The effect of intrathecally administered imiloxan and WB4101: possible role of alpha 2-adrenoceptor subtypes in the spinal cord.

To define the antagonist pharmacology of spinal antinociceptive effects of alpha 2-adrenoceptor agonists, the ability of WB4101 (2-(2,6-dimethoxyphenoxyethyl)-aminomethyl-1,4-benzodioxane) and imiloxan to antagonize the effect of spinal ST-91 (2-[2,6-diethylphenylamino]-2-imidazoline), clonidine and dexmedetomidine was examined. Antinociceptive effects of ST-91 were antagonized by WB4101 and imiloxan; clonidine only by WB4101; and, dexmedetomidine was not antagonized by either antagonist. It is hypothesized that the alpha 2 adrenoceptor subtype alpha 2A is involved in the antinociception of spinal clonidine and dexmedetomidine, and alpha 2B is involved in that of ST-91.