[Circadian and ultradian rhythms in gonadotropin secretion: regulation by ovarian steroids]

By virtue of the close temporal and functional coupling between intermittent hypothalamic GnRH stimulation and hypophyseal gonadotropin response, assessment of the serum gonadotropin pulsatility and its circadian variations may represent a feasible approach to accurately assess neuroendocrine regulation processes in human reproduction. In the absence of considerable ovarian steroid feedback, as in postmenopausal women, the LH pulsatility with maximal pulse frequencies and amplitudes constitutes an unrestrained basic pulse rhythm. During the menstrual cycle, the pulse characteristics of episodic LH secretion and their circadian excursions undergo characteristic alterations by the changing sex steroid environment. Yet, even during cycle phases of increased oestrogen and progesterone concentrations, the LH pulse frequency is limited to that of the basal rhythm. The LH secretory amplitudes increase from the follicular to the luteal phase, presumably as reflection of an altered pituitary sensitivity to endogenous GnRH stimulation. The tight coupling between the LH signal and the sex steroid response from the corpus luteum emphasizes the importance of central stimulatory rhythms for the target organ. The activity of the central "pulse generator" is profoundly modulated by ovarian steroid feedback and by interactions within the neuroendocrine regulatory units. These regulatory processes have been integrated in a hypothetic model of hypothalamic-pituitary interactions, presumably representing the biochemical basis for the regulation of the central pacemaker.