Uni- or bilateral injection of morphine (MO) (3-13 nmoles) into the substantia nigra pars reticulata (SNR) produced tonic activity in the electromyogram (EMG) recorded from the gastrocnemius-soleus (GS) muscle of non-anesthesized rats. This activity was antagonized by naloxone (NAL) (10 nmoles) coadministered with MO into the SNR. Bilateral lesion of the caudate nucleus (CN) with kainic acid did not prevent the tonic EMG activity occurring after the injection of MO into the SNR. Unilateral injection of MO (40 nmoles) into the CN also induced tonic EMG activity in the GS-muscle, which was antagonized by NAL (10 nmoles) administered into the SNR ipsilaterally and simultaneously to the intrastriatal injection of MO. The results suggest that enkephalinergic mechanisms in the SNR seem to play a crucial role in the function of striatal efferent pathways relayed in the SNR.
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