Plasma concentrations of progesterone and corticosterone during the ovulation cycle of the hen (Gallus domesticus).

The observation that injections of either ACTH or corticosterone induced ovulation in the hen has prompted speculation which implicates the adrenal gland in the mechanisms controlling ovulation. In this study, the role of the major adrenal steroid in birds was further examined by measuring the plasma concentration of corticosterone during the 24 hr period preceding mid-sequence ovipositions and the 28 hr period preceding terminal ovipositions. Progesterone, which is an accurate index of preovulatory ovarian activity, was also measured in the same samples. An increase in plasma progesterone preceded mid-sequence ovipositions which are accompanied by an ovulation. This preovulatory surge in plasma progesterone began approximately 7 hr before ovulation, reached a maximum of 6.9 ng/ml 3 to 2 hr before ovulation, and returned to baseline concentrations at the time of ovulation. A major preovulatory peak of progesterone was not observed during the interval between sequences, although a small but statistically significant rise in progesterone was observed between midnight and 0600 hr on the day of the last oviposition of the sequence. The concentration of corticosterone increased approximately two-fold during the dark portion of the photoperiod regardless of the position of the oviposition in the sequence. Baseline concentrations during the illuminated portion of the photoperiod ranged between 1 and 4 ng of corticosterone per ml of plasma whereas during the period of darkness, they ranged between 2 and 6 ng of corticosterone per ml of plasma. It was concluded, therefore, that the circulating concentration of corticosterone was regulated by a circadian rhythm which operates independently of follicular maturation.