Basal and stress-induced differences in HPA axis, 5-HT responsiveness, and hippocampal cell proliferation in two mouse lines

Preclinical Models of Chronic Stress: Adaptation or Pathology?

The experience of prolonged stress changes how individuals interact with their environment and process interoceptive cues, with the end goal of optimizing survival and well-being in the face of a now-hostile world. The chronic stress response includes numerous changes consistent with limiting further damage to the organism, including development of passive or active behavioral strategies and metabolic adjustments to alter energy mobilization. These changes are consistent with symptoms of pathology in humans, and as a result, chronic stress has been used as a translational model for diseases such as depression. While it is of heuristic value to understand symptoms of pathology, we argue that the chronic stress response represents a defense mechanism that is, at its core, adaptive in nature. Transition to pathology occurs only after the adaptive capacity of an organism is exhausted. We offer this perspective as a means of framing interpretations of chronic stress studies in animal models and how these data relate to adaptation as opposed to pathology.

Activity in a prefrontal-periaqueductal gray circuit overcomes behavioral and endocrine features of the passive coping stress response

The question of how the brain links behavioral and biological features of defensive responses has remained elusive. The importance of this problem is underscored by the observation that behavioral passivity in stress coping is associated with elevations in glucocorticoid hormones, and each may carry risks for susceptibility to a host of stress-related diseases. Past work implicates the medial prefrontal cortex (mPFC) in the top-down regulation of stress-related behaviors; however, it is unknown whether such changes have the capacity to buffer against the longer-lasting biological consequences associated with aversive experiences. Using the shock probe defensive burying test in rats to naturalistically measure behavioral and endocrine features of coping, we observed that the active behavioral component of stress coping is associated with increases in activity along a circuit involving the caudal mPFC and midbrain dorsolateral periaqueductal gray (PAG). Optogenetic manipulations of the caudal mPFC-to-dorsolateral PAG pathway bidirectionally modulated active (escape and defensive burying) behaviors, distinct from a rostral mPFC-ventrolateral PAG circuit that instead limited passive (immobility) behavior. Strikingly, under conditions that biased rats toward a passive coping response set, including exaggerated stress hormonal output and increased immobility, excitation of the caudal mPFC-dorsolateral PAG projection significantly attenuated each of these features. These results lend insight into how the brain coordinates response features to overcome passive coping and may be of importance for understanding how activated neural systems promote stress resilience.

Aggressive but not reproductive boldness in male green anole lizards correlates with baseline vasopressin activity

Across species, individuals within a population differ in their level of boldness in social encounters with conspecifics. This boldness phenotype is often stable across both time and social context (e.g., reproductive versus agonistic encounters). Various neural and hormonal mechanisms have been suggested as underlying these stable phenotypic differences, which are often also described as syndromes, personalities, and coping styles. Most studies examining the neuroendocrine mechanisms associated with boldness examine subjects after they have engaged in a social interaction, whereas baseline neural activity that may predispose behavioral variation is understudied. The present study tests the hypotheses that physical characteristics, steroid hormone levels, and baseline variation in Ile³-vasopressin (VP, a.k.a., Arg⁸-vasotocin) signaling predispose boldness during social encounters. Boldness in agonistic and reproductive contexts was extensively quantified in male green anole lizards (Anolis carolinensis), an established research organism for social behavior research that provides a crucial comparison group to investigations of birds and mammals. We found high stability of boldness across time, and between agonistic and reproductive contexts. Next, immunofluorescence was used to colocalize VP neurons with phosphorylated ribosomal protein S6 (pS6), a proxy marker of neural activity. Vasopressin-pS6 colocalization within the paraventricular and supraoptic nuclei of the hypothalamus was inversely correlated with boldness of aggressive behaviors, but not of reproductive behaviors. Our findings suggest that baseline vasopressin release, rather than solely context-dependent release, plays a role in predisposing individuals toward stable levels of displayed aggression toward conspecifics by inhibiting behavioral output in these contexts.

Behavioral effects of chronic stress in Carioca high- and low-conditioned freezing rats

Chronic unpredictable mild stress (CUMS) is a widely used model to study stress-coping strategies in rodents. Different factors have been shown to influence whether animals adopt passive or active coping responses to CUMS. Individual adaptation and susceptibility to the environment seem to play a critical role in this process. To further investigate this relationship, we examined the effects of CUMS on Carioca high- and low-conditioned freezing rats (CHF and CLF, respectively), bidirectional lines of animals selected for high and low freezing in response to contextual cues that were previously associated with footshocks. For this purpose, the behavior of CHF and CLF animals was evaluated in the contextual fear conditioning, open field, elevated T maze, and forced swimming tests before and after 21 days of CUMS. For all tests, CHF rats were more susceptible to the effects of CUMS compared to CLF. CHF animals exposed to CUMS displayed a reduction in freezing behavior, decreased number of entries and time spent in the center of the open field, greater latencies to become immobile, and increased avoidance and escaping behaviors in the elevated T maze. Overall, these findings support the hypothesis that a heightened susceptibility to the environment exerts a strong influence on coping responses to chronic stress.

Social boldness correlates with brain gene expression in male green anoles

Within populations, some individuals tend to exhibit a bold or shy social behavior phenotype relative to the mean. The neural underpinnings of these differing phenotypes - also described as syndromes, personalities, and coping styles - is an area of ongoing investigation. Although a social decision-making network has been described across vertebrate taxa, most studies examining activity within this network do so in relation to exhibited differences in behavioral expression. Our study instead focuses on constitutive gene expression in bold and shy individuals by isolating baseline gene expression profiles that influence social boldness predisposition, rather than those reflecting the results of social interaction and behavioral execution. We performed this study on male green anole lizards (Anolis carolinensis), an established model organism for behavioral research, which provides a crucial comparison group to investigations of birds and mammals. After identifying subjects as bold or shy through repeated reproductive and agonistic behavior testing, we used RNA sequencing to compare gene expression profiles between these groups within various forebrain, midbrain, and hindbrain regions. The ventromedial hypothalamus had the largest group differences in gene expression, with bold males having increased expression of neuroendocrine and neurotransmitter receptor and calcium channel genes compared to shy males. Conversely, shy males express more integrin alpha-10 in the majority of examined regions. There were no significant group differences in physiology or hormone levels. Our results highlight the ventromedial hypothalamus as an important center of behavioral differences across individuals and provide novel candidates for investigations into the regulation of individual variation in social behavior phenotype.

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.

Gaps to Address in Ecological Studies of Temperament and Physiology

Ecology is a diverse field with many researchers interested in drivers and consequences of variability within populations. Two aspects of variability that have been addressed are behavioral and physiological. While these have been shown to separately influence ecological outcomes such as survival, reproductive success and fitness, combined they could better predict within-population variability in survival and fitness. Recently there has been a focus on potential fitness outcomes of consistent behavioral traits that are referred to as personality or temperament (e.g. boldness, sociability, exploration, etc.). Given this recent focus, it is an optimal time to identify areas to supplement in this field, particularly in determining the relationship between temperament and physiological traits. To maximize progress, in this perspective paper we propose that the following two areas be addressed: (1) increased diversity of species, and (2) increased number of physiological processes studied, with an eye toward using more representative and relatively consistent measures across studies. We first highlight information that has been gleaned from species that are frequently studied to determine how animal personality relates to physiology and/or survival/fitness. We then shine a spotlight on important taxa that have been understudied and that can contribute meaningful, complementary information to this area of research. And last, we propose a brief array of physiological processes to relate to temperament, and that can significantly impact fitness, and that may be accessible in field studies.

Evolution of stress responses refine mechanisms of social rank

Social rank functions to facilitate coping responses to socially stressful situations and conditions. The evolution of social status appears to be inseparably connected to the evolution of stress. Stress, aggression, reward, and decision-making neurocircuitries overlap and interact to produce status-linked relationships, which are common among both male and female populations. Behavioral consequences stemming from social status and rank relationships are molded by aggressive interactions, which are inherently stressful. It seems likely that the balance of regulatory elements in pro- and anti-stress neurocircuitries results in rapid but brief stress responses that are advantageous to social dominance. These systems further produce, in coordination with reward and aggression circuitries, rapid adaptive responding during opportunities that arise to acquire food, mates, perch sites, territorial space, shelter and other resources. Rapid acquisition of resources and aggressive postures produces dominant individuals, who temporarily have distinct fitness advantages. For these reasons also, change in social status can occur rapidly. Social subordination results in slower and more chronic neural and endocrine reactions, a suite of unique defensive behaviors, and an increased propensity for anxious and depressive behavior and affect. These two behavioral phenotypes are but distinct ends of a spectrum, however, they may give us insights into the troubling mechanisms underlying the myriad of stress-related disorders to which they appear to be evolutionarily linked.

Experience and activity-dependent control of glucocorticoid receptors during the stress response in large-scale brain networks

The diversity of actions of the glucocorticoid stress hormones among individuals and within organs, tissues and cells is shaped by age, gender, genetics, metabolism, and the quantity of exposure. However, such factors cannot explain the heterogeneity of responses in the brain within cells of the same lineage, or similar tissue environment, or in the same individual. Here, we argue that the stress response is continuously updated by synchronized neural activity on large-scale brain networks. This occurs at the molecular, cellular and behavioral levels by crosstalk communication between activity-dependent and glucocorticoid signaling pathways, which updates the diversity of responses based on prior experience. Such a Bayesian process determines adaptation to the demands of the body and external world. We propose a framework for understanding how the diversity of glucocorticoid actions throughout brain networks is essential for supporting optimal health, while its disruption may contribute to the pathophysiology of stress-related disorders, such as major depression, and resistance to therapeutic treatments.

Relationship between exploratory activity and adrenocortical activity in the black rat (Rattus rattus)

The relationship between physiological and behavioral stress markers is documented in several rodent species. However, there is no information regarding the role of adrenocortical activity in behavior of the black rat (Rattus rattus). Therefore, we hypothesize that the adrenocortical activity of black rats varies between individuals and is related to some of the behaviors in a novel environment. To test this hypothesis, we (i) validated a method for quantifying glucocorticoid metabolites from feces (fGCMs) with an enzyme immunoassay (EIA); (ii) examined variation and diurnal rhythms of feces and GCM production; and (iii) examined the relationship between GCM levels and exploratory behavioral traits. We fulfilled the first aim (i) by successfully performing an ACTH challenge test to validate the use of a 5α-pregnane-3β,11β,21-triol-20-one EIA for measuring fGCMs. Second (ii) we detected considerable consistent interindividual variability in production of both feces and glucocorticoids. The peak production of feces occurred in the first hour of the dark cycle, the peak of fGCMs occurred approximately 3 h later. Lastly, (iii) there was no clear relationship between behavior in the hole board test and GCMs. Grooming, a typical behavioral stress marker, was negatively associated with stress reactivity, while head-dipping in the hole-board test (traditionally considered an exploratory behavior independent of stress) was not correlated with the GCMs. This study offers a first look at GCMs in the black rat, successfully validates a method for their measurement and opens possibilities for future research of the relationship between glucocorticoids and exploratory behavior in this species.

Vasopressin and post-traumatic stress disorder

Post-traumatic stress disorder (PTSD) is a debilitating psychiatric condition with a wide range of behavioral disturbances and serious consequences for both patient and society. One of the main reasons for unsuccessful therapies is insufficient knowledge about its underlying pathomechanism. In the search for centrally signaling molecules that might be relevant to the development of PTSD we focus here on arginine vasopressin (AVP). So far AVP has not been strongly implicated in PTSD, but different lines of evidence suggest a possible impact of its signaling in all clusters of PTSD symptomatology. More specifically, in laboratory rodents, AVP agonists affect behavior in a PTSD-like manner, while significant reduction of AVP signaling in the brain e.g. in AVP-deficient Brattleboro rats, ameliorated defined behavioral parameters that can be linked to PTSD symptoms. Different animal models of PTSD also show alterations in the AVP signaling in distinct brain areas. However, pharmacological treatment targeting central AVP receptors via systemic routes is hampered by possible side effects that are linked to the peripheral action of AVP as a hormone. Indeed, the V1a receptor, the most common receptor subtype in the brain, is implicated in vasoconstriction. Thus, systemic treatment with V1a receptor antagonists would be implicated in hypotonia. This implies that novel treatment concepts are needed to target AVP receptors not only at brain level but also in distinct brain areas, to offer alternative treatments for PTSD.

Activity of a vmPFC-DRN Pathway Corresponds With Resistance to Acute Social Defeat Stress

The ventromedial prefrontal cortex (vmPFC) plays a critical role in stress resilience through top-down inhibition of key stress-sensitive limbic and hindbrain structures, including the dorsal raphe nucleus (DRN). In a model of experience-dependent stress resistance, socially dominant Syrian hamsters display fewer signs of anxiety following acute social defeat when compared to subordinate or control counterparts. Further, dominants activate vmPFC neurons to a greater degree during stress than do subordinates and become stress-vulnerable following pharmacological inhibition of the vmPFC. Dominants also display fewer stress-activated DRN neurons than subordinates do, suggesting that dominance experience gates activation of vmPFC neurons that inhibit the DRN during social defeat stress. To test whether social dominance alters stress-induced activity of a vmPFC-DRN pathway, we injected a retrograde tracer, cholera toxin B (CTB), into the DRN of dominant, subordinate, and control hamsters and used a dual-label immunohistochemical approach to identify vmPFC neurons co-labeled with CTB and the defeat-induced expression of an immediate early gene, cFos. Results indicate that dominant hamsters display more cFos+ and dual-labeled cells in layers V/VI of infralimbic and prelimbic subregions of the vmPFC compared to other animals. Furthermore, vmPFC-DRN activation corresponded directly with proactive behavioral strategies during defeat, which is indicative of stress resilience. Together, results suggest that recruiting the vmPFC-DRN pathway during acute stress corresponds with resistance to the effects of social defeat in dominant hamsters. Overall, these findings indicate that a monosynaptic vmPFC-DRN pathway can be engaged in an experience-dependent manner, which has implications for behavioral interventions aimed at alleviating stress-related psychopathologies.

Does selection for behavioral and physiological performance traits alter glucocorticoid responsiveness in bank voles?

One of the key elements of an animal's Darwinian fitness is its ability to adequately respond to and cope with challenging situations. Glucocorticoid hormones, such as corticosterone, affect an organism's ability to overcome such challenges. We hypothesized that changes in the glucocorticoid response curve contribute to the evolution of increased performance during challenging conditions, and tested it on bank voles (Myodes glareolus) from a multidirectional artificial selection experiment, which involves lines selected for high aerobic exercise metabolism achieved during swimming (A - Aerobic), predatory behavior towards a cricket (P - Predatory) and ability to maintain body mass on a low-quality herbivorous diet (H - Herbivorous), as well as unselected control lines (C - Control). We elicited a glucocorticoid response either by restraining the animal or by maximum pharmacological stimulation, and measured plasma corticosterone levels at baseline, during the response and during the recovery phase. Response-level corticosterone was higher in females, and recovery from maximal level was faster than that of males. Selection did not affect baseline or stress-induced corticosterone levels, but it decreased the maximum corticosterone level in Aerobic and Predatory lines, reducing the difference between stress-induced and maximum levels. Recovery from restraint-induced corticosterone level tended to be slower in the Herbivorous than in the other lines, an effect that was stronger in females than in males. In conclusion, successful selection for increased performance in challenging conditions was not associated with changes in absolute values of the glucocorticoid response to stress, but can affect other characteristics of the glucocorticoid response curve.

Stress coping and evolution of aerobic exercise performance: corticosterone levels in voles from a selection experiment

The locomotor performance achieved in a challenging situation depends not only on physiological limitations, such as the aerobic exercise capacity, but also on behavioral characteristics, such as adequate coping with stress. The stress response is mediated largely by the hypothalamic-pituitary-adrenal (HPA) axis, through modulated release of glucocorticoids. We used a unique experimental evolution model system to test the hypothesis that the evolution of an increased aerobic exercise performance can be facilitated by modification of the glucocorticoid-related stress-coping mechanisms. Bank voles (Myodes glareolus) from 'aerobic' (A) lines, selected for 22 generations for high maximum swim-induced rate of oxygen consumption (V̇ _O2,swim), achieved a 64% higher V̇ _O2,swim than those from unselected, control lines. The temporal pattern of exercise during the swimming trial also evolved, and the A-line voles achieved V̇ _O2,swim later in the course of the trial, which indicates a modification of the stress response characteristics. Both V̇ _O2,swim and the average metabolic rate measured during the trial tended to increase with baseline corticosterone level, and decreased with the post-exercise corticosterone level. Thus, increased baseline corticosterone level promotes high metabolic performance, but a high corticosterone response to swimming acts as an inhibitor rather than stimulator of intense activity. However, neither of the corticosterone traits differed between the A-selected and control lines. Thus, the experiment did not provide evidence that evolution of increased aerobic performance is facilitated by the modification of glucocorticoid levels. The results, however, do not exclude the possibility that other aspects of the HPA axis function evolved in response to the selection.

Individual differences in inflammatory and oxidative mechanisms of stress-related mood disorders

Emotional stress leads to the development of peripheral disorders and is recognized as a modifiable risk factor for psychiatric disorders, particularly depression and anxiety. However, not all individuals develop the negative consequences of emotional stress due to different stress coping strategies and resilience to stressful stimuli. In this review, we discuss individual differences in coping styles and the potential mechanisms that contribute to individual vulnerability to stress, such as parameters of the immune system and oxidative state. Initial differences in inflammatory and oxidative processes determine resistance to stress and stress-related disorders via the alteration of neurotransmitter content in the brain and biological fluids. Differences in coping styles may serve as possible predictors of resistance to stress and stress-related disorders, even before stressful conditions. The investigation of natural variabilities in stress resilience may allow the development of new methods for preventive medicine and the personalized treatment of stress-related conditions.

Prefrontal-Bed Nucleus Circuit Modulation of a Passive Coping Response Set

One of the challenges facing neuroscience entails localization of circuits and mechanisms accounting for how multiple features of stress responses are organized to promote survival during adverse experiences. The rodent medial prefrontal cortex (mPFC) is generally regarded as a key site for cognitive and affective information processing, and the anteroventral bed nuclei of the stria terminalis (avBST) integrates homeostatic information from a variety of sources, including the mPFC. Thus, we proposed that the mPFC is capable of generating multiple features (endocrine, behavioral) of adaptive responses via its influence over the avBST. To address this possibility, we first optogenetically inhibited input to avBST from the rostral prelimbic cortical region of mPFC and observed concurrent increases in immobility and hypothalamo-pituitary-adrenal (HPA) output in male rats during tail suspension, whereas photostimulation of this pathway decreased immobility during the same challenge. Anatomical tracing experiments confirmed projections from the rostral prelimbic subfield to separate populations of avBST neurons, and from these to HPA effector neurons in the paraventricular hypothalamic nucleus, and to aspects of the midbrain periaqueductal gray that coordinate passive defensive behaviors. Finally, stimulation and inhibition of the prelimbic-avBST pathway, respectively, decreased and increased passive coping in the shock-probe defensive burying test, without having any direct effect on active coping (burying) behavior. These results define a new neural substrate in the coordination of a response set that involves the gating of passive, rather than active, coping behaviors while restraining neuroendocrine activation to optimize adaptation during threat exposure.SIGNIFICANCE STATEMENT The circuits and mechanisms accounting for how multiple features of responses are organized to promote adaptation have yet to be elucidated. Our report identifies a prefrontal-bed nucleus pathway that organizes a response set capable of gating passive coping behaviors while concurrently restraining neuroendocrine activation during exposure to inescapable stressors. These data provide insight into the central organization of how multiple features of responses are integrated to promote adaptation during adverse experiences, and how disruption in one neural pathway may underlie a broad array of maladaptive responses in stress-related psychiatric disorders.

The Lateral Habenula Directs Coping Styles Under Conditions of Stress via Recruitment of the Endocannabinoid System

BACKGROUND

The ability to effectively cope with stress is a critical determinant of disease susceptibility. The lateral habenula (LHb) and the endocannabinoid (ECB) system have independently been shown to be involved in the selection of stress coping strategies, yet the role of ECB signaling in the LHb remains unknown.

METHODS

Using a battery of complementary techniques in rats, we examined the localization of type-1 cannabinoid receptors (CB1Rs) and assessed the behavioral and neuroendocrine effects of intra-LHb CB1R manipulations. We further tested the extent to which the ECB system in the LHb is impacted following chronic unpredictable stress or social defeat stress, and whether manipulation of LHb CB1Rs can bias coping strategies in rats with a history of chronic stress.

RESULTS

Electron microscopy studies revealed CB1R expression on presynaptic axon terminals, postsynaptic membranes, mitochondria, and glial processes in the rat LHb. In vivo microdialysis experiments indicated that acute stress increased the amount of 2-arachidonoylglycerol in the LHb, while intra-LHb CB1R blockade increased basal corticosterone, augmented proactive coping strategies, and reduced anxiety-like behavior. Basal LHb 2-arachidonoylglycerol content was similarly elevated in rats that were subjected to chronic unpredictable stress or social defeat stress and positively correlated with adrenal weight. Finally, intra-LHb CB1R blockade increased proactive behaviors in response to a novel conspecific, increasing approach behaviors irrespective of stress history and decreasing the latency to be attacked during an agonistic encounter.

CONCLUSIONS

Alterations in LHb ECB signaling may be relevant for development of stress-related pathologies in which LHb dysfunction and stress-coping impairments are hallmark symptoms.

The Role of the Lateral Hypothalamus in Violent Intraspecific Aggression-The Glucocorticoid Deficit Hypothesis

This review argues for a central role of the lateral hypothalamus in those deviant forms of aggression, which result from chronic glucocorticoid deficiency. Currently, this nucleus is considered a key region of the mechanisms that control predatory aggression. However, recent findings demonstrate that it is strongly activated by aggression in subjects with a chronically downregulated hypothalamus-pituitary-adrenocortical (HPA) axis; moreover, this activation is causally involved in the emergence of violent aggression. The review has two parts. In the first part, we review human findings demonstrating that under certain conditions, strong stressors downregulate the HPA-axis on the long run, and that the resulting glucocorticoid deficiency is associated with violent aggression including aggressive delinquency and aggression-related psychopathologies. The second part addresses neural mechanisms in animals. We show that the experimental downregulation of HPA-axis function elicits violent aggression in rodents, and the activation of the brain circuitry that originally subserves predatory aggression accompanies this change. The lateral hypothalamus is not only an integral part of this circuitry, but can elicit deviant and violent forms of aggression. Finally, we formulate a hypothesis on the pathway that connects unfavorable social conditions to violent aggression via the neural circuitry that includes the lateral hypothalamus.

Anteroventral bed nuclei of the stria terminalis neurocircuitry: Towards an integration of HPA axis modulation with coping behaviors - Curt Richter Award Paper 2017

A network of interconnected cell groups in the limbic forebrain regulates hypothalamic-pituitary-adrenal (HPA) axis activation and behavioral responses to emotionally stressful experiences, and chronic disruption of these systems chronically is implicated in the pathogenesis of psychiatric illnesses. A significant challenge has been to unravel the circuitry and mechanisms providing for regulation of HPA activity, as these limbic forebrain regions do not provide any direct innervation of HPA effector cell groups in the paraventricular hypothalamus (PVH). Moreover, information regarding how endocrine and behavioral responses are integrated has remained obscure. Here we summarize work from our laboratory showing that anteroventral (av) bed nuclei of the stria terminalis (BST) acts as a point of convergence between the limbic forebrain and PVH, receiving and coordinating upstream influences, and restraining HPA axis output in response to inescapable stressors. Recent studies highlight a more expansive modulatory role for avBST as one that coordinates HPA-inhibitory influences while concurrently suppressing passive behavioral responses via divergent pathways. avBST is uniquely positioned to convey endocrine and behavioral alterations resulting from chronic stress exposure, such as HPA axis hyperactivity and increased passive coping strategies, that may result from synaptic reorganization in upstream limbic cortical regions. We discuss how these studies give new insights into understanding the systems-level organization of stress response circuitry, the neurobiology of coping styles, and BST circuit dysfunction in stress-related psychiatric disorders.

Glutamatergic stimulation of the left dentate gyrus abolishes depressive-like behaviors in a rat learned helplessness paradigm

BACKGROUND

Episodic experiences of stress have been identified as the leading cause of major depressive disorder (MDD). The occurrence of MDD is profoundly influenced by the individual's coping strategy, rather than the severity of the stress itself. Resting brain activity has been shown to alter in several mental disorders. However, the functional relationship between resting brain activity and coping strategies has not yet been studied. In the present study, we observed different patterns of resting brain activity in rats that had determined either positive (resilient to stress) or negative (vulnerable to stress) coping strategies, and examined whether modulation of the preset resting brain activity could influence the behavioral phenotype associated with negative coping strategy (i.e., depressive-like behaviors).

METHODS

We used a learned helplessness paradigm-a well-established model of MDD-to detect coping strategies. Differences in resting state brain activity between animals with positive and negative coping strategies were assessed using ¹⁸F-fluorodeoxyglucose positron emission tomography (FDG-PET). Glutamatergic stimulation was used to modulate resting brain activity.

RESULTS

After exposure to repeated uncontrollable stress, seven of 23 rats exhibited positive coping strategies, while eight of 23 rats exhibited negative coping strategies. Increased resting brain activity was observed only in the left ventral dentate gyrus of the positive coping rats using FDG-PET. Furthermore, glutamatergic stimulation of the left dentate gyrus abolished depressive-like behaviors in rats with negative coping strategies.

CONCLUSION

Increased resting brain activity in the left ventral dentate gyrus helps animals to select positive coping strategies in response to future stress.

The role of central and medial amygdala in normal and abnormal aggression: A review of classical approaches

The involvement of the amygdala in aggression is supported by overwhelming evidence. Frequently, however, the amygdala is studied as a whole, despite its complex internal organization. To reveal the role of various subdivisions, here we review the involvement of the central and medial amygdala in male rivalry aggression, maternal aggression, predatory aggression, and models of abnormal aggression where violent behavior is associated with increased or decreased arousal. We conclude that: (1) rivalry aggression is controlled by the medial amygdala; (2) predatory aggression is controlled by the central amygdala; (3) hypoarousal-associated violent aggression recruits both nuclei, (4) a specific upregulation of the medial amygdala was observed in hyperarousal-driven aggression. These patterns of amygdala activation were used to build four alternative models of the aggression circuitry, each being specific to particular forms of aggression. The separate study of the roles of amygdala subdivisions may not only improve our understanding of aggressive behavior, but also the differential control of aggression and violent behaviors of various types, including those associated with various psychopathologies.

Behavioral Effects of Systemic, Infralimbic and Prelimbic Injections of a Serotonin 5-HT2A Antagonist in Carioca High- and Low-Conditioned Freezing Rats

The role of serotonin (5-hydroxytryptamine [5-HT]) and 5-HT2A receptors in anxiety has been extensively studied, mostly without considering individual differences in trait anxiety. Our laboratory developed two lines of animals that are bred for high and low freezing responses to contextual cues that are previously associated with footshock (Carioca High-conditioned Freezing [CHF] and Carioca Low-conditioned Freezing [CLF]). The present study investigated whether ketanserin, a preferential 5-HT2A receptor blocker, exerts distinct anxiety-like profiles in these two lines of animals. In the first experiment, the animals received a systemic injection of ketanserin and were exposed to the elevated plus maze (EPM). In the second experiment, these two lines of animals received microinjections of ketanserin in the infralimbic (IL) and prelimbic (PL) cortices and were exposed to either the EPM or a contextual fear conditioning paradigm. The two rat lines exhibited bidirectional effects on anxiety-like behavior in the EPM and opposite responses to ketanserin. Both systemic and intra-IL cortex injections of ketanserin exerted anxiolytic-like effects in CHF rats but anxiogenic-like effects in CLF rats. Microinjections of ketanserin in the PL cortex also exerted anxiolytic-like effects in CHF rats but had no effect in CLF rats. These results suggest that the behavioral effects of 5-HT2A receptor antagonism might depend on genetic variability associated with baseline reactions to threatening situations and 5-HT2A receptor expression in the IL and PL cortices. Highlights -CHF and CLF rats are two bidirectional lines that are based on contextual fear conditioning.-CHF rats have a more "anxious" phenotype than CLF rats in the EPM.-The 5-HT2A receptor antagonist ketanserin had opposite behavioral effects in CHF and CLF rats.-Systemic and IL injections either decreased (CHF) or increased (CLF) anxiety-like behavior.-PL injections either decreased (CHF) anxiety-like behavior or had no effect (CLF).

Stress-responsiveness influences baseline glucocorticoid levels: Revisiting the under 3min sampling rule

Plasma glucocorticoid (CORT) levels collected within 3min of capture are commonly believed to reflect pre-stressor, baseline CORT levels. Differences in these "baseline" values are often interpreted as reflecting differences in health, or the amount of social and environmental stress recently experienced by an individual. When interpreting "baseline" values it is generally assumed that any effect of capture-and-handling during the initial sampling period is small enough and consistent enough among individuals to not obscure pre-capture differences in CORT levels. However, plasma CORT increases in less than 3min post-capture in many free-living, endothermic species in which timing has been assessed. In addition, the rate of CORT secretion and the maximum level attained (i.e., the degree of stress-responsiveness) during a severe stressor often differs among individuals of the same species. In Florida scrub-jays (Aphelocoma coerulescens), an individual's stress-responsiveness during a 30min post-capture stressor is correlated with CORT levels in samples collected within 1.5min of capture, suggesting there is an intrinsic connection between stress-responsiveness and pre-capture CORT levels. Although differences in stress-responsiveness accounted for just 11% of the variance in these samples, on average, higher stress-responsive jays (top third of individuals) had baseline values twice that of lower stress-responsive jays (bottom third). Further, plasma CORT levels begin to increase around 2min post-capture in this species, but the rate of increase between 2 and 3min differs markedly with CORT increasing more rapidly in jays with higher stress-responsiveness. Together, these data indicate that baseline CORT values can be influenced by an individual's stress response phenotype and the differences due to stress-responsiveness can be exaggerated during sample collection. In some cases, the effects of differences in stress-responsiveness and the increase in CORT during sample collection could obscure, or supersede, differences in pre-capture plasma CORT levels that are caused by extrinsic factors.

On the Role of Neurogenesis and Neural Plasticity in the Evolution of Animal Personalities and Stress Coping Styles

Individual variation in how animals react to stress and environmental change has become a central topic in a wide range of biological disciplines, from evolutionary ecology to biomedicine. Such variation manifests phenotypically as correlated trait-clusters (referred to as coping styles, behavioral syndromes, shyness-boldness, or personality traits). Thresholds for switching from active coping (fight-flight) to inhibition and passive behavior when exposed to stress depend on experience and genetic factors. Comparative research has revealed a range of neuroendocrine-behavioral associations which are conserved throughout the vertebrate subphylum, including factors affecting perception, learning, and memory of stimuli and events. Here we review conserved aspects of the contribution of neurogenesis and other aspects of neural plasticity to stress coping. In teleost fish, brain cell proliferation and neurogenesis have received recent attention. This work reveals that brain cell proliferation and neurogenesis are associated with heritable variation in stress coping style, and they are also differentially affected by short- and long-term stress in a biphasic manner. Routine-dependent and inflexible behavior in proactive individuals is associated with limited neural plasticity. These evolutionarily conserved relationships hold the potential to illuminate the biological background for stress-related neurobiological disorders.

The neurobiological basis of human aggression: A review on genetic and epigenetic mechanisms

Aggression is an evolutionary conserved behavior present in most species including humans. Inadequate aggression can lead to long-term detrimental personal and societal effects. Here, we differentiate between proactive and reactive forms of aggression and review the genetic determinants of it. Heritability estimates of aggression in general vary between studies due to differing assessment instruments for aggressive behavior (AB) as well as age and gender of study participants. In addition, especially non-shared environmental factors shape AB. Current hypotheses suggest that environmental effects such as early life stress or chronic psychosocial risk factors (e.g., maltreatment) and variation in genes related to neuroendocrine, dopaminergic as well as serotonergic systems increase the risk to develop AB. In this review, we summarize the current knowledge of the genetics of human aggression based on twin studies, genetic association studies, animal models, and epigenetic analyses with the aim to differentiate between mechanisms associated with proactive or reactive aggression. We hypothesize that from a genetic perspective, the aminergic systems are likely to regulate both reactive and proactive aggression, whereas the endocrine pathways seem to be more involved in regulation of reactive aggression through modulation of impulsivity. Epigenetic studies on aggression have associated non-genetic risk factors with modifications of the stress response and the immune system. Finally, we point to the urgent need for further genome-wide analyses and the integration of genetic and epigenetic information to understand individual differences in reactive and proactive AB. © 2015 Wiley Periodicals, Inc.

Altered Hippocampal Neurogenesis and Amygdalar Neuronal Activity in Adult Mice with Repeated Experience of Aggression

Repeated experience of winning in a social conflict setting elevates levels of aggression and may lead to violent behavioral patterns. Here, we use a paradigm of repeated aggression and fighting deprivation to examine changes in behavior, neurogenesis, and neuronal activity in mice with positive fighting experience. We show that for males, repeated positive fighting experience induces persistent demonstration of aggression and stereotypic behaviors in daily agonistic interactions, enhances aggressive motivation, and elevates levels of anxiety. When winning males are deprived of opportunities to engage in further fights, they demonstrate increased levels of aggressiveness. Positive fighting experience results in increased levels of progenitor cell proliferation and production of young neurons in the hippocampus. This increase is not diminished after a fighting deprivation period. Furthermore, repeated winning experience decreases the number of activated (c-fos-positive) cells in the basolateral amygdala and increases the number of activated cells in the hippocampus; a subsequent no-fight period restores the number of c-fos-positive cells. Our results indicate that extended positive fighting experience in a social conflict heightens aggression, increases proliferation of neuronal progenitors and production of young neurons in the hippocampus, and decreases neuronal activity in the amygdala; these changes can be modified by depriving the winners of the opportunity for further fights.

Habitat complexity, environmental change and personality: A tropical perspective

Tropical rainforests are species-rich, complex ecosystems. They are increasingly being negatively affected by anthropogenic activity, which is rapidly and unpredictably altering their structure and complexity. These changes in habitat state may expose tropical animals to novel and unpredictable conditions, potentially increasing their extinction risk. However, an animal's ability to cope with environmental change may be linked to its personality. While numerous studies have investigated environmental influences on animal personalities, few are focused on tropical species. In this review, we consider how behavioural syndromes in tropical species might facilitate coping under, and adapting to, increasing disturbance. Given the complexity of tropical rainforests, we first discuss how habitat complexity influences personality traits and physiological stress in general. We then explore the ecological and evolutionary implications of personality in the tropics in the context of behavioural flexibility, range expansion and speciation. Finally, we discuss the impact that anthropogenic environmental change may have on the ecological integrity of tropical rainforests, positing scenarios for species persistence. Maintaining tropical rainforest complexity is crucial for driving behavioural flexibility and personality type, both of which are likely to be key factors facilitating long term persistence in disturbed habitats.

Neurobiological mechanisms supporting experience-dependent resistance to social stress

Humans and other animals show a remarkable capacity for resilience following traumatic, stressful events. Resilience is thought to be an active process related to coping with stress, although the cellular and molecular mechanisms that support active coping and stress resistance remain poorly understood. In this review, we focus on the neurobiological mechanisms by which environmental and social experiences promote stress resistance. In male Syrian hamsters, exposure to a brief social defeat stressor leads to increased avoidance of novel opponents, which we call conditioned defeat. Also, hamsters that have achieved dominant social status show reduced conditioned defeat as well as cellular and molecular changes in the neural circuits controlling the conditioned defeat response. We propose that experience-dependent neural plasticity occurs in the prelimbic (PL) cortex, infralimbic (IL) cortex, and ventral medial amygdala (vMeA) during the maintenance of dominance relationships, and that adaptations in these neural circuits support stress resistance in dominant individuals. Overall, behavioral treatments that promote success in competitive interactions may represent valuable interventions for instilling resilience.

Individual differences in the effects of prenatal stress exposure in rodents

Exposure to prenatal stress alters the phenotype of the offspring in adulthood. When the prenatal and adult environments do not match, these alterations may induce pathology risk. There are, however, large individual differences in the effects of prenatal stress. While some individuals seem vulnerable, others appear to be relatively resistant to its effects. In this review we discuss potential mechanisms underlying these individual differences with a focus on animal models. Differences between rodent models selected for stress coping traits are discussed. In addition, the role of circulating factors, like glucocorticoids and cytokines, factors involved in brain development and influences of epigenetic and genetic factors in prenatal stress induced phenotype are covered.

Serotonin and stress coping

Coping is the necessary outcome of any stressful situation and the major determinant of stress resilience. Coping strategies can be divided into two broad categories, based on the presence (active) or absence (passive) of attempts to act upon the stressor. The role of brain serotonin (5-hydroxytryptamine, 5-HT) in coping behavior that is emerging from studies in animals and humans is the subject of this article. We have focused attention on studies that consider the coping behavior exhibited when the individual is faced with a new stressful experience. Coping styles characterize different species with different evolutionary histories, from fishes to mammals, and evidence shows that serotonin transmission in the central nervous system, with differences in transporter, receptor types and hormone or neurotransmitter influences is critical in determining coping behavior. Moreover, a major role of environmental challenges throughout the lifespan affects brain systems that control coping outcomes through 5-HT transmission. In particular early experiences, for their long-term effects in adulthood, and social experiences throughout the life span, for the effects on serotonin functioning, received attention in preclinical research because of their parallelism in humans and animals. Based on growing evidence pointing to a medial prefrontal cortex-amygdala system in mediating adaptive and maladaptive stress responses, we propose a brain circuit in which serotonin neurons in the dorsal raphe depending on the CRF (corticotropin releasing factor) regulatory action engage a prefrontal cortical-amygdala pathway through 5-HT1A receptors, GABA and Glutamate to moderate coping behavior.

Maintenance of dominance status is necessary for resistance to social defeat stress in Syrian hamsters

Resilience is an active process that involves a discrete set of neural substrates and cellular mechanisms and enables individuals to avoid some of the negative consequences of extreme stress. We have previously shown that dominant individuals show less stress-induced changes in behavior compared to subordinates using a conditioned defeat model in male Syrian hamsters (Mesocricetus auratus). To rule out pre-existing differences between dominants and subordinates, we examined whether 14 days of dominance experience is required to reduce the conditioned defeat response and whether the development of conditioned defeat resistance correlates with defeat-induced neural activation in select brain regions. We paired hamsters in daily 5-min aggressive encounters for 1, 7, or 14 days and then exposed animals to 3, 5-min social defeat episodes. The next day animals received conditioned defeat testing which involved a 5-min social interaction test with a non-aggressive intruder. In separate animals brains were collected after social defeat for c-Fos immunohistochemistry. We found that 14-day dominants showed a decreased conditioned defeat response compared to 14-day subordinates and controls, while 1-day and 7-day dominants did not differ from their subordinate counterparts. Also, the duration of dominance relationship was associated with distinct patterns of defeat-induced neural activation such that only 14-day dominants showed elevated c-Fos immunoreactivity in the ventral medial prefrontal cortex, medial amygdala, and lateral portions of the ventral medial hypothalamus. Our data suggest that resistance to social stress develops during the maintenance of dominance relationships and is associated with experience-dependent neural plasticity in select brain regions.

The glucocorticoid/aggression relationship in animals and humans: an analysis sensitive to behavioral characteristics, glucocorticoid secretion patterns, and neural mechanisms

Glucocorticoids control a wide array of biological processes from glucose homeostasis to neuronal function. The mechanisms mediating their effects are similarly varied and include rapid and transient nongenomic effects on calcium trafficking, various neurotransmitter receptors, and other membrane/cytoplasmic proteins, as well as slowly developing but durable genomic effects that are mediated by a large number of glucocorticoid-sensitive genes that are affected after variable lag-times. Given this complexity, we suggest that the aggression/glucocorticoid relationship cannot be reduced to the simple "stimulation/inhibition" question. Here, we review the effects of glucocorticoids on aggression by taking into account the complexities of glucocorticoid actions. Acute and chronic effects were differentiated because these are mediated by different mechanisms. The effects of chronic increases and decreases in glucocorticoid production were discussed separately, because the activation of mechanisms that are not normally activated and the loss of normal functions should not be confounded. Findings in healthy/normal subjects and those obtained in subjects that show abnormal forms of behavior or psychopathologies were also differentiated, because the effects of glucocorticoids are indirect, and largely depend on the properties of neurons they act upon, which are altered in subjects with psychopathologies. In addition, the conditions of glucocorticoid measurements were also thoroughly evaluated. Although the role of glucocorticoids in aggression is perceived as controversial by many investigators, a detailed analysis that is sensitive to glucocorticoid and behavioral measure as well as to the mediating mechanism suggests that this role is rather clear-cut; moreover, there is a marked similarity between animal and human findings.

Relations between peripheral and brain serotonin measures and behavioural responses in a novelty test in pigs

Pigs differ in their behavioural responses towards environmental challenges. Individual variation in maladaptive responses such as tail biting, may partly originate from underlying biological characteristics related to (emotional) reactivity to challenges and serotonergic system functioning. Assessing relations between behavioural responses and brain and blood serotonin parameters may help in understanding susceptibility to the development of maladaptive responses. The objective of the current study was, therefore, to assess the relationship between the pigs' serotonergic parameters measured in both blood and brain, and the behaviour of pigs during a novelty test. Pigs (n=31) were subjected to a novelty test at 11weeks of age, consisting of 5-min novel environment exposure after which a novel object (a bucket) was introduced for 5min. Whole blood serotonin, platelet serotonin level, and platelet serotonin uptake were determined at 13weeks of age. Levels of serotonin, its metabolite and serotonin turnover were determined at 19weeks of age in the frontal cortex, hypothalamus and hippocampus. The behaviour of the pigs was different during exposure to a novel object compared to the novel environment only, with more fear-related behaviours exhibited during novel object exposure. Platelet serotonin level and brain serotonergic parameters in the hippocampus were interrelated. Notably, the time spent exploring the test arena was significantly correlated with both platelet serotonin level and right hippocampal serotonin activity (turnover and concentration). In conclusion, the existence of an underlying biological trait - possibly fearfulness - may be involved in the pig's behavioural responses toward environmental challenges, and this is also reflected in serotonergic parameters.

Inflammatory pain and corticosterone response in infant rats: effect of 5-HT1A agonist buspirone prior to gestational stress

Our researches have shown that gestational stress causes exacerbation of inflammatory pain in the offspring; the maternal 5-HT1A agonist buspirone before the stress prevents the adverse effect. The serotonergic system and hypothalamo-pituitary-adrenal (HPA) axis are closely interrelated. However, interrelations between inflammatory pain and the HPA axis during the hyporeactive period of the latter have not been studied. The present research demonstrates that formalin-induced pain causes a gradual and prolonged increase in plasma corticosterone level in 7-day-old male rats; twenty-four hours after injection of formalin, the basal corticosterone level still exceeds the initial basal corticosterone value. Chronic treatments of rat dams with buspirone before restraint stress during gestation normalize in the offspring pain-like behavior and induce during the acute phase in the formalin test the stronger corticosterone increase as compared to the stress hormonal elevation in animals with other prenatal treatments. Negative correlation between plasma corticosterone level and the number of flexes+shakes is revealed in buspirone+stress rats. The new data enhance the idea about relativity of the HPA axis hyporeactive period and suggest that maternal buspirone prior to stress during gestation may enhance an adaptive mechanism of the inflammatory nociceptive system in the infant male offspring through activation of the HPA axis peripheral link.

The neurobiology of abnormal manifestations of aggression--a review of hypothalamic mechanisms in cats, rodents, and humans

Aggression research was for long dominated by the assumption that aggression-related psychopathologies result from the excessive activation of aggression-promoting brain mechanisms. This assumption was recently challenged by findings with models of aggression that mimic etiological factors of aggression-related psychopathologies. Subjects submitted to such procedures show abnormal attack features (mismatch between provocation and response, disregard of species-specific rules, and insensitivity toward the social signals of opponents). We review here 12 such laboratory models and the available human findings on the neural background of abnormal aggression. We focus on the hypothalamus, a region tightly involved in the execution of attacks. Data show that the hypothalamic mechanisms controlling attacks (general activation levels, local serotonin, vasopressin, substance P, glutamate, GABA, and dopamine neurotransmission) undergo etiological factor-dependent changes. Findings suggest that the emotional component of attacks differentiates two basic types of hypothalamic mechanisms. Aggression associated with increased arousal (emotional/reactive aggression) is paralleled by increased mediobasal hypothalamic activation, increased hypothalamic vasopressinergic, but diminished hypothalamic serotonergic neurotransmission. In aggression models associated with low arousal (unemotional/proactive aggression), the lateral but not the mediobasal hypothalamus is over-activated. In addition, the anti-aggressive effect of serotonergic neurotransmission is lost and paradoxical changes were noticed in vasopressinergic neurotransmission. We conclude that there is no single 'neurobiological road' to abnormal aggression: the neural background shows qualitative, etiological factor-dependent differences. Findings obtained with different models should be viewed as alternative mechanisms rather than conflicting data. The relevance of these findings for understanding and treating of aggression-related psychopathologies is discussed. This article is part of a Special Issue entitled 'Extrasynaptic ionotropic receptors'.

Fetal programming by maternal stress: Insights from a conflict perspective

Maternal stress during pregnancy has pervasive effects on the offspring's physiology and behavior, including the development of anxious, reactive temperament and increased stress responsivity. These outcomes can be seen as the result of adaptive developmental plasticity: maternal stress hormones carry useful information about the state of the external world, which can be used by the developing fetus to match its phenotype to the predicted environment. This account, however, neglects the inherent conflict of interest between mother and fetus about the outcomes of fetal programming. The aim of this paper is to extend the adaptive model of prenatal stress by framing mother-fetus interactions in an evolutionary conflict perspective. In the paper, I show how a conflict perspective provides many new insights in the functions and mechanisms of fetal programming, with particular emphasis on human pregnancy. I then take advantage of those insights to make sense of some puzzling features of maternal and fetal physiology and generate novel empirical predictions.

Poor sleep as a potential causal factor in aggression and violence

Clinical observations suggest that sleep problems may be a causal factor in the development of reactive aggression and violence. In this review we give an overview of existing literature on the relation between poor sleep and aggression, irritability, and hostility. Correlational studies are supporting such a relationship. Although limited in number, some studies suggest that treatment of sleep disturbances reduces aggressiveness and problematic behavior. In line with this is the finding that sleep deprivation actually increases aggressive behavior in animals and angriness, short-temperedness, and the outward expression of aggressive impulses in humans. In most people poor sleep will not evoke actual physical aggression, but certain individuals, such as forensic psychiatric patients, may be particularly vulnerable to the emotional dysregulating effects of sleep disturbances. The relation between sleep problems and aggression may be mediated by the negative effect of sleep loss on prefrontal cortical functioning. This most likely contributes to loss of control over emotions, including loss of the regulation of aggressive impulses to context-appropriate behavior. Other potential contributing mechanisms connecting sleep problems to aggression and violence are most likely found within the central serotonergic and the hypothalamic-pituitary-adrenal-axis. Individual variation within these neurobiological systems may be responsible for amplified aggressive responses induced by sleep loss in certain individuals. It is of great importance to identify the individuals at risk, since recognition and adequate treatment of their sleep problems may reduce aggressive and violent incidents.

Neural plasticity is affected by stress and heritable variation in stress coping style

Here we use a comparative model to investigate how behavioral and physiological traits correlate with neural plasticity. Selection for divergent post-stress cortisol levels in rainbow trout (Oncorhynchus mykiss) has yielded low- (LR) and high responsive (HR) lines. Recent reports show low behavioral flexibility in LR compared to HR fish and we hypothesize that this divergence is caused by differences in neural plasticity. Genes involved in neural plasticity and neurogenesis were investigated by quantitative PCR in brains of LR and HR fish at baseline conditions and in response to two different stress paradigms: short-term confinement (STC) and long-term social (LTS) stress. Expression of proliferating cell nuclear antigen (PCNA), neurogenic differentiation factor (NeuroD) and doublecortin (DCX) was generally higher in HR compared to LR fish. STC stress led to increased expression of PCNA and brain-derived neurotrophic factor (BDNF) in both lines, whereas LTS stress generally suppressed PCNA and NeuroD expression while leaving BDNF expression unaltered. These results indicate that the transcription of neuroplasticity-related genes is associated with variation in coping style, while also being affected by STC - and LTS stress in a biphasic manner. A higher degree of neural plasticity in HR fish may provide the substrate for enhanced behavioral flexibility.

Coping styles and behavioural flexibility: towards underlying mechanisms

A coping style (also termed behavioural syndrome or personality) is defined as a correlated set of individual behavioural and physiological characteristics that is consistent over time and across situations. This relatively stable trait is a fundamental and adaptively significant phenomenon in the biology of a broad range of species, i.e. it confers differential fitness consequences under divergent environmental conditions. Behavioural flexibility appears to be an important underlying attribute or feature of the coping style that might explain consistency across situations. Proactive coping is characterized by low flexibility expressed as rather rigid, routine-like behavioural tendencies and reduced impulse control (behavioural inhibition) in operant conditioning paradigms. This article summarizes some of the evidence that individual differentiation in behavioural flexibility emerges as a function of underlying variability in the activation of a brain circuitry that includes the prefrontal cortex and its key neurochemical signalling pathways (e.g. dopaminergic and serotonergic input). We argue that the multidimensional nature of animal personality and the terminology used for the various dimensions should reflect the differential pattern of activation of the underlying neuronal network and the behavioural control function of its components. Accordingly, unravelling the molecular mechanisms that give rise to individual differences in the coping style will be an important topic in biobehavioural neurosciences, ecology and evolutionary biology.

Neuroendocrinology of coping styles: towards understanding the biology of individual variation

Individual variation in behavior and physiology is a widespread and ecologically functional phenomenon in nature in virtually all vertebrate species. Due to domestication of laboratory animals, studies may suffer from a strong selection bias. This paper summarizes behavioral, neuroendocrine and neurobiological studies using the natural individual variation in rats and mice. Individual behavioral characteristics appear to be consistent over time and across situations. The individual variation has at least two dimensions in which the quality of the response to a challenging condition (coping style) is independent from the quantity of that response (stress reactivity). The neurobiology reveals important differences in the homeostatic control of the serotonergic neuron and the neuropeptides vasopressin and oxytocin in relation to coping style. It is argued that a careful exploitation of the broad natural and biologically functional individual variation in behavior and physiology may help in developing better animal models for understanding individual disease vulnerability.

Differential changes in steroid hormones before competition in bonobos and chimpanzees

A large body of research has demonstrated that variation in competitive behavior across species and individuals is linked to variation in physiology. In particular, rapid changes in testosterone and cortisol during competition differ according to an individual's or species' psychological and behavioral responses to competition. This suggests that among pairs of species in which there are behavioral differences in competition, there should also be differences in the endocrine shifts surrounding competition. We tested this hypothesis by presenting humans' closest living relatives, chimpanzees (Pan troglodytes) and bonobos (Pan paniscus), with a dyadic food competition and measuring their salivary testosterone and cortisol levels. Given that chimpanzees and bonobos differ markedly in their food-sharing behavior, we predicted that they would differ in their rapid endocrine shifts. We found that in both species, males showed an anticipatory decrease (relative to baseline) in steroids when placed with a partner in a situation in which the two individuals shared food, and an anticipatory increase when placed with a partner in a situation in which the dominant individual obtained more food. The species differed, however, in terms of which hormone was affected; in bonobo males the shifts occurred in cortisol, whereas in chimpanzee males the shifts occurred in testosterone. Thus, in anticipation of an identical competition, bonobo and chimpanzee males showed differential endocrine shifts, perhaps due to differences in perception of the situation, that is, viewing the event either as a stressor or a dominance contest. In turn, common selection pressures in human evolution may have acted on the psychology and the endocrinology of our competitive behavior.

Inter-individual differences in neurobiology as vulnerability factors for affective disorders: implications for psychopharmacology

Susceptibility to affective disorders is individually different, and determined both by genetic variance and life events that cause significant differences in the CNS structure and function between individual subjects. Therefore it is plausible that search for the inter-individual differences in endophenotypes that mediate the effects of causal factors, both genetic and environmental, will reveal the substrates for vulnerability, help to clarify pathogenetic mechanisms, and possibly aid in developing strategies to discover better, more personalized treatments. This review first examines comparatively a number of animal models of human affect and affect-related disorders that rely on persistent inter-individual differences, and then highlights some of the neurobiological findings in these models that are compatible with much of research in human behavioural and personality traits. Many behaviours occur in specific combinations in several models, but often remarkable dissociations are observed, providing a variety of constellations of traits. It is concluded that more systematic comparative experimentation on behaviour and neurobiology in different models is warranted to reveal possible "building blocks" of affect-related personality common in animals and humans. Looking into the perspectives in psychopharmacology the focus is placed on probable associations of inter-individual differences with brain structure and function, personality and coping strategies, and psychiatric vulnerability, highlighting some unexpected interactions between vulnerability endophenotypes, adverse life events, and behavioural traits. It is argued that further studies on inter-individual differences in affect and underlying neurobiology should include formal modeling of their epistatic, hierarchical and dynamic nature.