Aims: 

GTP-g-S has been shown to functionally up-regulate the persistent, tetrodotoxin-resistant (TTX-r) Na⁺ current in small-diameter sensory neurones, and cause a change in excitability associated with more negative voltage-thresholds. We made a null-mutant mouse, in which exons 4 and 5 of SCN11A were replaced with a neomycin resistance cassette, and subsequently tested the hypothesis that Na_V1.9 was the substrate for this change in excitability.

Methods: 

Sensory neurones were isolated from dorsal root ganglia of either knock-out mice or wild-type and heterozygote littermates, and maintained in culture (1-2 days). With the inclusion of 500 mM GTP-g-S in the pipette solution, Na⁺ current amplitudes and the voltage-thresholds of small-diameter (< 25 mm) neurones were measured in voltage-clamp and current-clamp, respectively, using the whole-cell patch-clamp technique.

Results: 

Knock-out of Na_V1.9 was associated with the loss of a component of TTX-r Na⁺ current (at -30 mV, p < 0.05), operating over more negative potentials than the major TTX-r Na⁺ current, Na_V1.8. We found no significant changes in voltage-threshold in Na_V1.9 knock-outs (n = 14), whereas some wild-type and heterozygote neurones showed substantial negative shifts in voltage-threshold (-13.63 ± 2.26 mV, p < 0.02 in 3 of 19 neurones), during recordings lasting up to 30 mins with an imposed negative holding potential.

Conclusion: 

These results are consistent with Na_V1.9 being the substrate for the persistent Na⁺ current in nociceptive neurones, and the effector of the threshold-change associated with intracellular dialysis of GTP-g-S. Functional up-regulation of Na_V1.9 may underlie a component of inflammatory pain.