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

The subiculum is the main output part of the hippocampal formation and is important for learning and memory. According to connection studies, the distal and proximal regions of the subiculum project to brain regions related to 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), whereas the dorsal subiculum (dSub) seems to comprise only one region (Sub1). However, the connection studies have reported that the dSub contains two regions. Accordingly, we hypothesized that the dSub may indicate "one region" and "two regions" at different dorsoventral levels. To confirm this hypothesis, serial sections of the dSub were prepared and labeled for nitric oxide synthase and Purkinje cell protein 4 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 observed in the dorsal sections. Accordingly, the same dSub indicated a different number of regions at different observation levels. To avoid confusion, we propose dividing the subiculum into Sub1 and Sub2 by immunoreactivities for subicular markers, instead of a rough division into the distal/proximal parts or the dorsal/ventral parts. Furthermore, we confirmed that Sub2 projected to the lateral septum. This finding is consistent with the fact that the proximal-ventral subiculum are involved in emotional memory.