Melatonin: a master hormone and a candidate for universal panacea.

The molecule of melatonin seems to have been evolutionarily conserved. Its presence has been demonstrated in almost all groups of organisms, from plants, protozoa to people. During evolution, melatonin is claimed to have mediated dark adaptation. The universal presence of melatonin may be because it is lipophilic in nature which enables it to cross all biological (lipid membrane) barriers and to diffuse into every compartment of the cell, and because it serves as an antioxidant and is used as a free radical scavenger. In vertebrates, the pineal gland is the single largest source of melatonin production although, especially in non-mammalian vertebrates, other organs (e.g. retina, harderian gland etc.) may contribute significantly to the blood melatonin levels. In invertebrates, on the other hand, the pineal gland is absent and, therefore, melatonin secretion is clearly derived from another source(s). Regardless of the site of synthesis and the nature of organisms (diurnal, nocturnal or crepuscular), melatonin is secreted in the night and melatonin biosynthetic pathway remains essentially the same. Tryptophan, an amino acid derived from dietary sources, undergoes a series of enzymatic reactions to produce melatonin. The rhythm in melatonin secretion is generated endogenously by the circadian pacemaker(s) in the suprachiasmatic nuclei (SCN), and regulated by environmental light:dark cycle. Melatonin through its action on the SCN synchronizes disrupted or free-running circadian rhythms, and regulates a variety of daily and seasonal changes in the physiology and behaviour of animals. Emerging scientific evidence for the role of melatonin as therapeutic agent in the treatment of circadian rhythm-associated sleep disorders in persons having normal social working hours and shift workers, in jet lag, in immunological functions etc. have considerably increased interest in this hormone molecule. The role of melatonin in organisms physiology has now been widely recognized, and the wealth of information accumulated in the past two decades indicate it to be the best hormone candidate to be investigated for a universal panacea.