Administration of ghrelin, an endogenous ligand for the GH secretagogue receptor 1a (GHSR 1a), induces potent stimulating effects on GH secretion and food intake. However, more than 7 yr after its discovery, the role of endogenous ghrelin remains elusive. Recently, a second peptide, obestatin, also generated from proteolytic cleavage of preproghrelin has been identified. This peptide inhibits food intake and gastrointestinal motility but does not modify in vitro GH release from pituitary cells. In this study, we have reinvestigated obestatin functions by measuring plasma ghrelin and obestatin levels in a period of spontaneous feeding in ad libitum-fed and 24-h fasted mice. Whereas fasting resulted in elevated ghrelin levels, obestatin levels were significantly reduced. Exogenous obestatin per se did not modify food intake in fasted and fed mice. However, it inhibited ghrelin orexigenic effect that were evident in fed mice only. The effects of obestatin on GH secretion were monitored in superfused pituitary explants and in freely moving rats. Obestatin was only effective in vivo to inhibit ghrelin stimulation of GH levels. Finally, the relationship between octanoylated ghrelin, obestatin, and GH secretions was evaluated by iterative blood sampling every 20 min during 6 h in freely moving adult male rats. The half-life of exogenous obestatin (10 microg iv) in plasma was about 22 min. Plasma obestatin levels exhibited an ultradian pulsatility with a frequency slightly lower than octanoylated ghrelin and GH. Ghrelin and obestatin levels were not strictly correlated. In conclusion, these results show that obestatin, like ghrelin, is secreted in a pulsatile manner and that in some conditions; obestatin can modulate exogenous ghrelin action. It remains to be determined whether obestatin modulates endogenous ghrelin actions.

This short review is focused on the neuroendocrine regulation of growth hormone (GH) pulsatile secretory pattern and GH gene expression. The neuronal network involved in the central control of GH includes extrahypothalamic neurons such as the noradrenergic and cholinergic systems, which regulate the two antagonistic neurohormonal systems: somatostatin (SRIH) and GH-releasing hormone (GHRH). Intrahypothalamic Proopiomelanocortin- and Galanin-containing interneurons also participate in the regulation of SRIH and GHRH neuronal activity, which also is dependent on sex steroids and GH and/or insulin-like growth factor I (IGF-I) feedback. cAMP (controlled mainly by GHRH and SRIH), thyroid and glucocorticoid hormones. IGF-I and activin are among the factors that regulate GH gene expression at the transcriptional level and may play a role in somatotroph differentiation and proliferation during ontogeny as well as physiological and pathological states such as acromegaly.

et al.. Pulsatile cerebrospinal fluid and plasma ghrelin in relation to growth hormone secretion and food intake in the sheep.. Journal of Neuroendocrinology, Wiley, 2008, 20 (10), pp.1138-46. 10.1111/j.1365-2826.2008 Pulsatile CSF and plasma ghrelin in relation with GH secretion and food intake in the sheep.Grouselle Dominique, * Chaillou Elodie, *Caraty Alain, Bluet-Pajot Marie-Therese, Zizzari Philippe, *Tillet Yves, Epelbaum Jacques. Key words : plasma, CSF, pulsatile patterns. AddressesElodie Chaillou and Dominique Grouselle contributed equally to this work.The definitive version is available at www.blackwell-synergy.com ABSTRACTAs in other species, exogenous administration of ghrelin, an endogenous ligand for the GH secretagogue receptors can stimulates feeding behaviour and GH secretion in the sheep. However the importance of endogenous ghrelin for these two functions as well as its central or peripheral origin remained to be established. In this study, CSF ghrelin concentrations were measured in five anoestrous ewes and found to be more than 1000-fold lower than circulating plasma levels, in keeping with the even lower concentration in hypothalamic as compared to abomasum tissue extracts. CSF and plasma ghrelin levels were measured every 10 minutes over a 6 hours sampling period in 5 unanesthetised ovariectomised-estradiol implanted ewes. Mean CSF ghrelin concentrations were 1400-fold lower than circulating plasma levels. Cluster analysis indicated that CSF ghrelin levels were markedly pulsatile with a greater number of peaks than plasma ghrelin. Pulsatility parameters were closer for GH and CSF ghrelin than between GH and plasma ghrelin. Plasma ghrelin and GH levels were significantly correlated in three out of five ewes but CSF ghrelin and GH in one ewe only. Half of the CSF ghrelin episodes were preceded by a ghrelin peak in plasma with a 22 min delay. Cross-correlations between plasma GH and plasma or CSF ghrelin did not reach significance but a trend towards cross-correlation was observed from 20 to 0 min between plasma and CSF ghrelin. At 09h00, when food was returned to ewes, voluntary food intake did not elicit a consistent change in plasma or CSF ghrelin levels. In contrast, a peripheral ghrelin injection (1 mg, i.v.) immediately stimulated food intake, feeding behaviour, and GH secretion. These effects were concomitant with a more than ten-fold increase in plasma ghrelin levels while CSF ghrelin values only doubled 40 to 50 minutes after the injection. This suggests that peripherally-injected ghrelin crosses the blood brain barrier but only in low amount 2/29 and with relatively slow kinetics when compared to its effects on GH release and food intake. Taken together, these results support the notion that, in the ovariectomised-oestradiol implanted sheep model, peripheral ghrelin injection rapidly induces GH secretion, food intake and feeding behaviour, probably by acting on GHS-R1 receptors located in brain regions in which the blood brain barrier is not complete such as, for instance, the arcu...

VIP stimulated prolactin secretion from incubated rat hemipituitaries. Under the same conditions, the secretion of GH, LH, FSH was not affected. The stimulation of prolactin was dose-dependent, with an apparent affinity of VIP of 10.9 ± 3.1 nM and a maximal stimulation of 57.7 ± 4.2%. Secretin, a structurally related peptide, was also active at higher concentrations whereas another partial analogue, glucagon, was ineffective. The effect of VIP was not blocked by α-flupentixol, a potent dopaminergic antagonist, at concentrations which antagonized the dopamine inhibition of prolactin secretion. Stimulation by VIP and TRH was additive. Neither Metenkephalin nor naloxone interfered with the response to VIP. It thus seems that specific VIP receptors are present on pituitary prolactin cells. VIP, present in the mediobasal hypothalamus and detected in the hypothalamo-hypophyseal portal blood therefore is a good candidate as a physiological PRF.

Ghrelin, the 28 amino acid peptide recently identified as the natural ligand for the growth hormone (GH) secretagogue (GHS) receptor, has multiple activities in addition to stimulation of GH secretion, including stimulation of feeding and weight gain. To utilize these actions for potential therapeutic benefit, we have produced analogs of human ghrelin with enhanced metabolic stability, affinity for the GHS receptor, and efficacy in stimulating weight gain. We have also discovered an analog of ghrelin, BIM-28163, that is an antagonist at the GHS receptor and that fully inhibits GHS receptor activation induced by native ghrelin. In vivo, BIM-28163 does not increase GH secretion but fully blocks ghrelin-induced GH secretion. In contrast, BIM-28163 acts as a full agonist with regard to the ghrelin actions of stimulating weight gain and food intake. These results suggest that a receptor other than the GHS receptor mediates the actions of ghrelin on feeding and weight gain. This concept is strengthened by our observation that at certain hypothalamic sites, BIM-28163 acts as an antagonist of ghrelin-induced neuronal activation, while at other sites, both ghrelin and BIM-28163 induce neuronal activation via the same receptor. Collectively, these results indicate the existence of a novel ghrelin receptor that may regulate the feeding activity of ghrelin. Using BIM-28163 as a tool to define the endogenous role of ghrelin in normal GH secretion, we have demonstrated that antagonism of the GHS receptor in normal rats does not impair the pulsatility of GH secretion but lowers the pulse amplitude and mean GH level. These results demonstrate that endogenous ghrelin acts to amplify the basic pattern of GH secretion established by the interplay of hypothalamic GH-releasing hormone and somatostatin. These studies demonstrate the feasibility of creating ghrelin analogs that are selective for specific activities, as well as their utility in dissecting the role of ghrelin in both normal physiology and specific pathologies.European Journal of Endocrinology 151 S71-S75

To test the hypothesis of the involvement of centrally expressed rat growth hormone receptors (rGH-R) in the ultradian rhythmicity of pituitary GH secretion, adult male rats were submitted to a 60 hr intracerebroventricular infusion of an antisense (AS) oligodeoxynucleotide (ODN) complementary to the sequence of rGH-R mRNA. Eight hour (10 A.M.-6 P.M.) GH secretory profiles, obtained from freely moving male rats infused with 2.0 nmol/hr of rGH-R AS, revealed a marked increase in GH peak amplitude (150 +/- 12 vs 101 +/- 10 ng/ml), trough levels (16.2 +/- 3.0 vs 5.4 +/- 1.4 ng/ml), and number of peaks (2.9 +/- 0.3 vs 1.8 +/- 0.2). No change was observed in rats treated with an ODN complementary to the prolactin receptor mRNA sequence (2.0 nmol/hr). Infusion of increasing ODN concentrations resulted in a dose-dependent stimulation of GH release. In parallel, somatogenic binding sites in the choroid plexus were decreased by 40%, and levels of rGH-R mRNA were increased in the periventricular nucleus (PeV) but unchanged in the arcuate nucleus (ARC). Levels of somatostatin mRNA, in the PeV but not in the ARC, were lowered by the treatment. Levels of GH-releasing hormone mRNA in the ARC were not affected. These data suggest that GH negative feedback results from a direct effect on central GH receptors and a subsequent activation of hypophysiotropic somatostatin neurons located in the anterior periventricular hypothalamus.

Effects of growth hormone (GH) hypersecretion on somatostatin-(SRIH) and GH-releasing hormone (GHRH) were studied by in situ hybridization and receptor autoradiography in rats bearing a GH-secreting tumor. 6 and 18 wk after tumor induction, animals displayed a sharp increase in body weight and GH plasma levels; pituitary GH content was reduced by 47 and 55%, while that of prolactin and thyrotropin was unchanged. At 18 wk, hypothalamic GHRH and SRIH levels had fallen by 84 and 52%, respectively. In parallel, the density of GHRH mRNA per arcuate neuron was reduced by 52 and 50% at 6 and 18 wk, while SRIH mRNA levels increased by 71 and 83% in the periventricular nucleus (with no alteration in the hilus of the dentate gyrus). The numbers of GHRH-and SRIH-synthetizing neurons in the hypothalamus were not altered in GH-hypersecreting rats. Resection of the tumor restored hypothalamic GHRH and SRIH mRNAs to control levels. GH hypersecretion did not modify 1211-SRIH binding sites on GHRH neurons. Thus, chronic GH hypersecretion affects the expression of the genes encoding for GHRH and SRIH. The effect is long lasting, not desensitizable and reversible. (J.

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