INFLUENCE OF 6-HYDROXYDOPAMINE LESION OF THE DOPAMINERGIC NIGROSTRIATAL PATHWAY ON THE MUSCLE TONE AND ELECTROMYOGRAPHIC ACTIVITY MEASURED DURING PASSIVE MOVEMENTS

The aim of the present study was to find out whether a 6-hydroxydopami ne-induced lesion of the substantia nigra in rats would evoke muscular rigidity of the parkinsonian type. Simultaneous measurements of muscl e resistance (mechanomyogram) of the hind foot to passive flexion and extension at the ankle joint, as well as of the electromyographic acti vity of the antagonistic muscles of the ankle joint-the gastrocnemius and tibialis anterior-in rats were carried out one, two and four weeks after bilateral injections of 6-hydroxydopamine (6.5 mu g/mu l) into the substantia nigra. After immunohistochemical staining of brain sect ions for tyrosine hydroxylase, the rats were divided into two groups i n which, on average, either 70% (63-80%) or 89% (81-96%) of nigral cel ls degenerated. Larger lesions increased the resistance (mechanomyogra m) of the rat's hind leg to passive movements two weeks after 6-hydrox ydopamine injection, whereas smaller lesions did not. Muscle rigidity was accompanied by an increase in the movement-induced reflex electrom yographic activity in both muscles, mainly in long-latency components which are most probably influenced by supraspinal mechanisms. However, in spite of relatively large lesions of nigral dopamine cells, alread y four weeks after the lesion, muscle rigidity and the respective elec tromyographic activity diminished dramatically, which seems to result from very effective compensatory mechanisms operating in young lesione d rats. The results suggest that the muscle rigidity induced by the 6- hydroxydopamine nigral lesion seems to be a good model of parkinsonian rigidity. Copyright (C) 1996 IBRO. Published by Elsevier Science Ltd.