Analysis of resurgent sodium-current expression in rat parahippocampal cortices and hippocampal formation

The resurgent Na+ current (I-NaR) is a component of neuronal voltage-dependent Na+ currents that is activated by repolarization and is believed to result from an atypical path of Na+ channel recovery from inactivation. So far, I-NaR has only been identified in a small number of central neuronal populations in the cerebellum, diencephalon, and brainstem. The possible presence and roles Of INaR in neurons of the cerebral cortex and temporal-lobe memory system are still uncharacterized. In this study whole-cell, patch-clamp experiments were carried out in acute rat brain slices to investigate IN R expression and properties in several neuronal populations of the parahippocampal region and hippocampal formation. Specifically, we examined pyramidal neurons ofperirhinal cortex areas 36 and 35 (layers II and V); neurons of superficial and deep layers of medial entorhinal cortex (mEC); dentate gyrus (DG) granule cells; and pyramidal cells of the CA3 and CAI hippocampal fields. INaR was found to be thoroughly expressed in parahippocampal cortices. The most consistent and prominent IN R expression was observed in mEC layer-II cells. A vast majority of areas 36 and 35 neurons (both in layers II and V) and mEC layer-III and -V neurons were also endowed with INaR, although at lower amplitude levels. INaR was expressed by similar to 60% of DG granule cells and similar to 35% of CAI pyramidal cells of the ventral hippocampus, whereas it was never observed in CA3 neurons (both in the ventral and dorsal hippocampus) and CAI neurons of the dorsal hippocampus. The biophysical properties of IN R were very similar in all of the neuronal types in which the current was observed, with a peak in the current-voltage relationship at -35/-40 mV. Our results show that the parahippocampal region and part of the hippocampal formation are sites of major INaR expression, and provide a new basis for further studies on the molecular correlates Of IN R. (c) 2007 Elsevier B.V. All rights reserved.