Melatonin: from the hormone to the drug?

Melatonin (MLT) is a methoxyindole secreted principally by the pineal gland. It is synthesized at night under normal environmental conditions. The endogenous rhythm of secretion is generated by the suprachiasmatic nuclei and entrained by the light/dark cycle. Light is able to both suppress or synchronize melatonin production according to the light schedule. The nycthohemeral rhythm of this hormone is determined by repeated measurement of plasma of saliva MLT or urine sulfatoxymelatonin, the main hepatic metabolite. Melatonin can be considered as the output (the hand) of the endogenous clock. Since the regulating system follows central and sympathetic nervous pathways, an abnormality at any level unspecifically modifies the melatonin secretion, especially in patients with sympathalgia or dysautonomia. Melatonin plays the role of an endogenous zeitgeber on sleep-wake cycle or core temperature. Exogenous melatonin is able to influence the endogenous secretion of the hormone according to a phase response curve. There are therapeutic implications for this property in situations when biological rhythms are disturbed (jet-lag syndrome, delayed sleep phase syndrome, insomnia in blind or elderly people, shift-work). Improvement of pharmaceutical forms studied in controlled trials under the responsibility of the medical community or development of melatonin analogs could lead to decisive progress in this field.