The Neuropeptide Tac2 Controls a Distributed Brain State Induced by Chronic Social Isolation Stress.

Chronic social isolation causes severe psychological effects in humans, but their neural bases remain poorly understood. 2 weeks (but not 24 hr) of social isolation stress (SIS) caused multiple behavioral changes in mice and induced brain-wide upregulation of the neuropeptide tachykinin 2 (Tac2)/neurokinin B (NkB). Systemic administration of an Nk3R antagonist prevented virtually all of the behavioral effects of chronic SIS. Conversely, enhancing NkB expression and release phenocopied SIS in group-housed mice, promoting aggression and converting stimulus-locked defensive behaviors to persistent responses. Multiplexed analysis of Tac2/NkB function in multiple brain areas revealed dissociable, region-specific requirements for both the peptide and its receptor in different SIS-induced behavioral changes. Thus, Tac2 coordinates a pleiotropic brain state caused by SIS via a distributed mode of action. These data reveal the profound effects of prolonged social isolation on brain chemistry and function and suggest potential new therapeutic applications for Nk3R antagonists.

Dynamic influences on the neural encoding of social valence.

Social signals can serve as potent emotional triggers with powerful impacts on processes from cognition to valence processing. How are social signals dynamically and flexibly associated with positive or negative valence? How do our past social experiences and present social standing shape our motivation to seek or avoid social contact? We discuss a model in which social attributes, social history, social memory, social rank and social isolation can flexibly influence valence assignment to social stimuli, termed here as 'social valence'. We emphasize how the brain encodes each of these four factors and highlight the neural circuits and mechanisms that play a part in the perception of social attributes, social memory and social rank, as well as how these factors affect valence systems associated with social stimuli. We highlight the impact of social isolation, dissecting the neural and behavioural mechanisms that mediate the effects of acute versus prolonged periods of social isolation. Importantly, we discuss conceptual models that may account for the potential shift in valence of social stimuli from positive to negative as the period of isolation extends in time. Collectively, this Review identifies factors that control the formation and attribution of social valence - integrating diverse areas of research and emphasizing their unique contributions to the categorization of social stimuli as positive or negative.

Social behaviour shapes hypothalamic neural ensemble representations of conspecific sex.

All animals possess a repertoire of innate (or instinctive) behaviours, which can be performed without training. Whether such behaviours are mediated by anatomically distinct and/or genetically specified neural pathways remains unknown. Here we report that neural representations within the mouse hypothalamus, that underlie innate social behaviours, are shaped by social experience. Oestrogen receptor 1-expressing (Esr1+) neurons in the ventrolateral subdivision of the ventromedial hypothalamus (VMHvl) control mating and fighting in rodents. We used microendoscopy to image Esr1+ neuronal activity in the VMHvl of male mice engaged in these social behaviours. In sexually and socially experienced adult males, divergent and characteristic neural ensembles represented male versus female conspecifics. However, in inexperienced adult males, male and female intruders activated overlapping neuronal populations. Sex-specific neuronal ensembles gradually separated as the mice acquired social and sexual experience. In mice permitted to investigate but not to mount or attack conspecifics, ensemble divergence did not occur. However, 30 minutes of sexual experience with a female was sufficient to promote the separation of male and female ensembles and to induce an attack response 24 h later. These observations uncover an unexpected social experience-dependent component to the formation of hypothalamic neural assemblies controlling innate social behaviours. More generally, they reveal plasticity and dynamic coding in an evolutionarily ancient deep subcortical structure that is traditionally viewed as a 'hard-wired' system.

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.

Alpha-synuclein pathology, microgliosis, and parvalbumin neuron loss in the amygdala associated with enhanced fear in the Thy1-aSyn model of Parkinson's disease.

In Parkinson's disease (PD), the second most common neurodegenerative disorder, non-motor symptoms often precede the development of debilitating motor symptoms and present a severe impact on the quality of life. Lewy bodies containing misfolded α-synuclein progressively develop in neurons throughout the peripheral and central nervous system, which may be correlated with the early development of non-motor symptoms. Among those, increased fear and anxiety is frequent in PD and thought to result from pathology outside the dopaminergic system, which has been the focus of symptomatic treatment to alleviate motor symptoms. Alpha-synuclein accumulation has been reported in the amygdala of PD patients, a brain region critically involved in fear and anxiety. Here we asked whether α-synuclein overexpression alone is sufficient to induce an enhanced fear phenotype in vivo and which pathological mechanisms are involved. Transgenic mice expressing human wild-type α-synuclein (Thy1-aSyn), a well-established model of PD, were subjected to fear conditioning followed by extinction and then tested for extinction memory retention followed by histopathological analysis. Thy1-aSyn mice showed enhanced tone fear across acquisition and extinction compared to wild-type littermates, as well as a trend to less retention of fear extinction. Immunohistochemical analysis of the basolateral nucleus of the amygdala, a nucleus critically involved in tone fear learning, revealed extensive α-synuclein pathology, with accumulation, phosphorylation, and aggregation of α-synuclein in transgenic mice. This pathology was accompanied by microgliosis and parvalbumin neuron loss in this nucleus, which could explain the enhanced fear phenotype. Importantly, this non-motor phenotype was detected in the pre-clinical phase, prior to dopamine loss in Thy1-aSyn mice, thus replicating observations in patients. Results obtained in this study suggest a possible mechanism by which increased anxiety and maladaptive fear processing may occur in PD, opening a door for therapeutic options and further early biomarker research.

Automated measurement of mouse social behaviors using depth sensing, video tracking, and machine learning.

A lack of automated, quantitative, and accurate assessment of social behaviors in mammalian animal models has limited progress toward understanding mechanisms underlying social interactions and their disorders such as autism. Here we present a new integrated hardware and software system that combines video tracking, depth sensing, and machine learning for automatic detection and quantification of social behaviors involving close and dynamic interactions between two mice of different coat colors in their home cage. We designed a hardware setup that integrates traditional video cameras with a depth camera, developed computer vision tools to extract the body "pose" of individual animals in a social context, and used a supervised learning algorithm to classify several well-described social behaviors. We validated the robustness of the automated classifiers in various experimental settings and used them to examine how genetic background, such as that of Black and Tan Brachyury (BTBR) mice (a previously reported autism model), influences social behavior. Our integrated approach allows for rapid, automated measurement of social behaviors across diverse experimental designs and also affords the ability to develop new, objective behavioral metrics.

Prefrontal microcircuit underlies contextual learning after hippocampal loss.

Specific brain circuits have been classically linked to dedicated functions. However, compensation following brain damage suggests that these circuits are capable of dynamic adaptation. Such compensation is exemplified by Pavlovian fear conditioning following damage to the dorsal hippocampus (DH). Although the DH normally underlies contextual fear and fear renewal after extinction, both can be learned in the absence of the DH, although the mechanisms and nature of this compensation are currently unknown. Here, we report that recruitment of alternate structures, specifically the infralimbic and prelimbic prefrontal cortices, is required for compensation following damage to the hippocampus. Disconnection of these cortices in DH-compromised animals and immediate early gene induction profiles for amygdala-projecting prefrontal cells revealed that communication and dynamic rebalancing within this prefrontal microcircuit is critical. Additionally, the infralimbic cortex normally plays a role in limiting generalization of contextual fear. These discoveries reveal that plasticity through recruitment of alternate circuits allows the brain to compensate following damage, offering promise for targeted treatment of memory disorders.

Neuronal ensembles in amygdala, hippocampus, and prefrontal cortex track differential components of contextual fear.

Although the circuit mediating contextual fear conditioning has been extensively described, the precise contribution that specific anatomical nodes make to behavior has not been fully elucidated. To clarify the roles of the dorsal hippocampus (DH), basolateral amygdala (BLA), and medial prefrontal cortex (mPFC) in contextual fear conditioning, activity within these regions was mapped using cellular compartment analysis of temporal activity using fluorescence in situ hybridization (catFISH) for Arc mRNA. Rats were delay-fear conditioned or immediately shocked (controls) and thereafter reexposed to the shocked context to test for fear memory recall. Subsequent catFISH analyses revealed that in the DH, cells were preferentially reactivated during the context test, regardless of whether animals had been fear conditioned or immediately shocked, suggesting that the DH encodes spatial information specifically, rather then the emotional valence of an environment. In direct contrast, neuronal ensembles in the BLA were only reactivated at test if animals had been fear conditioned, suggesting that the amygdala specifically tracks the emotional properties of a context. Interestingly, Arc expression in the mPFC was consistent with both amygdala- and hippocampus-like patterns, supporting a role for the mPFC in both fear and contextual processing. Collectively, these data provide crucial insight into the region-specific behavior of neuronal ensembles during contextual fear conditioning and demonstrate a dissociable role for the hippocampus and amygdala in processing the contextual and emotional properties of a fear memory.

Isomorphisms between psychological processes and neural mechanisms: from stimulus elements to genetic markers of activity.

Traditional learning theory has developed models that can accurately predict and describe the course of learned behavior. These "psychological process" models rely on hypothetical constructs that are usually thought to be not directly measurable or manipulable. Recently, and mostly in parallel, the neural mechanisms underlying learning have been fairly well elucidated. The argument in this essay is that we can successfully uncover isomorphisms between process and mechanism and that this effort will help advance our theories about both processes and mechanisms. We start with a brief review of error-correction circuits as a successful example. Then we turn to the concept of stimulus elements, where the conditional stimulus is hypothesized to be constructed of a multitude of elements only some of which are sampled during any given experience. We discuss such elements with respect to how they explain acquisition of associative strength as an incremental process. Then we propose that for fear conditioning, stimulus elements and basolateral amygdala projection neurons are isomorphic and that the activational state of these "elements" can be monitored by the expression of the mRNA for activity-regulated cytoskeletal protein (ARC). Finally we apply these ideas to analyze recent data examining ARC expression during contextual fear conditioning and find that there are indeed many similarities between stimulus elements and amygdala neurons. The data also suggest some revisions in the conceptualization of how the population of stimulus elements is sampled from.

SAUSI: a novel assay for measuring social anxiety and motivation.

Social anxiety is one of the most prevalent mental health disorders, though the underlying neurobiology is poorly understood. Progress in understanding the etiology of social anxiety has been hindered by the lack of comprehensive tools to assess social anxiety in model systems. Here, we created a new behavioral task - Selective Access to Unrestricted Social Interaction (SAUSI), which combines elements of social motivation, hesitancy, decision-making, and free interaction to enable the wholistic assessment of social anxiety-like behaviors in mice. Using this novel assay, we found that social isolation-induced social anxiety-like behaviors in female mice are largely driven by increases in social fear, social hesitancy, and altered ultrasonic vocalizations. Deep learning analyses were able to computationally identify a unique behavioral footprint underlying the state produced by social isolation, demonstrating the compatibility of modern computational approaches with SAUSI. Finally, we compared the results of SAUSI to traditionally social assays including the 3-chamber sociability assay and the resident intruder task. This revealed that behavioral changes induced by isolation were highly context dependent, and that while fragments of social anxiety measured in SAUSI were replicable across other tasks, a wholistic assessment was not obtainable from these alternative assays. Our findings debut a novel task for the behavioral toolbox - one which overcomes limitations of previous assays, allowing for both social choice as well as free interaction, and offers a new approach for assessing social anxiety in rodents.

Contextual fear memories formed in the absence of the dorsal hippocampus decay across time.

Mammals suffering damage to the hippocampus display a dramatic loss of explicit, recently formed memories (retrograde amnesia). In contrast, deficits in the ability to form new memories following hippocampal damage (anterograde amnesia) can be overcome with sufficient training. By combining contextual fear conditioning with lesions of the dorsal hippocampus in rats, we discovered that while animals can form long-term contextual fear memories in the absence of the hippocampus, these memories decay with time, lacking the permanence that is a hallmark characteristic of normal fear memories. These findings indicate that while it is initially possible to acquire explicit memories when the hippocampus is compromised, these memories cannot transfer from a recent to remote state. This suggests that memories formed outside the hippocampus may nevertheless require the hippocampus to undergo systems consolidation, which has important clinical implications for the treatment of memory disorders.

Anxiety in synucleinopathies: neuronal circuitry, underlying pathomechanisms and current therapeutic strategies.

Synucleinopathies are neurodegenerative disorders characterized by alpha-synuclein (αSyn) accumulation in neurons or glial cells, including Parkinson's disease (PD), dementia with Lewy bodies (DLB), and multiple system atrophy (MSA). αSyn-related pathology plays a critical role in the pathogenesis of synucleinopathies leading to the progressive loss of neuronal populations in specific brain regions and the development of motor and non-motor symptoms. Anxiety is among the most frequent non-motor symptoms in patients with PD, but it remains underrecognized and undertreated, which significantly reduces the quality of life for patients. Anxiety is defined as a neuropsychiatric complication with characteristics such as nervousness, loss of concentration, and sweating due to the anticipation of impending danger. In patients with PD, neuropathology in the amygdala, a central region in the anxiety and fear circuitry, may contribute to the high prevalence of anxiety. Studies in animal models reported αSyn pathology in the amygdala together with alteration of anxiety or fear learning response. Therefore, understanding the progression, extent, and specifics of pathology in the anxiety and fear circuitry in synucleinopathies will suggest novel approaches to the diagnosis and treatment of neuropsychiatric symptoms. Here, we provide an overview of studies that address neuropsychiatric symptoms in synucleinopathies. We offer insights into anxiety and fear circuitry in animal models and the current implications for therapeutic intervention. In summary, it is apparent that anxiety is not a bystander symptom in these disorders but reflects early pathogenic mechanisms in the cortico-limbic system which may even contribute as a driver to disease progression.

Temporal factors control hippocampal contributions to fear renewal after extinction.

Fear can be extinguished by repeated exposure to a cue that signals threat. However, extinction does not erase fear, as an extinguished cue presented in a context distinct from that of extinction results in renewed fear of that cue. The hippocampus, which is involved in the formation of contextual representations, is a natural candidate structure for investigations into the neural circuitry underlying fear renewal. Thus far, studies examining the necessity of the hippocampus for fear renewal have produced mixed results. We isolated the conditions under which the hippocampus may be required for renewal. Rats received lesions of the dorsal hippocampus either prior to tone fear conditioning or following extinction. Fear renewal was measured using discrete tone presentations or a long, continuous tone. The topography of fear responding at test was assessed by comparing "early" and "sustained" renewal, where early fear was determined by freezing to the first discrete tone or the equivalent initial segment of a continuous tone and sustained fear was determined by freezing averaged across all discrete tones or the entire continuous tone. We found that following pretraining damage of the hippocampus, early renewal remained intact regardless of lesion condition. However, sustained renewal only persisted in discrete, but not continuous, tone-tested animals. A more extensive analysis of the topography of fear responding revealed that the disruption of renewal was generated when the tone duration at test began to violate that used during extinction, suggesting that the hippocampus is sensitive to mismatches in CS-duration. Postextinction lesions resulted in an overall reduction of fear renewal. This pattern of results is consistent with those observed for contextual fear conditioning, wherein animals display a resistance to anterograde amnesia despite the presence of a strong retrograde amnesia for the same contextual information. Furthermore, the data support a role for the hippocampus in sustaining renewal when the CS duration at test does not match that used during extinction.

Neuroscience: The sting of social isolation.

Social isolation produces deleterious effects on the brain and behavior in many species. A new study on bumblebees uses a multimodal approach to further our understanding of the state produced by prolonged social isolation.

The emergence and influence of internal states.

Animal behavior is shaped by a variety of "internal states"-partially hidden variables that profoundly shape perception, cognition, and action. The neural basis of internal states, such as fear, arousal, hunger, motivation, aggression, and many others, is a prominent focus of research efforts across animal phyla. Internal states can be inferred from changes in behavior, physiology, and neural dynamics and are characterized by properties such as pleiotropy, persistence, scalability, generalizability, and valence. To date, it remains unclear how internal states and their properties are generated by nervous systems. Here, we review recent progress, which has been driven by advances in behavioral quantification, cellular manipulations, and neural population recordings. We synthesize research implicating defined subsets of state-inducing cell types, widespread changes in neural activity, and neuromodulation in the formation and updating of internal states. In addition to highlighting the significance of these findings, our review advocates for new approaches to clarify the underpinnings of internal brain states across the animal kingdom.

The Mouse Action Recognition System (MARS) software pipeline for automated analysis of social behaviors in mice.

The study of naturalistic social behavior requires quantification of animals' interactions. This is generally done through manual annotation-a highly time-consuming and tedious process. Recent advances in computer vision enable tracking the pose (posture) of freely behaving animals. However, automatically and accurately classifying complex social behaviors remains technically challenging. We introduce the Mouse Action Recognition System (MARS), an automated pipeline for pose estimation and behavior quantification in pairs of freely interacting mice. We compare MARS's annotations to human annotations and find that MARS's pose estimation and behavior classification achieve human-level performance. We also release the pose and annotation datasets used to train MARS to serve as community benchmarks and resources. Finally, we introduce the Behavior Ensemble and Neural Trajectory Observatory (BENTO), a graphical user interface for analysis of multimodal neuroscience datasets. Together, MARS and BENTO provide an end-to-end pipeline for behavior data extraction and analysis in a package that is user-friendly and easily modifiable.

Stress Varies Along the Social Density Continuum.

Social stress is ubiquitous in the lives of social animals. While significant research has aimed to understand the specific forms of stress imparted by particular social interactions, less attention has been paid to understanding the behavioral effects and neural underpinnings of stress produced by the presence and magnitude of social interactions. However, in humans and rodents alike, chronically low and chronically high rates of social interaction are associated with a suite of mental health issues, suggesting the need for further research. Here, we review literature examining the behavioral and neurobiological findings associated with changing social density, focusing on research on chronic social isolation and chronic social crowding in rodent models, and synthesize findings in the context of the continuum of social density that can be experienced by social animals. Through this synthesis, we aim to both summarize the state of the field and describe promising avenues for future research that would more clearly define the broad effects of social interaction on the brain and behavior in mammals.

Optimizing inhibitory learning during exposure therapy.

Prevailing models of exposure therapy for phobias and anxiety disorders construe level of fear throughout exposure trials as an index of corrective learning. However, the evidence, reviewed herein, indicates that neither the degree by which fear reduces nor the ending fear level predict therapeutic outcome. Developments in the theory and science of fear extinction, and learning and memory, indicate that 'performance during training' is not commensurate with learning at the process level. Inhibitory learning is recognized as being central to extinction and access to secondary inhibitory associations is subject to influences such as context and time, rather than fear during extinction training. Strategies for enhancing inhibitory learning, and its retrieval over time and context, are reviewed along with their clinical implications for exposure therapy and directions for future research.

Neuropeptidergic Control of an Internal Brain State Produced by Prolonged Social Isolation Stress.

Prolonged periods of social isolation can generate an internal state that exerts profound effects on the brain and behavior. However, the neurobiological underpinnings of protracted social isolation have been relatively understudied. Here, we review recent literature implicating peptide neuromodulators in the establishment and maintenance of such internal states. More specifically, we describe an evolutionarily conserved role for the neuropeptide tachykinin in the control of social isolation-induced aggression and review recent data that elucidate the manner by which Tac2 controls the widespread effects of social isolation on behavior in mice. Last, we discuss potential roles for additional neuromodulators in controlling social isolation and a more general role for Tac2 in the response to other forms of stress.

Ventromedial hypothalamic neurons control a defensive emotion state.

Defensive behaviors reflect underlying emotion states, such as fear. The hypothalamus plays a role in such behaviors, but prevailing textbook views depict it as an effector of upstream emotion centers, such as the amygdala, rather than as an emotion center itself. We used optogenetic manipulations to probe the function of a specific hypothalamic cell type that mediates innate defensive responses. These neurons are sufficient to drive multiple defensive actions, and required for defensive behaviors in diverse contexts. The behavioral consequences of activating these neurons, moreover, exhibit properties characteristic of emotion states in general, including scalability, (negative) valence, generalization and persistence. Importantly, these neurons can also condition learned defensive behavior, further refuting long-standing claims that the hypothalamus is unable to support emotional learning and therefore is not an emotion center. These data indicate that the hypothalamus plays an integral role to instantiate emotion states, and is not simply a passive effector of upstream emotion centers.