SINGLE-CHANNEL ANALYSIS OF 2 TYPES OF NA-ROOT GANGLIA( CURRENTS IN RAT DORSAL)

The properties of voltage-gated Na+ channels were studied in neurones isolated from rat dorsal root glia using the outside-out configuration of the patch-clamp technique. Two types of single-channel currents we re 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 sin gle-channel currents differed in their sensitivity to tetrodotoxin (TT X). The smaller channel current was insensitive to 1 mu M TTX (referre d to as TTX-I), while the larger channel current was blocked by 1 nM T TX (TTX-S). The unit amplitudes measured during a step depolarization to -30 mV (1.4 mM internal and 250 mM external Na+ concentrations) wer e 1.16 pA for TTX-S and 0.57 pA for TTX-I, respectively. The slope con ductance 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. Whe n the test pulse was preceded by a depolarizing prepulse, the prepulse positive to -50 mV preferentially inactivated TTX-S with a minimal ef fect on TTX-I. Activation and inactivation time courses of the average d ensemble currents computed from TTX-S showed remarkable resemblances to the time courses of the macroscopic TTX-sensitive Na+ current. Sim ilarly, the ensemble currents of TTX-I mimicked the macroscopic TTX-in sensitive Na+ current. It was concluded that the two types of Na+ chan nels in rat dorsal root ganglia differ not only in their sensitivity t o TTX, but also in their single-channel conductances.