Paracetamol exerts a spinal, tropisetron-reversible, antinociceptive effect in an inflammatory pain model in rats

Experiments were performed in carrageenin-treated rats to study, the antinociceptive and anti-inflammatory effects of paracetamol intravenously (i.v.) or intrathecally (i.t.) injected on rats submitted to a mechanical noxious stimulus. The influence of intrathecal tropisetron, a 5 hydroxytryptamine(3) (5-HT(3)) receptor antagonist, on the antinociceptive effects of paracetamol, was also studied. Paracetamol induced a significant antinociceptive effect after (100, 200 and 300 mg/kg) i.v. and (50, 100 and 200 microg/rat) i.t. injection, but no change occurred on edema volume. The effect of paracetamol was totally inhibited by tropisetron (10 microg/rat, i.t.). The foregoing results demonstrate that, in conditions of inflammatory pain, paracetamol exerts a central antinociceptive effect involving spinal 5-HT(3) receptors, without inducing any anti-inflammatory action. These data, give further arguments to consider paracetamol as a central analgesic drug which must be distinguished from non-steroidal anti-inflammatory drugs (NSAIDs), which justifies the usual combination of paracetamol in post-operative pain.