Intermittent hypoxia causes a suppressed pituitary growth hormone through somatostatin.

OBJECTIVES: The aim of this study was to investigate the response of the growth hormone (GH) in rat anterior pituitary to intermittent hypoxia (IH) and its modulation by hypothalamic somatostatin (SS). SETTING AND DESIGN: To observe the hypoxic response, rats were exposed to simulated altitude hypoxia (2 km or 5 km) in a hypobaric chamber for various days (4 h/d); to clarify SS-involvement, rats were pretreated with SS antagonist (cysteamine, CSH, 200 mg/kg/d, s.c.) then exposed to IH (5 km) for 2d. The GH mRNA and immunostaining GH in pituitary as well as immunostaining SS in median eminence (ME) of hypothalamus were assayed by RT-PCR and immuno-histochemistry, respectively. RESULTS: IH of 5 km altitude (IH5) markedly suppressed the body weight gain (BWG) of rats from 1d to 10d, and it was returned to control level henceforth, while no significant influence was showed in the group of IH of 2 km altitude (IH2). IH5 for 2 and 5d significantly decreased GH mRNA expression in the pituitary. The pituitary immunostaining GH was remarkably increased in groups of IH2 for 5, 10, and 15 d, and in groups of IH5 for 2, 5, and 10d. Immunostaining SS in ME was significantly reduced in group of IH2 for 5d, and in groups of IH5 for 2d and 5d. Pretreatments (s.c.) with SS antagonist (CSH) significantly reversed IH5-caused increase of immunostaining GH and reduction of mRNA levels in pituitary. CONCLUSIONS: IH may cause a short-term and recoverable suppression of GH release, and reduce GH mRNA expression in anterior pituitary, which may depend on the intensity and duration of the hypoxia. This suppression may be due to a modulation of hypoxia-activated SS.

Modulation of brain insulin-like growth factor I (IGF-I) binding sites and hypothalamic GHRH and somatostatin levels by exogenous growth hormone and IGF-I in juvenile rats.

The effect of exogenous growth hormone (GH) and insulin-like growth factor I (IGF-I) on brain IGF-I binding sites (IGF-IR), and on the levels of growth hormone-releasing hormone (GHRH) and somatostatin was studied in hypophysectomized and intact juvenile male rats. Animals were injected subcutaneously twice daily (n = 5 each) with recombinant GH (rGH) (2.5 U/kg per day) or rIGF-I (500 microg/kg per day). In the hypophysectomized rats, serum GH and IGF-I levels were markedly suppressed and IGF-I levels were partially restored by GH treatment. There was a significant increase in IGF-IR binding capacity in the IGF-I-treated hypophysectomized rats compared to the saline-treated hypophysectomized animals (150.61 +/- 45.66 vs 41.32 +/- 12.42 fmol/mg, p < 0.05) but no significant difference in IGF-IR mRNA levels. GHRH levels in the saline-treated hypophysectomized group were significantly lower than in the saline-treated intact rats (31.2 +/- 11.2 vs 140.6 +/- 48.1 pg/mg tissue, respectively, p < 0.01); no effect was induced by GH or IGF-I (37.5 +/- 26.8 and 53.8 +/- 22.5 pg/mg tissue, respectively). However, in the intact rats, GH and IGF-I injection led to a decrease in GHRH content, which was significant in the GH-treated compared to the saline-treated animals (33.1 +/- 16.2 vs 140.6 +/- 48.1 pg/mg tissue, p < 0.01). No difference was found in somatostatin levels between intact and hypophysectomized rats (631.2 +/- 81.2 and 625.0 +/- 62.5 pg/mg tissue, respectively). However, in the hypophysectomized animals, GH and IGF-I treatment induced a significant increase in somatostatin levels (1300 +/- 193.7 pg/mg tissue, p < 0.01, and 912.5 +/- 81.2 pg/mg tissue, p < 0.05, respectively). Our findings suggest that the bioavailability of exogenous IGF-I is greater than that of GH-stimulated endogenous IGF-I. Because IGF-I is a potent neurotrophic agent, this effect may have important implications for states of neurodegenerative diseases.

Somatostatin-14 neurons in the ovine hypothalamus: colocalization with estrogen receptor alpha and somatostatin-28(1-12) immunoreactivity, and activation in response to estradiol.

Pituitary gland growth hormone (GH) secretion is influenced by two hypothalamic neuropeptides: growth hormone-releasing hormone (GHRH) and somatostatin. Recent data also suggest that estrogen modulates GH release, particularly at the time of the preovulatory luteinizing hormone surge, when a coincident surge of GH is observed in sheep. The GHRH neurons do not possess estrogen receptor alpha (ERalpha), suggesting that estrogen does not act directly on GHRH neurons. Similarly, few somatotropes express ERalpha, suggesting a weak pituitary effect of estradiol on GH. It was hypothesized, therefore, that estradiol may affect somatostatin neurons to modulate GH release from the pituitary. Using immunocytochemical approaches, the present study revealed that although somatostatin neurons were located in several hypothalamic sites, only those in the arcuate nucleus (13% +/- 2%) and ventromedial nucleus (VMN; 29% +/- 1%) expressed ERalpha. In addition, we found that all neurons immunoreactive for somatostatin-14 were also immunoreactive for somatostatin-28(1-12). To determine whether increased GH secretion in response to estradiol is through modulation of GHRH and/or somatostatin neuronal activity, a final study investigated whether c-fos expression increased in somatostatin- and GHRH-immunoreactive cells at the time of the estradiol-induced LH surge in intact anestrous ewes. Estradiol significantly (P < 0.05) increased the percentage of GHRH (estradiol, 75% +/- 3%; no estradiol, 19% +/- 2%) neurons expressing c-fos in the hypothalamus. The percentage of somatostatin-immunoreactive neurons coexpressing c-fos in the estradiol-treated animals was significantly (P < 0.05) higher (periventricular, 44% +/- 3%; arcuate, 72% +/- 5%; VMN, 81% +/- 5%) than in the control animals (periventricular, 22% +/- 1%; arcuate, 29% +/- 3%; VMN, 31% +/- 3%). The present study suggests that estradiol modulates the activity of GHRH and somatostatin neurons but that this effect is most likely mediated through an indirect interneuronal pathway.

The use of nimodipine in the treatment of mood disorders.

Nimodipine, a dihydropyridine calcium entry blocker, has been shown to protect from neuronal damage due to ischemia by providing for increased postischemic perfusion. Further, it has also been demonstrated to have antiepileptic properties. These two properties--calcium channel blockade and anticonvulsant benefits have been applied with success to mood disorder treatment. Although found helpful nearly a decade ago for uncomplicated mania, nimodipine may have particular benefits for those diagnostic subclasses of bipolar disorder most resistant to therapy, e.g., ultra-rapid-cycling bipolars and brief recurrent depressions.

Effects of GH and IGF-I administration on GHRH and somatostatin mRNA levels: I. A study on ad libitum fed and starved adult male rats.

The individual role played by GH and IGF-I in the regulation of hypothalamic GHRH and SRIF gene expression is still object of debate. We have investigated the effect of exogenously administered recombinant hGH (rhGH) and recombinant hIGF-I (rhIGF-I) in ad libitum fed control and starved rats, the latter an animal model which is characterized by low circulating levels of endogenous GH and IGF-I. Adult male rats were fed ad libitum (C) or food-deprived (S) for 72 hours; rats in either C or S groups were treated with systemic administration of rhGH and rhIGF-I for 3 days. GHRH, SRIF and GH mRNA levels were evaluated by Northern and slot blot hybridization. Administration of rhGH (250 micrograms/kg/twice daily, sc) induced a significant inhibition of GHRH and a significant stimulation of SRIF mRNA levels in C rats; GH treatment was, however, ineffective on both neuropeptide mRNA levels in the S group. Continuous infusion of rhIGF-I (300 micrograms/kg/day, sc) induced a significant increase of SRIF levels in both C and S rats but did not modify GHRH mRNA levels in either group. In the pituitary, GH mRNA levels followed a pattern very similar to that of GHRH. These results provide evidence for a direct role of GH in the inhibition of GHRH mRNA levels; IGF-I appears more involved in the direct stimulation of SRIF mRNA levels.

Influence of type of dietary fat (olive and sunflower oil) upon gastric acid secretion and release of gastrin, somatostatin, and peptide YY in man.

The effects of adaptation to two diets differing in the type of dietary fat on the gastric acid secretory response to food and on the circulating levels of gastrin, somatostatin and peptide YY (PYY) were examined in humans. The study involved 18 cholecystectomized subjects previously submitted to a 30-day adaptation period to diets containing olive (group O) or sunflower oil (group S) as the fat source. During the experiments, physiological stimulation was achieved by ingestion of 200 ml of oleic acid- (group O) or linoleic acid-enriched (group S) liquid mixed meals. These resulted in an immediate rise in gastric pH. In group S, the return to the premeal value was completed within 60 min, and a further decline to values significantly lower than the basal ones was observed at the end of the study period. In contrast, ingestion of the meal containing olive oil attenuated and prolonged the pH decrease after the meal, this being associated with the suppression of postprandial gastrin response. Food ingestion induced no significant changes in plasma somatostatin concentration in either group, and no significant differences were revealed between them during the basal or postprandial situations. Plasma PYY levels were consistently higher in group O throughout the entire study period, although significance was reached only at resting. In conclusion, our results show that a 30-day adaptation period to diets containing olive oil as the main source of dietary fat results, compared with those containing sunflower oil, in an attenuated gastric secretory function in response to a liquid meal in humans. The effects of olive oil were associated with a suppression of serum gastrin and higher levels of PYY.

Growth hormone secretagogues: focus on the growth hormone-releasing peptides.

This review systematically analyses recent knowledge of the biology of the growth hormone-releasing peptides. Many years before native GHRH had been isolated and sequenced, the synthesis of an enkephalin analog, devoid of any opioid activity but capable of specifically releasing GH from in vitro pituitaries, prompted the design of a number of structurally interrelated GHRPs with improved GH-releasing activity. Nowadays, GHRPs are the most effective GH-secretagogues known and could be used profitably in humans with GH hyposecretory disturbances to promote a pattern of GH secretion that mimics physiology in a better way than the exogenously administered GH.

Chronic growth hormone (GH) hypersecretion induces reciprocal and reversible changes in mRNA levels from hypothalamic GH-releasing hormone and somatostatin neurons in the rat.

Effects of growth hormone (GH) hypersecretion on somatostatin-(SRIH) and GH-releasing hormone (GHRH) were studied by in situ hybridization and receptor autoradiography in rats bearing a GH-secreting tumor. 6 and 18 wk after tumor induction, animals displayed a sharp increase in body weight and GH plasma levels; pituitary GH content was reduced by 47 and 55%, while that of prolactin and thyrotropin was unchanged. At 18 wk, hypothalamic GHRH and SRIH levels had fallen by 84 and 52%, respectively. In parallel, the density of GHRH mRNA per arcuate neuron was reduced by 52 and 50% at 6 and 18 wk, while SRIH mRNA levels increased by 71 and 83% in the periventricular nucleus (with no alteration in the hilus of the dentate gyrus). The numbers of GHRH- and SRIH-synthetizing neurons in the hypothalamus were not altered in GH-hypersecreting rats. Resection of the tumor restored hypothalamic GHRH and SRIH mRNAs to control levels. GH hypersecretion did not modify 125I-SRIH binding sites on GHRH neurons. Thus, chronic GH hypersecretion affects the expression of the genes encoding for GHRH and SRIH. The effect is long lasting, not desensitizable and reversible.

Regulation of somatostatin and growth hormone-releasing factor by gonadal steroids in fetal rat hypothalamic cells in culture.

The mechanism underlying the sexually dimorphic pattern of growth hormone (GH) secretion in the rat has not been clearly elucidated. In the present study, we assayed the possible direct effect of gonadal steroids on both somatostatin (SS) and growth hormone-releasing factor (GRF) in fetal rat hypothalamic cells in culture. Hypothalamic cells, obtained by mechanical dispersion, were maintained as monolayer cultures in serum-supplemented medium. After 20 days in culture, cells were incubated with serum free medium containing testosterone (T, 10, 20, 40 ng/dl) or estradiol (E, 0.1, 1, 10 ng/dl) for 48 h. At the end of the experiments, immunoreactive SS (IR-SS) and immunoreactive GRF (IR-GRF) were measured by specific radioimmunoassays (RIAs) in media and cell extracts. After 48 h of incubation with testosterone, somatostatin in both media and cells was significantly reduced. On the contrary, this treatment lead to a dose-dependent increase in media and cell GRF content. When cells were incubated with estradiol for 48 h, a significant inhibition in medium SS release was observed, whereas intracellular SS slightly increased at the highest concentration of 10 ng/dl. Estradiol treatment resulted in an inconsistent decrease in media and cells IR-GRF. Our results indicate that both SS and GRF are under the influence of testosterone and estradiol acting at the hypothalamic level, and furthermore suggest that at this stage of brain development, gonadal steroids may regulate GH secretion through their ability to modulate hypothalamic SS and GRF.