Plasma gonadotropin, 17-estradiol, and 17,20-dihydroxy-4-pregnen-3-one levels during luteinizing hormone-releasing hormone analogue and gonadotropin induced ovulation in coho salmon (Oncorhynchus kisutch)

Intraperitoneal injections of des Gly 10 -(D-Ala 6 ) luteinizing hormone-releasing hormone ethylamide (LH-RHA DAla 6 ) alone and in combination with partially purified chinook salmon gonadotropin (SG-G100) were shown to be highly effective means of inducing ovulation in coho salmon. Two injections of LH-RHA DAla 6 72 h apart or LH-RHA DAla 6 injected with or 72 h following an injection of SG-G100 induced ovulation in 6–10 days. The induction of ovulation was associated with the duration rather than the initial magnitude of the increase in plasma gonadotropin levels. Plasma 17β-estradiol levels decreased whereas plasma 17α,20β-dihydroxy-4-pregnen-3-one (17α20βP) levels increased in response to elevated plasma gonadotropin levels. The time of ovulation was related to the magnitude and rate of change in plasma 17β-estradiol and 17α20βP levels. Ovulation occurred following a reduction of plasma 17β-estradiol levels to less than 2 ng/mL and this decline preceded or was concomitant with an increase in plasma 17αt20βP levels to 450–500 ng/mL. 17α20βP levels in fish which underwent oocyte maturation but not ovulation were lower (100 ng/mL), suggesting that high levels of 17α20βP may have a direct role in ovulation. Changes in 17β-estradiol production appear to determine the time of ovulation by influencing the magnitude and rate of increase of plasma 17α20βP levels.