Connections of the subiculum of the rat: Topography in relation to columnar and laminar organization

This paper summarizes published as well as yet unpublished data on the organization of the subiculum. Because of the complex three-dimensional structure of the hippocampus, all traditional planes of sectioning will result in sections that at some point or another do not cut through the hippocampus at an angle that is perpendicular to its long axis; particular focus therefore is on data using the so-called extended preparation. On the basis of our yet fragmented insights in the intrinsic network, as well as the known organization of major efferents and afferents, we propose that the subiculum is organized as a matrix of columnar modules along the transverse axis showing partial laminar connectivity. Although many pieces of the large-scale puzzle on the subicular neuronal network as part of an input-output network for the hippocampus are still missing, it appears that subicular organization is different from that known for CA1. This indicates that major functional differences between CA1 and the subiculum are to be expected.