Paradoxical excitatory impact of SK channels on dendritic excitability

Dendritic excitability regulates how neurons integrate synaptic inputs and thereby influences neuronal output. As active dendritic events are associated with significant calcium influx they are likely to be modulated by calcium-dependent processes, such as calcium-activated potassium channels. Here we investigate the impact of small conductance calcium-activated potassium channels, or SK channels, on dendritic excitability in male and female rat cortical pyramidal neurons in vitro and in vivo. Using local applications of the SK channel antagonist apamin in vitro, we show that blocking somatic SK channels enhances action potential output, whereas blocking dendritic SK channels paradoxically reduces the generation of dendritic calcium spikes and associated somatic burst firing. Opposite effects were observed using the SK channel enhancer NS309. The effect of apamin on dendritic SK channels was occluded when R-type calcium channels were blocked, indicating that the inhibitory impact of apamin on dendritic calcium spikes involved R-type calcium channels. Comparable effects were observed in vivo. Intracellular application of apamin via the somatic whole-cell recording pipette reduced the medium after-hyperpolarisation and increased action potential output during up-states. In contrast, extracellular application of apamin to the cortical surface to block dendritic SK channels shifted the distribution of action potentials within up-states from an initial burst to a more distributed firing pattern, while having no impact on overall action potential firing frequency or up- and down-states. These data indicate that somatic and dendritic SK channels have opposite effects on neuronal excitability, with dendritic SK channels counter-intuitively promoting rather than suppressing neuronal output. Significance Statement: Neurons typically receive input from other neurons onto processes called dendrites, and use electrical events such as action potentials for signalling. As electrical events in neurons are usually associated with calcium influx they can be regulated by calcium-dependent processes. One such process is through the activation of calcium-dependent potassium channels, which usually act to reduce action potential signalling. While this is the case for calcium-dependent potassium channels found at the cell body, we show here that calcium-dependent potassium channels in dendrites of cortical pyramidal neurons counter-intuitively promote rather than suppress action potential output.