Inhibition of electrically-induced vocalization in adjuvant arthritic rats as a novel method for evaluating analgesic drugs.

A new method for the evaluation of analgesic drugs in normal rats and in rats with hyperalgesia induced by adjuvant was developed using the vocalization response as an indicator of pain due to electrical shock. It was demonstrated that at ED50 doses, nonsteroidal anti-inflammatory drugs (NSAIDs), narcotic analgesic drugs and narcotic agonist/antagonist type analgesic drugs were effective, but pure narcotic antagonist drugs. CNS-acting drugs and anti-inflammatory steroid were ineffective. Acidic NSAIDs, except aspirin, were more effective in adjuvant arthritic rats than normals but other analgesic drugs had roughly the same effect in both. It was suggested that the acidic NSAIDs specifically inhibit inflammatory pain. Moreover, the vocalization response under adjuvant treatment is useful for the quantitative measurement of analgesics.