MELATONIN DECREASES MESSENGER-RNA FOR HISTONE H4 IN THYMUS OF YOUNG-RATS

The antiproliferative properties of melatonin have been previously dem onstrated for several normal and tumoral tissues. In a recent report w e have shown that melatonin is able to inhibit programmed cell death i n thymus both, in vivo and in vitro. Given that other authors have rel ated programmed cell death and cell proliferation and that no previous reports on melatonin and cell division exist on thymus, we decide to study the possible antiproliferative effect of melatonin in this organ measured as the levels of mRNA for the histone H4. We found that mela tonin inhibits cell division on thymus when administered chronically b oth, at high (500 mu g/body weight) and low (50 mu g/body weight) dose . We also found a circadian rhythm of the mRNA for histone H4, opposed to the one previously described for melatonin, supporting the negativ e regulation by this hormone of cell division on thymus. A single dose of melatonin (50 mu g/body weight) was not able to decrease the level s of mRNA for H4 in the time-points studied but after two hours of its administration. Finally, we report the inhibitory effect of melatonin in the cell proliferation of Harderian gland, brain, lung and kidney.