MELATONIN AND ITS PHYSIOLOGICAL AND THERAPEUTIC PROPERTIES

Melatonin is a hormone produced mainly by the pineal gland in most ver tebrate species, including humans. Recent metabolic, receptor and func tional studies created a picture of the melatoninergic system(s) in li ving organisms, its organization, physiology and a role in some pathol ogic conditions. The melatonin-generating system is characterized by t hree basic features: (1) photosensitivity (2) diurnal (or circadian) r hythmicity (with highest levels of melatonin production occurring at n ight in darkness), and oc (3) age-related decline in its activity. Cyc lic nocturnal increases of melatonin levels are proportional to the le ngth of nights (or dark periods of an imposed light-dark cycle); the h ormone thus conveys a photoperiodic message, and functions in an organ ism as an internal biochemical clock and calendar. Biological actions OF melatonin are mediated via specific melatonin receptors, whose dist ribution in the body is uneven, yet with decisively highest density in the suprachiasmatic nuclei of the hypothalamus, pars tuberalis of the pituitary, and the retina (particularly in birds and lower vertebrate s) Such a distribution of melatonin receptors suggests that the princi pal physiological role of the hormone is related to both chronobiology and modulating of the body hormonal milieu. This review surveys recen t developments in the melatonin field, and summarizes current knowledg e on the melatoninergic mechanisms, including the therapeutic aspect r elated to the hormone.