Optical measurement of physiological sodium currents in the axon initial segment

In most neurons of the mammalian central nervous system, the action potential (AP) is triggered in the axon initial segment (AIS) by a fast Na+ current mediated by voltage-gated Na+ channels. The intracellular Na+ increase associated with the AP has been measured using fluorescent Na+ indicators, but with insufficient resolution to resolve the Na+ current in the AIS. In this article, we report the critical improvement in temporal resolution of the Na+ imaging technique allowing the direct experimental measurement of Na+ currents in the AIS. We determined the AIS Na+ current, from fluorescence measurements at temporal resolution of 100 microseconds and pixel resolution of half a micron, in pyramidal neurons of the layer-5 of the somatosensory cortex from brain slices of the mouse. We identified a subthreshold current before the AP, a fast inactivating current peaking during the rise of the AP and a persistent current during the AP repolarisation. We correlated the kinetics of the current at different distances from the soma with the kinetics of the somatic AP. We quantitatively compared the experimentally measured Na+ current with the current obtained by computer simulation of published NEURON models and we show how the present approach can lead to the correct estimate of the native behaviour of Na+ channels. Thus, it is expected that the present method will be adopted to investigate the function of different channel types under physiological or pathological conditions.