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Influence of chronic melatonin on 9,10-dimethyl-1,2-benzanthracene-induced mammary tumors in female Holtzman rats exposed to continuous light.

Daily treatment of 'functionally' and surgically pinealectomized female Holtzman rats reared in continuous light (light:dark schedule, 24L:OD) with oral melatonin (100 micrograms/day/rat) initiated 30 days prior to and 90 days following 9,10-dimethyl-1,2-benzanthracene (DMBA) administration on day 55 of age, resulted in a highly significant suppression of mammary tumor incidence and prolongation of latency period of tumor appearance (p less than 0.001). An identical treatment schedule of melatonin to intact rats given DMBA, housed in short photoperiods (10L:14D), also resulted in a significant suppression of tumor incidence (p less than 0.05), though the latency period of tumor appearance was not prolonged. These results demonstrate for the first time that replacement or additive manipulations of melatonin from puberty to adulthood plays a critical role in the suppression of DMBA-induced mammary tumors.