• Frédéric Duflo, Xinhui Li, Carsten Bantel, Carlo Pancaro, Michelle Vincler, and James C Eisenach.
• Department of Anesthesiology and Center for the Study of Pharmacologic Plasticity in the Presence of Pain, Wake Forest University School of Medicine, Winston-Salem, North Carolina 27157, USA. fduflo@wfubmc.edu
• Anesthesiology. 2002 Sep 1; 97 (3): 636-41.

BackgroundPrevious studies suggest that the alpha adrenoceptor subtype is the target for spinally administered alpha -adrenergic agonists, clonidine, for pain relief. However, ST 91, a preferential alpha adrenoceptor subtype agonist, induces antinociception, and intrathecally administered alpha antisense oligodeoxynucleotide decreases antinociception induced by clonidine in the rat, suggesting non-A sites may be important as well. Therefore, the authors examined the subtype of alpha adrenoceptor activated by clonidine and ST 91 in normal rats and those with nerve injury-induced hypersensitivity.MethodsThe same mechanical stimulus was applied to normal rats and those following spinal nerve ligation, and the effect of intrathecal clonidine and ST 91 on withdrawal threshold to the stimulus was determined. To further examine subtypes, animals were spinally pretreated with vehicle, BRL 44408 (an alpha subtype-preferring antagonist), and ARC 239 (an alpha subtype-preferring antagonist).ResultsIn normal animals, clonidine's effect was diminished by pretreatment with either antagonist, whereas ST 91's antinociceptive effect was solely blocked by pretreatment with ARC 239. In nerve-injured animals, the antihypersensitivity action of both clonidine and ST 91 was blocked by administration of ARC 239, whereas BRL 44408 was ineffective.ConclusionsThese data agree with previous studies supporting that the alpha adrenoceptor is important to the antinociceptive effect of clonidine in normal animals. Nerve injury alters this and results in a total reliance on alpha adrenoceptors.

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