Suppression of apomorphine-induced oral stereotypy in rats by microinjection of muscimol into midbrain

Rats with large lesions of the superior colliculus do not display the oral stereotypy normally induced by high systemic doses of dopamine-agonists. It has been suggested that collicular lesions have such an effect because they destroy the GABAergic nigrotectal pathway. This suggestion was investigated by observing the effects of bilateral microinjections of the GABA-agonist muscimol into midbrain sites in rats given 8 mg/kg subcutaneous apomorphine. A low dose of muscimol (25 ng in 0.5 ul saline/side) injected into regions of the superior colliculus with nigrotectal innervation almost abolished apomorphine-induced licking and gnawing. Control microinjections of saline into the superior colliculus, or of muscimol into overlying cerebral cortex, were ineffective. This result is consistent with the GABAergic nigrotectal projection being important for the expression of dopamine-related oral stereotypy. It was also found, however, that 25 ng of muscimol suppressed oral stereotypy when microinjected into the mesencephalic reticular formation underlying the superior colliculus. The anatomical basis of this latter effect is uncertain.