DOPAMINE-RECEPTORS IN THE SUBSTANTIA-NIGRA ARE INVOLVED IN THE REGULATION OF MUSCLE TONE

The aim of the present study was to localize the dopamine receptors in volved in the regulation of muscle tone. A strategy was used whereby t he effects on muscle tone of injecting the irreversible dopamine recep tor antagonist N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) i n discrete brain regions were assessed. Increases in muscle tone were measured as changes in electromyographic activity of the gastrocnemius and tibialis muscles of conscious, unrestrained rats. No increases in muscle tone were found after injections of EEDQ into the anterior and posterior striatum, which produced marked reductions in dopamine rece ptor concentration. The effects on muscle tone of injecting EEDQ into the substantia nigra pars reticulata were also assessed. Large increas es in muscle tone were observed associated with inactivation of either D-1 or D-2 dopamine receptors in the substantia nigra. The increased muscle tone was not reduced by subcutaneous administration of apomorph ine, despite the presence of a normal population of striatal dopamine receptors. These findings provide evidence that dopamine receptors in the substantia nigra play an important role in the regulation of muscl e tone. Further, they challenge the hypothesis that the muscle rigidit y of Parkinson disease results primarily from loss of striatal dopamin e receptor stimulation.