Functional organization of entorhinal cortex layer V neurons : electrophysiological and morphological characterization

The entorhinal cortex occupies an important anatomical position in the temporal lobe as the interface between the hippocampus and the neocortex. Its superficial cell layers funnel the cortical input into the hippocampus while the deep layer V is the major target of hippocampal output, which it then directs back to the cortex. This connectivity alone makes the entorhinal cortex a crucial element in what is known as the neocortical-hippocampal-neocortical circuit, a system involved in the long-term storage of "declarative" memories. However, the detailed analysis of the morphological and electrophysiological characteristics of cells in entorhinal cortex layer V had not been carried out, information that is necessary to understand how this cortical layer may participate in the processing of cortically directed hippocampal information. The main goal of the present study was thus to carry out by means of intracellular recording and labelling in a "in vitro" rat entorhinal cortex slice preparation an extensive characterization of the intrinsic electrophysiological characteristics of entorhinal cortex layer V principal neurons in correlation with their morphological attributes. Our main working hypothesis was that electrophysiologically, similar to the neocortex, layer V principal neurons in both the medial and lateral subdivisions of the entorhinal cortex would constitute a heterogeneous population comprised of regularly spiking cells and intrinsically bursting neurons.We found layer V of the entorhinal cortex to be comprised of three main cellular subtypes, pyramidal, horizontal and polymorphic neurons, that were electrophysiologically non-distinguishable. Cells in both medial and lateral entorhinal areas had similar morphological and electrophysiological properties. No intrinsically bursting neurons were encountered; instead, cells were classified as regular spiking neurons with varying spike-frequency adaptation. We also found that most layer V neurons displayed persistent Na+ current (INap) dependent subthreshold membrane potential oscillations; a mechanism that might endow layer V cells with a functional contribution to network rhythmicity