Adequate interval of sGTH administration to induce vitellogenesis, and induction of ovulation by simultaneous administration of sGTH and 17α-hydroxyprogesterone in the Japanese eel

SUMMARY: In this study, the effects of weekly and biweekly administrations of salmon gonadotropin fraction (sGTH), via an emulsion prepared with lipophilized gelatin (LG emulsion), on the induction of vitellogenesis in the Japanese eel were compared to determine the adequate intervals of hormone administration. The effectiveness of simultaneous administration of sGTH and 17α-hydroxyprogesterone (17αP) using LG emulsion for the induction of final maturation and ovulation was also examined. Immature female Japanese eel (cultured eel and silver eel) were administered sGTH with LG emulsion weekly (2 mg/kg BW) or biweekly (4 mg/kg BW). In the fish injected biweekly with LG emulsion containing sGTH, biweekly cyclic fluctuations in the body weight (BW) and plasma levels of testosterone, estradiol-17β and sGTH II were repeated in response to the injection of sGTH. It took a longer period for the biweekly injected fish to complete vitellogenesis compared with the fish injected weekly. The weekly injections of sGTH were more effective than the biweekly injections in inducing vitellogenesis in the Japanese eel. The LG emulsion containing sGTH in the aqueous phase and 17αP in the oil phase was prepared. Hormone releasing properties were investigated by monitoring plasma profiles of hormones in immature eel. Significant differences in the plasma profiles of sGTH II and 17αP were not observed between simultaneous administration of sGTH and 17αP and single administration of sGTH or 17αP. Following induction of vitellogenesis, females that exhibited a rapid increase of BW and had oocytes at migratory nucleus stage were injected with sGTH and 17αP at the same time using LG emulsion. As a result, all females injected with sGTH and 17αP ovulated. Simultaneous administration of sGTH and 17αP can be useful for the induction of final maturation and ovulation in the Japanese eel.

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