Morphological and electrophysiological characteristics of layer V neurons of the rat medial entorhinal cortex

This study aimed to characterize the morphological and electrophysiological properties of neurons in layer V of the entorhinal cortex in the rat brain. Using the in vitro slice preparation and sharp electrode techniques, we recorded from layer V neurons located in the medial entorhinal cortex. Recorded cells were also labeled with biocytin. Based on morphological criteria, layer V of the entorhinal cortex is comprised of three categories of neurons: pyramidal cells, horizontal cells, and polymorphic cells. Horizontal cells could be easily distinguished from the pyramidal cells because the bulk of their dendritic plexus extended horizontally within layer V. Polymorphic cells vary in size and shape. Interestingly, they typically do not have apical dendrites, and some of them have dendrites that extend into the subiculum. Based on electrophysiological criteria alone, it was not possible to unequivocally distinguish the morphological cell types because they were somewhat heterogeneous with respect to several parameters including inward rectification, spike-frequency adaptation, and intrinsic oscillations. Nevertheless, although most horizontal cells displayed time-dependent inward rectification, most pyramidal cells displayed fast inward rectification exclusively. None of the entorhinal cortex layer V cells displayed oscillatory activity like that of neocortical layer V "bursting" cells, although neurons from all groups displayed rhythmic subthreshold membrane potential oscillations. In summary, we have found that layer V of the rat medial entorhinal cortex consists of three morphologically distinct neuronal subtypes that cannot be clearly distinguished from each other by traditional electrophysiological measures.