyramidal neurons in the mPFC. These results delineate the functional contribution of δ-GABAAR to learning and memory, suggesting their role extends beyond the mere maintenance of information.

Central amygdala-to-pre-Bötzinger complex neurotransmission is direct and inhibitory

Breathing behaviour is subject to emotional regulation, but the underlying mechanisms remain unclear. Here, we demonstrate a direct relationship between the central amygdala, a major output hub of the limbic system associated with emotional brain function, and the brainstem pre-Bötzinger complex, which generates the fundamental rhythm and pattern for breathing. The connection between these two sites is monosynaptic and inhibitory, involving GABAergic central amygdala neurons whose axonal projections act predominantly via ionotropic GABAA receptors to produce inhibitory postsynaptic currents in pre-Bötzinger neurons. This pathway may provide a mechanism to inhibit breathing in the context of freezing to assess threats and plan defensive action. The existence of this pathway may further explain how epileptic seizures invading the amygdala cause long-lasting apnea, which can be fatal. Although their ultimate importance awaits further behavioural tests, these results elucidate a link between emotional brain function and breathing, which underlies survival-related behaviour in mammals and pertains to human anxiety disorders.

Evaluation of the sedative-motor effects of novel GABAkine imidazodiazepines using quantitative observation techniques in rhesus monkeys

BACKGROUND

Benzodiazepines bind to γ-aminobutyric acid type A (GABAA) receptor subtypes identified by different α subunits (i.e., α1GABAA, α2GABAA, α3GABAA, and α5GABAA). Sedative-motor effects of benzodiazepines are thought to involve α1GABAA and α3GABAA subtypes.

AIMS

We evaluated observable measures of sedative-motor effects and species-typical behaviors in monkeys following acute administration of novel GABAkines (positive allosteric modulators of GABAA receptors), with varying degrees of selective efficacy at different GABAA receptor subtypes. We predicted that the induction of sedative-motor effects would depend on the degree of α1GABAA and α3GABAA efficacy.

METHODS

Adult female rhesus monkeys (N = 4) were implanted with chronic indwelling i.v. catheters. Quantitative behavioral observation was conducted by trained observers following administration of multiple doses of the conventional benzodiazepine alprazolam and the GABAkines MP-III-80 (preferential efficacy at α2/α3/α5GABAA subtypes), KRM-II-81, MP-III-24 (both with preferential efficacy for α2/α3GABAA subtypes), and MP-III-22 (preferential potency and efficacy for α5GABAA subtypes).

RESULTS

As with alprazolam, all GABAkines induced significant levels of mild sedation ("rest/sleep posture"). Deep sedation was observed with alprazolam, MP-III-80, and MP-III-22; motoric effects (observable ataxia) were obtained with alprazolam, KRM-II-81, and MP-III-22 only. Surprisingly, the order of potency for rest/sleep posture was significantly associated only with potency at α5GABAA subtypes.

CONCLUSIONS

GABAkines with preferential efficacy at α2/α3GABAA and/or α5GABAA subtypes engendered sedative-motor effects in monkeys, although only compounds with α5GABAA activity engendered deep sedation. Moreover, the significant relationship between potency obtained with in vitro electrophysiology data and the rest/sleep posture measure suggests a role for the α5GABAA subtype in this milder form of sedation.

Distinct ventral hippocampal inhibitory microcircuits regulating anxiety and fear behaviors

In emotion research, anxiety and fear have always been interconnected, sharing overlapping brain structures and neural circuitry. Recent investigations, however, have unveiled parallel long-range projection pathways originating from the ventral hippocampus, shedding light on their distinct roles in anxiety and fear. Yet, the mechanisms governing the emergence of projection-specific activity patterns to mediate different negative emotions remain elusive. Here, we show a division of labor in local GABAergic inhibitory microcircuits of the ventral hippocampus, orchestrating the activity of subpopulations of pyramidal neurons to shape anxiety and fear behaviors in mice. These findings offer a comprehensive insight into how distinct inhibitory microcircuits are dynamically engaged to encode different emotional states.

GABAergic implications in anxiety and related disorders

Evidence indicates that anxiety disorders arise from an imbalance in the functioning of brain circuits that govern the modulation of emotional responses to possibly threatening stimuli. The circuits under consideration in this context include the amygdala's bottom-up activity, which signifies the existence of stimuli that may be seen as dangerous. Moreover, these circuits encompass top-down regulatory processes that originate in the prefrontal cortex, facilitating the communication of the emotional significance associated with the inputs. Diverse databases (e.g., Pubmed, ScienceDirect, Web of Science, Google Scholar) were searched for literature using a combination of different terms e.g., "anxiety", "stress", "neuroanatomy", and "neural circuits", etc. A decrease in GABAergic activity is present in both anxiety disorders and severe depression. Research on cerebral functional imaging in depressive individuals has shown reduced levels of GABA within the cortical regions. Additionally, animal studies demonstrated that a reduction in the expression of GABAA/B receptors results in a behavioral pattern resembling anxiety. The amygdala consists of inhibitory networks composed of GABAergic interneurons, responsible for modulating anxiety responses in both normal and pathological conditions. The GABAA receptor has allosteric sites (e.g., α/γ, γ/β, and α/β) which enable regulation of neuronal inhibition in the amygdala. These sites serve as molecular targets for anxiolytic medications such as benzodiazepine and barbiturates. Alterations in the levels of naturally occurring regulators of these allosteric sites, along with alterations to the composition of the GABAA receptor subunits, could potentially act as mechanisms via which the extent of neuronal inhibition is diminished in pathological anxiety disorders.

A dopamine-dependent mechanism for Reward-induced modification of Fear memories

A positive mental state has been shown to modulate fear-related emotions associated with the recall of fear memories. These, and other observations suggest the presence of central brain mechanisms for affective states to interact. The neurotransmitter dopamine is important for both Reward- and fear-related processes, but it is unclear whether dopamine contributes to such affective interactions. Here, we show that precisely timed Reward-induced activation of dopamine neurons in mice potently modifies fear memories and enhances their extinction. This Reward-based switch in fear states is associated with changes in dopamine release and dopamine-dependent regulation of fear encoding in the central amygdala (CeA). These data provide a central mechanism for Reward-induced modification of fear states that have broad implications for treating generalized fear disorders.

Generation and Characterization of a Novel Prkcd-Cre Rat Model

Activity of central amygdala (CeA) PKCδ expressing neurons has been linked to appetite regulation, anxiety-like behaviors, pain sensitivity, and addiction-related behaviors. Studies of the role that CeA PKCδ+ neurons play in these behaviors have largely been carried out in mice, and genetic tools that would allow selective manipulation of PKCδ+ cells in rats have been lacking. Here, we used a CRISPR/Cas9 strategy to generate a transgenic Prkcd-cre knock-in rat and characterized this model using anatomical, electrophysiological, and behavioral approaches in both sexes. In the CeA, Cre was selectively expressed in PKCδ+ cells. Anterograde projections of PKCδ+ neurons to cortical regions, subcortical regions, several hypothalamic nuclei, the amygdala complex, and midbrain dopaminergic regions were largely consistent with published mouse data. In a behavioral screen, we found no differences between Cre+ rats and Cre- wild-type littermates. Optogenetic stimulation of CeA PKCδ+ neurons in a palatable food intake assay resulted in an increased latency to first feeding and decreased total food intake, once again replicating published mouse findings. Lastly, using a real-time place preference task, we found that stimulation of PKCδ+ neurons promoted aversion, without affecting locomotor activity. Collectively, these findings establish the novel Prkcd-Cre rat line as a valuable tool that complements available mouse lines for investigating the functional role of PKCδ+ neurons.

Stress-induced anxiety-related behavior in mice is driven by enhanced excitability of ventral tegmental area GABA neurons

Introduction

Stress and trauma are significant risk factors for many neuropsychiatric disorders and diseases, including anxiety disorders. Stress-induced anxiety symptoms have been attributed to enhanced excitability in circuits controlling fear, anxiety, and aversion. A growing body of evidence has implicated GABAergic neurons of the ventral tegmental area (VTA) in aversion processing and affective behavior.

Methods

We used an unpredictable footshock (uFS) model, together with electrophysiological and behavioral approaches, to investigate the role of VTA GABA neurons in anxiety-related behavior in mice.

Results

One day after a single uFS session, C57BL/6J mice exhibited elevated anxiety-related behavior and VTA GABA neuron excitability. The enhanced excitability of VTA GABA neurons was correlated with increased glutamatergic input and a reduction in postsynaptic signaling mediated via GABAA and GABAB receptors. Chemogenetic activation of VTA GABA neurons was sufficient to increase anxiety-related behavior in stress-naïve mice. In addition, chemogenetic inhibition of VTA GABA neurons suppressed anxiety-related behavior in mice exposed to uFS.

Discussion

These data show that VTA GABA neurons are an early substrate for stress-induced anxiety-related behavior in mice and suggest that approaches mitigating enhanced excitability of VTA GABA neurons may hold promise for the treatment of anxiety provoked by stress and trauma.

Foxg1 Modulation of the Prkcd Gene in the Lateral Habenula Mediates Trigeminal Neuralgia-Associated Anxiety-Like Behaviors in Mice

Trigeminal Neuralgia (TN) is a debilitating disorder frequently accompanied by mood complications such as depression and anxiety. The current study sought to elucidate the molecular underpinnings that contribute to the pathogenesis of TN and its associated anxiety. Employing a partial transection of the infraorbital nerve (pT-ION) in a murine model, we successfully induced sustained primary and secondary orofacial allodynia alongside anxiety-like behavioral manifestations. Transcriptome-wide gene microarray analyses revealed a marked upregulation of Foxg1 subsequent to pT-ION. Targeted knockdown of Foxg1, achieved through bilateral microinjection of adeno-associated virus harboring Foxg1-specific shRNA into the lateral habenula (LHb), resulted in a significant attenuation of both orofacial pain and anxiety-like behaviors. Subsequent RNA sequencing implicated Prkcd as a downstream effector gene modulated by Foxg1. Pharmacological inhibition of protein kinase C delta, encoded by Prkcd, within the LHb markedly ameliorated pT-ION-induced symptomatology. The dual luciferase assay revealed that Foxg1 substantially enhances the transcriptional activity of the Prkcd gene. Collectively, these findings indicate that trigeminal nerve injury leads to Foxg1 upregulation in the LHb, which in turn elevates the expression of Prkcd, culminating in the manifestation of orofacial pain and anxiety-like behaviors. This work offers promising therapeutic targets and a conceptual framework for the clinical management of TN and its psychological comorbidities.

NPY-mediated synaptic plasticity in the extended amygdala prioritizes feeding during starvation

Efficient control of feeding behavior requires the coordinated adjustment of complex motivational and affective neurocircuits. Neuropeptides from energy-sensing hypothalamic neurons are potent feeding modulators, but how these endogenous signals shape relevant circuits remains unclear. Here, we examine how the orexigenic neuropeptide Y (NPY) adapts GABAergic inputs to the bed nucleus of the stria terminalis (BNST). We find that fasting increases synaptic connectivity between agouti-related peptide (AgRP)-expressing 'hunger' and BNST neurons, a circuit that promotes feeding. In contrast, GABAergic input from the central amygdala (CeA), an extended amygdala circuit that decreases feeding, is reduced. Activating NPY-expressing AgRP neurons evokes these synaptic adaptations, which are absent in NPY-deficient mice. Moreover, fasting diminishes the ability of CeA projections in the BNST to suppress food intake, and NPY-deficient mice fail to decrease anxiety in order to promote feeding. Thus, AgRP neurons drive input-specific synaptic plasticity, enabling a selective shift in hunger and anxiety signaling during starvation through NPY.

Opioid receptor expressing neurons of the central amygdala gate behavioral effects of ketamine in mice

Ketamine has anesthetic, analgesic, and antidepressant properties which may involve multiple neuromodulatory systems. In humans, the opioid receptor (OR) antagonist naltrexone blocks the antidepressant effect of ketamine. It is unclear whether naltrexone blocks a direct effect of ketamine at ORs, or whether normal functioning of the OR system is required to realize the full antidepressant effects of treatment. In mice, the effect of ketamine on locomotion, but not analgesia or the forced swim test, was sensitive to naltrexone and was therefore used as a behavioral readout to localize the effect of naltrexone in the brain. We performed whole-brain imaging of cFos expression in ketamine-treated mice, pretreated with naltrexone or vehicle, and identified the central amygdala (CeA) as the area with greatest difference in cFos intensity. CeA neurons expressing both μOR (MOR) and PKCμ were strongly activated by naltrexone but not ketamine, and selectively interrupting MOR function in the CeA either pharmacologically or genetically blocked the locomotor effects of ketamine. These data suggest that MORs expressed in CeA neurons gate behavioral effects of ketamine but are not direct targets of ketamine.

An active inference perspective for the amygdala complex

The amygdala is a heterogeneous network of subcortical nuclei with central importance in cognitive and clinical neuroscience. Various experimental designs in human psychology and animal model research have mapped multiple conceptual frameworks (e.g., valence/salience and decision making) to ever more refined amygdala circuitry. However, these predominantly bottom up-driven accounts often rely on interpretations tailored to a specific phenomenon, thus preventing comprehensive and integrative theories. We argue here that an active inference model of amygdala function could unify these fractionated approaches into an overarching framework for clearer empirical predictions and mechanistic interpretations. This framework embeds top-down predictive models, informed by prior knowledge and belief updating, within a dynamical system distributed across amygdala circuits in which self-regulation is implemented by continuously tracking environmental and homeostatic demands.

Molecular diversity and functional dynamics in the central amygdala

The central amygdala (CeA) is crucial in integrating sensory and associative information to mediate adaptive responses to emotional stimuli. Recent advances in genetic techniques like optogenetics and chemogenetics have deepened our understanding of distinct neuronal populations within the CeA, particularly those involved in fear learning and memory consolidation. However, challenges remain due to overlapping genetic markers complicating neuron identification. Furthermore, a comprehensive understanding of molecularly defined cell types and their projection patterns, which are essential for elucidating functional roles, is still developing. Recent advancements in transcriptomics are starting to bridge these gaps, offering new insights into the functional dynamics of CeA neurons. In this review, we provide an overview of the expanding genetic markers for amygdala research, encompassing recent developments and current trends. We also discuss how novel transcriptomic approaches are redefining cell types in the CeA and setting the stage for comprehensive functional studies.

An ACC-VTA-ACC positive-feedback loop mediates the persistence of neuropathic pain and emotional consequences

The central mechanisms underlying pain chronicity remain elusive. Here, we identify a reciprocal neuronal circuit in mice between the anterior cingulate cortex (ACC) and the ventral tegmental area (VTA) that mediates mutual exacerbation between hyperalgesia and allodynia and their emotional consequences and, thereby, the chronicity of neuropathic pain. ACC glutamatergic neurons (ACCGlu) projecting to the VTA indirectly inhibit dopaminergic neurons (VTADA) by activating local GABAergic interneurons (VTAGABA), and this effect is reinforced after nerve injury. VTADA neurons in turn project to the ACC and synapse to the initial ACCGlu neurons to convey feedback information from emotional changes. Thus, an ACCGlu-VTAGABA-VTADA-ACCGlu positive-feedback loop mediates the progression to and maintenance of persistent pain and comorbid anxiodepressive-like behavior. Disruption of this feedback loop relieves hyperalgesia and anxiodepressive-like behavior in a mouse model of neuropathic pain, both acutely and in the long term.

A combinatory genetic strategy for targeting neurogliaform neurons in the mouse basolateral amygdala

The mouse basolateral amygdala (BLA) contains various GABAergic interneuron subpopulations, which have distinctive roles in the neuronal microcircuit controlling numerous behavioral functions. In mice, roughly 15% of the BLA GABAergic interneurons express neuropeptide Y (NPY), a reasonably characteristic marker for neurogliaform cells (NGFCs) in cortical-like brain structures. However, genetically labeled putative NPY-expressing interneurons in the BLA yield a mixture of interneuron subtypes besides NGFCs. Thus, selective molecular markers are lacking for genetically accessing NGFCs in the BLA. Here, we validated the NGFC-specific labeling with a molecular marker, neuron-derived neurotrophic factor (NDNF), in the mouse BLA, as such specificity has been demonstrated in the neocortex and hippocampus. We characterized genetically defined NDNF-expressing (NDNF+) GABAergic interneurons in the mouse BLA by combining the Ndnf-IRES2-dgCre-D transgenic mouse line with viral labeling, immunohistochemical staining, and in vitro electrophysiology. We found that BLA NDNF+ GABAergic cells mainly expressed NGFC neurochemical markers NPY and reelin (Reln) and exhibited small round soma and dense axonal arborization. Whole-cell patch clamp recordings indicated that most NDNF+ interneurons showed late spiking and moderate firing adaptation. Moreover, ∼81% of BLA NDNF+ cells generated retroaxonal action potential after current injections or optogenetic stimulations, frequently developing into persistent barrage firing. Optogenetic activation of the BLA NDNF+ cell population yielded both GABAA- and GABAB receptor-mediated currents onto BLA pyramidal neurons (PNs). We demonstrate a combinatory strategy combining the NDNF-cre mouse line with viral transfection to specifically target adult mouse BLA NGFCs and further explore their functional and behavioral roles.

Microglia govern the extinction of acute stress-induced anxiety-like behaviors in male mice

Anxiety-associated symptoms following acute stress usually become extinct gradually within a period of time. However, the mechanisms underlying how individuals cope with stress to achieve the extinction of anxiety are not clear. Here we show that acute restraint stress causes an increase in the activity of GABAergic neurons in the CeA (GABACeA) in male mice, resulting in anxiety-like behaviors within 12 hours; meanwhile, elevated GABACeA neuronal CX3CL1 secretion via MST4 (mammalian sterile-20-like kinase 4)-NF-κB-CX3CL1 signaling consequently activates microglia in the CeA. Activated microglia in turn inhibit GABACeA neuronal activity via the engulfment of their dendritic spines, ultimately leading to the extinction of anxiety-like behaviors induced by restraint stress. These findings reveal a dynamic molecular and cellular mechanism in which microglia drive a negative feedback to inhibit GABACeA neuronal activity, thus facilitating maintenance of brain homeostasis in response to acute stress.

Corticotropin-releasing hormone signaling from prefrontal cortex to lateral septum suppresses interaction with familiar mice

Social preference, the decision to interact with one member of the same species over another, is critical to optimize social interactions. Thus, adult rodents favor interacting with novel conspecifics over familiar ones, but whether this social preference stems from neural circuits facilitating interactions with novel individuals or suppressing interactions with familiar ones remains unknown. Here, we identify neurons in the infra-limbic area (ILA) of the mouse prefrontal cortex that express the neuropeptide corticotropin-releasing hormone (CRH) and project to the dorsal region of the rostral lateral septum (rLS). We show how release of CRH during familiar encounters disinhibits rLS neurons, thereby suppressing social interactions with familiar mice and contributing to social novelty preference. We further demonstrate how the maturation of CRH expression in ILA during the first 2 post-natal weeks enables the developmental shift from a preference for littermates in juveniles to a preference for novel mice in adults.

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.

GABAA receptors as targets for treating affective and cognitive symptoms of depression

In the past 20 years, our understanding of the pathophysiology of depression has evolved from a focus on an imbalance of monoaminergic neurotransmitters to a multifactorial picture including an improved understanding of the role of glutamatergic excitatory and GABAergic inhibitory neurotransmission. FDA-approved treatments targeting the glutamatergic [esketamine for major depressive disorder (MDD)] and GABAergic (brexanolone for peripartum depression) systems have become available. This review focuses on the GABAA receptor (GABAAR) system as a target for novel antidepressants and discusses the mechanisms by which modulation of δ-containing GABAARs with neuroactive steroids (NASs) or of α5-containing GABAARs results in antidepressant or antidepressant-like actions and discusses clinical data on NASs. Moreover, a potential mechanism by which α5-GABAAR-positive allosteric modulators (PAMs) may improve cognitive deficits in depression is presented.

New GABA-Targeting Therapies for the Treatment of Seizures and Epilepsy: I. Role of GABA as a Modulator of Seizure Activity and Recently Approved Medications Acting on the GABA System

γ-Aminobutyric acid (GABA) is the most prevalent inhibitory neurotransmitter in the mammalian brain and has been found to play an important role in the pathogenesis or the expression of many neurological diseases, including epilepsy. Although GABA can act on different receptor subtypes, the component of the GABA system that is most critical to modulation of seizure activity is the GABAA-receptor-chloride (Cl-) channel complex, which controls the movement of Cl- ions across the neuronal membrane. In the mature brain, binding of GABA to GABAA receptors evokes a hyperpolarising (anticonvulsant) response, which is mediated by influx of Cl- into the cell driven by its concentration gradient between extracellular and intracellular fluid. However, in the immature brain and under certain pathological conditions, GABA can exert a paradoxical depolarising (proconvulsant) effect as a result of an efflux of chloride from high intracellular to lower extracellular Cl- levels. Extensive preclinical and clinical evidence indicates that alterations in GABAergic inhibition caused by drugs, toxins, gene defects or other disease states (including seizures themselves) play a causative or contributing role in facilitating or maintaning seizure activity. Conversely, enhancement of GABAergic transmission through pharmacological modulation of the GABA system is a major mechanism by which different antiseizure medications exert their therapeutic effect. In this article, we review the pharmacology and function of the GABA system and its perturbation in seizure disorders, and highlight how improved understanding of this system offers opportunities to develop more efficacious and better tolerated antiseizure medications. We also review the available data for the two most recently approved antiseizure medications that act, at least in part, through GABAergic mechanisms, namely cenobamate and ganaxolone. Differences in the mode of drug discovery, pharmacological profile, pharmacokinetic properties, drug-drug interaction potential, and clinical efficacy and tolerability of these agents are discussed.

GABA tone regulation and its cognitive functions in the brain

γ-Aminobutyric acid (GABA) is the major inhibitory neurotransmitter released at GABAergic synapses, mediating fast-acting phasic inhibition. Emerging lines of evidence unequivocally indicate that a small amount of extracellular GABA - GABA tone - exists in the brain and induces a tonic GABA current that controls neuronal activity on a slow timescale relative to that of phasic inhibition. Surprisingly, studies indicate that glial cells that synthesize GABA, such as astrocytes, release GABA through non-vesicular mechanisms, such as channel-mediated release, and thereby act as the source of GABA tone in the brain. In this Review, we first provide an overview of major advances in our understanding of the cell-specific molecular and cellular mechanisms of GABA synthesis, release and clearance that regulate GABA tone in various brain regions. We next examine the diverse ways in which the tonic GABA current regulates synaptic transmission and synaptic plasticity through extrasynaptic GABAA-receptor-mediated mechanisms. Last, we discuss the physiological mechanisms through which tonic inhibition modulates cognitive function on a slow timescale. In this Review, we emphasize that the cognitive functions of tonic GABA current extend beyond mere inhibition, laying a foundation for future research on the physiological and pathophysiological roles of GABA tone regulation in normal and abnormal psychiatric conditions.

Generalized fear following acute stress is caused by change in co-transmitter identity of serotonergic neurons

Overgeneralization of fear to harmless situations is a core feature of anxiety disorders resulting from acute stress, yet the mechanisms by which fear becomes generalized are poorly understood. Here we show that generalized fear in mice in response to footshock results from a transmitter switch from glutamate to GABA in serotonergic neurons of the lateral wings of the dorsal raphe. We observe a similar change in transmitter identity in the postmortem brains of PTSD patients. Overriding the transmitter switch in mice using viral tools prevents the acquisition of generalized fear. Corticosterone release and activation of glucocorticoid receptors trigger the switch, and prompt antidepressant treatment blocks the co-transmitter switch and generalized fear. Our results provide new understanding of the plasticity involved in fear generalization.

Autophosphorylation of αCaMKII regulates alcohol consumption by controlling sedative effects of alcohol and alcohol-induced loss of excitatory synapses

Calcium/calmodulin-dependent kinase II (CaMKII) is a key enzyme at the glutamatergic synapses. CAMK2A gene variants have been linked with alcohol use disorder (AUD) by an unknown mechanism. Here, we looked for the link between αCaMKII autophosphorylation and the AUD aetiology. Autophosphorylation-deficient heterozygous αCaMKII mutant mice (T286A^+/- ) were trained in the IntelliCages to test the role of αCaMKII activity in AUD-related behaviours. The glutamatergic synapses morphology in CeA was studied in the animals drinking alcohol using 3D electron microscopy. We found that T286A^+/- mutants consumed less alcohol and were more sensitive to sedating effects of alcohol, as compared to wild-type littermates (WT). After voluntary alcohol drinking, T286A^+/- mice had less excitatory synapses in the CeA, as compared to alcohol-naive animals. This change correlated with alcohol consumption was not reversed after alcohol withdrawal and not observed in WT mice. Our study suggests that αCaMKII autophosphorylation affects alcohol consumption by controlling sedative effects of alcohol and preventing synaptic loss in the individuals drinking alcohol. This finding advances our understanding of the molecular processes that regulate alcohol dependence.

Expanding the canon: An inclusive neurobiology of thalamic and subthalamic fear circuits

Appropriate expression of fear in the face of threats in the environment is essential for survival. The sustained expression of fear in the absence of threat signals is a central pathological feature of trauma- and anxiety-related disorders. Our understanding of the neural circuitry that controls fear inhibition coalesces around the amygdala, hippocampus, and prefrontal cortex. By discussing thalamic and sub-thalamic influences on fear-related learning and expression in this review, we suggest a more inclusive neurobiological framework that expands our canonical view of fear. First, we visit how fear-related learning and expression is influenced by the aforementioned canonical brain regions. Next, we review emerging data that shed light on new roles for thalamic and subthalamic nuclei in fear-related learning and expression. Then, we highlight how these neuroanatomical hubs can modulate fear via integration of sensory and salient stimuli, gating information flow and calibrating behavioral responses, as well as maintaining and updating memory representations. Finally, we propose that the presence of this thalamic and sub-thalamic neuroanatomy in parallel with the tripartite prefrontal cortex-amygdala-hippocampus circuit allows for dynamic modulation of information based on interoceptive and exteroceptive signals. This article is part of the Special Issue on "Fear, Anxiety and PTSD".

Multimodal mapping of cell types and projections in the central nucleus of the amygdala

The central nucleus of the amygdala (CEA) is a brain region that integrates external and internal sensory information and executes innate and adaptive behaviors through distinct output pathways. Despite its complex functions, the diversity of molecularly defined neuronal types in the CEA and their contributions to major axonal projection targets have not been examined systematically. Here, we performed single-cell RNA-sequencing (scRNA-seq) to classify molecularly defined cell types in the CEA and identified marker genes to map the location of these neuronal types using expansion-assisted iterative fluorescence in situ hybridization (EASI-FISH). We developed new methods to integrate EASI-FISH with 5-plex retrograde axonal labeling to determine the spatial, morphological, and connectivity properties of ~30,000 molecularly defined CEA neurons. Our study revealed spatiomolecular organization of the CEA, with medial and lateral CEA associated with distinct molecularly defined cell families. We also found a long-range axon projection network from the CEA, where target regions receive inputs from multiple molecularly defined cell types. Axon collateralization was found primarily among projections to hindbrain targets, which are distinct from forebrain projections. This resource reports marker gene combinations for molecularly defined cell types and axon-projection types, which will be useful for selective interrogation of these neuronal populations to study their contributions to the diverse functions of the CEA.

Topographic representation of current and future threats in the mouse nociceptive amygdala

Adaptive behaviors arise from an integration of current sensory context and internal representations of past experiences. The central amygdala (CeA) is positioned as a key integrator of cognitive and affective signals, yet it remains unknown whether individual populations simultaneously carry current- and future-state representations. We find that a primary nociceptive population within the CeA of mice, defined by CGRP-receptor (Calcrl) expression, receives topographic sensory information, with spatially defined representations of internal and external stimuli. While Calcrl+ neurons in both the rostral and caudal CeA respond to noxious stimuli, rostral neurons promote locomotor responses to externally sourced threats, while caudal CeA Calcrl+ neurons are activated by internal threats and promote passive coping behaviors and associative valence coding. During associative fear learning, rostral CeA Calcrl+ neurons stably encode noxious stimulus occurrence, while caudal CeA Calcrl+ neurons acquire predictive responses. This arrangement supports valence-aligned representations of current and future threats for the generation of adaptive behaviors.

Acid-sensing ion channel 1a in the central nucleus of the amygdala regulates anxiety-like behaviors in a mouse model of acute pain

Pain is commonly comorbid with anxiety; however, the neural and molecular mechanisms underlying the comorbid anxiety symptoms in pain (CASP) have not been fully elucidated. In this study, we explored the role of acid-sensing ion channel 1a (ASIC1a), located in GABAergic neurons from the central nucleus of the amygdala (GABA^CeA), in the regulation of CASP in an acute pain mouse model. We found that the mice displayed significant mechanical pain sensitization and anxiety-like behaviors one day post injection of complete Freud's adjuvant (CFA1D). Electrophysiological recordings from acute brain slices showed that the activity of GABA^CeA neurons increased in the CFA1D mice compared with that in the saline mice. In addition, chemogenetic inhibition of GABA^CeA neurons relieved mechanical pain sensitization and anxiety-like behaviors in the CFA1D mice. Interestingly, through pharmacological inhibition and genetic knockdown of ASIC1a in the central nucleus amygdala, we found that downregulation of ASIC1a relieved the hypersensitization of mechanical stimuli and alleviated anxiety-related behaviors, accompanied with reversing the hyperactivity of GABA^CeA neurons in the CFA 1D mice. In conclusion, our results provide novel insights that ASIC1a in GABA^CeA neurons regulates anxiety-like behaviors in a mouse model of acute pain.

Pharmacological Signature and Target Specificity of Inhibitory Circuits Formed by Martinotti Cells in the Mouse Barrel Cortex

In the neocortex, fast synaptic inhibition orchestrates both spontaneous and sensory-evoked activity. GABAergic interneurons (INs) inhibit pyramidal neurons (PNs) directly, modulating their output activity and thus contributing to balance cortical networks. Moreover, several IN subtypes also inhibit other INs, forming specific disinhibitory circuits, which play crucial roles in several cognitive functions. Here, we studied a subpopulation of somatostatin-positive INs, the Martinotti cells (MCs) in layer 2/3 of the mouse barrel cortex (both sexes). MCs inhibit the distal portion of PN apical dendrites, thus controlling dendrite electrogenesis and synaptic integration. Yet, it is poorly understood whether MCs inhibit other elements of the cortical circuits, and the connectivity properties with non-PN targets are unknown. We found that MCs have a strong preference for PN dendrites, but they also considerably connect with parvalbumin-positive, vasoactive intestinal peptide-expressing, and layer 1 (L1) INs. Remarkably, GABAergic synapses from MCs exhibited clear cell type-specific short-term plasticity. Moreover, whereas the biophysical properties of MC-PN synapses were consistent with distal dendritic inhibition, MC-IN synapses exhibited characteristics of fast perisomatic inhibition. Finally, MC-PN connections used α5-containing GABA_A receptors (GABA_ARs), but this subunit was not expressed by the other INs targeted by MCs. We reveal a specialized connectivity blueprint of MCs within different elements of superficial cortical layers. In addition, our results identify α5-GABA_ARs as the molecular fingerprint of MC-PN dendritic inhibition. This is of critical importance, given the role of α5-GABA_ARs in cognitive performance and their involvement in several brain diseases.SIGNIFICANCE STATEMENT Martinotti cells (MCs) are a prominent, broad subclass of somatostatin-expressing GABAergic interneurons, specialized in controlling distal dendrites of pyramidal neurons (PNs) and taking part in several cognitive functions. Here we characterize the connectivity pattern of MCs with other interneurons in the superficial layers (L1 and L2/3) of the mouse barrel cortex. We found that the connectivity pattern of MCs with PNs as well as parvalbumin, vasoactive intestinal peptide, and L1 interneurons exhibit target-specific plasticity and biophysical properties. The specificity of α5-GABA_ARs at MC-PN synapses and the lack or functional expression of this subunit by other cell types define the molecular identity of MC-PN connections and the exclusive involvement of this inhibitory circuits in α5-dependent cognitive tasks.

Benzodiazepine Modulation of GABAA Receptors: A Mechanistic Perspective

Benzodiazepines (BZDs) are a class of widely prescribed psychotropic drugs that target GABAA receptors (GABAARs) to tune inhibitory synaptic signaling throughout the central nervous system. Despite knowing their molecular target for over 40 years, we still do not fully understand the mechanism of modulation at the level of the channel protein. Nonetheless, functional studies, together with recent cryo-EM structures of GABAA(α1)2(βX)2(γ2)1 receptors in complex with BZDs, provide a wealth of information to aid in addressing this gap in knowledge. Here, mechanistic interpretations of functional and structural evidence for the action of BZDs at GABAA(α1)2(βX)2(γ2)1 receptors are reviewed. The goal is not to describe each of the many studies that are relevant to this discussion nor to dissect in detail all the effects of individual mutations or perturbations but rather to highlight general mechanistic principles in the context of recent structural information.

The central extended amygdala guides survival-relevant tradeoffs: Implications for understanding common psychiatric disorders

To thrive in challenging environments, individuals must pursue rewards while avoiding threats. Extensive studies in animals and humans have identified the central extended amygdala (EAc)-which includes the central nucleus of the amygdala (Ce) and bed nucleus of the stria terminalis (BST)-as a conserved substrate for defensive behavior. These studies suggest the EAc influences defensive responding and assembles fearful and anxious states. This has led to the proliferation of a view that the EAc is fundamentally a defensive substrate. Yet mechanistic work in animals has implicated the EAc in numerous appetitive and consummatory processes, yielding fresh insights into the microcircuitry of survival- and emotion-relevant response selection. Coupled with the EAc's centrality in a conserved network of brain regions that encode multisensory environmental and interoceptive information, these findings suggest a broader role for the EAc as an arbiter of survival- and emotion-relevant tradeoffs for action selection. Determining how the EAc optimizes these tradeoffs promises to improve our understanding of common psychiatric illnesses such as anxiety, depression, alcohol- and substance-use disorders, and anhedonia.

The Pituitary Adenylate Cyclase-Activating Polypeptide (PACAP) System of the Central Amygdala Mediates the Detrimental Effects of Chronic Social Defeat Stress in Rats

Many psychiatric diseases stem from an inability to cope with stressful events, as chronic stressors can precipitate or exacerbate psychopathologies. The neurobiological mechanisms underlying the response to chronic stress and the resulting anxiety states remain poorly understood. Stress neuropeptides in the extended amygdala circuitry mediate the behavioral response to stress, and hyperactivity of these systems has been hypothesized to be responsible for the emergence of persistent negative outcomes and for the pathogenesis of anxiety-related and trauma-related disorders. Pituitary adenylate cyclase-activating polypeptide (PACAP) and its receptor PAC1R are highly expressed within the central amygdala (CeA) and play a key role in stress regulation. Here, we used chronic social defeat stress (CSDS), a clinically relevant model of psychosocial stress that produces robust maladaptive behaviors in rodents. We found that 10 days of CSDS cause a significant increase in PACAP levels selectively in the CeA of rats, as well as an increase in PAC1R mRNA. Using a viral vector strategy, we found that PAC1R knock-down in the CeA attenuates the CSDS-induced body weight loss and prevents the CSDS-induced increase in anxiety-like behavior. Notably, CSDS animals display reduced basal corticosterone (CORT) levels and PAC1R knock-down in CeA further reduce them. Finally, the CeA PAC1R knock-down blocks the increase in corticotropin-releasing factor (CRF) immunoreactivity induced by CSDS in CeA. Our findings support the notion that the persistent activation of the PACAP-PAC1R system in the CeA mediates the behavioral outcomes of chronic psychosocial stress independently of the hypothalamic-pituitary-adrenal axis, perhaps via the recruitment of the CRF system.

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.

GABAergic circuits of the basolateral amygdala and generation of anxiety after traumatic brain injury

Traumatic brain injury (TBI) has reached epidemic proportions around the world and is a major public health concern in the United States. Approximately 2.8 million individuals sustain a traumatic brain injury and are treated in an Emergency Department yearly in the U.S., and about 50,000 of them die. Persistent symptoms develop in 10-15% of the cases including neuropsychiatric disorders. Anxiety is the second most common neuropsychiatric disorder that develops in those with persistent neuropsychiatric symptoms after TBI. Abnormalities or atrophy in the temporal lobe has been shown in the overwhelming number of TBI cases. The basolateral amygdala (BLA), a temporal lobe structure that consolidates, stores and generates fear and anxiety-based behavioral outputs, is a critical brain region in the anxiety circuitry. In this review, we sought to capture studies that characterized the relationship between human post-traumatic anxiety and structural/functional alterations in the amygdala. We compared the human findings with results obtained with a reproducible mild TBI animal model that demonstrated a direct relationship between the alterations in the BLA and an anxiety-like phenotype. From this analysis, both preliminary insights, and gaps in knowledge, have emerged which may open new directions for the development of rational and more efficacious treatments.

Chronic pain-induced neuronal plasticity in the bed nucleus of the stria terminalis causes maladaptive anxiety

The comorbidity of chronic pain and mental dysfunctions such as depression and anxiety disorders has long been recognized, but the underlying mechanisms remain poorly understood. Here, using a mouse model of neuropathic pain, we demonstrated neuronal plasticity in the bed nucleus of the stria terminalis (BNST), which plays a critical role in chronic pain-induced maladaptive anxiety. Electrophysiology demonstrated that chronic pain increased inhibitory inputs to lateral hypothalamus (LH)-projecting BNST neurons. Chemogenetic manipulation revealed that sustained suppression of LH-projecting BNST neurons played a crucial role in chronic pain-induced anxiety. Furthermore, using a molecular genetic approach, we demonstrated that chronic pain elevated the excitability of a specific subpopulation of BNST neurons, which express cocaine- and amphetamine-regulated transcript (CART). The elevated excitability of CART-positive neurons caused the increased inhibitory inputs to LH-projecting BNST neurons, thereby inducing anxiety-like behavior. These findings shed light on how chronic pain induces psychiatric disorders, characterized by maladaptive anxiety.

Compartmentalized dendritic plasticity during associative learning

Experience-dependent changes in behavior are mediated by long-term functional modifications in brain circuits. Activity-dependent plasticity of synaptic input is a major underlying cellular process. Although we have a detailed understanding of synaptic and dendritic plasticity in vitro, little is known about the functional and plastic properties of active dendrites in behaving animals. Using deep brain two-photon Ca²⁺ imaging, we investigated how sensory responses in amygdala principal neurons develop upon classical fear conditioning, a form of associative learning. Fear conditioning induced differential plasticity in dendrites and somas regulated by compartment-specific inhibition. Our results indicate that learning-induced plasticity can be uncoupled between soma and dendrites, reflecting distinct synaptic and microcircuit-level mechanisms that increase the computational capacity of amygdala circuits.

PKCδ-positive GABAergic neurons in the central amygdala exhibit tissue-type plasminogen activator: role in the control of anxiety

Tissue plasminogen activator (tPA) is a serine protease expressed in several brain regions and reported to be involved in the control of emotional and cognitive functions. Nevertheless, little is known about the structure-function relationships of these tPA-dependent behaviors. Here, by using a new model of constitutive tPA-deficient mice (tPA^null), we first show that tPA controls locomotor activity, spatial cognition and anxiety. To investigate the brain structures involved in these tPA-dependent behavioral phenotypes, we next generated tPA^flox mice allowing conditional tPA deletion (cKO) following stereotaxic injections of adeno-associated virus driving Cre-recombinase expression (AAV-Cre-GFP). We demonstrate that tPA removal in the dentate gyrus of the hippocampus induces hyperactivity and partial spatial memory deficits. Moreover, the deletion of tPA in the central nucleus of the amygdala, but not in the basolateral nucleus, induces hyperactivity and reduced anxiety-like level. Importantly, we prove that these behaviors depend on the tPA present in the adult brain and not on neurodevelopmental disorders. Also, interestingly, our data show that tPA from Protein kinase-C delta-positive (PKCδ) GABAergic interneurons of the lateral/ capsular part of adult mouse central amygdala controls emotional functions through neuronal activation of the medial central amygdala. Together, our study brings new data about the critical central role of tPA in behavioral modulations in adult mice.

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.

Critical roles for breathing in the genesis and modulation of emotional states

Breathing can be classified into metabolic and behavioral categories. Metabolic breathing and voluntary behavioral breathing are controlled in the brainstem and in the cerebral motor cortex, respectively. This chapter places special emphasis on the reciprocal influences between breathing and emotional processes. As is the case with neural control of breathing, emotions are generated by multiple control networks, located primarily in the forebrain. For several decades, a respiratory rhythm generator has been investigated in the limbic system. The amygdala receives respiratory-related input from the piriform cortex. Excitatory recurrent branches are located in the piriform cortex and have tight reciprocal synaptic connections, which produce periodic oscillations, similar to those recorded in the hippocampus during slow-wave sleep. The relationship between olfactory breathing rhythm and emotion is seen as the gateway to interpreting the relationship between breathing and emotion. In this chapter, we describe roles of breathing in the genesis of emotion, neural structures common to breathing and emotion, and mutual importance of breathing and emotion. We also describe the central roles of conscious awareness and voluntary control of breathing, as effective methods for stabilizing attention and the contents in the stream of consciousness. Voluntary control of breathing is seen as an essential practice for achieving emotional well-being.

GABAA(δ) receptor hypofunction in the amygdala-hippocampal circuit underlies stress-induced anxiety

Dysregulated GABAergic inhibition in the amygdala has long been implicated in stress-related neuropsychiatric disorders. However, the molecular and circuit mechanisms underlying the dysregulation remain elusive. Here, by using a mouse model of chronic social defeat stress (CSDS), we observed that the dysregulation varied drastically across individual projection neurons (PNs) in the basolateral amygdala (BLA), one of the kernel amygdala subregions critical for stress coping. While persistently reducing the extrasynaptic GABA_A receptor (GABA_AR)-mediated tonic current in the BLA PNs projecting to the ventral hippocampus (BLA → vHPC PNs), CSDS increased the current in those projecting to the anterodorsal bed nucleus of stria terminalis (BLA → adBNST PNs), suggesting projection-based dysregulation of tonic inhibition in BLA PNs by CSDS. Transcriptional and electrophysiological analysis revealed that the opposite CSDS influences were mediated by loss- and gain-of-function of δ-containing GABA_ARs (GABA_A(δ)Rs) in BLA → vHPC and BLA → adBNST PNs, respectively. Importantly, it was the lost inhibition in the former population but not the augmentation in the latter population that correlated with the increased anxiety-like behavior in CSDS mice. Virally mediated maintenance of GABA_A(δ)R currents in BLA → vHPC PNs occluded CSDS-induced anxiety-like behavior. These findings clarify the molecular substrate for the dysregulated GABAergic inhibition in amygdala circuits for stress-associated psychopathology.

α2-containing γ-aminobutyric acid type A receptors promote stress resiliency in male mice

Brain α2-containing GABAA receptors play a critical role in the modulation of anxiety- and fear-like behavior. However, it is unknown whether these receptors also play a role in modulating resilience to chronic stress, and in which brain areas and cell types such an effect would be mediated. We evaluated the role of α2-containing GABAA receptors following chronic social defeat stress using male mice deficient in the α2 subunit globally or conditionally in dopamine D1- or D2-receptor-expressing neurons, e.g., within the nucleus accumbens (NAc). In addition, we examined the effect of the lack of the α2 subunit on intermediates of the glutathione synthesis pathway. We found that α2-containing GABAA receptors on D2-receptor-positive but not on D1-receptor-positive neurons promote resiliency to chronic social defeat stress, as reflected in social interaction tests. The pro-resiliency effects of α2-containing GABAA receptors on D2-receptor-positive neurons do not appear to be directly related to alterations in anxiety-like behavior, as reflected in the elevated plus-maze, light-dark box, and novel open field tests. Increases in indices of oxidative stress-reflected by increases in cystathionine levels and reductions in GSH/GSSG ratios-were found in the NAc and prefrontal cortex but not in the hippocampus of mice lacking α2-containing GABAA receptors. We conclude that α2-containing GABAA receptors within specific brain areas and cell populations promote stress resiliency independently of direct effects on anxiety-like behaviors. A potential mechanism contributing to this increased resiliency is the protection that α2-containing GABAA receptors provide against oxidative stress in NAc and the prefrontal cortex.

GAK and PRKCD are positive regulators of PRKN-independent mitophagy

The mechanisms involved in programmed or damage-induced removal of mitochondria by mitophagy remains elusive. Here, we have screened for regulators of PRKN-independent mitophagy using an siRNA library targeting 197 proteins containing lipid interacting domains. We identify Cyclin G-associated kinase (GAK) and Protein Kinase C Delta (PRKCD) as regulators of PRKN-independent mitophagy, with both being dispensable for PRKN-dependent mitophagy and starvation-induced autophagy. We demonstrate that the kinase activity of both GAK and PRKCD are required for efficient mitophagy in vitro, that PRKCD is present on mitochondria, and that PRKCD facilitates recruitment of ULK1/ATG13 to early autophagic structures. Importantly, we demonstrate in vivo relevance for both kinases in the regulation of basal mitophagy. Knockdown of GAK homologue (gakh-1) in C. elegans or knockout of PRKCD homologues in zebrafish led to significant inhibition of basal mitophagy, highlighting the evolutionary relevance of these kinases in mitophagy regulation.

GAK and PRKCD are positive regulators of PRKN-independent mitophagy

The mechanisms involved in programmed or damage-induced removal of mitochondria by mitophagy remains elusive. Here, we have screened for regulators of PRKN-independent mitophagy using an siRNA library targeting 197 proteins containing lipid interacting domains. We identify Cyclin G-associated kinase (GAK) and Protein Kinase C Delta (PRKCD) as regulators of PRKN-independent mitophagy, with both being dispensable for PRKN-dependent mitophagy and starvation-induced autophagy. We demonstrate that the kinase activity of both GAK and PRKCD are required for efficient mitophagy in vitro, that PRKCD is present on mitochondria, and that PRKCD facilitates recruitment of ULK1/ATG13 to early autophagic structures. Importantly, we demonstrate in vivo relevance for both kinases in the regulation of basal mitophagy. Knockdown of GAK homologue (gakh-1) in C. elegans or knockout of PRKCD homologues in zebrafish led to significant inhibition of basal mitophagy, highlighting the evolutionary relevance of these kinases in mitophagy regulation.

Neurobiology: Novel peptide pathways impact threat discrimination

Discriminating dangerous predictive stimuli from non-threatening stimuli is vital for maintaining optimal behavioral strategies. A new study finds that novel stress-related peptide pathways to the dopaminergic midbrain play a fundamental role in threat generalization.

SALM4 negatively regulates NMDA receptor function and fear memory consolidation

Many synaptic adhesion molecules positively regulate synapse development and function, but relatively little is known about negative regulation. SALM4/Lrfn3 (synaptic adhesion-like molecule 4/leucine rich repeat and fibronectin type III domain containing 3) inhibits synapse development by suppressing other SALM family proteins, but whether SALM4 also inhibits synaptic function and specific behaviors remains unclear. Here we show that SALM4-knockout (Lrfn3^-/-) male mice display enhanced contextual fear memory consolidation (7-day post-training) but not acquisition or 1-day retention, and exhibit normal cued fear, spatial, and object-recognition memory. The Lrfn3^-/- hippocampus show increased currents of GluN2B-containing N-methyl-D-aspartate (NMDA) receptors (GluN2B-NMDARs), but not α-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptors (AMPARs), which requires the presynaptic receptor tyrosine phosphatase PTPσ. Chronic treatment of Lrfn3^-/- mice with fluoxetine, a selective serotonin reuptake inhibitor used to treat excessive fear memory that directly inhibits GluN2B-NMDARs, normalizes NMDAR function and contextual fear memory consolidation in Lrfn3^-/- mice, although the GluN2B-specific NMDAR antagonist ifenprodil was not sufficient to reverse the enhanced fear memory consolidation. These results suggest that SALM4 suppresses excessive GluN2B-NMDAR (not AMPAR) function and fear memory consolidation (not acquisition).

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.

Neuronal basis for pain- and anxiety-like behaviors in the central nucleus of the amygdala

ABSTRACT

Chronic pain is often accompanied by anxiety and depression disorders. Amygdala nuclei play important roles in emotional responses, fear, depression, anxiety and pain modulation. The exact mechanism of how amygdala neurons are involved in pain and anxiety is not completely understood. The central nucleus of the amygdala (CeA) contains two major subpopulations of GABAergic neurons that express somatostatin (SOM+) or protein kinase Cδ (PKCδ+). In this study, we found about 70% of pERK-positive neurons colocalized with PKCδ+ neurons in the formalin-induced pain model in mice. Optogenetic activation of PKCδ+ neurons was sufficient to induce mechanical hyperalgesia without changing anxiety-like behavior in naïve mice. Conversely, chemogenetic inhibition of PKCδ+ neurons significantly reduced the mechanical hyperalgesia in the pain model. In contrast, optogenetic inhibition of SOM+ neurons induced mechanical hyperalgesia in naïve mice and increased pERK-positive neurons mainly in PKCδ+ neurons. Optogenetic activation of SOM+ neurons slightly reduced the mechanical hyperalgesia in the pain model but did not change the mechanical sensitivity in naïve mice. Instead, it induced anxiety-like behavior. Our results suggest that the PKCδ+ and SOM+ neurons in CeA exert different functions in regulating pain- and anxiety-like behaviors in mice.

Cellular, circuit and transcriptional framework for modulation of itch in the central amygdala

Itch is an unpleasant sensation that elicits robust scratching and aversive experience. However, the identity of the cells and neural circuits that organize this information remains elusive. Here, we show the necessity and sufficiency of chloroquine-activated neurons in the central amygdala (CeA) for both itch sensation and associated aversion. Further, we show that chloroquine-activated CeA neurons play important roles in itch-related comorbidities, including anxiety-like behaviors, but not in some aversive and appetitive behaviors previously ascribed to CeA neurons. RNA-sequencing of chloroquine-activated CeA neurons identified several differentially expressed genes as well as potential key signaling pathways in regulating pruritis. Finally, viral tracing experiments demonstrate that these neurons send projections to the ventral periaqueductal gray that are critical in modulation of itch. These findings reveal a cellular and circuit signature of CeA neurons orchestrating behavioral and affective responses to pruritus in mice.