CLUSTERING OF VOLTAGE-SENSITIVE SODIUM-CHANNELS ON AXONS IS INDEPENDENT OF DIRECT SCHWANN-CELL CONTACT IN THE DYSTROPHIC MOUSE

The distribution of voltage-sensitive sodium channels on axons in the dorsal and ventral spinal roots of the dystrophic mouse 129/ReJ-Lama(2 dy) was determined via immunocytochemistry, In these nerves there are regions in which Schwann cells fail To proliferate and myelinate axons in a normal manner, leaving bundles of closely packed large-diameter amyelinated axons. We have identified discrete and focal concentration s of sodium channel immunoreactivity on these axons by both confocal i mmunofluorescence and immunoelectron microscopy, using a peptide-deriv ed polyclonal antibody. In addition, simultaneous labeling with an ant ibody recognizing neuronal-specific ankyrin, revealed a distinct coloc alization with the sodium channels on both normal and amyelinated axon s. The presence of patches of sodium channels along with their anchori ng protein on amyelinated axons in the absence of intervening Schwann cells demonstrates that axons can form and maintain independently thes e initial aggregations. This confirms that direct contact between Schw ann cell and axon is not required for the formation of sodium channel patches of nodal dimensions and density. Furthermore, this strongly su ggests that local transfer of sodium channels from Schwann cells to ax ons is not required for this process.