Substance P-, vasoactive intestinal polypeptide-, and cholecystokinin-like immunoreactive fiber projections from the superior colliculus to the dorsal lateral geniculate nucleus in the rat.

Substance P (SP)-, vasoactive intestinal polypeptide (VIP)-, and cholecystokinin (CCK)-like immunoreactive (LI) neurons were found in the superior colliculus (SC) of the rat, and examined to ascertain whether they sent projection fibers to the dorsal lateral geniculate nucleus (LGNd). Immunocytochemical staining with antisera against SP, VIP, and CCK showed that many immunoreactive neuronal cell bodies were located in the superficial layers of the SC, especially in the stratum griseum superficiale. The pattern of distribution of these immunoreactive neuronal cell bodies in the SC was similar to that of neuronal cell bodies which were retrogradely labeled with WGA-HRP (wheat germ agglutinin-horseradish peroxidase conjugate) injected ipsilaterally into the LGNd. On the other hand, SP-, VIP- and CCK-LI axons were seen most densely in the lateral part of the LGNd, especially in the small-celled LGNd zone adjacent to the optic tract, where anterograde labeling was also observed after injection of WGA-HRP ipsilaterally into the superficial layers of the SC. When a lesion was produced by kainic acid injection into the superficial layers of the SC, axons showing SP-, VIP-, or CCK-LI in the LGNd ipsilateral to the lesion were markedly depleted. The results indicate that SC-LGNd projection neurons contain SP, VIP, and/or CCK in the rat.