Melatonin prolongs survival of immunodepressed mice infected with the Venezuelan equine encephalomyelitis virus

Male albino mice immunodepressed after the injection of dexamethasone (DEX) were inoculated intraperitoneally with the Guajira strain of Venezuelan eq uine encephalomyelitis (VEE) virus. Melatonin (MLT) was administered daily, at a dose of 500 mug/kg bodyweight, for 3 days before virus inoculation an d 10 days after. Serum levels of granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin-2 (IL-2) were determined in all the experim ental groups (control, DEX, DEX+MLT, DEX+VEE, DEX+VEE+MLT, VEE and MLT). At day 6 after the virus inoculation, the survival rate was significantly inc reased from 0% in group DEX+VEE to 32.5% in the group of immunodepressed in fected mice treated with MLT (DEX+VEE+MLT). By day 10 a survival rate of 10 % was found in group DEX+VEE+MLT and 0% in group VEE. No alterations in IL- 2 serum levels were observed. MLT increased GM-C SF in control and in DEX-t reated mice. In the VEE virus-infected mice treated with DEX, serum levels of GM-CSF increased progressively from day 1 to 5 postinoculation. In contr ast, the levels of GM-CSF in infected immunodepressed mice treated with MLT decreased significantly from day 1 to 5 postinoculation. At day 5 after vi ral inoculation, no differences were detected in the cerebral viral titres in groups VEE, DEX+VEE and DEX+MLT+VEE. These results show that MLT does no t inhibit VEE viral replication in the brain of immunodepressed mice.