Ovarian steroid synthesis during oocyte maturation and ovulation in Japanese catfish ( Silurus asotus)

Abstract

Several aspects of the endocrine regulation of final oocyte maturation (FOM) and ovulation in Japanese catfish ( Silurus asotus), was investigated. These were: (1) potency of various steroids on inducing FOM in vitro, (2) steroid production stimulated with human chorionic gonadotropin (hCG) in vitro, (3) effectiveness of hCG administration in vivo on inducing FOM and ovulation, and (4) plasma sex steroids profiles in female catfish during FOM and ovulation induced by hCG injections. Among steroids tested, 17,20β-dihydroxy-4-pregnen-3-one (17,20β-P) was the most potent steroid in inducing FOM and 17,20β,21-trihydroxy-4-pregnen-3-one (20β-S) was the second most effective. 17α- hydroxyprogesterone (17α-P) and 11-deoxycortisol (11-DC) did not induce FOM even at the highest dose tested. Both ovarian 17,20β-P and 20β-S production were induced by hCG in vitro and the 17,20β-P production was higher than 20β-S production. The ovarian fragments cultured in vitro did not convert 17α-P to 17,20β-P unless they were treated with hCG. hCG injection increased plasma 17,20β-P and 20β-S levels, causing FOM and partial ovulation. In addition, this hCG-induced 17,20β-P production was higher than the 20β-S production in vivo. These data suggest that 17,20β-P appears to be a major maturation-inducing steroid (MIS) in Japanese catfish and that MIS production may be regulated by GTH-induced activation or synthesis of 20β-hydroxysteroid dehydrogenase.