Morphologically identified cutaneous afferent DRG neurons express three different potassium currents in varying proportions

Outward K+ currents were recorded using a whole cell patch-clamp configuration, from acutely dissociated adult rat cutaneous afferent dorsal root ganglion (DRG) neurons (L-4 and L-5) identified by retrograde labeling with Fluorogold. Recordings were obtained 16-24 h after dissociation from cells between 39 and 49 mm in diameter with minimal processes. These cells represent medium-sized DRG neurons relative to the entire population, but are large cutaneous afferent neurons giving rise to myelinated axons. Voltage-activated K+ currents were recorded routinely during 300-ms depolarizing test pulses increasing in 10-mV steps from -40 to +50 mV; the currents were preceded by a 500-ms conditioning prepulse of either -120 or -40 mV. Coexpression of at least three components of K+ current was revealed. Separation of these components was achieved on the basis of sensitivities to the K+ channel blockers, 4-aminopyridine (4-AP) and dendrotoxin (DTx), and by the current responses to variation in conditioning voltage. Changing extracellular K+ concentration from 3 to 40 mM resulted in a shift to the right of the I-V curve commensurate with KC being the principal charge carrier. Presentation of 100 mM 4-AP revealed a rapidly activating K+ current sensitive to low concentrations of 4-AP. High concentrations of 4-AP (6 mM) extinguished all inactivating current, leaving almost pure sustained current (I-K). On the basis of the relative distribution of K+ currents neurons could be separated into three distinct categories: fast inactivating current (I-A), slow inactivating current (I-D), and sustained current (I-K); only I-A and I-K; and slow inactivating current and I-K. However, Is was always the dominant outward current component. These results indicate that considerable variation in K+ currents is present not only in the entire population of DRG neurons, as previously reported, but even within a restricted size and functional group (large cutaneous afferent neurons).