Stimulation of pituitary gonadotropin and ovarian development by chronic administration of testosterone in female Japanese silver eel, Anguilla japonica

Serial implantation of testosterone pellets, at 30-day intervals to day 75 or at 15-day intervals to day 105, stimulated the pituitary content of gonadotropin (GtH) to increase approximately 24 fold; however, no significant changes in serum GtH levels were detected at the sampling times used. Vitellogenesis was stimulated in the ovary of all testosterone-implanted animals; the ovary of at least one eel appeared to be at a preovulatory state (gonadosomatic index = 39.8%), as demonstrated by the presence of large yolky oocytes. Serial implantation of LHRH-A or domperidone along with testosterone did not further stimulate pituitary GtH content or ovarian development. The results demonstrate that chronic treatment with testosterone alone can stimulate the brain-pituitary-ovary axis of the Japanese silver eel to induce ovarian development to, or nearly to, the prespawning stage.