Masculinization of growth hormone (GH) secretory pattern by dihydrotestosterone is associated with augmentation of hypothalamic somatostatin and GH-releasing hormone mRNA levels in ovariectomized adult rats

Pubertal effects of 17α-methyltestosterone on GH-IGF-related genes of the hypothalamic-pituitary-liver-gonadal axis and other biological parameters in male, female and sex-reversed Nile tilapia

The influence of 17α-methyltestosterone (MT) on growth responses, biological parameters and the expression of genes involved in the GH-IGF pathway of the hypothalamic-pituitary-liver-gonadal axis were investigated in female, male, and sex-reversed Nile tilapia to evaluate the relationship between sex and MT-induced changes in these parameters. Female fish had a lower growth rate than male and sex-reversed fish, and MT increased growth performance and duodenal villi in females. Most but not all biological parameters of sex-reversed fish were similar to those of male fish. Male fish had higher red blood cell counts and hemoglobin levels than female and sex-reversed fish, suggesting that these hematological indices reflect a higher metabolic rate in male fish. Greater blood triglyceride levels indicated the vitellogenin process in female fish. MT increased the alternative complement activity in female fish (P<0.05). Sex and MT had no significant effects on the hypothalamic mRNAs of GHRH and PACAP. Although not statistically significant, females tended to have higher GH mRNA levels than male and sex-reversed fish. Additionally, MT tended to decrease and increase GH mRNA levels in female and male fish, respectively. There were significant differences among sexes in the expression of GHR, and IGF mRNAs at the peripheral level in the liver and gonads. Females had lower hepatic GHRs and higher ovarian GHRs than male and sex-reversed fish. While the mRNA levels of IGF-1 were lower in the ovary, the levels of IGF-2 were higher compared with those in testes. A significant correlation between GHRs and IGFs was demonstrated in the liver and gonad (except for IGF-1). Multiple regression analysis showed a significant relationship between GH mRNA and both GHRs and IGFs in the liver and gonad. MT exerted androgenic and, to some extent, estrogenic effects on several physiological parameters and GH-IGF action.

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.

Somatostatin and its receptors contribute in a tissue-specific manner to the sex-dependent metabolic (fed/fasting) control of growth hormone axis in mice

Somatostatin (SST) inhibits growth hormone (GH) secretion and regulates multiple processes by signaling through its receptors sst1-5. Differential expression of SST/ssts may contribute to sex-specific GH pattern and fasting-induced GH rise. To further delineate the tissue-specific roles of SST and sst1-5 in these processes, their expression patterns were evaluated in hypothalamus, pituitary, and stomach of male and female mice under fed/fasted conditions in the presence (wild type) or absence (SST-knockout) of endogenous SST. Under fed conditions, hypothalamic/stomach SST/ssts expression did not differ between sexes, whereas male pituitary expressed more SST and sst2A/2B/3/5A/5TMD2/5TMD1 and less sst1, and male pituitary cell cultures were more responsive to SST inhibitory actions on GH release compared with females. This suggests that local pituitary SST/ssts can contribute to the sexually dimorphic pattern of GH release. Fasting (48 h) reduced stomach sst2A/B and hypothalamic SST/sst2A expression in both sexes, whereas it caused a generalized downregulation of pituitary sst subtypes in male and of sst2A only in females. Thus, fasting can reduce SST sensitivity across tissues and SST input to the pituitary, thereby jointly contributing to enhance GH release. In SST-knockout mice, lack of SST differentially altered sst subtype expression levels in both sexes, supporting an important role for SST in sex-dependent control of GH axis. Evaluation of SST, IGF-I, and glucocorticoid effects on hypothalamic and pituitary cell cultures revealed that these hormones could directly account for alterations in sst2/5 expression in the physiological states examined. Taken together, these results indicate that changes in SST output and sensitivity can contribute critically to precisely define, in a tissue-dependent manner, the sex-specific metabolic regulation of the GH axis.

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.

Gender-dependent role of endogenous somatostatin in regulating growth hormone-axis function in mice

It has been previously reported that male and female somatostatin (SST) knockout mice (Sst-/-) release more GH, compared with Sst+/+ mice, due to enhanced GH-secretory vesicle release. Endogenous SST may also regulate GH secretion by directly inhibiting GHRH-stimulated GH gene expression and/or by modulating hypothalamic GHRH input. To begin to explore these possibilities and to learn more about the gender-dependent role of SST in modulating GH-axis function, hypothalamic, pituitary, and liver components of the GH-axis were compared in male and female Sst+/+ and Sst-/- mice. Pituitary mRNA levels for GH and receptors for GHRH and ghrelin were increased in female Sst-/- mice, compared with Sst+/+ controls, and these changes were reflected by an increase in circulating GH and IGF-I. Elevated levels of IGF-I in female Sst-/- mice were associated with elevated hepatic mRNA levels for IGF-I, as well as for GH and prolactin receptors. Consistent with the role of GH/IGF-I in negative feedback regulation of hypothalamic function, GHRH mRNA levels were reduced in female Sst-/- mice, whereas cortistatin (CST) mRNA levels were unaltered. In contrast to the widespread impact of SST loss on GH-axis function in females, only circulating GH, hypothalamic CST, and hepatic prolactin receptor expression were up-regulated in Sst-/- male mice, compared with Sst+/+ controls. These results confirm and extend the sexually dimorphic role of SST on GH-axis regulation, and suggest that CST, a neuropeptide that acts through SST receptors to inhibit GH secretion, may serve a compensatory role in maintaining GH-axis function in Sst-/- male mice.

Sexually dimorphic distribution of sst2A somatostatin receptors on growth hormone-releasing hormone neurons in mice

The pulsatile pattern of GH secretion exhibits sexual dimorphism that is likely to depend on somatostatin (SRIH) effects on somatoliberin (GHRH) neurons in the hypothalamus. Using transgenic GHRH-enhanced green fluorescent protein (eGFP) mice, no difference in the total number of GHRH-eGFP neurons or change in somatostatin receptor sst2 and SRIH mRNA levels in ventromedial hypothalamic nucleus-arcuate nucleus and periventricular nucleus regions and GHRH mRNA levels in the ventromedial hypothalamic-arcuate region were observed between male and female mice. However, the percentage of GHRH-eGFP neurons bearing sst2A receptors reached 78% in female vs. 26% in male GHRH-eGFP mice (P < 0.02). This sex difference in sst2A distribution on GHRH neurons may play an important role in the sexually differentiated pattern of GH secretion.

Androgen receptor null male mice develop late-onset obesity caused by decreased energy expenditure and lipolytic activity but show normal insulin sensitivity with high adiponectin secretion

Androgen receptor (AR) null male mice (AR(L-/Y)) revealed late-onset obesity, which was confirmed by computed tomography-based body composition analysis. AR(L-/Y) mice were euphagic compared with the wild-type male (AR(X/Y)) controls, but they were also less dynamic and consumed less oxygen. Transcript profiling indicated that AR(L-/Y) mice had lower transcripts for the thermogenetic uncoupling protein 1, which was subsequently found to be ligand-dependently activated by AR. We also found enhanced secretion of adiponectin, which is insulin sensitizing, from adipose tissue and a relatively lower expression of peroxisome proliferator-activated receptor-gamma in white adipose tissue in comparison to AR(X/Y) mice. Both factors might explain why the overall insulin sensitivity of AR(L-/Y) mice remained intact, despite their apparent obesity. The results revealed that AR plays important roles in male metabolism by affecting the energy balance, and it is negative to both adiposity and insulin sensitivity.

Microinjection of dihydrotestosterone into the medial preoptic area produces male-like pattern of growth hormone secretion in ovariectomized female rats

The pattern of growth hormone (GH) secretion is sexually dimorphic in rats. We have previously shown that the secretory pattern in adult ovariectomized (OVX) female rats is masculinized by the administration of a single dose of dihydrotestosterone (DHT), a nonaromatizable androgen. To investigate the primary site of action of DHT in the brain, a small amount of DHT was injected directly into a defined area of the brain, and the blood GH profile was observed for 18 h in conscious adult OVX female rats. The bilateral direct injection of 1 microg DHT into the medial preoptic area (MPA) produced a male-like secretory pattern of GH in OVX rats. The masculinizing effects became apparent at 9 h after injection, from which time the episodic GH secretion was produced regularly at intervals of about 150 min, the amplitude of the peak increased and baseline levels were lowered. These parameters, analyzed during 9-18 h after DHT injection, were not different from those in adult male rats. On the contrary, microinjection of DHT into the bed nucleus of the stria terminalis, the hypothalamic periventricular nucleus, or the hypothalamic arcuate-ventromedial nucleus did not affect the secretory pattern of GH. The data indicate that DHT primarily acts on cells in the MPA through androgen receptors and modulates the secretion of somatostatin and/or GH-releasing hormone secondarily to masculinize the GH secretory pattern in OVX rats.

Middle-age alterations in the sexually dimorphic plasma growth hormone profiles: involvement of growth hormone-releasing factor and effects on cytochrome p450 expression

Rat liver, as well as other species, contains numerous sex-dependent isoforms of cytochrome P450 (P450) that are regulated by the sexually dimorphic profiles of circulating growth hormone. During puberty, young adulthood, and senescence, changes in the hormonal profiles appear to be responsible for alterations in age-associated expression levels of selective P450 isoforms. In contrast, little is known about the growth hormone secretory profiles and their P450-dependent expression levels during middle age. In the present study, we observed subtle changes in the hormonal concentrations, and frequencies of peaks and interpulse periods in the sexually dimorphic growth hormone profiles of 1-year-old male and female rats correlated to suppression of male-specific isoforms CYP2C11 and CYP2C13 and female-predominant CYP2C7. To identify possible causes for the age-associated changes in the circulating growth hormone profiles, the responsiveness of the hypothalamic-pituitary axis to growth hormone secretagogues clonidine and growth hormone-releasing factor (GRF) were examined in middle-aged male and female rats. In spite of the same sexually dimorphic response in young adult and middle-aged rats to both secretogogues (males > females), the pituitary somatotrophs in the older animals exhibited a dramatic decrease in sensitivity to clonidine, characterized by subnormal growth hormone release levels and an inordinate delay in pituitary response to clonidine stimulation. Results from similar studies conducted on middle-aged arcuate nucleus-lesioned rats suggest that a decline in GRF secretion is a possible contributor to the age-associated alterations in plasma growth hormone profiles during middle age. These changes in GRF-induced, sexually dimorphic secretory growth hormone profiles and the accompanying decline in P450 expression levels may anticipate similar, but more profound, changes to occur during senescence.

Mechanisms of reduced body growth in the pubertal feminized male rat: unbalanced estrogen and androgen action on the somatotropic axis

It is well known that the sex difference in body growth at puberty is modulated by a complex interplay between sex steroids and somatotropic axis; however, the exact role played by sex steroids remains a matter of controversy. The aim of this study was to assess the mechanisms by which sex steroids regulate body growth during pubertal development. Flutamide, a non-steroid-blocking androgen receptor, was subcutaneously administered to 30-d-old male Wistar rats for 4 wk. The blockade of the androgen receptor led to a marked elevation in serum testosterone and an increment in serum estradiol. Flutamide administration decreased body weight gain, serum IGF-I levels, hepatic IGF-I mRNA, and GH receptor mRNA content. There were no significant changes in serum GH concentration, pituitary GH reserve, and pituitary GH mRNA content. Flutamide lowered hypothalamic somatostatin mRNA content and augmented hypothalamic immunoreactive somatostatin stores, but did not alter hypothalamic immunoreactive GH-releasing factor stores. Our findings indicate that during pubertal development of the male rat, the imbalance between androgen and estrogen actions determines an abnormal somatic growth, which is at least partly exerted through the peripheral or hepatic modification of the somatotropic axis that occurs under the high or exclusive action of estrogens. Potential implication of coincident sex-specific regulated mode of pulsatile GH secretion cannot be excluded from this random serum GH sample study.

Induction of growth hormone (GH) mRNA by pulsatile GH-releasing hormone in rats is pattern specific

Growth hormone-releasing hormone (GHRH) is a main inducer of growth hormone (GH) pulses in most species studied to date. There is no information regarding the pattern of GHRH secretion as a regulator of GH gene expression. We investigated the roles of the parameters of exogenous GHRH administration (frequency, amplitude, and total amount) upon induction of pituitary GH mRNA, GH content, and somatic growth in the female rat. Continuous GHRH infusions were ineffective in altering GH mRNA levels, GH stores, or weight gain. Changing GHRH pulse amplitude between 4, 8, and 16 microg/kg at a constant frequency (Q3.0 h) was only moderately effective in augmenting GH mRNA levels, whereas the 8 microg/kg and 16 microg/kg dosages stimulated weight gain by as much as 60%. When given at a 1.5-h frequency, GHRH doubled the amount of GH mRNA, elevated pituitary GH stores, and stimulated body weight gain. In the rat model, pulsatile but not continuous GHRH administration is effective in inducing pituitary GH mRNA and GH content as well as somatic growth. These studies suggest that the greater growth rate, pituitary mRNA levels, and GH stores seen in male compared with female rats are likely mediated, in part, by the endogenous episodic GHRH secretory pattern present in males.

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.

Growth hormone-releasing hormone receptor (GHRH-R) and growth hormone secretagogue receptor (GHS-R) mRNA levels during postnatal development in male and female rats

Experimental evidence suggests that differential pituitary sensitivity to hypothalamic signals exerts a role in mediating both age and sex dependent patterns of growth hormone (GH) release and synthesis. One mechanism by which pituitary sensitivity to hypothalamic GH regulators could be modified is by the differential synthesis of their pituitary receptors. In the present report we therefore studied the age and sex dependency of the expression of receptors for two known stimulators of GH release, growth hormone-releasing hormone (GHRH) and the synthetic peptidyl and non-peptidyl GH secretagogues (GHSs). Pituitary GHRH receptor (GHRH-R) and GHS receptor (GHS-R) mRNA levels were measured by reverse transcriptase-polymerase chain reaction (RT-PCR) in male and female rats at postnatal day 1, 10, 30 and 75. We also examined the age- and sex-dependent expression of the GHS-R in whole hypothalamic extracts, since the GHS-R is also expressed in a variety of nuclei within the hypothalamus and has been linked to central regulation of the GH-axis. Pituitary GHRH-R mRNA concentrations were age-dependent; the highest levels were observed in d1 pituitaries and then declined with age, reaching a nadir by d30. These results are in concordance with the age-related decline in pituitary GHRH sensitivity. In contrast, the ontogenic pattern of GHS-R expression was bimodal; GHS-R mRNA concentrations in dl and d30 pituitaries were approximately twice those at d10 and d75. These results mirror the transient increase in GHS sensitivity observed around the onset of puberty, suggesting that gonadal steroids mediate GHS-R expression. GHRH-R mRNA levels were comparable in males and females within each age while GHS-R mRNA levels were gender dependent. At d30, male GHS-R mRNA levels were 30% greater than in their female counterparts. This was reversed at d75, when females had 89% more GHS-R mRNA per pituitary and 65% more per somatotrope than did age-matched males. These sexual differences further support a role for gonadal steroids in the modulation of pituitary GHS-R synthesis. The ontogenic and gender-specific pattern of hypothalamic GHS-R expression differed from that observed for the pituitary. Hypothalamic GHS-R mRNA levels increased with age but exhibited no significant sex difference at each age tested. Taken together, these data demonstrate that changes in the levels of pituitary GHS-R mRNA, but not GHRH-R mRNA, are associated with changes in the gonadal steroid environment, thereby implicating the GHS/GHS-R signalling system as a control point in the establishment and maintenance of sexually dimorphic patterns of GH secretion.

Neuroendocrine control of growth hormone secretion

The secretion of growth hormone (GH) is regulated through a complex neuroendocrine control system, especially by the functional interplay of two hypothalamic hypophysiotropic hormones, GH-releasing hormone (GHRH) and somatostatin (SS), exerting stimulatory and inhibitory influences, respectively, on the somatotrope. The two hypothalamic neurohormones are subject to modulation by a host of neurotransmitters, especially the noradrenergic and cholinergic ones and other hypothalamic neuropeptides, and are the final mediators of metabolic, endocrine, neural, and immune influences for the secretion of GH. Since the identification of the GHRH peptide, recombinant DNA procedures have been used to characterize the corresponding cDNA and to clone GHRH receptor isoforms in rodent and human pituitaries. Parallel to research into the effects of SS and its analogs on endocrine and exocrine secretions, investigations into their mechanism of action have led to the discovery of five separate SS receptor genes encoding a family of G protein-coupled SS receptors, which are widely expressed in the pituitary, brain, and the periphery, and to the synthesis of analogs with subtype specificity. Better understanding of the function of GHRH, SS, and their receptors and, hence, of neural regulation of GH secretion in health and disease has been achieved with the discovery of a new class of fairly specific, orally active, small peptides and their congeners, the GH-releasing peptides, acting on specific, ubiquitous seven-transmembrane domain receptors, whose natural ligands are not yet known.

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.