The brain on stress: Insight from studies using the Visible Burrow System

Sex Differences in the Neurobiology of Stress

This review highlights the existing knowledge and data that explain the physiologic impacts of stress, especially pertaining to neurobiology, and how these impacts differ by sex. Furthermore, this review explains the benefits of interventions aimed at preventing or mitigating the adverse effects of stress, because of both the significant toll of stress on the body and the disproportionate impact of these changes experienced by women.

A paradigm shift in translational psychiatry through rodent neuroethology

Mental disorders are a significant cause of disability worldwide. They profoundly affect individuals' well-being and impose a substantial financial burden on societies and governments. However, despite decades of extensive research, the effectiveness of current therapeutics for mental disorders is often not satisfactory or well tolerated by the patient. Moreover, most novel therapeutic candidates fail in clinical testing during the most expensive phases (II and III), which results in the withdrawal of pharma companies from investing in the field. It also brings into question the effectiveness of using animal models in preclinical studies to discover new therapeutic agents and predict their potential for treating mental illnesses in humans. Here, we focus on rodents as animal models and propose that they are essential for preclinical investigations of candidate therapeutic agents' mechanisms of action and for testing their safety and efficiency. Nevertheless, we argue that there is a need for a paradigm shift in the methodologies used to measure animal behavior in laboratory settings. Specifically, behavioral readouts obtained from short, highly controlled tests in impoverished environments and social contexts as proxies for complex human behavioral disorders might be of limited face validity. Conversely, animal models that are monitored in more naturalistic environments over long periods display complex and ethologically relevant behaviors that reflect evolutionarily conserved endophenotypes of translational value. We present how semi-natural setups in which groups of mice are individually tagged, and video recorded continuously can be attainable and affordable. Moreover, novel open-source machine-learning techniques for pose estimation enable continuous and automatic tracking of individual body parts in groups of rodents over long periods. The trajectories of each individual animal can further be subjected to supervised machine learning algorithms for automatic detection of specific behaviors (e.g., chasing, biting, or fleeing) or unsupervised automatic detection of behavioral motifs (e.g., stereotypical movements that might be harder to name or label manually). Compared to studies of animals in the wild, semi-natural environments are more compatible with neural and genetic manipulation techniques. As such, they can be used to study the neurobiological mechanisms underlying naturalistic behavior. Hence, we suggest that such a paradigm possesses the best out of classical ethology and the reductive behaviorist approach and may provide a breakthrough in discovering new efficient therapies for mental illnesses.

Social deficits in BTBR T+ Itpr3tf/J mice vary with ecological validity of the test

Translational value of mouse models of neuropsychiatric disorders depends heavily on the accuracy with which they replicate symptoms observed in the human population. In mouse models of autism spectrum disorder (ASD) these include, among others, social affiliation, and communication deficits as well as impairments in understanding and perception of others. Most studies addressing these issues in the BTBR T+ Itpr3tf/J mouse, an idiopathic model of ASD, were based on short dyadic interactions of often non-familiar partners placed in a novel environment. In such stressful and variable conditions, the reproducibility of the phenotype was low. Here, we compared physical conditions and the degree of habituation of mice at the time of testing in the three chambered social affiliation task, as well as parameters used to measure social deficits and found that both the level of stress and human bias profoundly affect the results of the test. To minimize these effects, we tested social preference and network dynamics in mice group-housed in the Eco-HAB system. This automated recording allowed for long-lasting monitoring of differences in social repertoire (including interest in social stimuli) in BTBR T+ Itpr3tf/J and normosocial c57BL/6J mice. With these observations we further validate the BTBR T+ Itpr3tf/J mouse as a model for ASD, but at the same time emphasize the need for more ecological testing of social behavior within all constructs of the Systems for Social Processes domain (as defined by the Research Domain Criteria framework).

Multidimensional nature of dominant behavior: Insights from behavioral neuroscience

Social interactions for many species of animals are critical for survival, wellbeing, and reproduction. Optimal navigation of a social system increases chances for survival and reproduction, therefore there is strong incentive to fit into social structures. Social animals rely heavily on dominant-submissive behaviors in establishment of stable social hierarchies. There is a link between extreme manifestation of dominance/submissiveness and behavioral deviations. To understand neural substrates affiliated with a specific hierarchical rank, there is a real need for reliable animal behavioral models. Different paradigms have been consolidated over time to study the neurobiology of social rank behavior in a standardized manner using rodent models to unravel the neural pathways and substrates involved in normal and abnormal intraspecific social interactions. This review summarizes and discusses the commonly used behavioral tests and new directions for the assessment of dominance in rodents. We discuss the hierarchy inheritable nature and other critical issues regarding hierarchical rank manifestation which may help in designing social-rank-related studies that serve as promising pre-clinical tools in behavioral psychiatry.

Measuring Behavior in the Home Cage: Study Design, Applications, Challenges, and Perspectives

The reproducibility crisis (or replication crisis) in biomedical research is a particularly existential and under-addressed issue in the field of behavioral neuroscience, where, in spite of efforts to standardize testing and assay protocols, several known and unknown sources of confounding environmental factors add to variance. Human interference is a major contributor to variability both within and across laboratories, as well as novelty-induced anxiety. Attempts to reduce human interference and to measure more "natural" behaviors in subjects has led to the development of automated home-cage monitoring systems. These systems enable prolonged and longitudinal recordings, and provide large continuous measures of spontaneous behavior that can be analyzed across multiple time scales. In this review, a diverse team of neuroscientists and product developers share their experiences using such an automated monitoring system that combines Noldus PhenoTyper® home-cages and the video-based tracking software, EthoVision® XT, to extract digital biomarkers of motor, emotional, social and cognitive behavior. After presenting our working definition of a "home-cage", we compare home-cage testing with more conventional out-of-cage tests (e.g., the open field) and outline the various advantages of the former, including opportunities for within-subject analyses and assessments of circadian and ultradian activity. Next, we address technical issues pertaining to the acquisition of behavioral data, such as the fine-tuning of the tracking software and the potential for integration with biotelemetry and optogenetics. Finally, we provide guidance on which behavioral measures to emphasize, how to filter, segment, and analyze behavior, and how to use analysis scripts. We summarize how the PhenoTyper has applications to study neuropharmacology as well as animal models of neurodegenerative and neuropsychiatric illness. Looking forward, we examine current challenges and the impact of new developments. Examples include the automated recognition of specific behaviors, unambiguous tracking of individuals in a social context, the development of more animal-centered measures of behavior and ways of dealing with large datasets. Together, we advocate that by embracing standardized home-cage monitoring platforms like the PhenoTyper, we are poised to directly assess issues pertaining to reproducibility, and more importantly, measure features of rodent behavior under more ethologically relevant scenarios.

Gonadal steroid hormone receptors in the medial amygdala contribute to experience-dependent changes in stress vulnerability

Social experience can generate neural plasticity that changes how individuals respond to stress. Winning aggressive encounters alters how animals respond to future challenges and leads to increased plasma testosterone concentrations and androgen receptor (AR) expression in the social behavior neural network. In this project, our aim was to identify neuroendocrine mechanisms that account for changes in stress-related behavior following the establishment of dominance relationships over a two-week period. We used a Syrian hamster model in which acute social defeat stress increases anxiety-like responses in a conditioned defeat test in males and in a social avoidance test in females. First, we administered flutamide, an AR antagonist, via intraperitoneal injections daily during the establishment of dominance relationships in male hamsters. We found that pharmacological blockade of AR prevented a reduction in conditioned defeat in dominant males and blocked an upregulation of AR in the posterior dorsal medial amygdala (MePD) and posterior ventral medial amygdala (MePV), but not in the ventral lateral septum. Next, we administered flutamide into the posterior aspects of the medial amygdala (MeP) prior to acute social defeat stress or prior to conditioned defeat testing in males. We found that pharmacological blockade of AR in the MeP prior to social defeat, but not prior to testing, increased the conditioned defeat response in dominant males and did not alter behavior in subordinates. Finally, we developed a procedure to establish dominance relationships in female hamsters and investigated status-dependent changes in plasma steroid hormone concentrations, estrogen receptor alpha (ERα) immunoreactivity, and defeat-induced social avoidance. We found that dominant female hamsters showed reduced social avoidance regardless of social defeat exposure as well as increased ERα expression in the MePD, but no status-dependent changes in the concentration of plasma steroid hormones. Overall, these findings suggest that achieving and maintaining stable social dominance leads to sex-specific neural plasticity in the MeP that underlies status-dependent changes in stress vulnerability.

Stress in groups: Lessons from non-traditional rodent species and housing models

A major feature of life in groups is that individuals experience social stressors of varying intensity and type. Social stress can have profound effects on health, social behavior, and ongoing relationships. Relationships can also buffer the experience of exogenous stressors. Social stress has most commonly been investigated in dyadic contexts in mice and rats that produce intense stress. Here we review findings from studies of diverse rodents and non-traditional group housing paradigms, focusing on laboratory studies of mice and rats housed in visible burrow systems, prairie and meadow voles, and mole-rats. We argue that the use of methods informed by the natural ecology of rodent species provides novel insights into the relationship between social stress, behavior and physiology. In particular, we describe how this ethologically inspired approach reveals how individuals vary in their experience of and response to social stress, and how ecological and social contexts impact the effects of stress. Social stress induces adaptive changes, as well as long-term disruptive effects on behavior and physiology.

Analytic and Interpretational Pitfalls to Measuring Fecal Corticosterone Metabolites in Laboratory Rats and Mice

Minimization and alleviation of stress are generally viewed as desirable aspects of laboratory animal management and use. However, achieving that goal requires an unambiguous and valid measure of stress. Glucocorticoid concentrations are commonly used as a physiologic index of stress. Measurement of glucocorticoids in blood, serum or plasma clearly reflects many types of both acute and chronic stress. However, the rapid rise in concentrations of circulating glucocorticoids that occurs even with relatively simple manipulations such as handling has led to the increased use of fecal glucocorticoid metabolite (FCM) assays, which provide a temporally integrated measure that may allow a more accurate interpretation of chronic stressors. In this review, we consider 3 aspects of glucocorticoids as a measure of stress. First, we discuss the analytic and interpretational pitfalls of using FCM concentrations as an index of stress in mice and rats. Second, we consider evidence that some degree of stress may benefit animals by priming physiologic and behavioral adaptations that render the animals more resilient in the face of stress. Finally, we use 2 situations-social housing and food restriction-to illustrate the concept of hormesis-a biologic phenomenon in which a low dose or intensity of a challenge has a beneficial effect, whereas exposure to high doses or intensities is detrimental.

Immunostimulatory effect of kumquat (Fortunella crassifolia) and its constituents, β-cryptoxanthin andR-limonene

The active constituents of kumquat in NK cell activation and anti-stress effects are β-cryptoxanthin andR-limonene.

Dominance relationships in Syrian hamsters modulate neuroendocrine and behavioral responses to social stress

Stress is a well-known risk factor for psychopathology and rodent models of social defeat have strong face, etiological, construct and predictive validity for these conditions. Syrian hamsters are highly aggressive and territorial, but after an acute social defeat experience they become submissive and no longer defend their home territory, even from a smaller, non-aggressive intruder. This defeat-induced change in social behavior is called conditioned defeat (CD). We have shown that dominant hamsters show increased neural activity in the ventromedial prefrontal cortex (vmPFC) following social defeat stress and exhibit a reduced CD response at social interaction testing compared to subordinates. Although the vmPFC can inhibit the neuroendocrine stress response, it is unknown whether dominants and subordinates differ in stress-induced activity of the extended hypothalamic-pituitary-adrenal (HPA) axis. Here, we show that, following acute social defeat, dominants exhibit decreased submissive and defensive behavior compared to subordinates but do not differ from subordinates or social status controls (SSCs) in defeat-induced cortisol concentrations. Furthermore, both dominants and SSCs show greater corticotropin-releasing hormone (CRH) mRNA expression in the basolateral/central amygdala compared to subordinates, while there was no effect of social status on CRH mRNA expression in the paraventricular nucleus of the hypothalamus or bed nucleus of the stria terminalis. Overall, status-dependent differences in the CD response do not appear linked to changes in stress-induced cortisol concentrations or CRH gene expression, which is consistent with the view that stress resilience is not a lack of a physiological stress response but the addition of stress coping mechanisms. Lay summary Dominant hamsters show resistance to the behavioral effects of acute social defeat compared to subordinates, but it is unclear whether social status modulates the neuroendocrine stress response in Syrian hamsters. This study indicates that dominant social status does not alter stress-induced activity of the extended hypothalamic-pituitary-adrenal (HPA) axis, which suggests that the ability of dominants to cope with social defeat stress is not associated with changes in their neuroendocrine stress response.

Stress-Induced Depression: Is Social Rank a Predictive Risk Factor?

An intriguing question in the field of stress is what makes an individual more likely to be susceptible or resilient to stress-induced depression. Predisposition to stress susceptibility is believed to be influenced by genetic factors and early adversity. However, beyond genetics and life experiences, recent evidence has highlighted social rank as a key determinant of susceptibility to stress, underscoring dominant individuals as the vulnerable ones. This evidence is in conflict with epidemiological, clinical, and animal work pointing at a link between social subordination and depression. Here, we review and analyze rodent protocols addressing the relevance of social rank to predict vulnerability to chronic social stress. We also discuss whether a specific social status (i.e., dominance or subordination) is the appropriate predictor of vulnerability to develop stress-induced depression or rather, the loss of social rank and resources.

Redefining neuroendocrinology: Epigenetics of brain-body communication over the life course

The brain is the central organ of stress and adaptation to stress that perceives and determines what is threatening, as well as the behavioral and physiological responses to the stressor, and it does so somewhat differently in males and females. The expression of steroid hormone receptors throughout the brain has broadened the definition of 'neuroendocrinology' to include the reciprocal communication between the entire brain and body via hormonal and neural pathways. Mediated in part via systemic hormonal influences, the adult and developing brain possess remarkable structural and functional plasticity in response to stress, including neuronal replacement, dendritic remodeling, and synapse turnover. This article is both an account of an emerging field elucidating brain-body interactions at multiple levels, from molecules to social organization, as well as a personal account of my laboratory's role and, most importantly, the roles of trainees and colleagues, along with my involvement in interdisciplinary groups working on this topic.

Stress and Cognitive Reserve as independent factors of neuropsychological performance in healthy elderly

Exposure to high levels of cortisol and self-reported stress, as well as cognitive reserve, have been linked to Alzheimer's disease pathology. However, there are no studies on the interaction of these variables. The present study aims to assess the associations of measures of cortisol, self-reported stress, and cognitive reserve with neuropsychological performance in healthy elderly people; besides, to test the interactions between these variables. Cross-sectional analyzes were conducted using data on stress, cognitive reserve and clinical conditions in 145 healthy elderly adults. A neuropsychological battery was used to assess executive functions, verbal memory and processing speed. Measurement of salivary cortisol at the circadian nadir was taken. A negative association between different stress measures and performance on tasks of memory, executive functions and processing speed was observed. Elderly people with higher cognitive reserve showed superior performance on all neuropsychological measures. No significant interaction between stress and cognitive reserve to neuropsychological performance was observed. These results indicate that older adults with high levels of stress and reduced cognitive reserve may be more susceptible to cognitive impairment.

Randall Sakai, chronic social stress, and the research tradition of Curt Richter

This paper describes Randall Sakai's professional career from graduate school at the University of Pennsylvania, through postdoctoral work at Rockefeller University, and to being an independent investigator at the University of Cincinnati. He was fortunate in having Alan Epstein, Bruce McEwen, and Eliot Stellar as mentors. Early in Sakai's graduate work, Epstein and Stellar introduced him to Curt Richter, the legendary investigator at Johns Hopkins. This early introduction to Richter and his tradition of research was crucial for Sakai's scientific development. We review Sakai's research with the Visible Burrowing System (VBS) at Cincinnati. This was the most original of Sakai's research interests. His experimental proficiency in the investigation of salt appetite, food intake, and obesity led him to focus on the effect of chronic social stress on food intake, body composition, metabolism, and the distribution of fat. He and his colleagues, many of them his students, were able to demonstrate that chronic social stress produced changes in metabolism and fat distribution that were characteristic of an incipient metabolic syndrome that could lead to obesity. This did not solve the problem, but showed the way to further investigation. This opening up of problems to experimental investigation was a hallmark of Richter's research. Thus, Sakai worked in the mainstream of the research tradition of Richter. He did what he revered.

Social context-dependent relationships between mouse dominance rank and plasma hormone levels

The associations between social status and endogenous testosterone and corticosterone have been well-studied across taxa, including rodents. Dominant social status is typically associated with higher levels of circulating testosterone and lower levels of circulating corticosterone but findings are mixed and depend upon numerous contextual factors. Here, we determine that the social environment is a key modulator of these relationships in Mus musculus. In groups of outbred CD-1 mice living in stable dominance hierarchies, we found no evidence of simple linear associations between social rank and corticosterone or testosterone plasma levels. However, in social hierarchies with highly despotic alpha males that socially suppress other group members, testosterone levels in subordinate males were significantly lower than in alpha males. In less despotic hierarchies, where all animals engage in high rates of competitive interactions, subordinate males had significantly elevated testosterone compared to agonistically inhibited subordinates from despotic hierarchies. Subordinate males from highly despotic hierarchies also had elevated levels of corticosterone compared to alpha males. In pair-housed animals, the relationship was the opposite, with alpha males exhibiting elevated levels of corticosterone compared to subordinate males. Notably, subordinate males living in social hierarchies had significantly higher levels of plasma corticosterone than pair-housed subordinate males, suggesting that living in a large group is a more socially stressful experience for less dominant individuals. Our findings demonstrate the importance of considering social context when analyzing physiological data related to social behavior and using ethologically relevant behavioral paradigms to study the complex relationship between hormones and social behavior.

A users guide to HPA axis research

Glucocorticoid hormones (cortisol and corticosterone - CORT) are the effector hormones of the hypothalamic-pituitary-adrenal (HPA) axis neuroendocrine system. CORT is a systemic intercellular signal whose level predictably varies with time of day and dynamically increases with environmental and psychological stressors. This hormonal signal is utilized by virtually every cell and physiological system of the body to optimize performance according to circadian, environmental and physiological demands. Disturbances in normal HPA axis activity profiles are associated with a wide variety of physiological and mental health disorders. Despite numerous studies to date that have identified molecular, cellular and systems-level glucocorticoid actions, new glucocorticoid actions and clinical status associations continue to be revealed at a brisk pace in the scientific literature. However, the breadth of investigators working in this area poses distinct challenges in ensuring common practices across investigators, and a full appreciation for the complexity of a system that is often reduced to a single dependent measure. This Users Guide is intended to provide a fundamental overview of conceptual, technical and practical knowledge that will assist individuals who engage in and evaluate HPA axis research. We begin with examination of the anatomical and hormonal components of the HPA axis and their physiological range of operation. We then examine strategies and best practices for systematic manipulation and accurate measurement of HPA axis activity. We feature use of experimental methods that will assist with better understanding of CORT's physiological actions, especially as those actions impact subsequent brain function. This research approach is instrumental for determining the mechanisms by which alterations of HPA axis function may contribute to pathophysiology.

Adolescent Social Isolation as a Model of Heightened Vulnerability to Comorbid Alcoholism and Anxiety Disorders

Individuals diagnosed with anxiety-related illnesses are at increased risk of developing alcoholism, exhibit a telescoped progression of this disease and fare worse in recovery, relative to alcoholics that do not suffer from a comorbid anxiety disorder. Similarly, preclinical evidence supports the notion that stress and anxiety represent major risk factors for the development of alcohol use disorder (AUD). Despite the importance of understanding the link between anxiety and alcoholism, much remains unknown about the neurobiological substrates underlying this relationship. One stumbling block has been the lack of animal models that reliably reproduce the spectrum of behaviors associated with increased vulnerability to these diseases. Here, we review the literature that has examined the behavioral and neurobiological outcomes of a simple rodent adolescent social isolation procedure and discuss its validity as a model of vulnerability to comorbid anxiety disorders and alcoholism. Recent studies have provided strong evidence that adolescent social isolation of male rats leads to the expression of a variety of behaviors linked with increased vulnerability to anxiety and/or AUD, including deficits in sensory gating and fear extinction, and increases in anxiety measures and ethanol drinking. Neurobiological studies are beginning to identify mesolimbic adaptations that may contribute to the behavioral phenotype engendered by this model. Some of these changes include increased excitability of ventral tegmental area dopamine neurons and pyramidal cells in the basolateral amygdala and significant alterations in baseline and stimulated catecholamine signaling. A growing body of evidence suggests that adolescent social isolation may represent a reliable rodent model of heightened vulnerability to anxiety disorders and alcoholism in male rats. These studies provide initial support for the face, construct, and predictive validity of this model and highlight its utility in identifying neurobiological adaptations associated with increased risk of developing these disorders.

Neuroinflammation at the interface of depression and cardiovascular disease: Evidence from rodent models of social stress

A large body of evidence has emerged linking stressful experiences, particularly from one's social environment, with psychiatric disorders. However, vast individual differences emerge in susceptibility to developing stress-related pathology which may be due to distinct differences in the inflammatory response to social stress. Furthermore, depression is an independent risk factor for cardiovascular disease, another inflammatory-related disease, and results in increased mortality in depressed patients. This review is focused on discussing evidence for stress exposure resulting in persistent or sensitized inflammation in one individual while this response is lacking in others. Particular focus will be directed towards reviewing the literature underlying the impact that neuroinflammation has on neurotransmitters and neuropeptides that could be involved in the pathogenesis of comorbid depression and cardiovascular disease. Finally, the theme throughout the review will be to explore the notion that stress-induced inflammation is a key player in the high rate of comorbidity between psychosocial disorders and cardiovascular disease.