Role of GABAergic mechanisms in the substantia nigra pars reticulata in modulating morphine-induced muscular rigidity in rats.

Systemic administration of morphine (15 mg/kg i.p.) induced a muscular rigidity in rats, which was recorded from the gastrocnemius-soleus muscle as tonic activity in the electromyogram (EMG). Administration of muscimol (12.5 or 25 ng) into the substantia nigra pars reticulata (SNR) antagonized the rigidity in a dose-dependent manner, whereas bicuculline enhanced it. It was concluded that GABAergic mechanisms in the SNR play an important role in mediating or modulating the expression of the morphine-induced rigidity, which is known to be due to an alteration of striatal function via opioid receptors located in this brain area.