Melatonin supplementation restores cellular proliferation and DNA synthesis in the splenic and thymic lymphocytes of old rats.

Objectives: In this study we investigated the effect of melatonin treatment on the proliferative activity, the rate of DNA synthesis and the histopathological changes of splenic and thymic lymphocytes in old rats.

Methods: Two subgroups of old rats (25-months-old) were used in this study. One subgroup was given melatonin in the drinking water (250-300 mg/day/rat) for 3 months while the second subgroup was given water containing diluent. A third group consisted of young rats (3-months-old) which served as an additional control.

Results: A (3)H-thymidine autoradiographic investigation showed a reduction in both the proliferative activity and the rate of DNA synthesis in splenic and thymic lymphocytes in old rats. In addition, light and electron microscopy showed severe histopathological changes in these cells from diluent-treated old rats. Melatonin administration increased the proliferative activity and the rate of DNA synthesis in the lymphocytes of both the spleen and thymus of the old animals. Also, histopathological changes were partially reversed by melatonin treatment with the tissues appearing similar to those in the young rats.

Conclusion: The stimulation of the lymphocyte activity by melatonin is a beneficial response, especially in old rats, since aging results in an inhibition in lymphocytic functions.