MORPHOLOGICALLY IDENTIFIED CUTANEOUS AFFERENT DRG NEURONS EXPRESS 3 DIFFERENT POTASSIUM CURRENTS IN VARYING PROPORTIONS

Outward K+ currents were recorded using a whole cell patch-clamp confi guration, from acutely dissociated adult rat cutaneous afferent dorsal root ganglion (DRG) neurons (L-4 and L-5) identified by retrograde la beling with Fluorogold. Recordings were obtained 16-24 h after dissoci ation from cells between 39 and 49 mm in diameter with minimal process es. These cells represent medium-sized DRG neurons relative to the ent ire population, but are large cutaneous afferent neurons giving rise t o myelinated axons. Voltage-activated K+ currents were recorded routin ely during 300-ms depolarizing test pulses increasing in 10-mV steps f rom -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 com ponents 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 respons es to variation in conditioning voltage. Changing extracellular K+ con centration 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. Presen tation of 100 mM 4-AP revealed a rapidly activating K+ current sensiti ve to low concentrations of 4-AP. High concentrations of 4-AP (6 mM) e xtinguished all inactivating current, leaving almost pure sustained cu rrent (I-K). On the basis of the relative distribution of K+ currents neurons could be separated into three distinct categories: fast inacti vating current (I-A), slow inactivating current (I-D), and sustained c urrent (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 repor ted, but even within a restricted size and functional group (large cut aneous afferent neurons).