Molecular cloning and gene expression of growth hormone-releasing peptide receptor in rat tissues

New Aspects of Corpus Luteum Regulation in Physiological and Pathological Conditions: Involvement of Adipokines and Neuropeptides

The corpus luteum is a small gland of great importance because its proper functioning determines not only the appropriate course of the estrous/menstrual cycle and embryo implantation, but also the subsequent maintenance of pregnancy. Among the well-known regulators of luteal tissue functions, increasing attention is focused on the role of neuropeptides and adipose tissue hormones-adipokines. Growing evidence points to the expression of these factors in the corpus luteum of women and different animal species, and their involvement in corpus luteum formation, endocrine function, angiogenesis, cells proliferation, apoptosis, and finally, regression. In the present review, we summarize the current knowledge about the expression and role of adipokines, such as adiponectin, leptin, apelin, vaspin, visfatin, chemerin, and neuropeptides like ghrelin, orexins, kisspeptin, and phoenixin in the physiological regulation of the corpus luteum function, as well as their potential involvement in pathologies affecting the luteal cells that disrupt the estrous cycle.

The actions of ghrelin in the paraventricular nucleus: energy balance and neuroendocrine implications

Ghrelin is a peptide mainly produced and secreted by the stomach. Since its discovery, the impact of ghrelin on the regulation of food intake has been the most studied function of this hormone; however, ghrelin affects a wide range of physiological systems, many of which are controlled by the hypothalamic paraventricular nucleus (PVN). Several pathways may mediate the effects of ghrelin on PVN neurons, such as direct or indirect effects mediated by circumventricular organs and/or the arcuate nucleus. The ghrelin receptor is expressed in PVN neurons, and the peripheral or intracerebroventricular administration of ghrelin affects PVN neuronal activity. Intra-PVN application of ghrelin increases food intake and decreases fat oxidation, which chronically contribute to the increased adiposity. Additionally, ghrelin modulates the neuroendocrine axes controlled by the PVN, increasing the release of vasopressin and oxytocin by magnocellular neurons and corticotropin-releasing hormone by neuroendocrine parvocellular neurons, while possibly inhibiting the release of thyrotropin-releasing hormone. Thus, the PVN is an important target for the actions of ghrelin. Our review discusses the mechanisms of ghrelin actions in the PVN, and its potential implications for energy balance, neuroendocrine, and integrative physiological control.

Potential Therapeutic Effects of Gut Hormones, Ghrelin and Obestatin in Oral Mucositis

Chemotherapy and/or head and neck radiotherapy are frequently associated with oral mucositis. Oral pain, odynophagia and dysphagia, opioid use, weight loss, dehydration, systemic infection, hospitalization and introduction of a feeding tube should be mentioned as the main determinated effect of oral mucositis. Oral mucositis leads to a decreased quality of life and an increase in treatment costs. Moreover, oral mucositis is a life-threatening disease. In addition to its own direct life-threatening consequences, it can also lead to a reduced survival due to the discontinuation or dose reduction of anti-neoplasm therapy. There are numerous strategies for the prevention or treatment of oral mucositis; however, their effectiveness is limited and does not correspond to expectations. This review is focused on the ghrelin and obestatin as potentially useful candidates for the prevention and treatment of chemo- or/and radiotherapy-induced oral mucositis.

Capsaicin-Sensitive Sensory Nerves Are Necessary for the Protective Effect of Ghrelin in Cerulein-Induced Acute Pancreatitis in Rats

Ghrelin was shown to exhibit protective and therapeutic effect in the gut. Aim of the study was to investigate the role of sensory nerves (SN) in the protective effect of ghrelin in acute pancreatitis (AP). Studies were performed on male Wistar rats or isolated pancreatic acinar cells. After capsaicin deactivation of sensory nerves (CDSN) or treatment with saline, rats were pretreated intraperitoneally with ghrelin or saline. In those rats, AP was induced by cerulein or pancreases were used for isolation of pancreatic acinar cells. Pancreatic acinar cells were incubated in cerulein-free or cerulein containing solution. In rats with intact SN, pretreatment with ghrelin led to a reversal of the cerulein-induced increase in pancreatic weight, plasma activity of lipase and plasma concentration of tumor necrosis factor-α (TNF-α). These effects were associated with an increase in plasma interleukin-4 concentration and reduction in histological signs of pancreatic damage. CDSN tended to increase the severity of AP and abolished the protective effect of ghrelin. Exposure of pancreatic acinar cells to cerulein led to increase in cellular expression of mRNA for TNF-α and cellular synthesis of this cytokine. Pretreatment with ghrelin reduced this alteration, but this effect was only observed in acinar cells obtained from rats with intact SN. Moreover, CDSN inhibited the cerulein- and ghrelin-induced increase in gene expression and synthesis of heat shock protein 70 (HSP70) in those cells. Ghrelin exhibits the protective effect in cerulein-induced AP on the organ and pancreatic acinar cell level. Sensory nerves ablation abolishes this effect.

Growth Hormone Secretion: Molecular and Cellular Mechanisms and In Vivo Approaches

Growth hormone (GH) release is under the direct control of hypothalamic releasing hormones, some being also produced peripherally. The role of these hypothalamic factors has been understood by in vitro studies together with such in vivo approaches as stalk sectioning. Secretion of GH is stimulated by GH-releasing hormone (GHRH) and ghrelin (acting via the GH secretagogue [GHS] receptor [GHSR]), and inhibited by somatostatin (SRIF). Other peptides/proteins influence GH secretion, at least in some species. The cellular mechanism by which the releasing hormones affect GH secretion from the somatotrope requires specific signal transduction systems (cAMP and/or calcium influx and/or mobilization of intracellular calcium) and/or tyrosine kinase(s) and/or nitric oxide (NO)/cGMP. At the subcellular level, GH release (at least in response to GHS) is accomplished by the following. The GH-containing secretory granules are moved close to the cell surface. There is then transient fusion of the secretory granules with the fusion pores in the multiple secretory pits in the somatotrope cell surface.

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.

Effects of GHRP-2 and Cysteamine Administration on Growth Performance, Somatotropic Axis Hormone and Muscle Protein Deposition in Yaks (Bos grunniens) with Growth Retardation

The objective of this study was to investigate the effects of growth hormone-releasing peptide-2 (GHRP-2) and cysteamine (CS) administration on growth performance in yaks with growth retardation and try to elucidate its regulatory mechanisms. Trial 1, thirty-six 1-year-old Qinghai high plateau yaks (body weight 38–83.2 kg) were randomly chosen for body weight and jugular blood samples collection. The relationship between body weight and serum GHRH (P < 0.05, R = 0.45), GH (P < 0.05, R = 0.47), IGF-1 (P < 0.05, R = 0.62) was significantly correlated in yaks colonies with lighter body weights. Trial 2, fifteen 1-year-old Qinghai high plateau yaks with growth retardation (average body weight 54.8 ± 8.24 kg) were randomly selected and assigned to negative control group (NG), GHRP-2 injection group (GG) and cysteamine feeding group (CG), with 5 yaks per group. Another five 1-year-old Qinghai high plateau yaks with normal growth performance (average body weight 75.3 ± 2.43 kg) were selected as positive control group (PG). The average daily gain (ADG) of the GG and CG were significantly higher than those in the PG and NG (P < 0.05). Both GHRP-2 and CS administration significantly enhanced the myofiber diameter and area of skeletal muscle (P<0.05). GHRP-2 significantly enhanced the serum GH and IGF-1 levels (P < 0.05), and up-regulated GHR, IGF-1 and IGF-1R mRNA expression in the liver and skeletal muscle (P < 0.05), enhanced the mRNA expression of PI₃K, AKt and mTOR in the skeletal muscle (P<0.05). CS significantly reduced the serum SS levels and the hypothalamus SS mRNA expression (P < 0.05), and enhanced GHR and IGF-1 mRNA expression in the liver (P < 0.05), decreased the mRNA expression of muscle atrophy F-box (Atrogin-1) and muscle ring finger 1 (MuRF1) mRNA (P < 0.05). Conclusions: Growth retardation in yaks was primarily due to somatotropic axis hormones secretion deficiency. Both GHRP-2 and CS administration can accelerate growth performance and GH, IGF-1 secretion in yaks with growth retardation. GHRP-2 enhanced muscle protein deposition mainly by up-regulated the protein synthesis pathways, whereas CS worked mainly by down-regulated the ubiquitin-proteasome pathway.

Nutrient Sensing Overrides Somatostatin and Growth Hormone-Releasing Hormone to Control Pulsatile Growth Hormone Release

Pharmacological studies reveal that interactions between hypothalamic inhibitory somatostatin and stimulatory growth hormone-releasing hormone (GHRH) govern pulsatile GH release. However, in vivo analysis of somatostatin and GHRH release into the pituitary portal vasculature and peripheral GH output demonstrates that the withdrawal of somatostatin or the appearance of GHRH into pituitary portal blood does not reliably dictate GH release. Consequently, additional intermediates acting at the level of the hypothalamus and within the anterior pituitary gland are likely to contribute to the release of GH, entraining GH secretory patterns to meet physiological demand. The identification and validation of the actions of such intermediates is particularly important, given that the pattern of GH release defines several of the physiological actions of GH. This review highlights the actions of neuropeptide Y in regulating GH release. It is acknowledged that pulsatile GH release may not occur selectively in response to hypothalamic control of pituitary function. As such, interactions between somatotroph networks, the median eminence and pituitary microvasculature and blood flow, and the emerging role of tanycytes and pericytes as critical regulators of pulsatility are considered. It is argued that collective interactions between the hypothalamus, the median eminence and pituitary vasculature, and structural components within the pituitary gland dictate somatotroph function and thereby pulsatile GH release. These interactions may override hypothalamic somatostatin and GHRH-mediated GH release, and modify pulsatile GH release relative to the peripheral glucose supply, and thereby physiological demand.

Identification, tissue distribution and functional characterization of the ghrelin receptor in West African lungfish, Protopterus annectens

We identified two ghrelin receptor isoforms, the ghrelin receptor type-1a (GHS-R1a) and its alternative splice form (GHS-R1b) for West African lungfish, Protopterus annectens. Lungfish GHS-R1a and 1b comprised 361 and 281 amino acids, respectively. Lungfish GHS-R1a showed the highest identity to coelacanth GHS-R1a (80.4%). The highest expression of GHS-R1a mRNAs was seen in the brain, liver, ovary, heart, intestine, and gills. GHS-R1b mRNAs were also detected in the same tissues with GHS-R1a, but their expression level was 1/20 that of GHS-R1a. In human embryonic kidney 293 cells transiently expressing lungfish GHS-R1a, rat and bullfrog ghrelin, and two GHS-R1a agonists, GHRP-6 and hexarelin, increased intracellular Ca(2+) concentrations. The intensity of the Ca(2+) increases induced by GHS-R1a agonists was twice when compared to that induced by ghrelin, although the median effective doses (ED50) were similar, suggesting a long-lasting effect of GHS-R1a agonists with similar affinity. We also examined changes in the GHS-R gene expression during an eight-week estivation. Body weight was slightly lowered, but plasma sodium and glucose concentrations decreased; plasma urea concentration increased significantly 4weeks after the start of estivation. Overall, expression of GHS-R1a mRNA decreased, but changes in GHS-R1b mRNA expression were inconsistent with those of GHS-R1a during estivation, suggesting an involvement of GHS-R in energy homeostasis, as seen in mammals. Our results suggest that the ghrelin-GHS-R1a system is present in this lungfish although ghrelin has not yet been found. The structure of GHS-R1a is closer to that of tetrapods than Actinopterygian fish, indicating a process of evolution that follows the Crossopterygii such as coelacanth.

Gastric motility in ghrelin receptor knockout mice

The aim of this study was to investigate the effects and possible mechanisms of ghrelin receptor (GHS-R) deficiency on gastric motility in GHS-R deficient (Ghsr-/-) mice. Ghsr-/- and control (Ghsr+/+) mice were genotyped by PCR. The percentage of gastric emptying (GE%) was calculated following the intraperitoneal adminis-tration of ghrelin. In vitro, the contractile response of smooth muscle strips to ghrelin and electrical field stimulation (EFS) and the intraluminal pressure change of isolated stomach to carbachol were observed in an organ bath. The staining of nerve cells in the gastric muscle layer was performed by immunofluorescence. Delayed gastric emptying was observed in the Ghsr-/- mice; ghrelin enhanced the GE% in the Ghsr+/+ mice but had no effect on the GE% in the Ghsr-/- mice. In vitro, the response of the strips to ghrelin and EFS and the intraluminal pressure change to cabarchol was reduced in the Ghsr-/- mice. GHS-Rs were predominantly expressed on nerve cells in gastric muscle layers. The number of nerve cells was observed to be decreased in the Ghsr-/- mice. The delayed gastric emptying may relate to the loss of GHS-Rs and the reduction in the number of nerve cells in the gastric muscle layers of the GHS-R-deficient mice.

Growth hormone response to growth hormone-releasing peptide-2 in growth hormone-deficient little mice

OBJECTIVE

To investigate a possible direct, growth hormone-releasing, hormone-independent action of a growth hormone secretagogue, GHRP-2, in pituitary somatotroph cells in the presence of inactive growth hormone-releasing hormone receptors.

MATERIALS AND METHODS

The responses of serum growth hormone to acutely injected growth hormone-releasing P-2 in lit/lit mice, which represent a model of GH deficiency arising from mutated growth hormone-releasing hormone-receptors, were compared to those observed in the heterozygous (lit/+) littermates and wild-type (+/+) C57BL/6J mice.

RESULTS

After the administration of 10 mcg of growth hormone-releasing P-2 to lit/lit mice, a growth hormone release of 9.3±1.5 ng/ml was observed compared with 1.04±1.15 ng/ml in controls (p<0.001). In comparison, an intermediate growth hormone release of 34.5±9.7 ng/ml and a higher growth hormone release of 163±46 ng/ml were induced in the lit/+ mice and wild-type mice, respectively. Thus, GHRP-2 stimulated growth hormone in the lit/lit mice, and the release of growth hormone in vivo may be only partially dependent on growth hormone-releasing hormone. Additionally, the plasma leptin and ghrelin levels were evaluated in the lit/lit mice under basal and stimulated conditions.

CONCLUSIONS

Here, we have demonstrated that lit/lit mice, which harbor a germline mutation in the Growth hormone-releasing hormone gene, maintain a limited but statistically significant growth hormone elevation after exogenous stimulation with GHRP-2. The present data probably reflect a direct, growth hormone-independent effect on Growth hormone S (ghrelin) stimulation in the remaining pituitary somatotrophs of little mice that is mediated by growth hormone S-R 1a.

Electrophysiological effect of ghrelin and somatostatin on rat hypothalamic arcuate neurons in vitro

Growth hormone (GH) secretion from the pituitary gland is partly regulated by GH releasing hormone (GHRH)-containing neurons located in the hypothalamic arcuate nucleus (ARC). GHRH-containing neurons express the GH secretagogue (GHS) receptor (GHS-R) and the somatostatin (SRIF) receptor. Recently, an endogenous ligand for the GHS-R named ghrelin was found. Therefore, it seems that both ghrelin and SRIF are involved in the hypothalamic regulation of GH release via GHRH-containing neurons in the ARC. In extracellular single unit recordings from in vitro hypothalamic slice preparations from rats, application of 100 nM ghrelin substantially excited ARC neurons (82.5%), whereas 1 microM SRIF substantially inhibited them (81.8%). The ghrelin-induced excitatory and SRIF-induced inhibitory effects on ARC neurons were dose-dependent and persisted during synaptic blockade using low-Ca(2+)/high-Mg(2+) solution. In addition, the effects were antagonized by [D-Lys(3)]-GHRP-6, a GHS-R antagonist, and CYN154806, a SRIF receptor subtype sst2 antagonist, respectively. When ghrelin and SRIF were sequentially applied to ARC neurons, 95.2% were excited by ghrelin and inhibited by SRIF. Similarly, 85.0% of ARC neuroendocrine cells that project to the median eminence were excited by ghrelin and inhibited by SRIF. These results indicate that ARC neuroendocrine cells projecting to the median eminence are dose-dependently, postsynaptically and oppositely regulated by ghrelin through GHS-R and SRIF via the SRIF sst2 receptor subtype. Our results also suggest that most of these ARC neuroendocrine cells are presumably GHRH-containing neurons and are involved in the cellular processes through which ghrelin and SRIF participate in the hypothalamic regulation of GH release.

Relationship of arousal to circadian anticipatory behavior: ventromedial hypothalamus: one node in a hunger-arousal network

The mechanisms by which animals adapt to an ever-changing environment have long fascinated scientists. Different forces, conveying information regarding various aspects of the internal and external environment, interact with each other to modulate behavioral arousal. These forces can act in concert or, at times, in opposite directions. These signals eventually converge and are integrated to influence a common arousal pathway which, depending on all the information received from the environment, supports the activation of the most appropriate behavioral response. In this review we propose that the ventromedial hypothalamic nucleus (VMN) is part of the circuitry that controls food anticipation. It is the first nucleus activated when there is a change in the time of food availability, silencing of VMN ghrelin receptors decreases food-anticipatory activity (FAA) and, although lesions of the VMN do not abolish FAA, parts of the response are often altered. In proposing this model it is not our intention to exclude parallel, redundant and possibly interacting pathways that may ultimately communicate with, or work in concert with, the proposed network, but rather to describe the neuroanatomical requirements for this circuit and to illustrate how the VMN is strategically placed and connected to mediate this complex behavioral adaptation.

Stomach ghrelin-secreting cells as food-entrainable circadian clocks

Increases in arousal and activity in anticipation of a meal, termed "food anticipatory activity" (FAA), depend on circadian food-entrainable oscillators (FEOs), whose locations and output signals have long been sought. It is known that ghrelin is secreted in anticipation of a regularly scheduled mealtime. We show here that ghrelin administration increases locomotor activity in nondeprived animals in the absence of food. In mice lacking ghrelin receptors, FAA is significantly reduced. Impressively, the cumulative rise of activity before food presentation closely approximates a Gaussian function (r = 0.99) for both wild-type and ghrelin receptor knockout animals, with the latter having a smaller amplitude. For both groups, once an animal begins its daily anticipatory bout, it keeps running until the usual time of food availability, indicating that ghrelin affects response threshold. Oxyntic cells coexpress ghrelin and the circadian clock proteins PER1 and PER2. The expression of PER1, PER2, and ghrelin is rhythmic in light-dark cycles and in constant darkness with ad libitum food and after 48 h of food deprivation. In behaviorally arrhythmic-clock mutant mice, unlike control animals, there is no evidence of a premeal decrease in oxyntic cell ghrelin. Rhythmic ghrelin and PER expression are synchronized to prior feeding, and not to photic schedules. We conclude that oxyntic gland cells of the stomach contain FEOs, which produce a timed ghrelin output signal that acts widely at both brain and peripheral sites. It is likely that other FEOs also produce humoral signals that modulate FAA.

Ghrelin receptor (GHS-R)-like receptor and its genomic organisation in rainbow trout, Oncorhynchus mykiss

Ghrelin, a GH-releasing and appetite-regulating peptide that is released from the stomach is an endogenous ligand for growth hormone secretagogue-receptor (GHS-R). Two types of GHS-R are accepted to be present, a functional GHS-R1a and GHS-R1b with unknown function. In this study, we identified cDNA that encodes protein with close sequence similarity to GHS-R and exon-intron organization of the GHS-R genes in rainbow trout, Oncorhynchus mykiss. Two variants of GHS-R1a proteins with 387-amino acids, namely DQTA/LN-type and ERAT/IS-type, were identified. In 3'-RACE PCR and genomic PCR, we also identified three GHS-R1b orthologs that are consisted of 297- or 300-amino acids with different amino acid sequence at the C-terminus, in addition to the DQTA/LN-type and ERAT/IS-type variations. Genomic PCR revealed that the genes are composed of two exons separated by an intron, and that two GHS-R1a and three GHS-R1b variants are generated by three distinct genes. GHS-R1a and GHSR-1b mRNA were predominantly expressed in the pituitary, followed by the brain. Identified DQTA/LN-type or ERAT/IS-type GHS-R1a cDNA was transfected into mammalian cells, and intracellular calcium ion mobilization assay was carried out. However, we did not find any response to rat ghrelin and a homologous ligand, des-VRQ trout ghrelin, of either receptor in vitro. We found that unexpected mRNA splicing had occurred in the transfected cells, suggesting that the full-length, functional receptor protein might not be generated in the cells. Gene structure and characterization of protein sequence identified in this study were closely similar to other GHS-R, but to conclude that it is a GHS-R for rainbow trout, further study is required to confirm activation of GHS-R1a by ghrelin or GHS. Thus we designated the identified receptor proteins in this study as GHS-R-like receptor (GHSR-LR).

Ghrelin response to oral glucose load in hyperthyroidism, before and after treatment with antithyroid drugs

Hyperthyroidism is characterized by hyperphagia and increased basal metabolic rate. Ghrelin peptide is implicated in food intake through activation of the orexigenic neuropeptide Y/agouti related protein in the arcuate nucleus of hypothalamus. Also different studies suggested that ghrelin might play a role in states of energy insufficiency, controlling body weight. We therefore evaluate ghrelin levels in severe hyperthyroidism before and after medical treatment when euthyroidism was achieved, in order to evaluate its possible role in the increase of appetite and in the metabolic changes observed in hyperthyroidism. Serum ghrelin and insulin levels were measured after an oral glucose tolerance test (OGTT), in 7 severe hyperthyroid female patients, before and after medical treatment when euthyroidism was achieved. Body mass index (BMI), percentage of body fat and lean mass was also estimated in hyperthyroidism as well as in euthyroidism. Basal insulin levels were statistically higher in hyperthyroid patients with respect to euthyroid state after treatment (p=0.02, t=3.379), while homeostasis model assessment (HOMA) index for insulin sensitivity was statistically higher in hyperthyroidism (group 1) compared to euthyroidism (group 2) (1.64+/-0.69 vs 0.78+/-0.44, p=0.019, t=3.389). Fasting ghrelin concentrations were significantly reduced in group 1 compared to group 2 (938+/-578 pg/ml vs 1402+/-566 pg/ml, p<0.05, t=-2.489). Oral glucose loading induced suppression of ghrelin level in both groups, but the area under the curve for ghrelin during the OGTT in euthyroidism was greater compared to hyperthyroidism (p=0.05, t=-2.485). After medical treatment, a statistically significant increase in BMI (23.1+/-4.3 vs 25.9+/-5.1) (p=0.007, t=-4.399) was also observed. In hyperthyroidism, basal ghrelin levels showed a negative correlation with BMI (p=0.042, r=-0.829), insulin (p<0.001, r=-1.000), and HOMA index (p=0.019, r=-0.886). No correlation was found between ghrelin levels and thyroid hormone values. Ghrelin levels are decreased in hyperthyroidism and increase when euthyroidism is achieved. BMI and insulin are the main factors that influence ghrelin concentration in hyperthyroidism. T3 and T4 levels do not influence ghrelin levels. There is no evidence that ghrelin is responsible for the increase appetite seen in hyperthyroidism.

The Hepatocarcinoma Cell Line HepG2 Does Not Express a GHS-R1a-Type Ghrelin Receptor

The interaction of ghrelin, a 28-residue acylated peptide, with the growth hormone secretagogue receptor 1a (GHS-R1a) has been studied mostly in cells expressing a recombinant GHS-R1a. As awareness is growing on the importance to study G protein-coupled receptors in a natural environment, we studied the effect of ghrelin in the human hepatocellular HepG2 cell line because it has been described in literature to respond to ghrelin. Despite extensive efforts, we were not able to confirm mRNA expression of GHS-R1a by reverse transcription PCR, radioligand binding, or ghrelin-induced GHS-R1a receptor activation; therefore, we conclude that HepG2 cells do not express GHS-R1a. On the other hand, we confirmed a modest effect of ghrelin on the up-regulation of IRS-1 phosphorylation, which might suggest the existence of an alternative ghrelin receptor in HepG2 cells.

Effects of ghrelin on neuronal activity in the ventromedial nucleus of the hypothalamus in infantile rats: an in vitro study

Ghrelin is an endogenous ligand for the growth hormone (GH) secretagogue (GHS) receptor (GHS-R) and a potent stimulant for GH secretion even in infantile rats before puberty. Although the ventromedial nucleus of the hypothalamus (VMH) might be a site of action for ghrelin to induce GH release, the electrophysiological effect of ghrelin on VMH neurons in infantile rats remains to be elucidated. Thus, the purpose of the present study was to investigate the effect of ghrelin on VMH neurons using hypothalamic slices of infantile rats. Ghrelin excited a majority of VMH neurons in a concentration-dependent manner. VMH neurons that were excited by GH releasing peptide-6 (GHRP-6), a synthetic GHS, were also excited by ghrelin and vice versa. Repeated application of ghrelin to the same VMH neuron decreased progressively the excitatory responses depending on the number of times it was administered. The excitatory effect of ghrelin on VMH neurons in normal artificial cerebrospinal fluid (ACSF) persisted in low Ca2+-high Mg2+ ACSF. The present results indicate that (1) ghrelin excites a majority of VMH neurons dose-dependently and postsynaptically and (2) the excitatory effects of ghrelin are mimicked by GHRP-6 and desensitized by repeated applications of ghrelin.

Proliferative and protective effects of growth hormone secretagogues on adult rat hippocampal progenitor cells

Progenitor cells in the subgranular zone of the hippocampus may be of significance for functional recovery after various injuries because they have a regenerative potential to form new neuronal cells. The hippocampus has been shown to express the GH secretagogue (GHS) receptor 1a, and recent studies suggest GHS to both promote neurogenesis and have neuroprotective effects. The aim of the present study was to investigate whether GHS could stimulate cellular proliferation and exert cell protective effects in adult rat hippocampal progenitor (AHP) cells. Both hexarelin and ghrelin stimulated increased incorporation of (3)H-thymidine, indicating an increased cell proliferation. Furthermore, hexarelin, but not ghrelin, showed protection against growth factor deprivation-induced apoptosis, as measured by annexin V binding and caspase-3 activity and also against necrosis, as measured by lactate dehydrogenase release. Hexarelin activated the MAPK and the phosphatidylinositol 3-kinase/Akt pathways, whereas ghrelin activated only the MAPK pathway. AHP cells did not express the GHS receptor 1a, but binding studies could show specific binding of both hexarelin and ghrelin, suggesting effects to be mediated by an alternative GHS receptor subtype. In conclusion, our results suggest a differential effect of hexarelin and ghrelin in AHP cells. We have demonstrated stimulation of (3)H-thymidine incorporation with both hexarelin and ghrelin. Hexarelin, but not ghrelin, also showed a significant inhibition of apoptosis and necrosis. These results suggest a novel cell protective and proliferative role for GHS in the central nervous system.

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.

The orexigenic effect of peripheral ghrelin differs between rats of different age and with different baseline food intake, and it may in part be mediated by the area postrema

Ghrelin is mainly secreted during fasting. While an orexigenic effect of peripherally injected ghrelin has been reported, reproducing this effect has often proven difficult. Here, we hypothesized that ghrelin's effect to increase food intake may depend on the experimental conditions (e.g., age of animals). We therefore investigated the effect of an IP ghrelin injection (100 microg/kg) on food intake in rats of different age and at different times during the light-dark cycle, i.e. with different levels of baseline food intake. Ghrelin injected at dark onset in ad libitum fed young rats (body weight [BW] 92 g) slightly increased feeding while no such effect was observed in 12 h food deprived rats (BW 150 g). In the middle of the light phase, ghrelin significantly increased feeding up to 2 h after injection in ad libitum fed rats (BW 130 g; food intake 1 h after injection: NaCl 0.4 +/- 0.2 g versus ghrelin 1.2 +/- 0.3 g [p < 0.05]). In various subsequent experiments, older rats (BW 300-490 g) tested under the same conditions did not respond to a single ghrelin injection. However repeated ghrelin injection (15 microg/kg/day once daily at light onset) over 10 days significantly increased food intake in rats (BW 400-460 g) starting from day 4 of the experiment (24 h food intake: NaCl approx. 19.5 g, ghrelin 22.5 g). Interestingly, the latter effect was completely abolished in rats lesioned in the area postrema (AP). Cumulative food intake was also increased in SHAM but not in AP-X animals (e.g., after 7 days: SHAM/NaCl 135.1 +/- 5.3 g versus SHAM/ghrelin 149.7 +/- 3.5 g [p < 0.05], AP-X/NaCl 127.2 +/- 16.4 versus AP-X/ghrelin 127.9 +/- 5.3). We conclude that ghrelin's effect to increase food intake can best be demonstrated when basal food intake is low. Ghrelin increases feeding mainly in young, fast growing animals. Ghrelin may therefore link the high energy needs to body growth in young individuals. In older animals, peripheral ghrelin increased feeding when injected repeatedly over several days. At least under these conditions, ghrelin's effect was mediated by the AP/NTS region. Using repeated administration, ghrelin might be an interesting tool to increase feeding in patients suffering from wasting diseases such as cancer anorexia.

International Union of Pharmacology. LVI. Ghrelin receptor nomenclature, distribution, and function

Ghrelin is a 28-amino acid peptide originally isolated from rat stomach and is cleaved from a 117-amino acid precursor. The sequence of the mature peptide from rats and mice differs by two amino acids from that of human ghrelin. Alternative splicing of the ghrelin gene transcript can result in the translation of a second biologically active peptide, des-Gln14-ghrelin. Both peptides have a unique post-translational modification, octanoylation of Ser3, which is essential for the binding to receptors in hypothalamus and pituitary and stimulating the release of growth hormone from the pituitary. The growth hormone secretagogue receptor (GHS-R1a, Swiss-Prot code Q92847, LocusLink ID 2693), a rhodopsin-like seven transmembrane spanning G protein-coupled receptors belonging to Family A, was cloned in 1996 from the pituitary and hypothalamus and shown to be the target of growth hormone secretagogues (GHS), a class of synthetic peptide and nonpeptide compounds causing growth hormone release from the anterior pituitary. In 1999, ghrelin was identified as the endogenous cognate ligand for this receptor. The purpose of this review is to propose an official International Union of Pharmacology Committee on Receptor Nomenclature and Drug Classification (NC-IUPHAR) nomenclature designating GHS-R1a as the ghrelin receptor to follow the convention of naming receptors after the endogenous agonist, abbreviated where necessary to GRLN.

Stimulation of neurogenesis in rat nucleus of the solitary tract by ghrelin

Ghrelin, a gastric hormone, regulates growth hormone secretion and energy homeostasis. The present study shows that ghrelin promotes neural proliferation in vivo and in vitro in the rat nucleus of the solitary tract (NTS). Systemic administration of ghrelin significantly increased 5-bromo-2'-deoxyuridine (BrdU) incorporation in the NTS in adult rats with cervical vagotomy. Cultured NTS neurons contain immature precursor cells as shown by expression of Hu protein. Exposure of cultured NTS neurons to ghrelin significantly increased the percentage of BrdU incorporation into cells in both dose- and time-dependent manners. Co-localization of Hu immunoreactivity with BrdU labeling was demonstrated by double fluorescent staining, suggesting that cells labeled with BrdU are neuronal cells. Ghrelin receptor mRNA was detected in tissues from the NTS. The mitotic effect of ghrelin was abolished by treatment of cultured NTS neurons with ghrelin receptor antagonists: D-Lys-3-GHRP-6 and [D-Arg1, D-Phe-5, D-Trp-7, 9, Leu-11] substance P. Diltiazem, a L-type calcium channel blocker, significantly attenuated ghrelin-mediated increments in BrdU incorporation. Ghrelin acts directly on NTS neurons to stimulate neurogenesis.

Ghrelin and des-acyl ghrelin both inhibit isoproterenol-induced lipolysis in rat adipocytes via a non-type 1a growth hormone secretagogue receptor

Besides possessing a strong growth hormone (GH)-releasing activity, the gastrointestinal octanoylated peptide ghrelin has been reported to antagonize lipolysis in rat adipocytes. It is not yet clear whether this inhibitory activity on lipolysis is also shared by the major circulating isoform, des-acyl ghrelin, that does not activate the ghrelin receptor, namely the type 1a GH secretagogue-receptor (GHS-R1a) and lacks the endocrine effects of the acylated form. Here we show that des-acyl ghrelin, like ghrelin and some synthetic GHS (hexarelin and MK0677) and carboxy-terminally ghrelin fragments such as ghrelin-(1-5) and ghrelin-(1-10), all significantly reduced, over concentrations ranging from 1 to 1000 nM, the stimulation of glycerol release caused in rat epididymal adipocytes by the nonselective beta-adrenoceptor agonist isoproterenol in vitro. The order of potency on stimulated-lipolysis was: des-acyl ghrelin=ghrelin>MK0677=hexarelin>ghrelin-(1-5)=ghrelin-(1-10). This ranking was consistent with the binding experiments performed on membranes of epididymal adipose tissue or isolated adipocytes that did not express mRNA for GHS-R1a. A common high-affinity binding site was recognized in these cells by both acylated and des-acylated ghrelin and also by hexarelin, MK0677, ghrelin-(1-5) and ghrelin-(1-10). In conclusion, these findings provide the first evidence that des-acyl ghrelin, as well as ghrelin, short ghrelin fragments and synthetic GHS, may act directly as antilipolytic factors on the adipose tissue through binding to a specific receptor which is distinct from GHS-R1a.

Ghrelin stimulates neurogenesis in the dorsal motor nucleus of the vagus

Ghrelin, a gastric peptide hormone, has been reported to regulate growth hormone secretion and energy homeostasis. Here we show that ghrelin promotes neural proliferation in vivo and in vitro in the rat dorsal motor nucleus of the vagus (DMNV). Ghrelin receptor mRNA and immunoreactivity were detected in tissues from DMNV. Systemic administration of ghrelin (130 nmol kg(-1)) significantly increased 5-bromo-2'-deoxyuridine (BrdU) incorporation in the DMNV in adult rats with cervical vagotomy (BrdU positive cells; from 27 +/- 4 to 69 +/- 14 n = 5, P < 0.05). In vitro, exposure of cultured DMNV neurones to ghrelin significantly increased the percentage of BrdU incorporation into cells in both dose-dependent (10(-9) -10(-6)m), and time-dependent (6 h to 48 h) manners. Ghrelin significantly increased voltage-activated calcium currents in isolated single DMNV neurones from a mean maximal change of 141 +/- 26 pA to 227 +/- 37 pA. Upon removal of ghrelin, calcium currents slowly returned to baseline. Blocking L-type calcium channels by diltiazem (10 microm) significantly attenuated ghrelin-mediated increments in BrdU incorporation (n = 5, P < 0.05). Ghrelin acts directly on DMNV neurones to stimulate neurogenesis.

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.

Intracellular signaling mechanisms mediating ghrelin-stimulated growth hormone release in somatotropes

Ghrelin is a newly discovered peptide that binds the receptor for GH secretagogues (GHS-R). The presence of both ghrelin and GHS-Rs in the hypothalamic-pituitary system, together with the ability of ghrelin to increase GH release, suggests a hypophysiotropic role for this peptide. To ascertain the intracellular mechanisms mediating the action of ghrelin in somatotropes, we evaluated ghrelin-induced GH release from pig pituitary cells both under basal conditions and after specific blockade of key steps of cAMP-, inositol phosphate-, and Ca2+-dependent signaling routes. Ghrelin stimulated GH release at concentrations ranging from 10-10 to 10-6 m. Its effects were comparable with those exerted by GHRH or the GHS L-163,255. Combined treatment with ghrelin and GHRH or L-163,255 did not cause further increases in GH release, whereas somatostatin abolished the effect of ghrelin. Blockade of phospholipase C or protein kinase C inhibited ghrelin-induced GH secretion, suggesting a requisite role for this route in ghrelin action. Unexpectedly, inhibition of either adenylate cyclase or protein kinase A also suppressed ghrelin-induced GH release. In addition, ghrelin stimulated cAMP production and also had an additive effect with GHRH on cAMP accumulation. Ghrelin also increased free intracellular Ca2+ levels in somatotropes. Moreover, ghrelin-induced GH release was entirely dependent on extracellular Ca2+ influx through L-type voltage-sensitive channels. These results indicate that ghrelin exerts a direct stimulatory action on porcine GH release that is not additive with that of GHRH and requires the contribution of a multiple, complex set of interdependent intracellular signaling pathways.

Regulation of ghrelin secretion and action

The pulsatile release of growth hormone (GH) from anterior pituitary gland is regulated by the interplay of at least two hypothalamic hormones, GH-releasing hormone (GHRH) and somatostatin, via their engagement with specific cell surface receptors on the anterior pituitary somatotroph. Furthermore, release of GH in vivo may also be controlled by a third type of receptor, the growth hormone secretagogue receptor, a G-protein-coupled receptor, called GHS receptor type 1a (GHSR1a), which was identified in the pituitary and the hypothalamus in humans using a nonpeptidyl growth hormone secretagogue (MK-0677). Ghrelin, the endogenous ligand for the GHS-R1a, is a 28-amino-acid peptide isolated from human stomach that is modified by a straight chain octanoyl group covalently linked to Ser3, which is essential for its endocrine activity. This hormone, predominantly expressed and secreted by the stomach, has a dual action on GH secretion and food intake, showing interdependency between these actions. The finding that fasting and food intake, respectively, increase and decrease the secretion of ghrelin suggests that this hormone may be the bridge connecting somatic growth and body composition with energy metabolism, and appears to play a role in the alteration of energy homeostasis and body weight in pathophysiological states such as hypothyroidism and hyperthyroidism. Despite this, little is known about the intracellular signaling through which ghrelin exerts its regulatory actions. Activation of intracellular calcium mobilization is one of the earliest known cellular signals elicited by ghrelin. In HEK- 293 cells expressing the GHS-R1a, ghrelin induces a biphasic cytosolic calcium elevation characterized by a spike phase of the response, which reflects Ins(1,4,5)P3- dependent calcium mobilization of intracellular stores, and a sustained phase of the response, which is due to calcium influx across the plasma membrane triggered by aperture of capacitative calcium channels (store-operated calcium channels). Upon repeated administration, ghrelin showed a marked suppression of ghrelin-mediated elevations of intracellular calcium. This homologous desensitization represents an important physiological mechanism that modulates receptor responsiveness and acts as an information filter for intracellular signaling system. The discovery of ghrelin adds a new component to the complex machinery responsible for regulation of GH secretion in connection with the regulation of appetite and energy homeostasis.

Hypothalamic paraventricular injections of ghrelin: effect on feeding and c-Fos immunoreactivity

The paraventricular hypothalamic nucleus (PVN) appears to integrate orexigenic properties of a novel peptide, ghrelin. Thus, we examined central mechanisms underlying feeding generated by intra-PVN ghrelin. We established that 0.03 nmol of PVN-injected ghrelin was the lowest dose increasing food consumption and it induced c-Fos immunoreactivity (a marker of neuronal activation) in the PVN itself, as well as in other feeding-related brain areas, including the hypothalamic arcuate and dorsomedial nuclei, central nucleus of the amygdala, and nucleus of the solitary tract. We conclude that the PVN, as part of larger central circuitry, mediates orexigenic properties of ghrelin.

Neural basis of orexigenic effects of ghrelin acting within lateral hypothalamus

Ghrelin stimulates feeding when administered centrally and peripherally. The lateral hypothalamus (LH) is thought to mediate ghrelin-induced hyperphagia. Thus, we examined central mechanisms underlying feeding generated by LH ghrelin. We determined that 0.3nmol of LH-injected ghrelin was the lowest dose increasing food consumption and it induced Fos immunoreactivity (IR; a marker of neuronal activation) in feeding-related brain areas, including the hypothalamic paraventricular, arcuate, and dorsomedial nuclei, amygdala, and nucleus of the solitary tract. Also, LH ghrelin induced Fos IR in LH orexin neurons. We conclude that the LH, as part of larger central circuitry, integrates orexigenic properties of ghrelin.

Endogenous ghrelin is an orexigenic peptide acting in the arcuate nucleus in response to fasting

Ghrelin, a circulating growth-hormone releasing peptide derived from stomach, stimulates food intake through neuropeptide Y (NPY) neurons of the arcuate nucleus in the hypothalamus (ARC). We examined the effect of ghrelin microinjected into the ARC and the influence of intracerebroventricular (i.c.v.) pretreatment with a GHRH or NPY receptor antagonist on ghrelin-induced food intake in free-feeding male rats. Ghrelin (0.1-1 microg) stimulated food intake in a dose-dependent manner, and this effect was reduced by 55-60% by the Y(5) NPY receptor antagonist (10 microg i.c.v.), but not by the GHRH receptor antagonist MZ-4-71 (10 microg i.c.v.). We also evaluated the effects of passive ghrelin immunoneutralization by the microinjection of anti-ghrelin immunoglobulins (IgGs) intracerebroventricularly or directly into the ARC on food intake in free-feeding and fasted male rats. i.c.v. administration of anti-ghrelin IgGs decreased cumulative food intake over 24 h, whereas microinfusion of anti-ghrelin IgGs into the ARC induced only a short-lived (2 and 6 h) effect. Collectively, these data would indicate that centrally derived ghrelin has a major role in the control of food intake in rats and, in this context, blood-born ghrelin would be effective only in relation to its ability to reach the ARC, which is devoid of blood-brain barrier.

Research progress in the stimulatory inputs regulating growth hormone (GH) secretion

A review is presented on progress in the research of stimulatory inputs that regulate growth hormone secretion, including recent results on the action of the hypothalamic peptides growth-hormone releasing factor (GHRH) and pituitary adenylate cyclase-activating polypeptide (PACAP), as well as that of both peptidic (growth hormone-releasing hexapeptide; GHRP-6) and non-peptidyl (L-163,255) synthetic GHSs on somatotrope cell function.

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.

Ghrelin, an endogenous growth hormone secretagogue, is a novel orexigenic peptide that antagonizes leptin action through the activation of hypothalamic neuropeptide Y/Y1 receptor pathway

Ghrelin, an endogenous ligand for growth hormone secretagogue (GHS) receptor originally isolated from the stomach, occurs in the hypothalamic arcuate nucleus and may play a role in energy homeostasis. Synthetic GHSs have activated the hypothalamic arcuate neurons containing neuropeptide Y (NPY), suggesting the involvement of NPY in some of ghrelin actions. This study was designed to elucidate the role of ghrelin in the regulation of food intake. A single intracerebroventricular (ICV) injection of ghrelin (5-5,000 ng/rat) caused a significant and dose-related increase in cumulative food intake in rats. Ghrelin (500 ng/rat) was also effective in growth hormone-deficient spontaneous dwarf rats. Hypothalamic NPY mRNA expression was increased in rats that received a single ICV injection of ghrelin (500 ng/rat) (approximately 160% of that in vehicle-treated groups, P < 0.05). The ghrelin's orexigenic effect was abolished dose-dependently by ICV co-injection of NPY Y1 receptor antagonist (10-30 microg/rat). The leptin-induced inhibition of food intake was reversed by ICV co-injection of ghrelin in a dose-dependent manner (5-500 ng/rat). Leptin reduced hypothalamic NPY mRNA expression by 35% (P < 0.05), which was abolished by ICV co-injection of ghrelin (500 ng/rat). This study provides evidence that ghrelin is an orexigenic peptide that antagonizes leptin action through the activation of hypothalamic NPY/Y1 receptor pathway.

High plasma growth hormone (GH) levels inhibit expression of GH secretagogue receptor messenger ribonucleic acid levels in the rat pituitary

Synthetic GH secretagogues (GHSs) act via a receptor (GHS-R) distinct from that of GH-releasing hormone. The GHS-R has been cloned from the pituitary and is expressed not only in the pituitary but also in specific areas of the brain, including the hypothalamus. Recent studies suggest that hypothalamic GHS-R expression is regulated by GH. This study was designed to investigate whether pituitary GHS-R expression is modulated by GH. Female Wistar-Furth rats were injected sc with either saline (control) or GC tumor cells (GC) that secrete rat GH. The tumors were allowed to develop for 1-4 weeks. At weeks 1-4, control (n = 4-8) and GC rats (n = 3-8) were killed. Pituitary GHS-R messenger RNA (mRNA) was measured by a quantitative competitive PCR assay. The endogenous GHS-R mRNA levels were measured by determining the amount of competitive template RNA required to produce equimolar amounts of native and competitive template PCR products. The mean log plasma GH levels were significantly greater in the GC rat group than in the control group at weeks 2, 3, and 4. At these times, the mean log pituitary GHS-R mRNA contents were significantly lower in the GC rat group than in the control group. No relationship could be established between log estradiol levels and GHS-R levels. These data indicate that pituitary GHS-R expression is modulated by GH.

The growth hormone secretagogue hexarelin improves cardiac function in rats after experimental myocardial infarction

Several studies have shown that GH can enhance cardiac performance in rats after experimental myocardial infarction and in humans with congestive heart failure. In the present study, the hemodynamic effects of hexarelin (Hex), an analog of GH-releasing peptide-6 and a potent GH secretagogue, were compared with the effects of GH. Four weeks after ligation of the left coronary artery male rats were treated sc twice daily with hexarelin [10 microg/kg x day (Hex10) or 100 microg/kg x day (Hex100)], recombinant human GH (2.5 mg/kg x day), or 0.9% NaCl for 2 weeks. Transthoracic echocardiography was performed before and after the treatment period. GH, but not Hex, increased body weight gain. GH and Hex100 decreased total peripheral resistance (P < 0.05) and increased stroke volume (P < 0.05 and P < 0.01, respectively) and stroke volume index (P = 0.06 and P < 0.01, respectively) vs. NaCl. Cardiac output was increased by GH and Hex100 (P < 0.05), and cardiac index was increased by Hex100 with a borderline significance for GH (P = 0.06). In conclusion, Hex improves cardiac function and decreases peripheral resistance to a similar extent as exogenous GH in rats postmyocardial infarction. The mechanisms of these effects are unclear; they could be mediated by GH or a direct effect of Hex on the cardiovascular system.

Tissue distribution of prolactin-releasing peptide (PrRP) and its receptor

Prolactin-releasing peptide (PrRP) is a novel bioactive peptide, originally isolated from bovine hypothalamus by utilizing an orphan seven-transmembrane-domain receptor expressed in the human pituitary gland. In this paper, we analyzed the tissue distribution of rat and human PrRP and their receptor mRNAs by quantitative reverse transcription-polymerase chain reaction (RT-PCR) and Northern blotting. In RT-PCR analysis, rat PrRP receptor mRNA was detected in the central nervous system, and the highest expression was detected in the pituitary gland. In addition, in situ hybridization revealed that rat PrRP receptor mRNA was highly expressed in the anterior lobe of the pituitary. On the other hand, rat PrRP mRNA was most abundantly expressed in the medulla oblongata, while significant levels of expression were widely detected in other tissues. In Northern blot analyses, human PrRP receptor mRNA was detected only in the pituitary gland among tissues examined. Human PrRP mRNA was detected in the medulla oblongata and in the pancreas. In contrast to the pattern of mRNA expression, the highest content of bioactive PrRP was found in the hypothalamus rather than the medulla oblongata in the rat brain, indicating that PrRP mRNA does not always parallel with mature PrRP in tissue distribution. The wide distribution of PrRP and its receptor suggests that they have various functions not only in the pituitary gland but also in the other tissues.

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.

Pathophysiology of the neuroregulation of growth hormone secretion in experimental animals and the human

During the last decade, the GH axis has become the compelling focus of remarkably active and broad-ranging basic and clinical research. Molecular and genetic models, the discovery of human GHRH and its receptor, the cloning of the GHRP receptor, and the clinical availability of recombinant GH and IGF-I have allowed surprisingly rapid advances in our knowledge of the neuroregulation of the GH-IGF-I axis in many pathophysiological contexts. The complexity of the GHRH/somatostatin-GH-IGF-I axis thus commends itself to more formalized modeling (154, 155), since the multivalent feedback-control activities are difficult to assimilate fully on an intuitive scale. Understanding the dynamic neuroendocrine mechanisms that direct the pulsatile secretion of this fundamental growth-promoting and metabolic hormone remains a critical goal, the realization of which is challenged by the exponentially accumulating matrix of experimental and clinical data in this arena. To the above end, we review here the pathophysiology of the GHRH somatostatin-GH-IGF-I feedback axis consisting of corresponding key neurotransmitters, neuromodulators, and metabolic effectors, and their cloned receptors and signaling pathways. We propose that this system is best viewed as a multivalent feedback network that is exquisitely sensitive to an array of neuroregulators and environmental stressors and genetic restraints. Feedback and feedforward mechanisms acting within the intact somatotropic axis mediate homeostatic control throughout the human lifetime and are disrupted in disease. Novel effectors of the GH axis, such as GHRPs, also offer promise as investigative probes and possible therapeutic agents. Further understanding of the mechanisms of GH neuroregulation will likely allow development of progressively more specific molecular and clinical tools for the diagnosis and treatment of various conditions in which GH secretion is regulated abnormally. Thus, we predict that unexpected and enriching insights in the domain of the neuroendocrine pathophysiology of the GH axis are likely be achieved in the succeeding decades of basic and clinical research.

Identification of five novel germline mutations of the MEN1 gene in Japanese multiple endocrine neoplasia type 1 (MEN1) families

Multiple endocrine neoplasia type 1 (MEN1) is an autosomal dominant disorder characterised by tumours of the parathyroid glands, the anterior pituitary, and endocrine pancreas. The MEN1 gene has recently been cloned and germline mutations have been identified in MEN1 patients in the United States, Canada, and Europe. We examined MEN1 gene mutations in MEN1 and MEN1 related cases in eight unrelated Japanese families. These families include five familial MEN1 (FMEN1), two sporadic MEN1 (SMEN1), and one familial hyperparathyroidism (FHP). Direct sequence analysis of the protein coding regions was carried out in all the probands. We identified six different heterozygous mutations in the coding region, of which five were novel, including one missense mutation (E45G) in both FMEN1 and SMEN1, three deletions (569del, 711del, and 1350del3) in FMEN1 and FHP, and two nonsense mutations (R29X and Y312X) in FMEN1 and SMEN1. Only one of these mutations (Y312X) has previously been reported. One proband with FMEN1 had no mutation in the entire exon sequence including the 5' and 3' untranslated regions. A restriction digestion analysis of 19 relatives from the five families showed a close correlation between the existence of the MEN1 gene mutation and disease onset. Four different polymorphisms, including two novel ones, were identified. These findings imply that a diversity of MEN1 gene mutations exists in Japanese MEN1 and MEN1 related disease, suggesting that analysis of the entire coding region of the MEN1 gene is required for genetic counselling in Japan.