Point mutations in homology domain II modify the sensitivity of rat Nav1.8 sodium channels to the pyrethroid insecticide cismethrin

Two point mutations in homology domain II of the housefly Vsscl voltage-sensitive Sodium Channel subunit, M918T and L1014F are associated with resistance to pyrethroid insecticides and reduce the pyrethroid sensitivity of Vsscl sodium channels expressed in Xenopus laevis oocytes. To assess the impact of these residues as determinants of pyrethroid sensitivity in another sequence context, we mutated the corresponding positions of the rat pyrethroid-sensitive, TTX-resistant peripheral nerve Sodium Channel (rNav1.8; also called SNS or PN3) and determined the sensitivity of native and mutated channels expressed in Xenopus oocytes to the pyrethroid insecticide cismethrin. The rNav1.8 channel, like Other vertebrate Sodium Channel isoforms, contains a conserved isoleucine residue at sequence position 780 that aligns with the conserved methionine at position 918 of Vsscl and Other insect sodium channels. Channels mutated to contain methionine at position 780 (1780M) exhibited enhanced sensitivity to cismethrin and larger decay constants for pyrethroid-modified channel states. In contrast, the mutation corresponding to M918Tin the Vssc1 channel (1780T) profoundly decreased the cismethrin sensitivity of expressed channels. Insertion of the mutation corresponding to L1014F (L879F in rNav1.8) reduced the cismethrin sensitivity of channels having either isoleucine or methionine at position 780, whereas channels containing the 1780T/L879F double mutation were insensitive to this insecticide. Mutations at Ile780 and Leu879 also modified the voltage dependence of rNav1.8 channels, but these effects were not related to changes in pyrethroid sensitivity. These results confirm the importance of residues in homology domain II as fundamental determinants of the pyrethroid sensitivity of sodium channels.