Isoflurane inhibits the tetrodotoxin-resistant voltage-gated sodium channel Na_v1.8

Background: Voltage-gated sodium channels (Na_v) mediate neuronal action potentials. Tetrodotoxin inhibits all Na_v isoforms, but Na_v1.8 and Na_v1.9 are relatively tetrodotoxin-resistant (TTX-r) compared to other isoforms. Na_v1.8 is highly expressed in dorsal root ganglion neurons and is functionally linked to nociception, but the sensitivity of TTX-r isoforms to inhaled anesthetics is unclear. Methods: The sensitivities of heterologously expressed rat TTX-r Na_v1.8 and endogenous tetrodotoxin-sensitive (TTX-s) Na_v to the prototypic inhaled anesthetic isoflurane were tested in mammalian ND7/23 cells using patch-clamp electrophysiology. Results: From a holding potential of -70 mV, isoflurane (0.53 ± 0.06 mm, 1.8 minimum alveolar concentration at 24°C) reduced normalized peak Na⁺ current (IN_a) of Na_v1.8 to 0.55 ± 0.03 and of endogenous TTX-s Na_v to 0.56 ± 0.06. Isoflurane minimally inhibited I_Na from a holding potential of -140 mV. Isoflurane did not affect voltage-dependence of activation, but it significantly shifted voltage-dependence of steady-state inactivation by -6 mV for Na_v1.8 and by -7 mV for TTX-s Na _v IC₅₀ values for inhibition of peak I_Na were 0.67 ± 0.06 mm for Na_v1.8 and 0.66 ± 0.09 mm for TTX-s Na_v; significant inhibition occurred at clinically relevant concentrations as low as 0.58 minimum alveolar concentration. Isoflurane produced use-dependent block of Na_v1.8; at a stimulation frequency of 10 Hz, 0.56 ± 0.08 mm isoflurane reduced I_Na to 0.64 ± 0.01 versus 0.78 ± 0.01 for control. Conclusion Isoflurane inhibited the tetrodotoxin-resistant isoform Na_v1.8 with potency comparable to that for endogenous tetrodotoxin-sensitive Na_v isoforms, indicating that sensitivity to inhaled anesthetics is conserved across diverse Na _v family members. Block of Na_v1.8 in dorsal root ganglion neurons could contribute to the effects of inhaled anesthetics on peripheral nociceptive mechanisms.