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

Reduced neuronal sensitivity and susceptibility of the fall armyworm, Spodoptera frugiperda, to pyrethroids in the absence of known knockdown mutations

Neurophysiological recordings were employed to quantify neuronal sensitivity to neurotoxic insecticides and assessed toxicity across field and laboratory fall armyworm (FAW) populations. Topical toxicity resistance ratios (RR) in field-collected FAW was 767-fold compared to laboratory strains and, importantly, a 1750-fold reduction in potency was observed for λ-cyhalothrin in neurophysiological assays. Field collected FAW were found to have a RR of 12 to chlorpyrifos when compared to the susceptible strain and was 8-fold less sensitive in neurophysiological assays. Surprisingly, there were no point mutations identified in the voltage-gated sodium channel known to cause pyrethroid resistance. For acetylcholinesterase, FAW had more than 80% of their nucleotide sequences consistent with A201 and F290 of the susceptible strains although 60% of the tested population was heterozygous for the G227A mutation. These data indicate that point mutations did not contribute to the high level of pyrethroid resistance and nerve insensitivity in this population of field collected FAW. Additionally, these data suggest the kdr phenotype only explains a portion of the heritable variation in FAW resistance and indicates kdr is not the only predictor of high pyrethroid resistance. Phenotypic assays, such as toxicity bioassays or neurophysiological recordings, using field-collected populations are necessary to reliably predict resistant phenotypes and product failures.

Elucidation of pyrethroid and DDT receptor sites in the voltage-gated sodium channel

DDT and pyrethroid insecticides were among the earliest neurotoxins identified to act on voltage-gated sodium channels. In the 1960s, equipped with, at the time, new voltage-clamp techniques, Professor Narahashi and associates provided the initial evidence that DDT and allethrin (the first commercial pyrethroid insecticide) caused prolonged flow of sodium currents in lobster and squid giant axons. Over the next several decades, continued efforts by Prof. Narahashi's group as well as other laboratories led to a comprehensive understanding of the mechanism of action of DDT and pyrethroids on sodium channels. Fast forward to the 1990s, genetic, pharmacological and toxicological data all further confirmed voltage-gated sodium channels as the primary targets of DDT and pyrethroid insecticides. Modifications of the gating kinetics of sodium channels by these insecticides result in repetitive firing and/or membrane depolarization in the nervous system. This mini-review focuses on studies from Prof. Narahashi's pioneer work and more recent mutational and computational modeling analyses which collectively elucidated the elusive pyrethroid receptor sites as well as the molecular basis of differential sensitivities of insect and mammalian sodium channels to pyrethroids.

Functional reconstitution of rat Nav1.6 sodium channels in vitro for studies of pyrethroid action

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 Nav1.6 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.

Biophysical Adaptations of Prokaryotic Voltage-Gated Sodium Channels

This chapter describes the adaptive features found in voltage-gated sodium channels (NaVs) of prokaryotes and eukaryotes. These two families are distinct, having diverged early in evolutionary history but maintain a surprising degree of convergence in function. While prokaryotic NaVs are required for growth and motility, eukaryotic NaVs selectively conduct fast electrical currents for short- and long-range signaling across cell membranes in mammalian organs. Current interest in prokaryotic NaVs is stoked by their resolved high-resolution structures and functional features which are reminiscent of eukaryotic NaVs. In this chapter, comparisons between eukaryotic and prokaryotic NaVs are made to highlight the shared and unique aspects of ion selectivity, voltage sensitivity, and pharmacology. Examples of prokaryotic and eukaryotic NaV convergent evolution will be discussed within the context of their structural features.

An amino acid substitution (L925V) associated with resistance to pyrethroids in Varroa destructor

The Varroa mite, Varroa destructor, is an important pest of honeybees and has played a prominent role in the decline in bee colony numbers over recent years. Although pyrethroids such as tau-fluvalinate and flumethrin can be highly effective in removing the mites from hives, their intensive use has led to many reports of resistance. To investigate the mechanism of resistance in UK Varroa samples, the transmembrane domain regions of the V. destructor voltage-gated sodium channel (the main target site for pyrethroids) were PCR amplified and sequenced from pyrethroid treated/untreated mites collected at several locations in Central/Southern England. A novel amino acid substitution, L925V, was identified that maps to a known hot spot for resistance within the domain IIS5 helix of the channel protein; a region that has also been proposed to form part of the pyrethroid binding site. Using a high throughput diagnostic assay capable of detecting the mutation in individual mites, the L925V substitution was found to correlate well with resistance, being present in all mites that had survived tau-fluvalinate treatment but in only 8 % of control, untreated samples. The potential for using this assay to detect and manage resistance in Varroa-infected hives is discussed.

A residue in the transmembrane segment 6 of domain I in insect and mammalian sodium channels regulate differential sensitivities to pyrethroid insecticides

Voltage-gated sodium channels are critical for electrical signaling in the nervous system. Pyrethroid insecticides exert their toxic action by modifying the gating of sodium channels. A valine to methionine mutation in the transmembrane segment 6 of domain I (IS6) of sodium channels from tobacco budworms (Heliothis virescens) has been shown to alter channel gating and reduce insect sodium channel sensitivity to pyrethroids. A valine to leucine substitution was subsequently reported in pyrethroid-resistant bedbug populations. Intriguingly, pyrethroid-resistant mammalian sodium channels possess an isoleucine at the corresponding position. To determine whether different substitutions at this position alter channel gating and confer pyrethroid resistance, we made valine to methionine, isoleucine or leucine substitutions at the corresponding position, V409, in a cockroach sodium channel and examined the gating properties and pyrethroid sensitivity of the three mutants in Xenopus oocytes. All three mutations reduced the channel sensitivity to three pyrethroids (permethrin, cismethrin and deltamethrin). V409M, but not V409I or V409L, caused 6-7mV depolarizing shifts in the voltage dependences of both activation and inactivation. V409M and V409L slowed channel activation kinetics and accelerated open-state deactivation kinetics, but V409I did not. Furthermore, the substitution of isoleucine with valine, but not with methionine nor leucine, at the corresponding position in a rat skeletal muscle sodium channel, rNav1.4, enhanced channel sensitivity to deltamethrin. Collectively, our study highlights an important role of residues at 409 in regulating not only sodium channel gating, but also the differential sensitivities of insect and mammalian sodium channels to pyrethroids.

Differential state-dependent modification of rat Na(v)1.6 sodium channels expressed in human embryonic kidney (HEK293) cells by the pyrethroid insecticides tefluthrin and deltamethrin

We expressed rat Na(v)1.6 sodium channels in combination with the rat β1 and β2 auxiliary subunits in human embryonic kidney (HEK293) cells and evaluated the effects of the pyrethroid insecticides tefluthrin and deltamethrin on expressed sodium currents using the whole-cell patch clamp technique. Both pyrethroids produced concentration-dependent, resting modification of Na(v)1.6 channels, prolonging the kinetics of channel inactivation and deactivation to produce persistent "late" currents during depolarization and tail currents following repolarization. Both pyrethroids also produced concentration dependent hyperpolarizing shifts in the voltage dependence of channel activation and steady-state inactivation. Maximal shifts in activation, determined from the voltage dependence of the pyrethroid-induced late and tail currents, were ~25mV for tefluthrin and ~20mV for deltamethrin. The highest attainable concentrations of these compounds also caused shifts of ~5-10mV in the voltage dependence of steady-state inactivation. In addition to their effects on the voltage dependence of inactivation, both compounds caused concentration-dependent increases in the fraction of sodium current that was resistant to inactivation following strong depolarizing prepulses. We assessed the use-dependent effects of tefluthrin and deltamethrin on Na(v)1.6 channels by determining the effect of trains of 1 to 100 5-ms depolarizing prepulses at frequencies of 20 or 66.7Hz on the extent of channel modification. Repetitive depolarization at either frequency increased modification by deltamethrin by ~2.3-fold but had no effect on modification by tefluthrin. Tefluthrin and deltamethrin were equally potent as modifiers of Na(v)1.6 channels in HEK293 cells using the conditions producing maximal modification as the basis for comparison. These findings show that the actions of tefluthrin and deltamethrin of Na(v)1.6 channels in HEK293 cells differ from the effects of these compounds on Na(v)1.6 channels in Xenopus oocytes and more closely reflect the actions of pyrethroids on channels in their native neuronal environment.

Actions of Tefluthrin on Rat Na(v)1.7 Voltage-Gated Sodium Channels Expressed in Xenopus Oocytes

In rats expression of the Na(v)1.7 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 Na(v)1.7 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 100 µM modified of Na(v)1.7 channels to prolong inactivation of the peak current during a depolarizing pulse, resulting in a marked "late current" at the end of a 40-ms depolarization, and induced a sodium tail current following repolarization. Tefluthrin modification was enhanced up to two-fold by the application of a train of up to 100 5-ms depolarizing prepulses. These effects of tefluthrin on Na(v)1.7 channels were qualitatively similar to its effects on rat Na(v)1.2, Na(v)1.3 and Na(v)1.6 channels assayed previously under identical conditions. However, Na(v)1.7 sodium channels were distinguished by their low sensitivity to modification by tefluthrin, especially compared to Na(v)1.3 and Na(v)1.6 channels. It is likely that Na(v)1.7 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 Na(v)1.3, Na(v)1.6 and Na(v)1.8) and two pyrethroid-resistant isoforms (rat Na(v)1.2 and Na(v)1.7) 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.

Molecular mechanisms of pyrethroid insecticide neurotoxicity: recent advances

Synthetic pyrethroid insecticides were introduced into widespread use for the control of insect pests and disease vectors more than three decades ago. In addition to their value in controlling agricultural pests, pyrethroids are at the forefront of efforts to combat malaria and other mosquito-borne diseases and are also common ingredients of household insecticide and companion animal ectoparasite control products. The abundance and variety of pyrethroid uses contribute to the risk of exposure and adverse effects in the general population. The insecticidal actions of pyrethroids depend on their ability to bind to and disrupt voltage-gated sodium channels of insect nerves. Sodium channels are also important targets for the neurotoxic effects of pyrethroids in mammals but other targets, particularly voltage-gated calcium and chloride channels, have been implicated as alternative or secondary sites of action for a subset of pyrethroids. This review summarizes information published during the past decade on the action of pyrethroids on voltage-gated sodium channels as well as on voltage-gated calcium and chloride channels and provides a critical re-evaluation of the role of these three targets in pyrethroid neurotoxicity based on this information.

Divergent actions of the pyrethroid insecticides S-bioallethrin, tefluthrin, and deltamethrin on rat Na(v)1.6 sodium channels

We expressed rat Na(v)1.6 sodium channels in combination with the rat beta(1) and beta(2) 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 100 microM), 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 100 microM), 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 100 microM), 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 approximately 15-fold and that tefluthrin was approximately 10-fold more potent than deltamethrin as a use-dependent modifier of Na(v)1.6 sodium channels. Concentration-effect data from parallel experiments with the rat Na(v)1.2 sodium channel coexpressed with the rat beta(1) and beta(2) subunits in oocytes showed that the Na(v)1.6 isoform was at least 15-fold more sensitive to tefluthrin and deltamethrin than the Na(v)1.2 isoform. These results implicate sodium channels containing the Na(v)1.6 isoform as potential targets for the central neurotoxic effects of pyrethroids.

State-Dependent Modification of Voltage-Gated Sodium Channels by Pyrethroids

Pyrethroids disrupt nerve function by altering the rapid kinetic transitions between conducting and nonconducting states of voltage-gated sodium channels that underlie the generation of nerve action potentials. Recent studies of pyrethroid action on cloned insect and mammalian sodium channel isoforms expressed in Xenopus laevis oocytes show that in some cases pyrethroid modification is either absolutely dependent on or significantly enhanced by repeated channel activation. These use-dependent effects have been interpreted as evidence of preferential binding of at least some pyrethroids to the open, rather than resting, state of the sodium channel. This paper reviews the evidence for state-dependent modification of insect and mammalian sodium channels expressed in oocytes by pyrethroids and considers the implications of state-dependent effects for understanding the molecular mechanism of pyrethroid action and the development and testing of models of the pyrethroid receptor.

Evidence for dose-additive effects of pyrethroids on motor activity in rats

BACKGROUND

Pyrethroids are neurotoxic insecticides used in a variety of indoor and outdoor applications. Previous research characterized the acute dose-effect functions for 11 pyrethroids administered orally in corn oil (1 mL/kg) based on assessment of motor activity.

OBJECTIVES

We used a mixture of these 11 pyrethroids and the same testing paradigm used in single-compound assays to test the hypothesis that cumulative neurotoxic effects of pyrethroid mixtures can be predicted using the default dose-addition theory.

METHODS

Mixing ratios of the 11 pyrethroids in the tested mixture were based on the ED30 (effective dose that produces a 30% decrease in response) of the individual chemical (i.e., the mixture comprised equipotent amounts of each pyrethroid). The highest concentration of each individual chemical in the mixture was less than the threshold for inducing behavioral effects. Adult male rats received acute oral exposure to corn oil (control) or dilutions of the stock mixture solution. The mixture of 11 pyrethroids was administered either simultaneously (2 hr before testing) or after a sequence based on times of peak effect for the individual chemicals (4, 2, and 1 hr before testing). A threshold additivity model was fit to the single-chemical data to predict the theoretical dose-effect relationship for the mixture under the assumption of dose additivity.

RESULTS

When subthreshold doses of individual chemicals were combined in the mixtures, we found significant dose-related decreases in motor activity. Further, we found no departure from the predicted dose-additive curve regardless of the mixture dosing protocol used.

CONCLUSION

In this article we present the first in vivo evidence on pyrethroid cumulative effects supporting the default assumption of dose addition.

Human and rat Nav1.3 voltage-gated sodium channels differ in inactivation properties and sensitivity to the pyrethroid insecticide tefluthrin

Voltage-gated sodium channels are important sites for the neurotoxic actions of pyrethroid insecticides in mammals. The pore-forming alpha subunits of mammalian sodium channels are encoded by a family of 9 genes, designated Nav1.1-Nav1.9. Native sodium channels in the adult central nervous system (CNS) are heterotrimeric complexes of one of these 9 alpha subunits and two auxiliary (beta) subunits. Here we compare the functional properties and pyrethroid sensitivity of the rat and human Nav1.3 isoforms, which are abundantly expressed in the developing CNS. Coexpression of the rat Nav1.3 and human Nav1.3 alpha subunits in combination with their conspecific beta1 and beta2 subunits in Xenopus laevis oocytes gave channels with markedly different inactivation properties and sensitivities to the pyrethroid insecticide tefluthrin. Rat Nav1.3 channels inactivated more slowly than human Nav1.3 channels during a depolarizing pulse. The rat and human channels also differed in their voltage dependence of steady-state inactivation. Exposure of rat and human Nav1.3 channels to 100 microM tefluthrin in the resting state produced populations of channels that activated, inactivated and deactivated more slowly than unmodified channels. For both rat and human channels, application of trains of depolarizing prepulses enhanced the extent of tefluthrin modification approximately twofold; this result implies that tefluthrin may bind to both the resting and open states of the channel. Modification of rat Nav1.3 channels by 100 microM tefluthrin was fourfold greater than that measured in parallel assays with human Nav1.3 channels. Human Nav1.3 channels were also less sensitive to tefluthrin than rat Nav1.2 channels, which are considered to be relatively insensitive to pyrethroids. These data provide the first direct comparison of the functional and pharmacological properties of orthologous rat and human sodium channels and demonstrate that orthologous channels with a high degree of amino acid sequence conservation differ in both their functional properties and their sensitivities to pyrethroid insecticides.

Alternatively spliced sodium channel transcripts expressed in field strains of the diamondback moth

The frequencies of the L1014F and T929I mutations, both of which are involved in nerve insensitive resistance to pyrethroids, were examined in field and laboratory strains of the diamondback moth, Plutella xylostella at DNA and RNA levels. Results showed that the resistance allele frequencies at the L1014F and T929I sites in the field strains were respectively, 82.8-100% and 72.9-94.4%. No posttranscriptional regulation of the L1014F mutation was observed. The examined insects were classifiable into four groups according to the expression patterns of mutually exclusive exons 18a and 18b. Most insects in the field strains expressed transcripts containing exon 18b more abundantly than those containing exon 18a, although both transcripts were expressed with similar proportions in all insects of the laboratory strains. Some other insects expressed a chimeric transcript comprising parts of exons 18a and 18b. Deduced amino acid sequences of the chimeric transcript encoded amino substitution from Met to Ile at the site corresponding to the super-kdr mutation (M918T) in Musca domestica. The frequencies of the M918I mutation in the field strains were 5.0-19.4%. Analyses of the genomic organization revealed that the chimeric sequences are encoded in the genome.

Pyrethroids, knockdown resistance and sodium channels

Knockdown resistance to DDT and the pyrethrins was first described in 1951 in the housefly (Musca domestica L.). This trait, which confers reduced neuronal sensitivity to these insecticides, was subsequently shown to confer cross-resistance to all synthetic pyrethroid insecticides that have been examined to date. As a consequence, the worldwide commercial development of pyrethroids as a major insecticide class over the past three decades has required constant awareness that pyrethroid overuse has the potential to reselect this powerful resistance mechanism in populations that previously were resistant to DDT. Demonstration of tight genetic linkage between knockdown resistance and the housefly gene encoding voltage-sensitive sodium channels spurred efforts to identify gene mutations associated with knockdown resistance and understand how these mutations confer a reduction in the sensitivity of the pyrethroid target site. This paper summarizes progress in understanding pyrethroid resistance at the molecular level, with particular emphasis on studies in the housefly.

Structure–activity relationships for the action of 11 pyrethroid insecticides on rat Nav1.8 sodium channels expressed in Xenopus oocytes

Pyrethroid insecticides bind to voltage-sensitive sodium channels and modify their gating kinetics, thereby disrupting nerve function. This paper describes the action of 11 structurally diverse commercial pyrethroid insecticides on the rat Na v 1.8 sodium channel isoform, the principal carrier of the tetrodotoxin-resistant, pyrethroid-sensitive sodium current of sensory neurons, expressed in Xenopus laevis oocytes. All 11 compounds produced characteristic sodium tail currents following a depolarizing pulse that ranged from rapidly-decaying monoexponential currents (allethrin, cismethrin and permethrin) to persistent biexponential currents (cyfluthrin, cyhalothrin, cypermethrin and deltamethrin). Tail currents for the remaining compounds (bifenthrin, fenpropathrin, fenvalerate and tefluthrin) were monoexponential and decayed with kinetics intermediate between these extremes. Reconstruction of currents carried solely by the pyrethroid-modified subpopulation of channels revealed two types of pyrethroid-modified currents. The first type, found with cismethrin, allethrin, permethrin and tefluthrin, activated relatively rapidly and inactivated partially during a 40-ms depolarization. The second type, found with cypermethrin, cyfluthrin, cyhalothrin, deltamethrin, fenpropathrin and fenvalerate, activated more slowly and did not detectably inactivate during a 40-ms depolarization. Only bifenthrin did not produce modified currents that fit clearly into either of these categories. In all cases, the rate of activation of modified channels was strongly correlated with the rate of tail current decay following repolarization. Modification of Na v 1.8 sodium channels by cyfluthrin, cyhalothrin, cypermethrin and deltamethrin was enhanced 2.3- to 3.4-fold by repetitive stimulation; this effect appeared to result from the accumulation of persistently open channels rather than preferential binding to open channel states. Fenpropathrin was the most effective compound against Na v 1.8 sodium channels from the perspective of either resting or use-dependent modification. When use dependence is taken into account, cypermethrin, deltamethrin and tefluthrin approached the effectiveness of fenpropathrin. The selective expression of Na v 1.8 sodium channels in nociceptive neurons suggests that these channels may be important targets for pyrethroids in the production of paresthesia following dermal exposure.

Mammalian Voltage-Gated Calcium Channels Are Potently Blocked by the Pyrethroid Insecticide Allethrin

Pyrethroids are commonly used insecticides for both household and agricultural applications. It is generally reported that voltage-gated sodium channels are the primary target for toxicity of these chemicals to humans. The phylogenetic and structural relatedness between sodium channels and voltage-gated calcium (Ca) channels prompted us to examine the effects of the type 1 pyrethroid allethrin on the three major classes of mammalian calcium channels exogenously expressed in human embryonic kidney 293 cells. We report that all classes of mammalian calcium channels are targets for allethrin at concentrations very similar to those reported for interaction with sodium channels. Allethrin caused blockade with IC(50) values of 7.0 microM for T-type alpha(1G) (Ca(v)3.1), 6.8 microM for L-type alpha(1C) (Ca(v)1.2), and 6.7 microM for P/Q-type alpha(1A) (Ca(v)2.1) channels. Mechanistically, the blockade of calcium channels was found to be significantly different than the prolonged opening of mammalian sodium channels caused by pyrethroids. In all calcium channel subtypes tested, allethrin caused a significant acceleration of the inactivation kinetics and a hyperpolarizing shift in the voltage dependence of inactivation. The high-voltage-activated P/Q- and L-type channels showed a frequency of stimulation-dependent increase in block by allethrin, whereas the low-voltage-activated alpha(1G) subtype did not. Allethrin did not significantly modify the deactivation kinetics or current-voltage relationships of any of the calcium channel types. Our study indicates that calcium channels are another primary target for allethrin and suggests that blockade of different types of calcium channels may underlie some of the chronic effects of low-level pyrethroid poisoning.

Mutations of the para Sodium Channel of Drosophila melanogaster Identify Putative Binding Sites for Pyrethroids

The effects of two pyrethroids on recombinant wild-type and mutant (pyrethroid-resistant) Na+ channels of Drosophila melanogaster have been studied. Three mutations that confer resistance (kdr/superkdr) to pyrethroids were inserted, either individually or in combination, into the para Na+ channel of D. melanogaster: L1014F in domain IIS6, M918T in the IIS4-S5 linker, and T929I in domain IIS5. Channels were expressed in Xenopus laevis oocytes and the effects of the pyrethroids permethrin (type I) and deltamethrin (type II) on Na+ currents were investigated using voltage clamp. The Na+ channels deactivated slowly after deltamethrin treatment, the resultant "tail" currents being used to quantify the effects of this pyrethroid. The Hill slope of 2 for deltamethrin action on the wild-type channel and the mutant L1014F channel is indicative of cooperative binding at two or more sites on these channels. In contrast, binding to the mutants M918T and T929I is noncooperative. Tail currents for the wild-type channel and L1014F channel decayed biphasically, whereas those for M918T and T929I mutants decayed monophasically. The L1014F mutant was approximately 20-fold less sensitive than the wild-type to deltamethrin. Surprisingly, the sensitivity of the double mutant M918T+L1014F to deltamethrin was similar to that of M918T alone, whereas the sensitivity of T929I+L1014F was >30,000-fold lower than that of T929I. Permethrin was less potent than deltamethrin, and its binding to all channel types was noncooperative. The decays of permethrin-induced tail currents were exclusively monophasic. These findings are discussed in terms of the properties and possible locations of pyrethroid binding sites on the D. melanogaster Na+ channel.

The molecular biology of knockdown resistance to pyrethroid insecticides

The term "knockdown resistance" is used to describe cases of resistance to diphenylethane (e.g. DDT) and pyrethroid insecticides in insects and other arthropods that result from reduced sensitivity of the nervous system. Knockdown resistance, first identified and characterized in the house fly (Musca domestica) in the 1950's, remains a threat to the continued usefulness of pyrethroids in the control of many pest species. Research since 1990 has provided a wealth of new information on the molecular basis of knockdown resistance. This paper reviews these recent developments with emphasis on the results of genetic linkage analyses, the identification of gene mutations associated with knockdown resistance, and the functional characterization of resistance-associated mutations. Results of these studies identify voltage-sensitive sodium channel genes orthologous to the para gene of Drosophila melanogaster as the site of multiple knockdown resistance mutations and define the molecular mechanisms by which these mutations cause pyrethroid resistance. These results also provide new insight into the mechanisms by which pyrethroids modify the function of voltage-sensitive sodium channels.

Mechanisms of pyrethroid neurotoxicity: implications for cumulative risk assessment

The Food Quality Protection Act (FQPA) of 1996 requires the United States Environmental Protection Agency to consider the cumulative effects of exposure to pesticides having a 'common mechanism of toxicity.' This paper reviews the information available on the acute neurotoxicity and mechanisms of toxic action of pyrethroid insecticides in mammals from the perspective of the 'common mechanism' statute of the FQPA. The principal effects of pyrethroids as a class are various signs of excitatory neurotoxicity. Historically, pyrethroids were grouped into two subclasses (Types I and II) based on chemical structure and the production of either the T (tremor) or CS (choreoathetosis with salivation) intoxication syndrome following intravenous or intracerebral administration to rodents. Although this classification system is widely employed, it has several shortcomings for the identification of common toxic effects. In particular, it does not reflect the diversity of intoxication signs found following oral administration of various pyrethroids. Pyrethroids act in vitro on a variety of putative biochemical and physiological target sites, four of which merit consideration as sites of toxic action. Voltage-sensitive sodium channels, the sites of insecticidal action, are also important target sites in mammals. Unlike insects, mammals have multiple sodium channel isoforms that vary in their biophysical and pharmacological properties, including their differential sensitivity to pyrethroids. Pyrethroids also act on some isoforms of voltage-sensitive calcium and chloride channels, and these effects may contribute to the toxicity of some compounds. Effects on peripheral-type benzodiazepine receptors are unlikely to be a principal cause of pyrethroid intoxication but may contribute to or enhance convulsions caused by actions at other target sites. In contrast, other putative target sites that have been identified in vitro do not appear to play a major role in pyrethroid intoxication. The diverse toxic actions and pharmacological effects of pyrethroids suggest that simple additivity models based on combined actions at a single target are not appropriate to assess the risks of cumulative exposure to multiple pyrethroids.