[Melatonin and biological rhythms]

Melatonin is synthesized and secreted by the pineal gland during the dark period of the light-dark cycle. This rhythmic nocturnal melatonin secretion is generated directly by the circadian clock located, in mammals, within the suprachiasmatic nucleus of the hypothalamus (SCN) and entrained to a 24 h period by the light-dark cycle. The melatonin rhythm is thus an important efferent hormonal signal from the circadian clock. The periodic secretion of melatonin might thus be used as a circadian mediator of any system that can 'read' the message. The rhythmic nocturnal secretion is dependent on the SCN but the duration of the nocturnal production is directly proportional to the length of the dark period. It is through these changes in duration that the brain integrates the photoperiodic information, which explains the present use of melatonin on farms to control seasonal functions (e.g. fur, reproduction, milk production...). In essence, the melatonin rhythm appears to be an endocrine code of the environmental light-dark cycle conveying photic information that is used by an organism for both circadian and seasonal temporal organization. The major question arising out of this effect of melatonin concerns its precise mechanism of action. Our present knowledge is still fragmentary. As for other hormones, melatonin seems to exert its effects throughout different receptors. In mammals two types have already been determined and cloned (Mella and Mellb). Very probably others will soon be characterized and the list of melatonin receptor-containing structures or organs will lengthen. Most of our functioning (e.g. immune system, cardiovascular system...) displays temporal organization and disorders are known to result from a disturbance of such organization (e.g. during ageing). The potential therapeutic use of melatonin is great; however, this remains to be systematically evaluated.