Considering that the involvement of oxidative stress has been implicated in the toxicity of organophosphate insecticides, the objective of the present study was to investigate antioxidants and oxidative stress marker s in the liver and kidney of Wistar rats treated with chronic doses of diazinon. In addition, the effect of Tert -butylhydroquinone (TBHQ), a food additive antioxidant, on attenuation of diazinon -induced oxidative stress was evaluated. 28 rats were randomly assigned to 1 of 4 treatment groups: diazinon (10 mg/kg BW, once a day; n=7), TBHQ (0.028 g/kg of diet, once a day; n=7), TBHQ + diazinon (diazinon; 10 mg/kg BW, once a day + TBHQ; 0.028 g/kg of diet, once a day; n=7) and control (corn oil, as vehicle of diazinon and TBHQ, n=7). TBHQ, diazinon and corn oil were given to rats orally via gavage for 7 weeks. Total thiol groups, ferric reducing antioxidant power (FRAP) and malondialdehyde (MDA) levels in liver and kidney tissues were investigated at the end of 7th week. Treatment with diazinon significantly increased MDA level, as a marker of lipid peroxidation, in the kidney (p<0.05). Coadministration of TBHQ with diazinon significantly increased total thiol groups and FRAP levels in liver and kidney (p<0.05) . However, TBHQ did not reduce lipid peroxidation in the tissues. The results of the present study showed that administration of TBHQ at a dose of 0.028 g/kg of diet, increased antioxidant capacity of tissues of rats treated with diazinon , though this incr ease was not sufficiently effective to reduce lipid peroxidation caused by diazinon. Higher amounts of TBHQ might be more effective in attenuation of diazinon induced lipid peroxidation

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