The antinociceptive actions of dexmedetomidine on dorsal horn neuronal responses in the anaesthetized rat

The actions of the selective alpha 2-adrenoceptor agonist dexmedetomidine were examined on the nociceptive C and innocuous A beta fibre-evoked responses of dorsal horn neurones to transcutaneous electrical stimulation in the intact anaesthetized rat. C fibre-evoked responses were dose dependently reduced by intrathecal dexmedetomidine--to a maximum 86 +/- 6% inhibition by 10 micrograms of the agonist. The ED50 for inhibition of C fibre responses was estimated to be 2.5 micrograms. A beta-evoked responses were inhibited to a lesser degree--a maximum 54 +/- 8% inhibition after 10 micrograms dexmedetomidine. The antinociceptive effects of dexmedetomidine were reversed by the alpha 2-adrenoceptor antagonist atipamezole and the opioid antagonist naloxone. The results are discussed with reference to adrenergic and opioid mechanisms in the spinal cord.