Ovulation and Plasma Progesterone, Testosterone and Estradiol-17β Levels in Laying Hens after the Administration of Aminoglutethimide

In causing ovulation of the hen, the present experiments were performed to investigate the physiological role of P4, T and E2 during the ovulatory cycle. When 100mg of aminoglutethimide (AG), a steroidogenesis inhibitor, were singly injected into laying hens 12h before their ovulation of an intrasequential egg, the ovulation was delayed about 3 to 6h in all of the hens, while the preovulatory P4 rise was also postponed until the corresponding time to delayed ovulation. On the other hand, 200mg AG given 12h before ovulation completely suppressed ovulation and the preovulatory rises of P4, T and E2. In the case of 50mg AG administered 24h before ovulation, 5 of 12 hens exhibited delayed ovulation of 2 to 5h. 3 other hens failed to ovulate while the remaining 4 hens ovulated within the expected time of ovulation. In all 8 hens receiving 100mg AG 24h before ovulation, the ovulation was completely blocked. In a total of 16 hens exhibiting an affected ovulation by AG (including delay and block), the profile of plasma E2 was markedly low following AG administration. Plasma P4 of hens which failed to ovulate was kept under a significantly low level after the injection, while the P4 level in hens with delayed ovulation significantly increased during the preovulatory period corresponding to the time of delayed ovulation by AG. The present results indicate: 1) delayed ovulation is always accompanied by a delay in the preovulatory P4 rise and: 2) the ovulation inhibiting effect of AG injected immediately after ovulation is about twice more than when it is injected in the middle of an ovulatory cycle. Therefore, it is suggested that steroids, particularly P4 and E2, during the early half of the ovulatory cycle, play a significant role in causing ovulation.