Effect of clonidine on motoneuron excitability in spinalized rats.

In a dose of 0.1 mg/kg clonidine, an alpha-2 receptor agonist, depressed the spontaneous EMG activity of the biceps and quadriceps femoris in chronically-spinalized rats. It also antagonized in a dose-dependent manner the stimulating effect of 5-hydroxytryptophan (5-HTP, 100 mg/kg). Doses of more than 0.1 mg/kg were less potent in antagonizing the effect of 5-HTP. Clonidine reduced the tonic activity of the hindlimb muscles but allowed walking movements. The depressant effect of clonidine in animals pretreated with 5-HTP was prevented by yohimbine (1.25 mg/kg), while the depressant action of the serotonin antagonist, cyproheptadine was not. In chronically-spinalized rats, clonidine (0.1 mg/kg) increased the threshold of electrically-induced flexor and extensor reflexes and decreased their amplitude. No significant modification of reflexes was seen with this dose 24 hr after spinalization. Thus, clonidine in doses of 0.1 mg/kg or less reduced directly or indirectly the excitability of motoneurons. Clonidine may prove to be a useful therapeutic adjunct in the treatment of spasticity.