Effects of social context on endocrine function and Zif268 expression in response to an acute stressor in adolescent and adult rats

Cognitive and Emotional Symptoms Induced by Chronic Stress Are Regulated by EGR1 in a Subpopulation of Hippocampal Pyramidal Neurons

Chronic stress is a core risk factor for developing a myriad of neurological disorders, including major depression. The chronicity of such stress can lead to adaptive responses or, on the contrary, to psychological maladaptation. The hippocampus is one of the most affected brain regions displaying functional changes in chronic stress. Egr1, a transcription factor involved in synaptic plasticity, is a key molecule regulating hippocampal function, but its role in stress-induced sequels has been poorly addressed. Emotional and cognitive symptoms were induced in mice by using the chronic unpredictable mild stress (CUMS) protocol. We used inducible double-mutant Egr1-CreERT2 x R26RCE mice to map the formation of Egr1-dependent activated cells. Results show that short- (2 days) or long-term (28 days) stress protocols in mice induce activation or deactivation, respectively, of hippocampal CA1 neural ensembles in an Egr1-activity-dependent fashion, together with an associated dendritic spine pathology. In-depth characterization of these neural ensembles revealed a deep-to-superficial switch in terms of Egr1-dependent activation of CA1 pyramidal neurons. To specifically manipulate deep and superficial pyramidal neurons of the hippocampus, we then used Chrna7-Cre (to express Cre in deep neurons) and Calb1-Cre mice (to express Cre in superficial neurons). We found that specific manipulation of superficial but not deep pyramidal neurons of the CA1 resulted in the amelioration of depressive-like behaviors and the restoration of cognitive impairments induced by chronic stress. In summary, Egr1 might be a core molecule driving the activation/deactivation of hippocampal neuronal subpopulations underlying stress-induced alterations involving emotional and cognitive sequels.

Differential expression of 10 genes in the hypothalamus of two generations of rats selected for a reaction to humans

Individual behavioral differences are due to an interaction of the genotype and the environment. Phenotypic manifestation of aggressive behavior depends on the coordinated expression of gene ensembles. Nonetheless, the identification of these genes and of combinations of their mutual influence on expression remains a difficult task. Using animal models of aggressive behavior (gray rats that were selected for a reaction to humans; tame and aggressive rat strains), we evaluated the expression of 10 genes potentially associated with aggressiveness according to the literature: Cacna1b, Cacna2d3, Drd2, Egr1, Gad2, Gria2, Mapk1, Nos1, Pomc, and Syn1. To identify the genes most important for the manifestation of aggressiveness, we analyzed the expression of these genes in two generations of rats: 88th and 90th. Assessment of gene expression levels was carried out by real-time PCR in the hypothalamus of tame and aggressive rats. This analysis confirmed that 4 out of the 10 genes differ in expression levels between aggressive rats and tame rats in both generations. Specifically, it was shown that the expression of the Cacna1b, Drd2, Egr1, and Gad2 genes does not differ between the two generations (88th vs 90th) within each strain, but significantly differs between the strains: in the tame rats of both generations, the expression levels of these genes are significantly lower as compared to those in the aggressive rats. Therefore, these genes hold promise for further studies on behavioral characteristics. Thus, we confirmed polygenic causes of phenotypic manifestation of aggressive reactions.

Methods and Challenges in Investigating Sex-Specific Consequences of Social Stressors in Adolescence in Rats: Is It the Stress or the Social or the Stage of Development?

Adolescence is a time of social learning and social restructuring that is accompanied by changes in both the hypothalamic-pituitary-gonadal axis and the hypothalamic-pituitary-adrenal (HPA) axis. The activation of these axes by puberty and stressors, respectively, shapes adolescent development. Models of social stress in rats are used to understand the consequences of perturbations of the social environment for ongoing brain development. This paper reviews the challenges in investigating the sex-specific consequences of social stressors, sex differences in the models of social stress used in rats and the sex-specific effects on behaviour and provides an overview of sex differences in HPA responding to stressors, the variability in pubertal development and in strains of rats that require consideration in conducting such research, and directions for future research.

Transcriptomic analyses of humans and mice provide insights into depression

Accumulating studies have been conducted to identify risk genes and relevant biological mechanisms underlying major depressive disorder (MDD). In particular, transcriptomic analyses in brain regions engaged in cognitive and emotional processes, e.g., the dorsolateral prefrontal cortex (DLPFC), have provided essential insights. Based on three independent DLPFC RNA-seq datasets of 79 MDD patients and 75 healthy controls, we performed differential expression analyses using two alternative approaches for cross-validation. We also conducted transcriptomic analyses in mice undergoing chronic variable stress (CVS) and chronic social defeat stress (CSDS). We identified 12 differentially expressed genes (DEGs) through both analytical methods in MDD patients, the majority of which were also dysregulated in stressed mice. Notably, the mRNA level of the immediate early gene FOS ( Fos proto-oncogene) was significantly decreased in both MDD patients and CVS-exposed mice, and CSDS-susceptible mice exhibited a greater reduction in Fos expression compared to resilient mice. These findings suggest the potential key roles of this gene in the pathogenesis of MDD related to stress exposure. Altered transcriptomes in the DLPFC of MDD patients might be, at least partially, the result of stress exposure, supporting that stress is a primary risk factor for MDD.

Neuroendocrine and neuroimmune responses in male and female rats: evidence for functional immaturity of the neuroimmune system during early adolescence

Adolescence is a developmental period characterized by rapid behavioral and physiological changes, including enhanced vulnerability to stress. Recent studies using rodent models of adolescence have demonstrated age differences in neuroendocrine responses and blunted neuroimmune responding to pharmacological challenges. The present study was designed to test whether this neuroimmune insensitivity would generalize to a non-pharmacological stress challenge. Male and female adolescent (P29-33) and adult (P70-80) Sprague Dawley rats were exposed to intermittent footshock for one-, two-, or two-hours + recovery. Plasma corticosterone and progesterone levels as well as gene expression of several cytokines and c-Fos gene expression in the paraventricular nucleus of the hypothalamus (PVN), the medial amygdala (MeA), and the ventral hippocampus (vHPC) were analyzed. The results of the present study demonstrated differences in response to footshock, with these differences dependent on age, sex, and brain region of interest. Adult males and females demonstrated time-dependent increases in IL-1β and IL-1R2 in the PVN, with these changes not evident in adolescent males and substantially blunted in adolescent females. TNFα expression was decreased in all regions of interest, with adults demonstrating more suppression relative to adolescents and age differences more apparent in males than in females. IL-6 expression was affected by footshock predominantly in the vHPC of adolescent and adult males and females, with females demonstrating prolonged elevation of IL-6 gene expression. In summary, central cytokine responses to acute stressor exposure are blunted in adolescent rats, with the most pronounced immaturity evident for the brain IL-1 signaling system.

Early Growth Response Gene Upregulation in Epstein-Barr Virus (EBV)-Associated Myalgic Encephalomyelitis/Chronic Fatigue Syndrome (ME/CFS)

Myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) is a chronic multisystem disease exhibiting a variety of symptoms and affecting multiple systems. Psychological stress and virus infection are important. Virus infection may trigger the onset, and psychological stress may reactivate latent viruses, for example, Epstein-Barr virus (EBV). It has recently been reported that EBV induced gene 2 (EBI2) was upregulated in blood in a subset of ME/CFS patients. The purpose of this study was to determine whether the pattern of expression of early growth response (EGR) genes, important in EBV infection and which have also been found to be upregulated in blood of ME/CFS patients, paralleled that of EBI2. EGR gene upregulation was found to be closely associated with that of EBI2 in ME/CFS, providing further evidence in support of ongoing EBV reactivation in a subset of ME/CFS patients. EGR1, EGR2, and EGR3 are part of the cellular immediate early gene response and are important in EBV transcription, reactivation, and B lymphocyte transformation. EGR1 is a regulator of immune function, and is important in vascular homeostasis, psychological stress, connective tissue disease, mitochondrial function, all of which are relevant to ME/CFS. EGR2 and EGR3 are negative regulators of T lymphocytes and are important in systemic autoimmunity.

Sex differences in adult mood and in stress-induced transcriptional coherence across mesocorticolimbic circuitry

Women are approximately two times as likely to be diagnosed with major depressive disorder (MDD) compared to men. While sex differences in MDD might be driven by circulating gonadal hormones, we hypothesized that developmental hormone exposure and/or genetic sex might play a role. Mice were gonadectomized in adulthood to isolate the role of developmental hormones. We examined the effects of developmental gonadal and genetic sex on anhedonia-/depressive-like behaviors under non-stress and chronic stress conditions and performed RNA-sequencing in three mood-relevant brain regions. We used an integrative network approach to identify transcriptional modules and stress-specific hub genes regulating stress susceptibility, with a focus on whether these differed by sex. After identifying sex differences in anhedonia-/depressive-like behaviors (female > male), we show that both developmental hormone exposure (gonadal female > gonadal male) and genetic sex (XX > XY) contribute to the sex difference. The top biological pathways represented by differentially expressed genes were related to immune function; we identify which differentially expressed genes are driven by developmental gonadal or genetic sex. There was very little overlap in genes affected by chronic stress in males and females. We also identified highly co-expressed gene modules affected by stress, some of which were affected in opposite directions in males and females. Since all mice had equivalent hormone exposure in adulthood, these results suggest that sex differences in gonadal hormone exposure during sensitive developmental periods program adult sex differences in mood, and that these sex differences are independent of adult circulating gonadal hormones.

Social experience during adolescence in female rats increases 50 kHz ultrasonic vocalizations in adulthood, without affecting anxiety-like behavior

Adolescents are highly motivated to engage in social interactions, and researchers have hypothesized that positive social relationships during adolescence can have long term, beneficial effects on stress reactivity and mental well-being. Studies of laboratory rodents provide the opportunity to investigate the relationship between early social experiences and later behavioral and physiological responses to stressors. In this study, female Lister-hooded rats (N = 12 per group) were either (a) provided with short, daily encounters (10 min/day) with a novel partner during mid-adolescence (postnatal day 34-45; "social experience," SE, subjects) or (b) underwent the same protocol with a familiar cagemate during mid-adolescence ("control experience," CE, subjects), or (c) were left undisturbed in the home cage (non-handled "control," C, subjects). When tested in adulthood, the groups did not differ in behavioral responses to novel environments (elevated plus maze, open field, and light-dark box) or in behavioral and physiological (urinary corticosterone) responses to novel social partners. However, SE females emitted significantly more 50 kHz ultrasonic vocalizations than control subjects both before and after social separation from a familiar social partner, which is consistent with previous findings in male rats. Thus, enhanced adolescent social experience appears to have long-term effects on vocal communication and could potentially modulate adult social relationships.

Immediate Early Genes, Memory and Psychiatric Disorders: Focus on c-Fos, Egr1 and Arc

Many psychiatric disorders, despite their specific characteristics, share deficits in the cognitive domain including executive functions, emotional control and memory. However, memory deficits have been in many cases undervalued compared with other characteristics. The expression of Immediate Early Genes (IEGs) such as, c-fos, Egr1 and arc are selectively and promptly upregulated in learning and memory among neuronal subpopulations in regions associated with these processes. Changes in expression in these genes have been observed in recognition, working and fear related memories across the brain. Despite the enormous amount of data supporting changes in their expression during learning and memory and the importance of those cognitive processes in psychiatric conditions, there are very few studies analyzing the direct implication of the IEGs in mental illnesses. In this review, we discuss the role of some of the most relevant IEGs in relation with memory processes affected in psychiatric conditions.

The metamorphosis of adolescent hormonal stress reactivity: A focus on animal models

As adolescents transition from childhood to adulthood, many physiological and neurobehavioral changes occur. Shifts in neuroendocrine function are one such change, including the hormonal systems that respond to stressors. This review will focus on these hormonal changes, with a particular emphasis on the pubertal and adolescent maturation of the hypothalamic-pituitary-adrenal (HPA) axis. Furthermore, this review will concentrate on studies using animal models, as these model systems have contributed a great deal to our mechanistic understanding of how factors such as sex and experience with stressors shape hormonal reactivity during development. Continued study of the maturation of stress reactivity will undoubtedly shed much needed light on the stress-related vulnerabilities often associated with adolescence as well as providing us with possible strategies to mitigate these vulnerabilities. This area of research may lead to discoveries that enhance the well-being of adolescents, ultimately providing them with greater opportunities to mature into healthy adults.

The roots of empathy: Through the lens of rodent models

Empathy is a phenomenon often considered dependent on higher-order emotional control and an ability to relate to the emotional state of others. It is, by many, attributed only to species having well-developed cortical circuits capable of performing such complex tasks. However, over the years, a wealth of data has been accumulated showing that rodents are capable not only of sharing emotional states of their conspecifics, but also of prosocial behavior driven by such shared experiences. The study of rodent empathic behaviors is only now becoming an independent research field. Relevant animal models allow precise manipulation of neural networks, thereby offering insight into the foundations of empathy in the mammalian brains. Here we review the data on empathic behaviors in rat and mouse models, their neurobiological and neurophysiological correlates, and the factors influencing these behaviors. We discuss how simple rodent models of empathy enhance our understanding of how brain controls empathic behaviors.

The Role of Early Growth Response 1 (EGR1) in Brain Plasticity and Neuropsychiatric Disorders

It is now clearly established that complex interactions between genes and environment are involved in multiple aspects of neuropsychiatric disorders, from determining an individual's vulnerability to onset, to influencing its response to therapeutic intervention. In this perspective, it appears crucial to better understand how the organism reacts to environmental stimuli and provide a coordinated and adapted response. In the central nervous system, neuronal plasticity and neurotransmission are among the major processes integrating such complex interactions between genes and environmental stimuli. In particular, immediate early genes (IEGs) are critical components of these interactions as they provide the molecular framework for a rapid and dynamic response to neuronal activity while opening the possibility for a lasting and sustained adaptation through regulation of the expression of a wide range of genes. As a result, IEGs have been tightly associated with neuronal activity as well as a variety of higher order processes within the central nervous system such as learning, memory and sensitivity to reward. The immediate early gene and transcription factor early growth response 1 (EGR1) has thus been revealed as a major mediator and regulator of synaptic plasticity and neuronal activity in both physiological and pathological conditions. In this review article, we will focus on the role of EGR1 in the central nervous system. First, we will summarize the different factors influencing its activity. Then, we will analyze the amount of data, including genome-wide, that has emerged in the recent years describing the wide variety of genes, pathways and biological functions regulated directly or indirectly by EGR1. We will thus be able to gain better insights into the mechanisms underlying EGR1's functions in physiological neuronal activity. Finally, we will discuss and illustrate the role of EGR1 in pathological states with a particular interest in cognitive functions and neuropsychiatric disorders.

Translational relevance of rodent models of hypothalamic-pituitary-adrenal function and stressors in adolescence

Elevations in glucocorticoids that result from environmental stressors can have programming effects on brain structure and function when the exposure occurs during sensitive periods that involve heightened neural development. In recent years, adolescence has gained increasing attention as another sensitive period of development, a period in which pubertal transitions may increase the vulnerability to stressors. There are similarities in physical and behavioural development between humans and rats, and rats have been used effectively as an animal model of adolescence and the unique plasticity of this period of ontogeny. This review focuses on benefits and challenges of rats as a model for translational research on hypothalamic-pituitary-adrenal (HPA) function and stressors in adolescence, highlighting important parallels and contrasts between adolescent rats and humans, and we review the main stress procedures that are used in investigating HPA stress responses and their consequences in adolescence in rats. We conclude that a greater focus on timing of puberty as a factor in research in adolescent rats may increase the translational relevance of the findings.

Glucocorticoid receptor translocation and expression of relevant genes in the hippocampus of adolescent and adult male rats

We investigated whether pre-pubertal (postnatal day [P] 35) and post-pubertal adolescent (P45) and adult (P75) male rats differed in stressor-induced hormonal responses and in glucocorticoid receptor (GR) translocation because it has been proposed that negative feedback is maturing in adolescence and may be a basis for the prolonged activation of the HPA axis in adolescents compared with adults. The three age groups did not differ at baseline in plasma corticosterone or progesterone concentrations, and P35 had lower concentrations of testosterone than did both P45 and P75 rats, which did not differ. After 30min of restraint stress, plasma concentrations of corticosterone and progesterone increased to a greater extent in the adolescents than in the adults. Whereas restraint stress increased concentrations of testosterone in adult males, concentrations decreased in adolescents. In all three age groups, restraint stress reduced GR expression in the cytosol and increased expression in the nucleus within the hippocampus, and the increase in nuclear GR was greater in pre-pubertal adolescents compared with adults. In a separate set of rats we investigated age differences in hippocampal mRNA expression of corticosteroid receptors (MR and GR) and of chaperones (FKBP5, FKBP4, BAG-1), which are known to modulate their activity, at baseline and after restraint stress. Restraint stress decreased the expression of GR and increased the expression of FKBP5 mRNA, and age was not a significant factor. Higher expression of FKBP4 mRNA was found at P35 than at P75. Most research of HPA function in adolescent rats has involved pre-pubertal rats; the present findings indicate that despite their increase in gonadal function, responses to stressors in P45 rats are more like those of pre-pubertal than adult rats. The greater stressor-induced GR translocation in pre-pubertal adolescents parallels their greater release of corticosterone in response to stressors, which may contribute to the enhanced sensitivity of adolescent rats to the effects of chronic stress exposures compared with adults.

Effects of sex and housing on social, spatial, and motor behavior in adult rats exposed to moderate levels of alcohol during prenatal development

Persistent deficits in social behavior, motor behavior, and behavioral flexibility are among the major negative consequences associated with exposure to ethanol during prenatal development. Prior work from our laboratory has linked moderate prenatal alcohol exposure (PAE) in the rat to deficits in these behavioral domains, which depend upon the ventrolateral frontal cortex (Hamilton et al., 2014) [20]. Manipulations of the social environment cause modifications of dendritic morphology and experience-dependent immediate early gene expression in ventrolateral frontal cortex (Hamilton et al., 2010) [19], and may yield positive behavioral outcomes following PAE. In the present study we evaluated the effects of housing PAE rats with non-exposed control rats on adult behavior. Rats of both sexes were either paired with a partner from the same prenatal treatment condition (ethanol or saccharin) or from the opposite condition (mixed housing condition). At four months of age (∼3 months after the housing manipulation commenced), social behavior, tongue protrusion, and behavioral flexibility in the Morris water task were measured as in (Hamilton et al., 2014) [20]. The behavioral effects of moderate PAE were primarily limited to males and were not ameliorated by housing with a non-ethanol exposed partner. Unexpectedly, social behavior, motor behavior, and spatial flexibility were adversely affected in control rats housed with a PAE rat (i.e., in mixed housing), indicating that housing with a PAE rat has broad behavioral consequences beyond the social domain. These observations provide further evidence that moderate PAE negatively affects social behavior, and underscore the importance of considering potential negative effects of housing with PAE animals on the behavior of critical comparison groups.

Adolescence and the ontogeny of the hormonal stress response in male and female rats and mice

Adolescent development is marked by many changes in neuroendocrine function, resulting in both immediate and long-term influences on an individual's physiology and behavior. Stress-induced hormonal responses are one such change, with adolescent animals often showing different patterns of hormonal reactivity following a stressor compared with adults. This review will describe the unique ways in which adolescent animals respond to a variety of stressors and how these adolescent-related changes in hormonal responsiveness can be further modified by the sex and previous experience of the individual. Potential central and peripheral mechanisms that contribute to these developmental shifts in stress reactivity are also discussed. Finally, the short- and long-term programming effects of chronic stress exposure during adolescence on later adult hormonal responsiveness are also examined. Though far from a clear understanding of the neurobehavioral consequences of these adolescent-related shifts in stress reactivity, continued study of developmental changes in stress-induced hormonal responses may shed light on the increased vulnerability to physical and psychological dysfunctions that often accompany a stressful adolescence.

Adolescent and adult male rats habituate to repeated isolation, but only adolescents sensitize to partner unfamiliarity

We investigated whether adolescent male rats show less habituation of corticosterone release than adult male rats to acute vs repeated (16) daily one hour episodes of isolation stress, as well as the role of partner familiarity during recovery on social behavior, plasma corticosterone, and Zif268 expression in brain regions. Adolescents spent more time in social contact than did adults during the initial days of the repeated stress procedures, but both adolescents and adults that returned to an unfamiliar peer after isolation had higher social activity than rats returned to a familiar peer (p=0.002) or undisturbed control rats (p<0.001). Both ages showed evidence of habituation, with reduced corticosterone response to repeated than acute isolation (p=0.01). Adolescents, however, showed sensitized corticosterone release to repeated compared with an acute pairing with an unfamiliar peer during recovery (p=0.03), a difference not found in adults. Consistent with habituation of corticosterone release, the repeated isolation groups had lower Zif268 immunoreactive cell counts in the paraventricular nucleus (p<0.001) and in the arcuate nucleus (p=0.002) than did the acute groups, and adolescents had higher Zif268 immunoreactive cell counts in the paraventricular nucleus than did adults during the recovery period (p<0.001), irrespective of stress history and partner familiarity. Partner familiarity had only modest effects on Zif268 immunoreactivity, and experimental effects on plasma testosterone concentrations were only in adults. The results highlight social and endocrine factors that may underlie the greater vulnerability of the adolescent period of development.

Peer pressures: social instability stress in adolescence and social deficits in adulthood in a rodent model

Studies in animal models generate and test hypotheses regarding developmental stage-specific vulnerability that might inform research questions about human development. In both rats and humans, peer relationships are qualitatively different in adolescence than at other stages of development, and social experiences in adolescence are considered important determinants of adult social function. This review describes our adolescent rat social instability stress model and the long-lasting effects social instability has on social behaviour in adulthood as well as the possible neural underpinnings. Effects of other adolescent social stress experiences in rats on social behaviours in adulthood also are reviewed. We discuss the role of hypothalamic-pituitary-adrenal (HPA) function and glucocorticoid release in conferring differential susceptibility to social experiences in adolescents compared to adults. We propose that although differential perception of social experiences rather than immature HPA function may underlie the heightened vulnerability of adolescents to social instability, the changes in the trajectory of brain development and resultant social deficits likely are mediated by the heightened glucocorticoid release in response to repeated social stressors in adolescence compared to in adulthood.