RESUMO
Male albino mice immunodepressed after the injection of dexamethasone (DEX) were inoculated intraperitoneally with the Guajira strain of Venezuelan equine encephalomyelitis (VEE) virus. Melatonin (MLT) was administered daily, at a dose of 500 micrograms/kg bodyweight, for 3 days before virus inoculation and 10 days after. Serum levels of granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin-2 (IL-2) were determined in all the experimental 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 increased from 0% in group DEX + VEE to 32.5% in the group of immunodepressed infected 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-CSF in control and in DEX-treated mice. In the VEE virus-infected mice treated with DEX, serum levels of GM-CSF increased progressively from day 1 to 5 postinoculation. In contrast, the levels of GM-CSF in infected immunodepressed mice treated with MLT decreased significantly from day 1 to 5 postinoculation. At day 5 after viral 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 not inhibit VEE viral replication in the brain of immunodepressed mice.