Negative Feedback Control of Gonadotropins in Male Rhesus Monkeys: Effects of Time After Castration and Interactions of Testosterone and Estradiol-17β

Abstract

The ability of testosterone (T) to exert negative feedback action on systemic levels of LH [luteinizing hormone] and FSH [follicle stimulating hormone] was tested in long-term (> 2 yr) gonadectomized (GX) male (n = 10), female (n = 6) and prenatally androgenized female (n = 7) rhesus monkeys and in 12 recently GX males. In all 3 groups of long-term GX animals, estradiol-17.beta. (E2) administered in Silastic capsules for 6 days (serum levels of .apprx. 100 pg/ml) suppressed serum LH and FSH to approximately 50% of preimplantation levels. T [averaging 8.5 .+-. 0.3 (SE) ng/ml] administered simiarly for 1 wk, beginning 1 day before the E2 was removed, was unable to maintain the suppression initiated by E2 in males. Both FSH and LH gradually rose to pretreatment levels in both T-treated and control monkeys. In 2nd and 34r 3rd trials, similar and larger [11.7 .+-. 0.54 (SE) ng/ml] amounts of T were unable to suppress gonadotropins in males when given alone for 21 days without E2 pretreatment. Six males were then gonadectomized to test long-term vs. short-term GX effects. Half of the animals were given T for 21 days beginning 30 min before castration; the others were given blank implants. The treatments were then reversed, gor 24 additional days. Serum FSH and LH concentrations reached postcastration levels approximately 20 days after GX. As in long-term GX monkeys, T in physiological quantities was unable to exert a negative feedback action on gonadotropins. In 3 animals, small amounts of E2 (.apprx. 30 pg/ml serum) combined with T (.apprx. 6 ng/ml serum) beginning 30 min before castration maintained precastration levels of LH for 21 days. In 3 other males, this regimen of E2 without T was ineffective. A similar treatment of E2 and T in 6 monkeys starting 104 days after castration was able to suppress FSH but not LH. E2 concentration in the spermatic vein are 4 times higher than in the systemic circulation of the rhesus male, evidence for E2 secretion by the testes. These experiments suggest a synergistic role for E2 of testicular origin with T in negative feedback control of gonadotrpin that changes after castration in the male monkey.