Inactivation of macroscopic late Na+ current and characteristics of unitary late Na+ currents in sensory neurons

Na+ currents in adult rat large dorsal root ganglion neurons were recorded during long duration voltage-clamp steps by patch clamping whole cells and outside-out membrane patches. Na+ current present >60 ms after the onset of a depolarizing pulse (late Na+ current) underwent partial inactivation; it behaved as the sum of three kinetically distinct components, each of which was blocked by nanomolar concentrations of tetrodotoxin. Inactivation of one component (late-1) of the whole cell current reached equilibrium during the first 60 ms; repolarizing to -40 or -50 mV from potentials of -30 mV or more positive gave rise to a characteristic increase in current (tau greater than or equal to 5 ms), attributed to removal of inactivation. A second component (late-2) underwent slower inactivation ( tau > 80 ms) at potentials more positive than -80 mV, and steady-state inactivation appeared complete at -30 mV. In small membrane patches, bursts of brief openings (gamma = 13-18 pS) were usually recorded. The distribution of burst durations indicated that two populations of channel were present with inactivation rates corresponding to late-1 and late-2 macroscopic currents. The persistent Na+ current in the whole cell that extended to potentials more positive than -30 mV appeared to correspond to sporadic, brief openings that were recorded in patches (mean open time similar to 0.1 ms) over a wide potential range. None of the three types of gating described corresponded to activation/inactivation gating overlap of fast transient currents.