Induction of gonadotropin surge by steroid hormone implantation in ovariectomized and sexually regressed female goldfish.

Involvement of steroid hormones in the occurrence of the ovulatory gonadotropin (GtH) surge was studied in goldfish. Ovariectomized female goldfish were implanted with an empty Silastic capsule or a capsule containing testosterone or estradiol, and kept below 12 degrees for 3 months (Experiment 1). Some of the steroid implanted fish showed a GtH surge which was quite similar to the normal ovulatory GtH surge in response to a water temperature rise from 12 to 20 degrees, whereas no surge was observed in fish with empty capsules. When sexually regressed female goldfish were implanted with the capsules containing testosterone or estradiol and kept at 12 degrees for 6 weeks out of spawning season, the GtH surge was also observed in these fish after the water temperature rise to 20 degrees (Experiment 2). The GtH surge was observed in a larger number of testosterone-implanted fish than in the estradiol-treated fish in both experiments. These results strongly suggest that the high plasma level of testosterone observed before ovulation is an important physiological requisite for the occurrence of the ovulatory GtH surge in goldfish.