The antinociceptive action of tizanidine [5-chloro-(2-imidazolin-2-yl-amino)-2,1,3-benzothiadiazole], a centrally acting muscle relaxant, was evaluated after subcutaneous or peroral administration in mice and rats. Tizanidine strongly inhibited the writhing response induced by acetic acid, phenyl-p-benzoquinone and acetylcholine in mice, and its potency was found to be greater than that of morphine. Tizanidine showed antinociceptive action like morphine not only in tail pressure and electrical stimulation tests in mice but also in tail-flick tests in mice and rats. The antinociceptive action of tizanidine was unaffected by pretreatment with naloxone. These findings suggest that tizanidine develops relatively strong antinociceptive action by a nonopioid mechanism.
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