Fine Gating Properties of Channels Responsible for Persistent Sodium Current Generation in Entorhinal Cortex Neurons

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Abstract
The gating properties of channels responsible for the generation of persistent Na+ current (INaP) in entorhinal cortex layer II principal neurons were investigated by performing cell-attached, patch-clamp experiments in acutely isolated cells. Voltage-gated Na+-channel activity was routinely elicited by applying 500-ms depolarizing test pulses positive to −60 mV from a holding potential of −100 mV. The channel activity underlying INaP consisted of prolonged and frequently delayed bursts during which repetitive openings were separated by short closings. The mean duration of openings within bursts was strongly voltage dependent, and increased by e times per every ∼12 mV of depolarization. On the other hand, intraburst closed times showed no major voltage dependence. The mean duration of burst events was also relatively voltage insensitive. The analysis of burst-duration frequency distribution returned two major, relatively voltage-independent time constants of ∼28 and ∼190 ms. The probability of burst openings to occur also appeared largely voltage independent. Because of the above “persistent” Na+-channel properties, the voltage dependence of the conductance underlying whole-cell INaP turned out to be largely the consequence of the pronounced voltage dependence of intraburst open times. On the other hand, some kinetic properties of the macroscopic INaP, and in particular the fast and intermediate INaP-decay components observed during step depolarizations, were found to largely reflect mean burst duration of the underlying channel openings. A further INaP decay process, namely slow inactivation, was paralleled instead by a progressive increase of interburst closed times during the application of long-lasting (i.e., 20 s) depolarizing pulses. In addition, long-lasting depolarizations also promoted a channel gating modality characterized by shorter burst durations than normally seen using 500-ms test pulses, with a predominant burst-duration time constant of ∼5–6 ms. The above data, therefore, provide a detailed picture of the single-channel bases of INaP voltage-dependent and kinetic properties in entorhinal cortex layer II neurons.