Evidence for functional interactions of morphine in substantia nigra and striatum in relation to muscular rigidity in rats

Systemic administration of morphine (15 mg/kg i.p.) induced muscular rigidity in rats, which was recorded from the gastrocnemius-soleus muscle as tonic activity in the electromyogram. Administration of morphine (5 or 10 μg) into the pars compacta of the substantia nigra antagonized the rigidity in a dose-dependent manner, whereas administration of morphine at a dose of 5 μg into the pars reticulata of the substantia nigra enhanced the tonic EMG activity. When morphine and naloxone were co-administered intranigrally, the tonic EMG activity was not affected.