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

PBPK modeling to support risk assessment of pyrethroid exposure in French pregnant women

BACKGROUND

Pyrethroids are widely used pesticides and are suspected to affect children's neurodevelopment. The characterization of pyrethroid exposure during critical windows of development, such as fetal development and prenatal life, is essential to ensure a better understanding of pyrethroids potential effects within the concept of Developmental Origins of Health and Disease.

OBJECTIVE

The aim of this study was to estimate maternal exposure of French pregnant women from biomonitoring data and simulate maternal and fetal internal concentrations of 3 pyrethroids (permethrin, cypermethrin and deltamethrin) using a multi-substance pregnancy-PBPK (physiologically based pharmacokinetics) model. The estimated maternal exposures were compared to newly proposed toxicological reference values (TRV) children specific also called draft child-specific reference value to assess pyrethroid exposure risk during pregnancy i.e. during the in utero exposure period.

METHODS

A pregnancy-PBPK model was developed based on an existing adult pyrethroids model. The maternal exposure to each parent compound of pregnant women of the Elfe (French Longitudinal Study since Childhood) cohort was estimated by reverse dosimetry based on urinary biomonitoring data. To identify permethrin and cypermethrin contribution to their common urinary biomarkers of exposure, an exposure ratio based on biomarkers in hair was tested. Finally, exposure estimates were compared to current and draft child-specific reference values derived from rodent prenatal and postnatal exposure studies.

RESULTS

The main contributor to maternal pyrethroid diet intake is cis-permethrin. In blood, total internal concentrations main contributor is deltamethrin. In brain, the major contributors to internal pyrethroid exposure are deltamethrin for fetuses and cis-permethrin for mothers. Risk is identified only for permethrin when referring to the draft child-specific reference value. 2.5% of the population exceeded permethrin draft child-specific reference value.

CONCLUSIONS

A new reverse dosimetry approach using PBPK model combined with human biomonitoring data in urine and hair was proposed to estimate Elfe pregnant population exposure to a pyrethroids mixture with common metabolites.

An adverse outcome pathway-based approach to assess the neurotoxicity by combined exposure to current-use pesticides

Exposure to multiple pesticides in daily life has become an important public health concern. However, the combined effects of pesticide mixtures have not been fully elucidated by the conventional toxicological testing used for individual chemicals. Grouping of chemicals by mode of action using common key events (KEs) in the adverse outcome pathway (AOP) as endpoints could be applied for efficient risk assessment of combined exposure to multiple chemicals. The purpose of this study was to investigate whether exposure to multiple pesticides has synergistic neurotoxic effects on mammalian nervous systems. According to the AOP-based approach, we evaluated the effects of 10 current-use pesticides (4 neonicotinoids, 4 pyrethroids and 2 phenylpyrazoles) on the common KEs in AOPs for neurotoxicity, such as KEs involving mitochondrial and proteolytic functions, in a mammalian neuronal cell model. Our data showed that several pyrethroids and phenylpyrazoles partly shared the effects on several common KEs, including decreases in mitochondrial membrane potential and proteasome activity and increases in autophagy activity. Furthermore, we also found that combined exposure to a type-I pyrethroid permethrin or a type-II pyrethroid deltamethrin and the phenylpyrazole fipronil decreased the cell viability and the benchmark doses much more than either single exposure, indicating that the pair exhibited synergistic effects, since the combination indexes were less than 1. These findings revealed that novel pairs of different classes of pesticides with similar effects on common KEs exhibited synergistic neurotoxicity and provide new insights into the risk assessment of combined exposure to multiple chemicals.

Adverse effects of bifenthrin exposure on neurobehavior and neurodevelopment in a zebrafish embryo/larvae model

Bifenthrin, a third-generation synthetic pyrethroid, is widely used as an agricultural insecticide. However, it can flow into surface and groundwater, leading to adverse consequences such as immunotoxicity, hepatotoxicity, hormone dysregulation, or neurotoxicity. Nevertheless, the entire range of its neurotoxic consequences, particularly in aquatic organisms, remains unclear. In this study, we conducted an extensive examination of how exposure to bifenthrin affects the behavior and nervous system function of aquatic vertebrates, using a zebrafish model and multiple-layered assays. We exposed wild-type and transgenic lines [tg(elavl3:eGFP) and tg(mbp:mGFP)] to bifenthrin from <3 h post-fertilization (hpf) to 120 hpf. Our findings indicate that bifenthrin exposure concentrations of 103.9 and 362.1 μg/L significantly affects the tail-coiling response at 24 hpf and the touch-evoked responses at 72 hpf. Moreover, it has a significant effect on various aspects of behavior such as body contact, distance between subjects, distance moved, and turn angle. We attribute these effects to changes in acetylcholinesterase and dopamine levels, which decrease in a concentration-dependent manner. Furthermore, neuroimaging revealed neurogenesis defects, e.g., shortened brain and axon widths, and demyelination of oligodendrocytes and Schwann cells. Additionally, the transcription of genes related to neurodevelopment (e.g., gap43, manf, gfap, nestin, sox2) were significantly upregulated and neurotransmitters (e.g., nlgn1, drd1, slc6a4a, ache) was significantly downregulated. In summary, our data shows that bifenthrin exposure has detrimental effects on neurodevelopmental and neurotransmission systems in the zebrafish embryo/larvae model.

Critical review and analysis of literature on low dose exposure to chemical mixtures in mammalian in vivo systems

Anthropogenic chemicals are ubiquitous throughout the environment. Consequentially, humans are exposed to hundreds of anthropogenic chemicals daily. Current chemical risk assessments are primarily based on testing individual chemicals in rodents at doses that are orders of magnitude higher than that of human exposure. The potential risk from exposure to mixtures of chemicals is calculated using mathematical models of mixture toxicity based on these analyses. These calculations, however, do not account for synergistic or antagonistic interactions between co-exposed chemicals. While proven examples of chemical synergy in mixtures at low doses are rare, there is increasing evidence that, through non-conformance to current mixture toxicity models, suggests synergy. This review examined the published studies that have investigated exposure to mixtures of chemicals at low doses in mammalian in vivo systems. Only seven identified studies were sufficient in design to directly examine the appropriateness of current mixture toxicity models, of which three showed responses significantly greater than additivity model predictions. While the remaining identified studies were unable to provide evidence of synergistic toxicity, it became apparent that many results of such studies were not always explicable by current mixture toxicity models. Additionally, two data gaps were identified. Firstly, there is a lack of studies where individual chemical components of a complex mixture (>10 components) are tested in parallel to the chemical mixture. Secondly, there is a lack of dose-response data for mixtures of chemicals at low doses. Such data is essential to address the appropriateness and validity of future chemical mixture toxicity models.

In vitro cytotoxicity and genotoxicity of single and combined pesticides used by Bolivian farmers

We previously showed that farmers in Bolivia are exposed to many pesticides, some at elevated levels, and that this was associated with increased risk of genetic damage. To improve the understanding of possible mixture effects, the cytotoxicity and genotoxicity of pesticides were studied in vitro using human liver HepG2 cells. The studied pesticides were 2,4-D, chlorpyrifos, cypermethrin, glyphosate, methamidophos, paraquat, profenofos, and tebuconazole. Three mixtures (U1, U2, and U3) were based on profiles of urinary pesticide metabolites and one mixture on the most frequently used pesticides (S1). The results showed that paraquat and methamidophos were the most cytotoxic pesticides (EC₅₀ ≤0.3 mM). Paraquat, chlorpyrifos, tebuconazole, and the U1, U2, and U3 mixtures, which contained a large proportion of either chlorpyrifos or tebuconazole, significantly increased intracellular ROS levels. Most pesticides activated DNA damage signaling through proteins Chk1 and H2AX. Strongest responses were elicited by paraquat, profenofos, chlorpyrifos, cypermethrin, and the S1 mixture, which contained 25% paraquat. Comet assay revealed significant increases of DNA damage in response to paraquat, cypermethrin, and U2 and S1 mixtures, which contained high levels of cypermethrin and paraquat, respectively. In summary, we showed that the tested pesticides, alone or in mixtures, in general induced oxidative stress and that most pesticides, and especially paraquat and cypermethrin, were genotoxic in HepG2 cells. We could also show that mixtures dominated by these two pesticides displayed a marked genotoxic potency, which agreed with our previous population studies.

Use of subcutaneous transponders to monitor body temperature in laboratory rats

Implantable radiofrequency transponders may be adequate for the characterization of hazardous chemicals targeting body temperature control in experimental animals when colonic probes and automated monitoring systems based on intraperitoneal transmitters are not available, installable or applicable for any reason. In this work, we summarize a series of experiments showing the implantation protocol and utility of rice-grain size transponders to monitor subcutaneous temperature (T_sc) after exposure to pharmacological or toxicological treatments targeting body temperature control in laboratory rats. In addition, to explore the responsiveness of this thermometric system, the influence of physiological activity on T_sc readings was examined by monitoring T_sc after a motor exercise in a RotaRod system. Moreover, we characterized the effects of acute oral administration of the pyrethroid insecticide permethrin (PRM) in corn oil (1 mL/kg) on T_sc. PRM has been previously reported to cause dose-related increases in core temperature after administering oral doses ≥75 mg/kg, with peak effects at 2-4 h in adult rats. We monitored T_sc at 30 min intervals over a 4 h period after exposure to PRM (40-160 mg/kg). PRM caused a moderate increase in T_sc starting at ~3.5 h. Overall, T_sc assays showed minimal animal stress (if any) and rapid animal recovery from transponder implantation, simplicity to collect data, convenient testing room space requirements, and a competitive global cost per animal examined. However, various experimental factors may greatly influence the variability within and between individuals, some of which can be controlled by carefully setting up experimental conditions.

Assessing Combined Effects for Mixtures of Similar and Dissimilar Acting Neuroactive Substances on Zebrafish Embryo Movement

Risk assessment of chemicals is usually conducted for individual chemicals whereas mixtures of chemicals occur in the environment. Considering that neuroactive chemicals are a group of contaminants that dominate the environment, it is then imperative to understand the combined effects of mixtures. The commonly used models to predict mixture effects, namely concentration addition (CA) and independent action (IA), are thought to be suitable for mixtures of similarly or dissimilarly acting components, respectively. For mixture toxicity prediction, one important challenge is to clarify whether to group neuroactive substances based on similar mechanisms of action, e.g., same molecular target or rather similar toxicological response, e.g., hyper- or hypoactivity (effect direction). We addressed this by using the spontaneous tail coiling (STC) of zebrafish embryos, which represents the earliest observable motor activity in the developing neural network, as a model to elucidate the link between the mechanism of action and toxicological response. Our objective was to answer the following two questions: (1) Can the mixture models CA or IA be used to predict combined effects for neuroactive chemical mixtures when the components share a similar mode of action (i.e., hyper- or hypoactivity) but show different mechanism of action? (2) Will a mixture of chemicals where the components show opposing effect directions result in an antagonistic combined effect? Results indicate that mixture toxicity of chemicals such as propafenone and abamectin as well as chlorpyrifos and hexaconazole that are known to show different mechanisms of action but similar effect directions were predictable using CA and IA models. This could be interpreted with the convergence of effects on the neural level leading to either a collective activation or inhibition of synapses. We also found antagonistic effects for mixtures containing substances with opposing effect direction. Finally, we discuss how the STC may be used to amend risk assessment.

Estimating human exposure to pyrethroids' mixtures from biomonitoring data using physiologically based pharmacokinetic modeling

Human biomonitoring data provide evidence to exposure of environmental chemicals. Physiologically based pharmacokinetic (PBPK) modelling together with an adequate exposure scenario allows to transpose measured concentrations of chemicals or their metabolites into exposure levels, as daily intakes. In France, high levels of urinary pyrethroids metabolites have been measured in populations. Our work aims at estimating the exposure of the French ENNS cohort to mixtures of four pyrethroids (deltamethrin, permethrin, cypermethrin, and cyfluthrin) from the urinary concentrations of five pyrethroids' metabolites commonly measured in biomonitoring studies. We developed a modelling approach based on a global toxicokinetic model that accounts for the cumulative exposure to pyrethroids as some of the metabolites can be shared by several parent compounds and for human inter-individual variability in metabolism. The median of the individual daily intakes was estimated to 8.1 ng/kg bw/day for permethrin, 17.7 ng/kg bw/day for cypermethrin, 20.4 ng/kg bw/day for cyfluthrin and 34.3 ng/kg bw/day for deltamethrin leading to similar weights for the pair permethrin and cypermethrin (36%), cyfluthrin (31%) and deltamethrin (33%) to the cumulative exposure. Accounting for human variability enabled to explain some of the variations in the metabolites' levels within the cohort. The cumulative exposure was then weighted by their toxicities towards three neurotoxic effects to calculate margins of exposure (MOE). Low MOE values were always associated with high measured concentrations of metabolites in urine and the lowest MOEs were observed for the autonomic division. No risks associated with reconstructed mixtures of pyrethroids were expected for the ENNS cohort. Our approach is an asset to analyse the biomarkers of exposure to pyrethroids simultaneously and could be easily adapted to any local or national specificities in pyrethroids' exposure or populations.

Effectiveness of melatonin to restore fish brain activity in face of permethrin induced toxicity

Present study demonstrates permethrin induced oxidative damage in fish brain and explores effectiveness of melatonin to ameliorate brain function. Adult female Notopterus notopterus were exposed to nominal permethrin concentrations at 1/20th (0.34 μg/l) and 1/10th (0.68 μg/l) of LC₅₀ for 15 days. The measured permethrin concentrations using gas chromatography (GC-ECD) were 0.28 μg/l and 0.57 μg/l, respectively. Some fish were sacrificed to collect brain tissue after 15 days of exposure. Remaining fish from both groups were administered exogenous melatonin (50 μg/kg, 100 μg/kg body weight) for 7 days and brain tissues were collected. Brain enzymes, ntioxidant factors, HSP70, HSP90, nuclear factor-kappa binding (NFkB), melatonin receptor (MT1R) proteins were measured. Permethrin treatment significantly (P < 0.05) decreased the levels of glutathione and brain enzymes. Malondialdehyde (MDA), xanthine oxidase (XO), HSPs increased at each concentration of permethrin. However, superoxide dismutase, glutathione s-transferase levels increased at low permethrin concentration followed by sharp decrease at higher concentration. Expression of NFkB and MT1R increased significantly (P < 0.05). Melatonin administration reinstated activity of brain enzymes, reduced MDA, XO levels and modulated HSPs. Melatonin also increased expression of NFkB and MT1R. Exogenous melatonin improves oxidative status in permethrin stressed fish brain. Melatonin modulates expression of HSPs that enables brain to become stress tolerant and survive by initiating NFkB translocation. Melatonin could act through melatonin receptor protein to induce synthesis of antioxidant proteins. Therefore the study successfully evaluates the potential of melatonin application for better culture and management of fish against pesticide toxicity.

Impact of pesticide coexposure: an experimental study with binary mixtures of lambda-cyhalothrin (LCT) and captan and its impact on the toxicokinetics of LCT biomarkers of exposure

This study aimed at gaining more insights into the impact of pesticide coexposure on the toxicokinetics of biomarkers of exposure. This was done by conducting an in vivo experimental case-study with binary mixtures of lambda-cyhalothrin (LCT) and captan and by assessing its impact on the kinetic profiles of LCT biomarkers of exposure. Groups of male Sprague-Dawley rats were exposed orally by gavage to LCT alone (2.5 or 12.5 mg/kg bw) or to a binary mixture of LCT and captan (2.5/2.5 or 2.5/12.5 or 12.5/12.5 mg/kg bw). In order to establish the temporal profiles of the main metabolites of LCT, serial blood samples were taken, and excreta (urine and feces) were collected at predetermined intervals up to 48 h post-dosing. Major LCT metabolites were quantified in these matrices: 3-(2-chloro-3,3,3-trifluoroprop-1-enyl)-2,2-dimethyl-cyclopropane carboxylic (CFMP), 3-phenoxybenzoic acid (3-PBA), 4-hydroxy-3-phenoxybenzoic acid (4-OH3PBA). There was no clear effect of coexposure at the low LCT dose on the kinetics of CFMP and 3-PBA metabolites, based on the combined assessment of temporal profiles of these metabolites in plasma, urine and feces; however, plasma levels of 3-PBA were diminished in the coexposed high-dose groups. A significant effect of coexposure on the urinary excretion of 4-OH3PBA was also observed while fecal excretion was not affected. The temporal profiles of metabolites in plasma and in excreta were further influenced by the LCT dose. In addition, the study revealed kinetic differences between metabolites with a faster elimination of 3-PBA and 4-OH3BPA compared to CFMP. These results suggest that the pyrethroid metabolites CFMP and 3-PBA, mostly measured in biomonitoring studies, remain useful as biomarkers of exposure in mixtures, when pesticide exposure levels are below the reference values. However, the trend of coexposure effect observed in the benzyl metabolite pathway (in particular 4-OH3BPA) prompts further investigation.

Relationship Between the Dose Administered, Target Tissue Dose, and Toxicity Level After Acute Oral Exposure to Bifenthrin and Tefluthrin in Young Adult Rats

Most pyrethroid insecticides (PYRs) share a similar primary target site in mammals. However, the potency estimates of the lethal and sublethal effects of these compounds differ up to 103-fold. The aim of this study was to evaluate the relationship between the dose administered, the target tissue dose, and the effect of 2 highly toxic PYRs, tefluthrin (TEF; 0.1-9 mg/kg) and bifenthrin (BIF; 0.5-12 mg/kg), by using the oral route, a corn oil vehicle (1 ml/kg) and subcutaneous temperature (Tsc) monitoring assays in adult rats. The Tsc was determined at 30-min intervals for 5 h (TEF) or 4.5 h (BIF) after dosing. Rats were sacrificed at 6 h after dosing, and BIF and TEF concentrations were determined in blood (Bd), liver (Lv), and cerebellum (Cb) by using a GC-ECD system. The minimal effective dose of BIF (3 mg/kg) affecting Tsc was similar to that found in prior studies using other testing paradigms. Regarding TEF, a very steep relationship between the dose administered and toxicity was observed, with a near-threshold to low-effective range for Tsc at 0.1-6 mg/kg, and a near lethal syndrome at ≥ 7.5 mg/kg. At 6-7.5 mg/kg TEF, the Cb/Bd and Cb/Lv concentration ratios were both > 1. Conversely, for BIF, the Cb concentration was barely over the Bd concentration and the Cb/Lv concentration ratio remained < 1. Our results and previous findings call for more comprehensive consideration to establish the relevance of the distribution into target tissues and the tissue dosimetry for health risks through the exposure to PYRs in humans.

Behavioral, molecular and physiological responses of embryo-larval zebrafish exposed to types I and II pyrethroids

Pyrethroids are potent neurotoxicants that may elicit multiple pathways of toxicity in non-target organisms. Comparative studies on the mechanistic and developmental effects of types I and II pyrethroids against non-target aquatic species are limited. This study assessed the effects of the two pyrethroid types against embryo-larval zebrafish (Danio rerio) at environmentally relevant and laboratory concentrations. Zebrafish embryos were exposed to type-I (permethrin, bifenthrin) and type-II (deltamethrin, λ-cyhalothrin, fenvalerate, esfenvalerate) pyrethroids at 1000, 10, 0.1, 0.01, 0.0 μg/L, starting at 5-h post-fertilization (hpf) through 5-d post-fertilization (dpf) under static exposure conditions. Swimming behavior (distance traveled and velocity) was assessed at 5-dpf. The relative expression of Nrf2a, GST, Casp-9 and p53 mRNA transcripts, carboxyl esterase (CES) activity and total reactive oxygen species (ROS) were measured. The stability of the pyrethroids across 5 days was analyzed. Bifenthrin-(10 μg/L) and esfenvalerate-(1000 μg/L) significantly (p < 0.05) reduced total distance traveled by larvae while 1000 μg/L deltamethrin and λ-cyhalothrin were lethal causing body axis curvature and pericardial edema. At environmentally relevant concentrations-(μg/L) compared to control, permethrin-(0.122) upregulated Nrf2a and Casp-9 expressions while λ-cyhalothrin-(0.053) downregulated Nrf2a and fenvalerate-0.037 downregulated GST. At laboratory concentrations-(μg/L), permethrin-(1000) upregulated Nrf2a, Casp-9 and p53 expressions, bifenthrin-(10) upregulated Casp-9 while fenvalerate-(0.1) and esfenvalerate-(1000) downregulated GST. There was concentration dependent increase in CES activity which correlated positively with total ROS. Pyrethroid concentrations decreased significantly by day 5. This study showed disparity in the mechanistic effects across the pyrethroids types and their instability in aqueous media may underestimate toxicity against non-target aquatic species when exposed in their natural environment.

The immediate influence of deltamethrin on ion transport through rabbit skin. An in vitro study

Deltamethrin can be absorbed into the respiratory tract, the gastrointestinal tract and through the skin. The study was designed to assess the effect of deltamethrin on electrophysiological parameters of rabbit's skin, studied in vitro, to identify the mechanism of action and effects of short-term dermal exposure to deltamethrin. The objective of the study was to investigate changes in electrophysiological parameters after exposure to 0.01 M deltamethrin under unchanged conditions, in the presence of amiloride (sodium transport blocker) and bumetanide (chloride transport blocker). Exposure to deltamethrin reduced the electrophysiological reaction of examined tissue in unchanged conditions and during the sodium reabsorption phase but did not influence the chloride ion secretion phase. The presented data show that the pyrethroide affects transepithelial ion transport in the external layers of the skin. The inhibition of chloride and sodium ions enabled evaluation of the impact of the pesticide on dermal transport.

Single and mixture impacts of two pyrethroids on damselfly predatory behavior and physiological biomarkers

Direct mortality due to toxicity of single pesticide exposure along a concentration gradient, while the most common, is only one important parameter for assessing the effects of pesticide contamination on aquatic ecosystems. Sub-lethal toxicity can induce changes in an organism's behavior and physiology that may have population-level ramifications and consequences for ecosystem health. Additionally, the simultaneous detection of multiple contaminants in monitored watersheds stresses the importance of gaining a greater understanding of the toxicities of combined exposures, particularly at low, environmentally relevant concentrations. Using larvae of the Azure Damselfly (Coenagrion puella), we conducted a combined exposure investigation of two widely-used pyrethroid insecticides presumed to share the same neurotoxic mechanism of action, and estimated their effect on predatory ability, mobility and three physiological biomarkers (Glutathione S-transferase; GST, respiratory electron transport system; ETS, and malondialdehyde; MDA). Deltamethrin exposure (0.065μg/L and 0.13μg/L) was found to reduce the predatory ability, but it did not affect the larvae's mobility. Esfenvalerate exposure (0.069μg/L and 0.13μg/L), on the other hand, induced no significant changes in predatory ability or mobility. The decrease in predatory ability after the combination exposure (0.067μg/L deltamethrin and 0.12μg/L esfenvalerate) did not significantly differ from the impact of the single deltamethrin exposures. Glutathione-S-transferase was induced after single esfenvalerate exposure and the lower deltamethrin concentration exposure, but seemingly inhibited after exposure to the higher concentration of deltamethrin as well as the combination of both pyrethroids. Our data indicate that sub-lethal exposure to deltamethrin reduces predatory ability and suggest that sub-lethal combined exposure to deltamethrin and esfenvalerate inhibits the GST detoxification pathway. These effects can eventually result in a lower emergence of adults from contaminated ponds.

Effects of an environmentally-relevant mixture of pyrethroid insecticides on spontaneous activity in primary cortical networks on microelectrode arrays

Pyrethroid insecticides exert their insecticidal and toxicological effects primarily by disrupting voltage-gated sodium channel (VGSC) function, resulting in altered neuronal excitability. Numerous studies of individual pyrethroids have characterized effects on mammalian VGSC function and neuronal excitability, yet studies examining effects of complex pyrethroid mixtures in mammalian neurons, especially in environmentally relevant mixture ratios, are limited. In the present study, concentration-response functions were characterized for five pyrethroids (permethrin, deltamethrin, cypermethrin, β-cyfluthrin and esfenvalerate) in an in vitro preparation containing cortical neurons and glia. As a metric of neuronal network activity, spontaneous mean network firing rates (MFR) were measured using microelectorde arrays (MEAs). In addition, the effect of a complex and exposure relevant mixture of the five pyrethroids (containing 52% permethrin, 28.8% cypermethrin, 12.9% β-cyfluthrin, 3.4% deltamethrin and 2.7% esfenvalerate) was also measured. Data were modeled to determine whether effects of the pyrethroid mixture were predicted by dose-addition. At concentrations up to 10μM, all compounds except permethrin reduced MFR. Deltamethrin and β-cyfluthrin were the most potent and reduced MFR by as much as 60 and 50%, respectively, while cypermethrin and esfenvalerate were of approximately equal potency and reduced MFR by only ∼20% at the highest concentration. Permethrin caused small (∼24% maximum), concentration-dependent increases in MFR. Effects of the environmentally relevant mixture did not depart from the prediction of dose-addition. These data demonstrate that an environmentally relevant mixture caused dose-additive effects on spontaneous neuronal network activity in vitro, and is consistent with other in vitro and in vivo assessments of pyrethroid mixtures.

From the Cover: Vulnerability of C6 Astrocytoma Cells After Single-Compound and Joint Exposure to Type I and Type II Pyrethroid Insecticides

A primary mode-of-action of all pyrethroid insecticides (PYRs) is the disruption of the voltage-gated sodium channel electrophysiology in neurons of target pests and nontarget species. The neurological actions of PYRs on non-neuronal cells of the nervous system remain poorly investigated. In the present work, we used C6 astrocytoma cells to study PYR actions (0.1-50 μM) under the hypothesis that glial cells may be targeted by and vulnerable to PYRs. To this end, we characterized the effects of bifenthrin (BF), tefluthrin (TF), α-cypermethrin (α-CYP), and deltamethrin (DM) on the integrity of nuclear, mitochondrial, and lysosomal compartments. In general, 24- to 48-h exposures produced concentration-related impairment of cell viability. In single-compound, 24-h exposure experiments, effective concentration (EC)₁₅s 3-(4,5-dimethyl-thiazol-2-yl)-2,5-diphenyl-tetrazolium bromide (MTT assay) were computed as follows (in μM): BF, 16.1; TF, 37.3; α-CYP, 7.8; DM, 5.0. We found concentration-related damage in several C6-cell subcellular compartments (mitochondria, nuclei, and lysosomes) at ≥ 10^-1 μM levels. Last, we examined a mixture of all PYRs (ie, Σ individual EC₁₅) using MTT assays and subcellular analyses. Our findings indicate that C6 cells are responsive to nM levels of PYRs, suggesting that astroglial susceptibility may contribute to the low-dose neurological effects caused by these insecticides. This research further suggests that C6 cells may provide relevant information as a screening platform for pesticide mixtures targeting nervous system cells by expected and unexpected toxicogenic pathways potentially contributing to clinical neurotoxicity.

An analysis of lethal and sublethal interactions among type I and type II pyrethroid pesticide mixtures using standard Hyalella azteca water column toxicity tests

A novel 2-tiered analytical approach was used to characterize and quantify interactions between type I and type II pyrethroids in Hyalella azteca using standardized water column toxicity tests. Bifenthrin, permethrin, cyfluthrin, and lambda-cyhalothrin were tested in all possible binary combinations across 6 experiments. All mixtures were analyzed for 4-d lethality, and 2 of the 6 mixtures (permethrin-bifenthrin and permethrin-cyfluthrin) were tested for subchronic 10-d lethality and sublethal effects on swimming motility and growth. Mixtures were initially analyzed for interactions using regression analyses, and subsequently compared with the additive models of concentration addition and independent action to further characterize mixture responses. Negative interactions (antagonistic) were significant in 2 of the 6 mixtures tested, including cyfluthrin-bifenthrin and cyfluthrin-permethrin, but only on the acute 4-d lethality endpoint. In both cases mixture responses fell between the additive models of concentration addition and independent action. All other mixtures were additive across 4-d lethality, and bifenthrin-permethrin and cyfluthrin-permethrin were also additive in terms of subchronic 10-d lethality and sublethal responses. Environ Toxicol Chem 2016;35:2542-2549. © 2016 SETAC.

Validation of a Rapid and Sensitive UPLC-MS-MS Method Coupled with Protein Precipitation for the Simultaneous Determination of Seven Pyrethroids in 100 µL of Rat Plasma by Using Ammonium Adduct as Precursor Ion

United States Environmental Protection Agency has recommended estimating pyrethroids' risk using cumulative exposure. For cumulative risk assessment, it would be useful to have a bioanalytical method for quantification of one or several pyrethroids simultaneously in a small sample volume to support toxicokinetic studies. Therefore, in the present study, a simple, sensitive and high-throughput ultraperformance liquid chromatography-tandem mass spectrometry method was developed and validated for simultaneous analysis of seven pyrethroids (fenvalerate, fenpropathrin, bifenthrin, lambda-cyhalothrin, cyfluthrin, cypermethrin and deltamethrin) in 100 µL of rat plasma. A simple single-step protein precipitation method was used for the extraction of target compounds. The total chromatographic run time of the method was 5 min. The chromatographic system used a Supelco C18 column and isocratic elution with a mobile phase consisting of methanol and 5 mM ammonium formate in the ratio of 90 : 10 (v/v). Mass spectrometer (API 4000) was operated in multiple reaction monitoring positive-ion mode using the electrospray ionization technique. The calibration curves were linear in the range of 7.8-2,000 ng/mL with correlation coefficients of ≥ 0.99. All validation parameters such as precision, accuracy, recovery, matrix effect and stability met the acceptance criteria according to the regulatory guidelines. The method was successfully applied to the toxicokinetic study of cypermethrin in rats. To the best of our knowledge, this is the first LC-MS-MS method for the simultaneous analysis of pyrethroids in rat plasma. This validated method with minimal modification can also be utilized for forensic and clinical toxicological applications due to its simplicity, sensitivity and rapidity.

Characterization of synergistic embryotoxicity of nickel and buprofezin in zebrafish

Multiple pollutants, usually at low levels, coexist and may interact in the environment. It is therefore important to analyze the toxicity of mixtures of coexisting chemicals to evaluate the potential ecological risk. Concern regarding the co-occurrence and combined bioeffects of heavy metals and organic insecticides in aquatic settings has existed for many years, but a clear understanding of the interactions between and potential combined toxicity of these chemicals remains elusive. In the present study, the combined effects of the heavy metal nickel (NiSO4) and insect growth regulator buprofezin on the induction of embryo toxicity in zebrafish were assessed. By applying nonlinear regression to the concentration-response data with each of the chemicals using the Hill and Langmuir functions and computing the predictions using the model of concentration addition (CA), we confirmed that NiSO4 and buprofezin acted together to produce synergistic embryotoxicity in zebrafish. Subsequently, we further found that the combination of NiSO4 and buprofezin formed a complex that facilitated the uptake of nickel (Ni) and buprofezin by the embryos. Following this, we clarified that an oxidative mechanism of the complex might underlie the synergistic embryotoxicity of NiSO4 and buprofezin.

Evidence for dose-additive effects of a type II pyrethroid mixture. In vitro assessment

Despite the widespread use of pyrethroid insecticides that led to common exposure in the population, few studies have been conducted to quantitatively assess dose-additive effects of pyrethroids using a funcional measure involved in the common toxic mode of action. The aim of this study was to evaluate the potency and efficacy of 6 Type II pyretroids (α-cypermethrin, cyfluthrin, λ-cyhalothrin, deltamethrin, cyphenothrin and esfenvalerate) to evoke induction of both nitric oxide and lipid peroxides levels measured as malondialdehyde in three in vitro models (SH-SY5Y, HepG2 and Caco-2 human cells) as well as to test the hypothesis of dose additivity for mixtures of these same 6 pyrethroids. Concentration-responses for 6 pyrethroids were determined as well as the response to mixtures of all 6 pyrethroids. Additivity was tested assuming a dose-additive model. The human neuroblastoma SH-SY5Y cell line was the most sensitive in vitro model. The rank order of potency for cell SH-SY5Y viability MTT assay was deltamethrin>cyphenothrin>λ-cyhalothrin>cyfluthrin>esfenvalerate>α-cypermethrin. When 6 pyrethroids were present in the mixture at an equitoxic mixing ratio, the action on nitric oxide (NO) and lipid peroxides measured as malondialdehyde (MDA) production was consistent with a dose-additive model. The results of the present study are consistent with previous reports of additivity of pyrethroids in vivo e in vitro.

Characteristics and magnitude of acute pesticide-related illnesses and injuries associated with pyrethrin and pyrethroid exposures--11 states, 2000-2008

BACKGROUND

Excluding disinfectants, pyrethrins and pyrethroids are the pesticides used most commonly in and around homes. Respiratory effects and paresthesia are among the concerns about pyrethrin/pyrethroid exposures.

METHODS

Acute pesticide-related illness/injury cases were identified from the Sentinel Event Notification System for Occupational Risks-Pesticides Program and the California Department of Pesticide Regulation from 2000-2008. Characteristics and incidence rates were determined for acute pyrethrin/pyrethroid-related illness/injury cases. Logistic regression analyses were performed to determine odds of respiratory and dermal symptoms in persons with illness/injury following pyrethrin/pyrethroid exposure compared to persons with illness/injury following exposure to other pesticides.

RESULTS

A total of 4,974 cases of acute pyrethrin/pyrethroid-related illness were identified. Incidence rates increased over time, reaching 8 cases/million population in 2008. The majority of cases were low severity (85%) and 34% were work-related. Respiratory effects were the most common symptoms reported (48%). Risk of acute respiratory effects were significantly elevated among persons exposed only to pyrethrins (adjusted odds ratio [aOR] 1.79; 95% confidence interval [95% CI]: 1.49-2.16), only to pyrethroids (aOR 1.99 95% CI: 1.77-2.24), to a mixture of pyrethroids (aOR 2.36; 95% CI: 1.99-2.81) or to a mixture containing both pyrethrins and pyrethroids (aOR 2.99; 95% CI: 2.33-3.84) compared to those with illness arising from exposure to other pesticides. The most common factors contributing to pyrethrin/pyrethroid-related illness included exposure from spills/splashes, improper storage, and failure to evacuate during pesticide application.

CONCLUSIONS

The magnitude of acute pyrethrin/pyrethroid-related illness/injury is relatively low but is increasing. As such, additional measures to prevent them are needed.

An empirical approach to sufficient similarity: combining exposure data and mixtures toxicology data

When assessing risks posed by environmental chemical mixtures, whole mixture approaches are preferred to component approaches. When toxicological data on whole mixtures as they occur in the environment are not available, Environmental Protection Agency guidance states that toxicity data from a mixture considered "sufficiently similar" to the environmental mixture can serve as a surrogate. We propose a novel method to examine whether mixtures are sufficiently similar, when exposure data and mixture toxicity study data from at least one representative mixture are available. We define sufficient similarity using equivalence testing methodology comparing the distance between benchmark dose estimates for mixtures in both data-rich and data-poor cases. We construct a "similar mixtures risk indicator"(SMRI) (analogous to the hazard index) on sufficiently similar mixtures linking exposure data with mixtures toxicology data. The methods are illustrated using pyrethroid mixtures occurrence data collected in child care centers (CCC) and dose-response data examining acute neurobehavioral effects of pyrethroid mixtures in rats. Our method shows that the mixtures from 90% of the CCCs were sufficiently similar to the dose-response study mixture. Using exposure estimates for a hypothetical child, the 95th percentile of the (weighted) SMRI for these sufficiently similar mixtures was 0.20 (i.e., where SMRI <1, less concern; >1, more concern).

Impaired structural and functional development of cerebellum following gestational exposure of deltamethrin in rats: role of reelin

Reelin is an extracellular matrix molecule that is involved in the normal development of the cerebellar lamination, Bergmann glial fibres alignment, Purkinje cell monolayer arrangement and granule cell migration. In this study, we have examined the effects of maternal exposure of deltamethrin (DLT), a type II pyrethroid insecticide, on the structural and functional development of rat cerebellum during postnatal life. DLT (0.75 mg/kg body weight, intraperitoneally dissolved in dimethylsulphoxide) was administered in timed pregnant rats during two different gestational time periods, i.e. gestational days of 7-10 and 11-14, respectively. In DLT exposed rats, a significant overexpression of reelin was observed in the cells of the external granule cell layer (EGL) and internal granule cell layer along with an ectopic expression of reelin in the EGL as well as in the migrating granule cells just below the EGL, revealing an arrest of granule cell migration in this zone. Mis-orientation and hypertrophy of the Bergmann glial fibres further hampered the journey of the granule cells to their final destination. Possibly reelin overexpression also caused misalignment of the Purkinje cells and inhibited the neurite growth leading to a significant decrease in the spine density, main dendritic length and width of the dendritic arbour. Thus, it is proposed that the DLT exerts its neurotoxic effects possibly via the intracellular accumulation and low release of reelin leading to an impaired granule cell and Purkinje cell migration, inhibition of neurite outgrowth and reduced spine density. Such impaired cerebellar development leads to motor coordination deficits.

Critical consideration of the multiplicity of experimental and organismic determinants of pyrethroid neurotoxicity: a proof of concept

Pyrethroids (PYR) are pesticides with high insecticidal activity that may disrupt neuronal excitability in target and nontarget species. The accumulated evidence consistently showed that this neurophysiologic action is followed by alterations in motor, sensorimotor, neuromuscular, and thermoregulatory responses. Nevertheless, there are some equivocal results regarding the potency of PYR in lab animals. The estimation of potency is an important step in pesticide chemical risk assessment. In order to identify the variables influencing neurobehavioral findings across PYR studies, evidence on experimental and organismic determinants of acute PYR-induced neurotoxicity was reviewed in rodents. A comprehensive analysis of these studies was conducted focusing on test material and dosing conditions, testing conditions, animal models, and other determinants such as testing room temperature. Variations in the severity of the neurotoxicity, under lab-controlled conditions, was explained based upon factors including influence of animal species and age, test material features such as chemical structure and stereochemistry, and dosing conditions such as vehicle, route of exposure, and dose volume. If not controlled, the interplay of these factors may lead to large variance in potency estimation. This review examined the scope of acute toxicological data required to determine the safety of pesticide products, and factors and covariates that need to be controlled in order to ensure that predictivity and precaution are balanced in a risk assessment process within a reasonable time-frame, using acute PYR-induced neurotoxicity in rodents as an exemplar.

Increasing use of pyrethroids in Canadian households: should we be concerned?

Pyrethroids are a class of plant-derived insecticides and their man-made analogues that are increasingly applied in Canada as first choice for pest control in many agricultural and residential settings. Their popularity is partly due to their alleged safety compared to the older organochlorine and organophosphate insecticides. Application of pyrethroids is expanding because of recent increases in the level of pest infestations--such as bed bugs--and the decreased susceptibility of target species to many pest control products. Pyrethroid residues have been documented in homes, child care centres and food. While pyrethroids are considered of low health risk for humans, their increased use is of concern. Our current understanding of the adverse effects of pyrethroids derives mainly from studies of short-term effects in laboratory animals, case reports of self- and accidental poisonings, and high-dose occupational exposures, for which the levels and formulations of pyrethroid products differ from those relevant for long-term exposure in the general population. The available data suggest that the reproductive and nervous systems, endocrine signalling pathways, and early childhood development may be targets for adverse effects in the case of repeated exposure to pyrethroid formulations. Given uncertainty about the existence of long-term health effects of exposure to pyrethroids, particularly under realistic scenarios, we should be cautious when promoting pyrethroid products as safe methods for pest control.

Increasing use of pyrethroids in Canadian households: should we be concerned?

Pyrethroids are a class of plant-derived insecticides and their man-made analogues that are increasingly applied in Canada as first choice for pest control in many agricultural and residential settings. Their popularity is partly due to their alleged safety compared to the older organochlorine and organophosphate insecticides. Application of pyrethroids is expanding because of recent increases in the level of pest infestations--such as bed bugs--and the decreased susceptibility of target species to many pest control products. Pyrethroid residues have been documented in homes, child care centres and food. While pyrethroids are considered of low health risk for humans, their increased use is of concern. Our current understanding of the adverse effects of pyrethroids derives mainly from studies of short-term effects in laboratory animals, case reports of self- and accidental poisonings, and high-dose occupational exposures, for which the levels and formulations of pyrethroid products differ from those relevant for long-term exposure in the general population. The available data suggest that the reproductive and nervous systems, endocrine signalling pathways, and early childhood development may be targets for adverse effects in the case of repeated exposure to pyrethroid formulations. Given uncertainty about the existence of long-term health effects of exposure to pyrethroids, particularly under realistic scenarios, we should be cautious when promoting pyrethroid products as safe methods for pest control.

Urinary concentrations of pyrethroid metabolites in the convenience sample of an urban population of Northern Poland

Urinary concentrations of pyrethroid metabolites were measured in the first void urine samples collected from 132 healthy people living in the Gdańsk region of Northern Poland in 2010 and 2011. Four metabolites of synthetic pyrethroids: cis- and trans-3-(2,2-dichlorovinyl)-2,2-dimethylcyclopropane-1-carboxylic acids (cis-, trans-Cl2CA), cis-3-(2,2-dibromovinyl)-2,2-dimethylcyclopropane-1-carboxylic acid (Br2CA) and 3-phenoxybenzoic acid (3-PBA) were simultaneously liquid-liquid extracted, derivatized with hexafluoroisopropanol and analyzed by a gas chromatography ion-trap mass spectrometry. All the analytes were detected and quantified in the samples with various frequency, 3-phenoxybenzoic being the most often (80%) and the others less frequently (7-11%). Distribution of 3-PBA concentrations followed log-normal model, the mean concentration of 3-phenoxybenzoic acid: 0.393 μg/L (0.327 μg/g creatinine) is similar to those of the other general populations in various regions of the world. Neither sex nor age were predictors of urinary 3-PBA. Our findings suggest wide exposure to pyrethroid insecticides in the Polish general population. There is a continuous need to further study the exposure to synthetic pyrethroids among the general population since there is a strong, increasing trend in their usage.

Impact of chemical proportions on the acute neurotoxicity of a mixture of seven carbamates in preweanling and adult rats

Statistical design and environmental relevance are important aspects of studies of chemical mixtures, such as pesticides. We used a dose-additivity model to test experimentally the default assumptions of dose additivity for two mixtures of seven N-methylcarbamates (carbaryl, carbofuran, formetanate, methomyl, methiocarb, oxamyl, and propoxur). The best-fitting models were selected for the single-chemical dose-response data and used to develop a combined prediction model, which was then compared with the experimental mixture data. We evaluated behavioral (motor activity) and cholinesterase (ChE)-inhibitory (brain, red blood cells) outcomes at the time of peak acute effects following oral gavage in adult and preweanling (17 days old) Long-Evans male rats. The mixtures varied only in their mixing ratios. In the relative potency mixture, proportions of each carbamate were set at equitoxic component doses. A California environmental mixture was based on the 2005 sales of each carbamate in California. In adult rats, the relative potency mixture showed dose additivity for red blood cell ChE and motor activity, and brain ChE inhibition showed a modest greater-than additive (synergistic) response, but only at a middle dose. In rat pups, the relative potency mixture was either dose-additive (brain ChE inhibition, motor activity) or slightly less-than additive (red blood cell ChE inhibition). On the other hand, at both ages, the environmental mixture showed greater-than additive responses on all three endpoints, with significant deviations from predicted at most to all doses tested. Thus, we observed different interactive properties for different mixing ratios of these chemicals. These approaches for studying pesticide mixtures can improve evaluations of potential toxicity under varying experimental conditions that may mimic human exposures.

Biogeographical analysis of chemical co-occurrence data to identify priorities for mixtures research

A challenge with multiple chemical risk assessment is the need to consider the joint behavior of chemicals in mixtures. To address this need, pharmacologists and toxicologists have developed methods over the years to evaluate and test chemical interaction. In practice, however, testing of chemical interaction more often comprises ad hoc binary combinations and rarely examines higher order combinations. One explanation for this practice is the belief that there are simply too many possible combinations of chemicals to consider. Indeed, under stochastic conditions the possible number of chemical combinations scales geometrically as the pool of chemicals increases. However, the occurrence of chemicals in the environment is determined by factors, economic in part, which favor some chemicals over others. We investigate methods from the field of biogeography, originally developed to study avian species co-occurrence patterns, and adapt these approaches to examine chemical co-occurrence. These methods were applied to a national survey of pesticide residues in 168 child care centers from across the country. Our findings show that pesticide co-occurrence in the child care center was not random but highly structured, leading to the co-occurrence of specific pesticide combinations. Thus, ecological studies of species co-occurrence parallel the issue of chemical co-occurrence at specific locations. Both are driven by processes that introduce structure in the pattern of co-occurrence. We conclude that the biogeographical tools used to determine when this structure occurs in ecological studies are relevant to evaluations of pesticide mixtures for exposure and risk assessment.

Additivity of pyrethroid actions on sodium influx in cerebrocortical neurons in primary culture

BACKGROUND

Pyrethroid insecticides bind to voltage-gated sodium channels and modify their gating kinetics, thereby disrupting neuronal function. Although previous work has tested the additivity of pyrethroids in vivo, this has not been assessed directly at the primary molecular target using a functional measure.

OBJECTIVES

We investigated the potency and efficacy of 11 structurally diverse food-use pyrethroids to evoke sodium (Na+) influx in neurons and tested the hypothesis of dose additivity for a mixture of these same 11 compounds.

METHODS

We determined pyrethroid-induced increases in Na+ influx in primary cultures of cerebrocortical neurons using the Na+-sensitive dye sodium-binding benzofuran isophthalate (SBFI). Concentration-dependent responses for 11 pyrethroids were determined, and the response to dilutions of a mixture of all 11 compounds at an equimolar mixing ratio was assessed. Additivity was tested assuming a dose-additive model.

RESULTS

Seven pyrethroids produced concentration-dependent, tetrodotoxin-sensitive Na+ influx. The rank order of potency was deltamethrin > S-bioallethrin > β-cyfluthrin > λ-cyhalothrin > esfenvalerate > tefluthrin > fenpropathrin. Cypermethrin and bifenthrin produced modest increases in Na+ influx, whereas permethrin and resmethrin were inactive. When all 11 pyrethroids were present at an equimolar mixing ratio, their actions on Na+ influx were consistent with a dose-additive model.

CONCLUSIONS

These data provide in vitro relative potency and efficacy measurements for 7 pyrethroid compounds in intact mammalian neurons. Despite differences in individual compound potencies, we found the action of a mixture of all 11 pyrethroids to be additive when we used an appropriate statistical model. These results are consistent with a previous report of the additivity of pyrethroids in vivo.

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.