Chronobiology of prolactin secretion in women: diurnal and sleep-related variations in the pituitary lactotroph sensitivity.

While a nocturnal rise accounts for the marked circadian variability of prolactin (PRL) secretion in humans, the mechanisms subserving this neuroendocrine manifestation are still obscure. Since gonadotropin-releasing hormone (GnRH) stimulates PRL under physiological conditions, we questioned whether changes in the pituitary lactotroph sensitivity to GnRH during the 24-hour cycle may contribute to the expression of circadian PRL rhythmicity. Accordingly, 8 women were studied in the early follicular phase of their cycles (days 2-5) on 6 occasions in random order: during daytime between 10.00 and 14.00 h ('day' studies), at night between 22.00 and 02.00 h, when the women were awake ('night' studies), and finally, during the identical night hours, when the women were asleep ('sleep' studies). On all occasions, blood was collected at 10-min intervals for 4 h, while either GnRH (25 micrograms i.v. bolus) or saline (as control) was injected twice within 2 h. As assessed by the net PRL increments (differences between unstimulated nadir and stimulated peak) and the areas under the PRL response curves, the PRL secretion was not substantially altered following GnRH stimulations during the day studies. In contrast, PRL release was markedly enhanced (p < 0.05 or less vs. day studies), when GnRH was administered during the night studies. This GnRH-stimulated PRL release was even further increased (p < 0.01 vs. day, p < 0.05 vs. night or saline studies), when GnRH had been given during sleep.(ABSTRACT TRUNCATED AT 250 WORDS)