Negative valence systems: sustained threat and the predatory imminence continuum

Uncertainty and anxiety: Evolution and neurobiology

Anxiety is a complex phenomenon: Its eliciting stimuli and circumstances, component behaviors, and functional consequences are only slowly coming to be understood. Here, we examine defense systems from field studies; laboratory studies focusing on experimental analyses of behavior; and, the fear conditioning literature, with a focus on the role of uncertainty in promoting an anxiety pattern that involves high rates of stimulus generalization and resistance to extinction. Respectively, these different areas provide information on evolved elicitors of defense (field studies); outline a defense system focused on obtaining information about uncertain threat (ethoexperimental analyses); and, provide a simple, well-researched, easily measured paradigm for analysis of nonassociative stress-enhanced fear conditioning (the SEFL). Results suggest that all of these-each of which is responsive to uncertainty-play multiple and interactive roles in anxiety. Brain system findings for some relevant models are reviewed, with suggestions that further analyses of current models may be capable of providing a great deal of additional information about these complex interactions and their underlying biology.

A subiculum-hypothalamic pathway functions in dynamic threat detection and memory updating

Animals need to detect threats, initiate defensive responses, and, in parallel, remember where the threat occurred to avoid the possibility of re-encountering it. By probing animals capable of detecting and avoiding a shock-related threatening location, we were able to reveal a septo-hippocampal-hypothalamic circuit that is also engaged in ethological threats, including predatory and social threats. Photometry analysis focusing on the dorsal premammillary nucleus (PMd), a critical interface of this circuit, showed that in freely tested animals, the nucleus appears ideal to work as a threat detector to sense dynamic changes under threatening conditions as the animal approaches and avoids the threatening source. We also found that PMd chemogenetic silencing impaired defensive responses by causing a failure of threat detection rather than a direct influence on any behavioral responses and, at the same time, updated fear memory to a low-threat condition. Optogenetic silencing of the main PMd targets, namely the periaqueductal gray and anterior medial thalamus, showed that the projection to the periaqueductal gray influences both defensive responses and, to a lesser degree, contextual memory, whereas the projection to the anterior medial thalamus has a stronger influence on memory processes. Our results are important for understanding how animals deal with the threat imminence continuum, revealing a circuit that is engaged in threat detection and that, at the same time, serves to update the memory process to accommodate changes under threatening conditions.

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.

A multi-modal assessment of fear conditioning in adolescent anorexia nervosa

OBJECTIVE

Anorexia nervosa (AN) is a pernicious psychiatric disorder which is principally characterized by a fear of weight gain. Notwithstanding the centrality of fear in the psychopathology of AN, controlled assessments of negative valence systems are lacking. Herein we assess fear conditioning in adolescent females with AN.

METHOD

Adolescent girls (Mage  = 14.6 years, ±1.57) with DSM-5 diagnoses of AN (N = 25) and age-matched control girls (Mage  = 14.8 years, ±1.46) with no DSM-5 diagnoses (N = 25) completed structured clinical interviews and participated in a classical three-phase Pavlovian fear conditioning paradigm. Participants with comorbid anxiety disorders were excluded. Skin conductance response (SCR) was measured, alongside self-reported fear, valence, and fear expectancy ratings.

RESULTS

Both groups demonstrated significant differential acquisition across all four measures. Regarding group comparisons, no differences emerged for self-reported fear, valence, and fear expectancy ratings during acquisition, although for SCR, those with AN demonstrated reduced physiological arousal relative to controls. Both groups demonstrated significant differential extinction for unconditioned stimuli (US) expectancy, self-report fear, and self-report valence. No statistically significant group differences were evident during extinction to the conditioned stimuli (CS)+, on any outcome measure. However, controls reported more positive valence to the CS- than those with AN.

CONCLUSIONS

Contrary to our hypotheses, our preliminary assessment did not find support for elevated fear responding among adolescent girls with AN with regards to fear acquisition or extinction. These data suggest that AN in adolescent girls may not be associated with a heightened propensity to acquire fear, but conversely, may suggest that exposure treatments for AN may be helpful, since extinction learning is intact in AN.

PUBLIC SIGNIFICANCE

AN is characterized by fear-related symptoms, including food and weight-related fear, and behavioral avoidance, yet controlled studies assessing fear learning are limited. Our preliminary assessment of adolescent AN indicates no abnormalities in fear learning among adolescents with AN. These findings may inform existing mechanistic models of AN psychopathology, and the development of exposure-based treatments for AN.

Engrams: From Behavior to Brain-Wide Networks

Animals utilize a repertoire of behavioral responses during everyday experiences. During a potentially dangerous encounter, defensive actions such as "fight, flight, or freeze" are selected for survival. The successful use of behavior is determined by a series of real-time computations combining an animal's internal (i.e., body) and external (i.e., environment) state. Brain-wide neural pathways are engaged throughout this process to detect stimuli, integrate information, and command behavioral output. The hippocampus, in particular, plays a role in the encoding and storing of the episodic information surrounding these encounters as putative "engram" or experience-modified cellular ensembles. Recalling a negative experience then reactivates a dedicated engram ensemble and elicits a behavioral response. How hippocampus-based engrams modulate brain-wide states and an animal's internal/external milieu to influence behavior is an exciting area of investigation for contemporary neuroscience. In this chapter, we provide an overview of recent technological advancements that allow researchers to tag, manipulate, and visualize putative engram ensembles, with an overarching goal of casually connecting their brain-wide underpinnings to behavior. We then discuss how hippocampal fear engrams alter behavior in a manner that is contingent on an environment's physical features as well as how they influence brain-wide patterns of cellular activity. Overall, we propose here that studies on memory engrams offer an exciting avenue for contemporary neuroscience to casually link the activity of cells to cognition and behavior while also offering testable theoretical and experimental frameworks for how the brain organizes experience.

PAC1 receptor modulation of freezing and flight behavior in periaqueductal gray

The midbrain periaqueductal gray (PAG) region is a critical anatomical regulator of fear-related species-specific defensive reactions (SSDRs). Pituitary adenylate-cyclase-activating polypeptide (PACAP), and its main receptor PAC1, play an important role in fear-related behavior and anxiety disorders. However, the function of the PACAP-PAC1 system within the PAG with regards to SSDRs has received little attention. To address this gap, we used transgenic PAC1flox/flox mice to examine both conditional and unconditional defensive reactions. We performed conditional PAC1 gene deletion within the ventrolateral(vl)PAG of PAC1flox/flox mice using an adeno-associated virus (AAV) coding for Cre recombinase. Following viral expression, we used a white noise fear conditioning preparation that produces both an unconditional activity burst to the onset of noise that is followed by conditional freezing. On Day 1, mice received five white noise foot-shock pairings, whereas on Day 2, they were exposed to white noise five times without shock and we scored the activity burst and freezing to the white noise. Following behavioral testing, histology for immunofluorescent analysis was conducted in order to identify PACAP positive cells and stress-induced c-fos activity respectively. We found that PAC1 deletion in vlPAG increased the unconditional activity burst response but disrupted conditional freezing. PAC1 deletion was accompanied by higher c-fos activity following the behavioral experiments. Furthermore, a significant portion of PACAP-EGFP positive cells showed overlapping expression with VGAT, indicating their association with inhibitory neurons. The findings suggested that intact PACAP-PAC1 mechanisms are essential for SSDRs in vlPAG. Therefore, midbrain PACAP contributes to the underlying molecular mechanisms regulating fear responses.

Defensive responses: behaviour, the brain and the body

Most animals live under constant threat from predators, and predation has been a major selective force in shaping animal behaviour. Nevertheless, defence responses against predatory threats need to be balanced against other adaptive behaviours such as foraging, mating and recovering from infection. This behavioural balance in ethologically relevant contexts requires adequate integration of internal and external signals in a complex interplay between the brain and the body. Despite this complexity, research has often considered defensive behaviour as entirely mediated by the brain processing threat-related information obtained via perception of the external environment. However, accumulating evidence suggests that the endocrine, immune, gastrointestinal and reproductive systems have important roles in modulating behavioural responses to threat. In this Review, we focus on how predatory threat defence responses are shaped by threat imminence and review the circuitry between subcortical brain regions involved in mediating defensive behaviours. Then, we discuss the intersection of peripheral systems involved in internal states related to infection, hunger and mating with the neurocircuits that underlie defence responses against predatory threat. Through this process, we aim to elucidate the interconnections between the brain and body as an integrated network that facilitates appropriate defensive responses to threat and to discuss the implications for future behavioural research.

Bed nucleus of the stria terminalis GABA neurons are necessary for changes in foraging behaviour following an innate threat

Foraging is a universal behaviour that has co-evolved with predation pressure. We investigated the role of the bed nucleus of the stria terminalis (BNST) GABA neurons in robotic and live predator threat processing and their consequences in post-threat encounter foraging. Both robotic and live predator interactions increased BNST GABA neuron activity. Mice were trained to procure food in a laboratory-based foraging apparatus in which food pellets were placed at incrementally greater distances from a nest zone. After mice learned to forage, they were exposed to a robotic or live predator threat, while BNST GABA neurons were chemogenetically inhibited. Post-robotic threat encounter, mice spent more time in the nest zone, but other foraging parameters were unchanged compared with pre-encounter behaviour. Inhibition of BNST GABA neurons had no effect on foraging behaviour post-robotic threat encounter. Following live predator exposure, control mice spent significantly more time in the nest zone, increased their latency to successfully forage, and significantly altered their overall foraging performance. Inhibition of BNST GABA neurons during live predator exposure prevented changes in foraging behaviour from developing after a live predator threat. BNST GABA neuron inhibition did not alter foraging behaviour during robotic or live predator threats. We conclude that these results demonstrate that while both robotic and live predator encounters effectively intrude on foraging behaviour, the perceived risk and behavioural consequences of the threat are distinguishable. Additionally, BNST GABA neurons may play a role in the integration of prior innate predator threat experience that results in hypervigilance during post-encounter foraging behaviour.

Beyond Nature Versus Nurture: the Emergence of Emotion

Affective science is stuck in a version of the nature-versus-nurture debate, with theorists arguing whether emotions are evolved adaptations or psychological constructions. We do not see these as mutually exclusive options. Many adaptive behaviors that humans have evolved to be good at, such as walking, emerge during development - not according to a genetically dictated program, but through interactions between the affordances of the body, brain, and environment. We suggest emotions are the same. As developing humans acquire increasingly complex goals and learn optimal strategies for pursuing those goals, they are inevitably pulled to particular brain-body-behavior states that maximize outcomes and self-reinforce via positive feedback loops. We call these recurring, self-organized states emotions. Emotions display many of the hallmark features of self-organized attractor states, such as hysteresis (prior events influence the current state), degeneracy (many configurations of the underlying variables can produce the same global state), and stability. Because most bodily, neural, and environmental affordances are shared by all humans - we all have cardiovascular systems, cerebral cortices, and caregivers who raised us - similar emotion states emerge in all of us. This perspective helps reconcile ideas that, at first glance, seem contradictory, such as emotion universality and neural degeneracy.

Bed Nucleus of the Stria Terminalis GABA neurons are necessary for changes in foraging behavior following an innate threat

Foraging is a universal behavior that has co-evolved with predation pressure. We investigated the role of bed nucleus of the stria terminalis (BNST) GABA neurons in robotic and live predator threat processing and their consequences in post-threat encounter foraging. Mice were trained to procure food in a laboratory-based foraging apparatus in which food pellets were placed at discrete and incrementally greater distances from a nest zone. After mice learned to forage, they were exposed to either a robotic or live predator threat, while BNST GABA neurons were chemogenetically inhibited. Post-robotic threat encounter, mice spent more time in the nest zone, but other foraging parameters were unchanged compared to pre-encounter behavior. Inhibition of BNST GABA neurons had no effect on foraging behavior post-robotic threat encounter. Following live predator exposure, control mice spent significantly more time in the nest zone, increased their latency to successfully forage, and their overall foraging performance was significantly a ltered. I nhibition o f BNST GABA neurons during live predator exposure prevented changes in foraging behavior from developing after live predator threat. BNST GABA neuron inhibition did not alter foraging behavior during robotic or live predator threat. We conclude that while both robotic and live predator encounter effectively intrude on foraging behavior, the perceived risk and behavioral consequence of the threats are distinguishable. Additionally, BNST GABA neurons may play a role in the integration of prior innate predator threat experience that results in hypervigilance during post-encounter foraging behavior.

Editorial introduction: animal models relevant to mental health disorders

Mental health disorders affect a substantial proportion of the worldwide population, and currently available treatments do not work for all affected individuals. Understanding the psychological and biological mechanisms that underlie mental health disorders will facilitate treatment development, and the use of translational animal models is potentially transformative for this. Structured around the US National Institute of Mental Health's 'Research Domain Criteria' (RDoC) approach, this special issue showcases reviews that consider how animal models can best be used to understand and treat the processes that go awry in mental health disorders.