Hormonal regulation of final maturation of striped bass oocytes in vitro

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 17 alpha,20 beta-dihydroxy-4-pregnen-3-one (DHP) and the oocytes underwent germinal vesicle breakdown (GVBD). Slight increases in 17 alpha,20 beta,21-trihydroxy-4-pregnen-3-one (20 beta-S) production were also observed. Production of testosterone and estradiol-17 beta 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 20 beta-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 20 beta-S was added to the cultures. Structure-activity experiments revealed that DHP and 20 beta-S were more potent at inducing GVBD than 14 other structurally similar C21 steroids that were tested. These results demonstrate that FOM in striped bass is induced by gonadotropin-mediated production of a delta 4 steroid through an adenylate-cyclase pathway which requires protein synthesis. DHP and 20 beta-S are implicated as final oocyte maturation-inducing steroid hormones in striped bass.