Internal structure of the rat subiculum characterized by diverse immunoreactivities and septotemporal differences.

The subiculum is one of output structures of the hippocampal formation and plays a pivotal role in learning and memory. Because its morphological features are less investigated than those of the hippocampus proper, we explored the internal structure of the rat subiculum using immunohistochemistry. The septal subiculum comprised one region, whereas the temporal subiculum consisted of two subregions, the distal subiculum (Sub1) and proximal subiculum (Sub2). The Sub2 contained four layers: (1) molecular layer, (2) superficial cell layer containing three types of pyramidal neurons immunolabeled for either nitric oxide synthase, Purkinje cell protein 4 (PCP4), or calbindin, (3) middle cell layer where boutons labeled for zinc transporter 3 and those for vesicular glutamate transporter 2 accumulated in the proximal and distal part of the Sub2, respectively, and (4) deep cell layer containing PCP4-positive pyramidal cells, apical dendrites of which showed a characteristic bundling pattern. These features were not observed in either the Sub1 or septal subiculum. There were clear species differences between rats and mice in labeling patterns of pyramidal cells. Morphological differences along the septotemporal axis might be the basis of the functional diversity of the subiculum, such as spatial memory and emotional memory processed at different septotemporal levels.