Evidence for glutamatergic tectotectal neurons in the cat superior colliculus: a comparison with GABAergic tectotectal neurons.

The tectotectal commissural pathway is commonly regarded as responsible for the reciprocal inhibition that takes place between the two superior colliculi (SC). Although this hypothesis has received strong support from electrophysiological studies, more recent investigations have suggested that some collicular cells, e.g. fixation neurons, may establish excitatory connections with cells in the contralateral SC through the collicular commissure. The goal of the present study was to seek immunohistochemical evidence for glutamatergic tectotectal cells in the cat SC by using a double-labelling technique. Tectotectal cells were retrogradely labelled with wheat germ agglutinin (WGA) -horseradish peroxidase (HRP) coupled to colloidal gold injected in the contralateral SC, and neurons containing glutamate or gamma-aminobutyric acid (GABA) were then identified with immunohistochemical techniques. The present study provides evidence that, in the cat SC, equal numbers of tectotectal cells are immunopositive to glutamate and GABA, suggesting that the tectotectal pathway may consist of two distinct functional components. The finding that an equal number of tectotectal cells are GABAergic and glutamatergic is somewhat surprising as electrophysiological studies have invariantly indicated that the inhibitory component of the tectotectal projection predominates. Another striking feature of the GABAergic and glutamatergic tectotectal cell populations is their identical topographic distribution in the SC. These results suggest that not only cells in the rostral fixation zone establish excitatory connections with the contralateral SC. Tectotectal projections could be potentially important to shape the spatial pattern of saccade-related activity that may occur simultaneously in the two SC during vertical and oblique orienting movements.