Release of somatostatin-like immunoreactivity from incubated rat hypothalamus and cerebral cortex. Effects of glucose and glucoregulatory hormones

Role of Somatostatin in the Regulation of Central and Peripheral Factors of Satiety and Obesity

Obesity is one of the major social and health problems globally and often associated with various other pathological conditions. In addition to unregulated eating behaviour, circulating peptide-mediated hormonal secretion and signaling pathways play a critical role in food intake induced obesity. Amongst the many peptides involved in the regulation of food-seeking behaviour, somatostatin (SST) is the one which plays a determinant role in the complex process of appetite. SST is involved in the regulation of release and secretion of other peptides, neuronal integrity, and hormonal regulation. Based on past and recent studies, SST might serve as a bridge between central and peripheral tissues with a significant impact on obesity-associated with food intake behaviour and energy expenditure. Here, we present a comprehensive review describing the role of SST in the modulation of multiple central and peripheral signaling molecules. In addition, we highlight recent progress and contribution of SST and its receptors in food-seeking behaviour, obesity (orexigenic), and satiety (anorexigenic) associated pathways and mechanism.

Somatostatin system: molecular mechanisms regulating anterior pituitary hormones

The somatostatin (SRIF) system, which includes the SRIF ligand and receptors, regulates anterior pituitary gland function, mainly inhibiting hormone secretion and to some extent pituitary tumor cell growth. SRIF-14 via its cognate G-protein-coupled receptors (subtypes 1-5) activates multiple cellular signaling pathways including adenylate cyclase/cAMP, MAPK, ion channel-dependent pathways, and others. In addition, recent data have suggested SRIF-independent constitutive SRIF receptor activity responsible for GH and ACTH inhibition in vitro. This review summarizes current knowledge on ligand-dependent and independent SRIF receptor molecular and functional effects on hormone-secreting cells in the anterior pituitary gland.

Distinct modulation of microglial amyloid β phagocytosis and migration by neuropeptides (i)

Microglial activation plays an integral role in the development and course of neurodegeneration. Although neuropeptides such as bradykinin (BK), somatostatin (SST), and endothelin (ET) are known to be important mediators of inflammation in the periphery, evidence of a similar function in brain is scarce. Using immunocytochemistry, we demonstrate the expression of receptors for BK (B1, B2 subtypes), ET (ETA, ETB subtypes) and SST (SST 2, 3, 4 subtypes) in primary microglia and microglial cell lines. Exposure of BV2 and N9, as well as primary microglial cells to BK or SST increased Aβ uptake in a concentration-dependent manner, whereas endothelin decreased Aβ uptake. This was caused by increased phagocytosis of Aβ since the rate of intracellular Aβ degradation remained unaffected. All neuropeptides increased chemotactic activity of microglia. In addition, BK reduced Aβ-induced expression of proinflammatory genes including iNOS and COX-2. ET decreased the Aβ-induced expression of monocyte chemoattractant protein 1 and interleukin-6. These results suggest that neuropeptides play an important role in chemotaxis and Aβ clearance and modulate the brain's response to neuroinflammatory processes.

Hypoglycaemia-Insulin Test: Discordant Growth Hormone and Cortisol Response in Paediatric Patients Regarding Recovery from Hypoglycaemia with or without Oral Glucose Solution

BACKGROUND

Hypoglycaemia-insulin test (HIT) is the 'gold standard' for the diagnosis of adrenal-pituitary-hypothalamic axis disorders. Controversy exists on the convenience of recovery from an insulin-induced hypoglycaemia since this test is not risk-free.

OBJECTIVE

To ascertain whether recovery from insulin-induced hypoglycaemia with an oral glucose solution produces a different response of growth hormone (GH) and cortisol at different times of the study compared with spontaneous recovery from hypoglycaemia.

PATIENTS AND METHODS

Prospective study of 100 children and adolescents with growth delay who underwent an HIT. Patients were consecutively assigned to two groups of 50. In one group recovery from hypoglycaemia occurred spontaneously and in the other recovery was achieved with an oral glucose solution (20 g of glucose) when glycaemia was under 30 mg/dl. The two groups did not differ in age, sex, pubertal status, weight, height and IGF-I levels.

RESULTS

The response of GH at 30, 60, 90 and 120 min and cortisol at 10, 60, 90 and 120 min was lower and statistically significant in patients with recovery from hypoglycaemia with oral glucose solution. GH deficiency was diagnosed more frequently in patients recovered with glucose solutions (94%) compared to those with spontaneous recovery (68%).

CONCLUSIONS

Oral glucose solution administration when glycaemia was under 30 mg/dl in HIT produced a lower GH and cortisol response to insulin stimulus and a greater frequency of GH deficit diagnosis.

Somatostatin and somatostatin receptor physiology

Since the discovery of somatostatin (SST) over three decades ago, its ubiquitous distribution and manifold functions are still being documented. SST is synthesized in the hypothalamus and transported to the anterior pituitary gland where it tonicaly inhibits GH and TSH secretion as well as being responsible for GH pulsatile release. Several internal feedback loops, sleep, exercise, and chemical agents control and influence SST release. SST also impacts the function of a wide variety of cells and organ systems throughout the body. Knowledge of the structures of the SSTs has resulted in recognition of the essential four core conserved residues responsible for their actions. The SSTs act through six separate SST cell surface receptors (SSTRs), members of the family of G protein-coupled receptors. Receptor ligand binding (SST/SSTR) results in cellular activities specific for each receptor, or receptor combinations, and their tissue/cell localization. Understanding the structure/function relationship of the SSTs and their receptors, including the internalization of SST/SSTR complexes, has facilitated the development of a variety of novel pharmacologic agents for the diagnosis and treatment of neuroendocrine tumors and unfolding new applications.

The effect of growth hormone secretagogues and neuropeptide Y on hypothalamic hormone release from acute rat hypothalamic explants

Growth hormone (GH) secretagogues (GH-releasing peptides and their non-peptide analogues) stimulate growth hormone release via specific G-protein coupled receptors both directly from the pituitary gland and through stimulation of the hypothalamus. The exact mechanism of action in the hypothalamus is not known. The presence of endogenous GH releasing hormone (GHRH) seems to be necessary for the in-vivo actions of growth hormone secretagogues (GHSs), but data suggest that further factors must be involved as well. The effect of GHSs is not entirely specific for the GH axis; they release prolactin and stimulate the hypothalamo-pituitary-adrenal axis causing elevations in circulating ACTH and cortisol levels in both animal and human studies. Recently, it has also been suggested that GHSs stimulate hypothalamic neuropeptide Y (NPY) neurones. In the present study, we have therefore investigated the direct effect of several GHSs (GHRP-6, hexarelin and the non-peptide analogues L-692, 429 and L-692, 585) on GHRH, somatostatin (SS), corticotrophin-releasing hormone (CRH) and arginine vasopressin (AVP) release in vitro in an acute rat hypothalamic incubation system. We also assessed the effect of NPY on GHRH, SS and AVP release. Freshly removed hypothalami were incubated in control media for 20 min and then in 1-4 consecutive 20-min periods in each of the test substances at different concentrations. There was no significant change in either the basal or potassium-stimulated release of GHRH or SS at low concentrations of any of the secretagogues; however, at millimolar doses a paradoxical inhibition of GHRH was observed with GHRP-6, hexarelin and L-692 585 (data are expressed as the ratio of treated to preceding basal release; at 20 min control group: 0.97+/-0.02, GHRP-6: 0.55+/-0.04, P<0.001 compared to control group; hexarelin: 0. 56+/-0.06, P<0.001, L-692,585: 0.70+/-0.03, P<0.001), while SS was stimulated after 60 or 80 min (at 80 min control: 0.80+/-0.03, hexarelin: 1.23+/-0.07, P<0.05 and L-692,585: 1.37+/-0.11, P<0.05). GHSs stimulated hypothalamic AVP release (at 20 min control: 0. 99+/-0.06 ratio to basal release, 10-4 M concentration of GHRP-6: 6. 31+/-1, P<0.001, hexarelin: 1.88+/-0.4, P<0.01, L-692,429: 1.90+/-0. 5, P<0.05 and L-692,585: 2.34+/-0.96, P<0.01), while no stimulatory effect was found on CRH release. NPY significantly stimulated SS and inhibited basal and potassium-stimulated GHRH release, while potentiating potassium-evoked AVP secretion. The Y1 receptor antagonist BIBP 3226 did not inhibit the effects of NPY on SS, GHRH or AVP release. We therefore conclude that, in this in-vitro rat hypothalamic incubation model, growth hormone secretagogues stimulate the release of AVP but have no effect on either GHRH, SS or CRH at low doses; at high doses paradoxically they inhibit the hypothalamic GH axis similar to in-vivo data in the rat. We speculate that these effects might be mediated by NPY.

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.

Glutamate pathways mediate somatostatin responses to glucose in normal and diabetic rat hypothalamus

We investigated the role of hypothalamic glutamate receptors in mediating the stimulatory effect of low glucose (< 5 mM) on somatostatin release. We also studied whether alteration in glutamate release might contribute to the reduced hypothalamic somatostatin response to low glucose observed in diabetic (Goto-Kakizaki) rat hypothalami. Hypothalamic somatostatin release in response to incubation with 1 mM D-glucose was inhibited by the ionotropic glutamate receptor antagonists MK801, D-AP5 and DNQX but not by the metabotropic antagonists L-AP3 or MCPG. The release of somatostatin was increased by the ionotropic agonists NMDA, AMPA and kainate but not by metabotropic agonists t-ACPD or L-AP4. Basal and peak glutamate release in response to incubation with 1 mM glucose, were significantly lower from GK hypothalami There were no significant differences in the basal or stimulated release of serine and GABA. These data indicate that ionotropic NMDA/AMPA/kainate receptors and not metabotropic receptors mediate the effects of glucose on rat hypothalamic somatostatin release. Reduced hypothalamic somatostatin release in response to low glucose in diabetic (Goto-Kakizaki) rats may well be secondary, at least in part, to reduced glutamate release.

The regulation of growth hormone secretion

The regulation of GH secretion involves finely balanced systems with multiple components. As our knowledge of the physiology of GH regulation expands, so does our understanding of the bases for GH diseases. We now can identify several cellular loci that cause GH deficiency or GH excess. In addition, the recent increased understanding of GH physiology has resulted in an increase in potential therapies for growth disorders.

Effects of hyper- and hypoglycemia on blood growth hormone level in free-feeding rats with anterolateral deafferentation of the medial basal hypothalamus

In rats with anterolateral deafferentation of the medial basal hypothalamus, the growth hormone (GH) level in the blood showed irregular and small fluctuations instead of the usual high bursts and low trough level, and the baseline GH level was higher than that in sham-operated rats. Continuous infusion of a glucose solution to operated rats increased the baseline level, GH pulse and pulse amplitude. I.v. bolus injection of the glucose solution resulted in a significant but transient increase in GH level. Insulin-induced hypoglycemia decreased the blood GH level in operated rats more effectively than in sham-operated ones and that was prevented by simultaneous infusion of glucose. Since SS influence on GH secretion had been largely eliminated in rats with anterolateral deafferentation of the medial basal hypothalamus, it is highly unlikely that the effects of hyperglycemia or hypoglycemia on GH secretion were the consequence of altered SS secretion.

Central glucoprivation evoked by administration of 2-deoxy-D-glucose induces expression of the c-fos gene in a subpopulation of neuropeptide Y neurons in the rat hypothalamus

Central glucoprivation evoked by the intracerebroventricular administration of 2-deoxy-D-glucose (2DG) induces eating and suppresses growth hormone (GH) secretion in rats. To elucidate the hypothalamic mechanism of these phenomena, the induction of c-fos gene expression was examined by in situ hybridization using rats with centrally administered 2DG. Autoradiography on X-ray film showed that c-fos gene expression was transiently induced in discrete hypothalamic regions; namely the paraventricular nucleus, arcuate nucleus (ARC), the surrounding regions of the third ventricle dorsal to the ARC, and the periventricular nucleus (PeV). The time course of the expression was different in these nuclei. Double-label in situ hybridization for c-fos mRNA and neuropeptide Y (NPY) or somatostatin mRNAs revealed that 20% of the NPY neurons in the ARC expressed the c-fos gene, while a small population of somatostatin neurons (6.1% in the ARC and 2.6% in the PeV) expressed the c-fos gene following 2DG administration. Since NPY is an orexigenic neuropeptide and has an inhibitory effect on GH secretion, the data suggest that the activation of a subpopulation of NPY neurons in the ARC contributes, in part, to the increased food intake and suppression of GH secretion after central glucoprivation evoked by 2DG.

Central glucopenia induced by 2-deoxy-D-glucose stimulates somatostatin secretion in the rat

The mechanisms involved in 2-deoxy-D-glucose (2-DG)-induced growth hormone (GH) suppression in the rat were examined. Conscious male rats were given 2-DG by intracerebroventricular (icv) injection and the pulsatile GH secretion was monitored for 6 h. The single icv injection of 2-DG (8 mg/rat) eliminated pulsatile GH secretion in conscious rats. Pretreatment with somatostatin (SS) antiserum completely restored the suppressed GH secretion in the 2-DG treated rats. Hypothalamic GH-releasing hormone (GRH) and SS mRNA levels were not altered by single and multiple icv injections of 2-DG. These findings suggest that 2-DG-induced GH suppression is primarily due to hypersecretion of SS without a significant change at the transcription level in the rat.

Resistance of growth hormone secretion to hypoglycemia in the mouse

Although previous studies have demonstrated that acute hypoglycemia inhibits growth hormone (GH) secretion due to stimulation of hypothalamic somatostatin (SS) neurones in the rat, the effect of hypoglycemia on GH secretion has not yet been elucidated in the mouse. In this study, the effects of insulin-induced hypoglycemia on mouse GH secretion, hypothalamic c-fos expression, GH-releasing hormone (GRH) and SS mRNA levels were investigated in conscious male mice. Seven days after implantation of chronic atrial catheters, blood samples were taken every 20 min from 1200-1600 h under unrestrained conditions. Insulin was administered iv every 20 min from 1200-1240 h to induce moderate hypoglycemia (MH) and severe hypoglycemia (SH), respectively. Expression of hypothalamic c-fos protein was examined 30 min and 60 min after induction of hypoglycemia by immunohistochemistry. Hypothalamic GRH and SS mRNA levels were examined 1 h and 3 h after induction of hypoglycemia by Northern blot analysis. The lowest mean plasma glucose levels after insulin injections were 49.1 +/- 4.1 mg/dl and 34.2 +/- 5.6 mg/dl in conscious mice, respectively. However, pulsatile GH secretion was not significantly altered in either group. Although both MH and SH markedly stimulated c-fos expression in specific hypothalamic nuclei including the paraventricular nucleus, they did not induce c-fos protein in the periventricular nucleus. Neither MH nor SH altered hypothalamic GRH or SS mRNA levels. These results suggest that hypoglycemia does not activate SS neurons which inhibit GH secretion in the mouse.

Influence of metabolic substrates and obesity on growth hormone secretion

In addition to stimulating body growth, GH plays an important role in metabolism. In turn, various products of intermediary metabolism, such as glucose, free fatty acids, dietary proteins, and amino acids feed back on both the hypothalamus and the anterior pituitary to control the function of the somatotroph cell. Alterations in nutritional status, such as malnutrition and obesity, markedly influence GH secretion and/or GH actions at tissue level. Therefore, the interaction between metabolic substrates and GH secretion can be viewed as part of the overall regulation of feeding and fasting in order to maintain an adequate body weight and body composition.

The effect of hypocaloric diet with and without D-fenfluramine treatment on growth hormone release after growth hormone-releasing factor stimulation in patients with android obesity

Basal and stimulated growth hormone (GH) secretions are impaired in obesity, and partial restoration of the GH response to various stimuli is observed after weight loss. The aim of the present study was to investigate whether D-fenfluramine, a serotoninergic agent, would increase the GH response to growth hormone-releasing factor (GRF) as compared with placebo in obese android patients. The subjects were 17 patients with android obesity (four men and 13 women) aged 21 to 58 years with a body mass index (BMI) ranging from 32.0 to 52.2 kg/m2 and an abdominal-gluteal ratio greater than 1.0. The following four GRF (1-44) tests were performed: T-30 (control), T0 (after 30 days of a hypocaloric diet), T1 (after 30 days of either placebo or D-fenfluramine 15 mg twice daily), and T2 (after 30 additional days of placebo or D-fenfluramine). The hypocaloric diet was maintained during the T1 and T2 periods. At each test, the serum GH response to GRF was measured at frequent intervals, and the peak GH response and the GH area under the curve were calculated. Serum insulin concentrations were also assayed before GRF stimulation, and the insulin to GH ratio was obtained. The D-fenfluramine-treated group had a mean +/- SEM GH peak level after GRF significantly higher at T1 (43.3 +/- 8.2 micrograms/L) and T2 (50.9 +/- 9.2 micrograms/L) compared with the placebo group. Likewise, the mean integrated areas of GH response were significantly higher for the D-fenfluramine-treated group as compared with the placebo group at both T1 and T2 of therapy.(ABSTRACT TRUNCATED AT 250 WORDS)

Growth hormone response to GRF and insulin-induced hypoglycemia in Yorkshire and Meishan pigs

Plasma glucose and porcine growth hormone (GH) responses to insulin and/or growth hormone-releasing factor (GRF) challenges were determined in Yorkshire (Y) and Meishan (MS) castrated male pigs at 140 days of age. The decline in glucose levels observed during the first 30 min after insulin injection was similar between the two breeds. In Y pigs, GH concentrations increased sharply after insulin injection. The GH response to insulin was much smaller in MS than in Y pigs (101 +/- 82 vs. 1,077 +/- 124 ng.min.ml-1, P < 0.001 after 0.30 U/kg insulin). The GH response to GRF (10 micrograms/kg) did not differ significantly between the two breeds. The GH response was larger after combined insulin (0.20 U/kg) plus GRF than after GRF alone in Y pigs (1,512 +/- 229 vs. 463 +/- 401 ng.min.ml-1, P < 0.05), whereas it did not differ in MS pigs (714 +/- 164 vs. 507 +/- 127 ng.min.ml-1, P > 0.10). The present data demonstrate that the GH response to insulin is dramatically altered in MS pigs. A possible explanation to this result is that somatostatin tone would not be depressed by insulin-induced hypoglycemia in MS pigs.

Somatostatin and growth hormone-releasing factor release from zucker rat hypothalamic tissue

Plasma somatotropin (ST) levels are depressed in the genetically obese Zucker rat compared to those of their littermates. It is believed that this defect is associated with one or both of the hypothalamic neuropeptides that control ST release: growth hormone releasing factor (GRF) and somatostatin (SS). The mechanism by which SS and GRF neuropeptides are regulated remains uncertain. The objective of this study was to examine the effect of 2 deoxy-glucose (2DG), isoproterenol (ISO), tryptophan (TRP), and 5HT on SS and GRF release in hypothalamic tissue from lean and obese Zucker rats. An in vitro perifusion system was established to examine the release of SS and GRF from perifused hypothalami taken from 8- and 12-week-old Zucker rats under basal conditions and in response to 2DG, ISO, TRP, 5HT, and KCl administration. Hypothalami were perifused with Dulbecco's modified eagle's medium continuously at 37 degrees C for 5 h at a flow rate of 100 ml/min. ISO and 2DG significantly (p < 0.05) increased SS levels from the obese rat, but no effect was observed from the lean littermate. GRF was not affected by 2DG or ISO in either genotypes. TRP and 5HT failed to affect SS or GRF release in lean or obese Zucker rats. It is proposed that the obese Zucker rat is more sensitive to glucose deprivation and to beta-adrenergic stimulation of SS release than the lean littermate.

Arginine abolishes the inhibitory effect of glucose on the growth hormone response to growth hormone-releasing hormone in man

Acute hyperglycemia inhibits the growth hormone (GH) response to several stimuli including growth hormone-releasing hormone (GHRH), likely acting by stimulation of endogenous somatostatin release. The aim of our study was to verify whether arginine ([Arg] 30 g intravenously [IV] in 30 minutes), a well-known GH secretagogue likely acting via inhibition of hypothalamic somatostatin release, counteracts the inhibitory effect of oral glucose (OG) administration (100 mg orally) on the GH response to GHRH (1 micrograms/kg IV bolus) in seven normal subjects (aged 20 to 30 years). The GH response to GHRH (peak, 11.6 +/- 1.8 micrograms/L) was inhibited by previous OG load (peak, 7.4 +/- 0.8 micrograms/L; P less than .02 v GHRH alone) and potentiated by Arg coadministration (peak, 36.2 +/- 8.8 micrograms/L; P less than .03 v GHRH alone). The potentiating effect of Arg on the GHRH-induced GH increase was unaffected by previous OG load (peak, 30.4 +/- 6.9 micrograms/L). In conclusion, our results show that Arg abolishes the inhibitory effect of OG administration on the GHRH-induced GH response in man. These data, although indirect, suggest that both acute hyperglycemia and Arg act at the hypothalamic level, stimulating and inhibiting, respectively, the release of somatostatin.

The neurophysiological regulation of growth hormone secretion

With the advent of genetic engineering, the importance of GH in the regulation of growth and metabolism in domestic species has been clearly demonstrated. Ample evidence of an integral role for GH in the processes of growth and lactation exists in dairy cattle (1,2), sheep (3), beef cattle (4) and swine (5). For example, circulating GH levels are high during the period of rapid growth in several species including cattle (6), swine (7) and poultry (8). Endogenous GH secretion is primarily controlled by the central nervous system (CNS) via two specific hypothalamic neurohormones, growth hormone-releasing factor (GRF) and somatostatin (SRIF), an inhibitor of GH release. The secretion of GRF and SRIF is governed by a host of neuropeptides and neurotransmitters which provide a functional link between higher CNS centers and hypophysiotropic neurons. This review will focus on the CNS regulation of GH secretion and circulating factors which feedback to either stimulate or inhibit its release.

Interaction of glucose and pyridostigmine on the secretion of growth hormone (GH) induced by GH-releasing hormone (GHRH)

In order to investigate the mechanisms by which hyperglycaemia induces an inhibition of GHRH-induced GH release, we gave the following treatments to seven normal men: a) GHRH 100 micrograms iv; b) pyridostigmine (PD) 120 mg po 60 min before GHRH; c) glucose 250 mg/kg iv as a bolus (10 min before GHRH) plus 10 mg/kg/min until the end of the test; d) glucose pyridostigmine and GHRH as above. Glucose significantly reduced GHRH-stimulated GH levels, whereas PD significantly enhanced them. When PD and glucose were given together, the effect on GHRH-stimulated GH secretion was not different from the algebraic sum of the single effects of the two substances. Thus glucose seems to be able to exert its inhibition, at least partially, also when pyridostigmine is coadministered.

Measurement of growth hormone-releasing hormone and somatostatin in hypothalamic-portal plasma of unanesthetized sheep. Spontaneous secretion and response to insulin-induced hypoglycemia

To elucidate the role of growth hormone (GH)-releasing hormone (GRH) and somatostatin (SRIH) in the regulation of the growth hormone (GH) secretory pattern, we collected portal blood from five unanesthetized ovariectomized ewes for repeated measurements of GRH and SRIH simultaneous with those of peripheral GH. Hormones were measured at 10-min intervals for 5.5 h and their interrelationships analyzed. Mean portal GRH was 20.4 +/- 6.7 (SD) pg/ml and the estimated overall secretion rate was 13 pg/min. GRH secretion was pulsatile with peaks of 25-40 pg/ml and a mean pulse interval of 71 min. Mean portal SRIH was 72 +/- 33 pg/ml and the estimated overall secretion rate was 32 pg/min. SRIH secretion was also pulsatile with peaks of 65-160 pg/ml and a mean pulse interval of 54 min. The GH pulse interval was 62 min. A significant association was present between GRH and GH secretory peaks though not between GRH and SRIH or SRIH and GH. Insulin hypoglycemia resulted in a rapid and brief stimulation of SRIH secretion followed by a decline in GH levels. No effect was observed on GRH secretion until 90 min, when a slight increase occurred. The results suggest (a) the presence of an independent neural rhythmicity of GRH and SRIH secretion with a primary role of GRH in determining pulsatile GRH secretion, and (b) that the inhibitory effects of insulin hypoglycemia on GH in this species are attributable to a combination of enhanced SRIH secretion and possibly other factors, though without significant inhibition of GRH.

Effect of oral glucose on the late growth hormone rise and growth hormone responses to GHRH in normal subjects

A late rise in serum GH occurs 3-5 h following oral glucose in man. In order to investigate the mechanisms through which this occurs we have studied the late GH rise after oral glucose during administration of a supramaximal dose of GHRH. In eight normal subjects, oral glucose (100 g) greatly enhanced the GH responses to a supramaximal dose of GHRH (50 micrograms bolus, followed immediately by 100 micrograms/h infusion for 3 h) given 3.5 h after the glucose. GH peak (mean +/- SEM) elicited by GHRH (bolus + infusion) rose from 55.2 +/- 20.4 to 133.4 +/- 29.6 mU/l (P less than 0.02) after glucose pretreatment. In conclusion, it is likely that the late rise in GH secretion induced by oral glucose occurs via a non-GHRH-dependent mechanism. These data are consistent with the hypothesis that the delayed GH response to glucose is a consequence of reduced release of somatostatin from the hypothalamus.

Failure of glucose infusion to suppress the exaggerated GH response to GHRH in patients with anorexia nervosa

The growth hormone (GH) response to GH-releasing hormone (GHRH) is characteristically exaggerated in anorexia nervosa (AN). Hyperglycemia suppresses the GH response to GHRH in normal subjects. To test whether this inhibitory action of hyperglycemia is preserved in AN, we performed a GHRH (GHRH 1-40, 1 micrograms/kg) test under basal conditions (saline infusion) and during steady-state hyperglycemia (200 mg/dl, induced by the intravenous administration of 8 mg/min.kg of glucose) in 6 adolescent girls with acute-stage AN (as diagnosed by psychopathological, hormonal, and nutritional criteria) and in 5 age-matched female controls. In control subjects, GHRH stimulated GH release during saline, but not glucose, infusion. In the anorectic patients, the GH response to GHRH was exaggerated during both saline infusion (2.97 +/- 0.79 versus 0.52 +/- 0.22 micrograms.120 min.ml-1, p less than 0.02) and under hyperglycemic conditions (4.61 +/- 0.56 versus 0.33 +/- 0.10, p less than 0.001). We conclude that the inhibitory action of hyperglycemia on GHRH-induced GH release is lost in the acute phase of AN.

Effects of Glucose on Thyrotrophin-Releasing Hormone, Growth Hormone-Releasing Hormone, Somatostatin and Luteinizing Hormone-Releasing Hormone Release from Rat Hypothalamus in vitro

Abstract Increasing concentrations of D-glucose (1 to 25 mM) inhibited somatostatin, thyrotrophin-releasing hormone (TRH) and growth hormone-releasing hormone (GHRH) release from incubated adult rat hypothalami in a stereospecific manner. In contrast, the effects of D- and L-glucose on luteinizing hormone-releasing hormone release were virtually identical. Increasing concentrations of D-glucose also inhibited somatostatin release following depolarization with high K(+), but had no obvious effect on depolarization-induced TRH or GHRH release when compared with L-glucose. In conclusion, D-glucose exerts a potent, dose-related modulatory action on the release of rat hypothalamic TRH and GHRH as well as somatostatin in vitro. Further studies are required to establish any physiological relevance of glucose in the modulation of these hypothalamic neuropeptides.

Effects of insulin-induced hypoglycemia on somatostatin level and binding in rat cerebral cortex and hippocampus

The effects of severe insulin-induced hypoglycemia on somatostatin level and specific binding in the cerebral cortex and hippocampus were examined using 125I-Tyr11-somatostatin as a ligand. Severe insulin-induced hypoglycemia did not affect the level of somatostatin-like immunoreactivity in the brain areas studied. However, the number (but not the affinity) of specific somatostatin receptors was significantly decreased in membrane preparation from the hippocampus but not in the cerebral cortex at the onset of hypoglycemic coma (5-10 min). Administration of glucose at the onset of hypoglycemic coma brought about extensive recovery of hippocampal somatostatin receptor number. These results suggest that glucose modulates the somatostatin receptor in the rat hippocampus. The physiological significance of these findings remains to be clarified.

Somatostatin concentration and binding in the rat hypothalamus and striatum during severe insulin-induced hypoglycemia

Hypoglycemia was induced by administration of insulin (40 I.U./kg) to 24 h fasted rats. Somatostatin-like immunoreactivity (SLI) and 125I-Tyr11-somatostatin binding were measured in the striatum and hypothalamus at the onset of hypoglycemic coma (5-10 min). No significant changes in SLI concentration were detected in either site although the total number of specific somatostatin receptors in the striatum membranes, but not in the hypothalamus, decreased in insulin-injected rats when compared with the control group.

Effects of ingestion of glucose on GH and TSH secretion: evidence for stimulation of somatostatin release from the hypothalamus by acute hyperglycemia in normal man and its impairment in acromegalic patients

Ingestion of glucose is known to induce suppression of GH secretion in normal subjects and this phenomenon is often absent in acromegalic patients. To clarify the mechanism of GH suppression in acute hyperglycemia in normal subjects and disturbed GH response in acromegalic patients, the effects of acute hyperglycemia on plasma GH and TSH levels were examined in normal subjects and acromegalic patients. Plasma GH levels were significantly lowered 45-60 min after ingestion of 75 g glucose and elevated at 210 and 240 min in nine normal subjects. Plasma TSH levels were also significantly lowered between 45 and 120 min after ingestion; levels then gradually rose. Subcutaneous administration of 50 micrograms SMS 201-995, a long acting somatostatin analog, lowered plasma TSH levels in both normal subjects and acromegalic patients, and there was no significant difference in the degree of decrease in plasma TSH levels between the normal subjects and patients. These results, taken together with several reports that somatostatin suppresses TSH secretion as well as GH secretion, suggest that acute hyperglycemia stimulates somatostatin release from the hypothalamus, thus causing inhibition of GH and TSH secretion. However, in ten acromegalic patients, only two showed suppression of plasma GH levels to below 50% of basal level and the degree of suppression of TSH secretion was significantly less than in normal subjects in the glucose tolerance test. It is, therefore, suggested that somatostatin release in response to acute hyperglycemia is impaired in most acromegalic patients and that this abnormality may be one of causes for the absence of the normal GH response to acute hyperglycemia in this disorder.

Further studies on the effect of glucose concentrations and other oxidizable substrates upon ionic gradients and in vitro somatostatin release from rat mediobasal hypothalamus

During in vitro incubation of rat mediobasal hypothalamus (MBH), potassium and sodium gradients were high in the presence of glucose, pyruvate, lactate or the mixture glucose and pyruvate; in the absence of substrate, the ionic gradients were markedly lowered and corresponding somatostatin release from MBH was maximal. The specific effect of glucose on somatostatin release from MBH was tested under normal tissue polarization, i.e. in the presence of pyruvate. Under these more physiological conditions, somatostatin release was submaximal and inversely related to glucose concentrations (within the range 0-7 mM).

Somatostatin release from dispersed hypothalamic cells: effects of diabetes

We examined the release of growth hormone-release inhibiting factor (somatostatin) from dispersed hypothalamic cells obtained from mature diabetic rodents and normal age-matched controls, in an attempt to demonstrate a possible hypothalamic defect which might underlie some of the reported abnormalities in somatotrophic function in diabetes mellitus. Insulinopoenic diabetes was induced by either streptozotocin or alloxan. Somatostatin release from cells from diabetic rats was diminished both basally and after stimulation by membrane depolarisation. Stimulated release was calcium dependent in cells from both normal and diabetic animals. The defect was present in both streptozotocin and alloxan induced diabetes. We also compared hypothalamic somatostatin release from cells obtained from obese hyperinsulinaemic C57 BL/Ks db/db diabetic mice and non diabetic lean litter mates (db/-). Despite longstanding marked hyperglycaemia, no significant alteration in somatostatin release was apparent. Likewise, starvation of rats for 5 days did not result in significant diminution of somatostatin release. These observations document a defect in hypothalamic somatostatin release in experimentally induced insulinopoenic diabetes, which is not apparent in the db/db mouse, suggesting that glucose per se is not responsible. Rather than the anticipated increase in hypothalamic somatostatin release in insulinopoenic diabetes, a reduction in release was observed. These observations are compatible with the hypothesis that increased hypothalamic somatostatin release is not responsible for abnormal growth hormone secretion in this model.

Additive effects of growth hormone releasing factor and insulin hypoglycaemia on growth hormone release in man

We have measured GH and PRL changes following separate and combined administration of insulin and GH releasing factor (GRF) in six normal males. Peak GH responses to separate administration of insulin and GRF were comparable (71.4 +/- 10.2 vs 70.1 +/- 27.7 mU/l; mean +/- SEM). However, the peak GH response following combined administration was significantly higher (120.8 +/- 29.7, P less than 0.05) as was the total GH released as calculated by measuring the area under the curve (P less than 0.05). In contrast the PRL response to hypoglycaemia was not altered by the combined administration of insulin and GRF. This effect was not due to any direct action of hypoglycaemia or insulin at pituitary level since basal and 10(-8) M GRF stimulated GH release from rat anterior pituitary cells in vitro was not influenced by varying glucose and insulin levels. Our findings support the hypothesis that GRF and insulin-induced hypoglycaemia release GH via different pathways which are, at least in part, additive.

Glucose modulates the release of histamine from the mouse hypothalamus in vitro

The effect of glucose concentration on the in vitro release of histamine (HA) was examined, using two different preparations of the mouse hypothalamus. The HA and tele-methylhistamine released from whole blocks of the hypothalamus into the medium linearly increased during 2-h incubation in normal Krebs-Ringer bicarbonate solution in the absence of external depolarizing stimuli. The release of HA from this preparation depended on the temperature and Ca2+ in the medium and was progressively increased with decrease in the glucose concentration from 11.5 to 1 mM. The rate of the HA release was dependent on the absolute concentration of glucose and not on an abrupt change in the concentration. When slices of the hypothalamus were incubated in high K+ medium, a temperature- and Ca2+-dependent HA release was observed. At low concentrations of glucose, the K+ (20 mM)-induced HA release from the hypothalamic slices was also enhanced. Tetrodotoxin (10 microM) inhibited the enhancing effect of a low glucose concentration (2 mM) on the HA release by 60%, in both preparations of the hypothalamus. The possibility that the release of HA from the mouse hypothalamus is regulated by glucose concentration and that activation of neuronal Na+ channels is involved in the enhancement of the HA release by low glucose concentrations warrants further attention.

Growth hormone releasing hormone

Human growth hormone releasing hormone (GHRH) was originally extracted from two pancreatic tumours in patients with acromegaly, and is now known to consist of a 44 residue amidated peptide or its C-terminal-shortened derivatives. The sequence of rat GHRH has also been determined; this 43 residue peptide shows approximately 70% homology with human GHRH, and is located mainly in the arcuate nucleus of the hypothalamus. Pulsatile GH release in the rat is principally a consequence of the pulsatile release of hypothalamic GHRH, although this appears to be associated with a transient suppression of somatostatin release. Exogenous GHRH specifically increases circulating GH in many species, and in the long term may increase growth. In normal man, several analogues of GHRH have been shown to be safe, sensitive and specific stimuli to GH release; although there may be a variable prolactin response, this is usually of small magnitude. Continuous infusion of GHRH leads to a decrement in responsiveness, due at least in part to changes in hypothalamic somatostatin. The GH response to GHRH is also modulated by obesity, blood sugar, free fatty acids, and GH itself. Many children with 'GH deficiency' (idiopathic, radiation-induced, or secondary to hypothalamopituitary tumours) respond to intravenous GHRH with an acute rise in serum GH. Early studies also indicate that long-term therapy with subcutaneous GHRH may increase growth velocity in some of these children. It is concluded that analogues of GHRH are useful in the investigation of the hypothalamopituitary axis, and may be important in the therapy of short stature.

Growth hormone neurosecretory dysfunction

The basis for understanding clinical disorders in the neuroregulation of GH secretion is derived from the complexity of the CNS-hypothalamic-pituitary axis. Studies in animals and humans demonstrate an anatomic, physiological and pharmacological evidence for neurosecretory control over GH secretion including neurohormones (GRH, somatostatin), neurotransmitters (dopaminergic, adrenergic, cholinergic, serotonergic, histaminergic, GABAergic), and neuropeptides (gut hormones, opioids, CRH, TRH, etc). The observation of a defect in the neuroregulatory control of GH secretion in CNS-irradiated humans and animals led to the hypothesis of a disorder in neurosecretion, GHND, as a cause for short stature. We speculate that in this heterogeneous group of children a disruption in the neurotransmitter-neurohormonal functional pathway could modify secretion ultimately expressed as poor growth velocity and short stature.

Somatostatin release from dissociated cerebral cortical cell cultures

Primary monolayer cultures of dispersed fetal cerebral cortical cells can be used to measure the release of the neuropeptide, somatostatin. Three to five percent of cellular IRS is released basally into KRB in 10 min. Basal release is stable for at least 60 min and stimulated levels of release can be induced by introducing ionophores, neurotransmitters, or peptides. The peptide content of the incubation samples is readily measured by a well-characterized, sensitive RIA. Table II summarizes the major factors that must be taken into consideration when developing this system for measuring peptide release.

The effects of neurotensin, vasoactive intestinal polypeptide and other neuropeptides on the secretion of somatostatin from cerebral cortical cells

We have examined the effects of several neuropeptides on the release of immunoreactive somatostatin from cerebral cortical cells in vitro. Neurotensin and vasoactive intestinal polypeptide induced large increases in somatostatin release, whereas cholecystokinin, gonadotropin releasing hormone and Met-enkephalin induced only modest increases. Thyrotropin releasing hormone and insulin had no effect. These results demonstrate a complex interaction amongst neuropeptides in the cerebral cortex, which must be considered in future studies of the roles of peptides in cortical function.

Somatostatin and the cerebral cortex

A unique subset of interneurons which are rich in immunoreactive somatostatin (IRS) exists in the cerebral cortex. The regulation of IRS secretion by these cells is reviewed. Acetylcholine, glutamic acid and several neuropeptides including VIP, CCK, and metenkephalin have been identified as IRS secretagogues. The types of molecules which stimulate IRS release, the electrophysiologic effects of somatostatin, and the recognition of abnormal IRS levels in human CNS diseases were all used to formulate a working model of the role of the somatostatinergic cell in ongoing cerebral cortical function.

Impaired growth hormone responses to growth hormone-releasing factor in obesity. A pituitary defect reversed with weight reduction

To investigate whether the impaired growth hormone secretion associated with obesity is a result of a hypothalamic or a pituitary disorder and whether it is a cause or a consequence of obesity, we studied plasma growth hormone responses to growth hormone-releasing factor in morbidly obese patients before gastrointestinal surgical therapy, in formerly obese subjects who had lost considerable weight postoperatively, and in non-obese controls. Growth hormone secretion was also assessed in response to insulin-induced hypoglycemia (in seven patients preoperatively and four postoperatively). In patients studied preoperatively, growth hormone responses to growth hormone-releasing factor were markedly impaired (P less than 0.001 as compared with controls), whereas in patients studied postoperatively they were partially restored to normal (P less than 0.05 as compared with those studied preoperatively). Growth hormone responses to insulin-induced hypoglycemia were similarly diminished in obese patients studied before operation (P less than 0.02). The growth hormone response to growth hormone-releasing factor was inversely correlated with the percentage of ideal body weight (P less than 0.01) and directly correlated with the growth hormone response to insulin (P less than 0.01). The impaired responsiveness to growth hormone-releasing factor suggests that the diminished response to insulin hypoglycemia is mediated by an impaired pituitary response to endogenous growth hormone-releasing factor. The reversibility of the defect after weight reduction suggests that it is a consequence rather than a cause of obesity.

Glucose-induced changes in somatostatin-14 and somatostatin-28 released from rat hypothalamic fragments 'in vitro'

Previous data suggest that somatostatin is present and released from the hypothalamus in several molecular forms, basally and after K+ or electrical stimulation. In order to evaluate the proportions of somatostatin-14 (S14) and somatostatin-28 (S28) released during a stimulus which may be more closely related to the control of growth hormone secretion 'in vivo', we studied the molecular forms of somatostatin released from hypothalamic fragments ' in vitro', during incubations with different glucose concentrations (1.35 and 22mM), which we have previously shown to be inversely related to somatostatin release. Sephadex G-50 chromatography demonstrated that both forms are released in the same proportions (S14: 70%; S28: 30%) during incubation with different glucose concentrations; there is a parallel increase in both forms when low glucose is used. Although on a molar basis less S28 is released than S14, the higher potency, longer duration of action and higher affinity for pituitary receptors of S28 suggests that it may be of major physiological importance.

Influence of glucose on somatostatin synthesis and secretion in isolated cerebral cortical cells

Somatostatin-producing cerebral cortical cell cultures were grown in either high- (33 mM) or low-glucose (5 mM) medium and then exposed to short repetitive changes of high- or low-glucose Krebs-Ringer's bicarbonate buffer. Equivalent amounts of somatostatin were released in the high-to-high, the low-to-low, and the low-to-high paradigms. The high-to-low experiment produced a rapid rise in somatostatin release, followed by a decline. Cultures exposed to 2-deoxyglucose after high-glucose medium also released much greater amounts of immunoreactive somatostatin. Separate sets of cultures were grown in high- or low-glucose medium for up to 19 days. Cultures grown in high-glucose medium generally contained more somatostatin intracellularly than did those maintained in low glucose, although somatostatin in the medium was only different at day 19. These results identify extracellular glucose as an important determinant of cortical somatostatin production.

Somatostatin release from dispersed hypothalamic cells - effects of membrane depolarization, calcium and glucose deprivation

We have developed a new short term in vitro system to examine hypothalamic somatostatin (SRIF) release. Hypothalamic cells were obtained from normal rats after trypsin or collagenase aided dispersion and released immuno-reactive (IR) SRIF which eluted in 3 molecular weight (MW) forms on gel chromatography. The smallest MW form, which constituted the major peak, co-eluted with synthetic cyclic 1-14 SRIF on gel and reverse phase high pressure liquid chromatography (HPLC). After 24 h in culture in medium containing heat inactivated fetal calf serum, cell viability was demonstrated by two techniques, (1) vital staining with trypan blue, and (2) incorporation of 32Pi into phospholipids. SRIF release was also studied at this time which was optimal in terms of responsivity of the cells to depolarizing stimuli. SRIF release increased in a time dependent manner, over 3 h. Membrane depolarization, induced either by potassium chloride 56 mM or ouabain (the Na+, K+-ATPase inhibitor) 10(-6) M or greater, markedly stimulated SRIF release. Incubation at 4 degrees C, or in the presence of EDTA 0.05 M or verapamil, the calcium channel blocker, 50 microM abolished these stimulatory effects. Glucose deprivation was induced by the addition of 2-deoxy-D-glucose (2-DG) to the experimental medium. 2-DG, at concentrations of up to 200 mg%, had no significant effect on SRIF release during incubation periods of up to 1 h.