A NEW VIEW OF THE ROLE OF DOPAMINE-RECEPTORS IN THE REGULATION OF MUSCLE TONE

1. Clinical and experimental studies support the view that nigrostriatal dopaminergic neurons are involved in the maintenance of muscle tone. 2. The effects of inactivating dopamine receptors in the rat substantia nigra on muscle tone, assessed as tonic electromyographic (EMG) activity, was investigated. 3. Dopamine receptors were inactivated by injections of the irreversible dopamine receptor antagonist, N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) into the pars reticulata of the substantia nigra. 4. Significant increases in EMG activity were observed from 2 to 24 h after EEDQ injection and were associated with a loss of nigral dopamine receptors. 5. Increases in EMG activity were found only when both D-1 and D-2 receptors were inactivated. 6. Inactivation of alpha-adrenoceptors, 5HT(2C) and muscarinic receptors had no effect on EMG activity. 7. Injections of apomorphine did not reduce the increased EMG activity associated with nigral dopamine receptor loss. 8. The results are consistent with the hypothesis that dopamine receptors in the substantia nigra of the rat play an important role in the regulation of muscle tone.