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 r ecorded from layer V neurons located in the medial entorhinal cortex. Recor ded cells were also labeled with biocytin. Based on morphological criteria, layer V of the entorhinal cortex is comprised of three categories of neuro ns: pyramidal cells, horizontal cells, and polymorphic cells. Horizontal ce lls could be easily distinguished from the pyramidal cells because the bulk of their dendritic plexus extended horizontally within layer V. Polymorphi c cells vary in size and shape. Interestingly, they typically do not have a pical dendrites, and some of them have dendrites that extend into the subic ulum. Based on electrophysiological criteria alone, it was not possible to unequivocally distinguish the morphological cell types because they were so mewhat heterogeneous with respect to several parameters including inward re ctification, spike-frequency adaptation, and intrinsic oscillations. Nevert heless, although most horizontal cells displayed time-dependent inward rect ification, most pyramidal cells displayed fast inward rectification exclusi vely. None of the entorhinal cortex layer V cells displayed oscillatory act ivity like that of neocortical layer V "bursting" cells, although neurons f rom all groups displayed rhythmic subthreshold membrane potential oscillati ons. In summary, we have found that layer V of the rat medial entorhinal co rtex consists of three morphologically distinct neuronal subtypes that cann ot be clearly distinguished from each other by traditional electrophysiolog ical measures. (C) 2000 Wiley-Liss, Inc.