GABAergic and non-GABAergic neurons in the nucleus of the optic tract project to the superior colliculus: an ultrastructural retrograde tracer and immunocytochemical study in the rabbit.

Both the nucleus of the optic tract (NOT) and the superior colliculus (SC) are thought to play important roles in the regulation of eye movements. The superior colliculus contributes to visual orientation and saccades, and the nucleus of the optic tract contributes to the detection of slow movements of the visual surround. Recently, a GABAergic projection has been described between these two nuclei in the cat, a species with frontal vision. The present study aimed at determining whether a similar GABAergic pathway exists in the rabbit, a species with lateral vision. To study this pathway we used the retrograde tracer cholera-toxin (CTB) to identify NOT neurons projecting to the SC and GABA-antibody immunostaining to identify GABA-containing neurons and processes. CTB injections into the superficial laminae of the SC showed that GABAergic and non-GABAergic neurons in the NOT project to the SC. Both types of neurons have structural characteristics similar to other projection neurons in the NOT. In contrast to the NOT neurons projecting to the inferior olive (IO) which are mainly located in the rostral NOT, the GABAergic and non-GABAergic NOT-SC neurons are situated throughout the nucleus. The somata and principal dendrites of both neuron types receive numerous synaptic contacts from GABAergic terminals and only a few from retinals. The NOT projection neurons to the SC thus establish prominent excitatory and inhibitory links between the two structures, suggesting the existence of separate circuits that could interact through a GABAergic and non-GABAergic NOT-SC projection. It is further suggested that these circuits may be involved in the regulation of saccades in the SC during optokinetic nystagmus.