The role of the colliculus superior in the expression of muscular rigidity

Muscular rigidity, recorded as tonic activity in the electromyogram of the gastrocnemius-soleus muscle in the rat was produced by bilateral injections of muscimol (37.5-75 ng) into the intermediate or deep layers of the colliculus superior. This effect of muscimol was not only dose-dependent but also gamma-aminobutyric acid (GABA)-specific since it could be prevented by coadministration of the GABA antagonist bicuculline methiodide. Injections of muscimol into either the superficial layers of the colliculus superior or the underlying reticular formation were without effect in this respect. These results indicate that the region containing the terminals of the nigrocollicular pathway is involved in the expression of muscular rigidity (recorded as a tonic EMG activity). Muscimol injections into the colliculus superior did not lead to catalepsy thus providing additional evidence for the notion that muscular rigidity and catalepsy are two separate phenomena which are in part mediated by different neuronal pathways.