Subicular efferents are organized mostly as parallel projections: a double-labeling, retrograde-tracing study in the rat.

To understand the functional relevance of the subiculum as a major distributor of hippocampally processed information, detailed information about its neuronal organization is necessary. A striking feature of the subiculum is that it can be divided into four different areas, each characterized by a specific set of efferent connections. To establish whether the different areas of the subiculum are similar with respect to the organization of the origin of their respective efferents, the double-fluorescence retrograde-tracing technique was used to study the degree of collateralization. Because CA1 gives rise to a major input to the subiculum but also projects to some of the targets reached by subicular projections, we compared the subicular degree of collateralization with that of CA1. Throughout CA1, the percentages of double-labeled cells were high, ranging from 17% to 39%. In contrast, the percentages of double-labeled cells in the subiculum were much lower, ranging from 0% to 12%, and no differences were noted between the four areas of the subiculum. This indicates that the four regions of the subiculum are organized in the same way with regard to the output connectivity. Because all four different regions of the subiculum share this paucity of collateralized projections, we conclude that subicular outputs generally originate as parallel projections. This characteristic organization is in line with a proposed function of the subiculum in information storage.