Cellular Distribution and Regulation of Ghrelin Messenger Ribonucleic Acid in the Rat Pituitary Gland

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Abstract
Ghrelin, a 28-amino-acid acylated peptide, strongly stimulates GH release and food intake. In the present study, we found that ghrelin is expressed in somatotrophs, lactotrophs, and thyrotrophs but not in corticotrophs or gonadotrophs of rat pituitary. Persistent expression of the ghrelin gene is found during postnatal development in male and female rats, although the levels significantly decrease in both cases from pituitaries of 20-d-old rats onward, but at 60 d old, the levels were higher in male than female rats. This sexually dimorphic pattern appears to be mediated by estrogens because ovariectomy, but not orchidectomy, increases pituitary ghrelin mRNA levels. Taking into account that somatotroph cell function is markedly influenced by thyroid hormones, glucocorticoids, GH, and metabolic status, we also assessed such influence. We found that ghrelin mRNA levels decrease in hypothyroid- and glucocorticoid-treated rats, increase in GH-deficient rats (dwarf rats), and remain unaffected by food deprivation. In conclusion, we have defined the specific cell types that express ghrelin in the rat anterior pituitary gland. These data provide direct morphological evidence that ghrelin may well be acting in a paracrine-like fashion in the regulation of anterior pituitary cell function. In addition, we clearly demonstrate that pituitary ghrelin mRNA levels are age and gender dependent. Finally, we show that pituitary ghrelin mRNA levels are influenced by alteration on thyroid hormone, glucocorticoids, and GH levels but not by fasting, which indicates that the regulation of ghrelin gene expression is tissue specific.