Characterization of voltage- and Ca2+ -activated K+ channels in rat dorsal root ganglion neurons

Auxiliary P-subunits associated with pore-forming Slo I alpha-subunits play an essential role in regulating functional properties of large-conductance, voltage- and Ca2+-activated K+ channels commonly termed BK channels. Even though both noninactivating and inactivating BK channels are thought to be regulated by P-subunits (beta 1, 132, beta 3, or beta 4), the molecular determinants underlying inactivating BK channels in native cells have not been extensively demonstrated. In this study, r beta 2 (but not r beta 3-subunit) was identified as a molecular component in rat lumbar L4-6 dorsal root ganglia (DRG) by RT-PCR responsible for inactivating large-conductance Ca2+-dependent K+ currents (BKi currents) in small sensory neurons. The properties of native BKi currents obtained from both whole-cell and inside-out patches are very similar to inactivating BK channels produced by co-expressing mSlo I alpha- and h beta 2-subunits in Xenopus oocytes. Intracellular application of 0.5 mg/ml trypsin removes inactivation of BKi channels, and the specific blockers of BK channels, charybdotoxin (ChTX) and iberiotoxin (IbTX), inhibit these BKi currents. Single BKi channel currents derived from inside-out patches revealed that one BKi channel contained three r beta 2-subunits (on average), with a single-channel conductance about 217 pS under 160 K+ symmetrical recording conditions. Blockade of BKi channels by 100 nM IbTX augmented firing frequency, broadened action potential waveform and reduced afterhyperpolarization. We propose that the BKi channels in small diameter DRG sensory neurons might play an important role in regulating nociceptive input to the central nervous system (CNS).