Properties And Function Of Inactivating Calcium-activated Potassium Channels (BK_Ca) In Rat Dorsal Root Ganglia Neurons

Large conductance voltage- and Ca2+-activated K+ channels (BK or MaxiK channels) are almost ubiquitously expressed among mammalian tissues, which play a crucial role in coupling changes in submembrane calcium concentration with changes in membrane potential and excitability. BK channels are formed fromα(Slo1) gene with itsβfamily, two of which,β2 andβ3, show ability to inactivate currents. It remains to be demonstrated where the hβ3- subunit is expressed in native tissues and what are the functional roles. Even though the inactivation of BK channels has been extensively studied mostly in expression experiments, rarely do they exhibit inactivation in native cells, especially in neurons. It is unclear what kind of native BK channels exists in Dorsal root ganglia (DRG) neurons and what are their properties and functional roles.In the current study, we have identified and characterized an inactivating Ca2+-dependent potassium (KCa) current in small neurons from rat lumbar L4-L6 dorsal root ganglia (DRG). The KCa currents are verified as inactivating BK currents (BKi) due to their sensitivity to the specific BK channel blockers, Charybdotoxin (CTX) and Iberiotoxin (IbTX). We demonstrated that the inactivation of BKi could be removed by extracellular treatment with papain.Using whole-cell recordings and inside-out patches we found that BK channels in DRG neurons exhibited a rapid inactivation and/or a slow inactivation in 96% cells. Ensemble inactivating currents from single-channel recordings were best fitted to single- or double-exponential function, which yield inactivation time constant (τ). Two gaussian fits to distribution of inactivation time constant indicate that the inactivation time constants are 4.6 ms (fast) and 31.8 ms (slow). Based on the kinetics of inactivation and the properties of single channel currents, our results indicate the presence of bothβ2 andβ3b in small DRG neurons. In inside-out patches, we found inactivating medium conductance BK currents, and the single-channel conductance varied from 100 to 200 pS in symmetrical K+ solution, indicated that there might be a combination of slo2/slo1/β2/β3 in rat dorsal root ganglion neurons.At current-clamp, blockade of the BKi channels by applying 100 nM ChTX results in increased firing and broadened action potentials. The BK currents contribute to repolarization phase and excitability and we propose BKi in small DRG neurons might play an important role in modulating nociceptive input from the peripheral to the CNS.