Sex-Steroid Interaction with Somatostatin and GHRH in the Rat

Gonadal steroids modulate endocrine systems at both hypothalamic and pituitary levels (1). In particular, in the rat, testosterone (T) and estradiol (E2) have been shown to influence the synthesis and secretion of growth hormone (GH), as well as the pituitary cell response to somatostatin (SS) and GH- releasing hormone (GHRH) (2,3). These effects are exerted directly on the somatotroph (4) or through the alteration of the hypothalamic SS or GHRH (5,6), suggesting that in the rat hypothalamic GRF and SS neurons are targets for gonadal steroids. Previous studies done in vivo by in situ hybridization have indicated that T stimulates SS and GHRH gene expression (5,6), but not all studies have been in agreement with these findings (7). Also, differences in SS and GHRH concentrations in hypothalamic tissue have been reported in different developmental stages by several authors (1, 8–12).