Number of subfields of the rat dorsal subiculum defined by the NOS and PCP4 immunoreactivities changes according to the different levels of observation.

The subiculum is a main output part of the hippocampal formation and important for learning and memory. According to connection studies, the distal and proximal regions of the subiculum project to the brain regions related to the spatial and emotional memories, respectively. Our previous morphological studies indicated that the ventral subiculum (vSub) consists of two regions, the distal subiculum (Sub1) and the proximal subiculum (Sub2), while the dorsal subiculum (dSub) seemed to comprise only one region (Sub1). However, the connection studies reported that the dorsal subiculum also contained two regions. Accordingly, we hypothesized that the dorsal subiculum may indicate "one region" and "two regions" at the different dorsoventral levels. To confirm this hypothesis, serial sections of the dorsal subiculum were prepared and labeled for NOS and PCP4 as markers dividing the subiculum. As a result, vSub showed two regions, Sub1 and Sub2, whereas the dorsal tip of the subiculum showed one region (Sub1) as shown in our previous studies. However, two regions were also observed in the dorsal sections. Accordingly, the same dorsal subiculum indicated different number of regions at the different levels of observation. To avoid the confusion, we propose to divide the subiculum into Sub1 and Sub2 by immunoreactivities for subicular markers, instead of the rough division into the distal / proximal parts or the dorsal / ventral parts. Furthermore, we confirmed Sub2 projected to the lateral septum. That is consistent with the fact that the proximal and ventral parts of the subiculum are involved in emotional memory.