Functional microstructure of CaV-mediated calcium signaling in the axon initial segment

ABSTRACT The axon initial segment (AIS) is a specialized neuronal compartment in which synaptic input is converted into action potential output. This process is supported by a diverse complement of sodium, potassium, and calcium channels (Ca V ). Different classes of sodium and potassium channels are scaffolded at specific sites within the AIS, conferring unique functions, but how calcium channels are functionally distributed within the AIS is unclear. Here, we utilize conventional 2-photon laser scanning and diffraction-limited, high-speed spot 2-photon imaging to resolve action potential-evoked calcium dynamics in the AIS with high spatiotemporal resolution. In mouse layer 5 prefrontal pyramidal neurons, calcium influx was mediated by a mix of Ca V 2 and Ca V 3 channels that differentially localized to discrete regions. Ca V 3 functionally localized to produce nanodomain hotspots of calcium influx that coupled to ryanodine-dependent stores, whereas Ca V 2 localized to non-hotspot regions. Thus, different pools of Ca V s appear to play distinct roles in AIS function.