Morphological and immunocytochemical observations on the visual callosal projections in the cat.

The connections between the left and right 17-18 border regions of the cat's visual cortex were labeled by axonal transport of peroxidase-conjugated wheat-germ agglutinin (WGA-HRP) and examined by light and electron microscopy. The cells of origin of the pathway were further characterized by transport of fluorescent microspheres ("beads") followed by in vitro injection of cells with Lucifer Yellow, and by beads transport followed by immunocytochemistry with antibodies to gamma-aminobutyric acid (GABA). The cells of origin of the callosal pathway were located in the lower part of layer 2/3, the upper part of layer 4, and layer 6. In layers 2/3 and 6, they were pyramidal cells; in layer 4 they were star pyramids or spiny stellate cells. None of them were spinefree or sparsely spinous cells, and none were GABA-positive. The axon terminals of the callosal pathway formed type 1 (asymmetric) synapses, and most of them contacted dendritic spines. Both the cells of origin and the terminals were arranged in patches. The findings suggest that the direct action of the callosal pathway is excitatory. The callosal system appears to represent only a subset of the cell types that have intrinsic horizontal projections within areas 17 or 18.