Hormonal changes during meiotic maturation and ovulation in the brook trout (Salvelinus fontinalis)

The plasma levels of estradiol-17β (E2), 17α, 20β-dihydroxy-4-pregnen-3-one (17,20-P) and gonadotropin (GTH) were measured in brook trout (Salvelinus fontinalis) during the period from the end of vitellogenesis to postovulation. Blood samples were taken according to specific stages of maturation, including germinal vesicle breakdown (GVBD) and ovulation. E2 levels were quite high (∼45 ng/ml) at the end of vitellogenesis (and prior to GVBD) and dropped precipitously by GVBD (∼2 ng/ml). They remained low through ovulation and postovulation. 17,20-P levels were low prior to GVBD (∼0.7 ng/ml) and increased dramatically at GVBD (∼148 ng/ml). The levels of 17,20-P remained high at ovulation (∼142 ng/ml) and then dropped significantly within 24 h to approximately half of the ovulatory values. They decreased even further by 7 days postovulation. GTH levels rose gradually through GVBD and ovulation from a postvitellogenic level of approximately 3 ng/ml to a 7 day postovulatory value of approximately 10 ng/ml. The overall results; 1) decrease in estradiol prior to GVBD, 2) increase in 17,20-P at GVBD and 3) gradual GTH rise through GVBD and ovulation, are similar to those reported for other salmonids.