Depolarizing afterpotentials in rodent mEC layer II neurons vary gradually across all principal cell types

Principal cells in Layer II (LII) of the rodent medial entorhinal cortex (mEC) often discharge in short burst episodes, whose biophysical mechanism and biological function are not fully understood. As shown by in vitro recordings in rats and in vivo recordings in mice, action potentials of stellate cells (SCs) and pyramidal cells (PCs) in mEC LII can trigger depolarizing afterpotentials (DAPs) that facilitate spike doublets and burst firing. In vitro recordings in mice confirm these findings for SCs and two additional cell types intermediate between SCs and PCs but suggest that mouse PCs generate minute DAPs only. To better understand the diversity and physiology of the intermediate LII mEC cell types, we performed slice experiments in Long Evans rats of both sexes and analyzed the data using clustering techniques. The analysis suggests four distinct cell clusters, reminiscent of previous observations in mice. On purpose, DAP properties were not used for the cluster analysis so that they could be studied post-hoc in an unbiased manner. Our results show that characteristic features of DAPs and other physiological markers are broadly distributed within each cell class and vary gradually from cluster to cluster, forming a continuum of biophysical properties.