Other than growth hormone neuroendocrine actions of ghrelin

Abstract

Besides its growth hormone-releasing effect, ghrelin has been demonstrated to influence other hormonal systems, such as the hypothalamo-pituitary-adrenal axis, prolactin secretion, the thyroid axis as well as the gonadal axis. Ghrelin and its analogues stimulate the hypothalamo-pituitary-adrenal axis independent of the pituitary, via the hypothalamus, involving both corticotrophin-releasing hormone, arginine-vasopressin and neuropeptide Y stimulation. In adrenocortocotropic hormone (ACTH)-secreting tumors, the ghrelin receptor is pathologically expressed, thus accounting for especially high ACTH and cortisol responses to ghrelin and GH secretagogues in patients with Cushing's disease. Ghrelin stimulates prolactin release most probably from the somatomammotroph cells of the pituitary gland. The effect of ghrelin on the pituitary regulation of the thyroid axis is controversial and its role in the physiological control of thyroid function is still matter of investigation. On the other hand, ghrelin has been reported to exert an inhibitory effect on follicle-stimulating hormone and, in particular, on luteinizing hormone, probably via an inhibitory effect exerted at the hypothalamic level on gonadotropin-releasing hormone secretion.