Endogenous opioids contribute to insensitivity to pain in humans and mice lacking sodium channel Na(v)1.7

Loss-of-function mutations in the SCN9A gene encoding voltage-gated sodium channel Na(v)1.7 cause congenital insensitivity to pain in humans and mice. Surprisingly, many potent selective antagonists of Na(v)1.7 are weak analgesics. We investigated whether Na(v)1.7, as well as contributing to electrical signalling, may have additional functions. Here we report that Na(v)1.7 deletion has profound effects on gene expression, leading to an upregulation of enkephalin precursor Penk mRNA and met-enkephalin protein in sensory neurons. In contrast, Na(v)1.8-null mutant sensory neurons show no upregulated Penk mRNA expression. Application of the opioid antagonist naloxone potentiates noxious peripheral input into the spinal cord and dramatically reduces analgesia in both female and male Na(v)1.7-null mutant mice, as well as in a human Na(v)1.7-null mutant. These data suggest that Na(v)1.7 channel blockers alone may not replicate the analgesic phenotype of null mutant humans and mice, but may be potentiated with exogenous opioids.