Evidence that growth hormone-releasing factor stimulates somatostatin release in vitro via beta-endorphin

Links between the appetite regulating systems and the neuroendocrine hypothalamus: lessons from the sheep

The hypothalamus is integral to the regulation of energy homeostasis and the secretion of hormones from the pituitary gland. Consequently, hypothalamic systems may have a dual purpose in regulating both neuroendocrine function and appetite. To date, most studies investigating the interface between appetite and hormone secretion have been performed in rats or mice that have been acutely fasted or baring a genetic abnormality causing either obesity or aphagia. By contrast, various physiological models, including chronic food-restriction or photoperiodically driven changes in voluntary food intake, add further perspective to the issue. In this regard, sheep provide an innovative model whereby long-term changes in body weight or extended feeding rhythms can be investigated. This review compares and contrasts data obtained in different species with regard to the neuroendocrinology of appetite, and discusses the benefits and knowledge gained from using various nonrodent models with a particular emphasis on a ruminant species.

Hypothalamic derangement in traumatized patients: growth hormone (GH) and prolactin response to thyrotrophin-releasing hormone and GH-releasing hormone

OBJECTIVE

To study the impact of severe head injury on both basal pituitary hormone secretion and the response to exogenous synthetic hypothalamic releasing factors (TRH and GHRH) in order to evaluate sequential changes in the central control of hypophyseal secretion in the days following head injury.

DESIGN

Prospective clinical study

PATIENTS

21 comatose male patients with head injuries, each intubated and ventilated, intensively monitored and having no previous endocrine problems.

MEASUREMENTS

AND RESULTS The GH and PRL responses to TRH (200 microg iv), and the GH and PRL responses to GHRH (50 microg iv) were evaluated, respectively, on the days 1 and 16 and on days 2, 7and 15 after admission. Daily blood samples were also collected for GH, PRL, TSH, T3 and T4 evaluation. In the basal samples taken on days 2, 7 and 15, IGF-I and cortisol were also determined. Nitrogen balance was assessed daily. On the day 1, TRH increased GH levels from 9.8 +/- 2.2 to 22.4 +/- 6.5 mU/l but failed to induce GH release on day 16. The PRL response to TRH was normal. The GH peak response to GHRH was normal on the day 2 (35.7 +/- 13.9 mU/l), but was increased on days 7 and 15 (68.3 +/- 10.7 mU/l on day 7; 73.8 +/- 9.2 mU/l on day 15, P < 0.01 vs. day 2). We found a significant PRL response to GHRH during the whole period of observation. In the daily evaluation, nitrogen balance was negative in all patients from the day 1 to 5. On average, all patients reached a positive nitrogen balance on the day 8. Compared to the day 2, a statistical increase in IGF-I concentration was observed on days 7 and 15.

CONCLUSIONS

The evaluation of pituitary dynamics in the acute phase of a severe injury demonstrates an alteration of GH and PRL secretion, which correlate with the aminergic and/or peptidergic derangements. Taken together, our data suggest augmented tone of both GHRH and somatostatin in the very acute phase, while an imbalance of releasing factors is hypothesized in the following days. The metabolic consequences of this neuroendocrine pattern could be advantageous in the rapid recovery from the cascade of events produced by the trauma, as documented by the increase in IGF-1 levels and the positive nitrogen balance.

Interrelations between hypothalamic somatostatin and proopiomelanocortin neurons

Somatostatin receptors were visualized by [125I]-Tyr0-DTrp8-somatostatin radioautography on 35% of arcuate neurons containing proopiomelanocortin (POMC) mRNA, as identified by in situ hybridization using a [35S] labelled riboprobe on 5 microm-thick consecutive sections. Furthermore, double immunohistochemical staining revealed contacts of beta-endorphin or alpha-MSH containing fibres with a majority of somatostatin perikarya in the anterior hypothalamic periventricular nucleus. Taken together, these data indicate that hypothalamic somatostatin and POMC neurons are interconnected. The results are discussed in term of intrahypothalamic control of GH secretion.

Hypothalamic and hypophyseal regulation of growth hormone secretion

1. Regulation of pulsatile secretion of growth hormone (GH) relies on hypothalamic neuronal loops, major transmitters involved in their operation are growth hormone releasing hormone (GHRH) synthetized mostly in arcuate nucleus (ARC) neurons, and somatostatin (SRIH), synthetized both in hypothalamus periventricular (PVe) and ARC neurons. 2. Neurons synthetizing both peptides can inhibit each other in a reciprocal manner. Other neuropeptides synthetized in ARC neurons, such as galanin, or in ARC interneurons, such as neuropeptide Y (NPY), are able to modulate synthesis and release of GHRH and SRIH into the hypothalamohypophyseal portal system. 3. In addition, the hitherto uncharacterized endogenous ligand of the recently cloned growth hormone releasing peptide receptor, expressed mostly in the ARC, triggers GH release, presumably by actions on ARC interneurons. 4. Thyroid, gonadal, and adrenal steroid hormones also affect the GHRH-SRIH balance; a differential distribution of sex steroid receptors in the ARC and the PVe is likely to account for the different pattern of GH secretion in male and female animals. 5. Growth hormone itself is able to inhibit the amplitude of GH secretory episodes and to increase their frequency, by entering the brain (presumably by receptor-mediated internalization at the level of the choroid plexus) and acting subsequently on ARC neurons. 6. At the pituitary level, major neurotransmitters regulating GH cells act on receptors of the VIP/PACAP/GHRH family and of the somatostatin family, in particular, sst2 and sst3. Those are coupled to accumulation of cAMP as a second messenger. 7. In addition, patch-clamp experiments and measurement of intracellular Ca2+ indicate that GH cells present characteristic, GHRH-dependent, but self-maintained Ca2+ spikes and [Ca2+]i transients, which reflect adaptive mechanisms to constraints of episodic release. 8. Recent data on transcription factors affecting GH gene expression and somatotrope differentiation are also summarized. 9. Regulation and differentiation of somatotropes also depend upon paracrine processes within the pituitary itself and involve growth factors and several neuropeptides, for instance, vasoactive intestinal peptide, angiotensin 2, endothelin, and activin. 10. Finally, characteristic changes occur in the GH secretory pattern under discrete, pathological conditions, such as abnormal growth and dwarfism, diabetes, and acromegaly, as well as during inflammatory processes.

Growth hormone-regulatory peptides (GHRH and somatostatin) and feeding: a model for the integration of central and peripheral function

The present paper provides an overview of findings that implicate growth hormone-releasing hormone (GHRH) and somatostatin (SS), the two peptides that regulate growth hormone secretion, in the central regulation of feeding. Evidence is presented that GHRH and SS increase food intake, in the rat, via a common centrally mediated mechanism involving the suprachiasmatic nucleus. Food intake is increased by increasing motivation to eat as evidenced by facilitation of operant behavior. Macronutrient-choice studies indicate that GHRH (and possibly SS) selectively facilitate protein consumption. Time of day is also important, with evidence that endogenous GHRH and SS-induced feeding is most strong in the early nocturnal period. GHRH and SS, together with other nutrient-specific signals, such as neuropeptide Y, noradrenaline and galanin, may determine the circadian expression of food intake in animals. Other behavioral and physiological effects of these peptides, both central and peripheral, are reviewed in the context of a possible mechanism by which these peptides integrate diverse, but complimentary, central and peripheral functions related to nutrition, metabolism and growth.

Acute stress stimulates secretion of GHRH and somatostatin into hypophysial portal blood of conscious sheep

The effects of acute stress on growth hormone (GH) secretion and the mechanisms involved in its changes have been investigated in sheep. An acute isolation-restraint stress induced a rapid and significant increase in jugular GH levels in 12 out of 14 rams. GH-releasing hormone (GHRH) and somatostatin secretion during the same stress were studied in 5 animals prepared for hypophysial portal blood collection. A 3.5-fold increase in portal GHRH levels was observed concomitantly with a slight elevation in portal somatostatin. Portal corticotropin-releasing hormone (CRH) and jugular cortisol plasma levels increased during the same stress. Our data suggest that an isolation-restraint stress stimulates GH secretion in the sheep and that GHRH may be responsible for GH response.

Growth hormone-releasing factor increases somatostatin release and mRNA levels in the rat periventricular nucleus via nitric oxide by activation of guanylate cyclase

Previous work has shown that growth hormone-releasing factor (GRF) stimulates cGMP production and somatostatin [somatotropin (growth hormone)-release-inhibiting factor, SRIF] release without altering cAMP accumulation by fragments of median eminence incubated in vitro. Therefore, this study was undertaken to evaluate the effect of GRF and cGMP on SRIF mRNA and SRIF release in the periventricular nuclei of male rats in vitro. SRIF mRNA levels were determined in explants of periventricular nuclei incubated for 6 hr in Waymouth's medium in the presence of various substances. Steady-state levels of SRIF mRNA were measured by an S1 nuclease protection assay using a 32P-labeled rat SRIF RNA probe. SRIF release and cGMP formation were measured at 30 min and 6 hr by RIA. SRIF mRNA levels and SRIF release were significantly (P < 0.025) increased (approximately 2-fold) by 1 microM dibutyryl cGMP, whereas sodium butyrate had no effect. This augmentation was not influenced by cycloheximide, an inhibitor of protein synthesis. Sodium nitroprusside (10 microM), an activator of the guanylate cyclase pathway via its release of nitric oxide, augmented (P < 0.001) SRIF mRNA levels and significantly increased (P < 0.05) SRIF release. GRF (1 nM) increased SRIF mRNA (P < 0.001) and stimulated the release of SRIF at 30 min (P < 0.05) and 6 hr (P < 0.01). This stimulation was abolished by 10 microM NG-monomethyl-L-arginine (L-NMMA), a specific inhibitor of nitric oxide synthase, but not by NG-monomethyl-D-arginine (D-NMMA, the inactive isomer). GRF also increased cGMP formation. This effect was completely blocked by incubation with L-NMMA but not D-NMMA. These results indicate that GRF releases nitric oxide. The nitric oxide diffuses to the adjacent SRIF neurons, where it activates guanylate cyclase, leading to increased formation of cGMP. This cGMP increases SRIF mRNA and SRIF release in the periventricular nuclei of male rats.

Effects of long-term infusion of growth hormone (GH)-releasing factor on pulsatile GH secretion in the male rat

The effects of long-term infusion of GRF on pulsatile GH secretion were investigated in conscious, freely-moving male rats. GH-releasing factor (GRF) was intravenously administered for 14 days using an osmotic minipump. Six-hour iv infusion of 0.5 microgram/h GRF significantly increased mean GH concentration in the conscious rats. In contrast, 14-day iv infusion of 0.5 microgram/h GRF significantly inhibited pulsatile GH secretion. The pituitary GH content was significantly increased by the 14-day treatment. The GH response to bolus injection of 1 microgram GRF under urethane anesthesia was enhanced by the 14-day infusion of GRF. Pretreatment with somatostatin (SS) antiserum did not restore the inhibited GH secretion in rats treated with 14-day infusion of GRF. These results suggest that neither pituitary GH depletion nor high somatostatin tone is involved in the GRF-induced GH inhibition.

Periventricular nucleus lesioning modulates specific somatostatin binding in various brain regions and anterior pituitary

The effects of periventricular nucleus (Pe) lesioning on the plasma growth hormone (GH) levels and the anterior pituitary (A.P.) and brain somatostatin (SRIF) receptors were studied. A transient significant increase in plasma GH level in lesioned rats was detected 1 day after the operation. This elevated level of plasma GH started to decrease 3 days after lesioning. These changes were paralleled by an increase in binding of 125I-Tyr11-SRIF-14 to the A.P. 1 day after lesioning. This result could further confirm that the SRIF inhibitory action on GH release takes place at the A.P. level. Also, a transient increase in binding of the radioligand was detected in some brain areas 1 and 4 days after the lesion. However, the mechanism by which this increase takes place remains to be elucidated.

Mouse hypothalamic growth hormone-releasing hormone and somatostatin responses to probes of signal transduction systems

Signal transduction mechanisms involved in mouse growth hormone-releasing hormone (GRH) and somatostatin (SRIH) release were investigated using an in vitro perifusion system. Hypothalamic fragments were exposed to depolarizing agents, protein kinase A and C activators, and a calcium ionophore. The depolarizing agents, KCl (60 mM) and veratridine (50 microM), induced similar patterns of GRH and SRIH release. Somatostatin release in response to both agents was twofold greater than that of GRH. Forskolin (10 microM and 100 microM), an adenylate cyclase activator, stimulated both GRH and SRIH release, though with different secretory profiles. The SRIH response was prolonged and persisted beyond removal of the drug from the system, while the GRH response was brief, ending even prior to forskolin removal. Neither GRH nor SRIH were stimulated by 1,9-dideoxy-forskolin (100 microM), a forskolin analog with cAMP-independent actions. A23187 (5 microM), a calcium ionophore, stimulated the release of SRIH to a much greater extent than that of GRH. The GRH and SRIH secretory responses to PMA (1 microM), a protein kinase C activator, were similar, though delayed. The results suggest that 1) GRH and SRIH secretion are regulated by both protein kinase A and C pathways, and 2) depolarizing agents are important for the release of both hormones.

Rat growth hormone-releasing factor stimulates cyclic GMP formation and phosphatidylinositol metabolism in the median eminence

The effects of rat growth hormone releasing factor (rGRF) on somatostatin (SRIF) secretion, cyclic nucleotide production and phosphatidylinositol metabolism were investigated in the median eminence (ME), using an in vitro system. Medium was discarded and replaced by medium containing various concentrations of rGRF or rGRF plus epinephrine (E, 6 x 10(-7) M). rGRF had no effect on basal or E-stimulated release of cAMP. In the same experiments rGRF markedly stimulated SRIF release. These results suggested that cAMP is not involved in the stimulatory effect of GRF on SRIF release. However, GRF significantly stimulated release of both SRIF and cGMP in a dose-related manner. Maximal stimulation was observed at 10(-10) M GRF (p less than 0.005) which also produces maximal SRIF release. 2'0-monobutyrylguanosine 3'5' cyclic phosphate (mbcGMP, 10(-11) to 10(-10) M) stimulated SRIF release from ME fragments (p less than 0.001 at 10(-10) M) whereas the control, sodium butyrate (10(-6) M), had no effect. GRF caused significant elevation of 30.6% in the concentration of labelled inositol phosphates [( 3H]-IPs) in the ME. These data indicate that GRF stimulation of SRIF release is accompanied by increased cGMP production and phosphatidyl-inositol (PI) metabolism but does not alter cAMP production. Because mbcGMP can directly stimulate SRIF release, we suggest that GRF causes a receptor-mediated increase in the metabolism of phosphatidylinositol and cGMP formation. These actions therefore may be among the early metabolic events in the mechanism of GRF-stimulated SRIF release from the ME.

Central somatostatinergic regulation of growth hormone secretion in dwarf chickens

1. Basal circulating growth hormone (GH) concentrations in sex-linked-dwarf (SLD) chickens were unaffected by the intracerebroventricular (icv) injection of 10, 50 or 100 micrograms somatostatin (SRIF). 2. The GH response to systemic thyrotropin-releasing hormone (TRH; 10 micrograms/kg, iv) was, however, 'paradoxically' enhanced 20 min after icv SRIF administration. 3. A lower dose (1.0 micrograms) of SRIF had no effect on basal or TRH-induced GH release. 4. High-titre SRIF antisera (4 microliters) also had no acute effect on basal plasma GH concentrations, but augmented the GH response to TRH challenge. 5. SRIF would appear to act at central sites to modulate stimulated GH secretion in SLD chickens.

Methods for the study of somatostatin

It is clear from the above that there are a number of methods for study of SRIF release. From the standpoint of convenience, the in vitro static incubation of ME is the most practical technique at the present time. Using this preparation SRIF release has been found to be modified by a number of neurotransmitters and peptides, and studies on the mechanism of release of the peptide have been initiated. There is no doubt that such studies should be complemented by perifusion studies, by studies involving larger pieces of the hypothalamus which encompass the entire somatostatinergic neuron, and by in vivo studies to determine the correlation of in vivo and in vitro release. Among the in vivo techniques which have been utilized, the push-pull cannula technique employing cannulae implanted in hypothalamus or anterior pituitary gland offers the most promise. A summary of the effects of some neurotransmitters and neuropeptides on hypothalamic SRIF secretion is reported in Table. I.

GHRF causes biphasic stimulation of SRIF secretion from rat hypothalamic cells

The complex interactions of the hypothalamic releasing peptides somatostatin (SRIF) and growth hormone (GH)-releasing hormone (GHRF), which regulate GH secretion, are still incompletely understood. To further scrutinize these interactions, we have studied the effects of GHRF on SRIF secretion from dispersed adult rat hypothalamic cells. Rat GHRF caused calcium- and dose-dependent stimulation of SRIF release in static 1-h incubations. SRIF release was stimulated by GHRF in a concentration range of 1-100 nM. However, the extended dose-response curve was biphasic in nature, with a significantly lower SRIF response in the presence of 1 microM GHRF vs. 100 nM GHRF. SRIF release, stimulated by another secretagogue (10 microM veratridine), was not affected by the presence or absence of 1 microM GHRF, suggesting the lack of toxic impairment of hypothalamic cell function by GHRF at this concentration. In conclusion, a biphasic stimulatory pattern of SRIF secretion in response to GHRF was observed in experiments employing dispersed rat hypothalamic cells. The biphasic SRIF response pattern to GHRF may be relevant in the physiological regulation of GH secretion.