Developmentally-regulated sodium channel subunits are differentially sensitive to α-cyano containing pyrethroids

Juvenile rats have been reported to be more sensitive to the acute neurotoxic effects of the pyrethroid deltamethrin than adults. While toxicokinetic differences between juveniles and adults are documented, toxicodynamic differences have not been examined. Voltage-gated sodium channels, the primary targets of pyrethroids, are comprised of α and β subunits, each of which have multiple isoforms that are expressed in a developmentally-regulated manner. To begin to test whether toxicodynamic differences could contribute to age-dependent deltamethrin toxicity, deltamethrin effects were examined on sodium currents in Xenopus laevis oocytes injected with different combinations of rat α (Nav1.2 or Nav1.3) and β (β1 or β3) subunits. Deltamethrin induced tail currents in all isoform combinations and increased the percent of modified channels in a concentration-dependent manner. Effects of deltamethrin were dependent on subunit combination; Nav1.3-containing channels were modified to a greater extent than were Nav1.2-containing channels. In the presence of a β subunit, deltamethrin effects were significantly greater, an effect most pronounced for Nav1.3 channels; Nav1.3/β3 channels were more sensitive to deltamethrin than Nav1.2/β1 channels. Nav1.3/β3 channels are expressed embryonically, while the Nav1.2 and β1 subunits predominate in adults, supporting the hypothesis for age-dependent toxicodynamic differences. Structure-activity relationships for sensitivity of these subunit combinations were examined for other pyrethroids. Permethrin and tetramethrin did not modify currents mediated by either subunit combination. Cypermethrin, β-cyfluthrin, esfenvalerate and fenpropathrin all modified sodium channel function; effects were significantly greater on Nav1.3/β3 than on Nav1.2/β1 channels. These data demonstrate a greater sensitivity of Nav1.3 vs Nav1.2 channels to deltamethrin and other cyano-containing pyrethroids, particularly in the presence of a β subunit.