Modulation of biochemical circadian rhythms during long-term melatonin treatment in rats

The influences of chronic administration of low and high doses of melatonin on the characteristics of circadian rhythms of glucose, reduced glutathione, total protein were studied, in order to investigate whether melatonin could modulate these rhythms differently.

Pharmacological doses of melatonin (0.5 mg/kg and 1.0mg/kg body weight) were administered chronically for 45 days to Wistar rats, and 24-hour rhythms of glucose, reduced glutathione (GSH), total protein and melatonin (MLT) were studied under semi-natural (LD 12:12 hours) conditions.

Exogenous melatonin administered caused delays in the acrophase of glucose, total protein and melatonin rhythms, whereas advances in the acrophases of reduced glutathione were observed. This indicated that the chronic administration of melatonin could act as the modulated internal zeitgeber and this could be the reason for altered acrophase (peak time of the variable) and other characteristics of rhythms in the melatonin-treated groups. Significant dose-dependent effects of melatonin were absent in the study.

The present study demonstrates that the exogenous administration of melatonin could influence the biochemical rhythms.