Possible excitatory and inhibitory feedback to the superior colliculus: a combined retrograde and immunocytochemical study in the prepositus hypoglossi nucleus of the guinea pig.

We have investigated in the guinea pig the precise localization and the immunoreactivity of the neurones in the prepositus hypoglossi nucleus involved in a direct ascending projection onto the superior colliculus. The projecting neurones were characterized by a retrograde tracer (WGA-ApoHRP coupled to gold particles), injected in the intermediate and deep layers of the superior colliculus. After revealing gold particles, the sections were then treated using an antibody either against GABA or against glutamate, thus allowing identification of gold-filled-immunoreactive neurones. The retrogradely labelled cells were exclusively distributed on the contralateral side, and preferentially in the caudal two thirds of the prepositus hypoglossi nucleus, in its ventral and ventrolateral division. In addition, about 23% of these projecting neurones appear immunopositive when the sections are treated with a GABA antibody and around 27% are immunopositive to glutamate. Furthermore, these two classes of GABA-like or glutamate-like projecting neurones are intermingled within the prepositus hypoglossi nucleus. We conclude, in spite of a probable underestimation of these two populations, that the ascending projection is formed by an excitatory pathway that probably involves glutamate as well as an inhibitory pathway mediated by GABA. Thus we cannot consider this feedback as exclusively inhibitory as was suggested in theoretical models of the oculomotor system.