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Effects of Central Infusion of Ghrelin on Food Intake and Plasma Levels of Growth Hormone, Luteinizing Hormone, Prolactin, and Cortisol Secretion in Sheep

Prince Henry’s Institute of Medical Research (J.I., I.J.C.), Clayton, Victoria 3168, Australia; Laboratory of Animal Nutrient Metabolism (Y.K.), Kitasato University, Towada, Aomori 034, Japan; and Department of Physiology (B.C.), Monash University, Victoria 3800, Australia

Address all correspondence and requests for reprints to: Prof. Iain Clarke, Department of Physiology, P. O. Box 13F, Monash University, Victoria 3800, Australia. E-mail: iain.clarke{at}med.monash.edu.au.

Ghrelin is an endogenous ligand for the GH secretagogue/ghrelin receptor (GHS-R) and stimulates feeding behavior and GH levels in rodents and humans. A preprandial increase in plasma ghrelin levels is seen in sheep on programmed feeding, followed by a postprandial rise in plasma GH levels, but effects on food intake and endocrine function are not defined in this ruminant species. We administered ghrelin to female sheep in various modes and measured effects on voluntary food intake (VFI) and plasma levels of GH, LH, prolactin, and cortisol. Whether administered intracerebroventricularly or iv, ghrelin consistently failed to stimulate VFI. On the other hand, ghrelin invariably increased plasma GH levels and ,ß-diaminopropanoic acid-octanoyl³ human ghrelin was more potent than ovine ghrelin. Bolus injection of ghrelin into the third cerebral ventricle reduced plasma LH levels but did not affect levels of prolactin or cortisol. These findings suggested that the preprandial rise in plasma ghrelin that is seen in sheep on programmed feeding does not influence VFI but is likely to be important in the postprandial rise in GH levels. Thus, ghrelin does not appear to be a significant regulator of ingestive behavior in this species of ruminant but acts centrally to indirectly regulate GH and LH secretion.

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