Pharmacological characterization of a rat Nav1.7 loss-of-function model with insensitivity to pain.

Sodium channel Na(v)1.7, encoded by the SCN9A gene, is a well-validated target that plays a key role in controlling pain sensation. Loss-of-function mutations of Na(v)1.7 can cause a syndrome of profound congenital insensitivity to pain in humans. Better understanding of how the loss of Na(v)1.7 leads to loss of pain sensibility would help to decipher the fundamental mechanisms of nociception and inform strategies for development of novel analgesics. Using a recently described rat Na(v)1.7 loss-of-function model with deficient nociception but intact olfactory function, we investigated the involvement of endogenous opioid and cannabinoid systems in this rodent model of Na(v)1.7-related congenital insensitivity to pain. We found that both the opioid receptor antagonist naloxone and cannabinoid receptor blockers SR141716A (rimonabant) and SR144528 fail to restore acute pain sensitivity in Na(v)1.7 loss-of-function rats. We observed, however, that after rimonabant administration, Na(v)1.7 loss-of-function but not WT rats displayed abnormal behaviours, such as enhanced scratching, caudal self-biting, and altered facial expressions; the underlying mechanism is still unclear. Dorsal root ganglion neurons from Na(v)1.7 loss-of-function rats, although hypoexcitable compared with WT neurons, were still able to generate action potentials in response to noxious heat and capsaicin. Our data indicate that complete loss of dorsal root ganglion neuron excitability is not required for insensitivity to pain and suggest that endogenous opioid and cannabinoid systems are not required for insensitivity to pain in the absence of Na(v)1.7 channels in this rat Na(v)1.7 loss-of-function model.