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Marked Augmentation of Nocturnal Melatonin Secretion in Amenorrheic Athletes, but not in Cycling Athletes: Unaltered by Opioidergic or Dopaminergic Blockade^*

GAIL A. LAUGHLIN, ANNE B. LOUCKS and S. S. C. YEN

Department of Reproductive Medicine, School of Medicine 0802, and the General Clinical Research Center, University of California-San Diego La Jolla, California 92093-08

Address all correspondence and requests for reprints to: Dr. Samuel S. C. Yen, Department of Reproductive Medicine, University of California-San Diego School of Medicine, La Jolla, California 92093.

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 at30/µg/kg–h).

Elevated (P < 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 < 0.001 vs. CS and CA) and a 2-h delay in offset (P < 0.001 vs. CS and CA), which yielded a 2–fold amplification of the integrated nocturnal melatonin secretion (P < 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.

^* This work was supported by NIH Grants HD–21198–05, HD–12303–14, and Grant MOl–RR–00827 to the General Clinical Research Center. Research was conducted in part by the Clayton Foundation for Research.

Present address: Department of Zoological and Biomedical Sciences, Ohio University, Athens, Ohio 45701.

Clayton Foundation Investigator.

Received February 12, 1991.

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