Sodium Channel Expression and Localization at Demyelinated Sites in Painful Human Dental Pulp

Abstract

The expression of sodium channels (NaCh(s)) change after inflammatory and nerve lesions and this change has been implicated in the generation of pain states. Here we examine NaCh expression within nerve fibers from normal and painful extracted human teeth with special emphasis on their localization within large accumulations, like those seen at nodes of Ranvier. Pulpal tissue sections from normal wisdom teeth and from teeth with large carious lesions associated with severe and spontaneous pain were double-stained with pan-specific NaCh antibody and caspr (paranodal protein used to visualize nodes of Ranvier) antibody, while additional sections were triple-stained with NaCh, caspr and myelin basic protein (MBP) antibodies. Z-series of images were obtained with the confocal microscope and evaluated with NIH ImageJ software to quantify the density and size of NaCh accumulations, and to characterize NaCh localization at caspr-identified typical and atypical nodal sites. Although the results showed variability in the overall density and size of NaCh accumulations in painful samples, a common finding included the remodeling of NaChs at atypical nodal sites. This remodeling of NaChs included prominent NaCh expression within nerve regions that showed a selective loss of MBP staining in a pattern consistent with a demyelinating process.

Unmyelinated nerve fibers in the human dental pulp express markers for myelinated fibers and show sodium channel accumulations

“…This antibody has been widely used in previous studies to identify NaCh clusters at nodes of Ranvier in both CNS and PNS and the staining pattern seen in the current study is consistent with other studies [16,39,40,72]. …”

“…Results from these studies consistently showed a prominent demyelinating response of axons within the pulpitis samples that resulted in the increased incidence of atypical nodal forms. Our study with the pan-specific NaCh antibody included a quantification of NaCh clusters that lacked caspr and our results showed these "naked" NaCh accumulations were present in both normal and painful specimens [16]. Even though some of these "naked" clusters most likely result from the loss of myelin, their common occurrence in normal specimens as seen in our pan-specific NaCh study and the findings presented here provide additional evidence for their existence in unmyelinated fibers.…”

“…Results from these studies have included the identification of NaCh clusters at non-nodal sites in both normal and diseased/painful samples [16]. In this study we take the opportunity to characterize the fibers with NaCh accumulations at non-nodal sites to test the hypothesis that the unmyelinated segments of myelinated axons show an inherent ability to cluster NaChs.…”

Reciprocal relationship between membrane type 1 matrix metalloproteinase and the algesic peptides of myelin basic protein contributes to chronic neuropathic pain

“…Because myelin domains define both cytoarchitecture and molecular signature of A-afferents, demyelination causes electrical instability, ectopic insertion of ion channels, and ultimately, mechanical pain hypersensitivity [2, 5, 41, 42]. In addition to MMP-mediated demyelination, MMPs may regulate the pathogenesis of mechanical allodynia [4] via control of the structural and molecular assembly of myelinated axons.…”

“…In myelin gaps (nodes of Ranvier), voltage-gated sodium channel clustering warrants rapid, saltatory conduction [3]. Demyelination of A-afferents contributes to the pathogenesis of mechanical pain hypersensitivity via disruption in this precise molecular and structural signature, including ectopic insertion of ion channels [2, 4, 5]. Among such myelin-related changes contributing to mechanical allodynia we proposed is liberation of myelin auto-antigens, such as myelin basic protein (MBP), leading to the formation of the algesic complexes on A-afferents [6–8].…”