Pyrethroids of the most potent class antagonize GABA action at the crayfish neuromuscular junction

Pyrethroid mode(s) of action in the context of Food Quality Protection Act (FQPA) regulation

A Scientific Advisory Panel (SAP) in June 2009 concluded that a common mode of action existed for pyrethroids, with two subgroups. The purpose of this SAP was to advise the U.S. Environmental Protection Agency on the validity of regulation of pyrethroids as a single class under the Food Quality Protection Act of 1996. Two types of pyrethroid action were first described for clinical signs in the rat and clinical signs/nerve effects in the cockroach. In insects, Type I clinical signs correlate with repetitive firing in nerve axons, especially fine sensory axons. The Na(+) inward current is via a TTX-sensitive voltage-gated sodium channel (VGSC). Type II (α-CN) effects on VGSCs do not include repetitive firing following stimulation in these axons. Instead, Type II effects on VGSCs include prolonged Na(+) tail currents along with depolarization of nerve membrane. Other Type II effects have been measured on VG Ca(2+) and K(+) channels and VG and GABA-activated Cl(-) channels. In conclusion, in vivo pyrethroid effects in mammals should be linked with specific channel effects, allowing the use of specific clinical signs or ion channel effects for pyrethroid risk assessment.

Reduced susceptibility to pyrethroid insecticide treated nets by the malaria vector Anopheles gambiae s.l. in western Uganda

Background

Pyrethroid insecticide-treated mosquito nets are massively being scaled-up for malaria prevention particularly in children under five years of age and pregnant mothers in sub-Saharan Africa. However, there is serious concern of the likely evolution of widespread pyrethroid resistance in the malaria vector Anopheles gambiae s.l. due to the extensive use of pyrethroid insecticide-treated mosquito nets. The purpose of this study was to ascertain the status of pyrethroid resistance in An. gambiae s.l. in western Uganda.

Methods

Wild mosquitoes (1–2 days old) were exposed in 10 replicates to new nets impregnated with K-othrine (Deltamethrin 25 mg/m²), Solfac EW50 (Cyfluthrin 50 mg/m²) and Fendona 6SC (Cypermethrin 50 mg/m²) and observed under normal room temperature and humidity (Temperature 24.8°C–27.4°C, Humidity 65.9–45.7). A similar set of mosquitoes collected from the control area 80 km away were exposed to a deltamethrin 25 mg/m² impregnated net at the same time and under the same conditions. The 10-year mean KDT₅₀ and mortality rates for each of the three pyrethroid insecticides were compared using the Student t-test.

Results

A significant increase in the mean knockdown time (KDT₅₀) and mean mortality rate were observed in almost all cases an indication of reduced susceptibility. The overall results showed a four-fold increase in the mean knockdown time (KDT₅₀) and 1.5-fold decrease in mortality rate across the three pyrethroid insecticides. There was a significant difference in the 10-year mean KDT₅₀ between deltamethrin and cyfluthrin; deltamethrin and cypermethrin, but no significant difference between cyfluthrin and cypermethrin. The 10-year mean difference in KDT50 for mosquitoes exposed to deltamethrin from the control site was significantly different from that of mosquitoes from the intervention site (p<0.05, t=3.979, 9df). The 10-year mean difference in mortality rate between deltamethrin (84.64%); cyfluthrin (74.18%); cypermethrin (72.19%) and the control (90.45%) showed a significant decline in mortality across all the three insecticides.

Conclusion

Generally the results showed a trend of increase in mosquito resistance status with cross-resistance against all the three pyrethroid insecticides. This study reveals for the first time the development of pyrethroid resistance in An. gambiae s.l. in Western Uganda. It is therefore strongly recommended that the impact of this development on malaria control efforts be closely monitored and alternative fabric treatments be considered before this problem curtails community wide implementation of this malaria control strategy in Uganda.

Neuropharmacological effects of deltamethrin in rats

This study examined the effect of deltamethrin on some of the neuropharmacological paradigms in a rat brain such as the motor co-ordination test using a rotarod, the pentobarbitone-induced sleeping time and pentylenetetrazole (PTZ)-induced convulsion as well as the gamma aminobutyric acid (GABA) level. Albino Wistar rats were used as the experimental animals. Different neuropharmacological paradigms such as the motor co-ordination by the rotarod, pentobarbitone-induced sleeping time and the PTZ-induced convulsion were examined after administering deltamethrin orally at two doses, 150 mg/kg (LD₅₀) and 15 mg/kg (1/10 LD₅₀). The GABA level in the rat brain was estimated by HPLC after a single oral dose of 150mg/kg deltamethrin. Deltamethrin significantly reduced the motor coordination, decreased the onset time and increased the sleeping time duration induced by pentobarbitone. In addition, it also decreased the onset time and increased the duration of convulsions induced by PTZ at 150 mg/kg (LD₅₀) and 15 mg/kg (1/10 LD₅₀), respectively. Further deltamethrin administration decreased the GABA levels in the cerebellum as well as in the whole brain (except the cerebellum) significantly at the LD₅₀ dose level. There was some correlation between the effect of deltamethrin on the central GABA levels and its neuropharmacological effects.

Effect of pyrethroids on pentobarbital-induced sleeping time in relation to the chemical structure

The effect of pyrethroids on pentobarbital-induced sleeping time was examined in mice. Pentobarbital-induced sleeping time was defined as the time interval from loss of righting reflex to reappearance of righting reflex. Mice were administered pyrethroids orally 2 h before the intraperitoneal injection of pentobarbital at a dose of 45 mg/kg. Pyrethroids with a cyano group on the alcohol moiety, such as cyphenothrin, cypermethrin, fenvalerate and fenpropathrin shortened the pentobarbital-induced sleeping time, while pyrethroids with an ethynyl group on the acid moiety, such as prallethrin, furamethrin and empenthrin, prolonged sleeping time. Pyrethroids with neither a cyano nor an ethynyl group had no effect on sleeping time. We, thus, found a clear relationship between the chemical structure of pyrethroids and their effect on pentobarbital-induced sleeping time.

Effect of fenvalerate, a pyrethroid insecticide on membrane fluidity

Fenvalerate is a commonly used pyrethroid insecticide, used to control a wide range of pests. We have studied its interaction with the membrane using fluorescence polarization and differential scanning calorimetry (DSC) techniques. Fenvalerate was found to decrease the DPH fluorescence polarization value of synaptosomal and microsomal membrane, implicating that it makes the membrane more fluid. At different concentrations of fenvalerate, the activation energy of the probe molecule in the membrane also changes revealed from the change in slope of the Arrhenius plot. At higher concentrations the insecticide slowly saturates the membrane. The effects of fenvalerate on model membrane were also studied with liposomes reconstituted with dipalmitoylphosphatidylcholine (DPPC). Fenvalerate decreased the phase transition temperature (Tm) of DPPC by 1.5 C degrees at 40 microM concentration, but there was no effect on the cooperativity of the transition as interpreted from the DSC thermogram. From the change in the thermogram profile with fenvalerate it has been interpreted that it localizes in the acyl chain region of the lipid, possibly between C10 and C16 region and weakens the acyl chain packing. Fenvalerate was also found to interact with DPPC liposomes containing cholesterol to fluidize it.

Actions of insecticides on the insect GABA receptor complex

The actions of insecticides on the insect gamma-aminobutyric acid (GABA) receptor were investigated using [35S]t-butylbicyclophosphorothionate [( 35S]TBPS) binding and voltage-clamp techniques. Specific binding of [35S]TBPS to a membrane homogenate derived from the brain of Locusta migratoria locusts is characterised by a Kd value of 79.3 +/- 2.9 nM and a Bmax value of 1770 +/- 40 fmol/mg protein. [35S]TBPS binding is inhibited by mM concentrations of barbiturates and benzodiazepines. In contrast dieldrin, ivermectin, lindane, picrotoxin and TBPS are inhibitors of [35S]TBPS binding at the nanomolar range. Bicuculline, baclofen and pyrethroid insecticides have no effect on [35S]TBPS binding. These results are similar to those obtained in electrophysiological studies of the current elicited by GABA in both Locusta and Periplaneta americana central neurones. Noise analysis of the effects of lindane, TBPS, dieldrin and picrotoxin on the cockroach GABA responses reveals that these compounds decrease the variance of the GABA-induced current but have no effect on its mean open time. All these compounds, with the exception of dieldrin, significantly decrease the conductance of GABA-evoked single current.

The conformation of pyrethroids bound to lipid bilayers

A number of pyrethroids have been synthesised containing bromine or iodine atoms in the acid and alcohol moieties. Quenching of the fluorescence of a pyrene-containing fatty acid in phospholipid bilayers has been used to measure the partitioning of the pyrethroids into the bilayers. Comparison of the intensity of the fluorescence of the 3-phenoxybenzyl moiety of the pyrethroids in lipid bilayers with that in organic solvents shows that the 3-phenoxybenzyl moiety is located in a hydrophobic environment; this was confirmed by fluorescence quenching by spin-labelled fatty acids. Self-quenching of the fluorescence of pyrethroids which contain both a bromine-containing acid and a 3-phenoxybenzyl moiety is consistent with a 'horseshoe' conformation for the bound pyrethroid, with the ester group located at the lipid/water interface and the acid and alcohol moieties folded back with both penetrating into the lipid bilayer.

Pyrethroids and enhanced inhibition in the hippocampus of the rat

The pyrethroid insecticides have been divided into two classes on the basis of their biochemical actions and behavioral indices of toxicity. Both types of pyrethroids have effects on sodium conductance, and Type II pyrethroids have been reported to antagonize gamma-aminobutyric acid (GABA) by interacting with the t-butyl-bicyclophosphorothionate (TBPS)/picrotoxinin binding site. The dentate gyrus of the hippocampus is equipped with GABAergic recurrent inhibitory circuits. The present experiment was designed to demonstrate dissociation in the biochemistry of pyrethroids by activating the perforant path with pairs of stimulus pulses and monitoring the recurrent inhibition in this circuit. Antagonism of GABA leads to a reduction in inhibition, measured as an increase in the size of the population spike in response to the second pulse of the pair. The GABAergic properties of the pyrethroids were assessed by examining paired pulse inhibition before and after oral treatment with 20 mg/kg of cismethrin (Type I), 20 mg/kg of fenvalerate, or 10 mg/kg of deltamethrin (Type IIs). Input/output (I/O) functions revealed a reduction in excitatory postsynaptic potential (EPSP) following cismethrin and deltamethrin. Population spike height was unaffected. Fenvalerate had no effect on I/O functions. In contrast to the prediction of reduced inhibition following treatment with Type II pyrethroids, deltamethrin and fenvalerate increased inhibition up to 500 and 150 ms interpulse intervals, respectively. Cismethrin was without effect on paired pulse inhibition. These findings fail to provide evidence of GABA antagonistic properties of Type II pyrethroids and may be best explained by a differential effect of these three pyrethroids on sodium channel kinetics.

Responses to GABA by isolated insect neuronal somata: pharmacology and modulation by a benzodiazepine and a barbiturate

Mechanically dissociated neuronal somata from the thoracic ganglia of Locusta migratoria and Schistocerca gregaria were viable in vitro for hours and were current- and voltage-clamped to record the responses evoked by brief pressure applications of gamma-aminobutyric acid (GABA) in the presence of various modulators. The application of GABA and muscimol, but not baclofen, produced a hyperpolarization and concurrent increase in the membrane conductance. The current underlying this response reversed at -65 mV, was evoked in all cells tested and showed outward rectification. In 6 of 74 Locusta neurones but not in the neurones of Schistocerca, GABA and muscimol evoked a biphasic response. The initial, fast phase was indistinguishable from the GABA-evoked current seen in all neurones. The remaining predominant, slow and long-duration component of the response was an inward current over the membrane potential range 0 to -80 mV, increasing with hyperpolarization. The GABAA antagonists bicuculline and pitrazepin were without effect on the fast GABA response while picrotoxin was a potent blocker of both the fast and the slow GABA responses. Flunitrazepam enhanced the amplitude of the fast response by up to 70% without increasing its duration. Sodium pentobarbital enhanced both the amplitude and the duration of the fast GABA response. We conclude that the locust thoracic neuronal GABA receptor/channel complex resembles the vertebrate GABAA receptor in having associated modulatory receptor sites for benzodiazepines and barbiturates, but differs from it in terms of the pharmacology of the GABA receptor itself.

Actions of deltamethrin and tralomethrin on cholinergic synaptic transmission in the central nervous system of the cockroach (Periplaneta americana)

Deltamethrin (RU 22974) and tralomethrin isomers (RU 24784 and RU 24785) block transmission at the cercal-afferent giant-interneuron synapses of the cockroach, when bath-applied to the desheathed ganglion at micromolar concentrations. The time-course of the events leading to the blockage suggests two possible target sites: one located presynaptically and the other situated on postsynaptic membranes.

Polychlorocycloalkane insecticide-induced convulsions in mice in relation to disruption of the GABA-regulated chloride ionophore

The toxicity to mice of intraperitoneally-administered polychlorocycloalkane (PCCA) insecticides is generally correlated with their potency as in vitro inhibitors of the brain specific [35S]t-butylbicyclophosphorothionate [( 35S]TBPS) binding site with correction for metabolic activation and detoxification. These findings from our earlier studies are extended here to in vivo investigations relating convulsant action to inhibition of the TBPS binding site in poisoned mice. Radioligand binding assays involved brain P2 membranes washed three times with 1 mM EDTA to remove endogenous gamma-aminobutyric acid (GABA) or other modulator(s) which otherwise serves as a noncompetitive inhibitor of [35S]TBPS binding at the GABA-regulated chloride ionophore. Examination of lindane, technical toxaphene, toxaphene toxicant A, and 10 polychlorocyclodiene insecticides revealed 62 +/- 4% binding site inhibition 30 min after their LD50 doses with 32 +/- 3% inhibition at one-half and 6 +/- 3% inhibition at one-quarter of their LD50 doses. This correlation between binding site inhibition and convulsant action is also evident in dose- and time-dependency studies with endosulfan sulfate. The brain P2 membranes of treated mice contain the parent compound with each of the PCCAs plus activation products of some of the cyclodienes, i.e. endosulfan sulfate from alpha- and beta-endosulfan and 12-ketoendrin from isodrin and endrin. The finding that the brains of treated mice contain sufficient PCCA or its activation products to achieve a magnitude of [35S]TBPS binding site inhibition correlated with the severity of the poisoning signs supports the hypothesis that the acute toxicity of PCCA insecticides to mammals is due to disruption of the GABA-regulated chloride ionophore.

t-[3H]butylbicycloorthobenzoate: new radioligand probe for the gamma-aminobutyric acid-regulated chloride ionophore

t-[3H]Butylbicycloorthobenzoate [( 3H]TBOB; 22 Ci/mmol) was prepared by reductive dechlorination of its 4-chlorophenyl analog with tritium gas. This new radioligand binds reversibly to fresh washed rat brain P2 membranes in 500 mM NaCl plus 50 mM sodium-potassium phosphate buffer (pH 7.4) at 25 degrees C, with 80-90% specific relative to total binding, a KD of 61 +/- 15 nM, and a Bmax of 1.6 +/- 0.5 pmol/mg of protein. [3H]TBOB association with its binding site(s) is monophasic, but its dissociation is biphasic. The binding characteristics of [3H]TBOB are essentially identical to those of t-[35S]butylbicyclophosphorothionate [( 35S]TBPS) with respect to pH dependence, stimulation by anions, regional distribution in the brain, and pharmacological profile. Saturation analyses and dissociation studies further indicate that TBOB and TBPS have a common binding site. However, binding of the two radioligands differs in respect to temperature effects. In contrast to [35S]TBPS, which exhibits negligible binding at 0 degrees C, [3H]TBOB binds to rat brain membranes at 0, 25, and 37 degrees C with similar KD values. [3H]TBOB with its long radioactive half-life and temperature-independent KD is a valuable supplement to [35S]TBPS in further biochemical and pharmacological characterization of the gamma-aminobutyric acid receptor-ionophore complex.

Interactions of pyrethroids with phosphatidylcholine bilayers: comparisons in liposomal systems exhibiting large or small radii of curvature

Pyrethroid interactions with dipalmitoyl phosphatidylcholine (DPPC) vesicles have been characterized in bilayers having large and small radii of curvature. The abilities of pyrethroids to alter the gel-fluid phase transition profiles were determined by steady state fluorescence anisotropy and phase-modulation lifetime techniques using the fluorescent probes cis- and trans-parinaric acid. Using the geometric isomers of parinaric acid as membrane probes, pyrethroids were found to lower the phase transition temperature (Tc) of DPPC large multilamellar vesicles with the same order of comparative effectiveness as previously reported using the fluorescent probe 1,6-diphenyl-1,3,5-hexatriene (DPH). Permethrin had a greater depressive effect upon the Tc of DPPC in the small unilamellar vesicle (SUV) system than in the large multilamellar system. Conversely, allethrin was less effective in reducing the Tc of DPPC SUVs. The enhanced effect of permethrin in decreasing the Tc of DPPC SUVs was greatest in regions of more rigid lipid packing, as determined by trans-parinaric acid fluorescence parameters. The results indicate that changes in lipid packing configuration caused by differing bilayer radii of curvature may alter the interactive characteristics of pyrethroids with lipid membranes.

Interactions of pyrethroids with phosphatidylcholine liposomal membranes

Interactions of several pyrethroids with membrane lipids in the form of dipalmitoylphosphatidylcholine (DPPC) liposomes have been studied using fluorescent membrane probes. Fluorescence anisotropy values and lifetimes (determined by phase-shift and demodulation techniques) of the fluorescent probe, 1,6-diphenyl-1,3,5-hexatriene, were decreased in gel phase liposomes by pyrethroids at concentrations on the order of 10 microM. The pyrethroids containing a cyano substituent were also observed to cause collisional quenching of diphenylhexatriene fluorescence. Pyrethroids differed in their effectiveness at lowering the phase transition temperature of DPPC, and in their ability to broaden the temperature range of this transition. The fluorescence intensity of DPPC-incorporated chlorophyll a was used to monitor the pretransition of DPPC and the lateral diffusion of a membrane component located in the polar headgroup region. Permethrin did not affect chlorophyll a fluorescence intensity at any temperature. It may be concluded from these results that pyrethroids are preferentially located in the interior hydrophobic regions of the lipid bilayer, and that these compounds can disorder hydrocarbon packing in the bilayer core. However, polar headgroups were not disordered, and diffusion of membrane components in the polar headgroup region was not altered.

Similar properties of [35S]t-butylbicyclophosphorothionate receptor and coupled components of the GABA receptor-ionophore complex in brains of human, cow, rat, chicken and fish

No significant differences are evident in the specific binding characteristics of [35S]t-butylbicyclophosphorothionate ([35S]TBPS) to EDTA/water-dialyzed P2 membranes of human, cow, rat, chicken and fish brain. This species similarity includes dissociation constants of 61-77 nM at 37 degrees C, maximum receptor densities of 3-7 pmol/mg protein, and sensitivity to inhibition or displacement by gamma-aminobutyric acid (GABA), two cage convulsants (picrotoxinin and t-butylbicycloorthobenzoate) and the insecticide [1R,cis, alpha S]-cypermethrin, indicating a constancy during vertebrate evolution of the [35S]TBPS binding site and its coupling with other components of the GABA receptor-ionophore complex. As a possible exception, chicken and fish brain membranes appear to be less sensitive than the others to the insecticide alpha-endosulfan. Human and rat preparations are also essentially identical relative to the inhibition of radioligand binding by two GABA mimetics (muscimol and 3-amino-propanesulfonic acid), six other cage convulsants (including examples of three classes of polychlorocycloalkane insecticides), a potent anthelmintic agent (Ivermectin), dimethylbutylbarbiturate, the convulsant benzodiazepine Ro 5-3663, and ethanol. The findings to date with [35S]TBPS and the GABA receptor-ionophore complex in rat brain membranes are therefore generally applicable to human preparations. Cow brain is an appropriate source for large scale preparations in receptor purification studies since it is essentially identical to human and rat preparations in all parameters examined. Species differences in sensitivity to the toxic effects of the convulsants and polychlorocycloalkane insecticides considered are apparently not attributable to receptor site specificity.

Effects of two pyrethroid insecticides on motor activity and the acoustic startle response in the rat

To better characterize the behavioral toxicity of pyrethroid insecticides, comparisons were made of the effects of cismethrin and deltamethrin exposure on motor activity and the acoustic startle response in male Long-Evans rats. Acute dose-effect, acute time course, and 30-day repeated-exposure determinations of 1-hr motor activity were made using figure-eight mazes. The acoustic startle response was measured to a 13-kHz, 120-dB(A), 40-msec tone at each of three background white noise levels (50, 65, and 80 dB). Deltamethrin (0, 2, 6, or 8 mg/kg) or cismethrin (0, 6, 12, 18, or 24 mg/kg) were administered po in 0.2 ml/kg corn oil. Cismethrin and deltamethrin produced similar dosage-dependent decreases in motor activity. The time course of onset and recovery for this decreased activity was rapid (1 to 4 hr) No cumulative effects on motor activity of a 30-day exposure to 2 mg/kg/day deltamethrin or 6 mg/kg/day cismethrin were found. The effects of cismethrin and deltamethrin on the acoustic startle response were dissimilar: deltamethrin produced a dosage-dependent decrease in amplitude and an increase in latency, and cismethrin produced an increase in amplitude and no change in latency. The differential effects of cismethrin and deltamethrin on the acoustic startle response may be related to the contrasting effects previously shown with neurophysiological and/or neurochemical techniques.

Interactions of lindane, toxaphene and cyclodienes with brain-specific t-butylbicyclophosphorothionate receptor

Three major classes of chlorinated hydrocarbon insecticides, i.e., the lindane/hexachlorocyclohexane, toxaphene and aldrin/dieldrin types, are potent, competitive, and stereospecific inhibitors of t-butylbicyclophosphorothionate (TBPS) binding to brain-specific sites, thereby indicating an action at the gamma-aminobutyric acid (GABA)-regulated chloride channel. The most inhibitory and toxic of four isomers of hexachlorocyclohexane is lindane and of greater than 188 components of toxaphene is 2,2,5-endo, 6-exo,8,9,9,10-octachlorobornane. 12-Ketoendrin (IC50 = 36 nM) is twice as active as the most potent previously known inhibitor of TBPS binding and it is also the most inhibitory and toxic of 22 cyclodienes examined. Within each of these three series of polychlorocycloalkanes the mammalian toxicity is closely related to the potency for inhibition of TBPS binding. A modified receptor assay incorporating liver microsomes and reduced nicotinamide-adenine dinucleotide phosphate compensates in part for oxidative detoxification and bioactivation. Specific TBPS binding is reduced in a dose-dependent manner in dieldrin-poisoned rats. DDT, mirex and kepone are not inhibitors of TBPS binding, even at 10 microM.

Effects of pyrethroids on lymphocyte membrane lipid packing order

The effects of pyrethroids on the membrane lipid packing order of murine splenic lymphocytes have been investigated by utilizing the fluorescent probe 1,6-diphenyl-1,3,5-hexatriene. Fluorescence anisotropy and lifetime measurements indicated alterations in the environment of the probe at pyrethroid concentrations near 1 microM. Decreased packing order of membrane lipids was detected for permethrin and cypermethrin by decreased anisotropy values. Permethrin and cypermethrin decreased fluorescence lifetime over a concentration range similar to that causing anisotropy changes. Anisotropy was predominantly increased in the presence of fluvalinate and fenpropathrin, indicating pronounced quenching of fluorescence by these compounds. However, the anisotropy changes in response to fluvalinate and fenpropathrin occurred at concentrations which were similar to effective concentrations of permethrin and cypermethrin. Compared to the other pyrethroids tested, allethrin required approximately 10 fold higher concentrations to alter anisotropy and lifetime values. Effects of pyrethroids on the phase transition of dipalmitoyl phosphatidylcholine liposomes were also studied to give further insight into the membrane interactions of these compounds. Results from the liposomal system indicate that the effects of pyrethroids on membrane order which were observed in the lymphocyte may be generalizable to other cell types.

Stereospecific action of pyrethroid insecticides on the gamma-aminobutyric acid receptor-ionophore complex

The potent alpha-cyano-3-phenoxybenzyl pyrethroids, including cypermethrin, deltamethrin, and fenvalerate, act stereospecifically to inhibit binding to rat brain synaptic membranes of sulfur-35-labeled t-butylbicyclophosphorothionate, a new radioligand for the picrotoxinin binding site. Scatchard analysis indicates that picrotoxinin inhibition of t-butylbicyclophosphorothionate binding is competitive whereas cypermethrin inhibition possibly involves a closely associated site in the gamma-aminobutyric acid receptor-ionophore complex. Studies with 37 pyrethroids reveal an absolute correlation, that is, no false positives or negatives, between mouse intracerebral toxicity and in vitro inhibition: all toxic cyano compounds but none of their nontoxic stereoisomers are inhibitors; cis isomers are more potent than trans isomers as both toxicants and inhibitors; and noncyano pyrethroids are much less potent or are inactive.