Interactions of Estradiol with Gonadotropin-Releasing Hormone and Thyrotropin-Releasing Hormone in the Control of Growth Hormone Secretion in the Goldfish

Abstract

The effects of testosterone (T) and estradiol (E2) on serum growth hormone (GH) concentrations were investigated throughout the seasonal reproductive cycle of the female goldfish. Gonad-intact female goldfish were implanted intraperitoneally for 5 days with silastic pellets containing no steroid (blank), T(100 micrograms/g) or E2 (25-100 micrograms/g). In blank-implanted females, seasonal variations in serum GH were evident; maximal serum GH levels were found in spring while minimal GH levels were found in summer and early autumn. Implantation of E2-containing silastic capsules stimulated increases (2-4 times control) in serum GH levels throughout the reproductive cycle. Implantation of T did not affect serum GH at any time of the year. One possible mechanism by which E2 could exert its effects may be through alteration of pituitary sensitivity to GH-releasing factors. The decapeptide salmon gonadotropin-releasing hormone (sGnRH) is found in the brain and pituitary of goldfish and stimulates gonadotropin (GTH) and GH secretion. In contrast, thyrotropin-releasing hormone (TRH) stimulates GH, but not GTH, release from pars distalis fragments obtained from sexually regressed (ED50 = 5.7 +/- 3.8 nM; August) or sexually mature (ED50 = 0.53 +/- 0.28 nM; March) fish; in vivo E2 treatment resulted in a 3-fold increase in the in vitro GH response to TRH. Furthermore, E2 treatment increased sGnRH-stimulated GH release by approximately 4-fold. These results demonstrate that E2 but not T stimulates GH secretion throughout the reproductive cycle of female goldfish. Furthermore, sGnRH and TRH stimulate GH release in a teleost, and these stimulatory responses are enhanced by physiological levels of E2.