Effects of gonadectomy and steroid treatment on plasma gonadotropins and the response of superfused pituitaries to gonadotropin-releasing hormone in the turtle Sternotherus odoratus.

Gonadectomized (gonadex) turtles, Sternotherus odoratus, had significantly elevated plasma FSH, but LH was less consistently affected. Estradiol (E2)-implants suppressed plasma FSH in gonadex females but not in males: testosterone (T) partially suppressed FSH in males. In contrast, E2-treatment markedly suppressed pituitary LH content and in vitro LH secretion in gonadex and intact turtles (inhibitory effects of E2 were less in intact than ovariectomized females). These steroid effects were relatively specific for gonadotropin; pituitary TSH content was not altered. In vitro, pituitary LH secretion responded to doses of GnRH greater than or equal to 1 ng/ml and LH output remained elevated for at least 3 hr of continuous superfusion with gonadotropin-releasing hormone (GnRH). In general, gonadectomy elevated pituitary responsiveness to GnRH while E2 and T suppressed this responsiveness; the effects of E2 are greater in gonadectomized than intact turtles. Thus, negative gonadal feedback appears to be involved in the secretion of gonadotropins in turtles, and steroidal actions may be partly due to suppression of pituitary hormone content and responsiveness to GnRH.