Polarised localisation of the voltage-gated sodium channel Na v1.2 in cerebellar granule cells

Abstract

Voltage-gated sodium channels are responsible for action potential initiation and propagation in electrically excitable cells. In this study, we used biochemical, immunohistochemical and quantitative immunoelectron microscopy techniques to reveal the temporal and spatial expression of the Na v1.2 channel subunit in granule cells of cerebellum. Using histoblot, we detected Nav1.2 widely distributed in the adult brain, but prominently expressed in the cerebellum. During postnatal development, Na v1.2 mRNA and protein were detected low during the first and second postnatal week, increased to P15 and then continue to decrease until adult levels. At the light microscopic level, Nav1.2 immunoreactivity concentrated in the molecular layer of the cerebellar cortex. Using immunofluorescence, Nav1.2 colocalised with VGluT1, but not with VGluT2, demonstrating that the subunit was preferentially present in parallel fibre axons and axon terminals. At the electron microscopic level, Na v1.2 immunoparticles were exclusively detected at presynaptic sites in granule cell axons and axon terminals of granule cells, with occasional clustering in their axon initial segment. This was demonstrated using quantitative immunogold analysis. In the axon terminals, the distribution of Nav1.2 was relatively uniform along the extrasynaptic plasma membrane and never detected in the active zone. We could not find detectable levels of Nav1.2 at postsynaptic elements of granule cells or other cerebellar cell types. The present findings show a polarised distribution of Na v1.2 along the neuronal surface of granule cells and suggest its primary involvement in the transmission of information from granule cells to Purkinje cells. © 2012 Springer Science+Business Media, LLC.