Myorelaxant effect after intrathecal injection of antispastic drugs in rats

The effects of intrathecal (i.th.) injections of antispastic drugs were studied on spontaneous activity in the electromyogram (EMG) in genetically spastic rats and on spinal reflex transmission in anaesthetized normal rats. Baclofen, 0.2-2 nmol, and midazolam, 10-80 nmol, suppressed tonic activity in the EMG recorded from the gastrocnemius muscle in mutant rats, whereas tizanidine, 1-100 nmol, enhanced it. The action of baclofen was antagonized by i.th. co-administration of delta-aminovalerate but not by bicuculline, that of midazolam by systemic pretreatment with Ro 15-1788. The effect of i.th. tizanidine was antagonized by co-administration of prazosin but not by yohimbine. Baclofen, 2 nmol, exerted suppressant effects on Hoffman (H)-reflexes and spinal flexor reflexes in normal animals, midazolam, 80 nmol, only on flexor reflexes. Tizanidine, 100 nmol, failed to suppress H-reflexes and flexor reflexes. The present results demonstrate a myorelaxant effect of i.th. injections of baclofen and midazolam but not of i.th. tizanidine.