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

Novel ontogenetic patterns of sexual differentiation in arcuate nucleus GHRH neurons revealed in GHRH-enhanced green fluorescent protein transgenic mice

GH secretion and growth rates are developmentally regulated and sexually dimorphic, but the neuroregulatory mechanisms between birth and puberty are unclear. Using the GHRH-enhanced green fluorescent protein (eGFP) transgenic mouse, in which eGFP provides a strong surrogate signal for identifying GHRH neurons, we showed that numbers in the male arcuate nucleus were double those seen in females at x postnatal day (P)1 and P10, during which time numbers increased 2- to 3-fold. Thereafter (P20, P30, P60, P365) there was a significant trend for numbers to decrease in males and increase in females, such that sex differences were, surprisingly, absent in young and late adulthood. Conversely, we identified the emergence of male-dominant sex differences in the number of processes extended per GHRH perikarya across puberty. Intriguingly, prepubertal gonadectomy (P28), unlike adult gonadectomy, caused a dramatic 40% loss of GHRH cells in both sexes in adulthood and a significant (30%) increase in processes emanating from cell bodies only in females. These findings establish a novel ontogenetic profile for GHRH neurons and suggest previously undiscovered roles for peripubertal gonadal factors in establishing population size in both sexes. They also provide the first demonstration of emergent sex-specific GHRH architecture, which may signal the onset of sex-dependent regulation of activity reported for adult GHRH-eGFP neurons, and its differential regulation by gonadal factors in males and females. This information adds to our knowledge of processes that underpin the emergence of sex-specific GH secretory dynamics and hence biological activity of this pleiotropic hormone.

Estrogen actions in the brain and the basis for differential action in men and women: a case for sex-specific medicines

The classic view of estrogen actions in the brain was confined to regulation of ovulation and reproductive behavior in the female of all mammalian species studied, including humans. Burgeoning evidence now documents profound effects of estrogens on learning, memory, and mood as well as neurodevelopmental and neurodegenerative processes. Most data derive from studies in females, but there is mounting recognition that estrogens play important roles in the male brain, where they can be generated from circulating testosterone by local aromatase enzymes or synthesized de novo by neurons and glia. Estrogen-based therapy therefore holds considerable promise for brain disorders that affect both men and women. However, as investigations are beginning to consider the role of estrogens in the male brain more carefully, it emerges that they have different, even opposite, effects as well as similar effects in male and female brains. This review focuses on these differences, including sex dimorphisms in the ability of estradiol to influence synaptic plasticity, neurotransmission, neurodegeneration, and cognition, which, we argue, are due in a large part to sex differences in the organization of the underlying circuitry. There are notable sex differences in the incidence and manifestations of virtually all central nervous system disorders, including neurodegenerative disease (Parkinson's and Alzheimer's), drug abuse, anxiety, and depression. Understanding the cellular and molecular basis of sex differences in brain physiology and responses to estrogen and estrogen mimics is, therefore, vitally important for understanding the nature and origins of sex-specific pathological conditions and for designing novel hormone-based therapeutic agents that will have optimal effectiveness in men or women.

Role of endogenous somatostatin in regulating GH output under basal conditions and in response to metabolic extremes

Somatostatin (SST) was first described over 30 years ago as a hypothalamic neuropeptide which inhibits GH release. Since that time a large body of literature has accumulated describing how endogenous SST mediates its effects on GH-axis function under normal conditions and in response to metabolic extremes. This review serves to summarize the key findings in this field with a focus on recent progress, much of which has been made possible by the availability of genetically engineered mouse models and SST receptor-specific agonists.

Somatostatin in the rat periventricular nucleus: sex differences and effect of gonadal steroids

In the rat, the sexual dimorphism in growth hormone release is driven by sex steroids, and is suggested to result mainly from differences in somatostatin (SOM) release patterns from the median eminence. We studied the effect of gonadal steroids on SOM peptide-containing cells in the periventricular nucleus (PeVN) of ovariectomized (OVX) female rats, and compared these data with data from intact male rats. Adult female rats were treated with estradiol (E₂) and/or progesterone (P), 3 months (long-term) or 2 weeks (short-term) after ovariectomy (OVX). Perfusion-fixed brains were sliced and stained, and the number of SOM-immunoreactive (-ir) cells and total SOM-ir area (in μm²) were determined using computer assisted analysis. SOM-ir cells in the PeVN showed a very characteristic rostro-caudal distribution and localization in relation to the third ventricle. Both the number of SOM-ir cells and total SOM-ir area in the PeVN were higher in male compared to OVX female rats. Neither the number of SOM-ir cells, nor the total SOM-ir area in the PeVN was affected by E₂ or P treatment alone. Treatment with both gonadal steroids, however, did increase total SOM-immunoreactivity. This study is the first to describe SOM cell distribution within the rat PeVN in great detail. A clear sex difference exists in SOM peptide content in the rat PeVN. In addition, E₂ and P may act synergistically to affect SOM cells in the female PeVN, suggesting that both gonadal steroids may be involved in the generation of the typical feminine SOM release pattern.

Somatostatin is required for masculinization of growth hormone-regulated hepatic gene expression but not of somatic growth

Pulsatile growth hormone (GH) secretion differs between males and females and regulates the sex-specific expression of cytochrome P450s in liver. Sex steroids influence the secretory dynamics of GH, but the neuroendocrine mechanisms have not been conclusively established. Because periventricular hypothalamic somatostatin (SST) expression is greater in males than in females, we generated knockout (Smst(-/-)) mice to investigate whether SST peptides are necessary for sexually differentiated GH secretion and action. Despite marked increases in nadir and median plasma GH levels in both sexes of Smst(-/-) compared with Smst(+/+) mice, the mutant mice had growth curves identical to their sibling controls and retained a normal sexual dimorphism in weight and length. In contrast, the liver of male Smst(-/-) mice was feminized, resulting in an identical profile of GH-regulated hepatic mRNAs between male and female mutants. Male Smst(-/-) mice show higher expression of two SST receptors in the hypothalamus and pituitary than do females. These data indicate that SST is required to masculinize the ultradian GH rhythm by suppressing interpulse GH levels. In the absence of SST, male and female mice exhibit similarly altered plasma GH profiles that eliminate sexually dimorphic liver function but do not affect dimorphic growth.

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.

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.