THE MELATONIN RHYTHM - BOTH A CLOCK AND A CALENDAR

The paper briefly reviews the data which shows that the circadian prod uction and secretion of melatonin by the pineal gland can impart both daily, i.e., clock, and seasonal, i.e., calendar, information to the o rganism. The paper summarizes the 3 patterns of nocturnal melatonin pr oduction that have been described. Clearly, regardless of the pattern of nocturnal melatonin production a particular species normally displa ys, the duration; of nightime elevated melatonin is proportional to th e duration of the night length. Since daylength under natural conditio ns changes daily the melatonin rhythm, which adjusts to the photoperio d sends time of year information to the organism. The melatonin recept ors which subserve the clock message sent by the pineal gland in the f orm of a melatonin cycle may reside in the biological clock itself, na mely, the suprachiasmatic nuclei (SCN). The melatonin receptors that m ediate seasonal changes in reproductive physiology are presumably thos e that are located on the pars tuberalis cells of the anterior pituita ry gland. Besides these receptors which likely mediate clock and calen dar information, melatonin receptors have been described in other orga ns. Interestingly, the distribution of melatonin receptors is highly s pecies-specific. Whereas the clock and calendar information that the m elatonin cycle imparts to the organism relies on cell membrane recepto rs, a fact that is of some interest considering the high lipophilicity of melatonin, recent studies indicate that other functions of melaton in may require no receptor whatsoever.