Single-channel analysis of two types of Na+ currents in rat dorsal root ganglia

The properties of voltage-gated Na+ channels were studied in neurones isolated from rat dorsal root ganglia using the outside-out configuration of the patch-clamp technique. Two types of single-channel currents were identified from the difference in unit amplitudes. Neither type was evoked in the medium in which extracellular Na+ ions were replaced by an equimolar amount of tetramethylammonium ions. The two types of single-channel currents differed in their sensitivity to tetrodotoxin (TTX). The smaller channel current was insensitive to 1 microM TTX (referred to as TTX-I), while the larger channel current was blocked by 1 nM TTX (TTX-S). The unit amplitudes measured during a step depolarization to -30 mV (1.4 mM internal and 250 mM external Na+ concentrations) were 1.16 pA for TTX-S and 0.57 pA for TTX-I, respectively. The slope conductance measured at -30 mV was 16.3 pS for TTX-S and 8.5 pS for TTX-I. TTX-S could be activated by step depolarizations positive to -60 mV, while TTX-I could be activated at potentials positive to -40 mV. When the test pulse was preceded by a depolarizing prepulse, the prepulse positive to -50 mV preferentially inactivated TTX-S with a minimal effect on TTX-I. Activation and inactivation time courses of the averaged ensemble currents computed from TTX-S showed remarkable resemblances to the time courses of the macroscopic TTX-sensitive Na+ current. Similarly, the ensemble currents of TTX-I mimicked the macroscopic TTX-insensitive Na+ current. It was concluded that the two types of Na+ channels in rat dorsal root ganglia differ not only in their sensitivity to TTX, but also in their single-channel conductances.