The Melatonin (MLT), secreted rhythmically by the pineal, is an efferent hormonal signal of the circadian clock. MLT presents overall pleitropic effects but it is the role of MLT as a hormonal circadian signal which is the best documented. MLT-receptors are present in numerous structures/organs and the MLT is now considered as an endogenous synchronizer within the circadian system. The presence of MLT-receptors within the circadian clock, explains that exogenous MLT is a chronobiotic drug. Trials in humans, have confirmed the efficacy of MLT in circadian rhythm disorders. Subtypes of MLT-receptors have been characterized (MT1 and MT2). Striking differences are observed in the distribution pattern of these 2 subtypes. Up to now, MTL-analogues commercialized as drugs, are all non-specific MT1/MT2 agonists acting on the SCN. The development of new specific agonists/antagonists for both subtypes, the identification of the link between MLT target sites within different parts of the brain or the body and the association of specific MLT receptor subtypes and particular physiological effects open great therapeutic potential.
Keywords: chronobiotic effect; circadian clock; circadian rhythms; melatonin; suprachiasmatic nucleus.
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