Plasma Levels of Gonadal Steroids during Final Oocyte Maturation of Striped Bass, Morone saxatilis L.

Abstract

Levels of estradiol-17β (E 2), testosterone (T), 17α,20β-dihydroxy-4-pregnen-3-one (DHP), and 17α,20β,21-trihydroxy-4-pregnen-3-one (20β-S) were measured by radioimmunoassay (RIA) in blood plasma of striped bass undergoing final oocyte maturation (FOM). Females were captured just prior to, or in the early stages of, FOM and induced to complete maturation and ovulation with injected human chorionic gonadotropin, synthetic salmon gonadotropin-releasing hormone analogue (sGnRHa; [ d-Arg 6-Pro 9 NEt]-sGnRH), sGnRHa plus the dopamine receptor antagonist, domperidone (DOM), or OVAPRIM, a commercial preparation of sGnRHa + DOM. Their plasma levels of immunoreactive DHP and 20β-S were significantly greater at ovulation relative to the time of hormone injection, whereas the plasma levels of E 2 and T were greatest at injection and decreased by ovulation and 24 hr thereafter. Plasma levels of 20β-S, but not DHP, were sustained at high levels after ovulation. Fish injected only with DOM did not undergo FOM, its associated changes in plasma steroid levels, or ovulation. In females captured at various natural stages of FOM, plasma levels of 20β-S and DHP were low during germinal vesicle migration (GVM), peaked coincident with germinal vesicle breakdown, and then decreased near the time of ovulation. Plasma levels of E 2 and T were greatest during GVM and decreased as DHP and 20β-S levels increased. Analyses of conjugated versus free plasma steroids showed 64-79% of the various hormones to be in the free fraction. RIA of plasma fractionated by reversed-phase HPLC showed that half of the 20β-S immunoreactivity coeluted with 5β-pregnan-3α.17,20β,21-tetrol, a putative 20β-S metabolite with 99.7% cross-reactivity in the 20β-S RIA. These results indicate that striped bass follow the typical profile of changing plasma steroid levels seen in other teleosts during FOM, with a clear shift from C 18 and C 19 steroids to C 21 steroids. They suggest that both DHP and 20β-S, both potent inducers of striped bass FOM in vitro, may play a role in regulating FOM in this species.