EFFECTS OF 17-BETA-ESTRADIOL ON PITUITARY AND TESTICULAR FUNCTION IN MALE GOLDFISH

The potential involvement of 17beta-estradiol in the regulation of pituitary and testicular function was investigated in male goldfish at various times during the seasonal reproductive cycle. Serum levels of testosterone, 11-ketotestosterone, and 17beta-estradiol were 10, 16, and 4 times higher in mature males (tubercles on the pectoral fins and milt production; March) than in sexually regressed males (no tubercles or milt production; July). Intraperitoneal injection Of [D-Arg6,Trp7,Leu8,Pro9-NEt]gonadotropin releasing hormone (sGnRH-A, 0.01 mug/g) elevated serum gonadotropin II levels within 6 h, and there was a concomitant increase in testosterone but not 17beta-estradiol levels in sexually regressing male goldfish (October). Males in the early stage of testicular recrudescence (November) had increased serum levels of gonadotropin II and testosterone in response to [D-Ala6,Pro9-NEt]GnRH (LHRH-A, 0.1 mug/g) or the dopamine antagonist domperidone (5 mug/g). Males implanted intraperitoneally for 10 days with solid Silastic pellet implants containing 17beta-estradiol (100 mug/g) had lower basal testosterone levels and a reduced testosterone response to both LHRH-A and domperidone. The serum gonadotropin II response to LHRH-A but not domperidone was enhanced by 17beta-estradiol. Male goldfish in mid-recrudescence receiving 17beta-estradiol implants for 4 days had lower basal testosterone and 11-ketotestosterone levels than controls. Combined treatment with sGnRH-A (0.1 mug/g) and domperidone (10 mug/g) elevated serum testosterone and 11-ketotestosterone levels at 24 h; the serum androgen response was reduced in 17beta-estradiol-implanted fish. These studies support the concept that 17beta-estradiol plays a role in the regulation of reproduction in male goldfish by enhancing pituitary gonadotropin II release and by reducing testicular androgen production.