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

1. Clinical and experimental studies support the view that nigrostriat al dopaminergic neurons are involved in the maintenance of muscle tone . 2. The effects of inactivating dopamine receptors in the rat substan tia nigra on muscle tone, assessed as tonic electromyographic (EMG) ac tivity, was investigated. 3. Dopamine receptors were inactivated by in jections of the irreversible dopamine receptor antagonist, N-ethoxycar bonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) into the pars reticulata of the substantia nigra. 4. Significant increases in EMG activity were o bserved 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. Inacti vation of alpha-adrenoceptors, 5HT(2C) and muscarinic receptors had no effect on EMG activity. 7. Injections of apomorphine did not reduce t he increased EMG activity associated with nigral dopamine receptor los s. 8. The results are consistent with the hypothesis that dopamine rec eptors in the substantia nigra of the rat play an important role in th e regulation of muscle tone.