Effects of Gonadotropin on Ovarian Intrafollicular Processes during the Development of Oocyte Maturational Competence in a Teleost, the Atlantic Croaker: Evidence for Two Distinct Stages of Gonadotropic Control of Final Oocyte Maturation1

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Abstract
Full-grown oocytes of Atlantic croaker are insensitive to maturation-inducing steroid (MIS) unless they are primed with gonadotropin (GtH). The objective of this study was to examine the mechanism of GtH-induced maturational competence in croaker oocytes. Specifically, we determined the in vitro secretion of steroids by intact ovarian follicles of unprimed or hCG-primed fish, the direct effects of steroids on maturational competence, and the effects of steroid (cyanoketone), protein (cycloheximide), and RNA (actinomycin D) synthesis inhibitors on hCG-induced maturational competence and steroidogenesis in vitro. The steroid content of the incubation medium after hCG treatment was measured by RIA. The effects of hCG or exogenous steroid treatment on maturational competence were determined by recording the incidence of germinal vesicle breakdown (GVBD) after MIS-induced GVBD in a standard bioassay. Our major findings were: (1) induction of maturational competence occurred after exposure of ovarian follicles to hCG either in vivo or in vitro; (2) MIS secretion was detected in follicles of hCG-primed fish but not unprimed fish, and no MIS secretion was observed during hCG induction of maturational competence in vitro; (3) treatment with cyanoketone blocked the hCG-dependent secretion of testosterone and estradiol but not the development of maturational competence; (4) treatment with MIS or various other exogenous steroids in the absence of hCG did not induce maturational competence; and (5) hCG-induced maturational competence was inhibited by cycloheximide and actinomycin D. Therefore, the mechanisms of GtH induction of oocyte maturation in Atlantic croaker can be described in two distinct stages: a delta-4 steroid-(including MIS) and estrogen-independent priming stage followed by a MIS-mediated GVBD stage. The priming stage may involve mechanisms requiring RNA as well as protein synthesis.