Objectives: A strong positive coupling has been already documented between the adrenocortical axis and the gonadal axis at the time of the initiation of the preovulatory LH surge of the menstrual cycle in the human. The LH preovulatory surge starts in the morning at the time of the acrophase (maximal plasma cortisol values) of the cortisol circadian rhythm. Also, it was shown that morning maximal plasma cortisol values were higher during the follicular phase than during the luteal phase. The objective of the present study was designed to determine the exact day of the fall of morning maximal plasma cortisol values and the functional correlates which could exist between plasma 17beta-Estradiol, LH, ACTH and Cortisol at the time of the preovulatory LH surge.

Methods: We performed a detailed analysis of variations of plasma ACTH and cortisol concentrations during the various phases of the LH surge: 1) the day before the ascending phase, 2) the day of the ascending phase, 3) the day of the LH peak, 4) the day of the descending phase, 5) the day after the descending phase. 17beta-Estradiol, LH and FSH were determined by microparticle enzyme immunoassays kits and Progesterone determination was made using a radioimmunoassay kit. ACTH determination was made using a RIA kit and Cortisol was assayed by a RIA method.

Results: Plasma ACTH concentrations were at their highest the day before the day of the ascending phase of the LH surge and significantly higher than the day of the descending phase and the day after the day of the descending phase (p < 0.02). Plasma cortisol concentrations were at their highest, with almost similar values, the day before the day of the ascending phase, the day of the ascending phase and the day of the LH peak; then, plasma cortisol concentrations were significantly lower than those of the preceding days, the day of the descending phase (p = 0.0001) and the day after the day of the descending phase (p = 0.02), when plasma 17beta-Estradiol starts to decrease.

Conclusion: Overall, these results suggest that the positive ACTH-Cortisol-Estrogen dependency, well documented in the female rat, is also operating at midcycle during the menstrual cycle in the human.