Spinal adrenergic and cholinergic receptor interactions activated by clonidine in postincisional pain

Previous pharmacologic and molecular studies suggest that the alpha(2)-adrenoceptor subtype A is the target for spinally administered alpha(2)-adrenergic agonists, i.e., clonidine, for pain relief. However, intrathecally administered alpha(2) C antisense oligodeoxynucleotide was recently reported to decrease antinociception induced by clonidine in the rat, suggesting non-A sites may be important as well. The current study sought to determine the subtype of alpha(2) adrenoceptors activated by clonidine in a rodent model for human postoperative pain, and to examine its interaction with spinal cholinergic receptors.

Postoperative hypersensitivity was produced in rats by plantar incision of the hind paw and punctuate mechanical stimuli were applied around the wound 24 h after surgery. Effects of intrathecal clonidine and 2-(2,6-diethylphenylamino)-2-imidazoline (ST91) on withdrawal thresholds to the stimulus were determined. To examine the adrenoceptor subtype and its interaction with spinal cholinergic receptors, animals were intrathecally pretreated with vehicles BRL44408 (an alpha(2) A subtype-preferring antagonist), ARC239 (an alpha(2) non-A subtype-preferring antagonist), atropine (a muscarinic antagonist), and mecamylamine (a nicotinic antagonist).

Intrathecal ST91 showed a significantly greater efficacy when compared with clonidine. The analgesic effect of clonidine was diminished by pretreatment with either adrenoceptor antagonist, whereas the effect of ST91 was solely blocked by ARC239 pretreatment. Atropine and mecamylamine abolished the effect of clonidine effect but not the effect of ST91.

Both alpha(2) A and alpha(2) non-A adrenoceptors, as well as spinal cholinergic activation, are important to the antihypersensitivity effect of clonidine after surgery. ST91 is more efficacious in this model than clonidine and relies entirely on alpha(2) non-A adrenoceptors.