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, Na_v1.7, Na_v1.8, 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 Na_v1.7 (PN1) in pain pathways. A global Na_v1.7-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 Na_v1.8-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.