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The ability to reconstitute sodium channel function and pharmacology in vitro using cloned subunits of known structure has greatly enhanced our understanding of the action of pyrethroid insecticides at this target and the structural determinants of resistance and interspecies selectivity. However, the use of reconstituted channels raises three critical questions: (1) Which subunits and subunit combinations should be used? (2) Which heterologous expression system is preferred? (3) Which combination of subunits and expression system best represents the function of native neuronal channels in the organism of interest? This review considers these questions from the perspective of recent research in this laboratory on the action of pyrethroid insecticides on rat Na v 1.6 sodium channels by comparing the effects of heteroligomeric complex composition on channel function and insecticide response when channels are expressed in either Xenopus oocytes or stably-transformed HEK293 cells. These comparisons provide new insight into the influence of cellular context on the functional and pharmacological properties of expressed channels, the modulatory effects of sodium channel auxiliary subunits on the action of pyrethroids, and the relative fidelity of the Xenopus oocyte and HEK293 cell expression systems as model systems for studying of channel function and pyrethroid action. [ABSTRACT FROM AUTHOR]

Abstract: We expressed the rat Nav1.3 and Nav1.6 sodium channel α subunit isoforms in Xenopus oocytes either alone or with the rat β1 and β2 auxiliary subunits in various combinations and assessed the sensitivity of the expressed channels to resting and use-dependent modification by the pyrethroid insecticide tefluthrin using the two-electrode voltage clamp technique. Coexpression with the β1 and β2 subunits, either individually or in combination, did not affecting the resting sensitivity of Nav1.6 channels to tefluthrin. Modification by tefluthrin of Nav1.6 channels in the absence of β subunits was not altered by the application of trains of high-frequency depolarizing prepulses. By contrast, coexpression of the Nav1.6 channel with the β1 subunit enhanced the extent of channel modification 2-fold following repeated depolarization. Coexpression of Nav1.6 with the β2 subunit also slightly enhanced modification following repeated depolarization, but coexpression of Nav1.6 with both β subunits caused enhanced modification following repeated depolarization that was indistinguishable from that found with Nav1.6+β1 channels. In contrast to Nav1.6, the resting modification by tefluthrin of Nav1.3 channels expressed in the absence of β subunits was reduced by repeated depolarization. However, tefluthrin modification of the Nav1.3 α subunit expressed with both β subunits was enhanced 1.7-fold by repeated depolarization, thereby confirming that β subunit modulation of use-dependent effects was not confined to the Nav1.6 isoform. These results show that the actions of pyrethroids on mammalian sodium channels in the Xenopus oocyte expression system are determined in part by the interactions of the sodium channel α subunit with the auxiliary β subunits that are part of the heteromultimeric sodium channel complexes found in neurons and other excitable cells. [Copyright &y& Elsevier]

Abstract: In rats expression of the Nav1.7 voltage-gated sodium channel isoform is restricted to the peripheral nervous system and is abundant in the sensory neurons of the dorsal root ganglion. We expressed the rat Nav1.7 sodium channel α subunit together with the rat auxiliary β1 and β2 subunits in Xenopus laevis oocytes and assessed the effects of the pyrethroid insecticide tefluthrin on the expressed currents using the two-electrode voltage clamp method. Tefluthrin at 100μM modified of Nav1.7 channels to prolong inactivation of the peak current during a depolarizing pulse, resulting in a marked “late current” at the end of a 40ms depolarization, and induced a sodium tail current following repolarization. Tefluthrin modification was enhanced up to twofold by the application of a train of up to 100 5ms depolarizing prepulses. These effects of tefluthrin on Nav1.7 channels were qualitatively similar to its effects on rat Nav1.2, Nav1.3 and Nav1.6 channels assayed previously under identical conditions. However, Nav1.7 sodium channels were distinguished by their low sensitivity to modification by tefluthrin, especially compared to Nav1.3 and Nav1.6 channels. It is likely that Nav1.7 channels contribute significantly to the tetrodotoxin-sensitive, pyrethroid-resistant current found in cultured dorsal root ganglion neurons. We aligned the complete amino acid sequences of four pyrethroid-sensitive isoforms (house fly Vssc1; rat Nav1.3, Nav1.6 and Nav1.8) and two pyrethroid-resistant isoforms (rat Nav1.2 and Nav1.7) and found only a single site, located in transmembrane segment 6 of homology domain I, at which the amino acid sequence was conserved among all four sensitive isoform sequences but differed in the two resistant isoform sequences. This position, corresponding to Val410 of the house fly Vssc1 sequence, also aligns with sites of multiple amino acid substitutions identified in the sodium channel sequences of pyrethroid-resistant insect populations. These results implicate this single amino acid polymorphism in transmembrane segment 6 of sodium channel homology domain I as a determinant of the differential pyrethroid sensitivity of rat sodium channel isoforms. [Copyright &y& Elsevier]

Abstract: We expressed rat Nav1.6 sodium channels in combination with the rat β1 and β2 auxiliary subunits in Xenopus laevis oocytes and evaluated the effects of the pyrethroid insecticides S-bioallethrin, deltamethrin, and tefluthrin on expressed sodium currents using the two-electrode voltage clamp technique. S-Bioallethrin, a type I structure, produced transient modification evident in the induction of rapidly decaying sodium tail currents, weak resting modification (5.7% modification at 100μM), and no further enhancement of modification upon repetitive activation by high-frequency trains of depolarizing pulses. By contrast deltamethrin, a type II structure, produced sodium tail currents that were ~9-fold more persistent than those caused by S-bioallethrin, barely detectable resting modification (2.5% modification at 100μM), and 3.7-fold enhancement of modification upon repetitive activation. Tefluthrin, a type I structure with high mammalian toxicity, exhibited properties intermediate between S-bioallethrin and deltamethrin: intermediate tail current decay kinetics, much greater resting modification (14.1% at 100μM), and 2.8-fold enhancement of resting modification upon repetitive activation. Comparison of concentration–effect data showed that repetitive depolarization increased the potency of tefluthrin ~15-fold and that tefluthrin was ~10-fold more potent than deltamethrin as a use-dependent modifier of Nav1.6 sodium channels. Concentration–effect data from parallel experiments with the rat Nav1.2 sodium channel coexpressed with the rat β1 and β2 subunits in oocytes showed that the Nav1.6 isoform was at least 15-fold more sensitive to tefluthrin and deltamethrin than the Nav1.2 isoform. These results implicate sodium channels containing the Nav1.6 isoform as potential targets for the central neurotoxic effects of pyrethroids. [Copyright &y& Elsevier]

The article examines cross-resistance among 19 pyrethroid insecticides with different chemical structures using an isogenic metabolic resistance CMR strain isolated from a field pyrethroid-resistant population of Helicoverpa armigera from China. Results show that the most significant cross-resistance in the CMR strain happened between pyrethroids, like fenvalerate and flumethrin, which both have phenoxybenzyl and aromatic acid moieties. It was concluded that it is not impossible to find pyrethroids which are active against resistant populations.

Pyrethroid insecticide resistance due to reduced nerve sensitivity, known as knockdown resistance (kdr or kdr-type), is linked to multiple point mutations in the para-homologous sodium channel genes. Previously we demonstrated that two mutations (E434K and C764R) in the German cockroach sodium channel greatly enhanced the ability of the L993F mutation (a known kdr -type mutation) to reduce sodium channel sensitivity to deltamethrin, a pyrethroid insecticide. Neither E434K nor C764R alone, however, altered sodium channel sensitivity. To examine whether E434K and C764R also enhance the effect of pyrethroid resistance-associated sodium channel mutations identified in other insects, we introduced a V to M mutation (V409M) into the cockroach sodium channel protein at the position that corresponds to the V421M mutation in the Heliothis virescens sodium channel protein. We found that the V409M mutation alone modified the gating properties of the sodium channel and reduced channel sensitivity to deltamethrin by 10-fold. Combining the V409M mutation with either the E434K or C764K alone did not reduce the V409M channel sensitivity to deltamethrin further. However, the triple mutation combination (V409M, E434K and C764R) dramatically reduced channel sensitivity by 100-fold compared with the wild-type channel. These results suggest that the E434K and C764R mutations are important modifiers of sodium channel sensitivity to pyrethroid insecticides. [Copyright &y& Elsevier]