Hormone changes during ovulation and effects of steroid hormones on plasma gonadotropin levels and ovulation in goldfish

Abstract

Plasma hormone changes during ovulation and the effects of steroid hormones on plasma gonadotropin (GtH) levels and ovulation were studied in the female goldfish. Ovulation was induced by raising water temperature from 12 to 20°. Plasma gonadotropin levels exhibited a gradual rise during the latter half of the light phase. This was then followed by a surge in GtH, showing a peak at the time of ovulation in the dark phase. After ovulation, GtH levels decreased rapidly by the next light phase. Plasma 17α,20β-dihydroxy-4-pregnen-3-one (17α,20β-diOH-P) and testosterone showed a peak before ovulation, and then decreased by the time of ovulation. Estradiol-17β (E 2) showed moderate levels during the GtH surge. No marked decrease of E 2 levels was observed before the commencement of the GtH surge. Administration of testosterone or E 2 prior to elevating the water temperature did not inhibit the occurrence of the GtH surge and ovulation. Likewise, administration of neither 17α-hydroxyprogesterone nor 17α,20β-diOH-P affected plasma GtH levels at 20°. The present study shows a clear picture of the hormone changes that occur during spontaneous ovulation in goldfish. The results of administering steroids prior to ovulation does not support the hypothesis that a decline in E 2 levels triggers the GtH surge and ovulation.