STIMULATION OF OVULATION AND STEROID-SECRETION BY LHRHA INJECTION IN THE SEA BASS (DICENTRARCHUS-LABRAX) - EFFECT OF TIME OF DAY

To determine the influence of time of day on the ovarian activity of LHRHa-treated sea bass females, two groups were injected with 10 + 10-mu-g/kg body weight 24 h apart at either 10.00 or 22.00 h. The response was evaluated by the course of oocyte maturation, ovulation and plasma levels of 17-beta-estradiol (17-beta-E2), testosterone (T) and 17-alpha-20-beta-dihydroxyprogesterone (17-20-P) compared to saline-injected control groups. Both LHRHa-treated groups exhibited a marked increase in the proportion of germinal vesicle breakdown in oocytes at 16 (group 10.00) and 24 h (group 22.00) after the first LHRHa injection. Ovulation began at 32 h, in 80% of group 10.00 and 20% of group 22.00, reaching 100% in both groups at 48 h. Control groups did not ovulate throughout the experiment. Treated females showed a T peak at 8 and a 17-beta-E2 peak at 16 h after the first LHRHa injection, the 17-beta-E2 plasma levels being higher for the 22.00 than for the 10.00 group. Data obtained support the hypothesis that 17-beta-E2 acts as a negative factor on final oocyte maturation and ovulation, and that there is a daily change in sensitivity either for the pituitary to LHRHa or for the ovary to gonadotropin. Plasma levels of 17-alpha-20-beta-P were not detectable, confirming that this steriod does not act as maturation-inducing steroid (MIS) in the ovulating sea bass. Further research is necessary to establish the real advantage of giving the LHRHa injections in the morning.