Chronic central infusion of growth hormone secretagogues: Effects on Fos expression and peptide gene expression in the rat arcuate nucleus

Growth hormone (GH) secretagogues induce GH release, in part, by direct actions upon anterior pituitary somatotropes and, in part, by actions upon the neuroendocrine circuitry that regulates GH secretion. In particular, acute systemic administration of GH secretagogues results in increased neuronal activity and Fos protein expression in the arcuate nucleus of the hypothalamus. Prolonged administration of GH secretagogues has been reported to have long-lasting effects upon GH release, promoting increased pulsatile secretion. Here, we investigated how chronic central infusion of GH secretagogues affects the response of arcuate nucleus neurons to acute systemic administration of GH secretagogues. In male rats, after central infusion of GH secretagogues for 5 days, there was no sustained expression of Fos in the arcuate nucleus, no significant induction of Fos expression in response to acute GH secretagogue challenge, and a greatly attenuated secretion of GH in response to acute GH secretagogue challenge, all reflecting loss of functional responsiveness to GH secretagogues. In situ hybridisation revealed that, in the arcuate nucleus of GH secretagogue-infused rats, mRNA levels for GH-releasing hormone, neuropeptide Y and somatostatin were not different than in saline-infused animals. However, somatostatin mRNA levels in the periventricular nuclei of GH secretagogue-infused rats were significantly higher than those of saline-infused rats, indicating that this nucleus may play an important role in mediating the effects of chronic GH secretagogue administration.