Positive relationship between the nocturnal concentrations of melatonin and prolactin, and a stimulation of prolactin after melatonin administration in young men.

The relationship between the concentrations of melatonin and prolactin over the 24-h cycle has been investigated in a group of young men at three times in the year. Melatonin and prolactin showed a significant positive correlation (P less than 0.001) for all times during the 24-h period but with a greater contribution from concentrations during the nocturnal period, when both hormones were elevated. The positive correlation for nocturnal concentrations was evident in February and March (P less than 0.01) but was of greatest significance in June (P less than 0.001). In blood samples taken at 15-min intervals during the morning (0800-1200) and evening (2000-2400), melatonin and prolactin concentrations were not significantly correlated. Melatonin concentrations increased before prolactin during the evening and decreased before prolactin in the morning. Oral administration of 6 mg melatonin significantly stimulated prolactin release above concentrations measured after placebo administration, in both the morning (P less than 0.05) and evening (P less than 0.01) time periods; the prolactin response being greater in the evening. These results provide evidence for melatonin controlling the nocturnal increase of prolactin via its ability to stimulate prolactin release.