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 collic uli (SC). Although this hypothesis has received strong support from electro physiological studies, more recent investigations have suggested that some collicular cells, e.g. fixation neurons, may establish excitatory connectio ns with cells in the contralateral SC through the collicular commissure. Th e goal of the present study was to seek immunohistochemical evidence for gl utamatergic tectotectal cells in the cat SC by using a double-labelling tec hnique. Tectotectal cells were retrogradely labelled with wheat germ agglut inin (WGA) -horseradish peroxidase (HRP) coupled to colloidal gold injected in the contralateral SC, and neurons containing glutamate or gamma-aminobu tyric 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 gl utamatergic is somewhat surprising as electrophysiological studies have inv ariantly indicated that the inhibitory component of the tectotectal project ion predominates. Another striking feature of the GABAergic and glutamaterg ic tectotectal cell populations is their identical topographic distribution in the SC. These results suggest that not only cells in the rostral fixati on zone establish excitatory connections with the contralateral SC. Tectote ctal projections could be potentially important to shape the spatial patter n of saccade-related activity that may occur simultaneously in the two SC d uring vertical and oblique orienting movements.