Estradiol reduces pituitary responsiveness to somatostatin (SRIF-14) and down-regulates the expression of somatostatin sst2 receptors in female goldfish pituitary

Sex steroid hormones have been shown to regulate somatostatin (SRIF) gene expression in goldfish brain, which in turn influences the regulation of GH secretion. In this study, the influences of sex steroids on pituitary responsiveness to SRIF-14 and the pituitary expression of a type two SRIF receptor (sst(2)) were examined. Results from in vitro perifusion of pituitary fragments show that pituitaries from estradiol-primed sexually regressed female fish have significantly lower GH release responsiveness to pulse exposure to SRIF-14 than pituitaries from control or testosterone-treated sexually regressed females. Results from in vitro static culture show that pituitaries from sexually mature female fish have lower GH release responsiveness to SRIF-14 than those from sexually regressed females. In addition, the sst(2) receptor mRNA levels in pituitaries from mature and recrudescent female fish are significantly lower than in sexually regressed female fish. Our results indicate that estradiol acts at the level of the pituitary to regulate GH secretion by influencing the responsiveness to SRIF-14. The underlying mechanism includes, in part, reduction of the expression of sst(2) receptors, presumably leading to the lower number of the receptors available for SRIF binding.