The role of the substantia nigra in motility of the rat. Muscular rigidity, body asymmetry and catalepsy after injection of morphine into the nigra.

Unilateral injection of morphine (1.0, 2.5 or 5.0 micrograms/0.5 microliter) into the substantia nigra pars reticulata produced, in a dose-dependent way, tonic activity in the electromyogram (EMG) recorded from the ipsilateral gastrocnemius-soleus muscle of unanaesthetized rats. The effect produced by 5 micrograms of morphine was antagonized by co-administration of 5 micrograms of naloxone. Bilateral injection of morphine (5 micrograms each side) also produced a tonic activity in the EMG and catalepsy, but no asymmetry of posture. Unilateral injection of morphine (5 micrograms) into the substantia nigra pars compacta produced contralateral turning and/or stereotypy, but no tonic activity in the EMG. Naloxone (5 micrograms), when injected into the substantia nigra failed to produce any tonic activity in the EMG or any catalepsy. Larger doses of naloxone (10 or 20 micrograms) produced slight to moderate tonic activity in the EMG when injected into the pars reticulata. Injections of saline (0.5 microliter) into either the reticulata or compacta were ineffective. Bilateral lesions of the caudate nucleus with kainic acid (1 microgram each side) did not prevent the development either of the tonic activity in the EMG or of the catalepsy, which were both produced by unilateral injection of morphine (5 micrograms) into the pars reticulata. These results suggest that opioid mechanisms in the nigra might play a role in the regulation of posture and muscle tone and participate in the expression of motor functions controlled by the striatum (and probably also by the nucleus accumbens).