RECIPROCAL CONNECTIONS BETWEEN THE ZONA INCERTA AND THE PRETECTUM ANDSUPERIOR COLLICULUS OF THE CAT

The goal of the present experiments was to examine the relationships o f the zona incerta with two structures associated with visuomotor beha vior, the superior colliculus and pretectum. The experiments were carr ied out in the cat, a species commonly used in studies of visuomotor i ntegration, and utilized wheat germ agglutinin-horseradish peroxidase and biocytin as retrograde and anterograde neuronal tracers. Retrograd e axonal transport demonstrated that most cells in the ventral subdivi sion of the zona incerta project to the superior colliculus. Anterogra de tracers demonstrated that the incertotectal terminal field is most dense in the intermediate gray layer, which is the primary source of t he descending pathway from the superior colliculus to brainstem gaze c enters. Further experiments showed that scattered cells within the int ermediate gray layer give rise to a reciprocal pathway that terminates in both the dorsal and ventral subdivisions of the zona incerta. The distribution of both labeled incertotectal cells and tectoincertal ter minals extends dorsolateral to the zona incerta proper, between the re ticular thalamic nucleus and the external medullary lamina. Electron m icroscopic examination of labeled tectoincertal terminals demonstrated that they contain mainly spherical vesicles and have slightly asymmet ric to symmetric synaptic densities. Labeled terminals were observed c ontacting labeled cells in the zona incerta, suggesting that the recip rocal pathway may be monosynaptic. The zona incerta is also reciprocal ly interconnected with the pretectum. The anterior pretectal nucleus p rovides a dense projection to the ventral part of the zona incerta and receives a sparse reciprocal projection. The posterior pretectal nucl eus and nucleus of the optic tract may also project to the zona incert a. The pretectoincertal fibers form terminals that contain primarily s pherical vesicles and make distinctly asymmetric synaptic contacts. In summary, these results indicate that the deep layers of the superior colliculus, which are important for controlling saccades, are the targ et of a projection from the ventral subdivision of the zona incerta. L ike the substantia nigra, the zona incerta may play a permissive role in the tectal initiation of saccadic eye movements. The incertotectal terminal field in the cat is less dense than that observed previously in the rat, suggesting species differences in the development of this pathway. An additional finding of this study is that one of the main s ources of input to these incertotectal cells is the anterior prerectal nucleus. This pretectal-incertal-tectal pathway is likely to play a r ole in the guidance of tectally initiated saccades by somatosensory st imuli. (C) 1997 IBRO. Published by Elsevier Science Ltd.