Properties and functions of calcium-activated K+ channels in small neurones of rat dorsal root ganglion studied in a thin slice preparation

Properties, kinetics and functions of large conductance calcium-activated K + channels (BK Ca ) were investigated by the patch-clamp technique in small neurones (Aδ- and C-type) of a dorsal root ganglion (DRG) thin slice preparation without enzymatic treatment. Unitary conductance of BK Ca channels measured in symmetrical high K + solutions (155 m m ) was 200 pS for inward currents, and chord conductance in control solution was 72 pS. Potentials of half-maximum activation ( V 1/2 ) of the channels were linearly shifted by 43 mV per log 10 [Ca 2+ ] i unit (pCa) in the range of −28 mV (pCa 4) to +100 mV (pCa 7). Open probabilities increased e-times per 15–32 mV depolarization of potential. In mean open probability, fast changes with time were mainly observed at pCa > 6 and at potentials > +20 mV, without obvious changes in the experimental conditions. BK Ca channels were half-maximally blocked by 0.4 m m TEA, measured by apparent amplitude reductions. They were completely blocked by 100 n m charybdotoxin and 50 n m iberiotoxin by reduction of open probability. Two subtypes of small DRG neurones could be distinguished by the presence (type I) or absence (type II) of BK Ca channels. In addition, less than 10 % of small neurones showed fast (∼135 V s −1 ) and short (∼0.8 ms) action potentials (AP). The main functions of BK Ca channels were found to be shortening of AP duration, increasing of the speed of repolarization and contribution to the fast after-hyperpolarization. As a consequence, BK Ca channels may reduce the amount of calcium entering a neurone during an AP. BK Ca channel currents suppressed a subsequent AP and prolonged the refractory period, which might lead to a reduced repetitive activity. We suggest that the BK Ca current is a possible mechanism of the reported conduction failure during repetitive stimulation in DRG neurones.