GABA and non-GABA immunostained neurones in the nucleus prepositus and the periparabigeminal area projecting to the guinea pig superior colliculus.

We have investigated the possibility that GABAergic neurones may be involved in two ascending projections to the superior colliculus, originating in the nucleus prepositus hypoglossi and in the periparabigeminal area of the mesencephalon, respectively. The projecting neurones of both structures were identified using gold-WGA-apoHRP, a retrogradely transported tracer, injected unilaterally into the superior colliculus. GABA was detected in these neurones by means of immunocytochemical staining. The results show that 25% of the projecting neurones in the prepositus hypoglossi are indeed GABA-immunoreactive. They could exert a direct inhibitory influence on the colliculus. By contrast, only a few (7%) gold-filled-GABAergic cells were detected in the periparabigeminal area, which suggests that this region cannot participate in an important inhibitory afferent system to the colliculus.