Role of the bed nucleus of the stria terminalis in aversive learning and memory

Investigating the bed nucleus of the stria terminalis as a predictor of posttraumatic stress disorder in Black Americans and the moderating effects of racial discrimination

Altered functioning of the bed nucleus of the stria terminalis (BNST) may play a critical role in the etiology of posttraumatic stress disorder (PTSD). Chronic stressors such as racial discrimination and lifetime trauma are associated with an increased risk for PTSD, but it is unknown whether they influence the relationship between BNST functioning and PTSD. We investigated acute post-trauma BNST resting-state functional connectivity (rsFC) as a predictor of future PTSD symptoms in Black trauma survivors. We also examined whether racial discrimination and lifetime trauma moderated the relationship between BNST rsFC and PTSD symptoms. Black adults (N = 95; 54.7% female; mean age = 34.04) were recruited from an emergency department after experiencing a traumatic injury (72.6% were motor vehicle accidents). Two-weeks post-injury, participants underwent a resting-state fMRI scan and completed questionnaires evaluating their PTSD symptoms as well as lifetime exposure to racial discrimination and trauma. Six-months post-injury, PTSD symptoms were reassessed. Whole brain seed-to-voxel analyses were conducted to examine BNST rsFC patterns. Greater rsFC between the BNST and the posterior cingulate cortex, precuneus, left angular gyrus, and hippocampus prospectively predicted six-month PTSD symptoms after adjusting for sex, age, education, and baseline PTSD symptoms. Acute BNST rsFC was a stronger predictor of PTSD symptoms in individuals who experienced more racial discrimination and lifetime trauma. Thus, in the acute aftermath of a traumatic event, the BNST could be a key biomarker of risk for PTSD in Black Americans, particularly for individuals with a greater history of racial discrimination or previous trauma exposure.

Social threat alters the behavioral structure of social motivation and reshapes functional brain connectivity

Traumatic social experiences redefine socially motivated behaviors to enhance safety and survival. Although many brain regions have been implicated in signaling a social threat, the mechanisms by which global neural networks regulate such motivated behaviors remain unclear. To address this issue, we first combined traditional and modern behavioral tracking techniques in mice to assess both approach and avoidance, as well as sub-second behavioral changes, during a social threat learning task. We were able to identify previously undescribed body and tail movements during social threat learning and recognition that demonstrate unique alterations into the behavioral structure of social motivation. We then utilized inter-regional correlation analysis of brain activity after a mouse recognizes a social threat to explore functional communication amongst brain regions implicated in social motivation. Broad brain activity changes were observed within the nucleus accumbens, the paraventricular thalamus, the ventromedial hypothalamus, and the nucleus of reuniens. Inter-regional correlation analysis revealed a reshaping of the functional connectivity across the brain when mice recognize a social threat. Altogether, these findings suggest that reshaping of functional brain connectivity may be necessary to alter the behavioral structure of social motivation when a social threat is encountered.

The role and mechanism of 5-HTDRN-BNST neural circuit in anxiety and fear lesions

Central 5-hydroxytryptaminergic dorsal raphe nucleus (5-HTDRN)-bed nucleus of stria terminalis (BNST) neural circuit dysfunction is one of the important neurobiological basis of anxiety and fear disorders. Under stress, 5-hydroxytryptamine (5-HT) neurons act on BNST receptors to attenuate anxiety and fear responses or enhance anxiety and fear. In BNST, corticotropin releasing factor neurons play a role in regulating emotions by reversely regulating excitatory or inhibitory 5-HT neurons. The composition of 5-HTDRN-BNST neural circuit, the pathological changes of 5-HTDRN-BNST neural circuit function damage under stress, and the effects of 5-HTDRN-BNST neural circuit on anxiety disorder, panic disorder and post-traumatic stress disorder were analyzed and are summarized in this paper. The characteristics of functional changes of the neural circuit and its effects on brain functional activities provide a basis and ideas for the treatment of anxiety and fear disorders through the regulation of 5-HTDRN-BNST neural circuit, and they also provide a new perspective for understanding the pathological mechanism of such diseases.

Sex-specific threat responding and neuronal engagement in carbon dioxide associated fear and extinction: Noradrenergic involvement in female mice

Difficulty in appropriately responding to threats is a key feature of psychiatric disorders, especially fear-related conditions such as panic disorder (PD) and posttraumatic stress disorder (PTSD). Most prior work on threat and fear regulation involves exposure to external threatful cues. However, fear can also be triggered by aversive, within-the-body, sensations. This interoceptive signaling of fear is highly relevant to PD and PTSD but is not well understood, especially in the context of sex. Using female and male mice, the current study investigated fear-associated spontaneous and conditioned behaviors to carbon dioxide (CO2) inhalation, a potent interoceptive threat that induces fear and panic. We also investigated whether behavioral sensitivity to CO2 is associated with delayed PTSD-relevant behaviors. CO2 evoked heterogenous freezing behaviors in both male and female animals. However, active, rearing behavior was significantly reduced in CO2-exposed male but not female mice. Interestingly, behavioral sensitivity to CO2 was associated with compromised fear extinction, independent of sex. However, in comparison to CO2-exposed males, females elicited less freezing and higher rearing during extinction suggesting an engagement of active versus passive defensive coping. Persistent neuronal activation marker ΔFosB immuno-mapping revealed attenuated engagement of infralimbic-prefrontal areas in both sexes but higher activation of brain stem locus coeruleus (LC) area in females. Inter-regional co-activation mapping revealed sex-independent disruptions in the infralimbic-amygdala associations but altered LC associations only in CO2-exposed female mice. Lastly, dopamine β hydroxylase positive (DβH + ve) noradrenergic neuronal cell counts in the LC correlated with freezing and rearing behaviors during CO2 inhalation and extinction only in female but not male mice. Collectively, these data provide evidence for higher active defensive responding to interoceptive threat CO2-associated fear in females that may stem from increased recruitment of the brainstem noradrenergic system. Our findings reveal distinct contributory mechanisms that may promote sex differences in fear and panic associated pathologies.

Data-driven, connectome-wide analysis identifies psychosis-specific brain correlates of fear and anxiety

Decades of psychosis research highlight the prevalence and the clinical significance of negative emotions, such as fear and anxiety. Translational evidence demonstrates the pivotal role of the amygdala in fear and anxiety. However, most of these approaches have used hypothesis-driven analyses with predefined regions of interest. A data-driven analysis may provide a complimentary, unbiased approach to identifying brain correlates of fear and anxiety. The aim of the current study was to identify the brain basis of fear and anxiety in early psychosis and controls using a data-driven approach. We analyzed data from the Human Connectome Project for Early Psychosis, a multi-site study of 125 people with psychosis and 58 controls with resting-state fMRI and clinical characterization. Multivariate pattern analysis of whole-connectome data was used to identify shared and psychosis-specific brain correlates of fear and anxiety using the NIH Toolbox Fear-Affect and Fear-Somatic Arousal scales. We then examined clinical correlations of Fear-Affect scores and connectivity patterns. Individuals with psychosis had higher levels of Fear-Affect scores than controls (p < 0.05). The data-driven analysis identified a cluster encompassing the amygdala and hippocampus where connectivity was correlated with Fear-Affect score (p < 0.005) in the entire sample. The strongest correlate of Fear-Affect was between this cluster and the anterior insula and stronger connectivity was associated with higher Fear-Affect scores (r = 0.31, p = 0.0003). The multivariate pattern analysis also identified a psychosis-specific correlate of Fear-Affect score between the amygdala/hippocampus cluster and a cluster in the ventromedial prefrontal cortex (VMPFC). Higher Fear-Affect scores were correlated with stronger amygdala/hippocampal-VMPFC connectivity in the early psychosis group (r = 0.33, p = 0.002), but not in controls (r = -0.15, p = 0.28). The current study provides evidence for the transdiagnostic role of the amygdala, hippocampus, and anterior insula in the neural basis of fear and anxiety and suggests a psychosis-specific relationship between fear and anxiety symptoms and amygdala/hippocampal-VMPFC connectivity. Our novel data-driven approach identifies novel, psychosis-specific treatment targets for fear and anxiety symptoms and provides complimentary evidence to decades of hypothesis-driven approaches examining the brain basis of threat processing.

Threat Responses in Schizophrenia: A Negative Valence Systems Framework

PURPOSE OF REVIEW

Emotions are prominent in theories and accounts of schizophrenia but are largely understudied compared to cognition. Utilizing the Research Domain Criteria (RDoC) Negative Valence Systems framework, we review the current knowledge of emotions in schizophrenia. Given the pivotal role of threat responses in theories of schizophrenia and the substantial evidence of altered threat responses, we focus on three components of Negative Valence Systems tied to threat responses: responses to acute threat, responses to potential threat, and sustained threat.

RECENT FINDINGS

Individuals with schizophrenia show altered responses to neutral stimuli during acute threat, bed nucleus of the stria terminalis connectivity in response to potential threat, and threat responses associated with sustained threat. Our review concludes that Negative Valence Systems are altered in schizophrenia; however, the level and evidence of alterations vary across the types of threat responses. We suggest avenues for future research to further understand and intervene on threat responses in schizophrenia.

Brain-wide neuron quantification toolkit reveals strong sexual dimorphism in the evolution of fear memory

Fear responses are functionally adaptive behaviors that are strengthened as memories. Indeed, detailed knowledge of the neural circuitry modulating fear memory could be the turning point for the comprehension of this emotion and its pathological states. A comprehensive understanding of the circuits mediating memory encoding, consolidation, and retrieval presents the fundamental technological challenge of analyzing activity in the entire brain with single-neuron resolution. In this context, we develop the brain-wide neuron quantification toolkit (BRANT) for mapping whole-brain neuronal activation at micron-scale resolution, combining tissue clearing, high-resolution light-sheet microscopy, and automated image analysis. The robustness and scalability of this method allow us to quantify the evolution of activity patterns across multiple phases of memory in mice. This approach highlights a strong sexual dimorphism in recruited circuits, which has no counterpart in the behavior. The methodology presented here paves the way for a comprehensive characterization of the evolution of fear memory.

Somatostatin-Expressing Neurons in the Ventral Tegmental Area Innervate Specific Forebrain Regions and Are Involved in Stress Response

Expanding knowledge about the cellular composition of subcortical brain regions demonstrates large heterogeneity and differences from the cortical architecture. Previously we described three subtypes of somatostatin-expressing (Sst) neurons in the mouse ventral tegmental area (VTA) and showed their local inhibitory action on the neighboring dopaminergic neurons (Nagaeva et al., 2020). Here, we report that Sst+ neurons especially from the anterolateral part of the mouse VTA also project far outside the VTA and innervate forebrain regions that are mainly involved in the regulation of emotional behavior, including the ventral pallidum, lateral hypothalamus, the medial part of the central amygdala, anterolateral division of the bed nucleus of stria terminalis, and paraventricular thalamic nucleus. Deletion of these VTASst neurons in mice affected several behaviors, such as home cage activity, sensitization of locomotor activity to morphine, fear conditioning responses, and reactions to the inescapable stress of forced swimming, often in a sex-dependent manner. Together, these data demonstrate that VTASst neurons have selective projection targets distinct from the main targets of VTA dopamine neurons. VTASst neurons are involved in the regulation of behaviors primarily associated with the stress response, making them a relevant addition to the efferent VTA pathways and stress-related neuronal network.

Astrocyte expression in the extended amygdala of C57BL/6J mice is sex-dependently affected by chronic intermittent and binge-like ethanol exposure

Excessive ethanol drinking is a major problem within the United States, causing alterations in brain plasticity and neurocognitive function. Astrocytes are glial cells that regulate neurosynaptic plasticity, modulate neurochemicals, and monitor other homeostatic roles. Astrocytes have been found to play a part in modulating excessive ethanol consumption. The basolateral amygdala (BLA), central amygdala (CeA), and bed nucleus of the stria terminalis (BNST) are brain regions that process stress, anxiety, and reward; they are also implicated in modulating ethanol intake. Little is understood, however, about how astrocyte expression in each region is modulated by chronic and binge-like ethanol drinking patterns. In the present report, we utilized two separate animal models of excessive drinking: chronic intermittent ethanol (CIE) and "Drinking-in-the-dark" (DID). Following these paradigms, animal brains were processed through immunohistochemistry (IHC) and stained for glial fibrillary acidic protein (GFAP). Collected data illustrated a sex-dependent relationship between ethanol intake and GFAP immunoreactivity (IR) in the BLA and BNST, but not in the CeA. Specifically, CIE and DID ethanol drinking resulted in blunted GFAP-IR (specifically via GFAP-positive cell count) in the BLA and BNST, particularly in males. These findings may implicate sex-dependent ethanol-induced changes in BLA and BNST astrocytes, providing a potential therapeutic target for anxiety and stress disorders.

Nos1+ and Nos1- excitatory neurons in the BLA regulate anxiety- and depression-related behaviors oppositely

BACKGROUND

The basolateral amygdala (BLA) neurons are primarily glutamatergic and have been associated with emotion regulation. However, little is known about the roles of BLA neurons expressing neuronal nitric oxide synthase (nNOS, Nos1) in the regulation of emotional behaviors.

METHODS

Using Nos1-cre mice and chemogenetic and optogenetic manipulations, we specifically silenced or activated Nos1⁺ or Nos1^- neurons in the BLA, or silenced their projections to the anterdorsal bed nucleus of the stria terminalis (adBNST) and ventral hippocampus (vHPC). We measured anxiety behaviors in elevated plus maze (EPM) and open-field test (OFT), and measured depression behaviors in forced swimming test (FST) and tail suspension test (TST).

RESULTS

BLA Nos1⁺ neurons were predominantly glutamatergic, and glutamatergic but not GABAergic Nos1⁺ neurons were involved in controlling anxiety- and depression-related behaviors. Interestingly, by selectively manipulating the activities of BLA Nos1⁺ and Nos1^- excitatory neurons, we found that they had opposing effects on anxiety- and depression-related behaviors. BLA Nos1⁺ excitatory neurons projected to the adBNST, this BLA-adBNST circuit controlled the expression of anxiety- and depression-related behaviors, while BLA Nos1^- excitatory neurons projected to vHPC, this BLA-vHPC circuit contributed to the expression of anxiety- and depression-related behaviors. Moreover, excitatory vHPC-adBNST circuit antagonized the role of BLA-adBNST circuit in regulating anxiety- and depression-related behaviors.

CONCLUSIONS

BLA Nos1⁺ and Nos1^- excitatory neuron subpopulations exert different effects on anxiety- and depression-related behaviors through distinct projection circuits, providing a new insight of BLA excitatory neurons in emotional regulation.

LIMITATIONS

We did not perform retrograde labeling from adBNST and vHPC regions.

Lasting impact of postnatal maternal separation on the developing BNST: Lifelong socioemotional consequences

Nearly one percent of children in the US experience childhood neglect or abuse, which can incite lifelong emotional and behavioral disorders. Many studies investigating the neural underpinnings of maleffects inflicted by early life stress have largely focused on dysregulation of the hypothalamic-pituitary-adrenal (HPA) axis. Newer veins of evidence suggest that exposure to early life stressors can interrupt neural development in extrahypothalamic areas as well, including the bed nucleus of the stria terminalis (BNST). One widely used approach in this area is rodent maternal separation (MS), which typically consists of separating pups from the dam for extended periods of time, over several days during the first weeks of postnatal life - a time when pups are highly dependent on maternal care for survival. MS has been shown to incite myriad lasting effects not limited to increased anxiety-like behavior, hyper-responsiveness to stressors, and social behavior deficits. The behavioral effects of MS are widespread and thus unlikely to be limited to hypothalamic mechanisms. Recent work has highlighted the BNST as a critical arbiter of some of the consequences of MS, especially socioemotional behavioral deficits. The BNST is a well-documented modulator of anxiety, reward, and social behavior by way of its connections with hypothalamic and extra-hypothalamic systems. Moreover, during the postnatal period when MS is typically administered, the BNST undergoes critical neural developmental events. This review highlights evidence that MS interferes with neural development to permanently alter BNST circuitry, which may account for a variety of behavioral deficits seen following early life stress. This article is part of the Special Issue on 'Fear, Anxiety and PTSD'.

Social Synchronization of Conditioned Fear in Mice Requires Ventral Hippocampus Input to the Amygdala

BACKGROUND

Social organisms synchronize behaviors as an evolutionary-conserved means of thriving. Synchronization under threat, in particular, benefits survival and occurs across species, including humans, but the underlying mechanisms remain unknown because of the scarcity of relevant animal models. Here, we developed a rodent paradigm in which mice synchronized a classically conditioned fear response and identified an underlying neuronal circuit.

METHODS

Male and female mice were trained individually using auditory fear conditioning and then tested 24 hours later as dyads while allowing unrestricted social interaction during exposure to the conditioned stimulus under visible or infrared illumination to eliminate visual cues. The synchronization of the immobility or freezing bouts was quantified by calculating the effect size Cohen's d for the difference between the actual freezing time overlap and the overlap by chance. The inactivation of the dorsomedial prefrontal cortex, dorsal hippocampus, or ventral hippocampus was achieved by local infusions of muscimol. The chemogenetic disconnection of the hippocampus-amygdala pathway was performed by expressing hM4D(Gi) in the ventral hippocampal neurons and infusing clozapine N-oxide in the amygdala.

RESULTS

Mice synchronized cued but not contextual fear. It was higher in males than in females and attenuated in the absence of visible light. Inactivation of the ventral but not dorsal hippocampus or dorsomedial prefrontal cortex abolished fear synchronization. Finally, the disconnection of the hippocampus-amygdala pathway diminished fear synchronization.

CONCLUSIONS

Mice synchronize expression of conditioned fear relying on the ventral hippocampus-amygdala pathway, suggesting that the hippocampus transmits social information to the amygdala to synchronize threat response.

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.

The bed nucleus of the stria terminalis in threat detection: task choice and rodent experience

Behavioural reactivity to potential threat is used to experimentally refine models of anxiety symptoms in rodents. We present a short review of the literature tying the most commonly used tasks to model anxiety symptoms to functional recruitment of bed nucleus of the stria terminalis circuits (BNST). Using a review of studies that investigated the role of the BNST in anxiety-like behaviour in rodents, we flag the certain challenges for the field. These stem from inconsistent methods of reporting the neuroanatomical BNST subregions and the interpretations of specific behaviour across a wide variety of tasks as 'anxiety-like'. Finally, to assist in interpretation of the findings, we discuss the potential interactions between typically used 'anxiety' tasks of innate behaviour that are potentially modulated by the social and individual experience of the animal.

Hyperexcitability: From Normal Fear to Pathological Anxiety and Trauma

Hyperexcitability in fear circuits is suggested to be important for development of pathological anxiety and trauma from adaptive mechanisms of fear. Hyperexcitability is proposed to be due to acquired sensitization in fear circuits that progressively becomes more severe over time causing changing symptoms in early and late pathology. We use the metaphor and mechanisms of kindling to examine gains and losses in function of one excitatory and one inhibitory neuropeptide, corticotrophin releasing factor and somatostatin, respectively, to explore this sensitization hypothesis. We suggest amygdala kindling induced hyperexcitability, hyper-inhibition and loss of inhibition provide clues to mechanisms for hyperexcitability and progressive changes in function initiated by stress and trauma.

Chronic stress-induced synaptic changes to corticotropin-releasing factor-signaling in the bed nucleus of the stria terminalis

The sexually dimorphic bed nucleus of the stria terminalis (BNST) is comprised of several distinct regions, some of which act as a hub for stress-induced changes in neural circuitry and behavior. In rodents, the anterodorsal BNST is especially affected by chronic exposure to stress, which results in alterations to the corticotropin-releasing factor (CRF)-signaling pathway, including CRF receptors and upstream regulators. Stress increases cellular excitability in BNST CRF+ neurons by potentiating miniature excitatory postsynaptic current (mEPSC) amplitude, altering the resting membrane potential, and diminishing M-currents (a voltage-gated K+ current that stabilizes membrane potential). Rodent anterodorsal and anterolateral BNST neurons are also critical regulators of behavior, including avoidance of aversive contexts and fear learning (especially that of sustained threats). These rodent behaviors are historically associated with anxiety. Furthermore, BNST is implicated in stress-related mood disorders, including anxiety and Post-Traumatic Stress Disorders in humans, and may be linked to sex differences found in mood disorders.

Divergent Neural and Endocrine Responses in Wild-Caught and Laboratory-Bred Rattus Norvegicus

Although rodents have represented the most intensely studied animals in neurobiological investigations for more than a century, few studies have systematically compared neural and endocrine differences between wild rodents in their natural habitats and laboratory strains raised in traditional laboratory environments. In the current study, male and female Rattus norvegicus rats were trapped in an urban setting and compared to weight-and sex-matched conspecifics living in standard laboratory housing conditions. Brains were extracted for neural assessments and fecal boli were collected for endocrine [corticosterone and dehydroepiandrosterone (DHEA)] assays. Additionally, given their role in immune and stress functions, spleen and adrenal weights were recorded. A separate set of wild rats was trapped at a dairy farm and held in captivity for one month prior to assessments; in these animals, brains were processed but no hormone data were available. The results indicated that wild-trapped rats exhibited 31% heavier brains, including higher densities of cerebellar neurons and glial cells in the bed nucleus of the stria terminalis. The wild rats also had approximately 300% greater spleen and adrenal weights, and more than a six-fold increase in corticosterone levels than observed in laboratory rats. Further research on neurobiological variables in wild vs. lab animals will inform the extensive neurobiological knowledge base derived from laboratory investigations using selectively bred rodents in laboratory environments, knowledge that will enhance the translational value of preclinical laboratory rodent studies.

Involvement of nitric oxide in the neurobiology of fear-like behavior

Fear is an emotional reaction that arises in dangerous situations, inducing the adaptation to an existing condition. This behavior was conserved in all vertebrates throughout evolution and is observed in mammals, birds, fish, amphibians, and reptiles. The neurocircuitry of fear involves areas of the limbic system, cortical regions, midbrain, and brainstem. These areas communicate with each other so that there is an expression of fear and memory formation to deal with the same situation at another time. The effect of nitric oxide (NO) on fear modulation has been explored. NO is a gaseous compound that easily diffuses through the cell membrane and is produced through the oxidation reaction of l-Arginine to l-citrulline catalyzed by nitric oxide synthase (NOS). Activating the intracellular NO receptor (soluble guanylyl cyclase enzyme - sGC) triggers an enzymatic cascade that can culminate in plastic events in the neuron. NOS inhibitors induce anxiolytic-like responses in fear modulation, whereas NO donors promote fear- and anxiety-like behaviors. This review describes the neurobiology of fear in mammals and non-mammals, how NO is produced in the central nervous system, and how NO acts in fear-like behavior.

The Anterior Piriform Cortex and Predator Odor Responses: Modulation by Inhibitory Circuits

Rodents acquire more information from the sense of smell than humans because they have a nearly fourfold greater variety of olfactory receptors. They use olfactory information not only for obtaining food, but also for detecting environmental dangers. Predator-derived odor compounds provoke instinctive fear and stress reactions in animals. Inbred lines of experimental animals react in an innate stereotypical manner to predators even without prior exposure. Predator odors have also been used in models of various neuropsychiatric disorders, including post-traumatic stress disorder following a life-threatening event. Although several brain regions have been reported to be involved in predator odor-induced stress responses, in this mini review, we focus on the functional role of inhibitory neural circuits, especially in the anterior piriform cortex (APC). We also discuss the changes in these neural circuits following innate reactions to odor exposure. Furthermore, based on the three types of modulation of the stress response observed by our group using the synthetic fox odorant 2,5-dihydro-2,4,5-trimethylthiazoline, we describe how the APC interacts with other brain regions to regulate the stress response. Finally, we discuss the potential therapeutic application of odors in the treatment of stress-related disorders. A clearer understanding of the odor–stress response is needed to allow targeted modulation of the monoaminergic system and of the intracerebral inhibitory networks. It would be improved the quality of life of those who have stress-related conditions.

Activation of Lateral Parabrachial Nucleus (LPBn) PACAP-Expressing Projection Neurons to the Bed Nucleus of the Stria Terminalis (BNST) Enhances Anxiety-like Behavior

Anxiety disorders are among the most common psychiatric disorders, and understanding the underlying neurocircuitry of anxiety- and stress-related behaviors may be important for treatment. The bed nucleus of the stria terminalis (BNST) has been studied for its role in many stress-related pathologies, such as anxiety, pain, depression, and addiction. Our prior work has demonstrated that pituitary adenylate cyclase-activating polypeptide (PACAP) receptor activation in the BNST mediates many of the behavioral consequences of chronic stress. While the BNST contains local PACAP-expressing neurons, a major source of afferent PACAP is the lateral parabrachial nucleus (LPBn), and excitotoxic lesions of the LPBn substantially decreasess PACAP immunostaining in the BNST. Here, we first assessed Cre-dependent reporter expression by injecting AAV2-hSyn-DIO-mCherry into the LPBn of PACAP-IRES-Cre mice for circuit mapping studies and identified PACAP projections to the BNST, lateral capsular central nucleus of the amygdala (CeLC), and ventromedial hypothalamus (VMH). In a second study, we assessed the effects of chemogenetically activating LPBn PACAP afferents in the BNST by injecting AAV2-hSyn-DIO-hM3D(Gq)-mCherry into the LPBn of PACAP-IRES-Cre mice for Cre-dependent expression of excitatory designer receptors exclusively activated by designer drugs (DREADDs). Before behavioral testing, clozapine-N-oxide (CNO), the selective agonist of our DREADD, was infused directly into the BNST. We found that after specific activation of LPBn PACAP afferents in the BNST, mice had increased anxiety-like behavior compared with controls, while total locomotor activity was unaffected. These results indicate that activation of PACAPergic LPBn projections to the BNST may play an important role in producing anxiety-like behavior.

Threat imminence reveals links among unfolding of anticipatory physiological response, cortical-subcortical intrinsic functional connectivity, and anxiety

Excessive expression of fear responses in anticipation of threat occurs in anxiety, but understanding of underlying pathophysiological mechanisms is limited. Animal research indicates that threat-anticipatory defensive responses are dynamically organized by threat imminence and rely on conserved circuitry. Insight from basic neuroscience research in animals on threat imminence could guide mechanistic research in humans mapping abnormal function in this circuitry to aberrant defensive responses in pathological anxiety. 50 pediatric anxiety patients and healthy-comparisons (33 females) completed an instructed threat-anticipation task whereby cues signaled delivery of painful (threat) or non-painful (safety) thermal stimulation. Temporal changes in skin-conductance indexed anxiety effects on anticipatory responding as function of threat imminence. Multivariate network analyses of resting-state functional connectivity data from a subsample were used to identify intrinsic-function correlates of anticipatory-response dynamics, within a specific, distributed network derived from translational research on defensive responding. By considering threat imminence, analyses revealed specific anxiety effects. Importantly, pathological anxiety was associated with excessive deployment of anticipatory physiological response as threat, but not safety, outcomes became more imminent. Magnitude of increase in threat-anticipatory physiological responses corresponded with magnitude of intrinsic connectivity within a cortical-subcortical circuit. Moreover, more severe anxiety was associated with stronger associations between anticipatory physiological responding and connectivity that ventromedial prefrontal cortex showed with hippocampus and basolateral amygdala, regions implicated in animal models of anxiety. These findings link basic and clinical research, highlighting variations in intrinsic function in conserved defensive circuitry as a potential pathophysiological mechanism in anxiety.

Event-based control of autonomic and emotional states by the anterior cingulate cortex

Despite being an intensive area of research, the function of the anterior cingulate cortex (ACC) remains somewhat of a mystery. Human imaging studies implicate the ACC in various cognitive functions, yet surgical ACC lesions used to treat emotional disorders have minimal lasting effects on cognition. An alternative view is that ACC regulates autonomic states, consistent with its interconnectivity with autonomic control regions and that stimulation evokes changes in autonomic/emotional states. At the cellular level, ACC neurons are highly multi-modal and promiscuous, and can represent a staggering array of task events. These neurons nevertheless combine to produce highly event-specific ensemble patterns that likely alter activity in downstream regions controlling emotional and autonomic tone. Since neuromodulators regulate the strength of the ensemble activity patterns, they would regulate the impact these patterns have on downstream targets. Through these mechanisms, the ACC may determine how strongly to react to the very events its ensembles represent. Pathologies arise when specific event-related representations gain excessive control over autonomic/emotional states.

The maintained attention assessment in patients affected by Myalgic encephalomyelitis/chronic fatigue syndrome: a reliable biomarker?

The maintained attention is the cause of great functional limitations in CFS/ME, a disease that mainly affects women in the central period of life. Cognitive function is explored using the Montreal Cognitive Assessment, the maintained attention using the Toulouse-Piéron test with which the Global Index of Attention and Perception (GIAP) is obtained, the fatigue using the visual analog scale and the perception of effort using the modified Borg scale. The final sample were 84 patients (66 women/18 men) who met diagnostic criteria (Fukuda-1994, Carruthers-2011) and 22 healthy controls (14 women/8 men). Most of patients maintain normal cognitive function, showing low or very low attention score in the 70% of patients with a marked cognitive fatigue compared to the control group (p < 0.05). There were no significant differences between genders in GIAP or fatigue for CFS/ME; however, sick women perceive cognitive effort higher than men. Deficits in sustained attention and the perception of fatigue, so effort after performing the proposed test are a sensitive and reliable indicator that allows us to substantiate a clinical suspicion and refer patients for further studies in order to confirm or rule out CFS/ME.

Bed nucleus of the stria terminalis and amygdala responses to unpredictable threat in children

Substantial evidence from studies in humans suggests the amygdala is pivotal for anxiety. Findings from animal models and translational studies suggests the bed nucleus of the stria terminalis (BNST) is also critical for anxiety and the anticipation of unpredictable threat in adults. However, it remains unknown whether the BNST is involved in unpredictable threat anticipation in children. Forty-two 8-10-year-olds completed resting-state functional magnetic resonance imaging (fMRI) scans and an unpredictable threat fMRI task in which they were trained to associate cues with images. Intrinsic connectivity analyses were performed to establish functional BNST and amygdala networks. BNST and amygdala activation to cues and images was tested. Significant findings were followed by task-based functional connectivity analyses. Children showed evidence for BNST and amygdala intrinsic connectivity that was similar to previous patterns observed in adults. In response to unpredictable cues relative to neutral face cues, children had a significant amygdala response but no response in the BNST. The amygdala, but not the BNST, also showed a significantly greater response to fear face images relative to neutral images. Thus, unpredictable threat activated the amygdala, but not BNST, in children. This finding is contrary to studies showing robust BNST activation to unpredictable threat in adults and may suggest that the BNST's role in threat processing emerges later in development.

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

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

PACAP orchestration of stress-related responses in neural circuits

Pituitary adenylate cyclase activating polypeptide (PACAP) is a pleiotropic polypeptide that can activate G protein-coupled PAC1, VPAC1, and VPAC2 receptors, and has been implicated in stress signaling. PACAP and its receptors are widely distributed throughout the nervous system and other tissues and can have a multitude of effects. Human and animal studies suggest that PACAP plays a role responding to a variety of threats and stressors. Here we review the roles of PACAP in several regions of the central nervous system (CNS) as they relate to several behavioral functions. For example, in the bed nucleus of the stria terminalis (BNST), PACAP is upregulated following chronic stress and may drive anxiety-like behavior. PACAP can also influence both the consolidation and expression of fear memories, as demonstrated by studies in several fear-related areas, such as the amygdala, hippocampus, and prefrontal cortex. PACAP can also mediate the emotional component of pain, as PACAP in the central nucleus of the amygdala (CeA) is able to decrease pain sensitivity thresholds. Outside of the central nervous system, PACAP may drive glucocorticoid release via enhanced hypothalamic-pituitary-adrenal axis activity and may participate in infection-induced stress responses. Together, this suggests that PACAP exerts effects on many stress-related systems and may be an important driver of emotional behavior.

BNST and amygdala connectivity are altered during threat anticipation in schizophrenia

In schizophrenia, impairments in affect are prominent and anxiety disorders are prevalent. Neuroimaging studies of fear and anxiety in schizophrenia have focused on the amygdala and show alterations in connectivity. Emerging evidence suggests that the bed nucleus of the stria terminalis (BNST) also plays a critical role in anxiety, especially during anticipation of an unpredictable threat; however, previous studies have not examined the BNST in schizophrenia. In the present study, we examined BNST function and connectivity in people with schizophrenia (n = 31; n = 15 with comorbid anxiety) and controls (n = 15) during anticipation of unpredictable and predictable threat. A secondary analysis tested for differences in activation and connectivity of the central nucleus of the amygdala (CeA), which has also been implicated in threat anticipation. Analyses tested for group differences in both activation and connectivity during anticipation of unpredictable threat and predictable threat (p < .05). Relative to controls, individuals with schizophrenia showed stronger BNST-middle temporal gyrus (MTG) connectivity during unpredictable threat anticipation and stronger BNST-MTG and BNST-dorsolateral prefrontal connectivity during predictable threat anticipation. Comparing subgroups of individuals with schizophrenia and a comorbid anxiety disorder (SZ+ANX) to those without an anxiety disorder (SZ-ANX) revealed broader patterns of altered connectivity. During unpredictable threat anticipation, the SZ+ANX group had stronger BNST connectivity with regions of the salience network (insula, dorsal anterior cingulate cortex). During predictable threat anticipation, the SZ+ANX group had stronger BNST connectivity with regions associated with fear processing (insula, extended amygdala, prefrontal cortex). A secondary CeA analysis revealed a different pattern; the SZ+ANX group had weaker CeA connectivity across multiple brain regions during threat anticipation compared to the SZ-ANX group. These findings provide novel evidence for altered functional connectivity during threat anticipation in schizophrenia, especially in individuals with comorbid anxiety.

Mapping excessive "disgust" in the brain: Ventral pallidum inactivation recruits distributed circuitry to make sweetness "disgusting"

The ventral pallidum (VP) is an important structure in processing reward. The VP may be the only brain structure where localized lesions in rats replace normal facial "liking" expressions to sweetness with excessive "disgust" reactions, such as gapes and chin rubs, that are normally reserved for unpalatable tastes. The posterior half of the VP (pVP) contains a hedonic hot spot where opioid or related neurochemical stimulations can amplify positive "liking" reactions to sweet taste. This is the same site where lesions or pharmacological inactivations replace positive hedonic reactions to sucrose with intense negative "disgust." In the present study, we aimed to identify brain networks recruited by pVP inactivation to generate excessive "disgust," using neuronal Fos expression as a marker of neurobiological activation. Microinjections in pVP of inhibitory GABA_A/B agonists (muscimol and baclofen) caused rats to exhibit excessive "disgust" reactions to sucrose. Excessive "disgust" was accompanied by recruitment of neural Fos activation in several subcortical structures, including the posterior medial shell of nucleus accumbens (which also contains another GABAergic "disgust"-inducing "hedonic cold spot"), the bed nucleus of stria terminalis, lateral habenula, hypothalamus, and midbrain ventral tegmentum. Fos suppression was found in cortical limbic regions, including previously identified hedonic hot spots in the anteromedial orbitofrontal cortex and posterior insula. Finally, in addition to inducing excessive "disgust," pVP inactivation abolished motivational "wanting" to eat palatable food, reduced positive social interactions, and reordered sensorimotor relations. Our findings identify potential "disgust" generators in the brain that are released into excitation by pVP inhibition and may serve as targets for future research.

From Brain Organoids to Networking Assembloids: Implications for Neuroendocrinology and Stress Medicine

Brain organoids are three-dimensional cultures that contain multiple types of cells and cytoarchitectures, and resemble fetal human brain structurally and functionally. These organoids are being used increasingly to model brain development and disorders, however, they only partially recapitulate such processes, because of several limitations, including inability to mimic the distinct cortical layers, lack of functional neuronal circuitry as well as non-neural cells and gyrification, and increased cellular stress. Efforts to create improved brain organoid culture systems have led to region-specific organoids, vascularized organoids, glia-containing organoids, assembloids, sliced organoids and polarized organoids. Assembloids are fused region-specific organoids, which attempt to recapitulate inter-regional and inter-cellular interactions as well as neural circuitry development by combining multiple brain regions and/or cell lineages. As a result, assembloids can be used to model subtle functional aberrations that reflect complex neurodevelopmental, neuropsychiatric and neurodegenerative disorders. Mammalian organisms possess a highly complex neuroendocrine system, the stress system, whose main task is the preservation of systemic homeostasis, when the latter is threatened by adverse forces, the stressors. The main central parts of the stress system are the paraventricular nucleus of the hypothalamus and the locus caeruleus/norepinephrine-autonomic nervous system nuclei in the brainstem; these centers innervate each other and interact reciprocally as well as with various other CNS structures. Chronic dysregulation of the stress system has been implicated in major pathologies, the so-called chronic non-communicable diseases, including neuropsychiatric, neurodegenerative, cardiometabolic and autoimmune disorders, which lead to significant population morbidity and mortality. We speculate that brain organoids and/or assembloids could be used to model the development, regulation and dysregulation of the stress system and to better understand stress-related disorders. Novel brain organoid technologies, combined with high-throughput single-cell omics and gene editing, could, thus, have major implications for precision medicine.

Npas4a expression in the teleost forebrain is associated with stress coping style differences in fear learning

Learning to anticipate potentially dangerous contexts is an adaptive behavioral response to coping with stressors. An animal's stress coping style (e.g. proactive-reactive axis) is known to influence how it encodes salient events. However, the neural and molecular mechanisms underlying these stress coping style differences in learning are unknown. Further, while a number of neuroplasticity-related genes have been associated with alternative stress coping styles, it is unclear if these genes may bias the development of conditioned behavioral responses to stressful stimuli, and if so, which brain regions are involved. Here, we trained adult zebrafish to associate a naturally aversive olfactory cue with a given context. Next, we investigated if expression of two neural plasticity and neurotransmission-related genes (npas4a and gabbr1a) were associated with the contextual fear conditioning differences between proactive and reactive stress coping styles. Reactive zebrafish developed a stronger conditioned fear response and showed significantly higher npas4a expression in the medial and lateral zones of the dorsal telencephalon (Dm, Dl), and the supracommissural nucleus of the ventral telencephalon (Vs). Our findings suggest that the expression of activity-dependent genes like npas4a may be differentially expressed across several interconnected forebrain regions in response to fearful stimuli and promote biases in fear learning among different stress coping styles.

Endocannabinoids, cannabinoids and the regulation of anxiety

Cannabis has been used for hundreds of years, with its ability to dampen feelings of anxiety often reported as a primary reason for use. Only recently has the specific role cannabinoids play in anxiety been thoroughly investigated. Here we discuss the body of evidence describing how endocannabinoids and exogenous cannabinoids are capable of regulating the generation and termination of anxiety states. Disruption of the endogenous cannabinoid (eCB) system following genetic manipulation, pharmacological intervention or stress exposure reliably leads to the generation of an anxiety state. On the other hand, upregulation of eCB signaling is capable of alleviating anxiety-like behaviors in multiple paradigms. When considering exogenous cannabinoid administration, cannabinoid receptor 1 (CB1) agonists have a biphasic, dose-dependent effect on anxiety such that low doses are anxiolytic while high doses are anxiogenic, a phenomenon that is evident in both rodent models and humans. Translational studies investigating a loss of function mutation in the gene for fatty acid amide hydrolase, the enzyme responsible for metabolizing AEA, have also shown that AEA signaling regulates anxiety in humans. Taken together, evidence reviewed here has outlined a convincing argument for cannabinoids being powerful regulators of both the manifestation and amelioration of anxiety symptoms, and highlights the therapeutic potential of targeting the eCB system for the development of novel classes of anxiolytics. This article is part of the special issue on 'Cannabinoids'.

Cell-type diversity in the bed nucleus of the stria terminalis to regulate motivated behaviors

Over the past few decades, the bed nucleus of the stria terminalis (BNST) gained popularity as a unique brain region involved in regulating motivated behaviors related to neuropsychiatric disorders. The BNST, a component of the extended amygdala, consists of a variety of subnuclei and neuronal ensembles. Multiple studies have highlighted the BNST as playing a fundamental role in integrating information by interfacing with other brain regions to regulate distinct aspects of motivated behaviors associated with stress, anxiety, depression, and decision-making. However, due to the high molecular heterogeneity found within BNST neurons, the precise mechanisms by which this region regulates distinct motivational states remains largely unclear. Single-cell RNA sequencing data have revealed that the BNST consists of multiple genetically identifiable cell-type clusters. Contemporary tools can therefore be leveraged to target and study such cell-types and elucidate their precise functional role. In this review, we discuss the different subsets of neurons found in the BNST, their anatomical distribution, and what is currently known about BNST cell-types in regulating motivated behaviors.

Imaging of Functional Brain Circuits during Acquisition and Memory Retrieval in an Aversive Feedback Learning Task: Single Photon Emission Computed Tomography of Regional Cerebral Blood Flow in Freely Behaving Rats

Active avoidance learning is a complex form of aversive feedback learning that in humans and other animals is essential for actively coping with unpleasant, aversive, or dangerous situations. Since the functional circuits involved in two-way avoidance (TWA) learning have not yet been entirely identified, the aim of this study was to obtain an overall picture of the brain circuits that are involved in active avoidance learning. In order to obtain a longitudinal assessment of activation patterns in the brain of freely behaving rats during different stages of learning, we applied single-photon emission computed tomography (SPECT). We were able to identify distinct prefrontal cortical, sensory, and limbic circuits that were specifically recruited during the acquisition and retrieval phases of the two-way avoidance learning task.

Early-life sevoflurane exposure impairs fear memory by suppressing extracellular signal-regulated kinase signaling in the bed nucleus of stria terminalis GABAergic neurons

Sevoflurane exposure in neonates induces long-term impairment of learning and memory; however, its effect on cognition in the later developmental period and the underlying mechanisms remain unclear. In the present study, we showed that multiple sevoflurane exposures impaired fear memory at long retention delays in neonatal (postnatal day 7) and preadolescent mice (postnatal day 22), but not in mice at older ages. After the fear memory test, expression of phosphorylated extracellular signaling-regulated kinase (p-ERK) and c-fos were elevated in the bed nucleus of the stria terminalis (BNST) and central amygdala, but not in the hippocampus or prefrontal cortex. The upregulation of p-ERK was restricted to populations of γ-aminobutyric acid (GABAergic) neurons and was inhibited by multiple sevoflurane exposures. Intra-BNST injection of ERK inhibitor also impaired fear memory at long retention delays. In contrast, intra-BNST injection of ERK agonist attenuated impaired fear memory caused by repeated sevoflurane exposures. Injection of sevoflurane in the BNST but not the caudate putamen impaired the fear memory at long retention delays in preadolescent mice. Finally, chemogenetic activation of BNST GABAergic neurons by designer receptors exclusively activated by designer drug (DREADD) reversed the impaired fear memory at long retention delays by multiple sevoflurane exposures. These findings suggest that multiple sevoflurane exposures impaired fear memory at long retention delays in preadolescent mice by suppressing the ERK signaling in GABAergic neurons in the BNST.

Extended-amygdala intrinsic functional connectivity networks: A population study

Pre-clinical and human neuroimaging research implicates the extended-amygdala (ExtA) (including the bed nucleus of the stria terminalis [BST] and central nucleus of the amygdala [CeA]) in networks mediating negative emotional states associated with stress and substance-use behaviours. The extent to which individual ExtA structures form a functionally integrated unit is controversial. We utilised a large sample (n > 1,000 healthy young adult humans) to compare the intrinsic functional connectivity networks (ICNs) of the BST and CeA using task-free functional magnetic resonance imaging (fMRI) data from the Human Connectome Project. We assessed whether inter-individual differences within these ICNs were related to two principal components representing negative disposition and alcohol use. Building on recent primate evidence, we tested whether within BST-CeA intrinsic functional connectivity (iFC) was heritable and further examined co-heritability with our principal components. We demonstrate the BST and CeA to have discrete, but largely overlapping ICNs similar to previous findings. We found no evidence that within BST-CeA iFC was heritable; however, post hoc analyses found significant BST iFC heritability with the broader superficial and centromedial amygdala regions. There were no significant correlations or co-heritability associations with our principal components either across the ICNs or for specific BST-Amygdala iFC. Possible differences in phenotype associations across task-free, task-based, and clinical fMRI are discussed, along with suggestions for more causal investigative paradigms that make use of the now well-established ExtA ICNs.

Behavioral and brain mechanisms mediating conditioned flight behavior in rats

Environmental contexts can inform animals of potential threats, though it is currently unknown how context biases the selection of defensive behavior. Here we investigated context-dependent flight responses with a Pavlovian serial-compound stimulus (SCS) paradigm that evokes freeze-to-flight transitions. Similar to previous work in mice, we show that male and female rats display context-dependent flight-like behavior in the SCS paradigm. Flight behavior was dependent on contextual fear insofar as it was only evoked in a shock-associated context and was reduced in the conditioning context after context extinction. Flight behavior was only expressed to white noise regardless of temporal order within the compound. Nonetheless, rats that received unpaired SCS trials did not show flight-like behavior to the SCS, indicating it is associative. Finally, we show that pharmacological inactivation of two brain regions critical to the expression of contextual fear, the central nucleus of the amygdala (CeA) and bed nucleus of the stria terminalis (BNST), attenuates both contextual fear and flight responses. All of these effects were similar in male and female rats. This work demonstrates that contextual fear can summate with cued and innate fear to drive a high fear state and transition from post-encounter to circa-strike defensive modes.

Retrorubral field is a hub for diverse threat and aversive outcome signals

Adaptive fear scales to the degree of threat and requires diverse neural signals for threat and aversive outcome. We propose that the retrorubral field (RRF), a midbrain region containing A8 dopamine, is a neural origin of such signals. To reveal these signals, we recorded RRF single-unit activity while male rats discriminated danger, uncertainty, and safety. Many RRF neurons showed firing extremes to danger and safety that framed intermediate firing to uncertainty. The remaining neurons showed unique, threat-selective cue firing patterns. Diversity in firing direction, magnitude, and temporal characteristics led to the detection of at least eight functional neuron types. Neuron types defined with respect to threat showed unique firing patterns following aversive outcome. The result was RRF signals for foot shock receipt, positive prediction error, anti-positive prediction error, persistent safety, and persistent threat. The diversity of threat and aversive outcome signals points to a key role for the RRF in adaptive fear.

The effects of morphine withdrawal and conditioned withdrawal on memory consolidation and c-Fos expression in the central amygdala

The current study tested the hypothesis that drug withdrawal contributes to the addiction cycle in part because of an action on memory consolidation. Hence, four experiments in male Sprague-Dawley rats compared the effects of precipitated morphine withdrawal and conditioned morphine withdrawal on the consolidation of object memory and on activation of c-Fos in the central nucleus of the amygdala (CeA). It was found that immediate, but not 6 h delayed, post sample administration of 3 mg/kg of naltrexone significantly enhanced object memory in rats maintained, or previously maintained, on 10 mg/kg/day of morphine via osmotic minipumps. To establish whether conditioned withdrawal could also alter object memory, a contextual conditioning procedure was employed whereby morphine-maintained (10 mg/kg/day) animals received naltrexone (3 mg/kg) in a distinctive context (CS+) and vehicle in a separate context (CS-) for 10 days. During conditioning in the CS+, naltrexone suppressed locomotor activity, caused a rapid body weight loss and increased frequency of wet dog shakes. Interestingly, confinement to this CS+ immediately, but not 6 h, after the sample phase, also enhanced object memory. Finally, posttraining naltrexone and exposure to the CS+ both induced significant expression of c-Fos in the CeA. Therefore, this study reports for the first time that both acute precipitated withdrawal and conditioned withdrawal can facilitate memory consolidation, possibly through a common neural pathway that involves the central amygdala.

Functional Dissection of Glutamatergic and GABAergic Neurons in the Bed Nucleus of the Stria Terminalis

The bed nucleus of the stria terminalis (BNST)-a key part of the extended amygdala-has been implicated in the regulation of diverse behavioral states, ranging from anxiety and reward processing to feeding behavior. Among the host of distinct types of neurons within the BNST, recent investigations employing cell type- and projection-specific circuit dissection techniques (such as optogenetics, chemogenetics, deep-brain calcium imaging, and the genetic and viral methods for targeting specific types of cells) have highlighted the key roles of glutamatergic and GABAergic neurons and their axonal projections. As anticipated from their primary roles in excitatory and inhibitory neurotransmission, these studies established that the glutamatergic and GABAergic subpopulations of the BNST oppositely regulate diverse behavioral states. At the same time, these studies have also revealed unexpected functional specificity and heterogeneity within each subpopulation. In this Minireview, we introduce the body of studies that investigated the function of glutamatergic and GABAergic BNST neurons and their circuits. We also discuss unresolved questions and future directions for a more complete understanding of the cellular diversity and functional heterogeneity within the BNST.

The Raphe Dopamine System: Roles in Salience Encoding, Memory Expression, and Addiction

Dopamine (DA) neurons of the dorsal raphe nucleus (DRN) were traditionally viewed as an extension of the ventral tegmental area (VTA) DA population. While the VTA DA population is known to play important roles in reward processing, emerging evidence now supports the view that DRN DA neurons are a specialized midbrain DA subsystem that performs distinct functions in parallel to the VTA DA population. Recent studies have shed new light on the roles of DRN DA neurons in encoding incentive salience and in regulating memory expression and arousal. Here, we review recent findings using mouse models about the physiology and behavioral functions of DRN DA neurons, highlight the engagement of DRN DA neurons and their upstream circuits in opioid addiction, and discuss emerging lines of investigation that reveal multifaceted heterogeneity among DRN DA neurons.

Brain structures and networks responsible for stimulation-induced memory flashbacks during forniceal deep brain stimulation for Alzheimer's disease

Introduction

Fornix deep brain stimulation (fx‐DBS) is under investigation for treatment of Alzheimer's disease (AD). We investigated the anatomic correlates of flashback phenomena that were reported previously during acute diencephalic stimulation.

Methods

Thirty‐nine patients with mild AD who took part in a prior fx‐DBS trial (NCT01608061) were studied. After localizing patients’ implanted electrodes and modeling the volume of tissue activated (VTA) by DBS during systematic stimulation testing, we performed (1) voxel‐wise VTA mapping to identify flashback‐associated zones; (2) machine learning–based prediction of flashback occurrence given VTA overlap with specific structures; (3) normative functional connectomics to define flashback‐associated brain‐wide networks.

Results

A distinct diencephalic region was associated with greater flashback likelihood. Fornix, bed nucleus of stria terminalis, and anterior commissure involvement predicted memory events with 72% accuracy. Flashback‐inducing stimulation exhibited greater functional connectivity to a network of memory‐evoking and autobiographical memory‐related sites.

Discussion

These results clarify the neuroanatomical substrates of stimulation‐evoked flashbacks.

Limbic Neuropeptidergic Modulators of Emotion and Their Therapeutic Potential for Anxiety and Post-Traumatic Stress Disorder

Post-traumatic stress disorder (PTSD) is characterized by hypervigilance, increased reactivity to unpredictable versus predictable threat signals, deficits in fear extinction, and an inability to discriminate between threat and safety. First-line pharmacotherapies for psychiatric disorders have limited therapeutic efficacy in PTSD. However, recent studies have advanced our understanding of the roles of several limbic neuropeptides in the regulation of defensive behaviors and in the neural processes that are disrupted in PTSD. For example, preclinical studies have shown that blockers of tachykinin pathways, such as the Tac2 pathway, attenuate fear memory consolidation in mice and thus might have unique potential as early post-trauma interventions to prevent PTSD development. Targeting this pathway might also be beneficial in regulating other symptoms of PTSD, including trauma-induced aggressive behavior. In addition, preclinical and clinical studies have shown the important role of angiotensin receptors in fear extinction and the promise of using angiotensin II receptor blockade to reduce PTSD symptom severity. Additional preclinical studies have demonstrated that the oxytocin receptors foster accurate fear discrimination by facilitating fear responses to predictable versus unpredictable threats. Complementary human imaging studies demonstrate unique neural targets of intranasal oxytocin and compare its efficacy with well-established anxiolytic treatments. Finally, promising data from human subjects have demonstrated that a selective vasopressin 1A receptor antagonist reduces anxiety induced by unpredictable threats. This review highlights these novel promising targets for the treatment of unique core elements of PTSD pathophysiology.

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

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

Functional deletion of neuropeptide Y receptors type 2 in local synaptic networks of anteroventral BNST facilitates recall and increases return of fear

Return of previously extinguished fear memories presents a major hurdle in treatment of fear-related disorders. Neuropeptide Y receptors type 2 (Y2R) in the bed nucleus of stria terminalis (BNST) seem to play a crucial role in modulation of remote fear memories. Here, we targeted Cre-channelrhodopsin-2 to defined subregions of BNST or central amygdala (CeA) in floxed Y2R mice (Y2^lox/lox) for functional deletion of Y2R. We combined fear training and behavioral studies in vivo with optogenetic-electrophysiological analysis of BNST synaptic network activity ex vivo, in order to identify regional and cellular specificities of Y2R influence. Deletion of Y2R in the ventral section of anterior BNST (BNSTav) did not affect fear acquisition, but increased conditioned fear during recall and extinction learning, and aggravated remote fear return. By contrast, deletion of Y2R in the dorsal section of anterior BNST (BNSTad) or CeA did not influence acquisition, extinction or return of fear memories. Ex vivo optogenetic-electrophysiological analysis revealed Y2R-expressing local GABAergic inhibitory networks in BNST, both within (intraregional) and in-between (inter-regional) BNST subregions. Stimulation of Y2R resulted in a presynaptically mediated reduction of GABAergic responses, which did not differ between intraregional but predominantly affected inter-regional connections from BNSTav to BNSTad. Moreover, deletion of Y2R decreased the excitation/inhibition balance in BNSTav neurons, suggesting a regulatory influence of endogenous NPY via intraregional GABAergic microcircuits. This study reveals Y2R within local GABAergic networks in BNST as key elements in facilitating extinction and reducing return of remote fear memories, suggesting a potential avenue for translational purposes.

Roles of the bed nucleus of the stria terminalis and amygdala in fear reactions

The bed nucleus of the stria terminalis (BNST) plays a critical modulatory role in driving fear responses. Part of the so-called extended amygdala, this region shares many functions and connections with the substantially more investigated amygdala proper. In this chapter, we review contributions of the BNST and amygdala to subjective, behavioral, and physiological aspects of fear. Despite the fact that both regions are together involved in each of these aspects of fear, they appear complimentary in their contributions. Specifically, the basolateral amygdala (BLA), through its connections to sensory and orbitofrontal regions, is ideally poised for fast learning and controlling fear reactions in a variety of situations. The central amygdala (CeA) relies on BLA input and is particularly important for adjusting physiological and behavioral responses under acute threat. In contrast, the BNST may profit from more extensive striatal and dorsomedial prefrontal connections to drive anticipatory responses under more ambiguous conditions that allow more time for planning. Thus current evidence suggests that the BNST is ideally suited to play a critical role responding to distant or ambiguous threats and could thereby facilitate goal-directed defensive action.

Chemoarchitecture of the bed nucleus of the stria terminalis: Neurophenotypic diversity and function

The bed nucleus of the stria terminalis (BNST) is a compact but neurophenotypically complex structure in the ventral forebrain that is structurally and functionally linked to other limbic structures, including the amygdala nuclear complex, hypothalamic nuclei, hippocampus, and related midbrain structures, to participate in a wide range of functions, especially emotion, emotional learning, stress-related responses, and sexual behaviors. From a variety of sensory inputs, the BNST acts as a node for signal integration and coordination for information relay to downstream central neuroendocrine and autonomic centers for appropriate homeostatic physiological and behavioral responses. In contrast to the role of the amygdala in fear, the BNST has gained wide interest from work suggesting that it has main roles in mediating sustained responses to diffuse, unpredictable and/or long-duration threats that are typically associated with anxiety-related responses. Further, some BNST subregions are highly sexually dimorphic which appear contributory to the differential stress and social interactive behaviors, including reproductive responses, between males and females. Notably, maladaptive BNST neuroplasticity and function have been implicated in chronic pain, depression, anxiety-related abnormalities, and other psychopathologies including posttraumatic stress disorders. The BNST circuits are predominantly GABAergic-the glutaminergic neurons represent a minor population-but the complexity of the system results from an overlay of diverse neuropeptide coexpression in these neurons. More than a dozen neuropeptides may be differentially coexpressed in BNST neurons, and from variable G protein-coupled receptor signaling, may inhibit or activate downstream circuit activities. The mechanisms and roles of these peptides in modulating intrinsic BNST neurocircuit signaling and BNST long-distance target cell projections are still not well understood. Nevertheless, an understanding of some of the principal players may allow assembly of the circuit interactions.

A prefrontal-bed nucleus of the stria terminalis circuit limits fear to uncertain threat

In many cases of trauma, the same environmental stimuli that become associated with aversive events are experienced on other occasions without adverse consequence. We examined neural circuits underlying partially reinforced fear (PRF), whereby mice received tone-shock pairings on half of conditioning trials. Tone-elicited freezing was lower after PRF conditioning than fully reinforced fear (FRF) conditioning, despite an equivalent number of tone-shock pairings. PRF preferentially activated medial prefrontal cortex (mPFC) and bed nucleus of the stria terminalis (BNST). Chemogenetic inhibition of BNST-projecting mPFC neurons increased PRF, not FRF, freezing. Multiplexing chemogenetics with in vivo neuronal recordings showed elevated infralimbic cortex (IL) neuronal activity during CS onset and freezing cessation; these neural correlates were abolished by chemogenetic mPFC→BNST inhibition. These data suggest that mPFC→BNST neurons limit fear to threats with a history of partial association with an aversive stimulus, with potential implications for understanding the neural basis of trauma-related disorders.

GLP-1 receptors are involved in the GLP-1 (7-36) amide-induced modulation of glucose homoeostasis, emesis and feeding in Suncus murinus (house musk shrew)

GLP-1 receptor agonists are used for the treatment of type 2 diabetes but they may reduce appetite and cause nausea and emesis. We investigated if GLP-1 (7-36) amide can modulate glucose homoeostasis, emesis and feeding via an exendin (9-39)-sensitive mechanism in Suncus murinus. The effect of GLP-1 (7-36) amide on glucose homeostasis was examined using an intraperitoneal glucose tolerance test. In conscious fasted animals, food and water consumption and behavior were measured for 1 h following drug administration. c-Fos expression in the brain was measured using immunohistochemistry. GLP-1 (7-36) amide reduced blood glucose levels dose-dependently. Exendin (9-39) did not modify blood glucose levels but suppressed the glucose-lowering effect of GLP-1 (7-36) amide. GLP-1 (7-36) amide inhibited food and water intake, induced emesis and elevated c-Fos expression in the brainstem and hypothalamic nuclei in the brain. Exendin (9-39) antagonised the inhibition of food and water intake and emesis induced by GLP-1 (7-36) amide and the effects on c-Fos expression in the hypothalamus and brainstem, excepting for the bed nucleus of the stria terminalis. These data suggest that the action of GLP-1 (7-36) amide to modulate blood glucose, suppress food and water intake and induce emesis involve GLP-1 receptors in the hypothalamus and brainstem.

Brain mechanisms of HPA axis regulation: neurocircuitry and feedback in context Richard Kvetnansky lecture

Regulation of stress reactivity is a fundamental priority of all organisms. Stress responses are critical for survival, yet can also cause physical and psychological damage. This review provides a synopsis of brain mechanisms designed to control physiological responses to stress, focusing primarily on glucocorticoid secretion via the hypothalamo-pituitary-adrenocortical (HPA) axis. The literature provides strong support for multi-faceted control of HPA axis responses, involving both direct and indirect actions at paraventricular nucleus (PVN) corticotropin releasing hormone neurons driving the secretory cascade. The PVN is directly excited by afferents from brainstem and hypothalamic circuits, likely relaying information on homeostatic challenge. Amygdala subnuclei drive HPA axis responses indirectly via disinhibition, mediated by GABAergic relays onto PVN-projecting neurons in the hypothalamus and bed nucleus of the stria terminalis (BST). Inhibition of stressor-evoked HPA axis responses is mediated by an elaborate network of glucocorticoid receptor (GR)-containing circuits, providing a distributed negative feedback signal that inhibits PVN neurons. Prefrontal and hippocampal neurons play a major role in HPA axis inhibition, again mediated by hypothalamic and BST GABAergic relays to the PVN. The complexity of the regulatory process suggests that information on stressors is integrated across functional disparate brain circuits prior to accessing the PVN, with regions such as the BST in prime position to relay contextual information provided by these sources into appropriate HPA activation. Dysregulation of the HPA in disease is likely a product of inappropriate checks and balances between excitatory and inhibitory inputs ultimately impacting PVN output.

Emotional responses in monkeys differ depending on the stimulus type, sex, and neonatal amygdala lesion status

The amygdala plays an essential role in evaluating social information, threat detection, and learning fear associations. Yet, most of that knowledge comes from studies in adult humans and animals with a fully developed amygdala. Given the considerable protracted postnatal development of the amygdala, it is important to understand how early damage to this structure may impact the long-term development of behavior. The current study examined behavioral responses toward social, innate, or learned aversive stimuli among neonatal amygdala lesion (Neo-Aibo; males = 3, females = 3) or sham-operated control (Neo-C; males = 3, females = 4) rhesus macaques. Compared with controls, Neo-Aibo animals exhibited less emotional reactivity toward aversive objects, including faster retrieval of food reward, fewer fearful responses, and more manipulation of objects. This lower reactivity was only seen in response to social and innate aversive stimuli, whereas Neo-Aibo animals had similar responses to controls for learned aversive stimuli. The current study also detected sex differences in behavioral response to aversive stimuli, such that, as compared with males, females took longer to retrieve the food reward across all aversive stimuli types, but only expressed more hostility and more coo vocalizations during learned aversive trials. Early amygdala damage impacted the expression of some, but not all, sex differences. For example, neonatal amygdala damage eliminated the sex difference in object manipulation. These findings add important information that broaden our understanding of the role of the amygdala in the expression of sexually dimorphic behaviors, as well as its role in learning fear associations and threat detection. (PsycINFO Database Record (c) 2020 APA, all rights reserved).