Synergistic regulation of serotonin and opioid signaling contribute to pain insensitivity in Nav1.7 knockout mice

Genetic loss of the voltage-gated sodium channel Nav1.7 (Na(v)1.7(-/-)) results in lifelong insensitivity to pain inmice and humans. One underlying cause is an increase in the production of endogenous opioids in sensory neurons. We analyzed whether Na(v)1.7 deficiency altered nociceptive heterotrimeric guanine nucleotide-binding protein-coupled receptor (GPCR) signaling, such as initiated by GPCRs that respond to serotonin (pronociceptive) or opioids (antinociceptive), in sensory neurons. We found that the nociceptive neurons of Na(v)1.7 knockout (Na(v)1.7(-/-)) mice, but not those of Na(v)1.8 knockout (Na(v)1.8(-/-)) mice, exhibited decreased pronociceptive serotonergic signaling through the 5-HT4 receptors, which are G alpha(s)-coupled GPCRs that stimulate the production of cyclic adenosinemonophosphate resulting in protein kinase A (PKA) activity, as well as reduced abundance of the RIIb regulatory subunit of PKA. Simultaneously, the efficacy of antinociceptive opioid signaling mediated by the G alpha(i)-coupled mu opioid receptors was increased. Consequently, opioids inhibited more efficiently tetrodotoxin-resistant sodium currents, which are important for pain-initiating neuronal activity in nociceptive neurons. Thus, Na(v)1.7 controls the efficacy and balance of GPCR-mediated pro-and antinociceptive intracellular signaling, such that without Na(v)1.7, the balance is shifted toward antinociception, resulting in lifelong endogenous analgesia.