Two modes of analgesic action of aspirin, and the site of analgesic action of salicylic acid.

The analgesic activities of aspirin and salicylic acid were investigated by means of the lame-walking test in adjuvant-induced hind-paw-oedematous rats. Aspirin showed ca. 4 times more potent analgesic activity than did salicylic acid in the lame-walking test. The analgesic activity of aspirin was decreased to the level of that of salicylic acid by injection of prostaglandin E2 into the inflamed tissue. The analgesic activity of salicylic acid was not decreased by the same treatment. Salicylic acid inhibited the lame-walking reaction when given intracerebroventricularly. On the other hand, salicylic acid did not inhibit the lame-walking reaction by topical administration on the inflamed hind paw. However, with topical administration, salicylic acid inhibited the carrageenin-induced hind-paw oedema. These results suggest that aspirin has two analgesic effects on the inflammatory pain; one may be the inhibition of prostaglandin biosynthesis by acetylation of cyclo-oxygenase, and the other may be an action due to salicylic acid. Salicylic acid may produce its analgesic action mainly via a central mechanism.