The Impact of BK Channels on Cellular Excitability Depends on their Subcellular Location

Abstract

Large conductance calcium-activated potassium channels (or BK channels) fulfil a multitude of roles in the central nervous system. At the soma of many neuronal cell types they control the speed of action potential (AP) repolarization and therefore they can have an impact on neuronal excitability. Due to their presence in nerve terminals they also regulate transmitter release. BK channels have also been shown to be present in the dendrites of some neurons where they can regulate the magnitude and duration of dendritic spikes. Here, we investigate the impact of modulating the activation of BK channels at different locations on the cellular excitability of cortical layer 5 pyramidal neurons. We find that while somatic BK channels help to repolarize APs at the soma and mediate the fast after-hyperpolarization, dendritic BK channels are responsible for repolarization of dendritic calcium spikes and thereby regulate somatic AP burst firing. We found no evidence for a role of dendritic BK channels in the regulation of backpropagating AP amplitude or duration. These experiments highlight the diverse roles of BK channels in regulating neuronal excitability and indicate that their functional impact depends on their subcellular location.

Keywords: BK channel; calcium channel; cortex; dendrite; pyramidal neuron.

Figure 1

Effect of global large conductance calcium-activated potassium channels (BK channel) block on active and passive properties. (A) Single somatic action potential (AP) before (black) and after (blue) bath application of Iberiotoxin (IbTX). (B) Average half width of somatic APs before (black) and after (blue) block of BK channels by bath application of IbTX (n = 7). (C) Somatic and dendritic responses (660 μm from the soma) to somatic current injection before (black) and after (blue) bath application of IbTX. (D) Average F-I curve before (black) and after (blue) bath application of IbTX (n = 7). (E,F) Average inter-spike interval (ISI) between all somatic APs (E) or just the first two somatic APs (F) during a somatic current step injection of 1.5 nA before (black) and after (blue) bath application of IbTX (n = 7 cells). (G) Average resting membrane potential at the soma and dendrite recording site (600–800 μm from soma) before (black) and after (blue) bath application of IbTX. (H) Responses to sub-threshold current injections (−0.4 to +0.4 nA in steps of 0.1 nA) into the soma of an layer 5 (L5) pyramidal neuron before (black) and after (blue) bath application of IbTX. (I) Average input resistance at the soma and dendrite recording site (600–800 μm from soma) before (black) and after (blue) IbTX application (n = 7 cells). (J) Somatic and dendritic responses (660 μm from the soma) to dendritic current injection before (black) and after (blue) bath application of IbTX. (K) Average half width of dendritic calcium spikes before (black) and after (blue) block of BK channels by bath application of IbTX (dendritic recording site 600–800 μm from soma; n = 7). (L) Average number of APs per burst before (black) and after (blue) bath application of IbTX (n = 7). (M,N) Average ISI between all somatic APs (M) or just the first two somatic APs (N) during AP bursts evoked by dendritic current injections of 1.2 nA before (black) and after (blue) bath application of IbTX (n = 7 cells).

Figure 2

Effect of global BK channel block on backpropagating APs (bAPs). (A) Single somatic (bottom) and dendritic (top; 705 μm from the soma) AP evoked by somatic current injection before (black) and after (blue) bath application of IbTX. (B,C) bAP amplitude (B) and half width (C) before (black) and after (blue) bath application of IbTX (n = 7). (D) bAP amplitude plotted against the distance of the dendritic recording site from the soma before (black) and after (blue) bath application of IbTX (n = 7). (E,F) Average bAP amplitude (E) and half width (F) before (black) and after (red) bath application of paxilline (n = 6). (G) bAP amplitude plotted against the distance of the dendritic recording site from the soma before (black) and after (red) bath application of paxilline (n = 6).

Figure 3

Effect of somatic BK channels on neuronal excitability. (A) Single somatic AP before (black) and after (light green) local IbTX application to the soma. (B) Average half width of somatic APs before (black) and after (light green) local IbTX application to the soma (n = 7). (C,D) Average amplitude (C) and half width (D) of bAPs before (black) and after (light green) local IbTX application to the soma (n = 7). (E) Somatic and dendritic responses during dendritic current injection before (black) and after (light green) local IbTX application to the soma (dendritic recording site 610 μm from soma). Gray boxes indicate the part of the trace shown in (F). (F) Overlay of dendritic calcium spikes recorded before (black) and after (light green) local IbTX application to the soma (aligned at threshold). (G) Average half width of dendritic calcium spikes before (black) and after (light green) local IbTX application to the soma (n = 7). (H) Average number of APs per burst before (gray) and after (light green) local IbTX application to the soma (n = 7).

Figure 4

Effect of dendritic BK channels on neuronal excitability. (A) Single somatic AP before (black) and after (purple) local IbTX application to the dendritic recording site (640 μm from the soma). (B) Average half width of somatic APs before (black) and after (purple) local IbTX application to the dendritic recording site (n = 7). (C,D) Average amplitude (C) and half width (D) of bAPs before (black) and after (purple) local IbTX application to the dendrite recording site (600–750 μm from the soma; n = 7). (E) Somatic and dendritic responses during dendritic current injection before (black) and after (purple) local IbTX application to the dendritic recording site (640 μm from soma). Gray boxes indicate the part of the traces shown in (F). (F) Overlay of dendritic calcium spikes recorded before (black) and after (purple) local IbTX application to the dendritic recording site (aligned at threshold). (G) Average half width of dendritic calcium spikes before (black) and after (purple) local IbTX application to the dendritic recording site (600–750 μm from the soma; n = 7). (H) Average number of APs per burst before (black) and after (purple) local IbTX application to the dendritic recording site (600–750 μm from the soma; n = 7).