Tizanidine-induced depression of polysynaptic cutaneous reflexes in nonanesthetized monkeys is mediated by an alpha 2-adrenergic mechanism.

Previous studies in anesthetized or reduced preparations of nonprimate animals revealed that the alpha 2-adrenergic agonist tizanidine, clinically used as an antispastic drug, effectively reduces polysynaptic flexor reflexes. To further clarify the invoked adrenergic mechanism for physiological motor functions, and in view of the clinical relevance of tizanidine, the effect of this substance was reinvestigated in awake, nonanesthetized monkeys. Systemic applications of tizanidine dose-dependently reduced the magnitude of the electromyographic response of the flexor reflex that was induced by nonnoxious stimulation of cutaneous afferents. Whereas the effects on the flexor response were consistent, the changes of the background electromyogram were much more variable, often not paralleling those of the reflex. The reflex depression produced by tizanidine could be prevented by pretreatment with the alpha 2-antagonist yohimbine. It is concluded that the action of tizanidine on spinal reflexes, and therefore probably also on hyperactive reflexes of spastic patients, is mediated via the alpha 2-adrenergic properties of the drug. On the basis of the present results, taken together with previous observations that tizanidine transiently inactivates neurons of the nucleus locus coeruleus, it is proposed that the reflex depression may be caused by a removal of a descending noradrenergic facilitation exerted on spinal reflex transmission. This interpretation leaves open further possible actions of tizanidine exerted directly on spinal interneurons.