Identifying early behavioral and molecular markers of future stress sensitivity

Sex as a Biological Variable: Who, What, When, Why, and How

The inclusion of sex as a biological variable in research is absolutely essential for improving our understanding of disease mechanisms contributing to risk and resilience. Studies focusing on examining sex differences have demonstrated across many levels of analyses and stages of brain development and maturation that males and females can differ significantly. This review will discuss examples of animal models and clinical studies to provide guidance and reference for the inclusion of sex as an important biological variable relevant to a Neuropsychopharmacology audience.

Sex differences and stress across the lifespan

Sex differences in stress responses can be found at all stages of life and are related to both the organizational and activational effects of gonadal hormones and to genes on the sex chromosomes. As stress dysregulation is the most common feature across neuropsychiatric diseases, sex differences in how these pathways develop and mature may predict sex-specific periods of vulnerability to disruption and increased disease risk or resilience across the lifespan. The aging brain is also at risk to the effects of stress, where the rapid decline of gonadal hormones in women combined with cellular aging processes promote sex biases in stress dysregulation. In this Review, we discuss potential underlying mechanisms driving sex differences in stress responses and their relevance to disease. Although stress is involved in a much broader range of diseases than neuropsychiatric ones, we highlight here this area and its examples across the lifespan.

Developmental regulation of neuronal genes by DNA methylation: environmental influences

Steroid hormones have wide-ranging organizational, activational and protective actions in the brain. In particular, the organizational effects of early exposure to 17β-estradiol (E2) and glucocorticoids are essential for long-lasting behavioral and cognitive functions. Both steroid hormones mediate many of their actions through intracellular receptors that act as transcription factors. In the rodent cerebral cortex, estrogen receptor mRNA and protein expression are high early in postnatal life and declines dramatically as the animal approaches puberty. An understanding of the molecular mechanisms driving this developmental regulation of gene expression is critical for understanding the complex events that determine lasting brain physiology and prime the plasticity of neurons. Gene expression can be suppressed by the epigenetic modification of the promoter regions by DNA methylation that results in gene silencing. Indeed, the decrease in ERα mRNA expression in the cortex during development is accompanied by an increase in promoter methylation. Numerous environmental stimuli can alter the DNA methylation that occurs for ERα, glucocorticoid receptors, as well as many other critical genes involved in neuronal development. For example, maternal behavior toward pups can alter epigenetic regulation of ERα mRNA expression. Additionally perinatal stress and exposure to environmental estrogens can also have lasting effects on gene expression by modifying DNA methylation of these important genes. Taken together, there appears to be a critical window during development where, outside factors that alter epigenetic programming can have lasting effects on neuronal gene expression.

Steroid hormones, stress and the adolescent brain: a comparative perspective

Steroid hormones, including those produced by the gonads and the adrenal glands, are known to influence brain development during sensitive periods of life. Until recently, most brain organisation was assumed to take place during early stages of development, with relatively little neurogenesis or brain re-organisation during later stages. However, an increasing body of research has shown that the developing brain is also sensitive to steroid hormone exposure during adolescence (broadly defined as the period from nutritional independence to sexual maturity). In this review, we examine how steroid hormones that are produced by the gonads and adrenal glands vary across the lifespan in a range of mammalian and bird species, and we summarise the evidence that steroid hormone exposure influences behavioural and brain development during early stages of life and during adolescence in these two taxonomic groups. Taking a cross-species, comparative perspective reveals that the effects of early exposure to steroid hormones depend upon the stage of development at birth or hatching, as measured along the altricial-precocial dimension. We then review the evidence that exposure to stress during adolescence impacts upon the developing neuroendocrine systems, the brain and behaviour. Current research suggests that the effects of adolescent stress vary depending upon the sex of the individual and type of stressor, and the effects of stress could involve several neural systems, including the serotonergic and dopaminergic systems. Experience of stressors during adolescence could also influence brain development via the close interactions between the stress hormone and gonadal hormone axes. While sensitivity of the brain to steroid hormones during early life and adolescence potentially leaves the developing organism vulnerable to external adversities, developmental plasticity also provides an opportunity for the developing organism to respond to current circumstances and for behavioural responses to influence the future life history of the individual.

A pubertal immune challenge alters the antidepressant-like effects of chronic estradiol treatment in inbred and outbred adult female mice

Puberty is a period characterized by brain reorganization that contributes to the development of neural and behavioral responses to gonadal steroids. A single injection of the bacterial endotoxin, lipopolysaccharide (LPS), during the pubertal period decreases sexual receptivity in response to ovarian hormones in adulthood. Because chronic estradiol treatment alleviates depression-like symptoms in ovariectomized adult mice, we investigated the effect of pubertal LPS treatment on estradiol's antidepressant effects. We hypothesized that pubertal LPS treatment would decrease the antidepressant-like effect of estradiol in adult ovariectomized female mice, as it decreases other behavioral responses to ovarian hormones. As expected, chronic estradiol treatment decreased depression-like behavior, as measured by the duration of immobility, in saline-treated mice from two different strains, as well as in mice treated with LPS in adulthood. In contrast, in mice treated pubertally with LPS, estradiol strikingly increased the duration of immobility. No difference in body weight and in locomotion was found among the groups, suggesting that the differences in depression-like behavior were not due to differences in body weight or locomotor activity between LPS-treated and control mice. These results suggest that exposure to an immune challenge during the pubertal period alters the responsiveness of depression-like behavior to estradiol.

Neurobehavioral risk is associated with gestational exposure to stress hormones

The developmental origins of disease or fetal programming model predict that early exposures to threat or adverse conditions have lifelong consequences that result in harmful outcomes for health. The maternal endocrine 'fight or flight' system is a source of programming information for the human fetus to detect threats and adjust their developmental trajectory for survival. Fetal exposures to intrauterine conditions including elevated stress hormones increase the risk for a spectrum of health outcomes depending on the timing of exposure, the timetable of organogenesis and the developmental milestones assessed. Recent prospective studies, reviewed here, have documented the neurodevelopmental consequences of fetal exposures to the trajectory of stress hormones over the course of gestation. These studies have shown that fetal exposures to biological markers of adversity have significant and largely negative consequences for fetal, infant and child emotional and cognitive regulation and reduced volume in specific brain structures.

Anxiolytic effects and neuroanatomical targets of estrogen receptor-β (ERβ) activation by a selective ERβ agonist in female mice

The dichotomous anxiogenic and anxiolytic properties of estrogens have been reported to be mediated by two distinct neural estrogen receptors (ER), ERα and ERβ, respectively. Using a combination of pharmacological and genetic approaches, we confirmed that the anxiolytic actions of estradiol are mediated by ERβ and extended and these observations to demonstrate the neuroanatomical targets involved in ERβ activation in these behavioral responses. We examined the effects of the biologically active S-enantiomer of diarylpropionitrile (S-DPN) on anxiety-related behavioral measures, the corresponding stress hormonal response to hypothalamo-pituitary-adrenal axis reactivity, and potential sites of neuronal activation in mutant female mice carrying a null mutation for ERβ gene (βERKO). S-DPN administration significantly reduced anxiety-like behaviors in the open field, light-dark exploration, and the elevated plus maze (EPM) in ovariectomized wild-type (WT) mice, but not in their βERKO littermates. Stress-induced corticosterone (CORT) and ACTH were also attenuated by S-DPN in the WT mice but not in the βERKO mice. Using c-fos induction after elevated plus maze, as a marker of stress-induced neuronal activation, we identified the anterodorsal medial amygdala and bed nucleus of the stria terminalis as the neuronal targets of S-DPN action. Both areas showed elevated c-fos mRNA expression with S-DPN treatment in the WT but not βERKO females. These studies provide compelling evidence for anxiolytic effects mediated by ERβ, and its neuroanatomical targets, that send or receive projections to/from the paraventricular nucleus, providing potential indirect mode of action for the control of hypothalamo-pituitary-adrenal axis function and behaviors.

Exposure to prenatal psychobiological stress exerts programming influences on the mother and her fetus

BACKGROUND/AIMS

Accumulating evidence from a relatively small number of prospective studies indicates that exposure to prenatal stress profoundly influences the developing human fetus with consequences that persist into childhood and very likely forever.

METHODS

Maternal/fetal dyads are assessed at ∼20, ∼25, ∼31 and ∼36 weeks of gestation. Infant assessments begin 24 h after delivery with the collection of cortisol and behavioral responses to the painful stress of the heel-stick procedure and measures of neonatal neuromuscular maturity. Infant cognitive, neuromotor development, stress and emotional regulation are evaluated at 3, 6 12 and 24 months of age. Maternal psychosocial stress and demographic information is collected in parallel with infant assessments. Child neurodevelopment is assessed with cognitive tests, measures of adjustment and brain imaging between 5 and 8 years of age.

RESULTS

Psychobiological markers of stress during pregnancy, especially early in gestation, result in delayed fetal maturation, disrupted emotional regulation and impaired cognitive performance during infancy and decreased brain volume in areas associated with learning and memory in 6- to 8-year-old children. We review findings from our projects that maternal endocrine alterations that accompany pregnancy and influence fetal/infant/child development are associated with decreased affective responses to stress, altered memory function and increased risk for postpartum depression.

CONCLUSIONS

Our findings indicate that the mother and her fetus both are influenced by exposure to psychosocial and biological stress. The findings that fetal and maternal programming occur in parallel may have important implications for long-term child development and mother/child interactions.

Long-term effects of pubertal stressors on female sexual receptivity and estrogen receptor-α expression in CD-1 female mice

Exposure to stress during puberty can lead to long-term behavioral alterations. Female mice, of the inbred C57BL/6 strain, have been shown to display lower levels of sexual receptivity in adulthood when exposed to shipping stress or to an immune challenge during puberty. The present study investigated whether this effect can be extended to CD1 outbred mice and examined a possible mechanism through which exposure to stressors could suppress sexual receptivity. The results revealed that CD1 mice injected with lipopolysaccharide (LPS) or exposed to shipping stress at 6 weeks old display lower levels of sexual receptivity in response to estradiol and progesterone in adulthood than control mice. Moreover, mice exposed to shipping stress at 8 weeks old also displayed reduced sexual receptivity, but those injected with LPS at that time showed slightly reduced effects, suggesting that the sensitive pubertal period extends to 8 weeks of age in this strain of mice. The examination of estrogen receptor-α (ER-α) expression revealed that mice exposed to shipping stress during the sensitive period (6 weeks) display lower levels of ER-α expression in the medial preoptic area and the ventromedial nucleus and the arcuate nucleus of the hypothalamus than mice shipped at a younger age. These findings support the prediction that exposure to shipping stress or LPS during puberty decreases behavioral responsiveness to estradiol and progesterone in adulthood in an outbred strain of mice through enduring suppression of ER-α expression in some brain areas involved in the regulation of female sexual behavior.

Caloric restriction experience reprograms stress and orexigenic pathways and promotes binge eating

Long-term weight management by dieting has a high failure rate. Pharmacological targets have focused on appetite reduction, although less is understood as to the potential contributions of the stress state during dieting in long-term behavioral modification. In a mouse model of moderate caloric restriction in which a 10-15% weight loss similar to human dieting is produced, we examined physiological and behavioral stress measures. After 3 weeks of restriction, mice showed significant increases in immobile time in a tail suspension test and stress-induced corticosterone levels. Increased stress was associated with brain region-specific alterations of corticotropin-releasing factor expression and promoter methylation, changes that were not normalized with refeeding. Similar outcomes were produced by high-fat diet withdrawal, an additional component of human dieting. In examination of long-term behavioral consequences, previously restricted mice showed a significant increase in binge eating of a palatable high-fat food during stress exposure. Orexigenic hormones, melanin-concentrating hormone (MCH) and orexin, were significantly elevated in response to the high-fat diet only in previously restricted mice. Furthermore, administration of the MCH receptor-1 antagonist GSK-856464 [4-(4-ethyl-5-methylsulfanyl-1,2,4-triazol-3-yl)pyridine] significantly reduced total caloric intake in these mice during high-fat access. These results reveal reprogramming of key central pathways involved in regulating stress responsivity and orexigenic drives by moderate caloric restriction experience. In humans, such changes would be expected to reduce treatment success by promoting behaviors resulting in weight regain, and suggest that management of stress during dieting may be beneficial in long-term maintenance.

Framework for sex differences in adolescent neurobiology: a focus on cannabinoids

This review highlights the salient findings that have furthered our understanding of how sex differences are initiated during development and maintained throughout life. First we discuss how gonadal steroid hormones organize the framework for sex differences within critical periods of development-namely, during those exposures which occur in utero and post-partum, as well as those which occur during puberty. Given the extensive precedence of sex differences in cannabinoid-regulated biology, we then focus on the disparities within the endogenous cannabinoid system, as well as those observed with exogenously administered cannabinoids. We start with how the expression of cannabinoid CB(1) receptors is regulated throughout development. This is followed by a discussion of differential vulnerability to the pathological sequelae stemming from cannabinoid exposure during adolescence. Next we talk about sex differences in the interactions between cannabinoids and other drugs of abuse, followed by the organizational and activational roles of gonadal steroids in establishing and maintaining the sex dependence in the biological actions of cannabinoids. Finally, we discuss ways to utilize this knowledge to strategically target critical developmental windows of vulnerability/susceptibility and thereby implement more effective therapeutic interventions for afflictions that may be more prevalent in one sex vs. the other.

Gestational stress influences cognition and behavior

The developmental origins of disease or fetal programming model predicts that early exposures to threat or adverse conditions have lifelong consequences that result in harmful outcomes for health. The vast majority of the studies in support of the programming model in human beings are retrospective and most rely on surrogate measures of early experience such as birth weight or preterm birth. Recently, a small number of prospective studies have been reported that have documented the developmental consequences of exposures to stressful intrauterine conditions. These studies of gestational stress have clearly shown that fetal exposures to psychosocial and/or biological markers of adversity have significant and largely negative consequences for fetal, infant and child neurological development. Fetal exposure to stress, especially early in gestation, results in delayed fetal maturation and impaired cognitive performance during infancy and results in decreased brain volume in areas associated with learning and memory in children. The accumulating evidence supports the conclusion that fetal exposure to stress profoundly influences the nervous system, with consequences that persist into childhood and perhaps beyond.

Pubertal maturation and programming of hypothalamic-pituitary-adrenal reactivity

Modifications in neuroendocrine function are a hallmark of pubertal development. These changes have many short- and long-term implications for the physiological and neurobehavioral function of an individual. The purpose of the present review is to discuss our current understanding of how pubertal development and stress interact to affect the hypothalamic-pituitary-adrenal (HPA) axis, the major neuroendocrine axis that controls the hormonal stress response. A growing body of literature indicates that puberty is marked by dramatic transitions in stress reactivity. Moreover, recent studies indicate that exposure to stressors during pubertal maturation may result in enduring changes in HPA responsiveness in adulthood. As puberty is marked by a substantial increase in many stress-related psychological and physiological disorders (e.g., depression, anxiety, drug abuse), it is essential to understand the factors that regulate and modulate HPA function during this crucial period of development.

Sex differences in the serotonergic influence on the hypothalamic-pituitary-adrenal stress axis

Appropriate interactions between serotonin (5-HT) and stress pathways are critical for maintaining homeostasis. Dysregulation of hypothalamic-pituitary-adrenal (HPA) stress axis is a common feature in affective disorders in which an involvement of 5-HT neurocircuitry has been implicated in disease vulnerability and treatment responsiveness. Because there is a greater prevalence of affective disorders in women, sex differences in the 5-HTergic influence on stress pathways may contribute to disease disparity. Therefore, our studies compared stress or citalopram-induced corticosterone levels in male and female mice. To determine whether sex-dependent HPA axis responsiveness was mediated by the difference in testosterone levels, testosterone-treated females were also examined. Gene expression patterns in 5-HTergic and stress neurocircuitry were analyzed to determine sites of potential sex differences and mechanisms of testosterone action. As expected, restraint stress corticosterone levels were higher in intact females and were masculinized by testosterone. Interestingly, citalopram administration independent of stress resulted in a greater corticosterone response in females, which was also masculinized by testosterone. Analyses along the 5-HT-HPA axis revealed sex differences including greater pituitary 5-HT receptors and adrenal weights in females. Moreover, in stress-regulatory regions, we found sex differences in glucocorticoid receptor and glutamic acid decarboxylase expression supportive of greater inhibitory modulation and feedback potential in males. Taken together, these data suggest that multiple sites related to 5-HTergic stimulation, corticosterone production, and negative feedback of HPA neurocircuitry combine to produce higher female stress responsiveness. These studies support a potential for sex-specific involvement of 5-HT and stress pathways in the etiology of affective disorders.

CNS disorders--current treatment options and the prospects for advanced therapies

The development of new pharmaceutical products has successfully addressed a multitude of disease states; however, new product development for treating disorders of the central nervous system (CNS) has lagged behind other therapeutic areas. This is due to several factors including the complexity of the diseases and the lack of technologies for delivery through the blood-brain barrier (BBB). This article examines the current state of six major CNS disease states: depression, epilepsy, multiple sclerosis (MS), neurodegenerative diseases (specifically Alzheimer's disease [AD]), neuropathic pain, and schizophrenia. Discussion topics include analysis of the biological mechanisms underlying each disease, currently approved products, and available animal models for development of new therapeutic agents. Analysis of currently approved therapies shows that all products depend on the molecular properties of the drug or prodrug to penetrate the BBB. Novel technologies, capable of enhancing BBB permeation, are also discussed relative to improving CNS therapies for these disease states.

Examining the intersection of sex and stress in modelling neuropsychiatric disorders

Sex-biased neuropsychiatric disorders, including major depressive disorder and schizophrenia, are the major cause of disability in the developed world. Elevated stress sensitivity has been proposed as a key underlying factor in disease onset. Sex differences in stress sensitivity are associated with corticotrophin-releasing factor (CRF) and serotonin neurotransmission, which are important central regulators of mood and coping responses. To elucidate the underlying neurobiology of stress-related disease predisposition, it is critical to develop appropriate animal models of stress pathway dysregulation. Furthermore, the inclusion of sex difference comparisons in stress responsive behaviours, physiology and central stress pathway maturation in these models is essential. Recent studies by our laboratory and others have begun to investigate the intersection of stress and sex where the development of mouse models of stress pathway dysregulation via prenatal stress experience or early-life manipulations has provided insight into points of developmental vulnerability. In addition, examination of the maturation of these pathways, including the functional importance of the organisational and activational effects of gonadal hormones on stress responsivity, is essential for determination of when sex differences in stress sensitivity may begin. In such studies, we have detected distinct sex differences in stress coping strategies where activational effects of testosterone produced females that displayed male-like strategies in tests of passive coping, but were similar to females in tests of active coping. In a second model of elevated stress sensitivity, male mice experiencing prenatal stress early in gestation showed feminised physiological and behavioural stress responses, and were highly sensitive to a low dose of selective serotonin reuptake inhibitors. Analyses of expression and epigenetic patterns revealed changes in CRF and glucocorticoid receptor genes in these mice. Mechanistically, stress early in pregnancy produced a significant sex-dependent effect on placental gene expression that was supportive of altered foetal transport of key growth factors and nutrients. These mouse models examining alterations and hormonal effects on development of stress pathways provide necessary insight into how specific stress responses can be reprogrammed early in development resulting in sex differences in stress sensitivity and neuropsychiatric disease vulnerability.

Organizational and activational effects of testosterone on masculinization of female physiological and behavioral stress responses

The prevalence of affective disorders is two times greater in women than in men. The onset of anxiety and depression occurs at different ages that may correspond to key developmental periods when the brain is more vulnerable to hormonal and exogenous influences. Because stressful life events can precipitate disease onset, the development of greater stress sensitivity in females may contribute to their increased vulnerability. Gonadal hormone exposure in males during early development and again from puberty onward plays a prominent role in sexually dimorphic brain formation, possibly contributing to sex differences in stress responsivity. Therefore, organizational effects of testosterone propionate (TP) administered postnatally and activational effects of TP administered beginning at puberty on adult female physiological and behavioral stress responses were examined in mice. Although the activational effects of TP in females ameliorated the sex difference in the hypothalamic-pituitary-adrenal axis stress response, there was no effect of postnatal TP. Similarly, higher immobile time in intact females in the tail suspension test was blunted by activational TP in the absence of postnatal TP. However, in the marble-burying test of anxiety-like behaviors, organizational and activational TP independently resulted in increased burying behaviors. These results show that TP administration has distinct effects on reducing physiological and behavioral stress responsivity in rodent models and suggest that sex differences in these responses may partially result from the absence of testosterone in females.