Origin, distribution, and morphology of serotonergic afferents to the cat superior colliculus: a light and electron microscope immunocytochemistry study.

We have studied the serotonergic (5-HT) projection to the cat superior colliculus (SC) using serotonin antibody immunocytochemistry and retrograde transport of peroxidase-conjugated wheatgerm agglutinin (WGA-HRP). In 3 experiments, the two labels were combined in order to double label cells with both anti-5-HT and WGA-HRP. In the remaining experiments, the two labels were examined separately. Serotonin-like immunoreactive fibers were found throughout all layers of SC, but were most densely distributed within the zonal and upper superficial gray layers. Most 5-HT fibers were thin and had characteristic varicosities and terminal swellings. At the EM level, immunoreactive terminals and varicosities were found to contain small agranular vesicles and occasionally large granular vesicles (LGVs). Conventional synaptic densities were only rarely observed. Injections of WGA-HRP into SC resulted in labeling of neurons throughout the dorsal raphe nucleus and surrounding ventrolateral periaqueductal gray. Only a few cells were found in the raphe medianus and raphe pontis and none within the raphe magnus or other medullary raphe nuclei. Cells in the dorsal raphe giving rise to the SC projection varied in shape, size, and morphology and must represent more than one cell type. The morphology of these cells was indistinguishable from that of cells in the dorsal raphe which were double labeled by anti-5-HT and WGA-HRP. We conclude that the 5-HT innervation of the superior colliculus varies in density in different laminae, arises from several different cell types, and originates primarily from the dorsal raphe nucleus with minor projections from raphe medianus and raphe pontis.