Circulating levels and in vitro production of two maturation-inducing hormones in teleost: 17α,20β-dihydroxy-4-pregnen-3-one and 17α,20β,21-trihydroxy-4-pregnen-3-one, in a daily spawning wrasse, Pseudolabrus japonicus

The female bambooleaf wrasse, Pseudolabrus japonicus, spawns daily during the spawning season, and exhibits a diurnal rhythm of ovarian development. In the present study, we have investigated: (1) circulating levels of 17α,20β-dihydroxy-4-pregnen-3-one (17,20β-P) and 17α,20β,21-trihydroxy-4-pregnen-3-one (20β-S) in females sampled at different times of the day during spawning season in captivity, and (2) in vitro production of 17,20β-P and 20β-S by follicle-enclosed oocytes at seven different developmental stages. In addition, we developed a microtiter plate enzyme-linked immunosorbent assay (ELISA) for 17,20β-P. Serum levels of 17,20β-P and 20β-S showed similar diurnal changes; substantial increases in these levels occurred around the time of germinal vesicle breakdown (GVBD). In vitro experiments showed that massive production of 17,20β-P and 20β-S occurred in follicles collected just before or during GVBD. Further, acute decreases in 17,20β-P and 20β-S production were found in the ovarian follicles just prior to ovulation, suggesting inactivation of the maturation-inducing hormone (MIH). These results, taken together with our previous data on the occurrence of GVBD in vitro, suggest a role for both 17,20β-P and 20β-S as MIHs in the bambooleaf wrasse.