THE EFFECTS OF MORPHINE ON CARRAGEENAN-INDUCED SPINAL C-FOS EXPRESSION ARE COMPLETELY BLOCKED BY BETA-FUNALTREXAMINE, A SELECTIVE MU-OPIOIDRECEPTOR ANTAGONIST

We have demonstrated that pre-administered morphine (3 mg/kg, i.v.) de creased spinal c-Fos expression induced 2 h after intraplantar carrage enin (55 +/- 5% reduction, P < 0.0001). These effects were completely blocked by pre-administered beta-funaltrexamine (10 mg/kg, i.v., 24 h prior to stimulation) a selective long-lasting mu-opioid receptor anta gonist. In conclusion, these results clearly demonstrate that the effe cts of morphine on noxiously-evoked spinal c-Fos expression are essent ially mediated via mu-opioid receptors.