Effects of selected hormones and male cohorts on final oocyte maturation, ovulation, and steroid production in walleye ( Stizostedion vitreum)

Abstract

A series of in vivo and in vitro experiments were conducted to evaluate the effects of selected hormones and male cohorts on oocyte maturation and ovulation in walleye captured from the wild. In one experiment conducted 2 weeks prior to the normal spawning season, single intramuscular injections of human chorionic gonadotropin (hCG, 500 IU kg −1) and des-Gly 10 [D-Ala 6] LHRH-ethylamide (LHRHa, 100 μg kg −1) stimulated final oocyte maturation and ovulation. LHRHa induced oocyte maturation faster than hCG. The presence of spermiating males had a slight stimulatory effect on oocyte maturation in non-injected fish, but did not potentiate the effects of LHRHa. In a second experiment conducted 3 weeks prior to normal spawning, hCG (500 IU kg −1), LHRHa (100 μg kg −1), and 17,20-P (100 μg kg −1) all induced final oocyte maturation. In this experiment, however, hCG was more effective than LHRHa, and there was no male cohort effect. In maturing females, oestradiol-17β levels declined, and testosterone levels increased transiently prior to final oocyte maturation and ovulation. Levels of 17α,20β-dihydroxy-4-pregnen-3-one (17,20-P) were significantly elevated 2 and 3 days prior to ovulation. Cortisol levels were high (50–100 ng ml −1) in newly captured fish and remained elevated during the experimental period. No control fish in either experiment underwent final oocyte maturation. These findings suggest that capture and confinement stress may inhibit oocyte maturation in walleye. In vitro, human chorionic gonadotropin (hCG) was a potent inducer of final oocyte maturation whereas LHRHa had no effect. Of various steroids tested in vitro, 17,20-P and 17α,20β,21 -trihydroxy-4-pregnen-3-one were the two most effective inducers of final oocyte maturation and ovulation, and the lowest dose of 17,20-P tested (0.01 ng ml −1) was a consistent and potent inducer of final oocyte maturation. These data, together with our in vivo results, support the hypothesis that 17,20-P may be the maturation-inducing steroid in walleye. Cortisol, alone or in combination with hCG, had no effect on oocyte maturation or ovulation in vitro, indicating that any negative effects of cortisol on oocyte maturation in walleye probably occurs at higher levels of the hypothalamic-pituitary-gonadal axis.