Antinociceptive activity induced by tizanidine and alpha 2-adrenoreceptors.

Tizanidine [5-chloro-4-(2-imidazolin-2-yl-amino)-2,1,3-benzothiadiazole] is able to increase the pain threshold in the tail-flick test in mice. The effect of tizanidine was investigated after treatment of mice with drugs influencing central monoaminergic and GABAergic mechanisms. A drug that inhibits the synthesis and storage of monoamines and drugs that cause specific lesions of monoaminergic neurons had no consistent effect on the antinociceptive action of tizanidine. The action of tizanidine was antagonized by the alpha 2-adrenoreceptor antagonist, yohimbine, but not by the alpha 1 antagonist prazosin, nor by dopamine, serotonin and GABA receptor antagonists. These results indicate that the antinociceptive action induced by tizanidine may be mediated by alpha 2-adrenoreceptors.