Marked augmentation of nocturnal melatonin secretion in amenorrheic athletes, but not in cycling athletes: unaltered by opioidergic or dopaminergic blockade.

Exercise of sufficient intensity during daylight hours has been demonstrated to result in an acute elevation of circulating melatonin levels. The possibility that repeated elevations of daytime melatonin secretion may result in alterations of the nocturnal maxima of the circadian rhythm in highly trained athletic women with and without amenorrhea was investigated. Twenty-four-hour melatonin profiles in matched cyclic sedentary (CS; n = 10) women, cyclic athletic (CA; n = 10) women, and amenorrheic athletic (AA; n = 8) women were compared. The roles of endogenous opioids and dopamine as potential modulators of melatonin secretion were also evaluated by comparing the melatonin profiles during sequential 24-h infusions of saline, followed by either naloxone or metoclopramide (both at 30 micrograms/kg.h). Elevated (P less than 0.05) mean daytime (1000-1700 h) melatonin levels were observed in both groups of athletic women compared to sedentary women. In contrast, nocturnal melatonin levels in sedentary and athletic cycling women were indistinguishable, while amenorrheic athletic women displayed a marked increase in nocturnal peak amplitude (P less than 0.001 vs. CS and CA) and a 2-h delay in offset (P less than 0.001 vs. CS and CA), which yielded a 2-fold amplification of the integrated nocturnal melatonin secretion (P less than 0.001 vs. CS and CA). The onset of the nocturnal rise did not differ among the three groups. Opioidergic and dopaminergic blockade with naloxone and metoclopramide at the doses used did not alter any parameter of melatonin secretion in any of the three groups of women. In conclusion, athleticism in women is associated with an elevation of daytime melatonin levels independent of menstrual status. AA women, but not CA women, display a 2-fold amplification of nocturnal melatonin secretion with a 2-h delay of offset, which does not seem to be linked to athleticism per se. The significance and neuroendocrine basis for the expanded melatonin secretion in athletic amenorrheic women remains to be elucidated.