Immunohistochemical and electrophysiological characterization of principal cells in layer II of the lateral entorhinal cortex

Abstract

The entorhinal cortex (EC) is a part of the medial temporal lobe of the brain, and is considered to be important for learning and memory. The EC can be divided into two subdivisions; the lateral EC (LEC) and the medial EC (MEC). LEC and MEC have shown to be similar in many aspects, with the exception of properties of layer (L)II cells. These cells differ between the two subregions in terms of morphology and electrophysiology. Some ambiguity still exists whether the cells of LEC LII and MEC LII show similarities to the expression of the immunomarkers Reelin and Calbindin (CB). It has also been suggested that the reactivity to these immunomarkers in MEC LII might separate the cells into two populations with differences in projection patterns, cellular morphology and electrophysiology. The aim of this study is to investigate whether LEC LII cells can be separated by these two immunomarkers, and whether this coincides with differences in morphology and electrophysiology. In this study, electrophysiological responses of principal cells of LEC LII were recorded in rat brain slice preparations with the use of whole-cell multipatch recordings together with intracellular filling with Biocytin. Subsequent immunohistochemistry staining against CB and Reelin was performed, followed by post-hoc morphological analyses. A total of 102 out of the 104 morphologically identified cells, were found to be immunoreactive for Reelin but not for CB. The neuronal population included fan cells, multiform cells, oblique pyramidal cells and pyramidal cells. No correlation between morphologically identified cell type, and electrophysiological parameters was detected. The main findings of this study suggest that LEC LII cells can only be subdivided based on their morphology, and not by their immunoreactivity to Reelin and CB, nor can they be separated based on their electrophysiological responses. This implies that the cells of LEC LII are more homogenous than the cells of MEC LII. It remains to be seen whether the LEC LII cells differ in terms of connectivity and projection patterns.