Laminar organization of the pyramidal cell layer of the subiculum in the rat

The distribution of neurons in the subiculum of the rat that give rise to subcortical connections was studied using retrograde labeling with horseradish peroxidase conjugated to wheat germ agglutinin. Injections were made into the anteroventral thalamic nucleus, the medial mammillary nucleus, the nucleus accumbens, and the lateral septal nucleus. To facilitate the analysis, the hippocampal formation with adjacent cortices were “flattened,” which allowed the cutting of sections perpendicular to the full septotemporal axis. Cells projecting to the anteroventral thalamic nucleus (AV cells), the medial mammillary body (MMB cells), and the nucleus accumbens (ACC cells) were observed consistently throughout the entire septotemporal (dorsoventral) and transverse extent of the subiculum (from field CA1 of the hippocampus to the presubiculum). In the transverse plane, the three kinds of projection cells were arranged in a laminar fashion: The AV cells were observed in the deepest portion of the subicular pyramidal cell layer, the ACC cells were observed in the most superficial portion of the layer, and the MMB cells were observed in the middle portion of the layer. Although this laminar arrangement was observed at all septotemporal levels of the subiculum, it was most apparent at the septal level. At more temporal levels, the “laminae” shifted such that the superficially located ACC cells were more prominent in the proximal half of the subiculum, whereas the AV cells were shifted toward the distal half of the subiculum. The average size of somata of the AV cells was 72.3 μm2, that of the ACC cells was 105.2 μm2, and that of the MMB cells was 121.8 μm2. The connectional and cytoarchitectonic data indicate that there is a distinct sublamination of the subicular pyramidal cell layer, suggesting that the subiculum may be analogous to the infragranular layer (layers V and VI) of the isocortex. J. Comp. Neurol. 435:89–110, 2001. © 2001 Wiley‐Liss, Inc.