Sexually dimorphic expression of androgen receptor immunoreactivity by somatostatin neurones in rat hypothalamic periventricular nucleus and bed nucleus of the stria terminalis

Sex differences in androgen receptor, estrogen receptor alpha, and c-Fos co-expression with corticotropin releasing factor expressing neurons in restrained adult mice

Gonadal hormone actions through androgen receptor (AR) and estrogen receptor alpha (ERα) regulate sex differences in hypothalamic-pituitary-adrenal (HPA) axis responsivity and stress-related behaviors. Here we tested whether corticotropin releasing factor (CRF) expressing neurons, which are widely known to regulate neuroendocrine and behavioral stress responses, co-express AR and ERα as a potential mechanism for gonadal hormone regulation of these responses. Using Crh-IRES-Cre::Ai9 reporter mice we report high co-localization of AR in CRF neurons within the medial preoptic area (MPOA), bed nucleus of the stria terminalis (BST), medial amygdala (MeA), and ventromedial hypothalamus (VMH), moderate levels within the central amygdala (CeA) and low levels in the paraventricular hypothalamus (PVN). Sex differences in CRF/AR co-expression were found in the principal nucleus of the BST (BSTmpl), CeA, MeA, and VMH (males>females). CRF co-localization with ERα was generally lower relative to AR co-localization. However, high co-expression was found within the MPOA, AVPV, and VMH, with moderate co-expression in the arcuate nucleus (ARC), BST, and MeA and low levels in the PVN and CeA. Sex differences in CRF/ERα co-localization were found in the BSTmpl and PVN (males>females). Finally, we assessed neural activation of CRF neurons in restraint-stressed mice and found greater CRF/c-Fos co-expression in females in the BSTmpl and periaqueductal gray, while co-expression was higher in males within the ARC and dorsal CA1. Given the known role of CRF in regulating behavioral stress responses and the HPA axis, AR/ERα co-expression and sex-specific activation of CRF cell groups indicate potential mechanisms for modulating sex differences in these functions.

Whole-brain connectivity to the bed nucleus of the stria terminalis calretinin-expressing interneurons in male mice

The bed nucleus of the stria terminalis (BNST) is a neuropeptide-enriched brain region that modulates a wide variety of emotional behaviours and states, including stress, anxiety, reward and social interaction. The BNST consists of diverse subregions and neuronal ensembles; however, because of the high molecular heterogeneity within BNST neurons, the mechanisms through which the BNST regulates distinct emotional behaviours remain largely unclear. Prior studies have identified BNST calretinin (CR)-expressing neurons, which lack neuropeptides. Here, employing virus-based cell-type-specific retrograde and anterograde tracing systems, we mapped the whole-brain monosynaptic inputs and axonal projections of BNST CR-expressing neurons in male mice. We found that BNST CR-expressing neurons received inputs mainly from the amygdalopiriform transition area, central amygdala and hippocampus and moderately from the medial preoptic area, basolateral amygdala, paraventricular thalamus and lateral hypothalamus. Within the BNST, plenty of input neurons were primarily located in the oval and interfascicular subregions. Furthermore, numerous BNST CR-expressing neuronal boutons were observed within the BNST but not in other brain regions, thus suggesting that these neurons are a type of interneuron. These results will help further elucidate the neuronal circuits underlying the elaborate and distinct functions of the BNST.

Chronic stress-induced synaptic changes to corticotropin-releasing factor-signaling in the bed nucleus of the stria terminalis

The sexually dimorphic bed nucleus of the stria terminalis (BNST) is comprised of several distinct regions, some of which act as a hub for stress-induced changes in neural circuitry and behavior. In rodents, the anterodorsal BNST is especially affected by chronic exposure to stress, which results in alterations to the corticotropin-releasing factor (CRF)-signaling pathway, including CRF receptors and upstream regulators. Stress increases cellular excitability in BNST CRF+ neurons by potentiating miniature excitatory postsynaptic current (mEPSC) amplitude, altering the resting membrane potential, and diminishing M-currents (a voltage-gated K+ current that stabilizes membrane potential). Rodent anterodorsal and anterolateral BNST neurons are also critical regulators of behavior, including avoidance of aversive contexts and fear learning (especially that of sustained threats). These rodent behaviors are historically associated with anxiety. Furthermore, BNST is implicated in stress-related mood disorders, including anxiety and Post-Traumatic Stress Disorders in humans, and may be linked to sex differences found in mood disorders.

The hypothalamic paraventricular nucleus as a central hub for the estrogenic modulation of neuroendocrine function and behavior

Estradiol and hypothalamic paraventricular nucleus (PVN) help coordinate reproduction with body physiology, growth and metabolism. PVN integrates hormonal and neural signals originating in the periphery, generating an output mediated both by its long-distance neuronal projections, and by a variety of neurohormones produced by its magnocellular and parvocellular neurosecretory cells. Here we review the cyto-and chemo-architecture, the connectivity and function of PVN and the sex-specific regulation exerted by estradiol on PVN neurons and on the expression of neurotransmitters, neuromodulators, neuropeptides and neurohormones in PVN. Classical and non-classical estrogen receptors (ERs) are expressed in neuronal afferents to PVN and in specific PVN interneurons, projecting neurons, neurosecretory neurons and glial cells that are involved in the input-output integration and coordination of neurohormonal signals. Indeed, PVN ERs are known to modulate body homeostatic processes such as autonomic functions, stress response, reproduction, and metabolic control. Finally, the functional implications of the estrogenic modulation of the PVN for body homeostasis are discussed.

Sex and age influence gonadal steroid hormone receptor distributions relative to estrogen receptor β-containing neurons in the mouse hypothalamic paraventricular nucleus

Within the hypothalamic paraventricular nucleus (PVN), estrogen receptor (ER) β and other gonadal hormone receptors play a role in central cardiovascular processes. However, the influence of sex and age on the cellular and subcellular relationships of ERβ with ERα, G-protein ER (GPER1), as well as progestin and androgen receptors (PR and AR) in the PVN is uncertain. In young (2- to 3-month-old) females and males, ERβ-enhanced green fluorescent protein (EGFP) containing neurons were approximately four times greater than ERα-labeled and PR-labeled nuclei in the PVN. In subdivisions of the PVN, young females, compared to males, had: (1) more ERβ-EGFP neurons in neuroendocrine rostral regions; (2) fewer ERα-labeled nuclei in neuroendocrine and autonomic projecting medial subregions; and (3) more ERα-labeled nuclei in an autonomic projecting caudal region. In contrast, young males, compared to females, had approximately 20 times more AR-labeled nuclei, which often colocalized with ERβ-EGFP in neuroendocrine (approximately 70%) and autonomic (approximately 50%) projecting subregions. Ultrastructurally, in soma and dendrites, PVN ERβ-EGFP colocalized primarily with extranuclear AR (approximately 85% soma) and GPER1 (approximately 70% soma). Aged (12- to 24-month-old) males had more ERβ-EGFP neurons in a rostral neuroendocrine subregion compared to aged females and females with accelerated ovarian failure (AOF) and in a caudal autonomic subregion compared to post-AOF females. Late-aged (18- to 24-month-old) females compared to early-aged (12- to 14-month-old) females and AOF females had fewer AR-labeled nuclei in neuroendrocrine and autonomic projecting subregions. These findings indicate that gonadal steroids may directly and indirectly influence PVN neurons via nuclear and extranuclear gonadal hormone receptors in a sex-specific manner.

Functional anatomy of the bed nucleus of the stria terminalis-hypothalamus neural circuitry: Implications for valence surveillance, addiction, feeding, and social behaviors

The bed nucleus of the stria terminalis (BNST) is a medial basal forebrain structure that modulates the hypothalamo-pituitary-adrenal (HPA) axis. The heterogeneous subnuclei of the BNST integrate inputs from mood and reward-related areas and send direct inhibitory projections to the hypothalamus. The connections between the BNST and hypothalamus are conserved across species, promote activation of the HPA axis, and can increase avoidance of aversive environments, which is historically associated with anxiety behaviors. However, BNST-hypothalamus circuitry is also implicated in motivated behaviors, drug seeking, feeding, and sexual behavior. These complex and diverse roles, as well its sexual dimorphism, indicate that the BNST-hypothalamus circuitry is an essential component of the neural circuitry that may underlie various psychiatric diseases, ranging from anorexia to anxiety to addiction. The following review is a cross-species exploration of BNST-hypothalamus circuitry. First, we describe the BNST subnuclei, microcircuitry and complex reciprocal connections with the hypothalamus. We will then discuss the behavioral functions of BNST-hypothalamus circuitry, including valence surveillance, addiction, feeding, and social behavior. Finally, we will address sex differences in morphology and function of the BNST and hypothalamus.

ERα Signaling in GHRH/Kiss1 Dual-Phenotype Neurons Plays Sex-Specific Roles in Growth and Puberty

Gonadal steroids modulate growth hormone (GH) secretion and the pubertal growth spurt via undefined central pathways. GH-releasing hormone (GHRH) neurons express estrogen receptor α (ERα) and androgen receptor (AR), suggesting changing levels of gonadal steroids during puberty directly modulate the somatotropic axis. We generated mice with deletion of ERα in GHRH cells (GHRHΔERα), which displayed reduced body length in both sexes. Timing of puberty onset was similar in both groups, but puberty completion was delayed in GHRHΔERα females. Lack of AR in GHRH cells (GHRHΔAR mice) induced no changes in body length, but puberty completion was also delayed in females. Using a mouse model with two reporter genes, we observed that, while GHRHtdTom neurons minimally colocalize with Kiss1hrGFP in prepubertal mice, ∼30% of GHRH neurons coexpressed both reporter genes in adult females, but not in males. Developmental analysis of Ghrh and Kiss1 expression suggested that a subpopulation of ERα neurons in the arcuate nucleus of female mice undergoes a shift in phenotype, from GHRH to Kiss1, during pubertal transition. Our findings demonstrate that direct actions of gonadal steroids in GHRH neurons modulate growth and puberty and indicate that GHRH/Kiss1 dual-phenotype neurons play a sex-specific role in the crosstalk between the somatotropic and gonadotropic axes during pubertal transition.SIGNIFICANCE STATEMENT Late maturing adolescents usually show delayed growth and bone age. At puberty, gonadal steroids have stimulatory effects on the activation of growth and reproductive axes, but the existence of gonadal steroid-sensitive neuronal crosstalk remains undefined. Moreover, the neural basis for the sex differences observed in the clinical arena is unknown. Lack of ERα in GHRH neurons disrupts growth in both sexes and causes pubertal delay in females. Deletion of androgen receptor in GHRH neurons only delayed female puberty. In adult females, not males, a subset of GHRH neurons shift phenotype to start producing Kiss1. Thus, direct estrogen action in GHRH/Kiss1 dual-phenotype neurons modulates growth and puberty and may orchestrate the sex differences in endocrine function observed during pubertal transition.

Neuroprotective and neurotoxic outcomes of androgens and estrogens in an oxidative stress environment

BACKGROUND

The role of sex hormones on cellular function is unclear. Studies show androgens and estrogens are protective in the CNS, whereas other studies found no effects or damaging effects. Furthermore, sex differences have been observed in multiple oxidative stress-associated CNS disorders, such as Alzheimer's disease, depression, and Parkinson's disease. The goal of this study is to examine the relationship between sex hormones (i.e., androgens and estrogens) and oxidative stress on cell viability.

METHODS

N27 and PC12 neuronal and C6 glial phenotypic cell lines were used. N27 cells are female rat derived, whereas PC12 cells and C6 cells are male rat derived. These cells express estrogen receptors and the membrane-associated androgen receptor variant, AR45, but not the full-length androgen receptor. N27, PC12, and C6 cells were exposed to sex hormones either before or after an oxidative stressor to examine neuroprotective and neurotoxic properties, respectively. Estrogen receptor and androgen receptor inhibitors were used to determine the mechanisms mediating hormone-oxidative stress interactions on cell viability. Since the presence of AR45 in the human brain tissue was unknown, we examined the postmortem brain tissue from men and women for AR45 protein expression.

RESULTS

Neither androgens nor estrogens were protective against subsequent oxidative stress insults in glial cells. However, these hormones exhibited neuroprotective properties in neuronal N27 and PC12 cells via the estrogen receptor. Interestingly, a window of opportunity exists for sex hormone neuroprotection, wherein temporary hormone deprivation blocked neuroprotection by sex hormones. However, if sex hormones are applied following an oxidative stressor, they exacerbated oxidative stress-induced cell loss in neuronal and glial cells.

CONCLUSIONS

Sex hormone action on cell viability is dependent on the cellular environment. In healthy neuronal cells, sex hormones are protective against oxidative stress insults via the estrogen receptor, regardless of sex chromosome complement (XX, XY). However, in unhealthy (e.g., high oxidative stress) cells, sex hormones exacerbated oxidative stress-induced cell loss, regardless of cell type or sex chromosome complement. The non-genomic AR45 receptor, which is present in humans, mediated androgen's damaging effects, but it is unknown which receptor mediated estrogen's damaging effects. These differential effects of sex hormones that are dependent on the cellular environment, receptor profile, and cell type may mediate the observed sex differences in oxidative stress-associated CNS disorders.

The Neural Mechanisms of Sexually Dimorphic Aggressive Behaviors

Aggression is a fundamental social behavior that is essential for competing for resources and protecting oneself and families in both males and females. As a result of natural selection, aggression is often displayed differentially between the sexes, typically at a higher level in males than females. Here, we highlight the behavioral differences between male and female aggression in rodents. We further outline the aggression circuits in males and females, and compare their differences at each circuit node. Lastly, we summarize our current understanding regarding the generation of sexually dimorphic aggression circuits during development and their maintenance during adulthood. In both cases, gonadal steroid hormones appear to play crucial roles in differentiating the circuits by impacting on the survival, morphology, and intrinsic properties of relevant cells. Many other factors, such as environment and experience, may also contribute to sex differences in aggression and remain to be investigated in future studies.

Does somatostatin have a role to play in migraine headache?

Migraine is a condition without apparent pathology. Its cardinal symptom is the prolonged excruciating headache. Theories about this pain have posited pathologies which run the gamut from neural to vascular to neurovascular, but no observations have detected a plausible pathology. We believe that no pathology can be found for migraine headache because none exists. Migraine is not driven by pathology - it is driven by neural events produced by triggers - or simply by neural noise- noise that has crossed a critical threshold. If these ideas are true, how does the pain arise? We hypothesise that migraine headache is a consequence of withdrawal of descending pain control, produced by "noise" in the cerebral cortex. Nevertheless, there has to be a neural circuit to transform cortical noise to withdrawal of pain control. In our hypothesis, this neural circuit extends from the cortex, synapses in two brainstem nuclei (the periaqueductal gray matter and the raphe magnus nucleus) and ultimately reaches the first synapse of the trigeminal sensory system. The second stage of this circuit uses serotonin (5HT) as a neurotransmitter, but the neuronal projection from the cortex to the brainstem seems to involve relatively uncommon neurotransmitters. We believe that one of these is somatostatin (SST). Temporal changes in levels of circulating SST mirror the temporal changes in the incidence of migraine, particularly in women. The SST₂ receptor agonist octreotide has been used with some success in migraine and cluster headache. A cortical to PAG/NRM neural projection certainly exists and we briefly review the anatomical and neurophysiological evidence for it and provide preliminary evidence that SST may the critical neurotransmitter in this pathway. We therefore suggest that the withdrawal of descending tone in SST-containing neurons, might create a false pain signal and hence the headache of migraine.

Role of calcitonin gene-related peptide in energy metabolism

PURPOSE

Calcitonin gene-related peptide (CGRP) is a neuropeptide produced by alternative tissue-specific splicing of the primary transcript of the CALC genes. CGRP is widely distributed in the central and peripheral nervous system, as well as in several organs and tissues. The presence of CGRP in the liver and brown and white adipose tissue suggests an effect of this neuropeptide on regulation of energy homeostasis.

METHODS

In this review, we summarize the current knowledge of the effect of CGRP on the control of energy metabolism, primarily focusing on food intake, thermoregulation and lipid metabolism in adipose tissue, liver and muscle.

RESULTS

CGRP induces anorexia, stimulating anorexigenic neuropeptide and/or inhibiting orexigenic neuropeptide expression, through cAMP/PKA pathway activation. CGRP also induces energy expenditure, increasing the skin temperature and brown adipose tissue thermogenesis. It has been also suggested that information related to peripheral lipid stores may be conveyed to the brain via CGRP-sensory innervation from adipose tissue. More recently, it was demonstrated that mice lacking αCGRP are protected from obesity induced by high-fat diet and that CGRP regulates the content of lipid in liver, muscle and adipose tissue.

CONCLUSIONS

It is unclear the receptor responsible by CGRP effects, as well as whether this neuropeptide acts directly or indirectly in liver, muscle and adipose tissue.

Immunohistochemical localization of androgen receptors in female mole rat (Spalax leucodon) tissues

Androgens exert their effects through androgen receptors (AR) in tissues. We investigated the distribution of AR in female mole rat tissues. Tissues were excised, fixed with 10% formalin and embedded in paraffin. Sections were stained after microwave antigen retrieval for immunohistochemistry. Immunostaining of AR immunostaining was detected in the nucleus or cytoplasm of the cells in the cerebral cortex, cerebellum, anterior pituitary, lung, liver, uterus and skin. Granulosa and some thecal cells in the ovary, cardiac muscle cells and adipose cells exhibited a nuclear reaction for AR. In the kidney, labeling of AR was restricted to the cytoplasm of tubule cells. We found that AR could be detected using immunohistochemistry in the nucleus or cytoplasm or both in the presence of androgens.

Species differences in androgen receptor expression in the medial preoptic and anterior hypothalamic areas of adult male and female rodents

The medial preoptic and anterior hypothalamic areas (MPO/AH) are important androgen targets regulating homeostasis, neuroendocrinology and circadian rhythm as well as instinctive and sociosexual behaviors. Although species differences between rats and mice have been pointed out in terms of morphology and physiology, detailed distributions of androgen receptor (AR) have never been compared between the two rodents. In the present study, AR distribution was examined immunohistochemically in serial sections of the MPO/AH and compared for adult rats and mice. Western blotting and immunohistochemistry clearly demonstrated that AR expression in the brain was stronger in mice than in rats and was stronger in males than in females. In addition, we found (1) an "obliquely elongated calbindin-ir cell island" in mice medial preoptic nucleus (MPN) expressed AR intensely, as well as the sexually dimorphic nucleus in the MPN (SDN-MPN) in rats, strongly supporting a "putative SDN-MPN" previously proposed in mice; (2) AR expression in the suprachiasmatic nucleus (SCN) was much more prominent in mice than in rats and differed in localization between the two species; (3) a mouse-specific AR-ir cell cluster was newly identified as the "tear drop nucleus (TDN)", with male-dominant sexual dimorphism; and (4) two rat-specific AR-ir cell clusters were also newly identified as the "rostral and caudal nebular islands", with male-dominant sexual dimorphism. The present results may provide basic morphological evidence underlying species differences in androgen-modified psychological, physiological and endocrinergic responses. Above all, the findings of the mouse-specific TDN and differing AR expression in the SCN might explain not only species difference in gonadal modification of circadian rhythm, but also distinct structural bases in the context of transduction of SCN oscillation. The current study could also serve as a caution that data on androgen-sensitive functions obtained from one species should not always be directly applied to others among rodents.

Puberty and adolescence as a time of vulnerability to stressors that alter neurobehavioral processes

Puberty and adolescence are major life transitions during which an individual's physiology and behavior changes from that of a juvenile to that of an adult. Here we review studies documenting the effects of stressors during pubertal and adolescent development on the adult brain and behavior. The experience of complex or compound stressors during puberty/adolescence generally increases stress reactivity, increases anxiety and depression, and decreases cognitive performance in adulthood. These behavioral changes correlate with decreased hippocampal volumes and alterations in neural plasticity. Moreover, stressful experiences during puberty disrupt behavioral responses to gonadal hormones both in sexual performance and on cognition and emotionality. These behavioral changes correlate with altered estrogen receptor densities in some estrogen-concentrating brain areas, suggesting a remodeling of the brain's response to hormones. A hypothesis is presented that activation of the immune system results in chronic neuroinflammation that may mediate the alterations of hormone-modulated behaviors in adulthood.

Sex differences in circadian timing systems: implications for disease

Virtually every eukaryotic cell has an endogenous circadian clock and a biological sex. These cell-based clocks have been conceptualized as oscillators whose phase can be reset by internal signals such as hormones, and external cues such as light. The present review highlights the inter-relationship between circadian clocks and sex differences. In mammals, the suprachiasmatic nucleus (SCN) serves as a master clock synchronizing the phase of clocks throughout the body. Gonadal steroid receptors are expressed in almost every site that receives direct SCN input. Here we review sex differences in the circadian timing system in the hypothalamic-pituitary-gonadal axis (HPG), the hypothalamic-adrenal-pituitary (HPA) axis, and sleep-arousal systems. We also point to ways in which disruption of circadian rhythms within these systems differs in the sexes and is associated with dysfunction and disease. Understanding sex differentiated circadian timing systems can lead to improved treatment strategies for these conditions.

Neuroendocrine alterations in the fragile X mouse

The expression of GABA(A) receptors in the fragile X mouse brain is significantly downregulated. We additionally found that the expression of somatostatin and voltage-sensitive calcium channels (VSCCs) is also reduced. GABA(A) and the VSCCs, through a synergistic interaction, perform a critical role in mediating activity-dependent developmental processes. In the developing brain, GABA is excitatory and its actions are mediated through GABA(A) receptors. Subsequent to GABA-mediated depolarization, the VSCCs are activated and intracellular calcium is increased, which mediates gene transcription and other cellular events. GABAergic excitation mediated through GABA(A) receptors and the subsequent activation of the VSCCs are critically important for the establishment of neuronal connectivity within immature neuronal networks. Data from our laboratories suggest that there is a dysregulation of axonal pathfinding during development in the fragile X mouse brain and that this is likely due to a dysregulation of the synergistic interactions of GABA and VSCC. Thus, we hypothesize that the altered expression of these critical channels in the early stages of brain development leads to altered activity-dependent gene expression that may potentially lead to the developmental delay characteristic of the fragile X syndrome.

Spatiotemporal expression of androgen receptors in the female rat brain during the oestrous cycle and the impact of exogenous androgen administration: a comparison with gonadally intact males

Little is known about the regulation and cellular distribution of androgen receptors (ARs) in female rodent brains at various stages of the oestrous cycle. This information is critical for further studies of androgen signalling in the regulation of brain function under physiological and pathophysiological conditions. In this report, we show that the distribution of AR immunoreactivity in the female rat brain is consistent with reported AR mRNA hybridisation signals in the male brain, except for the dentate gyrus of the hippocampus. Immunohistochemical and Western blot analyses performed herein revealed that the onset of region-specific changes in AR proteins was strongly correlated with circulating and ovarian levels of estradiol and testosterone across the oestrous cycle. During the metestrus and diestrus stages, however, the highest levels of AR expression were abolished by chronic dihydrotestosterone (DHT) treatment. This demonstrates that fluctuations in endogenous androgens are required for the regulation of AR expression in the female rat brain. Colocalisation studies revealed that: (1) anatomical variations in AR protein localisation existed between female and male brains, (2) AR immunoreactivity was both neuronal and non-neuronal, and (3) AR protein expression was lower in female rat brains at all stages of the oestrous cycle compared to age-matched males. Our results indicate the presence of regional sex differences in AR expression and changes in the proportion of AR between different subcellular compartments. Furthermore, DHT was found to down-regulate the level of AR in the subcellular compartment in females in a region-specific manner. As a whole, the present study provides the first step toward understanding the dynamics of AR expression and regulation in the brain during normal physiological conditions and for differences in neuronal androgen effects based on sex.

Sexually dimorphic distribution of sst2A receptors on growth hormone-releasing hormone neurones in mice: modulation by gonadal steroids

The ultradian pulsatile pattern of growth hormone (GH) secretion is markedly sexually dimorphic in rodents as in primates, but the neuroanatomical mechanisms of this phenomenon are not clear. In the arcuate nucleus of the hypothalamus, GH-releasing hormone (GHRH) neurones receive somatostatinergic inputs through the sst2A receptor (sst2A-R) and the percentage of GHRH neurones bearing sst2A-R is higher in female than in male GHRH-enhanced green fluorescent protein (eGFP) mice. In the present study, we hypothesised that sst2A-R expression on GHRH neurones is modulated by gonadal steroids and constitutes a mechanism for sexually differentiated GH secretion. The distribution of sst2A-R on GHRH neurones was evaluated by immunohistochemistry in adult GHRH-eGFP mice gonadectomised and treated for 3 weeks with oestradiol or testosterone implants. In gonadectomised females supplemented with testosterone, sst2A-R distribution on GHRH neurones was reduced to the level seen in intact males, whereas oestradiol implants were ineffective. Conversely, orchidectomy induced a female 'sst2A phenotype', which was reversed by testosterone supplementation. Changes in the hepatic expression of GH-dependent genes for major urinary protein-3 and the prolactin receptor reflected the altered steroid influence on GH pulsatile secretion. In the ventromedial-arcuate region, GHRH and sst2-R, as well as GHRH and somatostatin expression as measured by the real-time polymerase chain reaction, were positively correlated in both sexes. By contrast, the positive correlation between ventromedial-arcuate GHRH and periventricular somatostatin expression in males was reversed to a negative one in females. Moreover, the positive correlation between periventricular somatostatin and ventromedial-arcuate sst2-R expressions in males was lost in females. These results suggest that, in the adult mouse, testosterone is a major modulator of sst2A distribution on GHRH neurones. This marked sex difference in sst2A-R distribution may constitute a key element in the genesis of the sexually differentiated pattern of GH secretion, possibly through testosterone-modulated changes in somatostatin inputs from hypophysiotrophic periventricular neurones.

Calcitonin gene-related peptide alters the firing rates of hypothalamic temperature sensitive and insensitive neurons

Background

Transient hyperthermic shifts in body temperature have been linked to the endogenous hormone calcitonin gene-related peptide (CGRP), which can increase sympathetic activation and metabolic heat production. Recent studies have demonstrated that these centrally mediated responses may result from CGRP dependent changes in the activity of thermoregulatory neurons in the preoptic and anterior regions of the hypothalamus (POAH).

Results

Using a tissue slice preparation, we recorded the single-unit activity of POAH neurons from the adult male rat, in response to temperature and CGRP (10 μM). Based on the slope of firing rate as a function of temperature, neurons were classified as either warm sensitive or temperature insensitive. All warm sensitive neurons responded to CGRP with a significant decrease in firing rate. While CGRP did not alter the firing rates of some temperature insensitive neurons, responsive neurons showed an increase in firing rate.

Conclusion

With respect to current models of thermoregulatory control, these CGRP dependent changes in firing rate would result in hyperthermia. This suggests that both warm sensitive and temperature insensitive neurons in the POAH may play a role in producing this hyperthermic shift in temperature.

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 effect of gonadal hormones and gender on anxiety and emotional learning

Disorders of anxiety and fear dysregulation are highly prevalent. These disorders affect women approximately 2 times more than they affect men, occur predominately during a woman's reproductive years, and are especially prevalent at times of hormonal flux. This implies that gender differences and sex steroids play a key role in the regulation of anxiety and fear. However, the underlying mechanism by which these factors regulate emotional states in either sex is still largely unknown. This review discusses animal studies describing sex-differences in and gonadal steroid effects on affect and emotional learning. The effects of gonadal hormones on the modulation of anxiety, with particular emphasis on progesterone's ability to reduce the responsiveness of female rats to corticotropin releasing factor and the sex-specific effect of testosterone in the reduction of anxiety in male rats, is discussed. In addition, gonadal hormone and gender modulation of emotional learning is considered and preliminary data are presented showing that estrogen (E2) disrupts fear learning in female rats, probably through the antagonistic effect of ERalpha and ERbeta activation.

Anatomy of the hypophysiotropic somatostatinergic and growth hormone-releasing hormone system minireview

The central control of growth hormone (GH) secretion from the pituitary gland is ultimately achieved by the interaction between two hypothalamic neurohormones, somatostatin which inhibits and growth hormone-releasing hormone (GHRH) which stimulates GH release. The regulation of the somatostatin and GHRH release from the hypothalamus is regulated by a range of other neuropeptides, neurotransmitters, neurohormones. In this mini review we attempt to provide a short summary covering the anatomy and chemical characteristics of the various cell populations regulating GH secretion as a tribute to Miklós Palkovits who pioneered the field of functional neuroanatomy of hypothalamic networks.

The expression of brain sexual dimorphism in artificial selection of rat strains

Central nervous system sex differences have two morphological patterns. In one pattern, males show larger measurements (volume, number of neurons) than females (male > female; m > f) and, in the other, the opposite is true (female > male; f > m). The bed nucleus of the stria terminalis (BST) is a unique model for the study of sex differences because it has dimorphic and isomorphic subdivisions, with the former showing the two sexually differentiated morphological patterns. Meanwhile, other CNS structures, like the locus coeruleus (LC), present the f > m pattern. The philogenetic maintenance of the two patterns of sexual differentiation can help to disentangle the functional meaning of sex differences. Laboratory rat strains, whether albino or pigmented, descend from the Wistar strain through artificial selection. The present work compares the BST and LC of Wistar and Long-Evans rats. The medial posterior subdivision of the BST (BSTMP) is sexually dimorphic (m > f pattern) in the original (Wistar) and derived (Long-Evans) strains, while the lateral anterior and medial anterior subdivisions of the BST and the LC only present sex differences (f > m pattern) in the ancestor Wistar strain. Isomorphic BST regions are the same in both strains. The fact that the BSTMP, which is implicated in male copulatory behavior, is sexually dimorphic in both strains, as well as in other species, including humans, indicates the relevance of this structure in male sexual behavior in mammals.

Species and sex differences in brain distribution of corticotropin-releasing factor receptor subtypes 1 and 2 in monogamous and promiscuous vole species

Corticotropin-releasing factor (CRF) receptor subtypes 1 and 2 have been implicated in rodent models of anxiety, but much less is known about the CRF system and social behavior. Both corticosterone and central CRF receptors modulate pair bonding in the monogamous prairie vole. Using receptor autoradiography, we mapped CRFR(1) and CRFR(2) in the brains of two monogamous vole species, the prairie vole and pine vole, and two promiscuous vole species, the meadow vole and montane vole. We found markedly different patterns of brain CRFR(1) and CRFR(2) binding among the four species, including species differences in the olfactory bulb, nucleus accumbens, lateral septum, hippocampus, laterodorsal thalamus, cingulate cortex, superior colliculus, and dorsal raphe. Interestingly, we also observed striking sex differences in voles: CRFR(2) binding was higher in the encapsulated bed nucleus of the stria terminalis in males than females for all four vole species. These results suggest possible sites of action for CRF-induced facilitation of pair bond formation in prairie voles, as well as potential sex differences in the CRF modulation of pair bonding. Further examination of CRF receptors in vole species may reveal a novel role for CRF in social behavior. Ultimately, our results identify several brain regions with conserved CRF receptor patterns across rodent and primate species, in contrast to several brain regions with phylogenetically plastic CRF receptor patterns, and have interesting implications for the evolution of CRF receptor patterns and behavior.

Hypothalamic STAT proteins: regulation of somatostatin neurones by growth hormone via STAT5b

Signal transducers and activators of transcription (STATs) are a family of transcription factors linked to class I cytokine receptors. In the present study, we investigated whether their distribution in the hypothalamus reflects the feedback regulation by growth hormone and what role they might play in the functioning of target neurones. We demonstrate that each of the seven known STATs has a distinct distribution in the hypothalamus. Notably, the STAT5 proteins, that are important in growth hormone (GH) and prolactin signalling in peripheral tissues, were expressed in somatostatin neurones of the periventricular nucleus and dopamine neurones of the arcuate nucleus. Because somatostatin neurones are regulated by feedback from circulating GH, we investigated the importance of STAT5 in these neurones. We demonstrate that STAT5b protein expression, similar to somatostatin mRNA, is sexually dimorphic in the periventricular nucleus of rats and mice. Furthermore, chronic infusion of male dwarf rats with GH increased the expression of STAT5b, while a single injection of GH into similar rats induced the phosphorylation of STAT5 proteins. The cellular abundance of somatostatin mRNA in STAT5b-deficient mice was significantly reduced in the periventricular nucleus, effectively reducing the sexually dimorphic expression. These results are consistent with the hypothesis that STAT5 proteins are involved in the feedback regulation of somatostatin neurones by GH, and that these neurones may respond to patterned GH secretion to reinforce sexual dimorphism in the GH axis.

Age-related changes in growth hormone (GH)-releasing hormone and somatostatin neurons in the hypothalamus and in GH cells in the anterior pituitary of female mice

We have observed growth hormone-releasing hormone (GHRH)-immunoreactive (ir) neurons in the arcuate nucleus (ARC), somatostatin (SS)-ir neurons in the periventricular nucleus (PeN), and pituitary growth hormone (GH)-ir cells in female C57BL/6J mice at 2 months old (2 M), 4, 12 and 23 M, using immunocytochemical and morphometric methods. The number of GHRH-ir neurons decreased with age. The number of SS-ir neurons increased from 2 to 4 M, but decreased after 4 M. The volume of the anterior pituitary and the number of adenohypophysial parenchymal cells fell from 12 to 23 M. The proportion of GH-ir cells decreased significantly from 2 to 4 M and decreased in number from 12 to 23 M as well as in size from 2 to 4 M and from 12 to 23 M. Our results show that both GHRH-ir neurons and SS-ir neurons are fewer in old female mice, but the ratio of the number of SS-ir neurons to GHRH-ir neurons increases in old females. We suggest that the fall in the number and size of GH-ir cells in the pituitary gland with age may be involved in the increase in the ratio of the number of SS-ir neurons to GHRH-ir neurons in the hypothalamus in female mice, as well as in males.

Sex differences, laterality, and hormonal regulation of androgen receptor immunoreactivity in rat hippocampus

Sex differences, laterality, and hormonal regulation of androgen receptor (AR) immunoreactivity in rat hippocampal CA1 pyramidal cells were examined using the PG21 antibody. Adult male rats were either castrated or sham-operated at least 2 weeks prior to sacrifice. Gonadally intact females were sacrificed on the day of proestrus. Animals received an injection of either testosterone propionate (TP) or vehicle 2 h prior to sacrifice. Within CA1, both the intensity of staining and the number of AR+ cells were assessed. AR immunostaining was detected in all the groups with marked variation among them. The overall ranking of staining intensity was: gonadally intact males > females given TP > castrated males given TP > females > castrated males given vehicle. The number of AR+cells within subregions of CA1 showed the same basic pattern: among control-treated animals, gonadally intact males have more than females, but castrated males have the least, and acute TP treatment increases the number in both sexes. The increased level of AR immunoreactivity in CA1 of castrated males following acute TP treatment suggests that testicular androgens in adulthood normally increase AR immunoreactivity there, producing a sex difference favoring males in gonadally intact animals. We also found a higher number of AR+ CA1 cells on the left than on the right, but only in gonadally intact males and in females given TP. These results suggest that a laterality of AR distribution in the rat hippocampus may lead to lateralities in hippocampal structure and function.

Androgen receptor messenger RNA and protein in adult rat sciatic nerve: implications for site of androgen action

Gonadal androgens exert a wide variety of effects on several neuromuscular systems, including controlling the developmental fate of motoneurons and neuromuscular synapses and promoting the growth of adult dendrites and axons. Paramount in understanding the molecular mechanisms behind androgen action is determining where androgen acts; does androgen act directly or indirectly on cells to change their fate and function? One step toward answering this question has been to determine which cells express androgen receptors (ARs). Motoneurons and skeletal muscles both have ARs and are, therefore, potential sites of androgen action. Recent evidence indicates that the sciatic nerve in rats also contains AR mRNA (Magnaghi et al. [1999] Brain Res. Mol. Brain Res. 70:36-44), although which cell type expresses ARs remains unanswered. In this study, we explored the question of which cell populations in the rat sciatic nerve express ARs. Using immunocytochemistry and reverse transcriptase-PCR, we confirmed the presence of AR protein and mRNA in sciatic nerve from adult rats and found a sex difference, favoring males, in the number of cell nuclei immunopositive for AR. This difference was not due to a sex difference in the overall number of cell nuclei. We also found a difference favoring males in AR mRNA, evidence also suggesting that AR expression is higher in males than in females. Results from double-immmunolabeling experiments in sciatic nerve from adult males suggest that, within the endoneurial compartment, endoneurial fibroblasts stain prominently for AR, with some endothelial cells also AR(+). Although Schwann cells showed light AR immunostaining, this staining is apparently nonspecific. We conclude that cells within peripheral nerve have ARs and may, therefore, mediate some of the effects of androgens on neuromuscular systems.

Are POMC neurons targets for sex steroids in the arcuate nucleus of the rat?

Testosterone alters the expression of proopiomelanocortin (POMC) mRNA in the neurons of the arcuate nucleus. While observations suggest that both estrogen and androgen receptors (AR) can mediate this action, only a negligible number of POMC neurons has previously been shown to contain estrogen receptor (ER)-alpha. To determine whether the putative action of testosterone is mediated via ER-beta or AR we double immuno- labeled hypothalamic sections from colchicine-pretreated male rats. Only few cells were immunostained for ER-beta and they were never found to co-localize POMC. In spite of the overlap in the anatomical distribution, only 3% of POMC cells appeared to contain AR. These results suggest that sex steroids have an indirect effect on most POMC neurons.

Androgen receptor immunolocalization in brains of courting and brooding male and female ring doves (Streptopelia risoria)

Nuclear androgen receptors (ARs) were localized immunocytochemically in the brains of courting and brooding male and female ring doves (Streptopelia risoria). AR immunoreactivity (AR-ir) in courting birds was localized in cell nuclei in the telencephalon, diencephalon, and mesencephalon. In the anterior hypothalamus, high density of AR-ir was concentrated in several nuclei including the nucleus lateralis hypothalami, nucleus periventricularis magnocellularis, nucleus preopticus anterior, nucleus preopticus medialis, and nucleus preopticus paraventricularis magnocellularis. In the posterior hypothalamus, areas showing high density of AR-ir included the nucleus lateralis hypothalami posterioris, nucleus medialis hypothalami posterior, nucleus ectomamillaris, nucleus mamillaris lateralis, and nucleus tuberis. No sex differences in the density or localization of AR-ir were observed. Compared to brains from courting birds, AR-ir density was either extremely low or absent in most brain regions of brooding birds. It is concluded that in the dove, central ARs are closely associated with the sexual stages of the reproductive cycle.

Sexually dimorphic effects of testosterone on preoptic area calcitonin gene-related peptide mRNA expression depend upon neuron location and differential estrogen and androgen receptor activation

Experiments examined activational roles of gonadal steroids on the sexually dimorphic, calcitonin gene-related peptide-expressing neurons of the rat preoptic area. Gonadectomy of male rats followed by treatment with testosterone, dihydrotestosterone, or estrogen demonstrated that the tonic suppressive influence of testosterone on cellular levels of calcitonin gene-related peptide mRNA expression in the medial preoptic nucleus and anteroventral periventricular nucleus occurred through either ER- or AR-mediated mechanisms (P < 0.05). The gonadectomy of adult female rats demonstrated little tonic influence of ovarian steroids upon calcitonin gene-related peptide mRNA levels. However, the administration of male levels of testosterone to ovariectomized rats resulted in reduced calcitonin gene-related peptide mRNA expression within the medial preoptic nucleus (P < 0.05) and, strikingly, a 3-fold induction in calcitonin gene-related peptide mRNA expression in the anteroventral periventricular nucleus (P < 0.01). Testosterone's effects in the medial preoptic nucleus and anteroventral periventricular nucleus of the female required both ER and AR activation. Dual labeling immunocytochemical studies revealed that less than 10% of calcitonin gene-related peptide neurons in the male expressed ARs compared with approximately 50% in the female. These investigations reveal that sexually differentiated region- and steroid receptor-specific mechanisms function in association with the sex differences in circulating gonadal steroids to maintain the sexually dimorphic nature of calcitonin gene-related peptide expression in the preoptic area of the adult rat.

Absence of androgen receptor in the growth hormone releasing hormone-containing neurones in the rat mediobasal hypothalamus

Growth hormone (GH) secretory patterns are influenced by gonadal steroids, at least in part, through modulation of hypothalamic somatostatin and GH releasing hormone (GHRH) secretion. In the adult male rat, testosterone appears to stimulate somatostatin gene expression by acting directly on androgen receptors in somatostatin neurones. The mechanism by which gonadal status influences hypothalamic GHRH gene expression is less clear. Gonadectomy reduces GHRH mRNA expression in rats, and this reduction can be prevented by the administration of testosterone or partly by a nonaromatizable androgen. While these observations suggest that androgen receptors mediate the actions of gonadal steroids on GHRH gene expression, they do not provide any information about the location of the androgen receptors involved in this process. To determine whether GHRH neurones themselves express androgen receptors, we double immunolabelled hypothalamic sections from colchicine-pretreated male rats. Although there was an overlap in the anatomical distribution of GHRH and androgen receptor-containing cell bodies, none of the nearly 900 GHRH immunolabelled cells we examined in each mediobasal hypothalamus appeared to contain androgen receptors. These results suggest that GHRH-expressing neurones are not direct targets for androgens and therefore the effects of testosterone on GHRH gene expression must be produced indirectly by some other neural or endocrine intermediary process.

Single exposure to testosterone in adulthood rapidly induces regularity in the growth hormone release process

The neonatal gonadal steroid milieu is known to be important in imprinting the striking sexual dimorphism of growth hormone (GH) secretion; however, the influence of the sex steroids on GH control in adult life and their mechanism/site of action are largely unknown. In the present study, we tested the hypothesis that testosterone (T) subserves the gender-specific regularity of the GH release process in adulthood. The approximate entropy statistic (ApEn) was used to quantify the degree of regularity of GH release patterns over time. Eighteen hours after a single subcutaneous injection of 1 mg T, both sham-operated and ovariectomized (OVX) female adult rats displayed plasma GH profiles that were strikingly similar to the regular male-like ultradian rhythm of GH secretion. The highest ApEn values, denoting greater disorderliness of GH secretion, were observed in the ovary-intact group, and T injection significantly (P < 0.001) reduced this irregularity whether or not the ovaries were present. Serial intravenous injections of GH-releasing hormone (GHRH) caused a similar increase in plasma GH levels in sham-operated females independently of time of administration. In contrast, female rats administered T exhibited a male-like intermittent pattern of GH responsiveness to GHRH, the latter known to be due to the cyclic release of endogenous somatostatin. These results demonstrate that acute exposure to T during adult life can rapidly and profoundly "masculinize" GH pulse-generating circuits in the female rat. Our findings suggest that the enhanced orderliness characteristic of the GH release process in males, compared with females, is regulated by T. We postulate that this T-induced regularity is mediated at the level of the hypothalamus by inducing regularity in somatostatin secretion, which in turn governs overall GH periodicity.

Masculinizing effect of dihydrotestosterone on growth hormone secretion is inhibited in ovariectomized rats with anterolateral deafferentation of the medial basal hypothalamus or in intact female rats

There is a striking sex difference in the pattern of growth hormone (GH) secretion in rats. Our previous studies showed that short-term administration of pharmacological doses of testosterone or dihydrotestosterone (DHT) masculinized the GH secretory pattern in ovariectomized (OVX) rats. The locus where testosterone or DHT interacts with the somatotropic axis is believed to be the hypothalamus. To obtain insights into this phenomenon, we administered a single dose of DHT s.c. to adult OVX rats at 0.01, 0. 1 or 1 mg/rat. Blood GH concentrations were measured in unanaesthetized rats. Six to 12 h after the s.c. administration of all three doses of DHT, the GH secretory pattern revealed a male-like secretory pattern as shown by episodic bursts occurring at 2-3-h intervals with low or undetectable trough levels. When anterolateral deafferentation of the medial basal hypothalamus (ALC) was performed, the blood concentrations revealed irregularly occurring small fluctuations, instead of the usual high bursts, but the basal GH concentration was significantly higher than that of OVX-sham-operated rats. DHT treatment did not elicit pulsatile GH secretion or alter GH concentrations in OVX rats with ALC. When intact adult female rats received DHT at a dose of 1 mg/rat, the male-like GH secretory pattern was not induced. These results suggest that neural inputs from the anterolateral direction to the medial basal hypothalamus are necessary for the masculinizing effect of DHT on the GH secretory pattern in OVX rats, and that oestrogen in intact female rats prevents the masculinizing effect of DHT.

Gender differences in brain and behavior: hormonal and neural bases

This article briefly discusses the difficulties in determining the brain-behavior relationship and reviews the literature on some potential mechanisms underlying gender differences in behavioral responses. Mechanisms that are discussed include genetic effects, organizational effects of gonadal hormones, genomic actions of steroids, nongenomic effects of steroids, and environmental influences. The review is an introduction to the articles presented in this special volume on gender differences in brain and behavior.

Sex differentiation of growth hormone-releasing hormone and somatostatin neurons in the mouse hypothalamus: an immunohistochemical and morphological study

We examine sexual dimorphism in growth hormone-releasing hormone (GHRH) in the arcuate nucleus (ARC), and somatostatin (SS) in the periventricular nucleus (PeN) of the hypothalamus, and investigate when it becomes evident. Using immunohistochemical staining and morphometry, we observed ARC GHRH-immunoreactive (ir) neurons, ARC SS-ir neurons and PeN SS-ir neurons in male and female mice at 5, 20, 30, 40 and 60 days old. The number of ARC GHRH-ir neurons was significantly higher in males than females, after 20 days old. ARC SS-ir neurons showed no significant differences between sexes. On the other hand, PeN SS-ir neurons were significantly more numerous in males at 30, 40 and 60 days than in females. During postnatal development, these GHRH- and SS-ir neurons changed in different patterns from ages 20 to 60 days. The number of ARC GHRH-ir neurons in both sexes decreased from 5 to 20 days, increased until day 40, and then decreased at day 60, while ARC SS-ir neurons in both sexes increased from day 5 to day 60. PeN SS-ir neurons in both sexes increased from days 5 to 20 to 116% in males and 189% in females. Furthermore, in male mice, the increase continued until 40 days of age, while in females, there was no significant difference from days 20 to 60. There were no apoptotic cells; a few proliferating cell nuclear antigen (PCNA) stained cells were found in the ARC and PeN. Our results suggest that the sex difference of ARC GHRH neurons and PeN SS neurons appears by stimulation with testosterone during the development life. The developmental fluctuation in the number of ARC GHRH-ir neurons may not be modulated by testosterone, but by ARC SS neurons.

Testosterone and its metabolites modulate 5HT1A and 5HT1B agonist effects on intermale aggression

Our understanding of the neurochemical and neuroendocrine systems' regulating the display of offensive intermale aggression has progressed substantially over the past twenty years. Pharmacological studies have shown that serotonin, via its action at 5HT1A and/or 5HT1B receptor sites, modulates the display of intermale aggressive behavior and that its effects serve to decrease behavioral expression. Neuroendocrine investigations, in turn, have demonstrated that male-typical aggression is testosterone-dependent and studies of genetic effects, metabolic function and steroid receptor binding have shown that facilitation of behavioral displays can occur via independent androgen-sensitive or estrogen-sensitive pathways. Remarkably, there have been virtually no studies that examined the interrelationship between these facilitative and inhibitory systems. As an initial step toward characterizing the interaction between the systems, studies were conducted that assessed hormonal modulation of serotonin function at 5HT1A and 5HT1B receptor sites. They demonstrated: (1) that the androgenic and estrogenic metabolites of testosterone differentially modulate the ability of systemically administered 8-OH-DPAT (a 5HT1A agonist) and CGS12066B (a 5HT1B agonist) to decrease offensive aggression; and (2) when microinjected into the lateral septum (LS) or medial preoptic area (MPO), the aggression-attenuating effects of 1A and 1B agonists differ regionally and vary with the steroidal milieu. In general, the results suggest that estrogens establish a restrictive environment for attenuation of T-dependent aggression by 8-OH-DPAT and CGS 12066B, while androgens either do not inhibit, or perhaps even facilitate, the ability of 5HT1A and 5HT1B agonists to reduce aggression. Potential mechanisms involved in the production of these steroidal effects are discussed and emerging issues that may impact on efforts to develop an integrative neurobiological model of offensive, intermale aggression are considered.

Correlation of hypothalamic somatostatin mRNA expression and peptide content with secretion: sexual dimorphism and differential regulation by gonadal factors

Sex differences in growth hormone (GH) secretion in the rat are thought to be determined, to a large extent, by gonadal steroid-dependent sex differences in somatostatin (SRIH) secretion from neurones in the periventricular nucleus (PeN) which project to the median eminence (ME). The present study aimed to obtain direct evidence for sex differences and gonadal regulation of SRIH release within this pathway and to determine the relationships between SRIH mRNA expression, SRIH peptide content and SRIH secretion in the adult rat. Somatostatin mRNA expression in the PeN and peptide content in both PeN and ME were higher in males than females (P<0.05). However, both basal and 56 mM K+-stimulated SRIH release in vitro from hypothalamic explants incorporating the PeN-ME pathway were higher (P<0.01) in females. The gonadectomy of female rats resulted in significantly reduced basal levels of SRIH release equivalent to that of males but had no effect on SRIH mRNA/peptide content or K+-stimulated release. In contrast, gonadectomy of male rats reduced SRIH mRNA and peptide contents and elevated K+-stimulated secretion (P<0.01) to levels similar to that seen in intact females, without affecting basal release. In summary, these results demonstrate that in the PeN-ME of the adult rat: (1) SRIH mRNA and peptide content is well correlated and sexually dimorphic but dependent on gonadal factors in the male only; (2) SRIH secretion is sexually dimorphic and dependent on gonadal factors; but (3) differences in mRNA/peptide content do not reflect secretory capacity; and (4) gonadal factors differentially modulate SRIH secretory dynamics in males and females.

Ontogeny and sexual differentiation of somatostatin biosynthesis and secretion in the hypothalamic periventricular-median eminence pathway

The biosynthesis of somatostatin (SRIH) in the hypothalamic periventricular nucleus (PeN) is sexually differentiated in neonatal and adult rats by virtue of the organizational and activational actions, respectively, of sex steroid hormones. Little information exists, however, on the normal pattern of maturation of these neurones or on how the sexually differentiated biosynthesis may relate to ontogenetic changes in somatostatin secretion during the neonatal and pubertal periods of development. Hence in the present study we determined the postnatal developmental profile of SRIH mRNA and peptide levels in the PeN-median eminence (ME) pathway as well as SRIH secretion, using an acute explant preparation, from the day of birth, through puberty and into adulthood in male and female rats. The results demonstrate that: (1) developmental sex differences in SRIH biosynthesis in PeN neurones occurred in an orderly cascade with differences observed for mRNA expression at postnatal day 5, for peptide content in the perikarya at postnatal day 10 and for peptide content in the nerve terminal (ME) by postnatal day 25; (2) sex differences in SRIH release were not evident prior to postnatal day 40; and (3) the developmental profile of SRIH biosynthesis in PeN neurones is unique compared with other hypothalamic (ventromedial nucleus) and extrahypothalamic (parietal cortex) populations. Specific developmental changes in the biosynthetic and secretory activity of the hypothalamic SRIH PeN-ME pathway may have a functional importance in the maturation of hypothalamic SRIH pathways involved in the regulation of GH secretion.

Pathophysiology of the neuroregulation of growth hormone secretion in experimental animals and the human

During the last decade, the GH axis has become the compelling focus of remarkably active and broad-ranging basic and clinical research. Molecular and genetic models, the discovery of human GHRH and its receptor, the cloning of the GHRP receptor, and the clinical availability of recombinant GH and IGF-I have allowed surprisingly rapid advances in our knowledge of the neuroregulation of the GH-IGF-I axis in many pathophysiological contexts. The complexity of the GHRH/somatostatin-GH-IGF-I axis thus commends itself to more formalized modeling (154, 155), since the multivalent feedback-control activities are difficult to assimilate fully on an intuitive scale. Understanding the dynamic neuroendocrine mechanisms that direct the pulsatile secretion of this fundamental growth-promoting and metabolic hormone remains a critical goal, the realization of which is challenged by the exponentially accumulating matrix of experimental and clinical data in this arena. To the above end, we review here the pathophysiology of the GHRH somatostatin-GH-IGF-I feedback axis consisting of corresponding key neurotransmitters, neuromodulators, and metabolic effectors, and their cloned receptors and signaling pathways. We propose that this system is best viewed as a multivalent feedback network that is exquisitely sensitive to an array of neuroregulators and environmental stressors and genetic restraints. Feedback and feedforward mechanisms acting within the intact somatotropic axis mediate homeostatic control throughout the human lifetime and are disrupted in disease. Novel effectors of the GH axis, such as GHRPs, also offer promise as investigative probes and possible therapeutic agents. Further understanding of the mechanisms of GH neuroregulation will likely allow development of progressively more specific molecular and clinical tools for the diagnosis and treatment of various conditions in which GH secretion is regulated abnormally. Thus, we predict that unexpected and enriching insights in the domain of the neuroendocrine pathophysiology of the GH axis are likely be achieved in the succeeding decades of basic and clinical research.

Sexual differentiation of aromatase activity in the rat brain: effects of perinatal steroid exposure

Androgens regulate aromatase activity in the medial preoptic area and other components of the brain circuit that mediates male sexual behavior. The levels of aromatase activity within these brain regions are greater in males than in females. As the activation of copulation requires aromatization of testosterone to estradiol, this quantitative enzymatic difference between sexes could contribute to the greater behavioral response displayed by males. The present study was designed to test the hypothesis that gender differences in brain aromatase activity of adult rats are dependent on the sexual differentiation of the brain that occurs during perinatal exposure to gonadal hormones. Aromatase activity was measured in vitro in microdissected brain samples using a sensitive radiometric assay. We examined the effect of pre- and postnatal treatment with testosterone propionate or diethylstilbestrol on basal levels and androgen responsiveness of aromatase in adults. In addition, we examined what effect prepubertal gonadectomy exerts on enzyme regulation. Our results demonstrate that perinatal treatments with gonadal hormones that are known to differentiate sexual behavior can completely masculinize the capacity for aromatization in the adult female. The process that differentiates aromatase expression appears to depend on androgen exposure and, in part, local estrogen synthesis, as diethylstilbestrol was able to substitute for testosterone propionate. We also observed that prepubertal gonadectomy reduced the levels of aromatase activity measured in adult brain, suggesting that gonadal hormones that are secreted during puberty may enhance the expression of aromatase activity in adulthood. From this study, we conclude that testosterone and/or its estrogenic metabolites act on the developing brain to determine the gender-specific capacity for aromatization and to regulate androgen responsiveness within components of the neural circuitry that mediates male sexual behavior.

In ovo treatment with an aromatase inhibitor masculinizes postnatal hormone levels, abdominal fat pad content, and GH pulsatility in broiler chickens

Vorozole, a selective aromatase inhibitor, was administered in ovo to test the specific embryonic role of estrogen in conferring the sex distinction in GH release and body phenotype in broilers. On Day 6 of incubation, eggs were injected with saline or with different concentrations of vorozole. Postnatal blood samples were analyzed for T3, T4, GH, estradiol (E2), and testosterone (T). At the age of 4 wk, control and vorozole-treated birds were cannulated, and serial blood samples were withdrawn every 10 min for 5 hr, wherein GH pulsatility characteristics were determined using deconvolution analysis. The proportional abdominal fat pad weight was reduced significantly in the treated groups, especially in female birds. The vorozole treatment increased plasma T3, E2, T, and GH concentrations, and decreased T4. The frequency of the GH pulses was lower and the interval between the bursts (min) was higher in the vorozole-treated group, as were the mass secreted per burst (ng/ml), the amplitude (ng/ml/min) and the production rate (ng/ml/5 hr). In conclusion, early in ovo treatment with a potent aromatase inhibitor is able to increase the mean serum T3 and GH concentration and masculinize the GH pulse pattern, resulting in an economically favorable decrease in abdominal fat pad content in male and female broilers at slaughter age.

Estrogen-dependent ontogeny of sex differences in somatostatin neurons of the hypothalamic periventricular nucleus

The sexually dimorphic profile of GH secretion is thought to be engendered by gonadal steroids acting in part on hypothalamic periventricular somatostatin (SOM) neurons. The present study set out to examine and characterize the development of sex differences in these SOM neurons. In the first series of experiments, we used in situ hybridization to examine SOM messenger RNA (mRNA) expression within the periventricular nucleus (PeN) of male and female rats on postnatal day 1 (P1), P5, and P10. Cellular SOM mRNA content was found to increase from P1 to P10 in both sexes (P < 0.01), but was 24% (P < 0.05) and 38% (P < 0.01) higher in males on P5 and P10, respectively. A second series of experiments examined the SOM peptide content of the PeN in developing rats and found increasing levels from P1 to P10, with a 44% higher SOM content in males compared with females on P10 (P < 0.05). The third series of experiments questioned the role of gonadal steroids in engendering sex differences in SOM mRNA expression by determining the effects of neonatal gonadectomy (GDX) and replacement of dihydrotestosterone or estradiol benzoate. The SOM mRNA content of PeN neurons in P5 males gonadectomized on the day of birth was the same as that in P5 females and was significantly reduced compared with that in sham-operated P5 males (P < 0.05). Male rats GDX on P1 and treated with estradiol benzoate from P1 to P5 had cellular SOM mRNA levels similar to those in intact males on P5, whereas dihydrotestosterone treatment had no effect. Treatment of intact males with an androgen receptor antagonist, cyproterone acetate, on P1 had no effect on cellular SOM mRNA on P5, whereas male rats given the aromatase inhibitor 1,4,6-androstatriene-3,17-dione from P1 to P5 had lower (P < 0.05) SOM mRNA levels than controls. In the final set of experiments, dual labeling immunocytochemistry showed that SOM neurons in the PeN of P5 rats did not contain estrogen receptor-alpha, but expressed androgen receptors in a sexually dimorphic manner. These results demonstrate that a sex difference in SOM biosynthesis, which persists into adulthood, develops between P1 and P5 in PeN neurons. Despite the absence of estrogen receptor-alpha in these neurons, the organizational influence of testosterone only occurs after its aromatization to estrogen.

Localization of the preprosomatostatin-mRNA by in situ hybridization in the ewe hypothalamus

The peptidergic neurohormone somatostatin (SRIF) derives from a precursor called preprosomatostatin (PPS) by proteolysis. We have isolated by RT-PCR and sequenced a partial cDNA coding for the ovine PPS. It contains a 348 base pairs coding sequence that shares strong similarities with previously cloned mammalian cDNAs. The ovine cDNA was used to synthesize radiolabeled cRNA to probe the PPS mRNA in the ewe hypothalamus by in situ hybridization. The PPS mRNA-containing cells are widely distributed in the hypothalamus. According to the number of silver grains over a cell, they show various staining intensities. The distribution of the PPS mRNA is in good agreement with that of the peptide previously determined using immunohistochemistry. The strongest labeled areas include the periventricular region of the paraventricular nucleus and the lateral division of the ventromedial nucleus. The difference in labeling intensity observed in the diverse populations of labeled neurons could reflect various levels of neuronal activity.