Nigral GABAergic mechanisms and EMG activity in rats: differences between pars reticulata and pars compacta.

The effects of GABAergic drugs, injected unilaterally either into the substantia nigra pars compacta (SNC) or pars reticulata (SNR) of rats, on motility (tonic activity in the gastrocnemius-soleus muscle, turning behaviour, catalepsy) were studied. Administration of the GABAergic antagonist bicuculline (12.5-50 ng) into the SNR produced tonic activity in the EMG, catalepsy and an ipsiversive posture. The tonic EMG activity was prevented by co-administration of muscimol (25 ng), but not by bilateral lesions of the striatum with kainic acid. Tonic activity in the EMG was also produced by injections of muscimol (25 ng) into the SNC, whereas injections into the SNR led to contraversive circling and no tonic activity in the EMG. Because a new parameter was used--the tonic activity in the EMG--our results support the assumption that GABAergic mechanisms in both parts of the substantia nigra play a different role in regulating and mediating striatal function.