Time-course response of plasma testosterone and estradiol-17β to salmon gonadotropin treatment in the Japanese eel during ovarian development

For the induction of ovarian maturation in the Japanese eel, Anguilla japonica under rearing conditions, we developed a method of hormone administration using an emulsion prepared with lipophilized gelatin (LG emulsion). We showed that the LG emulsion released the contained salmon gonadotropin fraction (sGTH) in a gradual manner, and weekly injections of sGTH using LG emulsion were more effective for inducing ovarian maturation than those using saline solution. Biweekly injections of sGTH caused biweekly cyclic fluctuations in plasma steroid levels. Such fluctuations may occur at shorter intervals when sGTH is injected weekly. For further analysis of the fluctuations, we monitored in detail the plasma profiles of testosterone (T) and estradiol-17b (E2) following weekly sGTH injections during vitellogenesis. Feminized Japanese eels (BW, 520–1066 g) were kindly supplied by Aichi Fisheries Research Institute, Aichi, Japan. Fish (n = 14) were acclimated to seawater at 20°C for 1 week in 3.4 m aquaria, and then received weekly intramuscular injections of LG emulsion (n = 4) or saline solution (n = 4), containing sGTH (2 mg/kg) prepared as described previously. Controls were injected with each vehicle without sGTH (n = 3 in each treatment). Experiments were conducted three times, after the first, second and eighth weekly injection. Blood samples were taken repeatedly from the same individuals at the caudal vessels with a heparinized syringe and needle just before, and at 1, 3, 5 and 7 days after the injection, following anesthesia with 1200 ppm 2phenoxyethanol. Plasma was separated by centrifugation at 12 000 ¥g for 5 min, and sGTH II, T and E2 levels were measured using specific RIA. Data were treated statically using the Student’s t-test and the Cochran-Cox test. Plasma sGTH II levels showed weekly fluctuations in each experimental period in both sGTH-treated groups: The levels peaked 1 day after the injection and then decreased (Fig. 1). There were no significant differences in the levels between the two groups. Plasma T and E2 levels also showed weekly fluctuations in each experimental period in both sGTH-treated groups. Plasma T levels in both groups peaked on day 3 after the first sGTH injection and then decreased (Fig. 2), whereas after the second and eighth injection, the levels peaked on the next day of injection and then decreased. Plasma T levels after the eighth injection were lower than those after the first and second injection. Weekly analysis experiment showed that plasma T levels were high after the first injection and were low during the period of vitellogenesis. This was confirmed by detailed analysis within a week of this experiment. Plasma T profiles in the LG emulsion group showed no significant differences from those in the saline solution group. Plasma E2 levels peaked on day 2 or 3 and gradually decreased on day 7 after the first and second injections. While after the eighth injection, the levels peaked on the next day of injection and then decreased (Fig. 3). Plasma E2 levels also increased after each sGTH injection and then decreased as found in plasma T levels. Both groups exhibited similar profiles of plasma E2. In controls, plasma sGTH II, T and E2 levels did not show significant changes. In this investigation, no significant difference in plasma sGTH II levels was observed between the LG emulsion and the saline solution group. However, it is Short Paper