Endocrine Control of Ovulation in Medaka (Oryzias latipes) and Ayu (Plecoglossus altivelis)

Abstract

This study was undertaken to elucidate the endocrine control of ovulation in the medaka (Oryzias latipes) and the ayu (Plecoglossus altivelis). Among gonadotropins, salmon gonadotropin gave the best response in inducing in vitro ovulation of Oryzias oocytes and demonstrated a higher potency than mammalian gonadotropin, suggesting a degree of zoological specificity. Of the steroids, cortisol was very effective in inducing in vitro ovulation of Oryzias oocytes. Furthermore, ovulation cannot be induced in Oryzias oocytes dissected from their follicular tissues prior to exposure to gonadotropin, suggesting that the hormone acts first on the follicular envelope to produce effective agents (corticoids?) which are then responsible for ovulation. Application of a specific inhibitor (SU-4885) of 11β-hydroxylase resulted in inhibition of gonadotropin-induced ovulation, but did not affect ovulation induced by cortisol. The results suggest that corticosteroidogenesis in the follicular tissues is required for gonadotropin-induced ovulation. However, cortisol failed to induce in vitro ovulation of Oryzias oocytes removed from their follicular tissues. Thus, ovulation may be induced by secondary agents that are produced during corticoid biosynthesis or metabolism within the follicular envelope. Hirose et al. showed in 1974 that gonadotropin-treated mature ayu imbibed a greater quantity of water than control ayu, and that body weight increased by 3% in the hormone-treated fish. These results indicate that water ingestion and hydration of oocytes induced by gonadotropin may be an important process at ovulation. It has also been shown that during maturation and ovulation hydration is associated with ion movement (Na+). These phenomena may be related to the action of gonadotropin and/or corticoids and also occur at ovulation in the medaka. Such information may possibly yield additional clues regarding the mechanism of ovulation in fish and provide a solution to the practical problems of regulating ovulation.