MELATONIN IS ABLE TO INFLUENCE ITS SECRETION IN HUMANS - DESCRIPTION OF A PHASE-RESPONSE CURVE

A single physiological dose of melatonin (20 mu g for 3 h given intrav enously at different times of the day (04.00-12.00, 16.00 and 20.00 h) was able to shift the endogenous plasma melatonin profile of healthy volunteers under entrained conditions according to a phase-response cu rve (PRC). ANOVA showed an effect of the time of administration on the onset, the acrophase or the offset of the melatonin profiles. These p rofiles were significantly delayed when the infusion was administered at 12.00 h and advanced when the infusion was given at 20.00 h. Furthe r, the AUCs evaluated on the nocturnal melatonin profiles were increas ed after the 04.00 h infusion (+20.5%, p < 0.05), whereas they were de creased after the 12.00 h infusion (-20%, p < 0.05). Lastly, no altera tion was observed for cortisol rhythm, whatever the time of melatonin administration. These results, which show that according to a PRC the system regulating melatonin secretion is sensitive to a single short-t erm administration of the hormone given at a low dose, support the par adigm of the endogenous synchronizer melatonin.