ABSTRACT
Single administration of sodium salicylate at dose of 400 mg/kg increased plasma total and free trytophan levels as well as brain levels of tryptophan, 5-hydroxytryptamine [5-HT] and 5-bydroxyindoleacetic acid [5-HIAA]. Administration of sodium salicylate at dose of 100 mg/kg daily for 5 days also increased plasma total, free tryptophan, brain tryptophan, 5-HT and 5-HIAA concentration 24th after the last administration. Administration of sodium salicylate at doses of 400 mg/kg to rats already injected 100mg/kg sodium salicylate for five days potentiated the effects of sodium salicylate on the enhancement of plasma tryptophan, brain tryptophan and 5-HIAA concentrations. These findings together with our previous reports show that repeated administration of analgesia drugs may increase the availability of tryptophan to the brain for the synthesis of 5-HT