Manipulation of androgens causes different energetic responses to cold in 60- and 40-day-old male rats

Sex differences in the HPA axis

The hypothalamic-pituitary-adrenal (HPA) axis is a major component of the systems that respond to stress, by coordinating the neuroendocrine and autonomic responses. Tightly controlled regulation of HPA responses is critical for maintaining mental and physical health, as hyper- and hypo-activity have been linked to disease states. A long history of research has revealed sex differences in numerous components of the HPA stress system and its responses, which may partially form the basis for sex disparities in disease development. Despite this, many studies use male subjects exclusively, while fewer reports involve females or provide direct sex comparisons. The purpose of this article is to present sex comparisons in the functional and molecular aspects of the HPA axis, through various phases of activity, including basal, acute stress, and chronic stress conditions. The HPA axis in females initiates more rapidly and produces a greater output of stress hormones. This review focuses on the interactions between the gonadal hormone system and the HPA axis as the key mediators of these sex differences, whereby androgens increase and estrogens decrease HPA activity in adulthood. In addition to the effects of gonadal hormones on the adult response, morphological impacts of hormone exposure during development are also involved in mediating sex differences. Additional systems impinging on the HPA axis that contribute to sex differences include the monoamine neurotransmitters norepinephrine and serotonin. Diverse signals originating from the brain and periphery are integrated to determine the level of HPA axis activity, and these signals are, in many cases, sex-specific.

Short-term fluoxetine treatment induces neuroendocrine and behavioral anxiogenic-like responses in adolescent male rats

Fluoxetine (FLX) is prescribed to treat depression and anxiety in adolescent patients. However, FLX has anxiogenic effects during the acute phase of treatment, and caution has been raised due to increased suicidal thinking and behavior. Herein, we sought to study in adolescent (35-day-old) male rats, the effects of short-term FLX treatment (10 mg/kg/day, i.p. for 3-4 days) on hypothalamic-pituitary-adrenal axis activity, serotonin (5-hidroxytriptamine, 5-HT) transporter (SERT) mRNA expression in the dorsal raphe nucleus (DRN), energy balance-related variables and behavioral profiles in the holeboard. Our results revealed that daily FLX administration increased plasma corticosterone (B) concentrations without affecting basal gene expression of corticotrophin releasing hormone in the hypothalamic paraventricular nucleus (PVN) nor of pro-opiomelanocortin in the anterior pituitary. However, FLX had significant effects increasing the mRNA expression of PVN arginine vasopressin (AVP) and reducing SERT mRNA levels in the dorsolateral subdivision of the DRN. In the holeboard, FLX-induced anxiety/emotionality-like behaviors. As expected, FLX treatment was endowed with anorectic effects and reduced body weight gain. Altogether, our study shows that short-term FLX treatment results in physiological, neuroendocrine and behavioral stress-like effects in adolescent male rats. More importantly, considering that the AVP- and 5-HTergic systems: (1) are intimately involved in regulation of the stress response; (2) are regulated by sex hormones and (3) are related to regulation of aggressive behaviors, our results highlight the potential significance of these systems mediating the anxiogenic/emotionality/stress-like responses of adolescent male rats to short-term FLX treatment.

Androgen receptors in the posterior bed nucleus of the stria terminalis increase neuropeptide expression and the stress-induced activation of the paraventricular nucleus of the hypothalamus

The posterior bed nuclei of the stria terminalis (BST) are important neural substrate for relaying limbic influences to the paraventricular nucleus (PVN) of the hypothalamus to inhibit hypothalamic-pituitary-adrenal (HPA) axis responses to emotional stress. Androgen receptor-expressing cells within the posterior BST have been identified as projecting to the PVN region. To test a role for androgen receptors in the posterior BST to inhibit PVN motor neurons, we compared the effects of the non-aromatizable androgen dihydrotestosterone (DHT), the androgen receptor antagonist hydroxyflutamide (HF), or a combination of both drugs implanted unilaterally within the posterior BST. Rats bearing unilateral implants were analyzed for PVN Fos induction in response to acute-restraint stress and relative levels of corticotrophin-releasing hormone and arginine vasopressin (AVP) mRNA. Glutamic acid decarboxylase (GAD) 65 and GAD 67 mRNA were analyzed in the posterior BST to test a local involvement of GABA. There were no changes in GAD expression to support a GABA-related mechanism in the BST. For PVN neuropeptide expression and Fos responses, basic effects were lateralized to the sides of the PVN ipsilateral to the implants. However, opposite to our expectations of an inhibitory influence of androgen receptors in the posterior BST, PVN AVP mRNA and stress-induced Fos were augmented in response to DHT and attenuated in response to HF. These results suggest that a subset of androgen receptor-expressing cells within the posterior BST region may be responsible for increasing the biosynthetic capacity and stress-induced drive of PVN motor neurons.

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.

Pmch expression during early development is critical for normal energy homeostasis

Postnatal development and puberty are times of strong physical maturation and require large quantities of energy. The hypothalamic neuropeptide melanin-concentrating hormone (MCH) regulates nutrient intake and energy homeostasis, but the underlying mechanisms are not completely understood. Here we use a novel rat knockout model in which the MCH precursor Pmch has been inactivated to study the effects of loss of MCH on energy regulation in more detail. Pmch(-/-) rats are lean, hypophagic, osteoporotic, and although endocrine parameters were changed in pmch(-/-) rats, endocrine dynamics were normal, indicating an adaptation to new homeostatic levels rather than disturbed metabolic mechanisms. Detailed body weight growth and feeding behavior analysis revealed that Pmch expression is particularly important during early rat development and puberty, i.e., the first 8 postnatal weeks. Loss of Pmch resulted in a 20% lower set point for body weight that was determined solely during this period and remained unchanged during adulthood. Although the final body weight is diet dependent, the Pmch-deficiency effect was similar for all diets tested in this study. Loss of Pmch affected energy expenditure in both young and adult rats, although these effects seem secondary to the observed hypophagia. Our findings show an important role for Pmch in energy homeostasis determination during early development and indicate that the MCH receptor 1 system is a plausible target for childhood obesity treatment, currently a major health issue in first world countries.

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.

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.

Exogenous testosterone attenuates the integrated central stress response in healthy young women

Animal research has shown that the androgen steroid testosterone, the end product of the hypothalamic-pituitary-gonadal (HPG) axis, down regulates the integrated stress response at multiple levels. These effects have been demonstrated at the level of the amygdala and the bed nucleus of the stria terminalis, and along the different nodes of the hypothalamic-pituitary-adrenal (HPA) axis. The present study was designed to assess effects of exogenous testosterone upon reactivity of the autonomic nervous system and modulation of the acoustic startle reflex in humans. Twenty healthy female participants received double-blind, placebo-controlled sublingual administrations of .5mg testosterone. Measurements were made of phasic electrodermal activity, cardiac responses, and startle reflexes to acoustic probes while participants were exposed to pictures with strongly aversive, neutral, or positive content. Subjective reports of mood and picture evaluations were also obtained. Results support the hypothesis of a generally decreased responsiveness of the stress system by showing reduced skin conductance responses as well as reduced affective startle modulation in anxiety-prone participants after administration of testosterone. Candidate neurobiological mechanisms of action are outlined and discussed, and it is argued that androgens promote dynamic regulation of the stress system through actions upon central neuropeptidergic pathways that control corticotropin releasing hormone (CRH) and arginine vasopressin (AVP) expression. The present findings highlight the importance of further investigation of the possible role of the HPG axis in disorders that are associated with HPA axis dysfunctions.

Central organization of androgen-sensitive pathways to the hypothalamic-pituitary-adrenal axis: implications for individual differences in responses to homeostatic threat and predisposition to disease

Despite clear evidence of the potency by which sex steroids operate on the hypothalamic-pituitary-adrenal (HPA) axis and genuine sex differences in disorders related to HPA dysfunction, the biological significance of this remains largely ignored. Stress-induced increases in circulating glucocorticoid levels serve to meet the metabolic demands of homeostatic threat head-on. Thus, the nature of the stress-adrenal axis is to protect the organism. As one develops, matures, and ages, still newer and competing physiological and environmental demands are encountered. These changing constraints are also met by shifts in sex steroid release, placing this class of steroids beyond the traditional realm of reproductive function. Here we focus on the dose-related and glucocorticoid-interactive nature by which testosterone operates on stress-induced HPA activation. This provides an overview on how to exploit these characteristics towards developing an anatomical framework of testosterone's actions in the brain, and expands upon the idea that centrally projecting arginine vasopressin circuits in the brain act to register and couple testosterone's effects on neuroendocrine and behavioural responses to stress. More generally, the work presented here underscores how a dual adrenal and gonadal systems approach assist in unmasking the bases by which individuals resist or succumb to stress.

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

Individual variations in hypothalamic-pituitary-adrenal (HPA) function are most evident at or beyond the time of puberty, when marked changes in sex steroid release occur. To explore the nature by which gender differences in HPA function emerge we examined in prepubertal (approximately 30-d-old) and postpubertal (approximately 60-d-old) male and female rats HPA activity under basal conditions and in response to 30 min of restraint. Within the ACTH-regulating, medial parvocellular portion of the paraventricular nucleus, restraint-induced Fos protein and arginine vasopressin heteronuclear RNA were lower in 60- than in 30-d-old males. No such age-related shift in the response of these synaptic and transcriptional markers of cellular activation occurred in female rats. Basal CRH mRNA expression levels in the paraventricular nucleus increased with age in female but not male rats. Conversely, only male rats showed an age-related increase in basal CRH mRNA in the central amygdala, suggesting that neuronal and neurosecretory CRH-expressing cell types are subject to different pubertal and gender influences. We conclude that gonadal regulation of the HPA axis develops via distinct mechanisms in males and females. Puberty-related shifts in parvocellular neurosecretory function in males are emphasized by stress-induced shifts in neuronal activation, whereas biosynthetic alterations dominate in female rats.

Thyroid hormone, glucocorticoids, and prolactin at the nexus of physiology, reproduction, and toxicology

A symposium at the 2003 Annual Meeting of the Society of Toxicology brought together an expert group of endocrinologists to review how non-reproductive hormones can affect the endocrine system. This publication captures the essence of those presentations. Paul Cooke and Denise Holsberger recapitulate the evidence of how thyroid hormones affect male and female reproduction, and reproductive development. Ray Witorsch summarizes the many effects of glucocorticoids on the reproductive system. Finally, Paul Sylvester reviews the mechanism of action of prolactin, and reminds us that this ancient hormone has many functions beyond lactation.

Repeated stress in young and old 5-HT(2C) receptor knockout mice

Serotonin (5-HT)(2C) receptor null mutant (knockout, KO) mice develop hyperphagia and midlife obesity. Based upon previous observations indicating altered responsiveness to stressful environmental conditions in these mice, we hypothesized that this KO mouse was hyperresponsive to repeated stress. To test this, we examined the effect of two intensities of repeated stress on food intake and body weight in 5-HT(2C) receptor KO and wild-type (WT) mice. The stressors involved daily cage change (including handling) for 3 days then daily restraint for 4 days. On the final day, mice were immediately decapitated after restraint to assess levels of plasma hormones. Two ages were used: young (12 weeks) and old (32-34 weeks). Basally, young KO were prehyperphagic and weighed the same as WT. In the old mice, KO were frankly hyperphagic and heavier than WT. In response to repeated cage change alone, the genotype-specific difference in food intake in the young group was enhanced, whereas in the old group it was diminished. This stressor did not significantly affect body weight change or caloric efficiency with respect to age or genotype. Repeated restraint had little effect on the young mice. However, in the old mice, KO had decreases in relative body weight and caloric efficiency compared with WT. In the old KO mice, adrenocorticotrophic hormone (ACTH), corticosterone and insulin were increased compared with WT mice. Together, these findings indicate that 5-HT(2C) receptor KO mice are hyperresponsive to repeated stress and this effect is influenced by stressor intensity and initial metabolic state of the mouse.

Functional cross-talk between the hypothalamic-pituitary-gonadal and -adrenal axes

Under normal conditions, the adrenal glucocorticoids, the endproduct of the hypothalamic-pituitary-adrenal (HPA) axis, provide a frontline of defence against threats to homeostasis (i.e. stress). On the other hand, chronic HPA drive and glucocorticoid hypersecretion have been implicated in the pathogenesis of several forms of systemic, neurodegenerative and affective disorders. The HPA axis is subject to gonadal influence, indicated by sex differences in basal and stress HPA function and neuropathologies associated with HPA dysfunction. Functional cross-talk between the gonadal and adrenal axes is due in large part to the interactive effects of sex steroids and glucocorticoids, explaining perhaps why several disease states linked to stress are sex-dependent. Realizing the interactive nature by which the hypothalamic-pituitary-gonadal and HPA systems operate, however, has made it difficult to model how these hormones act in the brain. Manipulation of one endocrine system is not without effects on the other. Simultaneous manipulation and assessment of both endocrine systems can overcome this problem. This dual approach in the male rat reveals that testosterone can act and interact on different aspects of basal and stress HPA function. Basal adrenocorticotropic hormone (ACTH) release is regulated by testosterone-dependent effects on arginine vasopressin synthesis, and corticosterone-dependent effects on corticotropin-releasing hormone (CRH) synthesis in the paraventricular nucleus (PVN) of the hypothalamus. In contrast, testosterone and corticosterone interact on stress-induced ACTH release and drive to the PVN motor neurones. Candidate structures mediating this interaction include several testosterone-sensitive afferents to the HPA axis, including the medial preoptic area, central and medial amygdala and bed nuclei of the stria terminalis. All of these relay homeostatic information and integrate reproductive and social behaviour. Because these modalities are affected by stress in humans, a dual systems approach holds great promise in establishing further links between the neuroendocrinology of stress and the central bases of sex-dependent disorders, including psychiatric, cardiovascular and metabolic disease.