MELATONIN AND SEASONAL RHYTHMS

The pineal hormone melatonin plays a ubiquitous role in biology as a c hemical mediator of the effects of season on animal physiology and beh avior. Seasonal changes in night length (scotoperiod) induce parallel changes in the duration of melatonin secretion (which occurs exclusive ly at night), so that it is longer in winter and shorter in summer. Th ese changes in duration of nocturnal melatonin secretion, in turn, tri gger seasonal changes in behavior. The retinohypothalamic-pineal (RHP) axis's responses to light are highly conserved in humans. Like other animals, humans secrete melatonin exclusively at night, and they inter rupt its secretion when they are exposed to light during the nocturnal period of its secretion. In many individuals, the RHP axis also is ca pable of detecting changes in the length of the night and making propo rtional adjustments in the duration of nocturnal melatonin secretion, producing the type of melatonin message that animals use to trigger se asonal changes in their behavior. This has been shown both in naturali stic studies in which melatonin profiles were compared in summer and w inter and in experimental studies in which melatonin profiles were com pared after chronic exposure to long and short artificial ''nights.'' Individuals who live in modern urban environments differ in the degree to which, or even whether, the intrinsic duration of melatonin secret ion (the duration measured in constant dim Light) responds to seasonal changes in the length of the solar night. Changes in the intrinsic du ration of melatonin secretion that are induced by changes in the scoto period are highly correlated with changes in the intrinsic timing of t he morning offset of secretion and are only weakly correlated with cha nges in the intrinsic timing of evening onset of secretion. This findi ng suggests that differences in the way in which individuals are expos ed to, or process, morning light may explain differences in their resp onsiveness to changes in duration of natural and experimental scotoper iods. Although the human RHP axis clearly is capable of detecting chan ges in the length of the night and in producing the melatonin message that other animals use to trigger seasonal changes in their behavior, it is not yet known whether or how the human reproductive system or ot her systems respond to this message.