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 x 10-2 PE ml-1). The response latency of sea bass oocytes to endogenous and exogenous gonadotropins was similar (t(1/2)≃45 and 48 h, respectively). Both 17a,20β-dihydroxy-4-pregnen-3-one (17a,20βP) and 17a,20β,21-trihydroxy-4-pregnen-3-one (20βS) stimulated maturation in a dose- and time-dependent manner with t(1/2) values less than those of hCG (15 h and 7.5 h at 100 ng ml-1, respectively); 17a,20a-dehydroxy-4-pregnen-3-one (17,20aP) 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.This work was supported by CICYT MAR96-1859 to M.C., a postdoctoral fellowshipfrom the Spanish government to L.A.S., and a predoctoral fellowship from the regional government of Valencia, Spain (Generalitat Valenciana) to J.F.A. Support from the British Council and the Spanish Ministry of Education and Culture under the frameworkof a Spanish–British Integrated Actions (HB97-235) to M.C. and D.E.K. is alsogratefully acknowledge