We evaluated the characteristics of the persistent sodium current (I NaP) in pyramidal neurons of layers II/III and V in slices of rat sensorimotor cortex using whole cell patch-clamp recordings. In both layers, INaP began activating around -60 mV and was half-activated at -43 mV. The INaP peak amplitude and density were significantly higher in layer V. The voltage-dependent INaP steady-state inactivation occurred at potentials that were significantly more positive in layer V (V 1/2: -42.3 ± 1.1 mV) than in layer II/III (V1/2: -46.8 ± 1.6 mV). In both layers, a current fraction corresponding to about 25% of the maximal peak amplitude did not inactivate. The time course of INaP inactivation and recovery from inactivation could be fitted with a biexponential function. In layer V pyramidal neurons the faster time constant of development of inactivation had variable values, ranging from 158.0 to 1,133.8 ms, but it was on average significantly slower than that in layer II/III (425.9 ± 80.5 vs. 145.8 ± 18.2 ms). In both layers, I NaP did not completely inactivate even with very long conditioning depolarizations (40 s at -10 mV). Recovery from inactivation was similar in the two layers. Layer V intrinsically bursting and regular spiking nonadapting neurons showed particularly prolonged depolarized plateau potentials when Ca2+ and K2+ currents were blocked and slower early phase of INaP development of inactivation. The biexponential kinetics characterizing the time-dependent inactivation of INaP in layers II/III and V indicates a complex inactivating process that is incomplete, allowing a residual "persistent" current fraction that does not inactivate. Moreover, our data indicate that INaP has uneven inactivation properties in pyramidal neurons of different layers of rat sensorimotor cortex. The higher current density, the rightward shifted voltage dependency of inactivation as well the slower kinetics of inactivation characterizing INaP in layer V with respect to layer II/III pyramidal neurons may play a significant role in their ability to fire recurrent action potential bursts, as well in the high susceptibility to generate epileptic events.
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