Hormonal dynamics during luteal-follicular transition.

To gain insights into the neuroendocrine basis for the initiation of folliculogenesis, the hormonal dynamics during the period from the late luteal to the early follicular phase of the menstrual cycle (the luteal-follicular transition) were examined. Blood samples were obtained at 2-h intervals for 5 consecutive days in seven women and at 15-min intervals for 8 h on each of 4 consecutive days in five women. The results indicate that the luteolytic process, as reflected by an exponential decline of both serum estradiol and progesterone levels, began at least 64 h before the onset of menses. During estradiol and progesterone withdrawal, there was a selective increase in mean serum FSH levels (P less than 0.001) beginning 24 h before and reaching a peak 24 h after the onset of menses. The frequency of LH pulses increased slightly but not significantly during this period, with a significant rise in mean serum LH levels on the day of menses. Thus, an acute rise in FSH concentrations the day before and in LH concentrations the day after the onset of menses occurs during luteal-follicular transition. The dissociation of FSH and LH secretion observed suggests that additional neuroendocrine events other than changes in pulsatile GnRH secretion may be operative during this period. These findings indicate that the initiation of folliculogenesis for the ensuing cycle occurs during the late luteal phase by a process of selective augmentation in FSH secretion independent of hypothalamic GnRH secretion. This event may ultimately prove to be a manifestation of the action of recently characterized ovarian peptides on FSH secretion.