Importance of ghrelin in hypothalamus-pituitary axis on growth hormone release during normal pregnancy in the rat

Ghrelin is a hormone mainly produced in the stomach and its first discovered action was connected with regulating growth hormone secretion. It was found that ghrelin injection increases growth hormone release and that this action is dose-dependent. Ghrelin may influence growth hormone secretion both by central and peripheral action. Ghrelin acts via its receptors named growth hormone secretagogue receptors (GHSR). Ghrelin receptors were found in almost all tissues including the central nervous system. Besides influence on growth hormone secretion, ghrelin also regulates food intake and energy metabolism centrally as well as peripherally. In our study, active ghrelin and growth hormone levels in serum were measured. We also investigated gene expression of proghrelin, growth hormone releasing hormone (GHRH) and growth hormone receptor (GH-R) in the hypothalamus and the active form of ghrelin receptor (GHSR-1a) in hypothalamus and pituitary. Expression of growth hormone and growth hormone releasing hormone receptor (GHRH-R) in the pituitary were also measured. The results of our study indicate that active ghrelin and growth hormone levels in serum increased during pregnancy. Expression of ghrelin in hypothalamus and its receptor also increased in hypothalamus and pituitary during pregnancy. We also observed that growth hormone gene expression rose in pituitary, while its receptor mRNA level in hypothalamus decreased. Additionally, growth hormone expression in placenta decreased during pregnancy. Moreover, GHRH in hypothalamus and its receptor in pituitary showed reduced levels during pregnancy. Our results may indicate that ghrelin is a important factor influencing growth hormone release during pregnancy.