The protective effect of melatonin against phenobarbital-induced oxidative stress in the rat liver was measured based on lipid peroxidation levels ( malondialedyde and 4-hydroxyalkenals). Cellular proliferation, DNA synthesis and cell cycle duration were quantitated by the incorporation of (3)H-thymidine, detected by autoradiography, into newly synthesized DNA. Two experiments were carried out in this study, each on four equal-sized groups of male rats ( control, melatonin [ 10 mg/kg], phenobabital [ 20 mg/kg] and phenobarbital plus melatonin). Experiment I was designed to study the proliferative activity and rate of DNA synthesis, and measure the levels of lipid peroxidation, while experiment II was for cell cycle time determination. Relative to the controls, the phenobarbital-treated rats showed a significant increase ( P < 0.01) in the lipid peroxidation levels ( 30.7%), labelling index ( 69.4%) and rate of DNA synthesis ( 37.8%), and a decrease in the cell cycle time. Administering melatonin to the phenobarbital-treated rats significantly reduced ( P < 0.01) the lipid peroxidation levels ( 23.5%), labelling index ( 38.2%)and rate of DNA synthesis ( 29.0%), and increased the cell cycle time. These results seem to indicate that the stimulatory effect of phenobarbital on the oxidized lipids, proliferative activity, kinetics of DNA synthesis and cell cycle time alteration in the liver may be one of the mechanisms by which the non-genotoxic mitogen induces its carcinogenic action. Furthermore, melatonin displayed powerful protection against the toxic effect of phenobarbital.
