SINGLE TRANSIENT K CHANNELS IN MAMMALIAN SENSORY NEURONS

A single-channel recording of the transient outward current (A-current) was obtained from dorsal root ganglion cells in culture using patch-clamp techniques. Depolarization of the membrane patch elicited pulse like currrent of a uniform amplitude in an outward direction, of which the unitary conductance was 20 pS. Alteration of extracellular ionic compositions indicated that the charge carriers were K ions. A systematic study was made on the voltage-dependence of the ensemble average current: (a) the activation started at a potential around-60 mV; (b) the time course of the activation was relatively rapid: (c) the channel was completely inactivated at a potential positive to -40 mV. Two time constants (.tau.f = 100 ms and .tau.s = 4,000 ms) were detected in the decay of the current indicating that the channels had two different states of inactivation. A convulsant, 4-aminopyridine (4-AP), acted on the channel only from the intracellular side of the membrane. 4-AP (5 mM) reduced not only open time (by 50%) but also the single-channel conductance (by 20%). The properties of the channel were independent of Ca ions in the intracellular space.