Enhanced alpha-2 adrenergic controls and spinal morphine potency in inflammation.

The antinociceptive potency of spinal morphine is enhanced in rats after carrageenan-induced inflammation. This electrophysiological study examines whether changes in alpha-2 adrenergic systems are responsible. Dorsal horn nociceptive neurones were recorded under halothane anaesthesia in normal animals and animals with carrageenan inflammation. There was a mild increase in potency of the selective alpha-2 adrenoceptor agonist dexmedetomidine following carrageenan (ED50 = 1 microgram). Intrathecal idazoxan (100 micrograms), an alpha-2 antagonist, produced a significant facilitation of the C-fibre evoked response in carrageenan-treated but not normal animals. However since neither idazoxan (100 micrograms) or atipamezole (50 micrograms, another antagonist) influenced the potency of spinal morphine, the increased alpha-2 adrenergic activity in inflammation does not contribute to the enhanced potency of spinal morphine.