Pyramidal and nonpyramidal callosal cells in the striate cortex of the adult rat

The aim of this study has been to determine the neuronal types (pyramidal and nonpyramidal) within the rat's visual cortex, which project through the corpus callosum. To this end, the morphology and laminar distribution of callosal cells have been investigated by combining Diamidino Yellow retrograde tracing with intracellular injection of Lucifer Yellow in slightly fixed tissue slices. The visual callosal projection arises from pyramidal cells of diverse morphology in layers II to VIb, as well as from several modified pyramids located mainly in layers II, IV (star pyramids) and VIb (horizontal or inverted pyramids and related forms of spiny stellate cells). Our results indicate that in rats, as in other mammals, several types of nonpyramidal neurons also contribute to the contralateral projection. Bitufted cells in layers II-III and V were found to project contralaterally. Moreover, a spine-free layer V cell and a sparsely spiny multipolar neuron of layer IV were also labeled. In both stellate cells, partial axonal labeling reveals that these callosal cells display a local axonal arborization. Finally, our results of retrograde transport with Diamidino Yellow and with another sensitive retrograde tracer, the beta subunit of the cholera toxin, demonstrate for the first time that the two main neuronal types of layer I participate in the callosal projection. In layer I, several small horizontal cells of the inner half of layer I and a large subpial cell displaying long radiating dendrites were also injected. The latter cell may correspond to the Cajal-Retzius cell of the adult rat. In spite of the important differences in the organization of the visual system between rodents and cats, the callosal projection in both mammals is composed of a large variety of pyramidal cells and several nonpyramidal neurons. This high morphological diversity suggests that the callosal projection is much more physiologically complex than the extracortical efferents of the visual cortex, resembling other cortico-cortical connections. The roles that the different callosal cells may play in the processing of visual information are discussed in relation to the known functions of the corpus callosum.