Haloperidol-increased muscle tone in rats as a model of parkinsonian rigidity.

The aim of the present study was to find out whether haloperidol-induced rigidity was similar to that seen in parkinsonism. Simultaneous measurements of the muscle resistance (mechanomyogram, MMG) of the hind foot to passive flexion and extension in the ankle joint, as well as determination of the electromyographic (EMG) activity of the gastrocnemius and tibialis anterior muscles of rats were carried out. Haloperidol was injected in doses of 0.5-10 mg/kg 1 h before the start of measurements. Haloperidol increased, in a dose-dependent manner, the muscle resistance of the rat's hind leg to passive movements. Muscle rigidity was accompanied with an increase resting, as well as in the stretch-induced long-latency EMG activity (in which supraspinal reflexes are most probably involved) in both those muscles, whereas the short-latency EMG activity (first large bursts of EMG activity, beginning ca. 9 ms after the start of a movement, probably of a spinal origin) was significantly decreased. The obtained results suggest that the haloperidol-increased MMG/EMG activity might be a good model of parkinsonian rigidity.