Hormonal regulation of final maturation of striped bass oocytes in vitro.

Abstract An in vitro culture system was developed to investigate hormonal control of final oocyte maturation (FOM) in striped bass ( Morone saxatilis ). Isolated ovarian fragments exposed to human chorionic gonadotropin (hCG), dibutyryl cAMP, or forskolin produced significant amounts of 17α,20β-dihydroxy-4-pregnen-3-one (DHP) and the oocytes underwent germinal vesicle breakdown (GVBD). Slight increases in 17α20β,21-trihydroxy-4-pregnen-3-one (20β-S) production were also observed. Production of testosterone and estradiol-17β was relatively high at the beginning of in vitro treatment with hCG but decreased as production of DHP increased and GVBD was initiated. Inhibitors of protein transcription (actinomycin-D), translation (cycloheximide), and steroidogenesis (trilostane) completely blocked hCG-induced DHP and 20β-S production and the associated GVBD. FOM, assessed from the progress of GVBD, proceeded in trilostane-treated but not in cycloheximide-treated follicle-enclosed oocytes when DHP or 20β-S was added to the cultures. Structure-activity experiments revealed that DHP and 20β-S were more potent at inducing GVBD than 14 other structurally similar C 21 , steroids that were tested. These results demonstrate that FOM in striped bass is induced by gonadotropin-mediated production of a Δ 4 steroid through an adenylate-cyclase pathway which requires protein synthesis. DHP and 20β-S are implicated as final oocyte maturation-inducing steroid hormones in striped bass.