Effects of intrathecally administered nociceptin, an opioid receptor-like1 (ORL1) receptor agonist, on the thermal hyperalgesia induced by carageenan injection into the rat paw.

Nociceptin is a 17 amino acid peptide and acts as a potent endogenous agonist of the opioid receptor-like1 (ORL1) receptor. Nociceptin is reported to depress glutamatergic transmission and to block the spinal facilitation which is thought to be mediated by the activation of N-methyl-D-aspartate (NMDA) receptor. Paw carageenan injection induces thermal hyperalgesia which has been reported to be maintained by the NMDA receptor dependent spinal facilitation. In the present study, we investigated the effect of intrathecally administered nociceptin on the level of thermal hyperalgesia induced by carageenan injection in the rat. Nociceptin depresses the level of thermal hyperalgesia in a dose-dependent manner. These data suggest that spinal ORL1 receptor activation depresses the spinal facilitation state induced by paw carageenan injection.