Encoding of conditioned fear in central amygdala inhibitory circuits

Traumatic brain injury-induced anxiety: Injury and plasticity of the central noradrenergic system

Long-term anxiety is a hallmark symptom following traumatic brain injury (TBI). Although the central noradrenergic system (CNAS) is known to play a critical role in anxiety by regulating the excitability of several intricately interconnected brain structures via its projections to them, critical questions remain regarding the nature and extent of TBI-induced neuroplastic alterations in the CNAS and how these alterations relate to anxiety disorders. Knowledge relative to these questions is pivotal to development and refinement of therapies for TBI-associated anxiety disorders, including post-traumatic stress disorder. To this end, this study was designed to determine the impacts of chronic TBI on neuroplasticity of the CNAS and their significance in anxiety disorders in a clinically relevant rodent model. A standardized weight-drop model was used to produce controlled impacts of mild-to-moderate TBI in rats. Following the elevated plus maze tests to longitudinally assess anxiety-like behavior at 2 and 18 weeks post-injury of TBI animals, brain tissues of naïve and TBI rats were coronally sectioned and immunostained for a noradrenergic (NA) marker (dopamine β-hydroxylase) and neuronal nuclei in the central NA production sites and critical anxiety-regulating brain structures. We discovered that TBI caused robust losses of NA cells in the locus coeruleus and NA innervation of the central nucleus of the amygdala, an emotional processing center. Conversely, TBI caused intense gains of NA cells in the A2/A1 cell groups and NA innervation of other major anxiety-regulating regions. These changes coincided with progressively elevated anxiety-like behavior. Possibly, NA properties of A2/A1 cells were upregulated to compensate for the TBI-induced severe cell losses in the locus coeruleus. We conclude that these bi-directional vast alterations in the CNAS following chronic TBI contribute to dysregulated anxiety and, in part, the pathophysiology of human post-traumatic stress disorder.

Mechanisms of alcohol influence on fear conditioning: A computational model

BACKGROUND

A connection between stress-related illnesses and alcohol use disorders is extensively documented. Fear conditioning is a standard procedure used to study stress learning and links it to the activation of amygdala circuitry. However, the connection between the changes in amygdala circuitry and function induced by alcohol and fear conditioning is not well established.

METHODS

We introduce a computational model to test the mechanistic relationship between amygdala functional and circuit adaptations during fear conditioning and the impact of acute vs. repeated alcohol exposure. Using firing rate formalism, the model generates electrophysiological and behavioral responses in fear conditioning protocols via plasticity of amygdala inputs. The influence of alcohol is modeled by accounting for known modulation of connections within amygdala circuits, which consequently affect plasticity. Thus, the model connects the electrophysiological and behavioral experiments. We hypothesize that alterations within amygdala circuitry produced by alcohol cause abnormal plasticity of amygdala inputs such that fear extinction is slower to achieve and less robust.

RESULTS

In accordance with prior experimental results, both acute and prior repeated alcohol decrease the speed and robustness of fear extinction in our simulations. The model predicts that, first, the delay in fear extinction caused by alcohol is mostly induced by greater activation of the basolateral amygdala (BLA) after fear acquisition due to alcohol-induced modulation of synaptic weights. Second, both acute and prior repeated alcohol shift the amygdala network away from the robust extinction regime by inhibiting activity in the central amygdala (CeA). Third, our model predicts that fear memories formed during acute or after chronic alcohol are more connected to the context.

CONCLUSIONS

The model suggests how circuit changes induced by alcohol may affect fear behaviors and provides a framework for investigating the involvement of multiple neuromodulators in this neuroadaptive process.

Neuromodulatory signaling contributing to the encoding of aversion

The appropriate and rapid encoding of stimuli bearing a negative valence enables behaviors that are essential for survival. Recent advances in neuroscience using rodents as a model system highlight the relevance of cell type-specific neuronal activities in diverse brain networks for the encoding of aversion, as well as their importance for subsequent behavioral strategies. Within these networks, neuromodulators influence cell excitability, adjust fast synaptic neurotransmission, and affect plasticity, ultimately modulating behaviors. In this review we first discuss contemporary findings leveraging the use of cutting-edge neurotechnologies to define aversion-related neural circuits. The spatial and temporal dynamics of the release of neuromodulators and neuropeptides upon exposure to aversive stimuli are described within defined brain circuits. Together, these mechanistic insights update the present neural framework through which aversion drives motivated behaviors.

Neural Circuits of Fear and Anxiety: Insights from a Neuroethological Perspective

The predatory imminence continuum (PIC) of antipredator defensive behavior has been a helpful strategy for modeling anxiety and fear-related disorders in nonclinical research. The PIC is divided into three different sequential stages that reflect defensive behavioral strategy in response to predatory imminence. However, the PIC was experimentally addressed with a series of shock-based fear conditioning experiments rather than predatory threats. In this article, we consider the PIC in a more naturalistic behavioral setting, focusing on analyzing the neural systems of animals responding to terrestrial and aerial predators. Of relevance, there is a sequential engagement of the distinct neural circuits along each phase of the PIC. In the preencounter phase, prefrontal cortical networks are particularly involved in planning and organizing behavioral responses to ambiguous threats. As the predatory cues or the real predator is detected, there is an engagement of amygdalar and hippocampal > hypothalamic pathways in conjunction with the periaqueductal gray, which organize fear responses. This dynamic particularly reveals how specific neural circuits are set into action to subserve distinct defensive responses. Moreover, we further explore the neural circuits governing other fearful situations outside the context of the PIC, including agonistic social encounters and interoceptive challenges. This analysis reveals an interesting overlap between the neural systems responding to these threats and those involved in response to predatory threats. The present review clarifies how defensive circuits respond to natural threats and provides a more realistic view of the neural systems underlying anxiety and fear responses.

Dopamine prediction error signaling in a unique nigrostriatal circuit is critical for associative fear learning

Learning by experience that certain cues in the environment predict danger is crucial for survival. How dopamine (DA) circuits drive this form of associative learning is not fully understood. Here, in male mice, we demonstrate that DA neurons projecting to a unique subregion of the dorsal striatum, the posterior tail of the striatum (TS), encode a prediction error (PE) signal during associative fear learning. These DA neurons are necessary specifically during acquisition of fear learning, but not once the fear memory is formed, and are not required for forming cue-reward associations. Notably, temporally-precise inhibition or excitation of DA terminals in TS impairs or enhances fear learning, respectively. Furthermore, neuronal activity in TS is crucial for the acquisition of associative fear learning and learning-induced activity patterns in TS critically depend on DA input. Together, our results reveal that DA PE signaling in a non-canonical nigrostriatal circuit is important for driving associative fear learning.

Dopamine modulation of basolateral amygdala activity and function

The basolateral amygdala (BLA) is central to emotional processing, fear learning, and memory. Dopamine (DA) significantly influences BLA function, yet its precise effects are not clear. We present a mathematical model exploring how DA modulation of BLA activity depends on the network's current state. Specifically, we model the firing rates of interconnected neural groups in the BLA and their responses to external stimuli and DA modulation. BLA projection neurons are separated into two groups according to their responses-fear and safety. These groups are connected by mutual inhibition though interneurons. We contrast 'differentiated' BLA states, where fear and safety projection neurons exhibit distinct activity levels, with 'non-differentiated' states. We posit that differentiated states support selective responses and short-term emotional memory. On the other hand, non-differentiated states represent either the case in which BLA is disengaged, or the activation of the fear and safety neurons is at a similar moderate or high level. We show that, while DA further disengages BLA in the low activity state, it destabilizes the moderate activity non-differentiated BLA state. We show that in the latter non-differentiated state the BLA is hypersensitive, and the polarity of its responses (fear or safety) to salient stimuli is highly random. We hypothesize that this non-differentiated state is related to anxiety and Post-Traumatic Stress Disorder (PTSD).

Aversive generalization in human amygdala neurons

Generalization around aversive stimuli is a key feature of learning and adaptive decision making,1,2,3,4,5 but it can be maladaptive if subjects overgeneralize and respond with fear to stimuli that are only loosely similar to the original experience.6,7,8,9,10,11,12 Human imaging studies indicate that the amygdala, a hub of emotional learning, is involved in such overgeneralization,2,9,13,14,15,16,17 and studies in animal models revealed neural correlates of generalized aversive stimuli and identified changes in response properties of single neurons.18,19,20,21,22,23,24,25,26,27,28,29 Yet, it remains unclear if human neurons contribute specifically in aversive situations and, importantly, if they contribute to subsequent behavior even in a safe environment. We recorded single neurons in human subjects while they engaged in probabilistic loss/gain conditioning, followed by a choice task that included additional stimuli and when the original conditioned stimulus no longer entails an aversive (loss) outcome. We find wider behavioral generalization around the aversive stimulus accompanied by a selective increase in amygdala neural responses that were correlated with the degree of individual generalization. In addition, neural activity in the amygdala was predictive of the later choice on a trial-by-trial basis and specific to loss trials. Whereas other brain regions also modulated their activity during generalization, only amygdala neurons signal a trial-specific and loss-specific generalization. The findings reveal that human amygdala neurons play a role in aversive overgeneralization and contribute to generalized choice behavior in a later safe environment and suggest a single-neuron substrate that might enhance anxious and traumatic behaviors.

Learning to fear novel stimuli by observing others in the social affordance framework

Fear responses to novel stimuli can be learned directly, through personal experiences (Fear Conditioning, FC), or indirectly, by observing conspecific reactions to a stimulus (Social Fear Learning, SFL). Although substantial knowledge exists about FC and SFL in humans and other species, they are typically conceived as mechanisms that engage separate neural networks and operate at different levels of complexity. Here, we propose a broader framework that links these two fear learning modes by supporting the view that social signals may act as unconditioned stimuli during SFL. In this context, we highlight the potential role of subcortical structures of ancient evolutionary origin in encoding social signals and argue that they play a pivotal function in transforming observed emotional expressions into adaptive behavioural responses. This perspective extends the social affordance hypothesis to subcortical circuits underlying vicarious learning in social contexts. Recognising the interplay between these two modes of fear learning paves the way for new empirical studies focusing on interspecies comparisons and broadens the boundaries of our knowledge of fear acquisition.

Astrocytic determinant of the fate of long-term memory

While some vivid memories are unyielding and unforgettable, others fade with time. Astrocytes are recognized for their role in modulating the brain's environment and have recently been considered integral to the brain's information processing and memory formation. This suggests their potential roles in emotional perception and memory formation. In this study, we delve into the impact of amygdala astrocytes on fear behaviors and memory, employing astrocyte-specific optogenetic manipulations in mice. Our findings reveal that astrocytic photoactivation with channelrhodopsin-2 (ChR2) provokes aversive behavioral responses, while archaerhodopsin-T (ArchT) photoactivation diminishes fear perception. ChR2 photoactivation amplifies fear perception and fear memory encoding but obstructs its consolidation. On the other hand, ArchT photoactivation inhibits memory formation during intense aversive stimuli, possibly due to weakened fear perception. However, it prevents the decay of remote fear memory over three weeks. Crucially, these memory effects were observed when optogenetic manipulations coincided with the aversive experience, indicating a deterministic role of astrocytic states at the exact moment of fear experiences in shaping long-term memory. This research underscores the significant and multifaceted role of astrocytes in emotional perception, fear memory formation, and modulation, suggesting a sophisticated astrocyte-neuron communication mechanism underlying basic emotional state transitions of information processing in the brain.

Amygdala intercalated cells form an evolutionarily conserved system orchestrating brain networks

The amygdala attributes valence and emotional salience to environmental stimuli and regulates how these stimuli affect behavior. Within the amygdala, a distinct class of evolutionarily conserved neurons form the intercalated cell (ITC) clusters, mainly located around the boundaries of the lateral and basal nuclei. Here, we review the anatomical, physiological and molecular characteristics of ITCs, and detail the organization of ITC clusters and their connectivity with one another and other brain regions. We describe how ITCs undergo experience-dependent plasticity and discuss emerging evidence demonstrating how ITCs are innervated and functionally regulated by neuromodulatory systems. We summarize recent findings showing that experience alters the balance of activity between different ITC clusters, thereby determining prevailing behavioral output. Finally, we propose a model in which ITCs form a key system for integrating divergent inputs and orchestrating brain-wide circuits to generate behavioral states attuned to current environmental circumstances and internal needs.

Neuronal cell type specific roles for Nprl2 in neurodevelopmental disorder-relevant behaviors

Loss of function in the subunits of the GTPase-activating protein (GAP) activity toward Rags-1 (GATOR1) complex, an amino-acid sensitive negative regulator of the mechanistic target of rapamycin complex 1 (mTORC1), is implicated in both genetic familial epilepsies and Neurodevelopmental Disorders (NDDs) (Baldassari et al., 2018). Previous studies have found seizure phenotypes and increased activity resulting from conditional deletion of GATOR1 function from forebrain excitatory neurons (Yuskaitis et al., 2018; Dentel et al., 2022); however, studies focused on understanding mechanisms contributing to NDD-relevant behaviors are lacking, especially studies understanding the contributions of GATOR1's critical GAP catalytic subunit, nitrogen permease regulator like-2 (Nprl2). Given the clinical phenotypes observed in patients with Nprl2 mutations, in this study, we sought to investigate the neuronal cell type contributions of Nprl2 to NDD behaviors. We conditionally deleted Nprl2 broadly in most neurons (Synapsin1cre), in inhibitory neurons only (Vgatcre), and in Purkinje cells within the cerebellum (L7cre). Broad neuronal deletion of Nprl2 resulted in seizures, social and learning deficits, and hyperactivity. In contrast, deleting Nprl2 from inhibitory neurons led to increased motor learning, hyperactive behavior, in addition to social and learning deficits. Lastly, Purkinje cell (PC) loss of Nprl2 also led to learning and social deficits but did not affect locomotor activity. These phenotypes enhance understanding of the spectrum of disease found in human populations with GATOR1 loss of function and highlight the significance of distinct cellular populations to NDD-related behaviors. SIGNIFICANCE STATEMENT: We aim to elucidate the neuronal-specific contributions of nitrogen permease regulator like-2 (Nprl2) to its neurodevelopmental disorder (NDD)-relevant phenotypes. We conditionally deleted Nprl2 broadly in neurons (Syn1cre), in inhibitory neurons (Vgatcre), and in cerebellar Purkinje cells (L7cre). We identify seizures only in the Syn1cre conditional mutant (cKO); hyperactivity, learning difficulties, social deficits, and impulsivity in the Syn1cre and Vgatcre cKOs; and social deficits, and fear learning deficits in L7cre cKOs. To our knowledge, we are the first to describe the behavioral contributions of Nprl2's function across multiple cell types. Our findings highlight both critical roles for Nprl2 in learning and behavior and also distinct contributions of select neuronal populations to these NDD-relevant behaviors.

Cannabinoid Modulation of Central Amygdala Population Dynamics During Threat Investigation

Cannabinoids modulate innate avoidance, threat-reactivity, and stress adaptations via modulation amygdala-associated circuits; however, the mechanisms by which cannabinoids modulate amygdala representation of threat-related behavior are not known. We show that cannabinoid administration increases the activity of central amygdala (CeA) somatostatin neurons (SOM) and alters basal network dynamics in a manner supporting generation of antagonistic sub-ensembles within the SOM population. Moreover, diverging neuronal population trajectory dynamics and enhanced antagonistic sub-ensemble representation of threat-related behaviors, and enhanced threat-related location representation, were also observed. Lastly, cannabinoid administration increased the proportion of SOM neurons exhibiting multidimensional representation of threat-related behaviors and behavior-location conjunction. While cannabinoid receptor activation ex vivo suppressed excitatory inputs to SOM neurons, our data suggest preferential suppression of local GABA release subserves cannabinoid activation of CeA SOM neurons. These data provide insight into how cannabinoid-mediated presynaptic suppression transforms postsynaptic population dynamics and reveal cellular mechanisms by which cannabinoids could affect threat-reactivity.

A localized tracing technique to explore intra-amygdala functional and structural correlates of individual variability in behavioral response

Introduction

The neurobiological basis for individual variability in behavioral responses to stimuli remains poorly understood. Probing the neural substrates that underlie individual variability in stress responses may open the door for preventive approaches that use biological markers to identify at-risk populations. New developments of viral neuronal tracing tools have led to a recent increase in studies on long range circuits and their functional role in stress responses and social behavior. While these studies are necessary to untangle largescale connectivity, most social behaviors are mediated and fine-tuned by local subregional circuitry.

Methods

In order to probe this local, interregional connectivity, we present a new combination of a neuronal tracing system with immediate early gene immunohistochemistry for examining structural and functional connectivity within the same animal. Specifically, we combine a retrograde transsynaptic rabies tracing system with cFos colocalization immediately after an acute stressor to elucidate local structural and stress-activated connectivity within the amygdala complex in female and male mice.

Results and discussion

We show how specific structural and functional connections can predict individual variability along a spectrum of social approach/avoidance following acute social defeat stress. We demonstrate how our robust method can be used to elucidate structural and functional differences in local connectivity that mediate individual variability in behavioral response.

Central amygdala NPBWR1 neurons facilitate social novelty seeking and new social interactions

The formation of new social interactions is vital for social animals, but the underlying neural mechanisms remain poorly understood. We identified CeANpbwr1 neurons, a population in central amygdala expressing neuropeptide B/W receptor-1 (NPBWR1), that play a critical role in these interactions. CeANpbwr1 neurons were activated during encounters with unfamiliar, but not with familiar, mice. Manipulations of CeANpbwr1 neurons showed that their excitation is essential for maintaining physical interactions with novel conspecifics. Activation of CeANpbwr1 neurons alleviated social deficits induced by chronic social defeat stress, suggesting therapeutic potential. Conversely, overexpression of human NPBWR1 in CeANpbwr1 neurons reduced activity of these neurons and impaired social interactions with unfamiliar mice. This effect was absent in a polymorphic variant of the human NPBWR1 gene (404A>T). These findings highlight how CeANpbwr1 neurons promote social novelty seeking and reveal a complex interplay between NPBWR1 genetic variations and social behavior.

Valence and salience encoding in the central amygdala

The central amygdala (CeA) has emerged as an important brain region for regulating both negative (fear and anxiety) and positive (reward) affective behaviors. The CeA has been proposed to encode affective information in the form of valence (whether the stimulus is good or bad) or salience (how significant is the stimulus), but the extent to which these two types of stimulus representation occur in the CeA is not known. Here, we used single cell calcium imaging in mice during appetitive and aversive conditioning and found that majority of CeA neurons (~65%) encode the valence of the unconditioned stimulus (US) with a smaller subset of cells (~15%) encoding the salience of the US. Valence and salience encoding of the conditioned stimulus (CS) was also observed, albeit to a lesser extent. These findings show that the CeA is a site of convergence for encoding oppositely valenced US information.

The Neurobehavioral State hypothesis

Since the early attempts to understand the brain made by Greek philosophers more than 2000 years ago, one of the main questions in neuroscience has been how the brain perceives all the stimuli in the environment and uses this information to implement a response. Recent hypotheses of the neural code rely on the existence of an ideal observer, whether on specific areas of the cerebral cortex or distributed network composed of cortical and subcortical elements. The Neurobehavioral State hypothesis stipulates that neurons are in a quasi-stable state due to the dynamic interaction of their molecular components. This increases their computational capabilities and electrophysiological behavior further than a binary active/inactive state. Together, neuronal populations across the brain learn to identify and associate internal and external stimuli with actions and emotions. Furthermore, such associations can be stored through the regulation of neuronal components as new quasi-stable states. Using this framework, behavior arises as the result of the dynamic interaction between internal and external stimuli together with previously established quasi-stable states that delineate the behavioral response. Finally, the Neurobehavioral State hypothesis is firmly grounded on present evidence of the complex dynamics within the brain, from the molecular to the network level, and avoids the need for a central observer by proposing the brain configures itself through experience-driven associations.

Computational modeling of fear and stress responses: validation using consolidated fear and stress protocols

Dysfunction in fear and stress responses is intrinsically linked to various neurological diseases, including anxiety disorders, depression, and Post-Traumatic Stress Disorder. Previous studies using in vivo models with Immediate-Extinction Deficit (IED) and Stress Enhanced Fear Learning (SEFL) protocols have provided valuable insights into these mechanisms and aided the development of new therapeutic approaches. However, assessing these dysfunctions in animal subjects using IED and SEFL protocols can cause significant pain and suffering. To advance the understanding of fear and stress, this study presents a biologically and behaviorally plausible computational architecture that integrates several subregions of key brain structures, such as the amygdala, hippocampus, and medial prefrontal cortex. Additionally, the model incorporates stress hormone curves and employs spiking neural networks with conductance-based integrate-and-fire neurons. The proposed approach was validated using the well-established Contextual Fear Conditioning paradigm and subsequently tested with IED and SEFL protocols. The results confirmed that higher intensity aversive stimuli result in more robust and persistent fear memories, making extinction more challenging. They also underscore the importance of the timing of extinction and the significant influence of stress. To our knowledge, this is the first instance of computational modeling being applied to IED and SEFL protocols. This study validates our computational model's complexity and biological realism in analyzing responses to fear and stress through fear conditioning, IED, and SEFL protocols. Rather than providing new biological insights, the primary contribution of this work lies in its methodological innovation, demonstrating that complex, biologically plausible neural architectures can effectively replicate established findings in fear and stress research. By simulating protocols typically conducted in vivo-often involving significant pain and suffering-in an insilico environment, our model offers a promising tool for studying fear-related mechanisms. These findings support the potential of computational models to reduce the reliance on animal testing while setting the stage for new therapeutic approaches.

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.

ErbB4 precludes the occurrence of PTSD-like fear responses by supporting the bimodal activity of the central amygdala

Post-traumatic stress disorder (PTSD) often arises after exposure to traumatic events and is characterized by dysregulated fear responses. Although the associations of erb-b2 receptor tyrosine kinase 4 (ErbB4) with various neuropsychiatric diseases, including schizophrenia and bipolar disorder, have been widely examined, the physiological roles of ErbB4 in PTSD and fear responses remain unclear. Using Cre-dependent ErbB4 knockout (KO) mice, we observed that PTSD-like fear behaviors emerged in ErbB4-deficient mice, particularly in inhibitory neurons. Specifically, the loss of ErbB4 in somatostatin-expressing (SST+) neurons was sufficient to induce PTSD-like fear responses. We also adopted the CRISPR/Cas9 system for region-specific KO of ErbB4, which revealed that ErbB4 deletion in SST+ neurons of the lateral division of the amygdala (CeL) caused elevated anxiety and PTSD-like fear generalization. Consistent with its physiological role, ErbB4 expression was diminished in CeLSST neurons from mice that exhibited PTSD-like phenotypes. While fear On and Off cells identified in the CeL displayed distinct responses to conditioned and novel cues, as previously shown, the selectivity of those On and Off cells was compromised in SSTErbB4-/- and stressed mice, which displayed strong fear generalization. Therefore, the bimodal activity that CeL On/Off cells display is likely required for proper discrimination of fearful stimuli from ambient stimuli, which should be sustained by the presence of ErbB4. Taken together, our data substantiate the correlation between PTSD-like fear responses and ErbB4 expression in CeLSST neurons and further underscore the functional effects of ErbB4 in CeLSST neurons, supporting the bimodal responses of CeL neurons.

Nitric oxide-signalling affects panic-like defensive behaviour and defensive antinociception neuromodulation in the prelimbic cerebral cortex

RATIONALE

The prelimbic division (PrL) of the medial prefrontal cortex (mPFC) is a key structure in panic.

OBJECTIVES

To evaluate the role of nitric oxide (NO) in defensive behaviour and antinociception.

METHODS

Either Nω-propyl-L-arginine (NPLA) or Carboxy-PTIO was microinjected in the PrL cortex, followed by hypothalamic treatment with bicuculline. The exploratory behaviours, defensive reactions and defensive antinociception were recorded. Encephalic c-Fos protein was immunolabelled after escape behaviour.

RESULTS

NPLA (an inhibition of nNOs) decreased panic-like responses and innate fear-induced antinociception. The c-PTIO (a membrane-impermeable NO scavenger) decreased the escape behaviour. PrL cortex pre-treatment with c-PTIO at all doses decreased defensive antinociception. c-Fos protein was labelled in neocortical areas, limbic system, and mesencephalic structures.

CONCLUSION

The NPLA and c-PTIO in the PrL/mPFC decreased the escape behaviour and defensive antinociception organised by medial hypothalamic nuclei. The oriented escape behaviour recruits neocortical areas, limbic system, and mesencephalic structures. These findings suggest that the organisation of defensive antinociception recruits NO-signalling mechanisms within the PrL cortex. Furthermore, the present findings also support the role of NO as a retrograde messenger in the PrL cortex during panic-like emotional reactions.

Sex Differences in Neural Circuits Underlying Fear Processing

Neural circuitry involved in anxiety and fear-related disorders exhibits strong sexual modulation. A limited number of studies integrating female and male data have revealed differences in neural networks, and distinct interconnectivity between these brain areas. Despite the efforts to incorporate female or mixed-sex data, there is compelling evidence that sex, as a biological variable, significantly influences fear processing. This chapter presents primary findings on sex differences in fear circuitry. It is imperative to consider this factor to ensure scientific research's integrity and understand how fear is processed in the central nervous system.

Disentangling the Neural Circuits of Arousal and Anxiety-Like Behavior

Anxiety disorders are prevalent and debilitating conditions characterized by excessive concern and fear, affecting thoughts, behaviors, and sensations. A critical component of anxiety is arousal, a complex process involving alertness regulation and stimulus salience modulation. While arousal is adaptive in normal circumstances, dysregulation can lead to hypoarousal or hyperarousal, affecting response selection and threat perception. This chapter reviews challenges in studying arousal in preclinical anxiety models, emphasizing the need for multicomponent measurement and analysis. Novel methodologies integrating physiological measurement with activity tracking of neurons with single-cell resolution in awake animals are discussed, with emphasis in current challenges. Understanding these mechanisms is crucial for developing effective treatments for anxiety disorders.

The Emerging Role of Brain Mitochondria in Fear and Anxiety

The functional complexity of brain circuits underlies the broad spectrum of behaviors, cognitive functions, and their associated disorders. Mitochondria, traditionally known for their role in cellular energy metabolism, are increasingly recognized as central to brain function and behavior. This review examines how mitochondria are pivotal in linking cellular energy processes with the functioning of neural circuits that govern fear and anxiety. Following an introductory section in which we summarize current knowledge about fear and anxiety neural circuits, we provide a brief summary of mitochondria fundamental roles (e.g., from energy production and calcium buffering to their involvement in reactive oxygen species (ROS) generation, mitochondrial dynamics, and signaling), particularly emphasizing their contribution to synaptic plasticity, neurodevelopment, and stress response mechanisms. The review's core focuses on the current state of knowledge regarding how mitochondrial function and dysfunction impact the neural substrates of fear and anxiety. Furthermore, we explore the implications of mitochondrial alterations in the context of posttraumatic stress disorder (PTSD) and anxiety disorders, underscoring the potential of mitochondrial pathways as new therapeutic targets. Integrating insights from genetic, biochemical, neurobiological, behavioral, and clinical studies, we propose a model in which mitochondrial function is critical for regulating the neural circuits that underpin fear and anxiety behaviors, highlighting how mitochondrial dysfunction can lead to their pathological manifestations. This integration emphasizes the potential for developing novel treatments targeting the biological roots of fear, anxiety, and related disorders. By merging mitochondrial biology with behavioral and circuit neuroscience, we enrich our neurobiological understanding of fear and anxiety, uncovering promising avenues for therapeutic intervention.

Common and divergent neuroimaging features in major depression, posttraumatic stress disorder, and their comorbidity

Posttraumatic stress disorder (PTSD) and major depressive disorder (MDD) are common stress-related psychiatric disorders. Genetic and neurobiology research has supported the viewpoint that PTSD and MDD may possess common and disorder-specific underlying mechanisms. In this systematic review, we summarize evidence for the similarities and differences in brain functional and structural features of MDD, PTSD, and their comorbidity, as well as the effects of extensively used therapies in patients with comorbid PTSD and MDD (PTSD + MDD). These functional magnetic resonance imaging (MRI) studies highlight the (i) shared hypoactivation in the prefrontal cortex during cognitive and emotional processing in MDD and PTSD; (ii) higher activation in fear processing regions including amygdala, hippocampus, and insula in PTSD compared to MDD; and (iii) distinct functional deficits in brain regions involved in fear and reward processing in patients with PTSD + MDD relative to those with PTSD alone. These structural MRI studies suggested that PTSD and MDD share features of reduced volume in focal frontal areas. The treatment effects in patients with PTSD + MDD may correlate with the normalization trend of structural alterations. Neuroimaging predictors of repetitive transcranial magnetic stimulation response in patients with PTSD + MDD may differ from the mono-diagnostic groups. In summary, neuroimaging studies to date have provided limited information about the shared and disorder-specific features in MDD and PTSD. Further research is essential to pave the way for developing improved diagnostic markers and eventually targeted treatment approaches for the shared and distinct brain alterations presented in patients with MDD and PTSD.

Acute stress activates basolateral amygdala neurons expressing corticotropin-releasing hormone receptor type 1 (CRHR1): Topographical distribution and projection-specific activation in male and female rats

Although the basolateral amygdala (BLA) and corticotropin releasing hormone receptor type I (CRHR1) signaling are both central to the stress response, the spatial and circuit-specific distribution of CRHR1 have not been identified in the BLA at a high resolution. We used transgenic male and female CRHR1-Cre-tdTomato rats to topographically map the distribution of BLACRHR1 neurons and identify whether they are activated by acute stress. Additionally, we used the BLA circuits projecting to the central amygdala (CeA) and nucleus accumbens (NAc) as a model to test circuit-specific expression of CRHR1 in the BLA. We established several key findings. First, CRHR1 had the strongest expression in the lateral amygdala and in caudal portions of the BLA. Second, acute restraint stress increased FOS expression of CRHR1 neurons, and stress-induced activation was particularly strong in medial subregions of the BLA. Third, stress significantly increased FOS expression on BLA-NAc, but not BLA-CeA projectors, and BLA-NAc activation was more robust in males than females. Finally, CRHR1 was expressed on a subset of BLA-CeA and BLA-NAc projection neurons. Collectively, this expands our understanding of BLA molecular- and circuit-specific activation patterns following acute stress.

Behavioral and neural correlates of diverse conditioned fear responses in male and female rats

Pavlovian fear conditioning is a widely used tool that models associative learning in rodents. For decades the field has used predominantly male rodents and focused on a sole conditioned fear response: freezing. However, recent work from our lab and others has identified darting as a female-biased conditioned response, characterized by an escape-like movement across a fear conditioning chamber. It is also accompanied by a behavioral phenotype: Darters reliably show decreased freezing compared to Non-darters and males and reach higher velocities in response to the foot shock ("shock response"). However, the relationship between shock response and conditioned darting is not known. This study investigated if this link is due to differences in general processing of aversive stimuli between Darters, Non-darters and males. Across a variety of modalities, including corticosterone measures, the acoustic startle test, and sensitivity to thermal pain, Darters were found not to be more reactive or sensitive to aversive stimuli, and, in some cases, they appear less reactive to Non-darters and males. Analyses of cFos activity in regions involved in pain and fear processing following fear conditioning identified discrete patterns of expression among Darters, Non-darters, and males exposed to low and high intensity foot shocks. The results from these studies further our understanding of the differences between Darters, Non-darters and males and highlight the importance of studying individual differences in fear conditioning as indicators of fear state.

Valence and Salience Encoding in the Central Amygdala

The central amygdala (CeA) has emerged as an important brain region for regulating both negative (fear and anxiety) and positive (reward) affective behaviors. The CeA has been proposed to encode affective information in the form of valence (whether the stimulus is good or bad) or salience (how significant is the stimulus), but the extent to which these two types of stimulus representation occur in the CeA is not known. Here, we used single cell calcium imaging in mice during appetitive and aversive conditioning and found that majority of CeA neurons (~65%) encode the valence of the unconditioned stimulus (US) with a smaller subset of cells (~15%) encoding the salience of the US. Valence and salience encoding of the conditioned stimulus (CS) was also observed, albeit to a lesser extent. These findings show that the CeA is a site of convergence for encoding oppositely valenced US information.

Divergent input patterns to the central lateral amygdala play a duet in fear memory formation

Somatostatin (SOM)-expressing neurons in the central lateral amygdala (CeL) are responsible for fear memory learning, but the circuit and molecular mechanisms underlying this biology remain elusive. Here, we found that glutamatergic neurons in the lateral parabrachial nucleus (LPB) directly dominated the activity of CeLSOM neurons, and that selectively inhibiting the LPBGlu→CeLSOM pathway suppressed fear memory acquisition. By contrast, inhibiting CeL-projecting glutamatergic neurons in the paraventricular thalamic nucleus (PVT) interfered with consolidation-related processes. Notably, CeLSOM-innervating neurons in the LPB were modulated by presynaptic cannabinoid receptor 1 (CB1R), and knock down of CB1Rs in LPB glutamatergic neurons enhanced excitatory transmission to the CeL and partially rescued the impairment in fear memory induced by CB1R activation in the CeL. Overall, our study reveals the mechanisms by which CeLSOM neurons mediate the formation of fear memories during fear conditioning in mice, which may provide a new direction for the clinical research of fear-related disorders.

Distinct roles of general anesthesia activated CeA neurons in acute versus late phase of neuropathic pain

A previous study discovered a distinct population of GABAergic neurons in the ce ntral a mygdala (CeA) that can be activated by g eneral a nesthesia (CeA GA ) and exert analgesic functions (Hua et al., 2020). To independently reproduce these prior findings and to investigate the electrophysiological properties of CeA GA neurons, we first used 1.2% isoflurane to induce c-Fos activation in the mouse brain and validated the Fos expression by RNAscope in situ hybridization. Indeed, isoflurane induced robust Fos expression in CeA and these Fos + CeA GA neurons are GABAergic neurons (Vgat + ). We next used Fos-TRAP2 method (different from the CANE method used in the prior study) to label CeA GA neurons (tdTomato + ). Our ex vivo electrophysiological recordings in brain slices revealed that compared to Fos-negative CeA neurons, CeA GA neurons had significantly higher excitability and exhibited distinct patterns of action potentials. Chemogenetic activation of Fos-TRAPed CeA GA neurons was effective at increasing pain thresholds in naïve mice and mice with early-phase neuropathic pain 2 weeks after spared nerve injury (SNI). However, the same chemogenetic activation of CeA GA neurons only had modest analgesia in the late phase of SNI at 8 weeks, although it was highly effective in reducing chronic pain-associated anxiety behaviors at this stage. We found that Fos-negative CeA neurons, but not CeA GA neurons, exhibited increased excitability in the late-phase of SNI, suggesting that chronic pain causes a shift in the relative activity of the CeA microcircuit. Interestingly, Fos-negative neurons exhibited much higher expression of K + -Cl - cotransporter-2 (KCC2), and KCC2 expression was downregulated in the CeA in the late-phase of neuropathic pain. These results support the idea that targeting CeA GA neurons may provide therapeutic benefits for pain relief and chronic pain-associated anxiety. Our findings also suggest distinct roles of CeA GA neurons in regulating physiological pain, acute pain, and chronic pain with a possible involvement of KCC2.

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.

Behavioral outputs and overlapping circuits between conditional fear and active avoidance

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

Transcranial Focused Ultrasound Targeting the Amygdala May Increase Psychophysiological and Subjective Negative Emotional Reactivity in Healthy Older Adults

Background

The amygdala is highly implicated in an array of psychiatric disorders but is not accessible using currently available noninvasive neuromodulatory techniques. Low-intensity transcranial focused ultrasound (TFUS) is a neuromodulatory technique that has the capability of reaching subcortical regions noninvasively.

Methods

We studied healthy older adult participants (N = 21, ages 48-79 years) who received TFUS targeting the right amygdala and left entorhinal cortex (active control region) using a 2-visit within-participant crossover design. Before and after TFUS, behavioral measures were collected via the State-Trait Anxiety Inventory and an emotional reactivity and regulation task utilizing neutral and negatively valenced images from the International Affective Picture System. Heart rate and self-reported emotional valence and arousal were measured during the emotional reactivity and regulation task to investigate subjective and physiological responses to the task.

Results

Significant increases in both self-reported arousal in response to negative images and heart rate during emotional reactivity and regulation task intertrial intervals were observed when TFUS targeted the amygdala; these changes were not evident when the entorhinal cortex was targeted. No significant changes were found for state anxiety, self-reported valence to the negative images, cardiac response to the negative images, or emotion regulation.

Conclusions

The results of this study provide preliminary evidence that a single session of TFUS targeting the amygdala may alter psychophysiological and subjective emotional responses, indicating some potential for future neuropsychiatric applications. However, more work on TFUS parameters and targeting optimization is necessary to determine how to elicit changes in a more clinically advantageous way.

Inhibitory fear memory engram in the mouse central lateral amygdala

Engrams, which are cellular substrates of memory traces, have been identified in various brain areas, including the amygdala. While most identified engrams are composed of excitatory, glutamatergic neurons, GABAergic inhibitory engrams have been relatively overlooked. Here, we report the identification of an inhibitory engram in the central lateral amygdala (CeL), a key area for auditory fear conditioning. This engram is primarily composed of GABAergic somatostatin-expressing (SST(+)) and, to a lesser extent, protein kinase C-δ-expressing (PKC-δ(+)) neurons. Fear memory is accompanied by a preferential enhancement of synaptic inhibition onto PKC-δ(+) neurons. Silencing this CeL GABAergic engram disinhibits the activity of targeted extra-amygdaloid areas, selectively increasing the expression of fear. Our findings define the behavioral function of an engram formed exclusively by GABAergic inhibitory neurons in the mammalian brain.

Behavioral and neural correlates of diverse conditioned fear responses in male and female rats

Pavlovian fear conditioning is a widely used tool that models associative learning in rodents. For decades the field has used predominantly male rodents and focused on a sole conditioned fear response: freezing. However, recent work from our lab and others has identified darting as a female-biased conditioned response, characterized by an escape-like movement across a fear conditioning chamber. It is also accompanied by a behavioral phenotype: Darters reliably show decreased freezing compared to Non-darters and males and reach higher velocities in response to the foot shock ("shock response"). However, the relationship between shock response and conditioned darting is not known. This study investigated if this link is due to differences in general processing of aversive stimuli between Darters, Non-darters and males. Across a variety of modalities, including corticosterone measures, the acoustic startle test, and sensitivity to thermal pain, Darters were found not to be more reactive or sensitive to aversive stimuli, and, in some cases, they appear less reactive to Non-darters and males. Analyses of cFos activity in regions involved in pain and fear processing following fear conditioning identified discrete patterns of expression among Darters, Non-darters, and males exposed to low and high intensity foot shocks. The results from these studies further our understanding of the differences between Darters, Non-darters and males and highlight the importance of studying individual differences in fear conditioning as indicators of fear state.

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.

Deciphering the role of brainstem glycinergic neurons during startle and prepulse inhibition

Prepulse inhibition (PPI) of the auditory startle response, a key measure of sensorimotor gating, diminishes with age and is impaired in various neurological conditions. While PPI deficits are often associated with cognitive impairments, their reversal is routinely used in experimental systems for antipsychotic drug screening. Yet, the cellular and circuit-level mechanisms of PPI remain unclear, even under non-pathological conditions. We recently showed that brainstem neurons located in the caudal pontine reticular nucleus (PnC) expressing the glycine transporter type 2 (GlyT2±) receive inputs from the central nucleus of the amygdala (CeA) and contribute to PPI but via an uncharted pathway. Here, using tract-tracing, immunohistochemistry and in vitro optogenetic manipulations coupled to field electrophysiological recordings, we reveal the neuroanatomical distribution of GlyT2± PnC neurons and PnC-projecting CeA glutamatergic neurons and we provide mechanistic insights on how these glutamatergic inputs suppress auditory neurotransmission in PnC sections. Additionally, in vivo experiments using GlyT2-Cre mice confirm that optogenetic activation of GlyT2± PnC neurons enhances PPI and is sufficient to induce PPI in young mice, emphasizing their role. However, in older mice, PPI decline is not further influenced by inhibiting GlyT2± neurons. This study highlights the importance of GlyT2± PnC neurons in PPI and underscores their diminished activity in age-related PPI decline.

Emotional vocalizations alter behaviors and neurochemical release into the amygdala

The basolateral amygdala (BLA), a brain center of emotional expression, contributes to acoustic communication by first interpreting the meaning of social sounds in the context of the listener's internal state, then organizing the appropriate behavioral responses. We propose that modulatory neurochemicals such as acetylcholine (ACh) and dopamine (DA) provide internal-state signals to the BLA while an animal listens to social vocalizations. We tested this in a vocal playback experiment utilizing highly affective vocal sequences associated with either mating or restraint, then sampled and analyzed fluids within the BLA for a broad range of neurochemicals and observed behavioral responses of adult male and female mice. In male mice, playback of restraint vocalizations increased ACh release and usually decreased DA release, while playback of mating sequences evoked the opposite neurochemical release patterns. In non-estrus female mice, patterns of ACh and DA release with mating playback were similar to males. Estrus females, however, showed increased ACh, associated with vigilance, as well as increased DA, associated with reward-seeking. Experimental groups that showed increased ACh release also showed the largest increases in an aversive behavior. These neurochemical release patterns and several behavioral responses depended on a single prior experience with the mating and restraint behaviors. Our results support a model in which ACh and DA provide contextual information to sound analyzing BLA neurons that modulate their output to downstream brain regions controlling behavioral responses to social vocalizations.

A widespread electrical brain network encodes anxiety in health and depressive states

In rodents, anxiety is charactered by heightened vigilance during low-threat and uncertain situations. Though activity in the frontal cortex and limbic system are fundamental to supporting this internal state, the underlying network architecture that integrates activity across brain regions to encode anxiety across animals and paradigms remains unclear. Here, we utilize parallel electrical recordings in freely behaving mice, translational paradigms known to induce anxiety, and machine learning to discover a multi-region network that encodes the anxious brain-state. The network is composed of circuits widely implicated in anxiety behavior, it generalizes across many behavioral contexts that induce anxiety, and it fails to encode multiple behavioral contexts that do not. Strikingly, the activity of this network is also principally altered in two mouse models of depression. Thus, we establish a network-level process whereby the brain encodes anxiety in health and disease.

Central amygdala CRF+ neurons promote heightened threat reactivity following early life adversity in mice

Failure to appropriately predict and titrate reactivity to threat is a core feature of fear and anxiety-related disorders and is common following early life adversity (ELA). A population of neurons in the lateral central amygdala (CeAL) expressing corticotropin releasing factor (CRF) have been proposed to be key in processing threat of different intensities to mediate active fear expression. Here, we use in vivo fiber photometry to show that ELA results in sex-specific changes in the activity of CeAL CRF+ neurons, yielding divergent mechanisms underlying the augmented startle in ELA mice, a translationally relevant behavior indicative of heightened threat reactivity and hypervigilance. Further, chemogenic inhibition of CeAL CRF+ neurons selectively diminishes startle and produces a long-lasting suppression of threat reactivity. These findings identify a mechanism for sex-differences in susceptibility for anxiety following ELA and have broad implications for understanding the neural circuitry that encodes and gates the behavioral expression of fear.

Prefrontal cortex neurons encode ambient light intensity differentially across regions and layers

While light can affect emotional and cognitive processes of the medial prefrontal cortex (mPFC), no light-encoding was hitherto identified in this region. Here, extracellular recordings in awake mice revealed that over half of studied mPFC neurons showed photosensitivity, that was diminished by inhibition of intrinsically photosensitive retinal ganglion cells (ipRGCs), or of the upstream thalamic perihabenular nucleus (PHb). In 15% of mPFC photosensitive neurons, firing rate changed monotonically along light-intensity steps and gradients. These light-intensity-encoding neurons comprised four types, two enhancing and two suppressing their firing rate with increased light intensity. Similar types were identified in the PHb, where they exhibited shorter latency and increased sensitivity. Light suppressed prelimbic activity but boosted infralimbic activity, mirroring the regions' contrasting roles in fear-conditioning, drug-seeking, and anxiety. We posit that prefrontal photosensitivity represents a substrate of light-susceptible, mPFC-mediated functions, which could be ultimately studied as a therapeutical target in psychiatric and addiction disorders.

A scalable spiking amygdala model that explains fear conditioning, extinction, renewal and generalization

The amygdala (AMY) is widely implicated in fear learning and fear behaviour, but it remains unclear how the many biological components present within AMY interact to achieve these abilities. Building on previous work, we hypothesize that individual AMY nuclei represent different quantities and that fear conditioning arises from error-driven learning on the synapses between AMY nuclei. We present a computational model of AMY that (a) recreates the divisions and connections between AMY nuclei and their constituent pyramidal and inhibitory neurons; (b) accommodates scalable high-dimensional representations of external stimuli; (c) learns to associate complex stimuli with the presence (or absence) of an aversive stimulus; (d) preserves feature information when mapping inputs to salience estimates, such that these estimates generalize to similar stimuli; and (e) induces a diverse profile of neural responses within each nucleus. Our model predicts (1) defensive responses and neural activities in several experimental conditions, (2) the consequence of artificially ablating particular nuclei and (3) the tendency to generalize defensive responses to novel stimuli. We test these predictions by comparing model outputs to neural and behavioural data from animals and humans. Despite the relative simplicity of our model, we find significant overlap between simulated and empirical data, which supports our claim that the model captures many of the neural mechanisms that support fear conditioning. We conclude by comparing our model to other computational models and by characterizing the theoretical relationship between pattern separation and fear generalization in healthy versus anxious individuals.

Intra-amygdala circuits of sleep disruption-induced anxiety in female mice

Combining mouse genetics, electrophysiology, and behavioral training and testing, we explored how sleep disruption may affect the function of anxiety-controlling circuits, focusing on projections from the basolateral nucleus of the amygdala (BLA) to CRF-positive cells in the lateral division of the central amygdala (CeL). We found in Crh-IRES-Cre::Ai14(tdTomato) reporter female mice that 6 hours of sleep disruption during their non-active (light) cycle may be anxiogenic. Notably, the AMPAR/NMDAR EPSC amplitude ratio at the BLA inputs to CRF-CeL cells (CRF CeL ), assessed with whole-cell recordings in ex vivo experiments, was enhanced in slices from sleep-disrupted mice, whereas paired-pulse ratio (PPR) of the EPSCs induced by two closely spaced presynaptic stimuli remained unchanged. These findings indicate that sleep disruption-associated synaptic enhancements in glutamatergic projections from the BLA to CRF-CeL neurons may be postsynaptically expressed. We found also that the excitation/inhibition (E/I) ratio in the BLA to CRF CeL inputs was increased in sleep-disrupted mice, suggesting that the functional efficiency of excitation in BLA inputs to CRF CeL cells has increased following sleep disruption, thus resulting in their enhanced activation. The latter could be translated into enhanced anxiogenesis as activation of CRF cells in the CeL was shown to promote anxiety-like behaviors.

Distinct engrams control fear and extinction memory

Memories are stored in engram cells, which are necessary and sufficient for memory recall. Recalling a memory might undergo reconsolidation or extinction. It has been suggested that the original memory engram is reactivated during reconsolidation so that memory can be updated. Conversely, during extinction training, a new memory is formed that suppresses the original engram. Nonetheless, it is unknown whether extinction creates a new engram or modifies the original fear engram. In this study, we utilized the Daun02 procedure, which uses c-Fos-lacZ rats to induce apoptosis of strongly activated neurons and examine whether a new memory trace emerges as a result of a short or long reactivation, or if these processes rely on modifications within the original engram located in the basolateral amygdala (BLA) and infralimbic (IL) cortex. By eliminating neurons activated during consolidation and reactivation, we observed significant impacts on fear memory, highlighting the importance of the BLA engram in these processes. Although we were unable to show any impact when removing the neurons activated after the test of a previously extinguished memory in the BLA, disrupting the IL extinction engram reactivated the aversive memory that was suppressed by the extinction memory. Thus, we demonstrated that the IL cortex plays a crucial role in the network involved in extinction, and disrupting this specific node alone is sufficient to impair extinction behavior. Additionally, our findings indicate that extinction memories rely on the formation of a new memory, supporting the theory that extinction memories rely on the formation of a new memory, whereas the reconsolidation process reactivates the same original memory trace.

Leveraging interindividual variability in threat conditioning of inbred mice to model trait anxiety

Trait anxiety is a major risk factor for stress-induced and anxiety disorders in humans. However, animal models accounting for the interindividual variability in stress vulnerability are largely lacking. Moreover, the pervasive bias of using mostly male animals in preclinical studies poorly reflects the increased prevalence of psychiatric disorders in women. Using the threat imminence continuum theory, we designed and validated an auditory aversive conditioning-based pipeline in both female and male mice. We operationalised trait anxiety by harnessing the naturally occurring variability of defensive freezing responses combined with a model-based clustering strategy. While sustained freezing during prolonged retrieval sessions was identified as an anxiety-endophenotype behavioral marker in both sexes, females were consistently associated with an increased freezing response. RNA-sequencing of CeA, BLA, ACC, and BNST revealed massive differences in phasic and sustained responders' transcriptomes, correlating with transcriptomic signatures of psychiatric disorders, particularly post-traumatic stress disorder (PTSD). Moreover, we detected significant alterations in the excitation/inhibition balance of principal neurons in the lateral amygdala. These findings provide compelling evidence that trait anxiety in inbred mice can be leveraged to develop translationally relevant preclinical models to investigate mechanisms of stress susceptibility in a sex-specific manner.

Morphological heterogeneity of neurons in the human central amygdaloid nucleus

The central amygdaloid nucleus (CeA) has an ancient phylogenetic development and functions relevant for animal survival. Local cells receive intrinsic amygdaloidal information that codes emotional stimuli of fear, integrate them, and send cortical and subcortical output projections that prompt rapid visceral and social behavior responses. We aimed to describe the morphology of the neurons that compose the human CeA (N = 8 adult men). Cells within CeA coronal borders were identified using the thionine staining and were further analyzed using the "single-section" Golgi method followed by open-source software procedures for two-dimensional and three-dimensional image reconstructions. Our results evidenced varied neuronal cell body features, number and thickness of primary shafts, dendritic branching patterns, and density and shape of dendritic spines. Based on these criteria, we propose the existence of 12 morphologically different spiny neurons in the human CeA and discuss the variability in the dendritic architecture within cellular types, including likely interneurons. Some dendritic shafts were long and straight, displayed few collaterals, and had planar radiation within the coronal neuropil volume. Most of the sampled neurons showed a few to moderate density of small stubby/wide spines. Long spines (thin and mushroom) were observed occasionally. These novel data address the synaptic processing and plasticity in the human CeA. Our morphological description can be combined with further transcriptomic, immunohistochemical, and electrophysiological/connectional approaches. It serves also to investigate how neurons are altered in neurological and psychiatric disorders with hindered emotional perception, in anxiety, following atrophy in schizophrenia, and along different stages of Alzheimer's disease.

Social buffering in rats reduces fear by oxytocin triggering sustained changes in central amygdala neuronal activity

The presence of a companion can reduce fear, but the neural mechanisms underlying this social buffering of fear are incompletely known. We studied social buffering of fear in male and female, and its encoding in the amygdala of male, auditory fear-conditioned rats. Pharmacological, opto,- and/or chemogenetic interventions showed that oxytocin signaling from hypothalamus-to-central amygdala projections underlied fear reduction acutely with a companion and social buffering retention 24 h later without a companion. Single-unit recordings with optetrodes in the central amygdala revealed fear-encoding neurons (showing increased conditioned stimulus-responses after fear conditioning) inhibited by social buffering and blue light-stimulated oxytocinergic hypothalamic projections. Other central amygdala neurons showed baseline activity enhanced by blue light and companion exposure, with increased conditioned stimulus responses that persisted without the companion. Social buffering of fear thus switches the conditioned stimulus from encoding "fear" to "safety" by oxytocin-mediated recruitment of a distinct group of central amygdala "buffer neurons".

Activation of the CXCR4 Receptor by Chemokine CXCL12 Increases the Excitability of Neurons in the Rat Central Amygdala

Primarily regarded as immune proteins, chemokines are emerging as a family of molecules serving neuromodulatory functions in the developing and adult brain. Among them, CXCL12 is constitutively and widely expressed in the CNS, where it was shown to act on cellular, synaptic, network, and behavioral levels. Its receptor, CXCR4, is abundant in the amygdala, a brain structure involved in pathophysiology of anxiety disorders. Dysregulation of CXCL12/CXCR4 signaling has been implicated in anxiety-related behaviors. Here we demonstrate that exogenous CXCL12 at 2 nM but not at 5 nM increased neuronal excitability in the lateral division of the rat central amygdala (CeL) which was evident in the Late-Firing but not Regular-Spiking neurons. These effects were blocked by AMD3100, a CXCR4 antagonist. Moreover, CXCL12 increased the excitability of the neurons of the basolateral amygdala (BLA) that is known to project to the CeL. However, CXCL12 increased neither the spontaneous excitatory nor spontaneous inhibitory synaptic transmission in the CeL. In summary, the data reveal specific activation of Late-Firing CeL cells along with BLA neurons by CXCL12 and suggest that this chemokine may alter information processing by the amygdala that likely contributes to anxiety and fear conditioning.

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.