Cholinergic agonist and antagonist drugs modulate the growth hormone response to growth hormone-releasing hormone in the rat: evidence for mediation by somatostatin

Multiple signaling pathways convey central and peripheral signals to regulate pituitary function: Lessons from human and non-human primate models

The anterior pituitary gland is a key organ involved in the control of multiple physiological functions including growth, reproduction, metabolism and stress. These functions are controlled by five distinct hormone-producing pituitary cell types that produce growth hormone (somatotropes), prolactin (lactotropes), adrenocorticotropin (corticotropes), thyrotropin (thyrotropes) and follicle stimulating hormone/luteinizing hormone (gonadotropes). Classically, the synthesis and release of pituitary hormones was thought to be primarily regulated by central (neuroendocrine) signals. However, it is now becoming apparent that factors produced by pituitary hormone targets (endocrine and non-endocrine organs) can feedback directly to the pituitary to adjust pituitary hormone synthesis and release. Therefore, pituitary cells serve as sensors to integrate central and peripheral signals in order to fine-tune whole-body homeostasis, although it is clear that pituitary cell regulation is species-, age- and sex-dependent. The purpose of this review is to provide a comprehensive, general overview of our current knowledge of both central and peripheral regulators of pituitary cell function and associated intracellular mechanisms, focusing on human and non-human primates.

Neuroendocrine Regulation of Growth Hormone Secretion

This article reviews the main findings that emerged in the intervening years since the previous volume on hormonal control of growth in the section on the endocrine system of the Handbook of Physiology concerning the intra- and extrahypothalamic neuronal networks connecting growth hormone releasing hormone (GHRH) and somatostatin hypophysiotropic neurons and the integration between regulators of food intake/metabolism and GH release. Among these findings, the discovery of ghrelin still raises many unanswered questions. One important event was the application of deconvolution analysis to the pulsatile patterns of GH secretion in different mammalian species, including Man, according to gender, hormonal environment and ageing. Concerning this last phenomenon, a great body of evidence now supports the role of an attenuation of the GHRH/GH/Insulin-like growth factor-1 (IGF-1) axis in the control of mammalian aging.

Neuronal M3 muscarinic acetylcholine receptors are essential for somatotroph proliferation and normal somatic growth

The molecular pathways that promote the proliferation and maintenance of pituitary somatotrophs and other cell types of the anterior pituitary gland are not well understood at present. However, such knowledge is likely to lead to the development of novel drugs useful for the treatment of various human growth disorders. Although muscarinic cholinergic pathways have been implicated in regulating somatotroph function, the physiological relevance of this effect and the localization and nature of the receptor subtypes involved in this activity remain unclear. We report the surprising observation that mutant mice that selectively lack the M(3) muscarinic acetylcholine receptor subtype in the brain (neurons and glial cells; Br-M3-KO mice) showed a dwarf phenotype associated with a pronounced hypoplasia of the anterior pituitary gland and a marked decrease in pituitary and serum growth hormone (GH) and prolactin. Remarkably, treatment of Br-M3-KO mice with CJC-1295, a synthetic GH-releasing hormone (GHRH) analog, rescued the growth deficit displayed by Br-M3-KO mice by restoring normal pituitary size and normal serum GH and IGF-1 levels. These findings, together with results from M(3) receptor/GHRH colocalization studies and hypothalamic hormone measurements, support a model in which central (hypothalamic) M(3) receptors are required for the proper function of hypothalamic GHRH neurons. Our data reveal an unexpected and critical role for central M(3) receptors in regulating longitudinal growth by promoting the proliferation of pituitary somatotroph cells.

CDP-choline increases plasma ACTH and potentiates the stimulated release of GH, TSH and LH: the cholinergic involvement

In the present study, we investigated the effect of intracerebroventricular (i.c.v.) administration of cytidine-5'-diphosphate (CDP) choline on plasma adrenocorticotropin (ACTH), serum growth hormone (GH), thyroid stimulating hormone (TSH), follicle stimulating hormone (FSH) and luteinizing hormone (LH) levels in conscious rats. The involvement of cholinergic mechanisms in these effects was also determined. In basal conditions, CDP-choline (0.5, 1.0 and 2.0 micromol, i.c.v.) increased plasma ACTH levels dose- and time-dependently, but it did not affect the TSH, GH, FSH and LH levels. In stimulated conditions, i.c.v. administration of CDP-choline (1 micromol, i.c.v.) produced an increase in clonidine-stimulated GH, thyrotyropin-releasing hormone (TRH)-stimulated TSH, LH-releasing hormone (LHRH)-stimulated LH, but not FSH levels. Injection of equimolar dose of choline (1 micromol, i.c.v.) produced similar effects on hormone levels, but cytidine (1 micromol, i.c.v.) failed to alter plasma levels of these hormones. Pretreatment with hemicholinium-3, a neuronal high affinity choline uptake inhibitor, (20 microg, i.c.v.) completely blocked the observed hormone responses to CDP-choline. The increase in plasma ACTH levels induced by CDP-choline (1 micromol, i.c.v.) was abolished by pretreatment with mecamylamine, a nicotinic receptor antagonist, (50 microg, i.c.v.) but not atropine, a muscarinic receptor antagonist, (10 microg, i.c.v.). The increase in stimulated levels of serum TSH by CDP-choline (1 micromol, i.c.v.) was blocked by atropine but not by mecamylamine pretreatment. However, CDP-choline induced increases in serum GH and LH levels were greatly attenuated by both atropine and mecamylamine pretreatments. The results show that CDP-choline can increase plasma ACTH and produce additional increases in serum levels of TSH, GH and LH stimulated by TRH, clonidine and LHRH, respectively. The activation of central cholinergic system, mainly through the presynaptic mechanisms, was involved in these effects. Central nicotinic receptors solely mediated the increase in plasma ACTH levels while the activation of central muscarinic receptors was involved in the increase in TSH levels. Both muscarinic and nicotinic receptor activations, separately, mediated the increases in serum GH and LH levels after CDP-choline.

Acute hyperglycemia and activation of the beta-adrenergic system do not exhibit synergistic inhibitory actions on thyrotropin-releasing hormone (TRH)-induced thyroid stimulating hormone (TSH) secretion

Thyrotropin-releasing hormone (TRH)-stimulated thyroid stimulating hormone (TSH) response in normal subjects is suppressed by oral glucose administration. Pharmacologic studies indicate that this suppressive action of glucose is mediated by an increase in hypothalamic somatostatin (SRIH) tone. Since activation of the beta-adrenergic system also suppresses basal TSH secretion by enhancing SRIH release we sought to determine whether isoproterenol alters the suppression of TRH-induced TSH response induced by the stimulation of glucose. Four tests were performed in seven healthy young men: Test 1: 200 microg TRH (iv) at 0 min; Test 2: 100 g oral glucose at -30 min and TRH at 0 min; Test 3: TRH at 0 min with isoproterenol (0.012 microg/kg, iv) infused continuously; Test 4: oral glucose at -30 min, TRH at 0 min with isoproterenol infused continuously. Pretreatment with glucose significantly suppressed TRH-induced TSH secretion. Isoproterenol infusion also suppressed the TRH-induced TSH secretion, but it did not enhance the inhibitory action of glucose on TSH secretion. The degree of suppression induced by glucose was significantly higher than that achieved by isoproterenol. These data suggest that combined administration of glucose and isoproterenol does not exhibit synergistic inhibitory actions on TRH-stimulated TSH secretion, and that the glucose-TRH test could be used for the evaluation of the hypothalamic somatostatinergic activity.

Nutritional status in the neuroendocrine control of growth hormone secretion: the model of anorexia nervosa

Growth hormone (GH) plays a key role not only in the promotion of linear growth but also in the regulation of intermediary metabolism, body composition, and energy expenditure. On the whole, the hormone appears to direct fuel metabolism towards the preferential oxidation of lipids instead of glucose and proteins, and to convey the energy derived from metabolic processes towards the synthesis of proteins. On the other hand, body energy stores and circulating energetic substrates take an important part in the regulation of somatotropin release. Finally, central and peripheral peptides participating in the control of food intake and energy expenditure (neuropeptide Y, leptin, and ghrelin) are also involved in the regulation of GH secretion. Altogether, nutritional status has to be regarded as a major determinant in the regulation of the somatotropin-somatomedin axis in animals and humans. In these latter, overweight is associated with marked impairment of spontaneous and stimulated GH release, while acute dietary restriction and chronic undernutrition induce an amplification of spontaneous secretion together with a clear-cut decrease in insulin-like growth factor I (IGF-I) plasma levels. Thus, over- and undernutrition represent two conditions connoted by GH hypersensitivity and GH resistance, respectively. Anorexia nervosa (AN) is a psychiatric disorder characterized by peculiar changes of the GH-IGF-I axis. In these patients, low circulating IGF-I levels are associated with enhanced GH production rate, highly disordered mode of somatotropin release, and variability of GH responsiveness to different pharmacological challenges. These abnormalities are likely due not only to the lack of negative IGF-I feedback, but also to a primary hypothalamic alteration with increased frequency of growth hormone releasing hormone discharges and decreased somatostatinergic tone. Given the reversal of the above alterations following weight recovery, these abnormalities can be seen as secondary, and possibly adaptive, to nutritional deprivation. The model of AN may provide important insights into the pathophysiology of GH secretion, in particular as regards the mechanisms whereby nutritional status effects its regulation.

Prolonged treatment with glycerophosphocholine, an acetylcholine precursor, does not disclose the potentiating effect of cholinesterase inhibitors on GHRH-induced somatotroph secretion in anorexia nervosa

Unlike normal subjects, in patients with anorexia nervosa (AN) the GH response to GHRH is refractory to the increasing and inhibitory effect of cholinergic agonists and antagonists, respectively. This cholinergic impairment could reflect malnutrition-induced exhaustion of acetylcholine (Ach) precursors. We studied whether treatment with glycerophosphocholine (GLY), an Ach precursor, could disclose the potentiating effect of pyridostigmine (PD) on the GH response to GHRH in AN. In 6 young women with AN (AW) we studied the GH response to iv GHRH (1.0 microg/kg) alone and combined with oral PD (120 mg) before and after 1 month of oral treatment with GLY (400 mg thrice daily). Eight age-matched normal women (NW) were studied as controls. Before GLY, basal GH levels in AW were higher (p < 0.05) than in NW. The GH response to GHRH in AW was higher (p < 0.05) than in NW. PD failed to modify the GHRH-induced GH rise in AW, while it enhanced it in NW (p < 0.05). One month treatment with GLY in AW did not modify the GH response to GHRH either alone or combined with PD. This study shows the existence of a derangement in the cholinergic control of somatotroph function in AN and indicates that treatment with Ach precursors does not exert any effect on this impairment. This could reflect primary alterations of cholinergic neurons, though the effectiveness of more prolonged treatment and/or higher doses of cholinergic precursors needs to be verified.

Effects of long-term estrogen replacement therapy on growth hormone response to pyridostigmine in healthy postmenopausal women

OBJECTIVE

There is growing evidence that estrogen may protect against age-related cognitive decline and reduce the risk of developing Alzheimer's disease (AD) in healthy, postmenopausal women. The underlying biological basis for this is not known but may include preservation of cholinergic systems. Cholinergic dysfunction has been implicated in the aetiology of age-related memory impairment and AD. We studied the effect of prolonged use of estrogen replacement therapy (ERT) on central cholinergic tone in healthy postmenopausal women.

METHOD

Growth hormone (GH) responses to oral pyridostigmine (120 mg) were measured over a 3 h period in thirty healthy postmenopausal women, 15 on long-term ERT and 15 ERT naïve.

RESULTS

GH release following pyridostigmine was significantly larger in ERT treated women than in ERT naïve women. In addition within the ERT treated group there was a significant positive correlation between duration of estrogen treatment and GH response.

CONCLUSIONS

Long-term ERT can enhance cholinergic function in postmenopausal women and this may be related to duration of estrogen treatment. Modulation of central cholinergic function may be one mechanism by which long-term ERT could preserve cognitive function in healthy, postmenopausal women.

Insulin-like growth factor-I restores the reduced somatostatinergic tone controlling growth hormone secretion in cirrhotic rats

BACKGROUND/AIMS

An altered growth hormone/insulin-like growth factor-I (GH/IGF-I) axis occurs in advanced liver cirrhosis, characterised by diminished serum levels of IGF-I and increased concentrations of GH. Under normal conditions, GH release is mediated by somatostatin (SS) inhibition. However, the influence of SS on GH release in cirrhosis is not well known. IGF-I supplementation has beneficial effects in experimental cirrhosis, and - under physiological conditions - IGF-I increases SS, inhibiting GH. The aims of this work were to study SS tone in cirrhotic animals and to evaluate whether IGF-I treatment influences SS tone, controlling GH secretion in cirrhosis.

METHODS

We studied the influence of SS on GH secretion by assessing GH response to pyridostigmine (PD) in cirrhotic rats treated and untreated with IGF-I. Liver cirrhosis was induced with CCl4-inhalation for 11 weeks in male Wistar rats. The animals were randomly divided into two groups: CI+IGF (n=12), which received IGF-I treatment for 12 days (2 microg/100 g body wt-1 x d-1) and CI (n=12), which received saline. Healthy controls (CO, n=12) were studied at the same time. On day 13, animals from each group were subdivided into two groups (n=6) in order to explore the effect of a PD intrajugular bolus (10 microg x 100 gbw-1) on serum GH levels (at 0,10,20,30 and 60 min), which were assessed by RIA.

RESULTS

PD bolus did not exert any effect on GH serum levels in the CI group, suggesting a low SS tone in cirrhotic rats. However, PD induced an increase in GH levels into CO and CI+IGF groups. In conclusion, as occurs under normal conditions, the cholinergic system is a significant modulator of GH secretion in experimental liver cirrhosis.

CONCLUSION

Cirrhotic rats have a reduced somatostatinergic tone which can be restored by IGF-I supplementation, suggesting that somatostatin is the main factor involved in the feed-back regulation between GH and IGF-I in cirrhosis.

Neuroregulation of growth hormone secretion in domestic animals

Growth hormone (GH) is essential for postnatal somatic growth, maintenance of lean tissue at maturity in domestic animals and milk production in cows. This review focuses on neuroregulation of GH secretion in domestic animals. Two hormones principally regulate the secretion of GH: growth hormone-releasing hormone (GHRH) stimulates, while somatostatin (SS) inhibits the secretion of GH. A long-standing hypothesis proposes that alternate secretion of GHRH and SS regulate episodic secretion of GH. However, measurement of GHRH and SS in hypophysial-portal blood of unanesthetized sheep and swine shows that episodic secretion of GHRH and SS do not account for all episodes of GH secreted. Furthermore, the activity of GHRH and SS neurons decreases after steers have eaten a meal offered for a 2-h period each day (meal-feeding) and this corresponds with reduced secretion of GH. Together, these data suggest that other factors also regulate the secretion of GH. Several neurotransmitters have been implicated in this regard. Thyrotropin-releasing hormone, serotonin and gamma-aminobutyric acid stimulate the secretion of GH at somatotropes. Growth hormone releasing peptide-6 overcomes feeding-induced refractoriness of somatotropes to GHRH and stimulates the secretion of GHRH. Norepinephrine reduces the activity of SS neurons and stimulates the secretion of GHRH via alpha(2)-adrenergic receptors. N-methyl-D,L-aspartate and leptin stimulate the secretion of GHRH, while neuropeptide Y stimulates the secretion of GHRH and SS. Activation of muscarinic receptors decreases the secretion of SS. Dopamine stimulates the secretion of SS via D1 receptors and inhibits the secretion of GH from somatotropes via D2 receptors. Thus, many neuroendocrine factors regulate the secretion of GH in livestock via altering secretion of GHRH and/or SS, communicating between GHRH and SS neurons, or acting independently at somatotropes to coordinate the secretion of GH.

Cholinergic modulation of growth hormone responses to growth hormone-releasing hormone in uraemic patients on peritoneal dialysis

BACKGROUND

Hypothalamic cholinergic neurotransmission plays a major role in the regulation of GH secretion. Pyridostigmine, a cholinesterase inhibitor, is able to decrease hypothalamic somatostatinergic tone and release GH in normal subjects. Blockade of muscarinic receptor with pirenzepine blunts the GH release in several clinical situations. However, little information is available on the role played by central cholinergic pathways in GH regulation in uraemic patients.

OBJECTIVE

We aimed to assess GH responses to GHRH after pretreatment with pyridostigmine and pirenzepine in a group of uraemic patients undergoing peritoneal dialysis (PD). GH responses of the patients treated with recombinant human erythropeitin (rhEPO) were compared to patients without treatment.

DESIGN

We studied 14 male patients on PD and nine control subjects. All subjects underwent three endocrine test in random order after an overnight fast. Each subject received GHRH (100 microg, i.v. in bolus at 0 minutes). Sixty minutes before the injection of GHRH subjects were given oral placebo, pyridostigmine (120 mg), or pirenzepine (100 mg).

MEASUREMENTS

Blood samples for GH were collected at -60, 0, 15, 30, 45, 60 and 90 minutes The hormonal secretory responses were studied by a time-averaged (area under the curves, AUC) and time-independent (peak values) analysis.

RESULTS

Baseline GH concentrations were similar in patients and controls. GH responses to placebo plus GHRH were also comparable in patients and controls (peak 26.6 +/- 3.8 vs. 33.2 +/- 4.4 mU/l, AUC 28.2 +/- 3.4 vs. 27.8 +/- 4.6 mU/h/l). Pyridostigmine administration induced a significant potentiation of GH responses to GHRH both in patients (peak 43.2 +/- 5.2 mU/l, AUC 47.6 +/- 6.0 mU/h/l; P < 0.01) and in control subjects (peak 79.2 +/- 8.6 mU/l, AUC 78.0 +/- 9.4 mU/h/l; P < 0.01). However, the increment in GH peak and AUC was significantly (P < 0.05) greater in controls in relation to values found in patients. Pirenzepine administration induced an abolishment of GH release after GHRH stimulation both in PD patients (peak 5.4 +/- 2.6 mU/l, AUC 6.0 +/- 2.4 mU/h/l; P < 0.01) and in healthy controls (peak 3.8 +/- 0.6 mU/l, AUC 4.0 +/- 0.4 mU/h/l; P < 0.05). Responses to pyridostigmine plus GHRH and pirenzepine plus GHRH were similar in patients on chronic therapy with recombinant human erythropeitin and in patients without rhEPO therapy.

CONCLUSION

These results suggest that the cholinergic regulation of GH release is preserved in uraemic patients on peritoneal dialysis. The significantly lower increase in GH response to GHRH induced by pyridostigmine suggests that cholinergic stimulatory tone is attenuated in patients in relation to control subjects. Long-term therapy with rhEPO seems not to affect GH responses to cholinergic stimulation or blockade.

The increased insulin sensitivity in growth hormone-deficient adults is reduced by growth hormone replacement therapy

BACKGROUND

Growth hormone deficiency is associated with increased morbidity and mortality from cardiovascular diseases, which might be related to changes in glucose and lipid metabolism.

DESIGN

To assess the influence of long-term growth hormone replacement therapy (GHRT) on glucose metabolism we examined eight growth hormone-deficient (GHD) adults (seven female/one male; age, 46 +/- 3 years; body mass index, 31 +/- 2 kg m-2) over a period of 18 months in comparison to an adequate control group consisting of eight obese subjects matched for age, sex, and body mass index. We performed frequently sampled intravenous glucose tolerance tests (FSIGT) with minimal model analysis before the study, and after 12 and 18 months.

RESULTS

Following GHRT, insulin-like growth factor-1 (IGF-1) increased significantly from a basal level of 75.9 +/- 18.9 to 200.8 +/- 31.0 microg L-1 after 12 months of therapy and remained stable, thereafter. GHRT did not affect fasting blood glucose, basal insulin, cholesterol, blood pressure and body weight. However, at 12 months, HbA1c (6.0 +/- 0.1 vs. 5.6 +/- 0.1% at basal, P < 0.05) and triglyceride (2.3 +/- 0.4 vs. 1.4 +/- 0.3 mmol L-1) significantly increased but returned to pretreatment values at 18 months. Insulin sensitivity was higher in GHD (8.2 +/- 3.1) compared to controls (3. 6 +/- 0.53 x 10-4 min-1/(microU mL-1), P = 0.06) and decreased significantly after 18 months of GHRT to 5.1 +/- 2.6, P < 0.05. Basal insulin secretion was similar to that in the control group and increased significantly after 12 and 18 months, total insulin secretion only after 12 months. SG (glucose effectiveness)was lower in GHD patients (0.0095 +/- 0.001 min-1) compared to controls (0.020 +/- 0.003 min-1, P < 0.05) and increased significantly after 12 and 18 months of GHRT (0.016 +/- 0.002, and 0.015 +/- 0.001 min-1, P < 0. 05), respectively. Hepatic insulin extraction rate was similar in both groups and remained unchanged following GHRT.

CONCLUSION

We conclude that long-term GHRT induces a significant decrease of the increased insulin sensitivity in GHD patients to levels observed in body mass index-matched control subjects. This is accompanied by an increase in basal and total insulin secretion as well as in glucose effectiveness as a possible compensatory mechanism.

The growth hormone axis and cognition: empirical results and integrated theory derived from giant transgenic mice

Sleep is required for the consolidation of memory for complex tasks, and elements of the growth-hormone (GH) axis may regulate sleep. The GH axis also up-regulates protein synthesis, which is required for memory consolidation. Transgenic rat GH mice (TRGHM) express plasma GH at levels 100-300 times normal and sleep 3.4 h longer (30%) than their normal siblings. Consequently, we hypothesized that they might show superior ability to learn a complex task (8-choice radial maze); 47% of the TRGHM learned the task before any normal mice. All 17 TRGHM learned the task, but 33% of the 18 normal mice learned little. TRGHM learned the task significantly faster than normal mice (p < 0.05) and made half as many errors in doing so, even when the normal nonlearners were excluded from the analysis. Whereas normal mice expressed a linear learning curve, TRGHM showed exponentially declining error rates. The contribution of the GH axis to cognition is conspicuously sparse in literature syntheses of knowledge concerning neuroendocrine mechanisms of learning and memory. This paper synthesizes the crucial role of major components of the GH axis in brain functioning into a holistic framework, integrating learning, sleep, free radicals, aging, and neurodegenerative diseases. TRGHM show both enhanced learning in youth and accelerated aging. Thus, they may provide a powerful new probe for use in gaining an understanding of aspects of central nervous system functioning, which is highly relevant to human health.

Age-related blunting of growth hormone secretion during exercise may not be soley due to increased somatostatin tone

Age-related declines in growth hormone (GH) secretion may result from augmented somatostatin (SRIH) tone and/or diminished GH-releasing hormone (GHRH) secretion. We assessed GH release during exercise without and with pyridostigmine (PYR), which indirectly suppresses SRIH. GH levels were measured throughout exercise and recovery in 12 young men (mean +/- SEM, 20.8 +/- 0.4 years) and seven old men (66.1 +/- 1.9). The area under the GH curve (GH-AUC) was greater in young versus old men during a short-term maximal exercise test (12.9 +/- 2.8 v 1.5 +/- 0.2 ng x min(-1) x mL(-1), P = .002) and a 1-hour 60% maximal (submaximal, 10.0 +/- 1.5 v 3.0 +/- 1.0 ng x min(-1) x mL(-1), P = .001) cycle exercise bout. PYR increased the GH-AUC in young and old men during maximal (20.9 +/- 5.2 v 4.9 +/- 1.8) and submaximal (12.3 +/- 1.6 v 4.7 +/- 1.5) exercise (P < .05). The greater GH response to maximal versus submaximal exercise suggests a role for adrenergic modulation of GHRH during exercise. However, the failure of PYR to restore the responses of the old to those of the young suggests that increased SRIH tone does not completely explain the age difference in GH secretion during exercise.

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.

Psychoneuroendocrinology of depression. Growth hormone

The release of growth hormone (GH) from the anterior pituitary is regulated by hypothalamic peptides especially GH-releasing hormone (GHRH) and somatostatin, which in turn are controlled by classic neurotransmitters such as noradrenaline, dopamine, and acetylcholine, as well as negative feedback from GH and insulin-like growth factor-1. There has been extensive investigation of this axis in patients with depression. The most consistently reported abnormality is in noradrenergic-mediated GH release, which probably occurs via GHRH containing neurones. ACh-induced GH release through the somatostatin system, GABA, and also GHRH-stimulated release are reported as abnormal by some researchers.

Enhanced response of growth hormone to growth hormone-releasing hormone and a decreased content of hypothalamic somatostatin in a stress-induced rat model of depression

This study was designed to evaluate changes in the hypothalamic somatostatin-growth hormone axis (SRIF-GH axis) in a stress-induced rat model of depression. We exposed male Wistar rats to intermittent walking stress for two weeks, and then measured their spontaneous running activities for 12 days. We divided the rats into the depression-model group and the partial recovery group according to their spontaneous running activities after the termination of exposure to stress. We examined the secretion of GH from the anterior pituitary by injecting human GH-releasing hormone (hGHRH) with intracardiac cannulae or by applying hGHRH or SRIF to isolated anterior pituitaries using a perifusion system. We also determined SRIF content in the stalk-median eminence (SME) and the plasma concentration of GH. In the depression-model group, intracardiac administration of hGHRH caused the enhanced release of GH into plasma, while application of hGHRH or SRIF to the anterior pituitary in vitro had similar effects on GH release in the control and partial recovery groups. Furthermore, the SRIF content was decreased in the SME and the GH concentration was increased in plasma. The partial recovery group gave similar values to the control group. The enhanced response of GH to hGHRH in the depression-model group might have been caused by the reduced content of SRIF in the SME in view of the unchanged response of GH to the infusion of hGHRH or SRIF in the perifusion system.

Plasma GH responses to GHRH, arginine, L-dopa, pyridostigmine, sequential administrations of GHRH and combined administration of PD and GHRH in Turner's syndrome

To investigate GH secretory capacities in patients with Turner's syndrome, GHRH, arginine, L-dopa and pyridostigmine (PD) were administered singly and GHRH was administered sequentially for 3 days. In addition, plasma GH and TSH responses to GHRH and TRH after pretreatment with PD were analyzed to investigate whether the hypothalamic cholinergic somatostatinergic system functioned normally. The maximal GH responses to GHRH, L-dopa and PD were significantly smaller in Turner's syndrome (no.=14) than in normal short children (NSC, no.=14). However, there was no difference in plasma GH responses to arginine between the two groups. In ten patients with Turner's syndrome, the plasma GH response to GHRH did not improve even after the sequential 3-day administrations. Although plasma GH and TSH responses to GHRH and TRH were significantly enhanced by the pretreatment of PD in NSC (no.=12), these responses were not enhanced in Turner's syndrome. Plasma GH response to GHRH in Turner's syndrome with normal body fat was still significantly lower than in NSC. It is therefore concluded that somatotroph sensitivity to GHRH is decreased in Turner's syndrome and that this may be due to the primary defects of the somatotrophs rather than to the increased body fat. In addition, the network of cholinergic-somatostatinergic systems seemed to be impaired in these patients, while the activity of hypothalamic somatostatin neurons was thought to be maintained.

Effect of cholinergic blockade on inhibited GH secretion by feeding and intraruminal SCFA infusion in sheep

The effect of cholinergic blockade on suppressed growth hormone (GH) secretion caused by feeding or the intraruminal infusion of an acetate, propionate, and butyrate mixture (107 and 214 mumol.kg-1.min-1 over 6 h) was examined in ovariectomized ewes. Intraruminal infusion at the rate of 107 mumol.kg-1.min-1 increased peripheral plasma short-chain fatty acid (SCFA) concentrations to approximately the physiological levels noted after feeding. Plasma GH was markedly suppressed by feeding and at both the 107 and 214 mumol.kg-1.min-1 SCFA infusion rates; however, cholinergic blocking agents completely blocked the suppressed GH secretion after feeding and only at the 107 mumol.kg-1.min-1 infusion rate. Plasma glucose increased at both infusion rates, and the plasma free fatty acids decreased after feeding and at both infusion rates. However, both metabolites were unchanged relative to the saline control after the injection of the cholinergic antagonists. It is suggested that the decrease in plasma GH observed after feeding and a near-physiological ruminal SCFA increment is mediated via the parasympathetic nerve and not by pharmacological ruminal SCFA increments attributed to other pathways.

Effects of cholinergic blockade on nocturnal thyrotropin and growth hormone (GH) secretion in type I diabetes mellitus: further evidence supporting somatostatin's involvement in GH suppression

To investigate the influence of cholinergic pathways on somatostatin (SS) tone in type I diabetes mellitus, we studied the effect of the muscarinic receptor antagonist pirenzepine ([PZP] 100 mg orally) on spontaneous nocturnal growth hormone (GH) and thyrotropin (TSH) secretion and on their response to GH-releasing hormone (GHRH) in the morning in a group of nine insulin-dependent diabetic patients with poor diabetic control. When the nocturnal period was divided into two phases (11:00 PM to 2:30 AM and 3:00 AM to 6:00 AM), both GH and TSH mean concentrations during the first phase were higher than those seen in the second half of the night following placebo administration (GH, 13.4 +/- 1.1 v 4.15 +/- 0.9 ng/mL, P < .001; TSH, 1.9 +/- 0.21 v 1.57 +/- 0.1 microU/mL, P < .05). Pretreatment with PZP induced a significant reduction of GH secretion (3.17 +/- 1.1 v 13.4 +/- 1.1 ng/mL, P < .001) and TSH secretion (1.61 +/- 0.21 microU/mL, P < .05) in the first phase of the night, accounting for a 64% and 11% reduction in the GH and TSH nocturnal peak, respectively. PZP reduced the GH response to GHRH in the morning (17.9 +/- 2.7 v 36.7 +/- 6.3 ng/mL, P < .05), but did not induce any change in TSH values at that time. A positive relationship (r = .73, P < .01) was observed between the percent reduction of the GH response to GHRH and that of the nocturnal GH peak following PZP administration. PZP caused a significant reduction in glucose levels during the second phase of the night (6.4 +/- 0.92 v 9.81 +/- 0.85 mmol/L, P < .05). These results demonstrate that administration of PZP reduces GH and TSH secretion, providing further support for the involvement of SS in the inhibition of GH secretion induced by cholinergic antagonists in type I diabetics. The inhibitory effect of PZP on GHRH-induced GH secretion may help to predict nocturnal GH behavior following administration of the drug.

Specificity of the pyridostigmine/growth hormone challenge in the diagnosis of depression

Acetylcholine is a neurotransmitter that has been implicated in the pathophysiology of major depression. This is supported by the enhanced growth hormone (GH) release in response to pyridostigmine (PYD) challenge in depressed subjects relative to healthy comparison subjects. The aim of this study is to examine the specificity of the PYD/GH challenge in the diagnosis of depression. Pyridostigmine 120 mg orally, was administered to a total of 116 physically healthy subjects. Growth hormone responses were studied in 38 patients with (DSM-III-R) major depression, 13 subjects with panic disorder, 9 subjects with schizophrenia, 10 recently detoxified alcoholics, and a comparison group of 46 healthy volunteers. Mean delta GH (the difference between basal and maximal GH following PYD) was significantly greater than comparison subjects in patients with major depression. Responses observed in patients with schizophrenia and alcohol dependence syndrome did not differ from the comparison group. Those patients with panic disorder and a high Hamilton depression score had an enhanced delta GH. The sensitivity of the PYD/GH test was 63% for major depression. These results indicate that the PYD/GH test may help distinguish depression from schizophrenia, alcohol-dependence syndrome, or panic disorder with a low Hamilton depression score.

Physiological role of the opioid-cholinergic interaction in growth hormone neuroregulation: effect of sex and food intake

Studies performed in animals and humans have suggested a functional interaction between opioid and cholinergic systems in the control of growth hormone (GH) secretion. Moreover, the sex-dependent modulation of GH secretion in humans is well established. To investigate the role of sex and food intake in the regulation of the reciprocal influences of opioids and acetylcholine in the modulation of GH secretion, we studied the GH response to pyridostigmine (PYR) alone and during a naloxone (NAL) infusion in a group of normal men and women before a meal (at 1:00 PM) and postprandially. In women, the response of GH to PYR alone before the meal was significantly lower than in the men (area under the curve [AUC], mean +/- SEM, 320.18 +/- 87.16 v 1,031.06 +/- 333.21 micrograms/L/90 min, P < .01). Before the meal, NAL completely abolished the response of GH to PYR in men (AUC, 1,031.06 +/- 333.21 v 16.50 +/- 7.50 micrograms/L/90 min, P < .01), whereas infusion of NAL did not significantly modify the GH response to PYR in women. Consumption of the meal significantly decreased PYR-induced GH release in both women (AUC, 21.75 +/- 12.75 v 320.18 +/- 87.16 micrograms/L/90 min, P < .05) and men (AUC, 45.75 +/- 18.75 v 1,031.06 +/- 333.21 micrograms/L/90 min, P < .01). Conversely, food intake did not change the effects of NAL infusion on the GH response to PYR either in women or in men. We conclude that the sex-dependent opioid modulation of PYR-induced GH secretion is observed before a meal but not in the postprandial state. Food intake may be hypothesized to influence the cholinergic regulation of GH secretion and the sex-dependent opioid modulation of central cholinergic tone.

Is the persistence of isolated GH deficiency in adulthood predicted by anatomical hypothalamic-pituitary alterations?

The aim of this study was to verify the persistence in adulthood of GH deficiency diagnosed in childhood and treated with hGH in childhood and to study whether anatomical hypothalamic-pituitary alterations evaluated by magnetic resonance (MR) imaging could predict it. To this goal, in six GHD adults (3 males and 3 females aged 17.2-24.5 yr, BMI 21.8 +/- 1.3), we studied anterior pituitary hormone response to GHRH (1 microgram/kg iv)+pyridostigmine (120 mg po)+ GnRH (100 micrograms iv) +TRH (400 micrograms iv)+hCRH (100 micrograms iv) as well as brain MR imaging. In childhood, the diagnosis of severe isolated GHD had been done based on auxological findings as well as on GH response < 7 micrograms/L after two classical provocative stimuli. In the present study, hormonal responses showed the persistence of severe isolated GHD in 4 out of 6 patients (peak, mean +/- SEM: 3.8 +/- 0.6, range 2.6-4.8 micrograms/L). In these patients, IGF-I levels were found low or low-normal. In other 2 patients, a clear GH response to stimulation (peak: 51.3 and 43.0 micrograms/L, respectively) together with normal IGF-I levels were found. No other anterior pituitary hormone deficiency was present in all subjects. MR imaging showed pituitary hypoplasia in all patients with persistent GHD; in 2 out of them, pituitary stalk interruption and ectopic neurohypophysis was also present. On the other hand, MR imaging showed normal hypothalamo-pituitary morphology in the 2 subjects with normal somatotrope response. In conclusion, our present data indicate that testing with a potent stimulus such as GHRH+pyridostigmine is a reliable method to assess the persistence of GH deficiency which associates with anatomical hypothalamic-pituitary alterations at the MR imaging. Patients with transient GH deficiency in childhood and normal pituitary GH reserve in adulthood have normal hypothalamic-pituitary MR imaging.

Pre- and postprandial pyridostigmine and oxiracetam effects on growth hormone secretion in anorexia nervosa

Previous studies have shown that food ingestion is not capable of inhibiting the GHRH-induced GH release in anorexia nervosa, at variance with what is observed in normal subjects. Moreover, a cholinergic alteration has been hypothesized in this disorder. In a group of 24 anorectic patients in a stabilized phase of the illness, we tested, before and after a standard meal, the GH response to GHRH alone and after pre-treatment with pyridostigmine, an inhibitor of acetylcholinesterase, and, on a different day, with oxiracetam, which stimulates the central cholinergic neurones. The GH response to GHRH was significantly increased by both drugs in a fasting state. The postprandial response was not significantly modified by pyridostigmine nor by oxiracetam. Neither of these compounds was able to enhance the postprandial GH 'paradoxical' response to GHRH in anorectic patients. The lack of effect of both groups postprandially also suggests a suppression of somatostatinergic activity.

Mitogenic and antiadipogenic properties of human growth hormone in differentiating human adipocyte precursor cells in primary culture

Children with GH deficiency have enlarged fat cells but a reduced number of fat cells compared with healthy children. After treatment with human GH (hGH) both fat cell volume and number are shifted toward normal. To clarify the role of hGH in fat cell formation in human adipose tissue, we investigated the effect of hGH on the proliferation and the differentiation of cultured human adipocyte precursor cells obtained from five children and 10 adults. In a chemically defined serum-free medium treatment of adipocyte precursor cells with hGH led to an increase in IGF-I production and a stimulation of cell proliferation, which could be blocked by a MAb raised against human IGF-I. hGH dose-dependently reduced the number of differentiating cells and suppressed the expression of glycerol-3-phosphate dehydrogenase (GPDH), a marker of adipose differentiation. No significant differences in the hGH effects on proliferation and differentiation capacities were seen between cultures obtained from children and adults. In newly differentiated adipocytes, hGH inhibited glucose uptake and lipogenesis, and stimulated lipolysis. Scatchard analysis of hGH competition experiments using 125I-labeled hGH yielded a linear plot with an apparent Kd of 1.08 nM and an estimated number of 7000 hGH receptors per cell. These data suggest that hGH is able to enlarge the human adipocyte precursor pool via induction of IGF-I synthesis but exhibits a direct antiadipogenic activity. hGH is also able to reduce fat cell volume by reducing lipogenesis and increasing lipolysis.

Aspects of the neuroendocrine control of somatotropic function in calorically restricted dogs and patients with eating disorders: studies with cholinergic drugs

A series of studies was devised in both an experimental model of food deprivation, i.e., beagle dogs undergoing a progressive reduction of calorie intake and adolescent females with anorexia nervosa (AN) in the acute and recovery phase, and in patients with atypical eating disorders. The studies were aimed at ascertaining whether the alleged function of the hypothalamic system inhibitory to growth hormone (GH) secretion, i.e., the somatostatinergic, may account for at least some of the abnormalities of GH secretion present in AN patients (e.g., elevated basal GH levels, paradoxical GH rise after glucose or thyrotropin releasing hormone, etc). Caloric restricted dogs or patients with eating disorders were given an intravenous injection of the physiologic GH-releasing peptide GHRH alone or preceded by pirenzepine, a muscarinic cholinergic antagonist reportedly capable of eliciting hypothalamic release of somatostatin (SS), or pyridostigmine, a muscarinic cholinergic agonist which, conversely, would restrain hypothalamic release of SS. In addition, dogs were challenged with acute administration of glucose or thyrotropin-releasing hormone, compounds also thought to act via somatostatinergic influences. Data obtained in dogs under caloric restriction or in AN patients in the acute phase of the disease with drugs affecting cholinergic transmission suggest that the latter is increased in both conditions (only partial suppression of the GHRH-induced GH rise with pirenzepine, failure of pyridostigmine to further enhance the GH response to GHRH). Instead, in the same AN patients in the acute phase tested during recovery, in AN patients during the recovery phase, and in patients with atypical eating disorders, pirenzepine completely suppressed the GH response to GHRH, as it did in controls. Finally, data obtained on basal and GHRH-stimulated GH release in dogs given glucose or thyrotropin-releasing hormone and in AN patients given arginine, another compound thought to act via inhibition of somatostatinergic influences, do not support the view that somatostatinergic function is impaired in states of food deprivation.

Adrenergic and cholinergic involvement in basal and growth hormone-releasing hormone-stimulated growth hormone secretion in glucocorticoid-treated rats

Glucocorticoids are known to inhibit GH secretion via somatostatin. The aim of our study was to elucidate the involvement of somatostatin in the GH-releasing action of the alpha 2 agonist clonidine and the cholinergic agent pyridostigmine in conscious, freely-moving rats chronically treated with dexamethasone. After seven days of chronic glucocorticoid treatment, animals received an i.v. injection of either saline (1 ml/kg) or clonidine (150 micrograms/kg) or pyridostigmine (100 micrograms/kg) at -15 min. Three blood samples were then drawn (-10 min, -5 min, and 0 min) to assess the GH response to either clonidine or pyridostigmine alone. After the 0 min sample, saline (1 ml/kg) or GNRH (500 ng/kg) was injected i.v. and additional blood samples were drawn from 5 to 30 min. The GH response to clonidine alone or combined with GNRH in rats treated with dexamethasone was significantly lower (p < 0.05) as compared to vehicle-treated rats. The GH response to pyridostigmine alone or combined with GNRH did not significantly differ between vehicle- and dexamethasone-treated rats. These data suggest that in the rat the mechanism of action of clonidine is mainly to stimulate endogenous GNRH secretion, while pyridostigmine appears to predominantly act by decreasing hypothalamic somatostatin.

Acipimox potentiates growth hormone response to growth hormone-releasing hormone by decreasing serum free fatty acid levels in hyperthyroidism

Hyperthyroidism is associated with an impairment of growth hormone (GH) responses to secretagogues. The aim of this study was to evaluate the effect of acipimox, an antilipolytic agent able to decrease free fatty acids (FFA), on GH response to GH-releasing hormone (GHRH) in hyperthyroid and normal control subjects. We studied six men with hyperthyroidism; seven normal men served as control subjects. Each subject underwent treatment with (1) 2 tablets of placebo orally or (2) 500 mg acipimox orally, 120 minutes before intravenous (IV) injection of 1 microgram/kg GHRH-(1-29)NH2. GH response to GHRH in hyperthyroid patients was markedly reduced; the mean peak GH response (9.6 +/- 1.0 microgram/L) and the area under the GH response curve (12.9 +/- 1.3 micrograms/L x 2 h) were lower than those of control subjects (25.7 +/- 1.8 micrograms/L, P < .05; 28.7 +/- 2.1 micrograms/L x 2 h, P < .05). Hyperthyroid patients had higher baseline levels of plasma FFA than control subjects (998.0 +/- 38.9 v 498.0 +/- 36.0 muEq/L, P < .01). Acipimox decreased FFA levels in both hyperthyroid and control subjects; the lowest FFA levels of hyperthyroid subjects induced by acipimox were similar to those of control subjects. After acipimox pretreatment, GH responses to GHRH increased significantly (P < .05); the mean peak plasma GH level (25.9 +/- 4.6 micrograms/L) was similar to the peak GH levels of control subjects during the GHRH test, and the area under the GH response curve (41.1 +/- 6.7 micrograms/L x 2 h) was even higher than that of control subjects with the GHRH test.(ABSTRACT TRUNCATED AT 250 WORDS)

Exogenous growth hormone administration does not inhibit the growth hormone response to hexarelin in normal men

Administration of exogenous human growth hormone (GH) blunts the GH response to physiological as well as pharmacological stimuli, including GH-releasing hormone (GHRH). Hexarelin (Hex) is a new synthetic GH-releasing peptide (GHRP) similar to GHRP-6 with potent GH-releasing activity in animals and men. To determine whether the short-term administration of GH inhibits the Hex-induced GH release, we measured the GH response to Hex (2 micrograms/kg iv) in five normal adult males (age 26-32 yr) three h after an iv bolus of rhGH (2 IU) or saline. Mean incremental change of serum GH from value at time 0 was 47.5 +/- 5.5 and 41.5 +/- 4.1 micrograms/l after saline + Hex and GH + Hex, respectively. Mean incremental area under the curve over baseline was 3216 +/- 586 and 3735 +/- 506 micrograms.min.1 after saline + Hex and GH + Hex, respectively. One of the proposed mechanism of action of GHRPs is to serve as functional somatostatin (SRIH) antagonists, and it is known that GH feeds backs on the hypothalamus to stimulate SRIH release. Therefore, we speculate that antagonisms of SRIH function by Hex prevented the inhibitory effect of exogenous GH, thus lending further support to the hypotheses that SRIH is involved in the feedback regulation of GH secretion, and that GHRPs action involves inhibition of SRIH function.

Effect of testosterone replacement therapy on the somatotrope responsiveness to GHRH alone or combined with pyridostigmine and on sympathoadrenal activity in patients with hypogonadism

There is evidence suggesting that androgens influence GH secretion in man. Our aim was to verify whether the GH releasable pool is preserved and influenced by testosterone replacement in male hypogonadism. To this goal, in eight male hypogonadal patients (HP, age 32.2 +/- 5.0 yr; Body Mass Index 23.9 +/- 1.1 kg/m2) before and after 3 months testosterone therapy, we studied the GH response to GHRH (1 microgram/kg iv) alone and combined with pyridostigmine (PD, 120 mg po), a cholinesterase inhibitor which likely inhibits hypothalamic somatostatin release allowing exploration of the maximal somatotrope secretory pool. Sixteen normal subjects (NS, age 30.1 +/- 3.5 yr; Body Mass Index 22.5 +/- 1.8 kg/m2) were studied as controls. The GH response to GHRH in HP was similar to that in NS (AUC, mean +/- SE: 1238 +/- 362 vs 1018 +/- 182 micrograms/L/h). PD potentiated to the same extent the GH response to GHRH in both groups (2092 +/- 807 and 2840 +/- 356 micrograms/L/h). After three month testosterone therapy, in HP the GH responses to GHRH alone (1352 +/- 612 micrograms/L/h) and combined with PD (1948 +/- 616 microgram/L/h) were unchanged. Also IGF-I levels in HP were similar to those in NS (222 +/- 42 vs 210.6 +/- 55.8 micrograms/L) and were unchanged during testosterone replacement (280 +/- 31 micrograms/L). As androgens have been reported to modulate sympathoadrenal activity in the rat, both before and during testosterone replacement, we also measured plasma catecholamine levels. Basal NE (p < 0.05) but not E levels were lower in HP than in NS; testosterone restored basal NE levels to normal without affecting basal E. delta absolute increase of NE and E (p < 0.05 and 0.01 vs baseline, respectively) after PD in HP were similar to those in NS and were unchanged during testosterone replacement. In conclusion, these results demonstrate that the GH releasable pool is preserved in male hypogonadism. As in this condition a reduction of spontaneous GH secretion has been reported, it could be due to neurosecretory dysfunction but not to pituitary impairment. Subtle alterations of sympathoadrenal activity seem to be present in male hypogonadism and reversed by testosterone replacement.

Effect of combined administration of growth hormone (GH)-releasing hormone, GH-releasing peptide-6, and pyridostigmine in normal and obese subjects

Growth hormone (GH) secretion in response to all provocative stimuli is decreased in patients with obesity. Recently, we found that the combined administration of GH-releasing hormone (GHRH) and the hexapeptide GH-releasing peptide-6 (GHRP-6) induced a large increase in plasma GH levels. To gain further insight into the disrupted mechanism of GH regulation in obesity, we investigated whether the inhibition of somatostatinergic tone with pyridostigmine could further increase the GH response to combined administration of GHRH and GHRP-6. In normal subjects, administration of GHRH plus GHRP-6 induced a marked increase in plasma GH with a peak at 30 minutes (mean +/- SEM, 76.7 +/- 9.7 micrograms/L), which was similar to that obtained after pretreatment with pyridostigmine (74.7 +/- 9.4 micrograms/L). In obese patients, combined administration of GHRH plus GHRP-6 induced a clear increase in GH secretion with a peak at 15 minutes of 42.2 +/- 10.0 micrograms/L, which was also unaffected after pretreatment with pyridostigmine (38.4 +/- 5.8 micrograms/L). The GH response was lower in obese patients than in controls as assessed by the area under the curve after administration of both GHRH plus GHRP-6 (1,846 +/- 396 v 4,773 +/- 653, P < .01) and pyridostigmine plus GHRH plus GHRP-6 (1,989 +/- 372 v 5,098 +/- 679, P < .005). In conclusion, these data suggest that GHRP-6 can behave as a functional somatostatin antagonist, and that somatotrope responsiveness to the combined administration of GHRH plus GHRP-6 is largely independent of somatostatinergic tone.(ABSTRACT TRUNCATED AT 250 WORDS)

Hypothalamic mediation of reduced GH secretion in diabetic rats: evidence for reduced cholinergic inhibition of somatostatin release

The Goto-Kakizaki (GK) rat is a new model of diabetes mellitus and in this study we have characterized the diabetic and growth hormone (GH) secretory status of male GK rats at 6 and 16 weeks of age. We have also investigated the role of endogenous somatostatin (SS) and cholinergic manipulation on the GH responses to GH-releasing hormone (GHRH). GK rats were non-obese with significant fasting hyperglycaemia, hyperinsulinaemia and absent insulin responses to IV glucose. The GH response to GHRH was reduced at 16 weeks compared with normal, age-matched Wistar rats but no differences were observed at 6 weeks. Pretreatment of older rats (16 weeks) with anti-somatostatin antibodies (SS-Ab) significantly increased GH responses to GHRH in both normal and GK groups. Cholinergic augmentation with pyridostigmine (PD) reversed the blunted GH responses to GHRH in older GK rats but had no effect in the normal or young (6 weeks) GK rats. These results indicate that SS release mediates the blunted GH response to GHRH in GK rats and that reduced hypothalamic cholinergic signalling to the somatostatinergic neurone may mediate the increase in SS release. This view is supported by the results from in vitro studies in which cholinergic muscarinic blockade with pirenzepine (PIR) caused dose-related stimulation of SS release from normal rat hypothalami but was without effect on GK rat hypothalami. The cause of this alteration in hypothalamic function is, at present, unknown.

Effects of cholinergic modulation on serum insulin-like growth factor-I and its binding proteins in normal and diabetic subjects

OBJECTIVE

We wished to study alterations in serum insulin-like growth factor-I (IGF-I) and its binding proteins in subjects with insulin dependent diabetes mellitus (IDDM) and possible relations with metabolic and GH secretory status, before and after cholinergic modulation. In addition, we have investigated whether cholinergic modulation exerts any effects on IGF-I secretion, independently of any actions on GH secretory status.

DESIGN

All subjects received GH releasing hormone (GHRH) 1-44; 80 micrograms i.v.) alone and 60 minutes following 120 mg of pyridostigmine orally or 200 mg of pierenzepine orally. The three tests were carried out in random order at least one week apart. Blood was sampled at 15-minute intervals over 120 minutes.

PATIENTS

Twelve male subjects with IDDM and no clinical evidence of complications were selected on the basis of HbA1 levels to provide a wide range of metabolic control. Six normal male subjects were also studied.

MEASUREMENTS

Serum IGF-I, IGF-binding protein 1 (IGFBP-1) and IGFBP-3 were measured at regular intervals throughout the study. Fasting plasma glucose and HbA1 were measured before each study to provide measures of metabolic control.

RESULTS

Serum IGF-I and IGFBP-3 levels were significantly lower while serum IGFBP-I levels were significantly higher in the diabetic subjects. Pirenzepine had no effect on serum IGF-I, IGFBP-1 or IGFBP-3 in diabetic subjects but caused a significant increase in serum IGF-I and IGFBP-3 levels in normal subjects. Pyridostigmine had no effect on IGF-I, IGFBP-1 or IGFBP-3 in either diabetic or normal subjects. IGFBP-1 levels were significantly correlated with fasting plasma glucose but no correlation was demonstrated between measures of diabetic control and serum IGF-I or IGFBP-3 levels in diabetic subjects, nor was there any correlation between GH responses to GHRH alone or after pirenzepine or pyridostigmine pretreatment and serum levels of IGF-I, IGFBP-1 or IGFBP-3.

CONCLUSION

These data confirm that subjects with IDDM have reduced serum IGF-I and IGFBP-3 and increased IGFBP-1 levels, the latter being directly related to the fasting plasma glucose concentrations. The absence of any relation between changes in the IGF-I system and altered GH neuroregulation after cholinergic modulation suggests that changes in IGF-I are not the sole contributors to the altered GH neuroregulation which occurs in IDDM. We have also shown an acute stimulatory effect of pirenzepine on serum IGF-I and IGFBP-3 in normal subjects which is not present in IDDM although the underlying mechanisms is unknown.

Glucagon stimulates GH secretion after intramuscular but not intravenous administration. Evidence against the assumption that glucagon per se has a GH-releasing activity

In order to verify the true GH-releasing effect of glucagon and to explain the mechanism underlying this effect, we studied the effect of glucagon (GLU, 1 mg) administered either iv or im on both basal and GHRH (1 microgram/kg)-induced GH rise in 48 normal short children and adolescents. Moreover, the in vitro effect of GLU on rat anterior pituitary cells was studied. Intravenous administration of GLU induced no significant GH rise. On the other hand, im GLU administration induced a clear-cut GH increase (mean +/- SE GH peak after GLU vs placebo = 25.7 +/- 3.9 vs 10.1 +/- 3.6 micrograms/L, p < 0.01). Intravenous administration of GLU failed to modify the GHRH-induced GH rise either when coadministered with the neurohormone (35.2 +/- 4.1 vs 34.1 +/- 6.0 micrograms/L) or when given 60 min earlier (20.2 +/- 5.8 vs 21.1 +/- 8.3 micrograms/L). Differently from iv GLU, im GLU strikingly potentiated the GH response to GHRH given 90 min later (57.5 +/- 6.3 vs 24.7 +/- 9.1 micrograms/L, p < 0.01). Mean plasma glucose levels increased 30 min after GLU, administered either iv or im, and returned to basal levels 60 min later. GH secretion from dispersed rat pituitary cells was unaffected by incubation with GLU (10(-10)-10(-4) mol/L). Incubation of the cells with 10(-7) mol/L GHRH induced instead a clear-cut stimulation of GH release. In conclusion, our data demonstrate that glucagon per se has not GH-releasing activity as indicated by its uneffectiveness to release GH in vitro and after intravenous administration.(ABSTRACT TRUNCATED AT 250 WORDS)

Arginine but not pyridostigmine, a cholinesterase inhibitor, enhances the GHRH-induced GH rise in patients with anorexia nervosa

Pirenzepine, a muscarinic antagonist probably acting via stimulation of hypothalamic somatostatin release, abolishes the growth hormone releasing hormone (GHRH)-stimulated growth hormone (GH) rise in normal subjects but only blunts it in patients with anorexia nervosa (AN). This finding suggested the existence in AN of an alteration of cholinergic system and/or somatostatinergic tone. To further investigate these mechanisms, in 11 AN women patients (age 18.8 +/- 0.9 years; BMI 13.4 +/- 0.4) we studied the GH response alone (1 microgram/Kg IV as a bolus at 0 min) and combined with pyridostigmine (PD, 120 mg orally, 60 min before GHRH administration), a cholinesterase inhibitor, or arginine (ARG 30 g infused over 30 min starting at 0 min), two compounds probably acting via inhibition of hypothalamic somatostatin (SS) release. The GH response to GHRH preceded by a previous (120 min before) neurohormone administration also was studied. All these tests also were performed in 20 normal age-matched women (age 22.0 +/- 1.8 yrs; BMI20.1 +/- 2.4). Basal serum GH levels were higher in AN patients than in normal volunteers (NV) (10.3 +/- 3.4 versus 2.8 +/- 0.3 microgram/L; p < 0.001), whereas plasma IGF-I levels were lower in AN patients than in NV (43.3 +/- 10.6 versus 172.4 +/- 13.9 micrograms/L; p < 0.00001). In AN patients, GHRH administration induced a GH rise higher, though not significantly, than that in NV [delta area under the curve (AUC) 1173.6 +/- 167.6 versus 834.6 +/- 188.1 micrograms/L/h]. The GH response to the second of two consecutive GHRH boluses was lower (p < 0.01) than that of the first one either in AN patients or in NV (67.6 +/- 27.4 and 53.1 +/- 25.7 micrograms/L/h, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

Growth hormone responses to pyridostigmine in schizophrenia: evidence for cholinergic dysfunction

The hypothesis that increased central cholinergic neurotransmitter function may be present in schizophrenic illness and may underlie negative symptoms was tested using a neuroendocrine challenge approach. The cholinergic challenge used was the anticholinesterase pyridostigmine, thought to cause the release of growth hormone (GH) from the anterior pituitary by diminishing inhibitory somatostatin tone. Eleven patients, six neuroleptic-naive and five neuroleptic-free, satisfying DSM-III-R criteria for schizophrenia and 11 matched controls took part. Subjects received pyridostigmine (120 mg orally) and blood was sampled at 0, 60, 90, 120, and 180 min for GH estimation. Peak GH responses were significantly increased in the schizophrenic group compared to controls. There was no relationship between individual peak GH values and negative symptom ratings (Scale for the Assessment of Negative Symptoms). Neither could a relationship be established between other aspects of psychopathology or dyskinesias and GH responses. An increased pyridostigmine/GH response is also found in affective disorders and could be related to nonspecific symptoms common to all these diagnostic groups. This study suggests that schizophrenia may be associated with increased cholinergic neurotransmitter function but the relationship between this cholinergic dysfunction and schizophrenia may involve psychopathology not specific to schizophrenia.

A review of GHRH stimulation test in psychiatry

We critically reviewed controlled investigations of the growth hormone releasing hormone (GHRH) stimulation test in depression, anorexia nervosa, bulimia, panic disorder, schizophrenia, and Alzheimer's disease. Comparisons of GH responsiveness between patients and controls within each diagnostic category were equivocal and in some cases contradictory. Factors that may contribute substantially to the inconsistent findings within diagnostic categories include (1) the variability of GHRH-simulated GH among control groups; (2) the lack of uniformity in test procedures and outcome measures; and (3) the age and gender of subjects. In addition, the individual reproducibility of the GHRH stimulation test has not been adequately investigated and until the test's stability within subjects can be determined, the validity of interpretations resulting from the GHRH simulation test are in question.

The evaluation of hypothalamic somatostatin tone using pyridostigmine and thyrotropin releasing hormone in patients with acromegaly

To indirectly evaluate the hypothalamic somatostatin (SS) tone in patients with acromegaly, the effects of pyridostigmine (PD), a cholinesterase inhibitor which can inhibit hypothalamic SS secretion, on TRH-induced TSH secretion and the effects of SMS 201-995 on TSH or GH secretion were studied in acromegalic patients (31-69 yr, n = 10), normal young (21-24 yr, n = 7) and normal old male subjects (62-71 yr, n = 7). After pretreatment with PD (60 mg po, -30 min), normal young subjects showed significantly enhanced TSH responses to TRH (500 micrograms i.v., 0 min) compared to single administration of TRH, whereas normal old and acromegalic patients did not show such enhancement. Plasma TSH response to a single administration of TRH in acromegalic patients was significantly lower than that of normal young and old subjects. Although normal young and old subjects showed significantly enhanced GH responses to GHRH (100 micrograms i.v. at 0 min) after the pretreatment with PD (60 mg, -30 min), no such enhancement was observed in acromegalic patients. In contrast, the decrement in plasma TSH after SMS 201-995 administration was similar between normal subjects (5 young 5 old) and 7 acromegalic patients. Further, the maximal plasma GH decrement after administration was significantly greater in acromegalic patients than in the 5 normal young and 5 old subjects p < 0.01). In conclusion, hypothalamic SS tone does not appear to be elevated in acromegalic patients compared to normal young and probably old subjects.

Effects of direct and indirect acetylcholine receptor agonists on growth hormone secretion in humans

Cholinergic pathways in the central nervous system positively influence growth hormone (GH) secretion. In fact pyridostigmine, a cholinesterase inhibitor, enhances both basal and GH-releasing hormone (GHRH)-induced GH secretion while, conversely, pirenzepine, an antagonist of muscarinic M1 receptors, inhibits the GH response to GHRH and to other physiological and pharmacological stimuli. The effect of the cholinergic system on GH secretion probably takes place via inhibition of the release of endogenous somatostatin. In this study in 36 normal adults (26 males and 10 females, age 22-35 years) we compared the effects of three cholinesterase inhibitors (pyridostigmine, 120 mg p.o., n = 19; neostigmine, 10 micrograms/kg i.v., n = 6; physostigmine, 12.5 micrograms/kg i.v., n = 6) and bethanechol, a direct muscarinic receptor agonist that is mainly active on muscarinic M3 receptors (25 micrograms/kg i.v., n = 5), on both basal and GHRH (1 microgram/kg i.v.)-stimulated GH secretion. Pyridostigmine, neostigmine and physostigmine induced a significant GH increase (peak vs. basal levels, mean +/- S.E.: 10.4 +/- 1.6 vs. 0.6 +/- 0.2 micrograms/l, P = 0.0001; 13.3 +/- 1.2 vs. 0.5 +/- 1.1 micrograms/l, P = 0.004; and 14.9 +/- 3.1 vs. 2.7 +/- 1.1 micrograms/l, P = 0.025;, respectively). These drugs also induced a similar potentiation of the GH response to GHRH (peak: 48.3 +/- 5.6 vs. 16.2 +/- 2.2 micrograms/l, P = 0.0001; 49.2 +/- 2.2 vs. 19.9 +/- 5.1 micrograms/l, P = 0.006; and 76.9 +/- 12.4 vs. 18.1 +/- 5.3 micrograms/l, P = 0.001, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation of glucose and growth hormone responses to meals and exercise in type 1 diabetes by cholinergic muscarinic blockade

Anticholinergic drugs suppress nocturnal and exercise-related growth hormone (GH) secretion in Type 1 diabetes; nocturnal GH suppression is associated with a fall in fasting plasma glucose levels. The aim of this study was to assess the effect of GH suppression on glucose levels following a period of meals and exercise in physiological pattern. Six Type 1 diabetic men recruited from the outpatient clinic were studied in random order at least 1 week apart. After an overnight fast subjects received two-thirds of their usual subcutaneous insulin and either 200 mg oral pirenzepine or placebo at time 0 min. Between 90 and 120 min subjects exercised continuously on an ergometric cycle. Standard meals or snacks were eaten at 30, 150, 270, and 390 min. Venous blood was collected from an indwelling cannula between 0 and 570 min. The mean incremental rise in plasma glucose after breakfast (delta peak/90 min) was 2.6 +/- 0.5 (mean +/- SEM mmol l-1 (pirenzepine) vs 4.5 +/- 0.8 (placebo)), p < 0.05. Following exercise the fall in plasma glucose (delta gluc90-240 min) was 6.4 +/- 1.9 (pirenzepine) vs 2.0 +/- 1.3 (placebo), p < 0.005. The exercise-related peak rise in GH was 12.6 +/- 3.3 (pirenzepine) vs 28.5 +/- 6.0 mU l-1 (placebo), p = 0.08. Excluding one outlying result there was an inverse correlation between the integrated exercise-related increase in GH between 90 and 240 min and the fall in glucose over the corresponding time period (n = 11, r = -0.75, p = 0.008).(ABSTRACT TRUNCATED AT 250 WORDS)

BAP/SKB Young Psychopharmacologist Award Towards a neuroendocrinology of obsessive-compulsive disorder

Neuroendocrine research has made an important contribution to the understanding of psychiatric illness in vivo. This review is a summary of the recent neuroendocrine studies on obsessive-compulsive disorder (OCD) patients carried out at the Departments of Psychiatry at Trinity College Medical School and St. Patrick's Hospital, Dublin, Ireland. We found that both serotonergic and cholinergic abnormalities were present in OCD, while noradrenergic responsivity and hypothalmic-pituitary axis function were normal. The data suggests that OCD may have a unique neuroendocrinology.

Growth hormone response to L-dopa and pyridostigmine in women with polycystic ovarian syndrome

OBJECTIVE

To evaluate the GH secretion and clarify the factors influencing the GH secretion in women with polycystic ovarian syndrome (PCOS).

DESIGN

Comparison of the GH response to L-dopa with or without pyridostigmine (inhibitor of acetylcholinesterase) pretreatment and insulin response to oral glucose tolerance test in patients with PCOS and matched controls.

SETTING

Outpatients and healthy volunteers studied at a clinical research unit of a university hospital.

PATIENTS, PARTICIPANTS

Ten women with PCOS and 9 controls with regular cycles were recruited.

INTERVENTIONS

After an overnight fast, each subject underwent a GH stimulation test with L-dopa with or without pyridostigmine pretreatment. Plasma insulin and glucose levels were measured after a 75-g glucose load.

MAIN OUTCOME MEASURES

Plasma GH, insulin-like growth factor I (IGF-I), insulin, and nonesterified fatty acids.

RESULTS

Growth hormone responses and GH area under the response curve (AUC) to L-dopa were significantly lower in PCOS than those in controls. Pyridostigmine enhanced the GH response to L-dopa significantly in PCOS. Insulin responses and insulin AUC to oral glucose load were significantly higher in PCOS than those in controls. Plasma IGF-I levels of PCOS were significantly higher than controls. Insulin AUC had a positive correlation with plasma IGF-I levels but an inverse correlation with GH AUC in PCOS and controls.

CONCLUSION

Our result indicated that decreased GH secretion of PCOS may be associated with a high somatostatin activity and a high plasma IGF-I level.

Effect of the enhancement of the cholinergic tone by pyridostigmine on the exercise-induced growth hormone release in man

We have evaluated the effect of pyridostigmine (PD), a cholinergic agonist, on the growth hormone (GH) response to physical exercise (EXC) in nine healthy volunteers. PD administration and EXC caused a similar increase of GH secretion to mean (+/- SE) peak values of 5.3 +/- 0.9 and 6.5 +/- 1.2 micrograms/l, respectively. Pretreatment with PD caused a significant augmentation of the EXC-induced GH release evaluated both as maximum peak (13.5 +/- 2.1 micrograms/l, p < 0.01 vs EXC) and as area under the secretory curve (EXC = 292.6 +/- 41.9 micrograms.min.l; PD + EXC = 587.3 +/- 68.9 micrograms.min.l, p < 0.005). The action of PD on GH secretion was additive to that of EXC since the sum of the GH responses to PD and EXC was not significantly different from the response obtained during PD + EXC. Whether PD and EXC act through a common final pathway, i.e. inhibition of endogenous somatostatin release, or the EXC-induced GH secretion involves stimulation of endogenous GHRH remains matter of investigation.

Gene expression and chemical diversity in hypothalamic neurosecretory neurons

Hypothalamic neurosecretory neurons transcribe, translate, store, and secrete a large number of chemical messengers. The neurons contain hypothalamic signal substances that regulate the secretion of anterior pituitary hormones as well as the neurohypophysial peptides vasopressin and oxytocin. In addition to the classical hypophysiotropic hormones, a large number of neuropeptides and classical transmitters of amine and amino acid nature are present in the same cells. This is particularly evident in the magnocellular neurons of the supraoptic and paraventricular nuclei, and in parvocellular neurons of the arcuate and paraventricular nuclei. The changes in gene expression induced by experimental manipulations and the colocalization chemical messengers in hypothalamic neurosecretory neurons and its possible significance is summarized in this review.

Galanin-induced growth hormone secretion in conscious rats: evidence for a possible involvement of somatostatin

Intracerebroventricular administration of galanin (GAL) potently evoked growth hormone (GH) secretion in conscious male rats. Pretreatments with neostigmine and cysteamine blunted the GAL-induced GH secretion. Pretreatment of animals with a specific anti-somatostatin serum significantly inhibited the GAL-induced GH secretion. On the contrary, GH-releasing hormone-induced GH secretion was significantly enhanced with cysteamine and anti-somatostatin serum. These results suggest that somatostatin is involved in GAL-induced GH secretion in rats.

Effect of galanin on the growth hormone (GH) response to GH-releasing hormone in patients with Cushing's disease

Attenuated plasma GH secretion during sleep and blunted GH responses to provocative stimuli have been observed in patients with Cushing's disease. Synthetic porcine galanin elicits GH secretion when given alone, and enhances the GH response to GHRH in normal human subjects. The aim of our study was to investigate the effects of galanin on the GH response to GHRH in patients with Cushing's disease. We studied 5 female subjects with untreated active Cushing's disease caused by micro-pituitary adenomas (age 43 +/- 6.7 years; BMI 30 +/- 0.7 kg/m2). Four normal adult females, matched for age and body weight with the patients with Cushing's disease, were studied as controls. Subjects underwent in random order: (1) infusion of synthetic porcine galanin IV, 500 micrograms in 100 mL; (2) infusion of saline, IV, 100 mL. A bolus of human GHRH(1-29)NH2 (Geref, Serono, Italy), 100 micrograms in 1 mL saline, was injected IV at 0 minutes. Patients with Cushing's disease showed blunted GH peaks after GHRH (1.2 +/- 0.4 micrograms/L) during saline infusion, as compared to normal controls (24.6 +/- 4.6 micrograms/L; p < 0.05). During galanin infusion a significantly enhanced GH response to GHRH, as compared with saline infusion, was observed in control subjects (GH peak: 51.4 +/- 9.8 micrograms/L; p < 0.05), but not in patients with Cushing's disease (GH peak: 2.3 +/- 0.6 micrograms/L). GH levels were significantly lower both after saline and after galanin in patients with Cushing's disease as compared to normal controls. Our data demonstrate that galanin is not able to enhance the GH response to GHRH in patients with Cushing's disease. That galanin cannot reverse this effect suggests that the mechanism of action of galanin is not via a decrease in somatostatin release by the hypothalamus.

Eptastigmine augments basal and GHRH-stimulated growth hormone release in young and old dogs

The aim of the present work was to evaluate the effect on the growth hormone (GH) secretion of eptastigmine, a new long-acting cholinesterase inhibitor, in unanesthetized beagle dogs. In a first study, 5 young dogs were given single doses (0.5, 1.0, and 2.0 mg/kg, i.m.) of the drug or saline in a randomized cross-over manner. Blood samples were collected immediately before and, at regular intervals, until 150 min after drug injection. GH plasma concentrations were determined by radioimmunoassay. Plasma cholinesterase activity was measured with a potentiometric method. There was a significant logistic relationship (r = 0.601, P < 0.01) between the administered dose of eptastigmine and the log-transformed areas under the GH plasma concentration-time curve (AUC) with a calculated ED50 for eptastigmine of 0.63 +/- 0.36 mg/kg. There was also a significant linear relationship (r = 0.630, P < 0.01) between log-transformed AUC of GH levels and AUC of plasma cholinesterase activity. In a second study we evaluate the ability of eptastigmine (2.0 mg/kg, i.m.) to potentiate the GH-releasing effect of the GH-releasing hormone (GHRH, 2.0 micrograms/kg, i.v.) in young and old dogs. Eptastigmine was administered 45 min before GHRH and blood collected every 15 min until 90 min after GHRH injection. In young dogs, maximum GH plasma levels (Cmax) were 6.1 +/- 1.0 ng/ml after GHRH compared to 22.5 +/- 2.3 ng/ml after GHRH preceded by eptastigmine (P < 0.01). In old animals, Cmax were 4.6 +/- 1.4 ng/ml after GHRH vs 13.2 +/- 7.4 ng/ml after combined administration of GHRH and eptastigmine (P < 0.05). These data indicate that eptastigmine is very effective in augmenting basal and stimulated GH secretion in old dog. The good activity also shown in old animals suggests a potential use of this drug to reverse the age-dependent decline in GH secretion responsible for many involutional changes of aging.