Gender and puberty interact on the stress-induced activation of parvocellular neurosecretory neurons and corticotropin-releasing hormone messenger ribonucleic acid expression in the rat

The Adaptive Calibration Model of stress responsivity

This paper presents the Adaptive Calibration Model (ACM), an evolutionary-developmental theory of individual differences in the functioning of the stress response system. The stress response system has three main biological functions: (1) to coordinate the organism's allostatic response to physical and psychosocial challenges; (2) to encode and filter information about the organism's social and physical environment, mediating the organism's openness to environmental inputs; and (3) to regulate the organism's physiology and behavior in a broad range of fitness-relevant areas including defensive behaviors, competitive risk-taking, learning, attachment, affiliation and reproductive functioning. The information encoded by the system during development feeds back on the long-term calibration of the system itself, resulting in adaptive patterns of responsivity and individual differences in behavior. Drawing on evolutionary life history theory, we build a model of the development of stress responsivity across life stages, describe four prototypical responsivity patterns, and discuss the emergence and meaning of sex differences. The ACM extends the theory of biological sensitivity to context (BSC) and provides an integrative framework for future research in the field.

The medial preoptic nucleus integrates the central influences of testosterone on the paraventricular nucleus of the hypothalamus and its extended circuitries

Testosterone contributes to sex differences in hypothalamic-pituitary-adrenal (HPA) function in humans and rodents, but the central organization of this regulation remains unclear. The medial preoptic nucleus (MPN) stands out as an important candidate in this regard because it contains androgen receptors and projects to forebrain nuclei integrating cognitive-affective information and regulating HPA responses to homeostatic threat. These include the HPA effector neurons of the paraventricular nucleus (PVN) of the hypothalamus, medial amygdala, and lateral septum. To test the extent to which androgen receptors in the MPN engage these cell groups, we compared in adult male rats the effects of unilateral microimplants of testosterone and the androgen receptor antagonist hydroxyflutamide into the MPN on acute restraint induced activation and/or neuropeptide expression levels. The basic effects of these implants were lateralized to the sides of the nuclei ipsilateral to the implants. Testosterone, but not hydroxyflutamide implants, decreased stress-induced Fos and arginine vasopressin (AVP) heteronuclear RNA expression in the PVN, as well as Fos expression in the lateral septum. In unstressed animals, AVP mRNA expression in the PVN decreased and increased in response to testosterone and hydroxflutamide MPN implants, respectively. The differential influences of these implants on AVP mRNA expression were opposite in the medial amygdala. These results confirm a role for androgen receptors in the MPN to concurrently modulate neuropeptide expression and activational responses in the PVN and its extended circuitries. This suggests that the MPN is capable of bridging converging limbic influences to the HPA axis with changes in gonadal status.

Sex differences in corticotropin-releasing factor receptor signaling and trafficking: potential role in female vulnerability to stress-related psychopathology

Although the higher incidence of stress-related psychiatric disorders in females is well documented, its basis is unknown. Here, we show that the receptor for corticotropin-releasing factor (CRF), the neuropeptide that orchestrates the stress response, signals and is trafficked differently in female rats in a manner that could result in a greater response and decreased adaptation to stressors. Most cellular responses to CRF in the brain are mediated by CRF receptor (CRFr) association with the GTP-binding protein, G(s). Receptor immunoprecipitation studies revealed enhanced CRFr-G(s) coupling in cortical tissue of unstressed female rats. Previous stressor exposure abolished this sex difference by increasing CRFr-G(s) coupling selectively in males. These molecular results mirrored the effects of sex and stress on sensitivity of locus ceruleus (LC)-norepinephrine neurons to CRF. Differences in CRFr trafficking were also identified that could compromise stress adaptation in females. Specifically, stress-induced CRFr association with beta-arrestin2, an integral step in receptor internalization, occurred only in male rats. Immunoelectron microscopy confirmed that stress elicited CRFr internalization in LC neurons of male rats exclusively, consistent with reported electrophysiological evidence for stress-induced desensitization to CRF in males. Together, these studies identified two aspects of CRFr function, increased cellular signaling and compromised internalization, which render CRF-receptive neurons of females more sensitive to low levels of CRF and less adaptable to high levels of CRF. CRFr dysfunction in females may underlie their increased vulnerability to develop stress-related pathology, particularly that related to increased activity of the LC-norepinephrine system, such as depression or post-traumatic stress disorder.

Adolescence: a central event in shaping stress reactivity

The magnitude and duration of the hormonal stress response change dramatically throughout an organism's lifespan. Although much is known about the factors that modulate stress reactivity during adulthood and how neonatal development and aging influence stress responsiveness, we know relatively little about how stress reactivity changes during the juvenile to adult transition. Recent studies in adolescent boys and girls have suggested that stress is an important factor contributing to an individual's vulnerability to various neuropsychological dysfunctions, including anxiety, depression, and drug abuse. Thus, understanding how exposure to stressors during this crucial period of development lead to negative consequences is of paramount importance. A growing body of literature indicates that pubertal organisms react differentially, both physiologically and behaviorally, to a stressor compared to adults. The purpose of this review, therefore, is to discuss the recent findings regarding the pubertal maturation of stress reactivity, while also highlighting future research directions that will aid in our understanding of stress and adolescent mental health and development.

Stress-induced suppression of hippocampal neurogenesis in adult male rats is altered by prenatal ethanol exposure

In adulthood, both alcohol (ethanol) and stress are known to suppress hippocampal neurogenesis in male rats. Similarly, most studies report that prenatal alcohol exposure (PAE) reduces cell proliferation and/or cell survival in the hippocampus of adult males. Furthermore, PAE is known to have marked effects on behavioral and hypothalamic-pituitary-adrenal (HPA) responsiveness to stressors. However, no studies have examined the modulation of adult hippocampal neurogenesis by stress in PAE animals. We hypothesized that, in accordance with previous data, PAE would suppress basal levels of adult hippocampal neurogenesis, and further that stress acting on a sensitized HPA axis would have greater adverse effects on adult hippocampal neurogenesis in PAE than in control rats. Adult male offspring from PAE, pair-fed (PF) control, and ad libitum-fed control (C) groups were subjected to restraint stress (9 days, 1 h/day) or left undisturbed. Rats were then injected with bromodeoxyuridine (BrdU) on day 10, perfused 24 h (proliferation) or 3 weeks (survival) later, and brains processed for BrdU immunohistochemistry. We found that (1) under non-stressed conditions, PAE rats had a small but statistically significant suppressive effect on levels of hippocampal neurogenesis and (2) unexpectedly, repeated restraint stress significantly reduced neurogenesis in C and PF, but not PAE rats. We speculate that the failure of PAE males to mount an appropriate (i.e. suppressive) neurogenic response to stressors, implies reduced plasticity and adaptability or resilience, which could impact negatively on hippocampal structure and function.

Adolescent development, hypothalamic-pituitary-adrenal function, and programming of adult learning and memory

Chronic exposure to stress is known to affect learning and memory in adults through the release of glucocorticoid hormones by the hypothalamic-pituitary-adrenal (HPA) axis. In adults, glucocorticoids alter synaptic structure and function in brain regions that express high levels of glucocorticoid receptors and that mediate goal-directed behaviour and learning and memory. In contrast to relatively transient effects of stress on cognitive function in adulthood, exposure to high levels of glucocorticoids in early life can produce enduring changes through substantial remodeling of the developing nervous system. Adolescence is another time of significant brain development and maturation of the HPA axis, thereby providing another opportunity for glucocorticoids to exert programming effects on neurocircuitry involved in learning and memory. These topics are reviewed, as is the emerging research evidence in rodent models highlighting that adolescence may be a period of increased vulnerability compared to adulthood in which exposure to high levels of glucocorticoids results in enduring changes in adult cognitive function.

Interactions of stress and CRF in ethanol-withdrawal induced anxiety in adolescent and adult rats

BACKGROUND

Repeated stress or administration of corticotropin-releasing factor (CRF) prior to ethanol exposure sensitizes anxiety-like behavior in adult rats. Current experiments determined whether adolescent rats were more sensitive to these challenges in sensitizing ethanol withdrawal-induced anxiety and altering CRF levels in brain during withdrawal.

METHODS

Male adult and adolescent Sprague-Dawley rats were restraint stressed (1 hour) twice 1 week apart prior to a single 5-day cycle of ethanol diet (ED; stress/withdrawal paradigm). Other rats received control diet (CD) and three 1-hour restraint stress sessions. Rats were then tested 5, 24, or 48 hours after the final withdrawal for anxiety-like behavior in the social interaction (SI) test. In other experiments, adolescent rats were given two microinjections of CRF icv 1 week apart followed by 5 days of either CD or ED and tested in social interaction 5 hours into withdrawal. Finally, CRF immunoreactivity was measured in the central nucleus of the amygdala (CeA) and paraventricular nucleus (PVN) after rats experienced control diet, repeated ethanol withdrawals, or stress/withdrawal.

RESULTS

Rats of both ages had reduced SI following the stress/withdrawal paradigm, and this effect recovered within 24 hours. Higher CRF doses were required to reduce SI in adolescents than previously reported in adults. CRF immunohistochemical levels were higher in the PVN and CeA of CD-exposed adolescents. In adolescent rats, repeated ethanol withdrawals decreased CRF in the CeA but was not associated with decreased CRF cell number. There was no change in CRF from adult treatments.

CONCLUSIONS

In the production of anxiety-like behavior, adolescent rats have equal sensitivity with stress and lower sensitivity with CRF compared to adults. Further, adolescents had higher basal levels of CRF within the PVN and CeA and reduced CRF levels following repeated ethanol withdrawals. This reduced CRF within the CeA could indicate increased release of CRF, and future work will determine how this change relates to behavior.

Enduring and sex-specific effects of adolescent social isolation in rats on adult stress reactivity

In adolescence, gender differences in rates of affective disorders emerge. For both adolescent boys and girls, peer relationships are the primary source of life stressors though adolescent girls are more sensitive to such stressors. Social stressors are also powerful stressors for non-human social species like rodents. In a rat model, we examined how social isolation during adolescence impacts stress reactivity and specific neural substrates in adult male and female rats. Rats were isolated during adolescence by single housing from day 30 to 50 of age and control rats were group housed. On day 50, isolated rats and control rats were re-housed in same-treatment same-sex groups. Adult female rats isolated as adolescents exhibited increased adrenal responses to acute and to repeated stress and exhibited increased hypothalamic vasopressin mRNA and BDNF mRNA in the CA3 hippocampal subfield. In contrast, adult male rats isolated as adolescents exhibited a lower corticosterone response to acute stress, exhibited a reduced state of anxiety as assessed in the elevated plus maze and reduced Orexin mRNA compared to adult males group-housed as adolescents. These data point to a markedly different impact of isolation experienced in adolescence on endocrine and behavioral endpoints in males compared to females and identify specific neural substrates that may mediate the long-lasting effects of stress in adolescence.

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.

A comparison of two repeated restraint stress paradigms on hypothalamic-pituitary-adrenal axis habituation, gonadal status and central neuropeptide expression in adult male rats

The available evidence continues to illustrate an inhibitory influence of male gonadal activity on the hypothalamic-pituitary-adrenal (HPA) axis under acute stress. However, far less is known about how these systems interact during repeated stress. Because HPA output consistently declines across studies examining repeated restraint, the potential mechanisms mediating this habituation are often inferred as being equivalent, even though these studies use a spectrum of restraint durations and exposures. To test this generalisation, as well as to emphasise a potential influence of the male gonadal axis on the process of HPA habituation, we compared the effects of two commonly used paradigms of repeated restraint in the rodent: ten daily episodes of 0.5 h of restraint and five daily episodes of 3 h of restraint. Both paradigms produced comparable declines in adrenocorticotrophic hormone and corticosterone between the first and last day of testing. However, marked differences in testosterone levels, as well as corticotrophin-releasing hormone (CRH) and arginine vasopressin (AVP) expression, occurred between the two stress groups. Plasma testosterone levels remained relatively higher in animals exposed to 0.5 h of restraint compared to 3 h of restraint, whereas forebrain gonadotrophin-releasing hormone (GnRH) cell counts increased in both groups. AVP mRNA was increased after 3 h, but not after 0.5 h of repeated restraint, in the medial parvicellular paraventricular nucleus and in the posterior bed nucleus of the stria terminalis (BST), and increased with 0.5 h of repeated restraint in the medial amygdala. CRH mRNA was increased after 3 h, but not after 0.5 h of repeated restraint, in the central amygdala and anterior BST. The data obtained illustrate that, despite comparable declines in HPA responses, the pathways recruited for stress adaptation appear to be distinct between restraint groups. Given the extreme sensitivity of limbic AVP to testosterone, and conversely CRH to circulating glucocorticoids, whether differences in endocrine profiles might explain these neuropeptide differences remains to be seen. Nonetheless, the present study provides several new entry points for testing gonadal influences on stress-specific HPA habituation.

Corticotrophin-releasing factor alters the timing of puberty in the female rat

Puberty is a developmental process that is dependent upon activation of the hypothalamic gonadotrophin-releasing hormone (GnRH) pulse generator. It is well established that the stress neuropeptide, corticotrophin-releasing factor (CRF), has a profound inhibitory action on GnRH pulse generator frequency. Although stress is known to affect the timing of puberty, the role of CRF is unknown. The present study aimed to test the hypothesis that CRF plays a critical role in the timing of puberty. On postnatal day (pnd) 28, female rat pups were chronically implanted with i.c.v. cannulae and received 14 days of administration of either CRF, CRF receptor antagonist (astressin-B) or artificial cerebrospinal fluid via an osmotic mini-pump. A separate group of rats served as nonsurgical controls. As a marker of puberty, rats were monitored for vaginal opening and first vaginal oestrus. Levels of CRF, CRF receptor types 1 and 2 (CRF-R1, CRF-R2) mRNA expression in micropunches of the medial preoptic area (mPOA), hypothalamic paraventricular nucleus (PVN) and arcuate nucleus (ARC) were determined across pubertal development; brain tissue was collected from a naive group of rats on pnd 14, 32, on the day of vaginal opening, and pnd 77 (Adult). Administration of CRF resulted in a delay in the onset of puberty, whereas astressin-B advanced pubertal onset. Additionally, CRF and CRF-R1 mRNA expression was reduced in the mPOA, but not ARC, at puberty. In the PVN, expression of CRF, but not CRF-R1 mRNA, was reduced at the time of puberty. These data support the hypothesis that CRF signalling may play an important role in modulating the timing of puberty in the rat.

Naturally occurring variations in defensive burying behavior are associated with differences in vasopressin, oxytocin, and androgen receptors in the male rat

Largely ignored in tests of defensive burying is the capacity for individual animals to display marked variations in active coping behaviors. To expose the neurobiological correlates of this behavioral differentiation rats were exposed to a mousetrap that was remotely triggered upon approach to remove the quality of pain. Relative to animals showing no significant levels of defensive burying activity, rats showing sustained elevations in defensive burying displayed higher levels of arginine vasopressin (AVP) mRNA and increased numbers of androgen receptor positive cells in the medial amygdala and posterior bed nuclei of the stria terminalis, brain regions that integrate emotional appraisal and sensory information. In contrast, animals showing little to no defensive burying responses displayed relatively higher levels of AVP and oxytocin (OT) mRNA within the supraoptic nucleus and subregions of the paraventricular nucleus of the hypothalamus responsible for neuroendocrine and autonomic function. Finally, animals showing sustained levels of burying also displayed increased levels of testosterone and pituitary-adrenal hormones under stress conditions. These findings implicate roles for central AVP and OT in mediating differential avoidance behaviors and demonstrate the utility of using a pain-free test of defensive burying as a framework for exploring naturally occurring differences in coping style and neuroendocrine capacity.

Investigations of HPA function and the enduring consequences of stressors in adolescence in animal models

Developmental differences in hypothalamic-pituitary-adrenal (HPA) axis responsiveness to stressors and ongoing development of glucocorticoid-sensitive brain regions in adolescence suggest that similar to the neonatal period of ontogeny, adolescence may also be a sensitive period for programming effects of stressors on the central nervous system. Although research on this period of life is scarce compared to early life and adulthood, the available research indicates that effects of stress exposure during adolescence differ from, and may be longer-lasting than, effects of the same stress exposure in adulthood. Research progress in animal models in this field is reviewed including HPA function and the enduring effects of stress exposures in adolescence on sensitivity to drugs of abuse, learning and memory, and emotional behaviour in adulthood. The effects of adolescent stress depend on a number of factors, including the age, gender, the duration of stress exposure, the type of stressor, and the time between stress exposure and testing.

Prozac during puberty: distinctive effects on neurogenesis as a function of age and sex

Neurogenesis is a possible substrate through which antidepressants alleviate symptoms of depression. In adult male rodents and primates, chronic treatment with fluoxetine increases neurogenesis in the hippocampal formation. Little is known about the effects of the antidepressant on neurogenesis during puberty or in female animals at any age. Therefore we examined the effects of chronic fluoxetine treatment on cell proliferation and survival in male and female rats during puberty and adulthood. Adult and peri-pubescent male and female rats were treated chronically with fluoxetine (Prozac, 5 mg/kg) or saline. Subsequently rats received a single injection of 5-bromo-2'-deoxyuridine (BrdU; 200 mg/kg) to label DNA synthesis. Rats were sacrificed 2 h, 24 h, or 28 days after BrdU injection to examine cell proliferation, survival and cell fate. Fluoxetine increased cell proliferation in adult male rats but not in peri-pubescent males or female rats of any age or stage of the estrous cycle. Treatment did not alter the number of surviving cells in the male hippocampus but decreased survival in the female hippocampus. Thus, fluoxetine has distinctive effects on neurogenesis as a function of age and sex. Circulating levels of the stress hormone corticosterone were also examined. Treatment of female rats with fluoxetine during puberty decreased circulating levels of corticosterone in adults, even in the absence of the drug suggesting disruption of maturation of the hypothalamic-pituitary-adrenal axis.

Organizational role for pubertal androgens on adult hypothalamic-pituitary-adrenal sensitivity to testosterone in the male rat

The inhibitory effect of androgens on the hypothalamic-pituitary-adrenal (HPA) axis in basal and stress conditions in adult male rats is well documented. Major sex-related neuroendocrine changes take place during puberty. There is a robust rise in production and secretion of gonadal steroids, which is thought to underlie numerous neural and behavioural changes brought on after puberty. The present study investigated the effect of the pubertal rise in gonadal steroid levels on the subsequent adult corticosterone profile, particularly the sensitivity of the adult HPA axis to testosterone. Animals were castrated either prepubertally (28 days) or in adulthood (11 weeks) and adult animals were subsequently treated with subcutaneous implants containing either testosterone or cholesterol. Using an automated blood sampling system, blood was collected from each freely moving, conscious rat every 10 min (i) over a 24 h period; (ii) in response to 10 min of noise stress, and (iii) following an immunological challenge with lipopolysaccharide (LPS). Analysis revealed that testosterone treatment did not significantly affect overall corticosterone release over the 24 h period in adult animals castrated before puberty in contrast to animals castrated in adulthood in which testosterone significantly suppressed corticosterone secretion. Following either a noise stress or LPS injection, testosterone treatment did not affect the hypothalamic or adrenal stress response in animals castrated prepubertally. Testosterone significantly suppressed the corticotrophin-releasing hormone and arginine vasopressin mRNA as well as the corticosterone response to LPS in castrated animals that had had their testes intact over puberty. These data provide evidence that puberty is a critical organizational period during which rising levels of gonadal steroids programme the sensitivity of the adult HPA axis to gonadal steroids in adulthood.

Social and non-social anxiety in adolescent and adult rats after repeated restraint

Adolescence is associated with potentially stressful challenges, and adolescents may differ from adults in their stress responsivity. To investigate possible age-related differences in stress responsiveness, the consequences of repeated restraint stress (90 min/day for 5 days) on anxiety, as indexed using the elevated plus-maze (EPM) and modified social interaction (SI) tests, were assessed in adolescent and adult Sprague-Dawley male and female rats. Control groups at each age included non-stressed and socially deprived animals, with plasma corticosterone (CORT) levels also measured in another group of rats on days 1 and 5 of stress (sampled 0, 30, 60, 90, and 120 min following restraint onset). While repeatedly restrained animals exhibited similar anxiety levels compared to non-stressed controls in the EPM, restraint stress increased anxiety at both ages in the SI test (as indexed by reduced social investigation and social preference). Daily weight gain measurements, however, revealed more marked stress-related suppression of body weight in adolescents versus adults. Analysis of stress-induced increases in CORT likewise showed that adolescents demonstrated less habituation than adults, embedded within typical sex differences in CORT magnitude (females greater than males) and age differences in CORT recovery (adolescents slower than adults). Despite no observable age-related differences in the behavioral response to restraint, adolescents were more sensitive to the repeated stressor in terms of physiological indices of attenuated weight gain and habituation of stress-induced CORT.

Effects of prenatal ethanol exposure on hypothalamic-pituitary-adrenal function across the estrous cycle

BACKGROUND

Rats prenatally exposed to ethanol (E) typically show increased hypothalamic-pituitary-adrenal (HPA) responses to stressors in adulthood. Importantly, prenatal ethanol may differentially alter stress responsiveness in male and female offspring, suggesting a role for the gonadal hormones in mediating the effects of ethanol on HPA activity. We investigated the role of ethanol-induced changes in hypothalamic-pituitary-gonadal (HPG) activity in the differential HPA regulation observed in E compared to control females across the estrous cycle.

METHODS

Peripheral hormones and changes in central neuropeptide mRNA levels were measured across the estrous cycle in adult female offspring from E, pair-fed (PF) and ad libitum-fed control (C) dams.

RESULTS

Ethanol females showed normal estrous cyclicity (vaginal smears) but delayed sexual maturation (vaginal opening). Both HPG and HPA activity were differentially altered in E (and in some cases, PF) compared to control females as a function of estrous cycle stage. In relation to HPG activity, E and PF females had higher basal and stress estradiol (E(2)) levels in proestrus compared to other phases of the cycle, and decreased GnRH mRNA levels compared to C females in diestrus. Further, E females had greater variation in LH than PF and C females across the cycle, and in proestrus, only E females showed a significant LH increase following stress. In relation to HPA activity, both basal and stress CORT levels and overall ACTH levels were greater in E than in C females in proestrus. Furthermore, AVP mRNA levels were increased overall in E compared to PF and C females.

CONCLUSIONS

These data demonstrate ethanol-induced changes in both HPG and HPA activity that are estrous phase-specific, and support the possibility that changes in HPA activity in E females may reflect differential sensitivity to ovarian steroids. E females appear to have an increased HPA sensitivity to E(2), and a possible shift toward AVP regulation of HPA activity. That PF were similar to E females on some measures suggests that nutritional effects of diet or food restriction played a role in mediating at least some of the changes observed.

A role for the androgen metabolite, 5alpha-androstane-3beta,17beta-diol, in modulating oestrogen receptor beta-mediated regulation of hormonal stress reactivity

Activation of the hypothalamic-pituitary-adrenal (HPA) axis is a basic response of animals to environmental perturbations that threaten homeostasis. These responses are regulated by neurones in the paraventricular nucleus of the hypothalamus (PVN) that synthesise and secrete corticotrophin-releasing hormone (CRH). Other PVN neuropeptides, such as arginine vasopressin and oxytocin, can also modulate activity of CRH neurones in the PVN and enhance CRH secretagogue activity of the anterior pituitary gland. In rodents, sex differences in HPA reactivity are well established; females exhibit a more robust activation of the HPA axis after stress than do males. These sex differences primarily result from opposing actions of sex steroids, testosterone and oestrogen, on HPA function. Ostreogen enhances stress activated adrenocorticotrophic hormone (ACTH) and corticosterone (CORT) secretion, whereas testosterone decreases the gain of the HPA axis and inhibits ACTH and CORT responses to stress. Data show that androgens can act directly on PVN neurones in the male rat through a novel pathway involving oestrogen receptor (ER)beta, whereas oestrogen acts predominantly through ERalpha. Thus, we examined the hypothesis that, in males, testosterone suppresses HPA function via an androgen metabolite that binds ERbeta. Clues to the neurobiological mechanisms underlying such a novel action can be gleaned from studies showing extensive colocalisation of ERbeta in oxytocin-containing cells of the PVN. Hence, in this review, we address the possibility that testosterone inhibits HPA reactivity by metabolising to 5alpha-androstane-3beta,17beta-diol, a compound that binds ERbeta and regulates oxytocin containing neurones of the PVN. These findings suggest a re-evaluation of studies examining pathways for androgen receptor signalling.

Gender differences in corticotropin and corticosterone secretion and corticotropin-releasing factor mRNA expression in the paraventricular nucleus of the hypothalamus and the central nucleus of the amygdala in response to footshock stress or psychological stress in rats

Anorexia nervosa is mostly seen in adolescent females, although the gender-differentiation mechanism is unclear. Corticotropin-releasing factor (CRF), a key peptide for stress responses such as inhibition of food intake, increases in arousal and locomotor activity, and gonadal dysfunction, is thought to be involved in the pathophysiology of anorexia nervosa. CRF in the paraventricular nucleus of the hypothalamus (PVN) and CRF in the central nucleus of the amygdala (CeA) are involved in the regulation of stress responses, and gender differences in CRF mRNA expression in these regions in response to various stressors are controversial. We therefore examined CRF gene expression in the PVN and CeA as well as corticotropin (ACTH) and corticosterone secretion in response to a 60-min period of electric footshock (FS) or psychological stress (PS) induced by a communication box in both male and female rats in proestrus or diestrus in an effort to elucidate the mechanism underlying the gender difference in the activity of the hypothalamic-pituitary-adrenal (HPA) axis and the mechanism underlying the remarkable prevalence of anorexia nervosa in females. Female rats in proestrus showed higher basal plasma ACTH and CRF mRNA expression levels in the PVN and CeA than males. Females more rapidly showed higher plasma ACTH and corticosterone levels and a higher CRF mRNA expression level in the PVN in response to FS than males. Although females in both proestrus and diestrus showed significant increases in plasma ACTH and corticosterone and CRF mRNA expression in the PVN in response to PS, no significant responses of the HPA axis to PS were found in males. FS significantly increased CRF mRNA expression in the CeA in both females and males, with significantly higher peaks in females in proestrus than in males, while PS significantly increased CRF mRNA expression in the CeA only in males. These results suggest that gender affects differentially the function of the stress-related regions such as the PVN and CeA. The finding that CRF gene expression in the PVN responds to PS only in females may be a clue to elucidation of the neurobiological mechanism underlying the gender-differential prevalence of anorexia nervosa.

Stress response and the adolescent transition: performance versus peer rejection stressors

Little is known about normative variation in stress response over the adolescent transition. This study examined neuroendocrine and cardiovascular responses to performance and peer rejection stressors over the adolescent transition in a normative sample. Participants were 82 healthy children (ages 7-12 years, n = 39, 22 females) and adolescents (ages 13-17, n = 43, 20 females) recruited through community postings. Following a habituation session, participants completed a performance (public speaking, mental arithmetic, mirror tracing) or peer rejection (exclusion challenges) stress session. Salivary cortisol, salivary alpha amylase (sAA), systolic and diastolic blood pressure (SBP, DBP), and heart rate were measured throughout. Adolescents showed significantly greater cortisol, sAA, SBP, and DBP stress response relative to children. Developmental differences were most pronounced in the performance stress session for cortisol and DBP and in the peer rejection session for sAA and SBP. Heightened physiological stress responses in typical adolescents may facilitate adaptation to new challenges of adolescence and adulthood. In high-risk adolescents, this normative shift may tip the balance toward stress response dysregulation associated with depression and other psychopathology. Specificity of physiological response by stressor type highlights the importance of a multisystem approach to the psychobiology of stress and may also have implications for understanding trajectories to psychopathology.

Selective contributions of the medial preoptic nucleus to testosterone-dependant regulation of the paraventricular nucleus of the hypothalamus and the HPA axis

Previous data have consistently demonstrated an inhibitory effect of androgens on stress-induced hypothalamic-pituitary-adrenal (HPA) responses. Several brain regions may influence androgen-mediated inhibition of the HPA axis, including the medial preoptic area. To test the role of the medial preoptic nucleus (MPN) specifically, we examined in high- and low-testosterone-replaced gonadectomized rats bearing discrete bilateral lesions of the MPN basal and stress-induced indexes of HPA function, and the relative levels of corticotropin-releasing hormone (CRH) and arginine vasopressin (AVP) mRNA in the amygdala. High testosterone replacement decreased plasma adrenocorticotropin hormone (ACTH) and paraventricular nucleus (PVN) Fos responses to restraint exposure in sham- but not in MPN-lesioned animals. AVP-, but not CRH-immunoreactivity staining in the external zone of the median eminence was increased by testosterone in sham animals, and MPN lesions blocked this increment in AVP. A similar interaction between MPN lesions and testosterone occurred on AVP mRNA levels in the medial nucleus of the amygdala. These findings support an involvement of MPN projections in mediating the AVP response to testosterone in both the medial parvocellular PVN and medial amygdala. We conclude that the MPN forms part of an integral circuit that mediates the central effects of gonadal status on neuroendocrine and central stress responses.

Novelty-induced locomotion is positively associated with cocaine ingestion in adolescent rats; anxiety is correlated in adults

The present studies assessed the roles of sex, age, novelty-seeking and plus-maze behavior on cocaine drinking in rats. Cocaine/saccharin solution was available in three daily, 5-hour sessions then a saccharin-only solution was also available in following sessions. In the one-bottle drinking phase, early and late adolescent males, post-natal day 28 (PN28) and PN42, consumed more cocaine/saccharin solution than young adults (PN65), but females did not exhibit significant age differences. Adolescents of both sexes consumed more cocaine/saccharin than adults during choice drinking. Saccharin availability in the two-bottle trials decreased cocaine/saccharin consumption in PN28 and PN65 rats. After a drug-free period, cocaine-stimulated locomotion was lower in cocaine/saccharin drinking than saccharin-only males, indicating tolerance. We tested the hypothesis that individual differences in pre-screened behavioral traits would correlate with cocaine/saccharin consumption in PN28 and PN65 male rats. High locomotor responses to novelty were associated with greater cocaine/saccharin drinking in adults in one-bottle sessions. In the subsequent choice drinking phase, correlations were age-specific. Adolescents with high novelty-induced locomotion and adults that spent less time on open arms of the elevated plus-maze drank more cocaine/saccharin. Thus, behavioral phenotypes correlated with individual differences in cocaine/saccharin consumption in an age-related manner.

CRHR1 polymorphisms predict bone density in survivors of acute lymphoblastic leukemia

PURPOSE

Corticosteroids are a critical component of therapy for acute lymphoblastic leukemia (ALL) but are associated with late effects, such as osteoporosis. Risk factors remain poorly defined. Because CRHR1 polymorphisms have been associated with other corticosteroid effects, our goal was to define whether CRHR1 polymorphisms predict which patients with ALL are likely to develop bone mineral deficits.

PATIENTS AND METHODS

The mean bone mineral density z scores of 309 long-term survivors of ALL were determined by quantitative computed tomography of the trabecular lumbar spine. We analyzed whether CRHR1 genotypes, adjusted for sex, ALL treatment regimen, and weight, could predict bone density.

RESULTS

We found that three single nucleotide polymorphisms (SNPs), all in linkage disequilibrium, were associated with bone density in a sex-specific manner. Bone density was lower in males (P = .001), in nonblack patients (P < .08), in those who were not overweight (P < .001), and in those who received intensive antimetabolites and glucocorticoids (P < .001). After adjustment for these features, the G allele at the rs1876828 SNP was associated with lower z scores (P = .02) in males but tended to have the opposite association in females (P = .09).

CONCLUSION

CRHR1 polymorphisms may impact the risk of bone density deficits in patients treated with corticosteroids and antimetabolites in a sex-specific manner.

Hypothalamic neuropeptide expression of juvenile and middle-aged rats after early postnatal food restriction

Rats subjected to early postnatal food restriction (FR) show persistent changes in energy balance. The hypothalamus plays a major role in the regulation of energy balance. Therefore, we hypothesized that early postnatal food restriction induces developmental programming of hypothalamic gene expression of neuropeptides involved in this regulation. In the hypothalamus of juvenile and middle-aged rats that were raised in control (10 pups) or FR litters (20 pups), gene expression was investigated for neuropeptide Y (NPY), agouti-related protein (AgRP), proopiomelanocortin (POMC), and cocaine- and amphetamine-regulated transcript (CART) in the arcuate nucleus (ARC); CRH and TRH in the paraventricular nucleus; and melanin-concentrating hormone (MCH) and orexin in the lateral hypothalamic area. Early postnatal FR acutely and persistently reduced body size. Juvenile FR rats had significantly reduced CART gene expression and increased MCH expression. In middle-aged FR rats, POMC and CART mRNA levels were significantly reduced. The ratio between expression of the ARC orexigenic peptides (NPY and AgRP) and anorexigenic peptides (POMC and CART) was increased in juvenile, but not in middle-aged, FR rats. These results suggest that in neonatal rats, FR already triggers the ARC, and to a lesser extent the lateral hypothalamic area, but not the paraventricular nucleus, to increase expression of orexigenic relative to anorexigenic peptides. In addition, with enduring small body size and normalized hypothalamic gene expression, the adult FR rats appeared to have accepted this smaller body size as normal. This suggests that the body weight set-point was differently programmed in animals with early postnatal FR.

Neonatal gonadectomy and adult testosterone replacement suggest an involvement of limbic arginine vasopressin and androgen receptors in the organization of the hypothalamic-pituitary-adrenal axis

Testosterone exposure during critical periods of development exerts major organizing effects on the hypothalamic-pituitary-adrenal (HPA) axis. Here we examined how neonatal gonadectomy (GDX) with or without testosterone treatment during the first week of life alters the HPA response to adult testosterone replacement in 65-d-old male rats. As adults, neonatal GDX rats showed higher levels of plasma corticosterone and Fos activation in medial parvocellular part of the paraventricular nucleus of the hypothalamus under basal conditions and during 30 min of restraint exposure. These responses were normalized with testosterone treatment on postnatal d 1-5 but were not restored with adult testosterone replacement. As adults, neonatal GDX rats also showed a decrease in the number of androgen receptor and arginine vasopressin-positive cells in the bed nucleus of the stria terminalis and in the medial nucleus of the amygdala, and both of these responses were reversed with postnatal testosterone treatment. In stressed and unstressed animals, the number of androgen receptors and arginine vasopressin-expressing neurons in both of these nuclei correlated negatively with corticosterone concentrations in plasma and Fos levels in the paraventricular nucleus. Taken together, our findings suggest that testosterone exposure during the neonatal period primes the adult HPA response to testosterone by altering androgen receptor levels and function within afferent mediators of basal and stress-related input to the HPA axis.

The role of the arginine vasopressin Avp1b receptor in the acute neuroendocrine action of antidepressants

In times of stress the hypothalamic-pituitary-adrenal (HPA) axis is activated and releases two neurohormones, corticotropin-releasing hormone (Crh) and arginine vasopressin (Avp), to synergistically stimulate the secretion of adrenocorticotropin hormone (ACTH) from the anterior pituitary, culminating in a rise in circulating glucocorticoids. Avp mediates its actions at the Avp V1b receptor (Avpr1b) present on pituitary corticotropes. Dysregulation of the stress response is associated with the pathophysiology of depression and a major treatment involves increasing the availability of monamines at the synaptic cleft. Acute administration of selective serotonin reuptake inhibitors (SSRI) and tricyclic antidepressants (TCA) has previously been shown to activate the HPA axis. The present study was undertaken to evaluate the involvement of the Avpr1b in the HPA axis response to acute SC administration of an SSRI (fluoxetine 10mg/kg) and a TCA (desipramine 10mg/kg). We measured plasma ACTH and corticosterone (CORT) levels and neuropeptide mRNA expression in the hypothalamic paraventricular nucleus (PVN) of Avpr1b knockout (KO) mice and wild-type controls. Fluoxetine and desipramine administration significantly attenuated plasma ACTH and CORT levels in male and female Avpr1b KO mice when compared to their wild-type counterparts. Avp, oxytocin (Oxt) and Crh mRNA expression in the PVN did not change in fluoxetine-treated male Avpr1b KO or wild-type mice. In contrast, fluoxetine treatment increased PVN Avp mRNA levels in female Avpr1b wild type but not KO animals. PVN Oxt mRNA levels increased in fluoxetine-treated female mice of both genotypes. The data suggests that the Avpr1b is required to drive the HPA axis response to acute antidepressant treatment and provides further evidence of a sexual dichotomy in the regulation of PVN Avp/Oxt gene expression following antidepressant administration.

An alternate pathway for androgen regulation of brain function: activation of estrogen receptor beta by the metabolite of dihydrotestosterone, 5alpha-androstane-3beta,17beta-diol

The complexity of gonadal steroid hormone actions is reflected in their broad and diverse effects on a host of integrated systems including reproductive physiology, sexual behavior, stress responses, immune function, cognition, and neural protection. Understanding the specific contributions of androgens and estrogens in neurons that mediate these important biological processes is central to the study of neuroendocrinology. Of particular interest in recent years has been the biological role of androgen metabolites. The goal of this review is to highlight recent data delineating the specific brain targets for the dihydrotestosterone metabolite, 5alpha-androstane, 3beta,17beta-diol (3beta-Diol). Studies using both in vitro and in vivo approaches provide compelling evidence that 3beta-Diol is an important modulator of the stress response mediated by the hypothalmo-pituitary-adrenal axis. Furthermore, the actions of 3beta-Diol are mediated by estrogen receptors, and not androgen receptors, often through a canonical estrogen response element in the promoter of a given target gene. These novel findings compel us to re-evaluate the interpretation of past studies and the design of future experiments aimed at elucidating the specific effects of androgen receptor signaling pathways.

Glucocorticoid receptor mRNA expression in the hippocampal formation of male rats before and after pubertal development in response to acute or repeated stress

Numerous studies have established that adolescence is marked by substantial changes in stress reactivity and hippocampal function. Glucocorticoid receptors (GRs) in the hippocampus are imperative in corticosterone-dependent gene transcription when glucocorticoid levels are relatively high, such as during periods of stress. As reported previously, in reaction to acute stress, prepubertal animals show a significantly more protracted corticosterone response compared to adults. Chronic stress, however, results in a higher peak response, but a faster return to baseline in prepubertal compared to adult animals. Thus, depending on the developmental stage and experience of the animal, the hippocampus is exposed to different concentrations and durations of corticosterone. The present set of experiments assessed the effects of acute or repeated stress on GR mRNA expression in the dorsal and ventral hippocampal formation either before or after pubertal maturation in male rats. We found that acute stress results in a significant decrease in GR mRNA in the CA1 pyramidal cell layer and dentate gyrus in the dorsal and ventral hippocampal formation of both prepubertal and adult males. In response to repeated stress, we found no differences in GR expression in either the dorsal or ventral hippocampus. Thus, despite the dramatic differences in corticosterone concentration following stress at these two developmental stages, the stress-induced changes in GR expression in the hippocampus before and after pubertal maturation were more similar than different. These data point to a dissociation between differential stress-induced corticosterone responses and regulation of hippocampal GR levels in prepubertal and adult animals.

Identifying early behavioral and molecular markers of future stress sensitivity

Puberty is a plastic period of neurological development when critical maturation of stress pathways occurs. Abnormal maturation may be predictive of future stress sensitivity and affective disorder risk. To identify potential early markers of stress-related disease predisposition, we examined physiological and behavioral stress responses in male pubertal mice compared with adults, using a genetic model of elevated stress sensitivity, CRF receptor-2 (CRFR2)-deficient mice. Juvenile mice of both genotypes exhibited greater basal and stress-induced corticosterone levels than adult mice, indicating that overall hypothalamic-pituitary-adrenal axis sensitivity diminishes in adulthood. However, juvenile CRFR2-deficient mice displayed a delayed stress recovery typical of adults of this genotype, suggesting an early marker of stress sensitivity. The adult phenotype of reduced hippocampal glucocorticoid receptor expression in these sensitive mice was also detected during puberty. This reduction may account for an impaired hypothalamic-pituitary-adrenal axis negative feedback and as such be an early indicator of a stress-sensitive phenotype. Examination of behavioral responses to stress revealed that CRFR2-deficient mice show exaggerated postpubertal maturation. Although wild-type mice did not alter their burying response to stress-provoking marbles after puberty, CRFR2-deficient mice showed a dramatic increase in burying behavior. We conclude that identification of abnormal pubertal stress pathway maturation may be predictive of adult heightened stress sensitivity and future susceptibility to stress-related affective disorders.

Androgen receptor expressing neurons that project to the paraventricular nucleus of the hypothalamus in the male rat

Androgen receptors are distributed throughout the central nervous system and are contained by a variety of nuclei that are known to project to or regulate the paraventricular nucleus (PVN) of the hypothalamus, the final common pathway by which the brain regulates the hypothalamic-pituitary-adrenal (HPA) response to homeostatic threat. Here we characterized androgen receptor staining within cells identified as projecting to the PVN in male rats bearing iontophoretic or crystalline injections of the retrograde tracer FluoroGold aimed at the caudal two-thirds of the nucleus, where corticotropin-releasing hormone-expressing neurons are amassed. Androgen receptor (AR) and FluoroGold (FG) double labeling was revealed throughout the limbic forebrain, including scattered numbers of cells within the anterior and posterior subdivisions of the bed nuclei of the stria terminalis; the medial zone of the hypothalamus, including large numbers of AR-FG-positive cells within the anteroventral periventricular and medial preoptic cell groups. Strong and consistent colabeling was also revealed throughout the hindbrain, predominantly within the periaqueductal gray and the lateral parabrachial nucleus, and within various medullary cell groups identified as catecholaminergic, predominantly C1 and A1 neurons of the ventral medulla. These connectional data predict that androgens can act on a large assortment of multimodal inputs to the PVN, including those involved with the processing of various types of sensory and limbic information, and provide an anatomical framework for understanding how gonadal status could contribute to individual differences in HPA function.

Social control of brain morphology in a eusocial mammal

Social status impacts reproductive behavior in diverse vertebrate species, but little is known about how it affects brain morphology. We explore this in the naked mole-rat, a species with the most rigidly organized reproductive hierarchy among mammals. Naked mole-rats live in large, subterranean colonies where breeding is restricted to a single female and small number of males. All other members of the colony, known as subordinates, are reproductively suppressed. Subordinates can become breeders if removed from the colony and placed with an opposite sex partner, but in nature most individuals never attain reproductive status. We examined the brains of breeding and subordinate naked mole-rats of both sexes, including several regions linked to reproduction and shown to be sexually dimorphic in other mammals. Stereological analyses revealed that neural morphology depends on status, such that breeders, regardless of sex, had more cells than subordinates in the ventromedial nucleus of the hypothalamus and a larger volume of the bed nucleus of the stria terminalis, paraventricular nucleus, and medial amygdala. Several other brain regions examined were unaffected. Surprisingly, males and females did not differ on any measure. These findings provide evidence that a change in social status triggers considerable neural remodeling and indicate that status, rather than sex, has a predominant role in determining neural structure in this remarkably social mammal.

Region- and sex-specific modulation of anxiety behaviours in the rat

Recent studies in both animals and humans indicate that gonadal hormones have profound control over emotional states, and certainly contribute to the increased occurrence of psychiatric illness in women. Reports, as reviewed here, suggest that two important regions of the limbic system, the central nucleus of the amygdala (CeA) and the bed nucleus of the stria terminalis (BNST), control different aspects of emotional behaviour. Short-term cue-specific emotional responses, like Pavlovian fear conditioning, require activation of the CeA, while long-duration and contextual emotional responses, are dependant on the BNST. There is accumulating experimental evidence that gender and sex hormones specifically modulate BNST-mediated anxiety behaviours. Moreover, the functional separation between the CeA and the BNST may be exaggerated during lactation in the rat, a time of profound hormonal and behavioural change. In this study, the effects of sex hormones on fear and anxiety are reviewed with an emphasis on the differential effects of these hormones on functions subserved by the BNST as opposed to the CeA. Studies, as highlighted here, looking at sex hormone and gender effects on the ability of corticotrophin-releasing factor and bright ambient light to enhance startle, emphasise the importance of understanding both the effect of, and brain region where, gonadal hormones exert their control over emotional behaviour.

The effects of acute stress and pubertal development on metabolic hormones in the rat

A dramatic change in stress responsiveness occurs during pubertal development such that stress-induced corticosterone secretion in prepubertal animals takes 45-60 min longer to return to baseline compared to adults. Though corticosterone is known to influence energy mobilization, it is presently unknown whether stressors affect other hormones important in energy utilization and metabolism differentially in animals before and after pubertal development. Therefore, we exposed prepubertal (28 days of age) and adult (77 days of age) male rats to a single 30 min session of restraint stress in either the light or dark phase of the animals' light-dark (LD) cycle and measured plasma glucose, insulin and thyroid hormones (thyroxine (T4) and triiodothyronine (T3)). We found similar stress-induced increases in plasma glucose levels in prepubertal and adult animals in the LD phase of the LD cycle. We also found that prepubertal animals have lower circulating insulin and total and free T4 levels, but higher total and free T3 levels compared to adults in both the light and dark phases (LD). Interestingly, insulin and thyroid hormone levels were unaffected by acute stress at either age or time of day. These data indicate that, despite prepubertal animals showing an extended glucocorticoid stress response after a single acute exposure to stress, glucose levels are similarly affected by acute stress in prepubertal and adult animals. Furthermore, though stage of development significantly affects the levels of peripheral metabolic hormones such as insulin, T4 and T3, acute stress does not appreciably influence their secretion before or after puberty.

Androgen and estrogen receptor-beta distribution within spinal-projecting and neurosecretory neurons in the paraventricular nucleus of the male rat

Activation of the hypothalamic-pituitary-adrenal (HPA) axis is initiated by neurosecretory neurons residing within the medial parvicellular part of the hypothalamic paraventricular nucleus (PVN). Despite the potency by which sex steroids operate on HPA and medial parvocellular responses to stress, previous topographic and phenotypic studies suggest that gonadal steroid hormone receptors are scarcely, if at all, expressed by PVN neurons controlling anterior pituitary corticotropes. Guided by the pattern of retrograde accumulation of fluorogold, we used a direct connectional approach to define the distribution of androgen receptors (AR) and estrogen-beta receptors (ER-beta) within populations of neurosecretory vs. nonneurosecretory neurons in the PVN. Juxtaposition of AR-immunoreactivity (ir) and ER-beta mRNA to the pattern of intravenous fluorogold labeling showed these steroid hormone receptors to be concentrated within portions of the PVN devoid of neurosecretory neurons. Superimposing receptor profiles onto the pattern of spinal retrograde labeling confirmed a selective distribution of AR-ir within autonomic-related cells of the medial parvocellular division, including its dorsal, lateral, and ventral medial components. ER-beta mRNA expression was likewise concentrated within regions accumulating spinal tracer, highest within the ventral aspect of the PVN. These results indicate a direct influence of gonadal hormones on preautonomic effector neurons and remain in keeping with an indirect influence of androgens on adrenocorticotropin-regulating neurons in the PVN.

HPA function in adolescence: Role of sex hormones in its regulation and the enduring consequences of exposure to stressors

The hypothalamic-pituitary-adrenal (HPA) axis is one of the physiological systems involved in coping with stressors. There are functional shifts in the HPA axis and its regulation by sex hormones over the lifespan that allow the animal to meet the challenges of the internal and external environment that are specific to each stage of development. Sex differences in HPA function emerge over adolescence, a phenomenon reflecting the concomitant initiation of regulatory effects of sex hormones. The focus of this review is recent research on differences between adolescents and adults in HPA function and the enduring effects of exposure to stressors in adolescence. During adolescence, HPA function is characterized by a prolonged activation in response to stressors compared to adulthood, which may render ongoing development of the brain vulnerable. Although research has been scarce, there is a growing evidence that exposure to stressors in adolescence may alter behavioural responses to drugs and cognitive performance in adulthood. However, the effects reported appear to be stressor-specific and sex-specific. Such research may contribute toward understanding the increased risk for drug abuse and psychopathology that occurs over adolescence in people.

Social instability in adolescence alters the central and peripheral hypothalamic-pituitary-adrenal responses to a repeated homotypic stressor in male and female rats

There has been little research on effects of chronic stressors on neuroendocrine function in adolescence despite increasing evidence of enduring effects of stressors during this period on behaviour in adulthood. We previously reported that social stress (SS: daily 1 h isolation and new cage partner for 16 days) in adolescence altered locomotor responses to psychostimulants in adulthood. Here, we investigated neuroendocrine responses over the duration of the procedure that may underlie the enduring effects of SS. SS rats were compared to rats undergoing daily isolation only (ISO) and controls (CTL) to determine responses to acute and repeated isolation with and without social instability. At 30 days of age (first isolation), higher plasma corticosterone and corticotrophin-releasing hormone (CRH) mRNA expression in the paraventricular nucleus (PVN) of the hypothalamus and in the central nucleus of the amygdala (CeA) were found in males caged with a new partner (SS) after isolation than those returned to their original partner (ISO). On day 45, SS males and females showed less habituation (higher bioactive levels of corticosterone based on plasma corticosterone and corticosteroid binding globulin levels) to the 16th episode of isolation than did ISO. SS and ISO had higher baseline expression of CRH mRNA in the PVN on day 45 than did CTL, and only CTL had increased levels after isolation. CRH mRNA expression in the CeA increased to a first isolation in CTL and to a 16th isolation in SS but not in ISO males. Modest differences in social interactions were observed between SS and ISO when returned to their cages after isolation. The results suggest that mild social stressors in adolescence impede neuroendocrine adaptation to homotypic stressors. The resultant increase in exposure to glucocorticoids over adolescence may alter ongoing brain development and increase vulnerability to psychopathology.

The influence of 17beta-estradiol on annexin 1 expression in the anterior pituitary of the female rat and in a folliculo-stellate cell line

Annexin 1 (ANXA1) is a Ca2+- and phospholipid-binding protein that plays an important role as a mediator of glucocorticoid action in the host-defence and neuroendocrine systems. Sex differences in hypothalamo-pituitary-adrenal (HPA) axis activity are well documented and a number of studies have demonstrated that gonadal steroids act as regulators of HPA activity. The aim of this study was to investigate the effect of ovariectomy and 17beta-estradiol replacement, and estrous cycle stage, on anterior pituitary ANXA1 content. The amount of anterior pituitary ANXA1 determined by western blotting varied with estrous cycle stage with a peak at estrus declining to a trough at proestrus. Ovariectomy resulted in a significant (P<0 x 05) decrease in anterior pituitary ANXA1 content. Administration of 17beta-estradiol (1 microg/100 g) significantly (P<0 x 01) increased anterior pituitary ANXA1 expression in the ovariectomized animals. In contrast, there was no change in pituitary ANXA1 content in response to 17beta-estradiol in adrenalectomized and adrenalectomized/ovariectomized rats. Treatment of TtT/GF cells, a folliculo-stellate cell line, with 17beta-estradiol (1 x 8-180 nM) increased ANXA1 mRNA expression and increased the amount of ANXA1 protein externalized in response to a dexamethasone stimulus. These results indicate that 17beta-estradiol stimulates ANXA1 expression in the anterior pituitary and in vivo an adrenal factor contributes to the mechanism of action.

Age- and stress-induced changes in corticotropin-releasing hormone mRNA expression in the paraventricular nucleus of the hypothalamus

In reaction to acute stress, prepubertal (25-28 days of age) animals demonstrate a prolonged adrenocorticotropic hormone (ACTH) and corticosterone response compared to adults (>65 days of age), while after chronic stress, prepubertal animals show a higher peak ACTH and corticosterone response, but a faster return to baseline compared to adults. Differential activation of corticotropin-releasing hormone (CRH) neurons in the paraventricular nucleus of the hypothalamus (PVN) of prepubertal and adult animals have been suggested to mediate these changes in stress responsiveness. The purpose of the present set of experiments was to further elucidate possible differences in PVN structure and function in prepubertal (28 days of age) and adult (77 days of age) male rats. The results indicate that PVN volume and somal size and cell number are similar in the parvocellular and magnocellular subdivision of the PVN before and after pubertal development. Furthermore, after a peripheral injection of the retrograde tracer Fluoro-Gold (FG), prepubertal and adult males demonstrate similar numbers of anterior pituitary projecting neurosecretory neurons in the parvocellular region of the PVN. Finally, using in situ hybridization we show that in response to acute stress, CRH mRNA in the PVN was affected by both age and stress such that prepubertal males have greater CRH expression than adults and both prepubertal and adult males show significant stress-induced increases in CRH mRNA. Interestingly, in response to repeated restraint, neither age nor stress significantly influence CRH expression. Together, these data indicate that both age and experience with stress interact to modulate CRH expression in the PVN.

Puberty, hormones, and sex differences in alcohol abuse and dependence

Sex differences in patterns of drinking and rates of alcohol abuse and dependence begin to emerge during the transition from late puberty to young adulthood. Increases in pubertal hormones, including gonadal and stress hormones, are a prominent developmental feature of adolescence and could contribute to the progression of sex differences in alcohol drinking patterns during puberty. This paper reviews experimental and correlational studies of gonadal and stress-related hormone changes and their effects on alcohol drinking and other associated actions of alcohol. Mechanisms are suggested by which reproductive hormones and stress-related hormones may modulate neural circuits within the brain reward system to produce sex differences in alcohol drinking patterns and vulnerability to alcohol abuse and dependence which become apparent during the late pubertal period.

Involvement of the endocannabinoid system in the ability of long-term tricyclic antidepressant treatment to suppress stress-induced activation of the hypothalamic-pituitary-adrenal axis

The efficacy of antidepressants has been linked in part to their ability to reduce activity of the hypothalamic-pituitary-adrenal (HPA) axis; however, the mechanism by which antidepressants regulate the HPA axis is largely unknown. Given that recent research has demonstrated that endocannabinoids can regulate the HPA axis and exhibit antidepressant potential, we examined the hypothesis that the endocannabinoid system is regulated by long-term antidepressant treatment. Three-week administration of the tricyclic antidepressant desipramine (10 mg/kg/day) resulted in a significant increase in the density of the cannabinoid CB(1) receptor in the hippocampus and hypothalamus, without significantly altering endocannabinoid content in any brain structure examined. Furthermore, chronic desipramine treatment resulted in a reduction in both secretion of corticosterone and the induction of the immediate early gene c-fos in the medial dorsal parvocellular region of the paraventricular nucleus of the hypothalamus (PVN) following a 5 min exposure to swim stress. Acute treatment with the CB(1) receptor antagonist, AM251 (1 mg/kg), before exposure to swim stress, completely occluded the ability of desipramine to reduce both corticosterone secretion and induction of c-fos expression in the PVN. Collectively, these data demonstrate that CB(1) receptor density in the hippocampus and hypothalamus is increased by chronic tricyclic antidepressant treatment, and suggest that this upregulation could contribute to the ability of tricyclic antidepressants to suppress stress-induced activation of the HPA axis.

Early deprivation, but not maternal separation, attenuates rise in corticosterone levels after exposure to a novel environment in both juvenile and adult female rats

Separation from the maternal nest alters the hypothalamic-pituitary-adrenal (HPA) axis stress response in adult male rats, but little research has addressed how separation affects female rats. The following experiments investigated how early maternal separation from postnatal day (PND) 2 to 14 affected stress-induced corticosterone and ACTH after exposure to an open field in juvenile and adult female rats. Female rats were separated for 5 h daily from mother and littermates (early deprivation: ED), separated from mother but not littermates (maternal separation: MS), or animal facility reared (AFR). Male siblings were left with the mother rat during separation. Female rats were exposed to an open field arena either during the juvenile period (PND 30) or during adulthood (PND 80-100). Results show that ED juvenile female rats showed a lower corticosterone stress response than MS and AFR female rats when measured at 5 min post-stress, but no difference at 20 or 60 min post-stress. In adulthood, ED female rats showed comparable elevations of corticosterone as MS and AFR rats at 5 min post-stress but lower elevations at 20 min. In terms of behavior, there were no significant effects of early experience. However, in adulthood, ED and MS rats tended to show a decreased proportion of inner grid crossings of the open field compared to AFR rats, suggesting a tendency for increased anxiety in these two separation groups.

Lack of annexin 1 results in an increase in corticotroph number in male but not female mice

Annexin 1 (ANXA1) is a member of the annexin family of phospholipid- and calcium-binding proteins with a well demonstrated role in early delayed (30 min to 3 h) inhibitory feedback of glucocorticoids in the pituitary. We have examined corticotrophs in wild-type and ANXA1 knockout mice to determine the effects of lack of ANXA1 in male and female animals. Anterior pituitary tissue from ANXA1 wild-type, heterozygote and null mice was fixed and examined (i) by confocal immunocytochemistry to determine the number of corticotrophs and (ii) by electron microscopy to examine the size, secretory granule population and secretory machinery of corticotrophs. No differences in these parameters were detected in female mice. In male ANXA1 null mice, there were approximately four-fold more corticotrophs than in wild-type animals. However, the corticotrophs in ANXA1 null mice were smaller and had reduced numbers of secretory granules (the reduction in granules paralleled the reduction in cell size). No differences in the numerical density of folliculo-stellate, gonadotroph, lactotroph or somatotroph cells were detected in male ANXA1 null mice. Plasma corticosterone, adrenocorticotrophic hormone (ACTH) and pituitary pro-opiomelanocortin mRNA were unchanged but pituitary ACTH content was increased in male ANXA1 null mice. Interleukin (IL)-6 pituitary content was significantly elevated in male and reduced in female ANXA1 null mice compared to wild-type. In conclusion, these data indicate that ANXA1 deficiency is associated with gender-specific changes in corticotroph number and structure, via direct actions of ANXA1 and/or indirect changes in factors such as IL-6.

Associations between behavior, hormones, and Fos responses to novelty differ in pre- and post-pubertal grass rats

As animals progress from one developmental stage to the next, the nature of the challenges they face can change in systematic ways, as do the mechanisms that enable them to deal effectively with them. Here we examined the changes in the behavioral patterns and neuroendocrine mechanisms associated with exposure to a novel environment before and after the transition from a pre- to a post-pubertal stage of development in the unstriped Nile grass rat (Arvicanthis niloticus), a murid rodent that appears to live in family groups in the wild. We introduced juvenile (28 days old) and adult (60 days old) grass rats to a novel glass aquarium where they were kept for 1 h; controls were maintained in their home cages during this time. Animals were then killed, blood was sampled, and plasma corticosterone and testosterone were measured. Brains were collected and processed for immunohistochemical detection of Fos. Although behavior in the novel environment did not differ as a function of age, corticosterone secretion and Fos expression in a variety of stress-related brain regions were increased by the manipulation to a greater extent in the juveniles compared to the adults. The data suggest a pattern of development in which a novel environment that elicits the same levels of exploratory behaviors in young and adult animals triggers a greater response in stress-related brain regions as well as corticosterone secretion in the more vulnerable young ones.

Pubertal maturation and time of day differentially affect behavioral and neuroendocrine responses following an acute stressor

Puberty markedly influences stress responsiveness such that prepubertal animals show a more protracted corticosterone (CORT) and progesterone response following acute stress compared to adults. In both adult and juvenile rats, circadian time modulates adrenocortical steroids with basal CORT and progesterone levels rising prior to the onset of the dark phase of the light-dark cycle (i.e., active period). How time of day affects the pubertal difference in stress responsiveness and if the behaviors of prepubertal and adult animals are differentially affected by stress and time of testing remain unknown. Thus, we exposed group housed (3 per cage) prepubertal (28d) and adult (77d) male rats to 30 min of restraint in either the early portion of the behaviorally inactive, light (circadian nadir of CORT and progesterone) or behaviorally active, dark (circadian peak) phase of their light-dark cycle and measured ACTH, CORT, progesterone, and home cage behavior before and after the stressor. We found that the extended hormonal stress response demonstrated by prepubertal males occurred at both times of day. However, differences in post-stress behavior were dependent on time of testing. Specifically, although pre- and post-stress behaviors were similarly affected by the stressor in the light phase in prepubertal and adult males, during the dark phase, stress suppressed play behavior in the prepubertal males, and increased their time spent resting together (huddling), while these behaviors were unaffected by stress in the adults. These data indicate that pubertal development and time of day interact to modulate post-stress behavior and demonstrate a dissociation between post-stress hormonal and behavioral responses.

Adolescent rats are protected from the conditioned aversive properties of cocaine and lithium chloride

In humans, most drug use is initiated during adolescence and adolescent users are more likely to become drug-dependent than adult users. Repeated, high levels of use are required for the transition from use to addiction. Individual levels of drug use are thought to result from a balance between the pleasant or rewarding and the unpleasant or aversive properties of the drug. Repeated high levels of drug use are required for the transition from drug use to dependence. We hypothesized that diminished aversive effects of drugs of abuse during adolescence might be one reason for higher rates of use and addiction during this phase. We therefore tested adolescent and adult CD rats in single-dose cocaine conditioned taste aversion (CTA) at a range of doses (10-40 mg/kg), and examined whether various behavioral markers of addiction vulnerability were correlated to outcome in cocaine CTA. We found that adolescents are indeed less susceptible to cocaine CTA. In fact, age was the predominant predictor of CTA outcome, predominating over measures of novelty-seeking, anxiety, and stress hormone levels, which are all known to be related to drug intake in other models. Furthermore, we found that adolescent rats are also less susceptible to conditioned taste aversion to a low dose of a non-addictive substance, lithium chloride. These results suggest that one explanation for elevated drug use and addiction among adolescents is reduced aversive or use-limiting effects of the drugs. This contributes to our understanding of why adolescence is a particularly vulnerable period for development of drug abuse.

Stress history and pubertal development interact to shape hypothalamic-pituitary-adrenal axis plasticity

Both the magnitude and the duration of the hormonal stress response change dramatically during neonatal development and aging as well as with prior experience with a stressor. However, surprisingly little is known with regard to how pubertal maturation and experience with stress interact to affect hypothalamic-pituitary-adrenal axis responsiveness. Because adolescence is a period of neurodevelopmental vulnerabilities and opportunities that may be especially sensitive to stress, it is imperative to more fully understand these interactions. Thus, we examined hormonal and neural responses in prepubertal (28 d of age) and adult (77 d of age) male rats after exposure to acute (30 min) or more chronic (30 min/d for 7 d) restraint stress. We report here that after acute stress, prepubertal males exhibited a significantly prolonged hormonal stress response (e.g. ACTH and total and free corticosterone) compared with adults. In contrast, after chronic stress, prepubertal males exhibited a higher response immediately after the stressor, but a faster return to baseline, compared with adults. Additionally, we demonstrate that this differential stress reactivity is associated with differential neuronal activation in the paraventricular nucleus of the hypothalamus, as measured by FOS immunohistochemistry. Using triple-label immunofluorescence histochemistry, we found that a larger proportion of CRH, but not arginine vasopressin, cells are activated in the arginine vasopressin in response to both acute and chronic stress in prepubertal animals compared with adults. These data indicate that experience-dependent plasticity of the hypothalamic-pituitary-adrenal neuroendocrine axis is significantly influenced by pubertal maturation.