Evidence for suppression of spinal glial activation by dexmedetomidine in a rat model of monoarthritis

P>1. Spinal glial cells play a key role in developing and maintaining allodynia and hyperalgesia following tissue inflammation. Dexmedetomidine, a highly selective alpha(2)-adrenoceptor (alpha(2)-AR) agonist, has exhibited a significant analgesic effect in various rodent models of chronic pain. However, whether spinal glial activation is involved in the analgesic effect of dexmedetomidine remains unknown. The present study investigated whether spinal administration of dexmedetomidine could antagonize glial activation in the spinal dorsal horn and attenuate thermal hyperalgesia in complete Freund's adjuvant (CFA)-induced ankle joint monoarthritic (MA) rats. 2. Unilateral intra-articular injection of CFA produced a robust activation of microglia and astrocytes in the spinal cord, which was associated with the development and maintenance of thermal hyperalgesia. Repeated lumbar puncture (LP) administration of dexmedetomidine (10 mu g) significantly attenuated MA-induced thermal hyperalgesia in a cumulative manner. Monoarthritis-induced spinal glial activation was also suppressed following dexmedetomidine application. The alpha(2A)-AR, essential for the antinociceptive effects of alpha(2)-AR agonists, was detected in spinal neurons and glia, as well as in dorsal root ganglion primary afferent neurons, which may be implicated in dexmedetomidine-induced suppression of spinal glial activation and antihyperalgesic effects. 3. These data provide the first evidence that blocking spinal glial activation is involved in the analgesic action of dexmedetomidine.