Nociceptor-specific gene deletion reveals a major role for Na_v1.7 (PN1) in acute and inflammatory pain

Abstract

Nine voltage-gated sodium channels are expressed in complex patterns in mammalian nerve and muscle. Three channels, Nav1.7, Nav1.8, and Na v1.9, are expressed selectively in peripheral damage-sensing neurons. Because there are no selective blockers of these channels, we used gene ablation in mice to examine the function of Nav1.7 (PN1) in pain pathways. A global Nav1.7-null mutant was found to die shortly after birth. We therefore used the Cre؊loxP system to generate nociceptor-specific knockouts. Na v1.8 is only expressed in peripheral, mainly nociceptive, sensory neurons. We knocked Cre recombinase into the Na v1.8 locus to generate heterozygous mice expressing Cre recombinase in Nav1.8-positive sensory neurons. Crossing these animals with mice where Na v1.7 exons 14 and 15 were flanked by loxP sites produced nociceptor-specific knockout mice that were viable and apparently normal. These animals showed increased mechanical and thermal pain thresholds. Remarkably, all inflammatory pain responses evoked by a range of stimuli, such as formalin, carrageenan, complete Freund's adjuvant, or nerve growth factor, were reduced or abolished. A congenital pain syndrome in humans recently has been mapped to the Na v1.7 gene, SCN9A. Dominant Na v1.7 mutations lead to edema, redness, warmth, and bilateral pain in human erythermalgia patients, confirming an important role for Na v1.7 in inflammatory pain. Nociceptor-specific gene ablation should prove useful in understanding the role of other broadly expressed genes in pain pathways.

Dysregulation of voltage-gated sodium channels by ubiquitin ligase NEDD4-2 in neuropathic pain

“…Increasing their expression in slow neurons might promote hyperexcitability. However, in sensory neuron-specific knockouts of Na v 1.7, Na v 1.8 or double-knockout mice, neuropathic pain-like behavior still develops after nerve injury (18)(19)(20). It must be noted that compensatory effects in the expression of the different Na v isoforms in genetically modified animals during development cannot be excluded.…”

“…Not only are Na v 1.7 and Na v 1.8 important in inherited pain disorders, but also in acquired pain disorders, where their increased expression has already been linked to diverse chronic pain symptoms (14)(15)(16). Studies using knockout mice have implicated Na v 1.7 and Na v 1.8 in acute and inflammatory pain (17)(18)(19)(20), but their involvement in hyperexcitability and neuropathic pain remains to be determined.…”

Nociceptors: the sensors of the pain pathway

“…The challenges encountered in assigning a role for transducers of noxious mechanical stimuli include the efficacy of stimulation protocols applied in behavioral and ex vivo tissue assays (e.g., phenotypes can differ when challenged with thin calibrated von Frey nylon filaments vs. distributed pinch that may also induce local ischemia; ref. 95), the suppressive influence of innocuous A-fiber mechanosensitive inputs at a systems level (18), and the extrapolation of cellular assays to nociception (e.g., poking, ref. This topic is reviewed in more detail elsewhere (12,19,(91)(92)(93).…”

“…Cell excitability and firing behavior (e.g., threshold for action potential generation, action potential and undershoot amplitude and duration, and maximal firing frequency) depend on the complement of these channels as well as those contributing to frequency modulation (e.g., hyperpolarization-activated cyclic nucleotide-gated cation channel [HCN] and A-type K v 4.3 and K v 3.4 channels) (54). For instance, nociceptors responsive to noxious cold require the expression of the tetrodotoxin-resistant (TTX-resistant) Na v 1.8 channel at the peripheral terminal (100), and mice lacking Na v 1.8 and Na v 1.7 display deficits in mechanosensation (95,101). Peripheral CGRP release by inflammatory mediators is unaffected by TTX, suggesting an important role of TTX-resistant Na v in regulated pain thresholds, consistent with their robust modulation by bradykinin (BK) and PGE 2 (102) (see below).…”

Targeting Neuroprotection as an Alternative Approach to Preventing and Treating Neuropathic Pain

“…[73][74][75][76] Surprisingly, neuropathic pain is still present in mice lacking Na v 1.7 or Na v 1.8 (established using a Na v 1.8-driven knock-out strategy) and neuropathic pain behavior is even maintained in mice in which nearly all Na v 1.8-positive nociceptors have been ablated. [77][78][79] Because Na v 1.8-expressing nociceptors also express and release BDNF in the spinal cord, perhaps it is the absence of BDNF that changes pain behavior in mice lacking Na v 1.8-positive sensory neurons, given the role of BDNF and TrkB in the establishment of some types of persistent pain behavior (although maybe not all types of neuropathic pain-at least not in mice). 62 Nevertheless, increased expression of sodium channels on nerve terminals and axons would be one mechanism contributing to hyperexcitability and, through increased depolarization, increased intracellular calcium due to activation of voltage-dependent calcium channels.…”