The effects of morphine on carrageenin-induced spinal c-Fos expression are completely blocked by beta-funaltrexamine, a selective mu-opioid receptor antagonist.

We have demonstrated that pre-administered morphine (3 mg/kg, i.v.) decreased spinal c-Fos expression induced 2 h after intraplantar carrageenin (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 antagonist. In conclusion, these results clearly demonstrate that the effects of morphine on noxiously-evoked spinal c-Fos expression are essentially mediated via mu-opioid receptors.