The Journal of Neurophysiology Vol. 80 No. 5 November 1998, pp. 2538-2549
Copyright ©1998 The American Physiological Society

Inactivation of Macroscopic Late Na⁺ Current and Characteristics of Unitary Late Na⁺ Currents in Sensory Neurons

Mark D. Baker and Hugh Bostock

Sobell Department of Neurophysiology, Institute of Neurology, London WC1N 3BG, United Kingdom

Baker, Mark D. and Hugh Bostock. Inactivation of macroscopic late Na⁺ current and characteristics of unitary late Na⁺ currents in sensory neurons. J. Neurophysiol. 80: 2538-2549, 1998. 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 (  5 ms), attributed to removal of inactivation. A second component (late-2) underwent slower inactivation ( > 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 ( = 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 ~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.

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