The temporal relationship between melatonin and prolactin in women

The relationship between the concentrations of melatonin and prolactin (PRL) over 24 hours has been investigated. Two peaks in PRL concentrations were distinguished: an evening peak (at 8:00 P.M.) and a nocturnal peak (at 6:00 A.M.). Melatonin concentrations showed a single peak (at 2:00 A.M.). In both the follicular and luteal phases of the menstrual cycle, melatonin increased 2 to 3 hours after the evening increase in PRL and 4 hours before the nocturnal rise in PRL concentrations. Oral administration of melatonin stimulated PRL release. The PRL response varied with menstrual cycle stage being significantly greater in the ovulatory (P less than 0.001) and luteal (P less than 0.01) phases than in the follicular phase. The results suggest that melatonin may be involved in control of the nocturnal PRL increase.