Substance P immunoreactivity identifies a projection from the cat's superior colliculus to the principal tectorecipient zone of the lateral posterior nucleus.

Cells in the superficial layers of the superior colliculus innervate multiple visual regions within the pulvinar-lateral posterior complex of the cat. To characterize these neurons we have examined their immunocytochemical properties in conjunction with their projection patterns. In the present study, we show that a monoclonal antibody for substance P recognizes a morphologically diverse population of neurons, which can be classified as granular, stellate, angular, and horizontal or nonhorizontal fusiform cell types. These neurons are distributed throughout the superficial layers of the colliculus, with a peak density corresponding to sublayer 2 of the stratum griseum superficiale. Injections of rhodamine latex micropheres into the pulvinar-lateral posterior complex demonstrate that a substantial proportion of these collicular cells (at least 35%) project to this region of the posterior thalamus. The overall population of substance P-containing cells, as well as the immunoreactive projection neurons, is composed of the same proportions of cell classes, with the exception that granular cells were not found to be projection neurons. A distinct wedge of substance P immunoreactivity, consisting of fiber and diffuse extracellular labeling, was discovered in the pulvinar-lateral posterior complex. This staining was demonstrated to be confined entirely within the medial division of the lateral posterior nucleus, which is considered to be the principal tectorecipient zone of the extrageniculate visual thalamus. Lesions of the superior colliculus largely abolished the substance P immunoreactivity in the ipsilateral tectorecipient zone. These results are consistent with the view that substance P plays a role in the functional organization of the principal tectothalamic pathway of the cat's extrageniculate visual system.