Melatonin: possible mechanisms involved in its 'radioprotective' effect.

Peripheral blood samples were collected from four human volunteers before, and at 1 and 2 h after a single oral dose of 300 mg of melatonin. From each sample, separate aliquots of whole blood were exposed to 100-150 cGy gamma radiation in vitro to determine the extent of genetic damage. Irradiated lymphocytes from all volunteers which were collected after melatonin ingestion exhibited a significantly decreased extent of primary DNA damage and reduced frequencies of chromosomal aberrations and micronuclei, as compared with similarly irradiated cells collected before the oral dose of melatonin. The extent of the melatonin-associated decrease in primary DNA damage was less than the corresponding decrease observed in the incidence of chromosomal aberrations and micronuclei; the latter assays required an additional post-irradiation incubation of the cells at 37+/-1 degreesC for 48 and 72 h, respectively. The possible mechanisms involved in the radioprotective influence of melatonin are discussed.