Prolonged oral treatment with MK-677, a novel growth hormone secretagogue, improves sleep quality in man

A Sleep-Specific Midbrain Target for Sevoflurane Anesthesia

Sevoflurane has been the most widely used inhaled anesthetics with a favorable recovery profile; however, the precise mechanisms underlying its anesthetic action are still not completely understood. Here the authors show that sevoflurane activates a cluster of urocortin 1 (UCN1+ )/cocaine- and amphetamine-regulated transcript (CART+ ) neurons in the midbrain involved in its anesthesia. Furthermore, growth hormone secretagogue receptor (GHSR) is highly enriched in sevoflurane-activated UCN1+ /CART+ cells and is necessary for sleep induction. Blockade of GHSR abolishes the excitatory effect of sevoflurane on UCN1+ /CART+ neurons and attenuates its anesthetic effect. Collectively, their data suggest that anesthetic action of sevoflurane necessitates the GHSR activation in midbrain UCN1+ /CART+ neurons, which provides a novel target including the nucleus and receptor in the field of anesthesia.

The Safety and Efficacy of Growth Hormone Secretagogues

INTRODUCTION

Growth hormone (GH) increases lean body mass, decreases fat mass, increases exercise tolerance and maximum oxygen uptake, enhances muscle strength, and improves linear growth. Long-term studies of GH administration offer conflicting results on its safety, which has led to strict Food and Drug Administration criteria for GH use. The potential drawbacks of exogenous GH use are believed to be due in part to impaired regulatory feedback.

AIM

To review the literature on GH secretagogues (GHSs), which include GH-releasing peptides and the orally available small-molecule drug ibutamoren mesylate.

METHODS

Review of clinical studies on the safety and efficacy of GHSs in human subjects.

MAIN OUTCOME MEASURE

Report on the physiologic changes from GHS use in human subjects including its safety profile.

RESULTS

GHSs promote pulsatile release of GH that is subject to negative feedback and can prevent supra-therapeutic levels of GH and their sequelae. To date, few long-term, rigorously controlled studies have examined the efficacy and safety of GHSs, although GHSs might improve growth velocity in children, stimulate appetite, improve lean mass in wasting states and in obese individuals, decrease bone turnover, increase fat-free mass, and improve sleep. Available studies indicate that GHSs are well tolerated, with some concern for increases in blood glucose because of decreases in insulin sensitivity.

CONCLUSION

Further work is needed to better understand the long-term impact of GHSs on human anatomy and physiology and more specifically in the context of a diversity of clinical scenarios. Furthermore, the safety of these compounds with long-term use, including evaluation of cancer incidence and mortality, is needed. Sigalos JT, Pastuszak AW. The Safety and Efficacy of Growth Hormone Secretagogues. Sex Med Rev 2018;6:45-53.

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.

Central but not systemic administration of ghrelin induces wakefulness in mice

Ghrelin is a brain-gut peptide hormone widely known for its orexigenic and growth hormone-releasing activities. Findings from our and other laboratories indicate a role of ghrelin in sleep regulation. The effects of exogenous ghrelin on sleep-wake activity in mice are, however, unknown. The aim of the present study was to determine the sleep-modulating effects of ghrelin after central and systemic administrations in mice. Sleep-wake activity after intracerebroventricular (i.c.v.) administration of 0.2, 1 and 5 µg ghrelin and intraperitoneal injections of 40, 100, and 400 µg/kg ghrelin prior to light onset were determined in C57BL/6 mice. In addition, body temperature, motor activity and 1-hour food intake was measured after the systemic injections. Sleep effects of systemic ghrelin (40 and 400 µg/kg) injected before dark onset were also determined. I.c.v. injection of ghrelin increased wakefulness and suppressed non-rapid-eye-movement sleep and electroencephalographic slow-wave activity in the first hour after injections. Rapid-eye-movement sleep was decreased for 2-4 hours after each dose of ghrelin. Sytemic administration of ghrelin did not induce changes in sleep-wake activity in mice at dark or light onset. Motor activity and body temperature remained unaltered and food intake was significantly increased after systemic injections of ghrelin given prior the light period. These findings indicate that the activation of central, but not peripheral, ghrelin-sensitive mechanisms elicits arousal in mice. The results are consistent with the hypothesis that the activation of the hypothalamic neuronal circuit formed by ghrelin, orexin, and neuropeptide Y neurons triggers behavioral sequence characterized by increased wakefulness, motor activity and feeding in nocturnal rodents.

Role of ghrelin in the pathophysiology of eating disorders: implications for pharmacotherapy

Ghrelin is the only known circulating orexigenic hormone. It increases food intake by interacting with hypothalamic and brainstem circuits involved in energy balance, as well as reward-related brain areas. A heightened gut-brain ghrelin axis is an emerging feature of certain eating disorders such as anorexia nervosa and Prader-Willi syndrome. In common obesity, ghrelin levels are lowered, whereas post-meal ghrelin levels remain higher than in lean individuals. Agents that interfere with ghrelin signalling have therapeutic potential for eating disorders, including obesity. However, most of these drugs are only in the preclinical phase of development. Data obtained so far suggest that ghrelin agonists may have potential in the treatment of anorexia nervosa, while ghrelin antagonists seem promising for other eating disorders such as obesity and Prader-Willi syndrome. However, large clinical trials are needed to evaluate the efficacy and safety of these drugs.

Ghrelin in mental health, sleep, memory

Ghrelin acts as a neuropeptide. It participates in sleep-wake regulation. After systemic ghrelin treatment nonREM sleep is promoted in male humans and mice. This effect is influenced by gender, time of administration and depression. Ghrelin does not modulate sleep in healthy women and during the early morning in male subjects. In depressed women REM sleep is diminished after ghrelin. In elderly men and depressed men sleep promotion by ghrelin was preserved. In rats after central ghrelin feeding and wakefulness increased. The nocturnal secretion pattern of cortisol, GH, LH, FSH and hypothalamo-pituitary-thyroid hormones are influenced by ghrelin. Furthermore ghrelin appears to be related to memory and to be involved in the pathophysiology of CNS disorders, particularly depression.

Ghrelin increases slow wave sleep and stage 2 sleep and decreases stage 1 sleep and REM sleep in elderly men but does not affect sleep in elderly women

Ghrelin increases non-REM sleep and decreases REM sleep in young men but does not affect sleep in young women. In both sexes, ghrelin stimulates the activity of the somatotropic and the hypothalamic-pituitary-adrenal (HPA) axis, as indicated by increased growth hormone (GH) and cortisol plasma levels. These two endocrine axes are crucially involved in sleep regulation. As various endocrine effects are age-dependent, aim was to study ghrelin's effect on sleep and secretion of GH and cortisol in elderly humans. Sleep-EEGs (2300-0700 h) and secretion profiles of GH and cortisol (2000-0700 h) were determined in 10 elderly men (64.0+/-2.2 years) and 10 elderly, postmenopausal women (63.0+/-2.9 years) twice, receiving 50 microg ghrelin or placebo at 2200, 2300, 0000, and 0100 h, in this single-blind, randomized, cross-over study. In men, ghrelin compared to placebo was associated with significantly more stage 2 sleep (placebo: 183.3+/-6.1; ghrelin: 221.0+/-12.2 min), slow wave sleep (placebo: 33.4+/-5.1; ghrelin: 44.3+/-7.7 min) and non-REM sleep (placebo: 272.6+/-12.8; ghrelin: 318.2+/-11.0 min). Stage 1 sleep (placebo: 56.9+/-8.7; ghrelin: 50.9+/-7.6 min) and REM sleep (placebo: 71.9+/-9.1; ghrelin: 52.5+/-5.9 min) were significantly reduced. Furthermore, delta power in men was significantly higher and alpha power and beta power were significantly lower after ghrelin than after placebo injection during the first half of night. In women, no effects on sleep were observed. In both sexes, ghrelin caused comparable increases and secretion patterns of GH and cortisol. In conclusion, ghrelin affects sleep in elderly men but not women resembling findings in young subjects.

Neurochemical regulation of sleep

This review summarizes recent developments in the field of sleep regulation, particularly in the role of hormones, and of synthetic GABA(A) receptor agonists. Certain hormones play a specific role in sleep regulation. A reciprocal interaction of the neuropeptides growth hormone (GH)-releasing hormone (GHRH) and corticotropin-releasing hormone (CRH) plays a key role in sleep regulation. At least in males GHRH is a common stimulus of non-rapid-eye-movement sleep (NREMS) and GH and inhibits the hypothalamo-pituitary adrenocortical (HPA) hormones, whereas CRH exerts opposite effects. Furthermore CRH may enhance rapid-eye-movement sleep (REMS). Changes in the GHRH:CRH ratio in favor of CRH appear to contribute to sleep EEG and endocrine changes during depression and normal ageing. In women, however, CRH-like effects of GHRH were found. Besides CRH somatostatin impairs sleep, whereas ghrelin, galanin and neuropeptide Y promote sleep. Vasoactive intestinal polypeptide appears to be involved in the temporal organization of human sleep. Beside of peptides, steroids participate in sleep regulation. Cortisol appears to promote REMS. Various neuroactive steroids exert specific effects on sleep. The beneficial effect of estrogen replacement therapy in menopausal women suggests a role of estrogen in sleep regulation. The GABA(A) receptor or GABAergic neurons are involved in the action of many of these hormones. Recently synthetic GABA(A) agonists, particularly gaboxadol and the GABA reuptake inhibitor tiagabine were shown to differ distinctly in their action from allosteric modulators of the GABA(A) receptor like benzodiazepines as they promote slow-wave sleep, decrease wakefulness and do not affect REMS.

Aging and the growth hormone/insulin like growth factor-I axis

Growth hormone release and IGF-I synthesis decrease with increasing age. The regulation of the GH/IGF-I system is dependent on the integrity of the hypothalamus, pituitary and liver. During aging there are several changes which contribute to the decline in GH/IGF-I including changes in signal to the somatotrophs from growth hormone releasing hormone, somatostatin and other factors such as body composition, exercise, diet and sleep. All of these factors are discussed in detail within this review. The phenotypic similarities between aging and adult growth hormone deficiency syndrome combined with this decrease in GH/IGF-I with aging have prompted the question whether aging is a GH deficient state. The advent of recombinant growth hormone has led to a number of studies treating elderly patients with GH alone or in combination with sex steroids or exercise. The results of these studies would not back up the use of GH in elderly non-hypopituitary patients as they did not show efficacy, showed high rates of adverse events and there is also some evidence associating GH/IGF-I and risk of neoplasia. If GH therapy is to be used in this cohort of patients further long term efficacy and safety studies are required.

Roles of peptides and steroids in sleep disorders

A bidirectional interaction exists between the electrophysiological and neuroendocrine components of sleep. The first is represented by the nonrapid eye movement sleep (NREMS) and rapid eye movement sleep (REMS) cycles, the latter by distinct patterns of the secretion of various hormones. Certain hormones (neuropeptides and steroids) play a specific role in sleep regulation. Changes in their activity contribute to the pathophysiology of sleep disorders. A reciprocal interaction of the peptides growth hormone-releasing hormone (GHRH) and corticotropin-releasing hormone (CRH) plays a key role in sleep regulation. GHRH promotes growth hormone secretion and, at least in males, NREMS, whereas CRH impairs NREMS, promotes REMS and stimulates the secretion of adrenocorticotropic hormone and cortisol. Changes in the CRH:GHRH ratio in favor of CRH contribute to impaired sleep, elevated cortisol secretion and blunted GH levels during depression and normal aging. However, in women, GHRH exerts CRH-like effects. Galanin, ghrelin and neuropeptide Y are other sleep-promoting peptides, whereas somatostatin impairs sleep. A decline of orexin activity causes narcolepsy. In addition to CRH overactivity, hypercortisolism appears to be involved in the pathophysiology of sleep- electroencephalogram (EEG) changes in depression. Various neuroactive steroids exert specific effects on sleep. The changes of sleep EEG in women after the menopause are related to the decline of estrogen and progesterone. Furthermore, sleep-EEG changes in dwarfism, acromegaly, Addison's disease, Cushing's disease, brain injury, sleep apnea syndrome, primary insomnia, prolactinoma and dementia appear to be related to changes in the activity of peptides and steroids.

[Trp3, Arg5]-ghrelin(1-5) stimulates growth hormone secretion and food intake via growth hormone secretagogue (GHS) receptor

Ghrelin, a 28 amino acid peptide identified as an endogenous ligand for growth hormone secretagogue (GHS) receptor, stimulates food intake and growth hormone (GH) secretion. We designed low molecular weight peptides with affinity for the GHS receptor based on the primary structure of ghrelin. We found that [Trp3, Arg5]-ghrelin(1-5) (GSWFR), a novel pentapeptide composed of all L-amino acids, had affinity for the GHS receptor (IC50 = 10 microM). GSWFR stimulated GH secretion after intravenous or oral administration. Centrally administered GSWFR increased food intake in non-fasted mice. The orexigenic action of GSWFR was inhibited by a GHS receptor antagonist, [D-Lys3]-GH-releasing peptide-6, suggesting that GSWFR stimulated food intake through the GHS receptor. The orexigenic action of GSWFR was also inhibited by a neuropeptide Y (NPY) Y1 receptor antagonist, BIBO3304. These results suggest that the GSWFR-induced feeding is mediated by the NPY Y1 receptor.

Ghrelin stimulates appetite, imagination of food, GH, ACTH, and cortisol, but does not affect leptin in normal controls

Ghrelin, a growth hormone (GH) secretagogue receptor ligand was isolated from the stomach and hypothalamus of rats and humans. In rodents, ghrelin exerts distinct orexigenic action, probably as counterpart of the anorexigenic leptin. In humans, ghrelin infusion enhances appetite. It is unknown whether single intravenous (i.v.) injections of ghrelin affect human eating behavior. Therefore, we investigated the influence of a single i.v. bolus injection of 100 microg ghrelin on appetite, ideas about food, hormone levels, and glucose concentration in young control subjects. In order to test gender differences, we included five women and four men. After ghrelin administration, appetite was enhanced in eight of nine subjects. Seven probands reported a vivid, plastic image of their preferred meal. Furthermore, ghrelin stimulated an immediate increase in plasma levels of GH (area under the curve, mean+/-SEM 35+/-16 ng/ml x min after placebo [P] to 2808+/-533 ng/ml x min after ghrelin [G]; p<0.001), cortisol (5908+/-984 ng/ml x min [P] to 10179+/-1293 ng/ml x min [G]; p<0.001), and ACTH (922+/-103 pg/ml x min [P] to 3030+/-763 pg/ml x min [G]; p<0.02), whereas leptin levels remained unchanged. Contrary to placebo, glucose concentration did not decrease markedly after administration of ghrelin. Our data suggest that i.v. ghrelin stimulates appetite and images of food in young women and men. Obviously, leptin is not involved in these effects.

Molecular endocrinology and physiology of the aging central nervous system

Aging is associated with a progressive decline in physical and cognitive functions. The impact of age-dependent endocrine changes regulated by the central nervous system on the dynamics of neuronal behavior, neurodegeneration, cognition, biological rhythms, sexual behavior, and metabolism are reviewed. We also briefly review how functional deficits associated with increases in glucocorticoids and cytokines and declining production of sex steroids, GH, and IGF are likely exacerbated by age-dependent molecular misreading and alterations in components of signal transduction pathways and transcription factors.

Signal transduction mechanism of the seabream growth hormone secretagogue receptor

We have recently cloned the full-length cDNAs of the two growth hormone secretagogue receptor (GHSR) subtypes from a teleost species, the black seabream (Acanthopagrus schlegeli) [Mol. Cell. Endocrinol. 214 (2004) 81], namely sbGHSR-1a and sbGHSR-1b. Functional expression of these two receptor constructs in human embryonic kidney 293 (HEK293) cells indicated that stimulation of sbGHSR-1a by growth hormone secretagogues (GHS) could evoke increases in intracellular Ca2+ concentration ([Ca2+]i), whereas sbGHSR-1b appeared to play an inhibitory role on the signal transduction activity of sbGHSR-1a. In the present study, we have further investigated the signal transduction mechanism of sbGHSR-1a. The peptide GHS GHRP-6 and the non-peptide GHS L163,540 were able to trigger a receptor specific and phospholipase C (PLC)-dependent elevation of [Ca2+]i in HEK293 cells stably expressing sbGHSR-1a. This GHS-induced calcium mobilization was also dependent on protein kinase C activated L-type calcium channel opening. It was found that sbGHSR-1a could function in an agonist-independent manner as it exhibited a high basal activity of inositol phosphate production in the absence of GHS, indicating that the fish receptor is constitutively active. In addition, the extracellular signal-regulated kinases 1 and 2 (ERK1/2) were found to be activated upon stimulation of sbGHSR-1a by GHRP-6. This observation provides direct evidence in the coupling of sbGHSR-1a to ERK1/2 activation. Neither Gs nor Gi proteins are coupled to the receptor, as GHS did not induce cAMP production nor inhibit forskolin-stimulated cAMP accumulation in the sbGHSR-1a bearing cells. Furthermore, the ability of the GHSR antagonist D-Lys3-GHRP-6 to inhibit basal PLC and basal ERK1/2 activity suggests that this compound is an inverse agonist. In summary, the sbGHSR-1a appears to couple through the G(q/11)-mediated pathway to activate PLC, resulting in increased IP3 production and Ca2+ mobilization from both intracellular and extracellular stores. Moreover, sbGHSR-1a may trigger multiple signal transduction cascades to exert its physiological functions.

Ghrelin: more than a natural GH secretagogue and/or an orexigenic factor

Ghrelin, an acylated peptide produced predominantly by the stomach, has been discovered to be a natural ligand of the growth hormone secretagogue receptor type 1a (GHS-R1a). Ghrelin has recently attracted considerable interest as a new orexigenic factor. However, ghrelin exerts several other neuroendocrine, metabolic and also nonendocrine actions that are explained by the widespread distribution of ghrelin and GHS-R expression. The likely existence of GHS-R subtypes and evidence that the neuroendocrine actions, but not all the other actions, of ghrelin depend on its acylation in serine-3 revealed a system whose complexity had not been completely explored by studying synthetic GHS. Ghrelin secretion is mainly regulated by metabolic signals and, in turn, the modulatory action of ghrelin on the control of food intake and energy metabolism seems to be among its most important biological actions. However, according to a recent study, ghrelin-null mice are neither anorectics nor dwarfs and this evidence clearly depicts a remarkable difference from leptin null mice. Nevertheless, the original and fascinating story of ghrelin, as well as its potential pathophysiological implications in endocrinology and internal medicine, is not definitively cancelled by these data as GHS-R1a null aged mice show significant alterations in body composition and growth, in glucose metabolism, cardiac function and contextual memory. Besides potential clinical implications for natural or synthetic ghrelin analogues acting as agonists or antagonists, there are several open questions awaiting an answer. How many ghrelin receptor subtypes exist? Is ghrelin 'the' or just 'a' GHS-R ligand? That is, are there other natural GHS-R ligands? Is there a functional balance between acylated and unacylated ghrelin forms, potentially with different actions? Within the next few years suitable answers to these questions will probably be found, making it possible to gain a better knowledge of ghrelin's potential clinical perspectives.

Night sleep EEG and daytime sleep propensity in adult hypopituitary patients with growth hormone deficiency before and after six months of growth hormone replacement

BACKGROUND

Hypopituitary patients with growth hormone deficiency (GHD) complain of reduced vitality, general fatigue, lack of concentration, irritability and reduced alertness during daytime. It is unclear whether these symptoms are primarily due to GH-deficiency and/or secondary to GHD related sleep impairments. Bi-directional interactions between the somatotropic system and human sleep patterns are well established. However, data on the effect of GH either in subjects without GHD or in patients with GHD under GH replacement therapy on the sleep electroencephalogram (EEG) are controversial. No reports exist about objective measures of daytime sleepiness in GH deficient patients before and during GH-therapy.

OBJECTIVE

To assess the effects of GH on nocturnal and daytime sleep in adult patients with GHD before and during recombinant human GH (rhGH, Somatropin) replacement therapy.

METHODS

Eighteen adult patients with GHD (4 women and 14 men) participated in the study. Mean age at the beginning of the study was 48.5 years (range 27-64 years). Ten patients were recruited from a double-blind, randomized placebo controlled trial over 6 months, followed by an open treatment period of 6 additional months (Group I). In all patients from this group, only the effects of the first 6 months of GH treatment were assessed. Eight additional patients were treated in an open study design for 6 months (Group II). Nocturnal sleep recordings and daytime sleep EEGs with a multiple sleep latency test were performed at baseline and after 6 months of additional GH replacement therapy.

RESULTS

One patient dropped out due to side effects and was not included in sleep analysis. IGF-1 levels were increased in all patients, partially in a supraphysiologic range. Side effects were mainly mild but in one patient (from group II), general muscle pain led to interruption of the study. Therefore sleep analysis was only done in 17 patients. Sleep parameters were comparable to healthy control groups from the literature. GH substitution over 6 months did neither affect total sleep time nor times spent in different sleep stages. REM sleep density was also not changed. Daytime sleep propensity as measured by the multiple sleep latency test was not influenced by GH treatment.

CONCLUSIONS

GH replacement does neither affect night sleep nor daytime sleep propensity in GH deficient hypopituitary adults. GH substitution has no sleep disturbing effect.

Rhythms of ghrelin, leptin, and sleep in rats: effects of the normal diurnal cycle, restricted feeding, and sleep deprivation

To determine the relationships among plasma ghrelin and leptin concentrations and hypothalamic ghrelin contents, and sleep, cortical brain temperature (Tcrt), and feeding, we determined these parameters in rats in three experimental conditions: in free-feeding rats with normal diurnal rhythms, in rats with feeding restricted to the 12-h light period (RF), and in rats subjected to 5-h of sleep deprivation (SD) at the beginning of the light cycle. Plasma ghrelin and leptin displayed diurnal rhythms with the ghrelin peak preceding and the leptin peak following the major daily feeding peak in hour 1 after dark onset. RF reversed the diurnal rhythm of these hormones and the rhythm of rapid-eye-movement sleep (REMS) and significantly altered the rhythm of Tcrt. In contrast, the duration and intensity of non-REMS (NREMS) were hardly responsive to RF. SD failed to change leptin concentrations, but it promptly stimulated plasma ghrelin and induced eating. SD elicited biphasic variations in the hypothalamic ghrelin contents. SD increased plasma corticosterone, but corticosterone did not seem to influence either leptin or ghrelin. The results suggest a strong relationship between feeding and the diurnal rhythm of leptin and that feeding also fundamentally modulates the diurnal rhythm of ghrelin. The variations in hypothalamic ghrelin contents might be associated with sleep-wake activity in rats, but, unlike the previous observations in humans, obvious links could not be detected between sleep and the diurnal rhythms of plasma concentrations of either ghrelin or leptin in the rat.

Stimulation of growth hormone secretion from seabream pituitary cells in primary culture by growth hormone secretagogues is independent of growth hormone transcription

The action of a number of growth hormone secretagogues (GHS) on growth hormone (GH) secretion and gene expression was studied in a primary culture of pituitary cells isolated from the black seabream Acanthopagrus schlegeli. The peptide GHS employed included growth hormone-releasing peptide (GHRP)-2, ipamorelin, and human ghrelin. The nonpeptide GHS employed included the benzolactam GHS L692,585 and the spiropiperidine GHS L163,540. Secreted GH was measured in the culture medium by an enzyme-linked immunosorbent assay (ELISA) method using a specific antibody against seabream GH. The GH mRNA content in the incubated cells was assessed by reverse transcription polymerase chain reaction (RT-PCR) using a pair of gene-specific primers designed from the cloned black seabream GH cDNA sequence. A dose-dependent stimulation of GH release was demonstrated by all the GHS tested, except human ghrelin, with EC(50) values in the nanomolar range. Simultaneous measurement of GH mRNA levels in the incubated seabream pituitary cells indicated that the GHS-stimulated increase in GH secretion was not paralleled by corresponding changes in GH gene expression. In contrast to the situation previously reported in the rat, no change in GH gene expression was noticed in the seabream pituitary cells even though the time of stimulation by GHS was increased up to 48 h, confirming that the GHS-stimulated GH secretion in seabream is independent of GH gene transcription.

Hexarelin decreases slow-wave sleep and stimulates the secretion of GH, ACTH, cortisol and prolactin during sleep in healthy volunteers

Ghrelin, the endogenous ligand of the growth hormone (GH) secretagogue (GHS) receptor and some GHSs exert different effects on sleep electroencephalogram (EEG) and sleep-related hormone secretion in humans. Similar to GH-releasing hormone (GHRH) ghrelin promotes slow-wave sleep in humans, whereas GH-releasing peptide-6 (GHRP-6) enhances stage 2 nonrapid-eye movement sleep (NREMS). As GHRP-6, hexarelin is a synthetic GHS. Hexarelin is superior to GHRH and GHRP-6 in stimulating GH release. The influence of hexarelin on sleep-endocrine activity and the immune system is unknown. We investigated simultaneously the sleep EEG and nocturnal profiles of GH, ACTH, cortisol, prolactin, leptin, tumor necrosis factor (TNF)-alpha, and soluble TNF-alpha receptors in seven young normal volunteers after repetitive administration of 4 x 50 microg hexarelin or placebo at 22.00, 23.00, 24.00 and 01.00 h. Following hexarelin, stage 4 sleep during the first half of the night, and EEG delta power during the total night decreased significantly. Significant increases of the concentrations of GH and prolactin during the total night, and of ACTH and of cortisol during the first half of the night were found. Leptin levels, TNF-alpha and soluble TNF receptors remained unchanged. We hypothesize that sleep is impaired after hexarelin since the GHRH/corticotropin-releasing hormone (CRH) ratio is changed in favour of CRH. There are no hints for an interaction of hexarelin and the immune system.

Ghrelin, appetite, and gastric motility: the emerging role of the stomach as an endocrine organ

Recent progress in the field of energy homeostasis was triggered by the discovery of adipocyte hormone leptin and revealed a complex regulatory neuroendocrine network. A late addition is the novel stomach hormone ghrelin, which is an endogenous agonist at the growth hormone secretagogne receptor and is the motilin-related family of regulatory peptides. In addition to its ability to stimulate GH secretion and gastric motility, ghrelin stimulates appetite and induces a positive energy balance leading to body weight gain. Leptin and ghrelin are complementary, yet antagonistic, signals reflecting acute and chronic changes in energy balance, the effects of which are mediated by hypothalamic neuropeptides such as neuropeptide Y and agouti-related peptide. Endocrine and vagal afferent pathways are involved in these actions of ghrelin and leptin. Ghrelin is a novel neuroendocrine signal possessing a wide spectrum of biological activities that illustrates the importance of the stomach in providing input into the brain. Defective ghrelin signaling from the stomach could contribute to abnormalities in energy balance, growth, and associated gastrointestinal and neuroendocrine functions.

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.

Identification and functional characterization of two alternatively spliced growth hormone secretagogue receptor transcripts from the pituitary of black seabream Acanthopagrus schlegeli

Two cDNA transcripts, namely sbGHSR-1a and sbGHSR-1b, for growth hormone secretagogue receptor (GHSR), were identified from the seabream pituitary. When translated, the sbGHSR-1a encodes for a protein of 385 amino acids (aa) with seven putative transmembrane domains and the sbGHSR-1b contains 295 aa with five putative transmembrane domains. Tissue distribution studies indicated that the two receptors are mainly expressed in the central nervous system of the fish. The sbGHSR-1a transcript has the highest expression level in the pituitary. The sbGHSR-1b transcript, on the other hand, has the highest expression level in the telencephalon. Genomic Southern analysis indicated that there is a single gene for GHSR in the seabream genome. Comparison of the cDNA sequences of sbGHSR1a and sbGHSR1b with the seabream genomic sequence indicated that the presence of the two receptor transcripts is a result of alternative splicing of the single GHSR gene. The two receptor cDNAs were expressed in cultured eukaryotic cells for functional analyses. A variety of structurally diverse growth hormone secretogogues (GHS), including the peptide GHS (GHRP-6 and ghrelin), the benzolactam GHS (L692,585) and the spiropiperidine GHS (L163,255), were able to trigger an elevation of intracellular Ca(2+) ion concentration in HEK293 cells expressing sbGHSR-1a, but not in cells expressing sbGHSR-1b. Microphysiometry revealed that an increase in extracellular acidification rate (EAR) could be detected in CHO cells expressing the sbGHSR-1a receptor when stimulated with GHRP-6. On the contrary, CHO cells expressing the sbGHSR-1b receptor registered no detectable EAR changes. However, when sbGHSR-1b was co-expressed with sbGHSR-1a in HEK293 cells, the signal transduction capacity of sbGHSR-1a was attenuated. This is the first report on the identification of a GHSR-1b transcript from species other than mammals and the demonstration that receptor interaction might provide a possible explanation for the existence and biological significance of the sbGHSR-1b transcript.

Ghrelin promotes slow-wave sleep in humans

Ghrelin, an endogenous ligand of the growth hormone (GH) secretagogue (GHS) receptor, stimulates GH release, appetite, and weight gain in humans and rodents. Synthetic GHSs modulate sleep electroencephalogram (EEG) and nocturnal hormone secretion. We studied the effect of 4 x 50 microg of ghrelin administered hourly as intravenous boluses between 2200 and 0100 on sleep EEG and the secretion of plasma GH, ACTH, cortisol, prolactin, and leptin in humans (n = 7). After ghrelin administration, slow-wave sleep was increased during the total night and accumulated delta-wave activity was enhanced during the second half of the night. Rapid-eye-movement (REM) sleep was reduced during the second third of the night, whereas all other sleep EEG variables remained unchanged. Furthermore, GH and prolactin plasma levels were enhanced throughout the night, and cortisol levels increased during the first part of the night (2200-0300). The response of GH to ghrelin was most distinct after the first injection and lowest after the fourth injection. In contrast, cortisol showed an inverse pattern of response. Leptin levels did not differ between groups. Our data show a distinct action of exogenous ghrelin on sleep EEG and nocturnal hormone secretion. We suggest that ghrelin is an endogenous sleep-promoting factor. This role appears to be complementary to the already described effects of the peptide in the regulation of energy balance. Furthermore, ghrelin appears to be a common stimulus of the somatotropic and hypothalamo-pituitary-adrenocortical systems. It appears that ghrelin is a sleep-promoting factor in humans.

Sleep in mice with nonfunctional growth hormone-releasing hormone receptors

The role of the somatotropic axis in sleep regulation was studied by using the lit/lit mouse with nonfunctional growth hormone (GH)-releasing hormone (GHRH) receptors (GHRH-Rs) and control heterozygous C57BL/6J mice, which have a normal phenotype. During the light period, the lit/lit mice displayed significantly less spontaneous rapid eye movement sleep (REMS) and non-REMS (NREMS) than the controls. Intraperitoneal injection of GHRH (50 microg/kg) failed to promote sleep in the lit/lit mice, whereas it enhanced NREMS in the heterozygous mice. Subcutaneous infusion of GH replacement stimulated weight gain, increased the concentration of plasma insulin-like growth factor-1 (IGF-1), and normalized REMS, but failed to restore normal NREMS in the lit/lit mice. The NREMS response to a 4-h sleep deprivation was attenuated in the lit/lit mice. In control mice, intraperitoneal injection of ghrelin (400 microg/kg) elicited GH secretion and promoted NREMS, and intraperitoneal administration of the somatostatin analog octretotide (Oct, 200 microg/kg) inhibited sleep. In contrast, these responses were missing in the lit/lit mice. The results suggest that GH promotes REMS whereas GHRH stimulates NREMS via central GHRH-Rs and that GHRH is involved in the mediation of the sleep effects of ghrelin and somatostatin.

Ghrelin and synthetic GH secretagogues

Ghrelin, a 28-amino-acid acylated peptide, produced mainly by the stomach, displays strong growth hormone-(GH)-releasing activity mediated by the hypothalamus-pituitary growth hormone potential secretagogue (GHS) receptor which had been shown to be specific for a family of synthetic, orally active GHS. GHS are reliable provocative tests for the diagnosis of GH deficiency but, as orally active growth-promoting agents, they are not comparable with human recombinant GH in terms of efficacy. The usefulness of GHS in anabolic, anti-ageing drug intervention in the somatopause is still unclear. GHS also act on central and peripheral receptors and show other actions, including an orexigenic effect, an influence on gastroentero-pancreatic functions, and cardiovascular and anti-proliferative effects. Ghrelin mediates the neuroendocrine and metabolic response to starvation. Taking into account its orexigenic effect, GHS analogues acting as agonists or antagonists on appetite could represent a new drug intervention for eating disorders.

Neuroendocrine and peripheral activities of ghrelin: implications in metabolism and obesity

Ghrelin, a 28-amino acid acylated peptide predominantly produced by the stomach, displays strong growth hormone (GH)-releasing activity mediated by the hypothalamus-pituitary GH secretagogue (GHS)-receptors specific for synthetic GHS. The discovery of ghrelin definitely changes our understanding of GH regulation but it is also already clear that ghrelin is much more than simply a natural GHS. Ghrelin acts also on other central and peripheral receptors and shows other actions including stimulation of lactotroph and corticotroph secretion, orexia, influence on gastro-entero-pancreatic functions, metabolic, cardiovascular and anti-proliferative effects. GHS were born more than 20 years ago as synthetic molecules suggesting the option that GH deficiency could be treated by orally active GHS as an alternative to recombinant human GH (rhGH). Up to now, this has not been the case and also their usefulness as anabolic anti-aging intervention restoring GH/insulin-like growth factor-I axis in somatopause is still unclear. We are now confronted with the theoretical possibility that GHS analogues could become candidate drugs for treatment of pathophysiological conditions in internal medicine totally unrelated to disorders of GH secretion. Particularly, GHS receptor agonists or antagonists acting on appetite could represent new drug intervention in eating disorders.

Growth hormone-releasing hormone and growth hormone secretagogue-receptor ligands: focus on reproductive system

Growth hormone-releasing hormone (GHRH) and somatostatin are the most important hypothalamic neurohormones controlling growth hormone (GH) secretion. Several neurotransmitters and neuropeptides also play an important role in the control of GH secretion, mainly acting via modulation of GHRH and somatostatin. In the past two decades, particular attention has been given to a new family of substances showing a strong GH-releasing effect: GH secretagogues (GHSs). GHSs increase GH secretion in a dose- and age-related manner after iv and even oral administration. The endocrine effects of GHSs, are not fully specific for GH; they show, in fact, prolactin- (PRL), adenocorticotropic hormone- and cortisol-releasing effects. Specific GHS receptors are present in both the central nervous system and peripheral tissues, where they mediate several extraendocrine effects of GHSs. The isolation of these "orphan" receptors suggested the existence of an endogenous GHS-like ligand that could be represented by a recently discovered gastric peptide, named ghrelin. The interaction between GHSs and GHRH at the central level and in the pituitary gland, but not at peripheral level, has clearly been shown. Because GHRH and GHS receptors share the same localization in some peripheral tissues, they may have some interactions even at this level.

Adenosine: A partial agonist of the growth hormone secretagogue receptor

The growth hormone secretagogue receptor (GHS-R) is involved in the regulation of pulsatile GH release. However, until recently, natural endogenous ligands for the receptor were unknown. We fractionated porcine hypothalamic extracts and assayed fractions for activity on HEK293 cells expressing GHS-R and aequorin. A partial agonist was isolated and identified using microspray tandem mass spectrometry as adenosine. GHS-R activation by adenosine and synthetic adenosine agonists is inhibited by the GHS-R selective antagonists L-765,867, D-Lys(3)-GHRP-6, and by theophylline and XAC. Cross desensitization of the GHS-R occurs with both MK-0677 and adenosine. Ligand binding and site directed mutagenesis studies show that adenosine binds to a binding site that is distinct from the previously characterized MK-0677 and GHRP-6 binding pocket. We propose, that adenosine is a physiologically important endogenous GHS-R ligand and speculate that GHS-R ligands modulate dopamine release from hypothalamic neurons.

Growth hormone secretagogue binding sites in peripheral human tissues

The family of GH secretagogues (GHS) includes peptidyl (hexarelin) and nonpeptidyl (MK 0677) molecules possessing specific receptors in the brain, pituitary, and thyroid. GHS receptor subtypes have also been identified in the heart; and a gastric-derived peptide, named ghrelin, has recently been proposed as a natural ligand. Our aim was to investigate the presence of GHS receptors in a wide range of human tissues, by radioreceptor assay with [125I]Tyr-Ala-hexarelin. GHS receptors were detected mainly in the myocardium, but they were also present (in order of decreasing binding activity) in adrenal, gonads, arteries, lung, liver, skeletal muscle, kidney, pituitary, thyroid, adipose tissue, veins, uterus, skin, and lymphnode. In contrast, negligible binding was found in parathyroid, pancreas, placenta, mammary gland, prostate, salivary gland, stomach, colon, and spleen. Hexarelin, MK 0677, and human ghrelin completely displaced the radioligand from binding sites of endocrine tissues, but MK 0677 and ghrelin were less potent than hexarelin. In nonendocrine tissues, both MK 0677 and ghrelin were inactive in displacement of [125I]Tyr-Ala-hexarelin, whereas hexarelin was as active as a displacing agent in endocrine tissues. This study provides the first detailed analysis of the tissue localization of GHS receptors and suggests that a still unknown receptor subtype, specific for peptidyl GHS, may exist in the heart and in other tissues.

The aging process: where are the drug opportunities?

New data support a role for growth hormone secretagogue receptor agonists as rejuvenating agents. Two enzymes critical for the formation of beta-amyloid plaques in Alzheimer's disease have been identified. Estrogen receptor beta continues to emerge as a potential drug target. The orphan nuclear receptor Nurr1 appears to be a target for treatment of Parkinson's disease, and propargylamines are emerging as inhibitors of oxidative damage in neurons.

[New growth hormone secretagogues: from physiology to the first therapeutic applications]

Growth hormone releasing peptides (GHRP) are synthetic hexapeptides that physiologically stimulate GH release through two different pathways: 1) central and 2) direct action on somatotropic cells. Animal experiments and first clinical trials show that synthetic GHRP and synthetic analogues could be useful substitutes to recombinant GH in the treatment of GH deficiency, and in pathological conditions which may benefit from amplification of the GH-IGF I axis activity.

Interrelationships between growth hormone and sleep

In healthy young adults, the 24-hour profile of plasma growth hormone (GH) levels consists of stable low levels abruptly interrupted by bursts of secretion. In normal women, daytime GH secretory pulses are frequent. However, in normal men, a sleep-onset-associated pulse is generally the major or even the only daily episode of active secretion. Extensive evidence indicates the existence of a consistent relationship between slow-wave (SW) sleep and increased GH secretion. There is a linear relationship between the amount of SW sleep (measured by either visual scoring or spectral analysis of the EEG) and the amount of concomitant GH secretion. During ageing, SW sleep and GH secretion decrease exponentially and with the same chronology. Pharmacological stimulation of SW sleep results in increased GH release, and compounds that increase SW sleep may therefore represent a novel class of GH secretagogues.

Insulin-Like growth factor I: implications in aging

According to the somatomedin model, growth hormone (GH)-dependent hepatic synthesis is responsible for maintaining circulating insulin-like growth factor (IGF)-I levels. On the other hand, the local autocrine/paracrine IGF-I expression in peripheral tissue is generally GH-independent and reflects the effects of various and tissue-specific trophic hormones. Circulating IGF-I levels undergo important age-related variations increasing at puberty and decreasing, thereafter, to low levels in the elderly. Low IGF-I levels in the elderly mainly reflect impaired somatotroph secretion but the decline in gonadal sex steroid levels, some protein and micronutrients malnutrition as well as age-dependent variations in IGF-binding proteins may also play a role in the age-related decrease in IGF-I activity. This, in turn, partially accounts for age-related changes in bones, muscles, cardiovascular system, central nervous system and the immune system. However, it is currently unclear whether treatment with exogenous IGF-I can retard or reverse age-related changes in body structure and function.

Treatment of obese subjects with the oral growth hormone secretagogue MK-677 affects serum concentrations of several lipoproteins, but not lipoprotein(a)

Obesity is associated with blunted GH secretion and an unfavorable lipoprotein pattern. The objective of this study was to investigate the effects of treatment with the oral GH secretagogue MK-677 on lipoproteins in otherwise healthy obese males. The study was randomized, double blind, and parallel. Twenty-four obese males, aged 18-50 yr, with body mass index greater than 30 kg/m2 and waist/hip ratio above 0.95 were treated with 25 mg MK-677 (n = 12) or placebo (n = 12) daily for 8 weeks. MK-677 treatment did not significantly change serum lipoprotein(a) [Lp(a)] levels. Serum apolipoprotein A-I and E (apoA-I and apoE) were increased at 2 weeks (P < 0.001 and P < 0.01 vs. placebo, respectively), but were not changed at study end. Serum total cholesterol and low density lipoprotein (LDL) cholesterol (LDL-C) levels were not significantly changed by MK-677 treatment. Serum high density lipoprotein (HDL) cholesterol (HDL-C) was increased at 2 weeks of MK-677 treatment (P < 0.01 vs. placebo), but not at 8 weeks. The LDL-C/HDL-C ratio was reduced after 8 weeks of MK-677 treatment (P < 0.05 vs. placebo). Mean LDL particle diameter was decreased at 2 weeks (P < 0.05 vs. placebo), but was unchanged compared with baseline values at 8 weeks (P = NS vs. placebo). The level of serum triglycerides was increased at 2 (P < 0.05 vs. placebo), but not at 8, weeks. Lipoprotein lipase activity in abdominal and gluteal sc adipose tissue was not affected by active treatment. In conclusion, treatment with the oral GH secretagogue MK-677 affected circulating lipoproteins. The effects on serum apoA-1, apoE, triglycerides, and mean LDL particle diameter were transient. At study end, the LDL-C/HDL-C ratio was decreased. MK-677 treatment did not significantly affect serum Lp(a) concentrations at the present dose and administration protocol.

Effects of growth hormone-releasing peptide-6 on the nocturnal secretion of GH, ACTH and cortisol and on the sleep EEG in man: role of routes of administration

After repeated intravenous (i.v.) boluses of growth hormone-releasing peptide-6 (GHRP-6) we found recently increases of growth hormone (GH), corticotropin (ACTH) and cortisol levels and of the amount of stage 2 sleep. In clinical use, oral (p.o.), intranasal (i.n.) and sublingual (s.l.) routes of administration have advantages over i.v. administration. We compared the sleep-endocrine effects of 300 microg/kg of body weight (b.w.) GHRP-6 in enteric-coated capsules given p.o. at 21.00 h and of 30 microg/kg GHRP-6 i.n. or 30 microg/kg GHRP-6 sl. given at 22.45 h in normal young male controls with placebo conditions. After GHRP-6 p.o. secretion of GH, ACTH and cortisol remained unchanged. The only effect of GHRP-6 s.l. was a trend toward an increase in GH in the first half of the night. GHRP-6 i.n. prompted a significant increase in GH concentration during the total night and a trend toward an increase in ACTH secretion during the first half of the night, whereas cortisol secretion remained unchanged. Furthermore, after GHRP-6 i.n., sleep stage 2 increased in the second half of the night by trend, and spectral analysis of total night non-rapid eye movement (REM) sleep revealed a decrease of delta power by trend. In contrast sleep stage 2 decreased during the second half of the night after GHRP-6 p.o. Our data demonstrate that GHRP-6 is capable of modulating GH and ACTH secretion as well as sleep. However, the effects depend upon dosage, duration and route of administration.

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.

Evidence against a role for the growth hormone-releasing peptide axis in human slow-wave sleep regulation

A complex interrelationship exists between sleep and somatotropic activity. In humans, intravenous injections of growth hormone-releasing hormone (GHRH) given during sleep consistently stimulate slow-wave (SW) sleep, particularly when given in the latter part of the night. In the present study, the possible somnogenic effects induced under similar conditions by GH-releasing peptide (GHRP) were investigated in seven young healthy men. Bolus intravenous injections of GHRP-2 (1 microgram/kg body wt) or saline, in randomized order, were given after 60 s of the third rapid-eye-movement period. All GHRP injections were immediately followed by transient prolactin elevations and by GH pulses of a magnitude within or around the upper limit of the physiological range. Except for a nonsignificant tendency to increased amounts of wakefulness during the 1st h after the injection, no effects of GHRP-2 administration on sleep were detected. There was in particular no enhancement of SW sleep. Thus, in contrast to GHRH, late-night single injections of GHRP-2 at a dosage resulting in similar GH elevations have no stimulatory effects on SW sleep. The present data provide evidence against the involvement of the GHRP axis in human SW sleep regulation.