Evidence for the involvement of the mu but not delta opioid receptor subtype in the synergistic interaction between opioid and alpha 2 adrenergic antinociception in the rat spinal cord.

The interaction between the spinal antinociceptive effects of selective mu or delta opioid agonists morphine and DSTBULET (Tyr-D-Ser(OtBu)-Gly-Phe-Leu-Thr), respectively, and the selective alpha 2 adrenergic agonist dexmedetomidine was examined on convergent dorsal horn neuronal responses in the intact anaesthetized rat. The coadministration of intrathecal morphine (0.5 microgram, 2.5 micrograms) and dexmedetomidine (0.5 microgram) produced a greater than additive inhibition of C fibre-evoked responses. Inhibitions were reversed by either the opioid antagonist naloxone or the alpha 2 adrenergic antagonist atipamezole. The coadministration of intrathecal DSTBULET (1 microgram, 2.5 micrograms) and dexmedetomidine did not result in a supra-additive inhibition of C fibre-evoked responses. The results suggest that mu rather than delta opioid receptors are involved in the synergism of spinal opioid and alpha 2 adrenergic antinociception.