In vitro oocyte maturation in the sea bass: effects of hCG, pituitary extract and steroids

Exposure of sea bass Dicentrarchus labrax oocytes to 25, 50, 100, and 300 IU ml−1 of human chorionic gonadotropin (hCG) resulted in a dose‐dependent increase (ED50=60 IU ml−1) in the maximal maturation and volume response rate over a 120‐h incubation period. Oocytes responded in a similar dose‐dependent manner to graded doses of homologous sea bass pituitary extract (ED50=3·3#10−2 PE ml−1). The response latency of sea bass oocytes to endogenous and exogenous gonadotropins was similar (t1/2≃45 and 48 h, respectively). Both 17α,20β‐dihydroxy‐4‐pregnen‐3‐one (17,20βP) and 17α,20β,21‐trihydroxy‐4‐pregnen‐3‐one (20βS) stimulated maturation in a dose‐ and time‐dependent manner with t1/2 values less than those of hCG (15 h and 7·5 h at 100 ng ml−1, respectively); 17α,20α‐dehydroxy‐4‐pregnen‐3‐one (17,20αP) was ineffective. 17,20βP was more potent at all doses tested. The stimulatory actions of gonadotropin and maturation‐inducing steroid (MIS) involved both transcription and translation since maturation was significantly inhibited in the presence of actinomycin D and cycloheximide. Moreover, pituitary extract and hCG stimulated release of higher levels of 17,20βP in a dose‐ and time‐dependent manner in follicle incubations as compared to 20βS suggesting that this steroid may in fact be the MIS in this species.