Ghrelin, a 28-amino acid octanoylated peptide, has recently been identified in rat stomach as an endogenous ligand for the GH secretagogue receptor. In addition to GH-releasing properties, exogenous ghrelin injections exert orexigenic effects in both rodents and humans. As the endogenous peptide appears directly related to feeding behavior, we assessed its plasma levels in anorexia nervosa (AN) patients before and after renutrition and in constitutionally thin subjects with body mass indexes (BMIs) equivalent to those of AN women but with no abnormal feeding behavior. The relationships between plasma ghrelin levels and other neuroendocrine and nutritional parameters, such as GH, leptin, T3, and cortisol, were also investigated. In AN patients, morning fasting plasma ghrelin levels were doubled compared with levels in controls, constitutionally thin subjects, and AN patients after renutrition. Twenty-four-hour plasma ghrelin, GH, and cortisol levels determined every 4 h were significantly increased, whereas 24-h plasma leptin levels were decreased in AN patients compared with controls and constitutionally thin subjects. Both plasma ghrelin and leptin levels returned to control values in AN patients after renutrition. Constitutionally thin subjects displayed intermediate 24-h plasma ghrelin and leptin levels, significantly different from controls and AN patients, whereas GH and cortisol were not modified. Ghrelin was negatively correlated with BMI, leptin, and T(3) in controls, constitutionally thin subjects, and AN patients, whereas no correlation was found between GH and ghrelin or between cortisol and ghrelin. Ghrelin and BMI or T3 were still correlated after renutrition, suggesting that ghrelin is also a good nutritional indicator. Basal and GHRH-stimulated GH release were significantly increased in AN patients only. In conclusion, ghrelin is increased in AN and constitutionally thin subjects who display very low BMI but different eating behaviors, suggesting that not only is ghrelin dependent on body fat mass, but it is also influenced by nutritional status. Even though endogenous ghrelin is not strictly correlated with basal GH secretion, it may be involved in the magnitude of GHRH-induced GH release in AN patients.

Ghrelin, an endogenous ligand for the GHS receptor, stimulates GH secretion and gastrointestinal motility and has orexigenic effects. In this study, the relationships between ghrelin, GH secretion, feeding behavior, and sleep-wake patterns were investigated in adult male rats. The half-life of exogenous ghrelin (10 microg i.v.) in plasma was about 30 min. Repeated administration of ghrelin at 3- to 4-h intervals (one during lights-on and two during lights-off periods) increased GH release and feeding activity, and decreased rapid eye movement sleep duration. Endogenous plasma ghrelin levels exhibited pulsatile variations that were smaller and less regular compared with those of GH. No significant correlation between GH and ghrelin circulating levels was found, although mean interpeak intervals and pulse frequencies were close for the two hormones. In contrast, ghrelin pulse variations were correlated with food intake episodes in the lights off period, and plasma ghrelin concentrations decreased by 26% in the 20 min following the end of the food intake periods. A positive correlation between ghrelin levels and active wake was found during the first 3 h of the dark period only. In conclusion, ghrelin, in addition to affecting GH secretion, gastrointestinal motility, and feeding activity, also modifies sleep-wake patterns. However, a direct action of ghrelin per se or the indirect effects of feeding (and all of its attendant metabolic sequelae) on sleep cannot be differentiated. Moreover, ghrelin secretion is pulsatile and directly related to feeding behavior only.

The original strain of proopiomelanocortin (POMC)-deficient mice (Pomc Ϫ/Ϫ ) was generated by homologous recombination in 129X1/SvJ (A w /A w )-derived embryonic stem cells using a targeting construct that deleted exon 3, encoding all the known functional POMC-derived peptides including ␣MSH, from the Pomc gene. Although these Pomc Ϫ/Ϫ mice exhibited adrenal hypoplasia and obesity similar to the syndrome of POMC deficiency in children, their agouti coat color was only subtly altered. To further investigate the mechanism of hair pigmentation in the absence of POMC peptides, we studied wild-type (Pomc ϩ/ϩ ), heterozygous (Pomc ϩ/Ϫ ), and homozygous (Pomc Ϫ/Ϫ ) mice on a nonagouti (a/a) 129;B6 hybrid genetic background. All three genotypes had similar black fur pigmentation with yellow hairs behind the ears, around the nipples, and in the perianal area characteristic of inbred C57BL/6 mice. Histologic and electron paramagnetic resonance spectrometry examination demonstrated that hair follicles in back skin of Pomc Ϫ/Ϫ mice developed with normal structure and eumelanin pigmentation; corresponding molecular analyses, however, excluded local production of ␣MSH and ACTH because neither Pomc nor putative Pomc pseudogene mRNAs were detected in the skin. Thus, 129;B6 Pomc null mutant mice produce abundant eumelanin hair pigmentation despite their congenital absence of melanocortin ligands. These results suggest that either the mouse melanocortin receptor 1 has sufficient basal activity to trigger and sustain eumelanogenesis in vivo or that redundant nonmelanocortin pathway(s) compensate for the melanocortin deficiency. Whereas the latter implies feedback control of melanogenesis, it is also possible that the two mechanisms operate jointly in hair follicles. (Endocrinology 146: M ELANIN BIOSYNTHESIS IS initiated both in vitro andin vivo from the obligatory hydroxylation of ltyrosine to l-dihydroxyphenylalanine (L-DOPA), catalyzed by tyrosinase (EC 1.14.18.1) (1-3). Once L-DOPA is formed, further steps of melanogenesis consisting of a series of oxidoreduction reactions and intramolecular transformations can occur spontaneously and at varying rates, depending on local concentrations of hydrogen ions, metal cations, reducing agents, thiols, and oxygen (1). Eumelanogenesis involves the transformation of dopaquinone to leukodopachrome, followed by a series of oxidoreduction reactions with production of the intermediates dihydroxyindole and dihydroxyindole carboxylic acid, which undergo polymerization to form eumelanin (1,4,5). Pheomelanogenesis also starts with dopaquinone, which is conjugated to cysteine or glutathione to yield cysteinyldopa and glutathionyldopa for further transformation into pheomelanin (1,5,6).Although tyrosinase activity is the rate-limiting step among the melanogenesis-related enzymes (1,7,8), pigmentation is under complex genetic control regulated by more than 150 alleles representing more than 90 gene loci (3, 9 -14). Protein products of these loci include enzymes, structural proteins, transcriptional ...

Ghrelin (Ghr), a 28 amino acid gastric peptide with an n-octanoylation on Ser 3, has recently been identified as an endogenous ligand of the growth hormone secretagogue (GHS) receptor. A cDNA was also isolated from a mouse stomach library encoding a protein named prepromotilin-related peptide (ppMTLRP) which shares sequence similarities with prepromotilin. Mouse and rat ppMTLRP sequences (rGhr) are identical and show 89% identity with human ghrelin (hGhr). By analogy with promotilin, cleavage of proMTLRP into an 18 amino acid endogenous processed peptide can be assumed on the basis of a conserved dibasic motif in position 9–10 of its sequence. In the present work, we compared the GH-releasing activity of rGhr28/MTLRP and of hGhr28/MTRLP with that of a shorter form of the peptide, hGhr18. A short peptide devoid of Ser-3 n-octanoylation hGhr18[–] was also tested. Addition of rGhr28, hGhr28 and hGhr18 stimulated GH release to the same extent from superfused pituitaries. The effect was dose dependent in a 10^–8 to 10^–6M concentration range. In contrast, hGhr 18[–] was inactive. In freely moving animals, both rGhr28 and hGhr28 (10 µg, i.v.) stimulated GH release, whereas the same dose of hGhr18 or of hGhr18[–] was ineffective. After rGhr28, GH plasma levels increased as early as 5 min after injection and returned to basal values within 40–60 min. Expressed as percent stimulation, administration of rGhr28 was equally effective when injected during troughs or peaks of GH. Plasma concentrations of prolactin, adrenocorticotropin and leptin were not modified. Spontaneous GH secretory episodes were no longer observed within 3 h of rGhr28 treatment, but repeated administration of the secretagogue at 3- to 4-hour intervals resulted in a similar GH response. Activation of somatostatin (SRIH) release by ether stress did not blunt the GH response to rGhr28. This suggests that the secretagogue acts in part by inhibiting endogenous SRIH, as further substantiated by the ability of rGhr28 (10^–6M), to decrease the amplitude of 25 mM K⁺-induced SRIH release from perifused hypothalami. In conclusion, (1) n-octanoylation of Ghrs and the shorter form hGhr18 is essential for the direct pituitary GH-releasing effect of this new family of endogenous GHSs; (2) only the longer forms are active in vivo and (3) inhibition of SRIH release appears involved in the mechanism of Ghr action.

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.