The molecular basis of analgesia in congenital insensitivity to pain associated with loss of Nav1.7 function

Loss of function mutations in the SCN9a gene encoding voltage-gated sodium channel Nav1.7 cause congenital insensitivity to pain (CIP) and anosmia in otherwise normal humans and mice, suggesting that this channel may be a good analgesic drug target. Surprisingly, potent selective antagonists of Nav1.7 are weak analgesics. We therefore investigated whether Nav1.7 , as well as contributing to electrical signalling may have an additional function. Here we report that Nav1.7 deletion has profound effects on the sensory neuron transcriptome, leading to dysregulation of a number of transcription factors as well as upregulation of enkephalin precursor PENK mRNA and down regulation of CEACAM10 mRNA, a protein involved in noxious thermosensation. PENK mRNA is transcriptionally upregulated in Nav1.7 null mutant female sensory neurons, resulting in increased enkephalin expression in the dorsal horn of the spinal cord. PENK expression is down-regulated by addition of the sodium ionophore monensin, suggesting that sodium may play a role as a second messenger. Application of the opioid antagonist naloxone strongly enhances noxious peripheral input into the spinal cord, and dramatically reduces analgesia in both male and female Nav1.7 null mutant mice, as well as in human Nav1.7 null mutants. These data show that loss of Nav1.7 expression increases opioid drive over the lifetime of mice and humans. They further suggest that Nav1.7 channel blockers alone may not replicate the phenotype of null mutant humans and mice, but should be potentiated with exogenous opioids.

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Minett MS, Pereira V, Sikandar S, Matsuyama A, Lolignier S, Kanellopoulos AH, Mancini F, Iannetti GD, Bogdanov YD, Santana-Varela S, Millet Q, Baskozos G, MacAllister R, Cox JJ, Zhao J, Wood JN