Distinct potassium channels on pain-sensing neurons

Abstract

Differential expression of ion channels contributes functional diversity to sensory neuron signaling. We find nerve injury induced by the Chung model of neuropathic pain leads to striking reductions in voltage-gated K(+) (Kv) channel subunit expression in dorsal root ganglia (DRG) neurons, suggesting a potential molecular mechanism for hyperexcitability of injured nerves. Moreover, specific classes of DRG neurons express distinct Kv channel subunit combinations. Importantly, Kv1.4 is the sole Kv1 alpha subunit expressed in smaller diameter neurons, suggesting that homomeric Kv1.4 channels predominate in A delta and C fibers arising from these cells. These neurons are presumably nociceptors, because they also express the VR-1 capsaicin receptor, calcitonin gene-related peptide, and/or Na(+) channel SNS/PN3/Nav1.8. In contrast, larger diameter neurons associated with mechanoreception and proprioception express high levels of Kv1.1 and Kv1.2 without Kv1.4 or other Kv1 alpha subunits, suggesting that heteromers of these subunits predominate on large, myelinated afferent axons that extend from these cells.

Figure 1

Double-labeling of contralateral L5/L6, ipsilateral L4 (unligated), and ipsilateral L5/L6 (ligated) DRG, using antibodies against Kv1.1, Kv1.2, Kv1.4, Kvβ2.1, NaCh, VR-1, and NF-M. All tissue was processed simultaneously for each set of antibodies, and exposures for ipsilateral and contralateral sections were identical. (Scale bars, 100 μm.)

Figure 2

Immunofluorescence staining of rat DRG and histograms showing diameters of cells labeled with ion channel-specific antibodies. (Scale bars, 100 μm.)

Figure 3

Double- (A–D) and triple-labeling (E) of ion channels in DRG cryosections. (A) Kv1.2 (green) and pan-NaCh (red). Anti-pan-NaCh antibodies intensely labeled small-diameter cells (arrowheads), and weakly stained a few large-diameter cells that had overlapping Kv1.2 immunoreactivity (yellow). (B) Kv1.4 (green) and pan-NaCh (red). Most staining colocalized in small-diameter cells (arrowheads, yellow), but some larger-diameter cells were also labeled. (C) VR-1 (green) and pan-NaCh (red). Staining usually colocalized (arrowhead), but distinct subpopulations existed with either only VR-1 (arrow) or pan-NaCh (asterisk) immunoreactivity. (D) Nav1.8 (green) and pan-NaCh (red); overlap is yellow. (E) Kv1.1 (red), Kv1.2 (blue), and Kv1.4 (green). Most medium- and large-diameter cells had Kv1.1 and Kv1.2 staining that colocalized (arrow, violet). Some cells had immunoreactivity for all three subunits (arrowhead, white). A subpopulation of large diameter cells had primarily Kv1.1 immunoreactivity (asterisk, red). (Scale bars, 100 μm.)

Figure 4

Subcellular localization of Kv1 subunits in dorsal roots (A), ventral roots (C), Sciatic nerve (B), and unmyelinated axons in rat bladder (D–F). (A) Kv1.1 (red), Kv1.2 (blue), and Kv1.4 (green). Most juxtaparanodes had Kv1.1 and Kv1.2 (arrows), but some had all three Kv1 subunits colocalized (arrowheads). (B) Kv1.4 (red) and NaChs (green). Unmyelinated axons have a high density of Kv1.4 subunits (arrowhead). (C) Juxtaparanodes in ventral roots double-labeled with Kv1.4 and Kv1.2 (arrowheads, yellow) or Kv1.2 (arrows, red). (D–F) CGRP (green) and VR-1 (E), Kv1.4 (F), or Kv1.2 (G). (Scale bars, 10 μm.)