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 dis tributor of hippocampally processed information, detailed information about its neuronal organization is necessary. A striking feature of th e 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 d ouble-fluorescence retrograde-tracing technique was used to study the degree of collateralization. Because CA1 gives rise to a major input t o the subiculum but also projects to some of the targets reached by su bicular projections, we compared the subicular degree of collateraliza tion with that of CA1. Throughout CA1, the percentages of double-label ed cells were high, ranging from 17% to 39%. In contrast, the percenta ges of double-labeled cells in the subiculum were much lower, ranging from 0% to 12%, and no differences were noted between the four areas o f 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 paucit y of collateralized projections, we conclude that subicular outputs ge nerally originate as parallel projections. This characteristic organiz ation is in line with a proposed function of the subiculum in informat ion storage. (C) 1998 Wiley-Liss, Inc.