Serotonin and opiate involvement in the antinociceptive effect of acetylsalicylic acid.

Acetylsalicylic acid (ASA), 400 mg/kg i.p., displayed antinociceptive activity in both the hot-plate and the formalin test. ASA significantly increased brain serotonin (5-HT) content and reduced the number of 5-HT2 receptors in cortical brain membranes 30 min after drug administration. Pretreatment with naloxone abolished the antinociceptive activity of both ASA and morphine in the hot-plate and formalin tests and prevented the increase in cerebral 5-HT concentration and the reduction in 5-HT2 receptors in cortical membranes induced by ASA. The serum salicylate concentrations were not affected by pretreatment with naloxone. These data indicate a central antinociceptive activity of ASA and suggest that ASA may exert its antinociceptive action through serotonergic and opiatergic pathways.