EFFECTS OF THYROID-HORMONE ON GONADOTROPIN-INDUCED STEROID-PRODUCTIONIN MEDAKA, ORYZIAS-LATIPES, OVARIAN FOLLICLES

Blood and ovarian samples were collected at intervals of 4h prior to s pawning time from medaka (Oryzias latipes) that were maturationally sy nchronized with artificial photoperiod (14h light: 10h dark). Plasma e stradiol-17beta (E2) levels increased rapidly from 16h before spawning and peaked at 8h before spawning. Follicle-enclosed oocytes (ovarian follicles) at different stages of development were isolated from the o varies and used to study the in vitro effects of thyroid hormone (trii odothyronine; T3) on pregnant mare serum gonadotropin (GTH)-induced E2 production. GTH at a concentration of 100 IU/ml stimulated E2 product ion by ovarian follicles collected between 32 and 16h before spawning. At 32h before spawning, T3 (5 ng/ml) administered along with GTH (100 IU/ml) resulted in a 3.5 fold increase in E2 production, compared wit h GTH administered alone. These results suggest that T3 can act on ova rian follicles directly to modulate GTH-stimulated E2 production in th e medaka.