ROLE OF AROMATIZATION IN TESTOSTERONE-INDUCED INHIBITION OF LUTEINIZING-HORMONE SECRETION IN FEMALE TURTLES, TRACHEMYS-SCRIPTA

Both 17 beta-estradiol (E(2)) and testosterone (T) were shown to inhib it in vitro pituitary LH secretion in the turtle Trachemys scripta. Si nce T was approximately 500 times less potent than E(2), and 5 alpha-d ihydrotestosterone was even less active than T, the inhibitory action of T may result from its aromatization to estrogen. We utilized both i n vivo and in vitro approaches to elucidate the roles of T and estroge n in the negative feedback of pituitary LH secretion. Gonadectomy of a dult (vitellogenic) females significantly elevated plasma LH. Adult fe males treated with fadrozole (an aromatase inhibitor) with or without daily injections of keoxifene (an antiestrogen) also showed an increas e in plasma LH to a level comparable to that observed in gonadectomize d females, whereas plasma LH levels of juvenile females treated with f adrozole remained undetectable. In vitro LH secretion In response to G nRH in juvenile females was significantly inhibited by 48-h exposure t o 50 ng/ml T or 100 pg/ml E(2). Bath fadrozole (200 mu M) and keoxifen e (200 nM) significantly blocked this T-induced inhibition of LH secre tion, demonstrating that T lacks intrinsic inhibitory activity. Confir mation of the inhibition of aromatase activity by fadrozole comes from metabolic studies of 1 beta-[H-3]androstenedione using turtle brain, ovary, and pituitary. In vitro, fadrozole altered the metabolism of 1 beta-[H-3]androstenedione and inhibited aromatase activities in these tissues. These results indicate that the inhibitory effect of T is lar gely mediated through its aromatization to estrogen, and that estrogen is primarily responsible for the suppressed LH secretion in vitelloge nic adult turtles.