Increased sodium channel immunofluorescence at myelinated and demyelinated sites following an inflammatory and partial axotomy lesion of the rat infraorbital nerve

The localization of sodium channels (NaChs) change following nerve lesions and this change may contribute to the development of increased pain states. Here we examine the change in distribution of NaChs within the rat infraorbital nerve (ION) two weeks after a combined inflammatory/partial axotomy lesion that results in behavior showing increased sensitivity to mechanical stimuli. Sections from experimental and normal control IONs were double-stained for indirect immunofluorescence using an antibody that identifies all NaCh isoforms and caspr-antibody to identify nodes of Ranvier, and a confocal microscope z-series of optically sectioned images were then obtained. ImageJ (NIH) software was used to quantify the area of pixels showing maximum NaCh intensity within both caspr and non-caspr associated accumulations. Analysis showed that the lesioned IONs had many more split nodes, heminodes and caspr-negative “naked” accumulations, a significantly increased area of NaCh staining within typical nodes and “naked” accumulations, as well as an increased density and size of significant accumulations when compared to normal IONs. This study demonstrates a dramatic redistribution and increased immunofluorescence of NaChs especially at myelinated and demyelinated sites in fibers located just proximal to the lesion. The remodeling of NaChs seen in this study may represent an important event associated with the development of increased nerve excitability after lesions.