A novel non-peptidyl growth hormone secretagogue

An insight into the multifunctional role of ghrelin and structure activity relationship studies of ghrelin receptor ligands with clinical trials

Ghrelin is a multifunctional gastrointestinal acylated peptide, primarily synthesized in the stomach and regulates the secretion of growth hormone and energy homeostasis. It plays a central role in modulating the diverse biological, physiological and pathological functions in vertebrates. The synthesis of ghrelin receptor ligands after the finding of growth hormone secretagogue developed from Met-enkephalin led to reveal the endogenous ligand ghrelin and the receptors. Subsequently, many peptides, small molecules and peptidomimetics focusing on the ghrelin receptor, GHS-R1a, were derived. In this review, the key features of ghrelin's structure, forms, its bio-physiological functions, pathological roles and therapeutic potential have been highlighted. A few peptidomimetics and pseudo peptide synthetic perspectives have also been discussed to make ghrelin receptor ligands, clinical trials and their success.

Ghrelin as a treatment for amyotrophic lateral sclerosis

Ghrelin is a gut hormone best known for its role in regulating appetite and stimulating the secretion of the anabolic hormone growth hormone (GH). However, there is considerable evidence to show wider-ranging biological actions of ghrelin that favour improvements in cellular and systemic metabolism, as well as neuroprotection. Activation of these ghrelin-mediated pathways may alleviate pathogenic processes that are assumed to contribute to accelerated progression of disease in patients with neurodegenerative disease. Here, we provide a brief overview on the history of discoveries that led to the identification of ghrelin. Focussing on the neurodegenerative disease amyotrophic lateral sclerosis (ALS), we also present an overview of emerging evidence that suggests that ghrelin and ghrelin mimetics may serve as potential therapies for the treatment of ALS. Given that ALS is a highly heterogeneous disease, where multiple disease mechanisms contribute to variability in disease onset and rate of disease progression, we speculate that the wide-ranging biological actions of ghrelin might offer therapeutic benefit through modulating multiple disease-relevant processes observed in ALS. Expanding on the well-known actions of ghrelin in regulating food intake and GH secretion, we consider the potential of ghrelin-mediated pathways in improving body weight regulation, metabolism and the anabolic and neuroprotective actions of GH and insulin-like growth factor-1 (IGF-1). This is of clinical significance because loss of body weight, impairments in systemic and cellular metabolism, and reductions in IGF-1 are associated with faster disease progression and worse disease outcome in patients with ALS.

Effect of the Orally Active Growth Hormone Secretagogue MK-677 on Somatic Growth in Rats

PURPOSE

Growth hormone secretagogues (GHSs) possess the ability to release growth hormone (GH) in the body. This study aimed to investigate the effects of MK-677, an orally active GHS, on somatic growth in rats.

MATERIALS AND METHODS

The serum levels of GH were measured after oral administration of MK-677 to confirm GH stimulatory effects. Body weight, body length, tibia length, epiphyseal plate width, and serum levels of insulin-like growth factor (IGF)-I were measured after oral administration of 4 mg/kg of MK-677 for 6 weeks to investigate growth-promoting effects.

RESULTS

Oral administration of MK-677 at 4 mg/kg increased peak GH concentrations by 1.8-fold, compared to baseline. However, oral administration of MK-677 for 6 weeks did not increase body growth or serum levels of IGF-I. At 6 weeks after treatment, the GH response to MK-677 was abolished. Pituitary GH mRNA and hypothalamic GH-releasing hormone mRNA, and GH secretagogue receptor (GHSR) mRNA expression in the pituitary and hypothalamus did not differ between the control and treatment group. Somatostatin (SST) mRNA expression in the hypothalamus was markedly increased in the treatment group, whereas SST receptor (SSTR)-2 mRNA expression in the pituitary gland was decreased. Protein expression of hypothalamic GHSR, SST, and pituitary SSTR-2 showed patterns similar to those for mRNA expression.

CONCLUSION

Our results suggest that prolonged administration of MK-677 in rats does not promote growth despite the GH stimulatory effect of MK-677, which may be related to increased expression of SST in the hypothalamus. Further studies are needed to overcome the observed desensitization to GHS.

Ghrelin and the Cardiovascular System

Ghrelin is a small peptide released primarily from the stomach. It is a potent stimulator of growth hormone secretion from the pituitary gland and is well known for its regulation of metabolism and appetite. There is also a strong relationship between ghrelin and the cardiovascular system. Ghrelin receptors are present throughout the heart and vasculature and have been linked with molecular pathways, including, but not limited to, the regulation of intracellular calcium concentration, inhibition of proapoptotic cascades, and protection against oxidative damage. Ghrelin shows robust cardioprotective effects including enhancing endothelial and vascular function, preventing atherosclerosis, inhibiting sympathetic drive, and decreasing blood pressure. After myocardial infarction, exogenous administration of ghrelin preserves cardiac function, reduces the incidence of fatal arrhythmias, and attenuates apoptosis and ventricular remodeling, leading to improvements in heart failure. It ameliorates cachexia in end-stage congestive heart failure patients and has shown clinical benefit in pulmonary hypertension. Nonetheless, since ghrelin's discovery is relatively recent, there remains a substantial amount of research needed to fully understand its clinical significance in cardiovascular disease.

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.

Semagacestat, a γ-secretase inhibitor, activates the growth hormone secretagogue (GHS-R1a) receptor

OBJECTIVES

Semagacestat, is a γ-secretase inhibitor, which belongs to a class of drugs that are being developed as therapeutic agents for Alzheimer's disease (AD). This study aims to evaluate another potential effect of semagacestat, namely its ability to stimulate the growth hormone secretagogue receptor (GHS-R1a), which may also contribute to its therapeutic efficacy.

METHODS

The GHS-R1a-activating potential of semagacestat and its synthetic precursor was assessed in an in vitro calcium mobilization assay in cells expressing the GHS-R1a receptor and compared with that of the endogenous peptide GHS-R1a agonist, acyl-ghrelin, as well as the non-peptidyl synthetic GHS-R1a agonist, MK0677. In addition, semagacestat-mediated cellular trafficking of the GHS-R1a receptor, expressed as an enhanced green fluorescent protein tagged fusion protein, was analysed.

KEY FINDINGS

Semagacestat and its precursor were shown to activate the GHS-R1a receptor, as demonstrated by an increased GHS-R1a-mediated intracellular calcium influx. Moreover, a synergistic GHS-R1a receptor activation was shown following a combined exposure to ghrelin and semagacestat. In addition, GHS-R1a receptor internalization was observed upon exposure to semagacestat and its precursor.

CONCLUSION

These data suggest a novel molecular mechanism of action for semagacestat via modest GHS-R1a receptor activation. Studies focusing on the relative functional consequence of such effects in vivo are now warranted.

The role of ghrelin in the control of energy balance

Ghrelin is the only potent orexigenic peptide in circulation. It stimulates food intake and leads to positive energy balance, adipogenesis, and body weight gain. However, the physiological significance of ghrelin in the regulation of energy homeostasis is controversial, since loss of ghrelin function in rodents does not necessarily lead to anorexia and weight loss. In this chapter, we discuss the metabolic function of ghrelin and are highlighting recent findings including the discovery and function of ghrelin-acylating enzyme ghrelin O-acyltransferase (GOAT). Based on available published data, we conclude that ghrelin is a principally important endogenous regulator of energy balance, which however may affect both food intake and systemic metabolism via independent mechanisms. Importantly, ghrelin, when acylated by GOAT, might represent a key molecular link between the sensing of consumed calories and the neuroendocrine control of energy homeostasis. Thus, agents antagonizing the action of ghrelin may have therapeutic potential in the therapy of obesity.

The effects of ghrelin/GHSs on AVP mRNA expression and release in cultured hypothalamic cells in rats

Ghrelin, the endogenous ligand for growth hormone secretagogues (GHSs) receptor (GHS-R), increases adrenocorticotropin (ACTH) and cortisol (corticosterone) as well as GH secretion in humans and animals. However, the site of GHSs action to induce ACTH secretion is not fully understood. To clarify the mechanisms of the action of ghrelin/GHSs on ACTH secretion, we analyzed the effects of KP-102 and ghrelin on the mRNA expression and release of corticotropin releasing factor (CRF) and arginine vasopressin (AVP), ACTH secretagogues, in monolayer-cultured hypothalamic cells of rats. Incubation of cells with KP-102 for 4h and 8h and with ghrelin for 4h significantly increased AVP mRNA expression and release without changing CRF mRNA expression. CRF levels in culture media were undetectable. Suppression of GHS-R expression by siRNA blocked ghrelin- and KP-102-induced AVP mRNA expression and release. NPY significantly increased AVP mRNA expression and release. Furthermore, treatment of cells with anti-NPY IgG blocked KP-102-induced AVP mRNA expression and release. We previously reported that KP-102 significantly increases NPY mRNA expression in cultured hypothalamic cells. Taken together, these results suggest that ACTH secretion by ghrelin/GHSs is induced mainly through hypothalamic AVP, and that NPY mediates the action of ghrelin/GHSs.

Reduction in hypophyseal growth hormone and prolactin expression due to deficiency in ghrelin receptor signaling is associated with Pit-1 suppression: relevance to the immune system

In mice and in rats, reduced levels of the growth hormone secretagogue receptor (GHS-R1a) results in reduced body weight and lower levels of serum insulin-like growth factor I (IGF-I). However, the mechanism leading to these impairments has not been elucidated. Studies in primary cultures of pituitary cells from very young mice have shown that GHS-R1a agonists, including ghrelin, increase expression of the pituitary-specific transcription factor (Pit-1) that is critical for differentiation of pituitary cells into somatotrophs, lactotrophs, and thyrotrophs. Hence, we hypothesized that ablation of Ghsr would reduce Pit-1 expression and as a consequence reduce growth hormone (GH) production explaining the lower body weight of Ghsr-/- mice. Here, we now show that Pit-1 mRNA levels are significantly lower in the pituitary gland of Ghsr-/- mice compared to wild-type littermates and also with advancing age. This Pit-1 loss is associated with reduced GH mRNA and fewer GH producing cells. To determine whether reduced GH is caused by reduced expression of Pit-1 in Ghsr-/- mice, we also measured prolactin (PRL) expression in the pituitary gland and in the circulation. PRL mRNA was significantly reduced in Ghsr-/- mice compared to wild-type littermates and fewer cells expressed PRL. The reduction in expression of both GH and PRL is consistent with a Pit-1 regulated pathway and demonstrates that the GHS-R has an important role in the pituitary gland as a modulator of Pit-1 expression and provides a possible mechanism to explain the lower plasma IGF-1 and modestly reduced body weight exhibited by Ghsr-/- mice. We also believe that lower systemic and lymphoid hormone expression may also account, in part, for the enhanced thymic involution and reduced thymic output in Ghsr-/- mice.

Recent Developments in Ghrelin Receptor Ligands

The 28-amino acid peptide ghrelin is a neuroendocrine hormone synthesized primarily in the stomach. It stimulates growth hormone secretion and appetite, thus promoting food intake and body-weight gain. The pharmacological properties of this peptide are mediated by the growth hormone secretagogue receptor type 1a (GHS-R1a). Given its wide spectrum of biological activities, it is evident that the discovery of ghrelin and its receptor has opened up many perspectives in the fields of neuroendocrine and metabolic research and has had an influence on such fields of internal medicine as gastroenterology, oncology, and cardiology. It is therefore increasingly likely that synthetic, peptidyl, and nonpeptidyl GHS-R1a ligands, acting as agonists, partial agonists, antagonists, or inverse agonists, could have both clinical and therapeutic potential. This review summarizes the various types of GHS-R1a ligands that have been described in the literature and discusses the recent progress made in this research area.

Discovery and pharmacological evaluation of growth hormone secretagogue receptor antagonists

The discovery and pharmacological evaluation of potent, selective, and orally bioavailable growth hormone secretagogue receptor (GHS-R) antagonists are reported. Previously, 2,4-diaminopyrimidine-based GHS-R antagonists reported from our laboratories have been shown to be dihydrofolate reductase (DHFR) inhibitors. By comparing the X-ray crystal structure of DHFR docked with our GHS-R antagonists and GHS-R modeling, we designed and synthesized a series of potent and DHFR selective GHS-R antagonists with good pharmacokinetic (PK) profiles. An amide derivative 13d (Ca2+ flux IC50 = 188 nM, [brain]/[plasma] = 0.97 @ 8 h in rat) showed a 10% decrease in 24 h food intake in rats, and over 5% body weight reduction after 14-day oral treatment in diet-induced obese (DIO) mice. In comparison, a urea derivative 14c (Ca2+ flux IC50 = 7 nM, [brain]/[plasma] = 0.0 in DIO) failed to show significant effect on food intake in the acute feeding DIO model. These observations demonstrated for the first time that peripheral GHS-R blockage with small molecule GHS-R antagonists might not be sufficient for suppressing appetite and inducing body weight reduction.

Structure–activity relationship studies on tetralin carboxamide growth hormone secretagogue receptor antagonists

The structure-activity relationship studies on a series of tetralin carboxamide growth hormone secretagogue receptor (GHS-R) antagonists are discussed. It was found that certain 2-alkoxycarbonylamino substituted tetralin carboxamides are potent, selective, and orally bioavailable GHS-R antagonists.

Synthesis and structure–activity relationships of isoxazole carboxamides as growth hormone secretagogue receptor antagonists

A series of isoxazole carboxamide derivatives has been developed as potent ghrelin receptor antagonists. The synthesis and structure-activity relationship (SAR) are described.

Discovery of Tetralin Carboxamide Growth Hormone Secretagogue Receptor Antagonists via Scaffold Manipulation

A case study of rational design of an efficient, specific, and proprietary molecular scaffold based on the structure-activity relationship (SAR) information on a screening hit is described. Potent, selective, and orally bioavailable tetralin carboxamide growth hormone secretagogue receptor (GHS-R) antagonists were discovered. Union of rational design and high throughput synthesis provided a quick access to high quality chemical leads.

Novel isoxazole carboxamides as growth hormone secretagogue receptor (GHS-R) antagonists

Novel isoxazole carboxamides have been identified as growth hormone secretagogue receptor (GHS-R) antagonists. Substituent modification off the 5-position of the isoxazole ring led to analogues with potent binding affinity and functional antagonism of GHS-R. A potent analogue (32) with high aqueous solubility and good GPCR selectivity was also identified as a potential pharmacological tool for in vivo studies.

Biological, physiological, pathophysiological, and pharmacological aspects of ghrelin

Ghrelin is a peptide predominantly produced by the stomach. Ghrelin displays strong GH-releasing activity. This activity is mediated by the activation of the so-called GH secretagogue receptor type 1a. This receptor had been shown to be specific for a family of synthetic, peptidyl and nonpeptidyl GH secretagogues. Apart from a potent GH-releasing action, ghrelin has other activities including stimulation of lactotroph and corticotroph function, influence on the pituitary gonadal axis, stimulation of appetite, control of energy balance, influence on sleep and behavior, control of gastric motility and acid secretion, and influence on pancreatic exocrine and endocrine function as well as on glucose metabolism. Cardiovascular actions and modulation of proliferation of neoplastic cells, as well as of the immune system, are other actions of ghrelin. Therefore, we consider ghrelin a gastrointestinal peptide contributing to the regulation of diverse functions of the gut-brain axis. So, there is indeed a possibility that ghrelin analogs, acting as either agonists or antagonists, might have clinical impact.

High constitutive signaling of the ghrelin receptor--identification of a potent inverse agonist

Ghrelin is a GH-releasing peptide that also has an important role as an orexigenic hormone-stimulating food intake. By measuring inositol phosphate turnover or by using a reporter assay for transcriptional activity controlled by cAMP-responsive elements, the ghrelin receptor showed strong, ligand-independent signaling in transfected COS-7 or human embryonic kidney 293 cells. Ghrelin and a number of the known nonpeptide GH secretagogues acted as agonists stimulating inositol phosphate turnover further. In contrast, the low potency ghrelin antagonist, [D-Arg1,D-Phe5,D-Trp7,9,Leu11]-substance P was surprisingly found to be a high potency (EC50 = 5.2 nm) full inverse agonist as it decreased the constitutive signaling of the ghrelin receptor down to that observed in untransfected cells. The homologous motilin receptor functioned as a negative control as it did not display any sign of constitutive activity; however, upon agonist stimulation the motilin receptor signaled as strongly as the unstimulated ghrelin receptor. It is concluded that the ghrelin receptor is highly constitutively active and that this activity could be of physiological importance in its role as a regulator of both GH secretion and appetite control. It is suggested that inverse agonists for the ghrelin receptor could be particularly interesting for the treatment of obesity.

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.

Skeletal growth acceleration with growth hormone secretagogues in transgenic growth retarded rats: pattern-dependent effects and mechanisms of desensitization

The transgenic growth retarded (Tgr) rat is the first genetic model of growth hormone (GH) deficiency whose growth can be accelerated with exogenous GH secretagogues (GHSs). In this study, we have demonstrated that GHS-receptor (GHS-R) mRNA expression in the arcuate nucleus of Tgr rats was not significantly different to that in wild-type littermates. We have confirmed that GHS-induced elevation in body weight gain was accompanied by acceleration of skeletal growth, and that the effects of the GHS, GHRP-6, were both dose- and pattern-dependent. The growth response with continuous infusion of GHRP-6 was transient, accompanied by suppression of GH and corticosterone responses to bolus injection of GHRP-6. This desensitization occurred without downregulation of arcuate GHS-R mRNA expression, but was accompanied by elevated periventricular somatostatin mRNA expression. In contrast, pulsatile (3-hourly) infusion of GHRP-6 produced sustained growth and GH responses, which were accompanied by suppression of corticosterone responses and elevated arcuate GH-releasing factor (GRF) mRNA expression. Skeletal growth was further accelerated by coinfusion of GRF, but significant depletion of pituitary GH stores suggested that this growth rate may not be sustainable. These experiments confirm the importance of the Tgr rat for investigating the growth promoting potential of the GHSs in the context of GH-deficient dwarfism, and suggest that elevated somatostatin expression may mediate the suppression of the GRF-GH and hypothalamo-pituitary-adrenal axes following continuous GHRP-6 treatment.

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.

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

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

Novel orally active growth hormone secretagogues

A novel class of growth hormone-releasing compounds with a molecular weight in the range from 500 to 650 has been discovered. The aim of this study was to obtain growth hormone secretagogues with oral bioavailability. By a rational approach we were able to reduce the size of the lead compound ipamorelin (4) and simultaneously to reduce hydrogen-bonding potential by incorporation of backbone isosters while retaining in vivo potency in swine. A rat pituitary assay was used for screening of all compounds and to evaluate which compounds should be tested further for in vivo potency in swine and oral bioavailability, fpo, in dogs. Most of the tested compounds had fpo in the range of 10-55%. In vivo potency in swine after iv dosing is reported, and ED50 was found to be 30 nmol/kg of body weight for the most potent compound.

Pituitary and extrapituitary action sites of the novel nonpeptidyl growth hormone (GH) secretagogue L-692,429 in the chicken

Chickens were used as a model to further analyze the efficacy and specificity of L-692,429, a novel nonpeptidyl mimic of growth hormone (GH)-releasing peptide-6 (GHRP-6), which is a specific GH-releasing secretagogue in mammals. Actions at the level of the pituitary and the hypothalamus were studied. Pituitaries isolated from 1-day-old (C1) chicks responded in a dose-dependent manner to L-692,429 (ED50 = 10 nM). Using equimolar concentrations of thyrotropin-releasing hormone (TRH), human GH-releasing hormone (hGHRH1-29), and L-692,429 (10 nM), L-692,429 had 20-25% the in vitro potency of the two endogenous releasing factors. There was an additive effect between hGHRH1-29 (10 nM) and L-692,429 (10 or 100 nM) on GH release from C1 pituitaries but no such additive effect was observed when pituitaries were exposed to both TRH (10 nM) and L-692,429 (100 nM). An acute challenge with 50 microg L-692,429 resulted in increased plasma GH levels within 5 min, which remained elevated for up to 15 min (C1 chickens). This increase in GH was accompanied by a drop in hypothalamic TRH content by 5 min. Hypothalamic somatostatin (SRIH) content did not change. Plasma corticosterone concentrations were increased following L-692,429 treatment, whereas plasma alpha-subunit, T4, and T3 levels were unchanged. To confirm the role of the decreased hypothalamic TRH concentrations in the GH-releasing activity of L-692,429 in the chicken, chickens (C1) were pretreated with normal rabbit serum (NRS) or a TRH antiserum (1/50) 1 h prior to the L-692,429 challenge. Both groups showed an increase in circulating GH but the increase was within 5 min inhibited by the TRH antiserum pretreatment, whereas no differences were noted in plasma corticosterone levels. It is concluded that in the chicken the GH secretagogue L-692,429 has a dual action site: (1) directly at the level of the pituitary and (2) centrally through an increase in hypothalamic TRH release.

Growth hormone-releasing peptides and their analogs

Growth hormone-releasing peptides (GHRPs) are a series of hepta (GHRP-1)- and hexapeptides (GHRP-2, GHRP-6, Hexarelin) that have been shown to be effective releasers of GH in animals and humans. More recently, a series of nonpeptidyl GH secretagogues (L-692,429, L-692,585, MK-0677) were discovered using GHRP-6 as a template. Some cyclic peptides as well as penta-, tetra-, and pseudotripeptides have also been described. This review summarizes recent developments in our understanding of the GHRPs, as well as the current nonpeptide pharmacologic analogs. GHRPs and their analogs have no structural homology with GHRH and act via specific receptors present at either the pituitary or the hypothalamic level. The GHRP receptor has recently been cloned and it does not show sequence homology with other G-protein-coupled receptors known so far. This evidence strongly suggests the existence of a natural GHRP-like ligand which, however, has not yet been found. Although the exact mechanism of action of GHRPs has not been fully established, there is probably a dual site of action on both the pituitary and the hypothalamus, possibly involving regulatory factors in addition to GHRH and somatostatin. Moreover, the possibility that GHRPs act via an unknown hypothalamic factor (U factor) is still open. The marked GH-releasing activity of GHRPs is reproducible and dose-related after intravenous, subcutaneous, intranasal, and even oral administration. The GH-releasing effect of GHRPs is the same in both sexes, but undergoes age-related variations. It increases from birth to puberty and decreases in aging. The GH-releasing activity of GHRPs is synergistic with that of GHRH and not affected by opioid receptor antagonists, while it is only blunted by inhibitory influences that are known to nearly abolish the effect of GHRH, such as neurotransmitters, glucose, free fatty acids, glucocorticoids, rhGH, and even exogenous somatostatin. GHRPs maintain their GH-releasing effect in somatotrope hypersecretory states, such as acromegaly, anorexia nervosa, and hyperthyroidism. On the other hand, GHRPs and their analogs have been reported to be effective in idiopathic short stature, in some situations of GH deficiency, in obesity, and in hypothyroidism, while in patients with pituitary stalk disconnection and in Cushing's syndrome the somatotrope responsiveness to GHRPs is almost absent. A potential role in the treatment of short stature, aging, catabolic states, and dilated cardiomyopathy has been envisaged.

Distribution of mRNA encoding the growth hormone secretagogue receptor in brain and peripheral tissues

Growth hormone release is under tight control by two hypothalamic hormones: growth hormone-releasing hormone and somatostatin. In addition, synthetic growth hormone secretagogues have also been shown to regulate growth hormone release through the growth hormone secretagogue receptor (GHS-R), suggesting the existence of an additional physiological regulator for growth hormone release. To understand the physiological role of the GHS-R in more detail, we mapped the expression of mRNA for the receptor by in situ hybridization and RNase protection assays using rat and human tissues. In the rat brain, the major signals were detected in multiple hypothalamic nuclei as well as in the pituitary gland. Intense signals were also observed in the dentate gyrus of the hippocampal formation. Other brain areas that displayed localized and discrete signals for the receptor include the CA2 and CA3 regions of the hippocampus, the substantia nigra, ventral tegmental area, and dorsal and median raphe nuclei. In resemblance to the results from rat brain, RNase protection assays using human tissues revealed specific signals in pituitary, hypothalamus and hippocampus. Moreover, a weak signal was noted in the pancreas. The demonstration of hypothalamic and pituitary localization of the GHS-R is consistent with its role in regulating growth hormone release. The expression of the receptor in other central and peripheral regions may implicate its involvement in additional as yet undefined physiological functions.

Hepatic extraction of hexarelin, a new peptidic GH secretagogue, in the isolated perfused rat liver

PURPOSE

To assess the hepatic extraction of hexarelin (HEX), a novel peptidyl GH secretagogue, in the isolated perfused rat model and document the in vitro binding of HEX to plasma proteins using plasma from rats, dogs, pigs, and humans.

METHODS

Rat liver was perfused in situ using a recirculating system. The recirculating perfusate consisted of a Krebs Henseleit buffer containing 20% (v/v) prewashed bovine red blood cells, 1% albumin, and lg/L dextrose. Three HEX concentrations of 5, 50, and 500 ng/ml were examined. In vitro plasma binding was determined by the ultrafiltration method.

RESULTS

The disappearance rate constant (K), half-life (t1/2), clearance (Cl), and hepatic extraction ratio (E) were: K = 0.013-0.014 min-1, t1/2 = 45-55 min, Cl = 0.345-0.401 ml/min/g liver, and E = 19-21% for the different concentrations of HEX. A linear increase in AUC (270-24334 min pmol/ml) was observed with increasing concentrations. Binding of HEX to plasma proteins of rats, dogs, pigs, and humans was 68.7 +/- 0.8%, 78.7 +/- 0.6%, 67.3 +/- 0.7%, and 65.2 +/- 0.6% respectively. Plasma binding was concentration-independent in the range between 0.003-3 microM for the four species examined.

CONCLUSIONS

These results show that 1) the hepatic extraction of HEX is low, 2) the hepatic clearance is concentration independent up to 500 ng HEX/ml of perfusate, and 3) the plasma protein binding of HEX is significant over the dose range studied. HEX exhibits a low hepatic extraction ratio, allowing us to predict that its hepatic clearance may be limited upon HEX protein binding.

Activation of the hypothalamo-pituitary-adrenal axis by the growth hormone (GH) secretagogue, GH-releasing peptide-6, in rats

GH-releasing hexapeptide (GHRP-6) is a synthetic secretagogue that stimulates the release of GH by acting at both hypothalamic and pituitary sites. GHRPs also consistently elicit small, but significant, increases in plasma concentrations of ACTH and adrenal steroids. As these secretagogues do not release ACTH directly, they probably interact with the hypothalamic peptidergic systems controlling ACTH release, such as CRH and arginine vasopressin (AVP). We have now examined the activation of the hypothalamo-pituitary-adrenal axis by GHRP-6 in conscious rats. In a series of experiments, rats were injected i.v. with 10 microg GHRP-6, 2 microg CRH, 0.5 microg AVP, or saline, alone or in combination, and serial plasma samples withdrawn and assayed for ACTH, corticosterone, and GH. CRH and AVP increased plasma ACTH levels in all rats, whereas ACTH and corticosterone responses to GHRP-6 were variable and were dependent on the prevailing activity of the hypothalamo-pituitary-adrenal axis. GHRP-6 stimulated the largest ACTH responses in rats that had the lowest basal plasma ACTH and corticosterone levels before GHRP-6 administration. GHRP-6 given in combination with CRH did not increase ACTH levels beyond the response to CRH alone (change in ACTH, 1570 +/- 207 vs. 1714 +/- 245 pg/ml), whereas the combination of GHRP-6 and AVP markedly increased ACTH levels compared with the effects of AVP alone (change in ACTH, 5587 +/- 669 vs. 2338 +/- 451 pg/ml; P < 0.05). The GH responses to GHRP-6 were significantly greater in rats with low basal plasma ACTH and corticosterone levels than in rats with elevated ACTH and corticosterone levels (change in GH response, 119 +/- 27 vs. 29 +/- 7 ng/ml; P < 0.01). CRH alone significantly inhibited GH release (pre- vs. 40 min post-CRH, 11.9 +/- 3.8 vs. 1.7 +/- 0.4 ng/ml; P < 0.05), whereas AVP alone had no effect on GH levels. Neither CRH nor AVP had any effect on the GH response to GHRP-6. We suggest that GHRP-6 acts via the hypothalamus to mediate the release of ACTH, and that these effects are probably mediated at least in part via the release of endogenous CRH and are subject to regulation by circulating glucocorticoids.