Nav1.3 sodium channels: Rapid repriming and slow closed-state inactivationdisplay quantitative differences after expression in a mammalian cell lineand in spinal sensory neurons

Although rat brain Nav1.3 voltage-gated sodium channels have been expressed and studied in Xenopus oocytes, these channels have not been studied after their expression in mammalian cells. We characterized the properties of th e rat brain Nav1.3 sodium channels expressed in human embryonic kidney (HEK ) 293 cells. Nav1.3 channels generated fast-activating and fast-inactivatin g currents. Recovery from inactivation was relatively rapid at negative pot entials (<-80 mV) but was slow at more positive potentials. Development of closed-state inactivation was slow, and, as predicted on this basis, Nav1.3 channels generated large ramp currents in response to slow depolarizations . Coexpression of <beta>3 subunits had small but significant effects on the kinetic and voltage-dependent properties of Nav1.3 currents in HEK 293 cel ls, but coexpression of beta1 and beta2 subunits had little or no effect on Nav1.3 properties. Nav1.3 channels, mutated to be tetrodotoxin-resistant ( TTX-R), were expressed in SNS-null dorsal root ganglion (DRG) neurons via b iolistics and were compared with the same construct expressed in HEK 293 ce lls. The voltage dependence of steady-state inactivation was similar to7 mV more depolarized in SNS-null DRG neurons, demonstrating the importance of background cell type in determining physiological properties. Moreover, con sistent with the idea that cellular factors can modulate the properties of Nav1.3, the repriming kinetics were twofold faster in the neurons than in t he HEK 293 cells. The rapid repriming of Nav1.3 suggests that it contribute s to the acceleration of repriming of TTX-sensitive (TTX-S) sodium currents that are seen after peripheral axotomy of DRG neurons. The relatively rapi d recovery from inactivation and the slow closed-state inactivation kinetic s of Nav1.3 channels suggest that neurons expressing Nav1.3 may exhibit a r educed threshold and/or a relatively high frequency of firing.