Regulatory and research issues related to cholinesterase inhibition

Current status and future directions for a neurotoxicity hazard assessment framework that integrates in silico approaches

Neurotoxicology is the study of adverse effects on the structure or function of the developing or mature adult nervous system following exposure to chemical, biological, or physical agents. The development of more informative alternative methods to assess developmental (DNT) and adult (NT) neurotoxicity induced by xenobiotics is critically needed. The use of such alternative methods including in silico approaches that predict DNT or NT from chemical structure (e.g., statistical-based and expert rule-based systems) is ideally based on a comprehensive understanding of the relevant biological mechanisms. This paper discusses known mechanisms alongside the current state of the art in DNT/NT testing. In silico approaches available today that support the assessment of neurotoxicity based on knowledge of chemical structure are reviewed, and a conceptual framework for the integration of in silico methods with experimental information is presented. Establishing this framework is essential for the development of protocols, namely standardized approaches, to ensure that assessments of NT and DNT based on chemical structures are generated in a transparent, consistent, and defendable manner.

Positron emission tomography evaluation of oxime countermeasures in live rats using the tracer O-(2-[18 F]fluoroethyl)-O-(p-nitrophenyl)methylphosphonate [18 F]-VXS

Oxime antidotes regenerate organophosphate-inhibited acetylcholinesterase (AChE). Although they share a common mechanism of AChE reactivation, the rate and amount of oxime that enters the brain are critical to the efficacy, a process linked to the oxime structure and charge. Using a platform based on the organophosphate [¹⁸ F]-VXS as a positron emission tomography tracer for active AChE, the in vivo distribution of [¹⁸ F]-VXS was evaluated after an LD₅₀ dose (250 μg/kg) of the organophosphate paraoxon (POX) and following oximes as antidotes. Rats given [¹⁸ F]-VXS tracer alone had significantly higher radioactivity (two- to threefold) in the heart and lung than rats given LD₅₀ POX at 20 or 60 min prior to [¹⁸ F]-VXS. When rats were given LD₅₀ POX followed by 2-PAM (cationic), RS194b (ionizable), or monoisonitrosoacetone (MINA) (neutral), central nervous system (CNS) radioactivity returned to levels at or above untreated naive rats (no POX), whereas CNS radioactivity did not increase in rats given the dication oximes HI-6 or MMB-4. MINA showed a significant, pairwise increase in CNS brain radioactivity compared with POX-treated rats. This new in vivo dynamic platform using [¹⁸ F]-VXS tracer measures and quantifies peripheral and CNS relative changes in AChE availability after POX exposure and is suitable for comparing oxime delivery and AChE reactivation in rats.

Neurotoxic and respiratory effects of human use drugs on a Neotropical fish species, Phalloceros harpagos

Pharmaceutical drugs are usually and continuously carried to the aquatic environment in different ways. Thus, they are pseudo-persistent in the environment, and they may exert deleterious effects on aquatic organisms. The objective of the present study was to investigate the acute and chronic effects of two widely used pharmaceutical drugs, paracetamol (analgesic and antipyretic) and propranolol (β-blocker) on the activity of specific biomarkers (namely cholinesterase enzymes and lactate dehydrogenase) of the neotropical fish Phalloceros harpagos. The obtained results indicate an inhibition of the activity of the enzyme lactate dehydrogenase (LDH) after acute exposure to paracetamol, and an increase in cholinesterase activity in acutely propranolol-exposed fish. Chronic exposure to both drugs did not modify the enzymatic activities. Such short-term changes in enzymatic activities may be harmful to organisms, altering the preferential pathway of energy metabolism, and may induce behavioral changes that may compromise prey capture and predator escape, and in the longer term may induce population declines.

Behavior and histopathology as biomarkers for evaluation of the effects of paracetamol and propranolol in the neotropical fish species Phalloceros harpagos

Pharmaceutical drugs in the aquatic environment can induce adverse effects on nontarget organisms. This study aimed to assess the short-term effects of sublethal concentrations of both paracetamol and propranolol on the fish Phalloceros harpagos, specifically light/dark preference, swimming patterns, skin pigmentation, histopathology, and liver glycogen levels. Fish were acutely exposed to sublethal concentrations of both paracetamol (0.008, 0.08, 0.8, 8, 80 mg L^-1) and propranolol (0.0001, 0.001, 0.01, 0.1, 1 mg L^-1) under controlled conditions. For scototaxis, a significant preference for the dark compartment was observed for the group exposed to the highest concentration of paracetamol (80 mg L^-1). Propranolol exposure significantly altered the swimming pattern, especially in fish exposed to the 0.001 mg L^-1 concentration. Pigmentation was reduced in propranolol-exposed fish (0.1, 1 mg L^-1). The lowest concentration of propranolol (0.0001 mg L^-1) induced a decrease of histochemical reaction for hepatic glycogen. These data demonstrate that pharmaceuticals can induce sublethal effects in nontarget organisms, even at low concentrations, compromising specific functions of the individual with ecological relevance, such as energy balance and behavior.

Oxidative stress and damage to erythrocytes in patients with chronic obstructive pulmonary disease--changes in ATPase and acetylcholinesterase activity

The study indicates, for the first time, the changes in both ATPase and AChE activities in the membrane of red blood cells of patients diagnosed with COPD. Chronic obstructive pulmonary disease (COPD) is one of the most common and severe lung disorders. We examined the impact of COPD on redox balance and properties of the membrane of red blood cells. The study involved 30 patients with COPD and 18 healthy subjects. An increase in lipid peroxidation products and a decrease in the content of -SH groups in the membrane of red blood cells in patients with COPD were observed. Moreover, an increase in the activity of glutathione peroxidase and a decrease in superoxide dismutase, but not in catalase activity, were found as well. Significant changes in activities of erythrocyte membrane enzymes in COPD patients were also evident demonstrated by a considerably lowered ATPase activity and elevated AChE activity. Changes in the structure and function of red blood cells observed in COPD patients, together with changes in the activity of the key membrane enzymes (ATPases and AChE), can result from the imbalance of redox status of these cells due to extensive oxidative stress induced by COPD disease.

Use of in vitro data in developing a physiologically based pharmacokinetic model: Carbaryl as a case study

In vitro-derived information has been increasingly used to support and improve human health risk assessment for exposure to chemicals. Physiologically based pharmacokinetic (PBPK) modeling is a key component in the movement toward in vitro-based risk assessment, providing a tool to integrate diverse experimental data and mechanistic information to relate in vitro effective concentrations to equivalent human exposures. One of the challenges, however, in the use of PBPK models for this purpose has been the need for extensive chemical-specific parameters. With the remarkable advances in in vitro methodologies in recent years, in vitro-derived parameters can now be easily incorporated into PBPK models. In this study we demonstrate an in vitro data based parameterization approach to develop a physiologically based pharmacokinetic and pharmacodynamic (PBPK/PD) model, using carbaryl as a case study. In vitro experiments were performed to provide the chemical-specific pharmacokinetic (PK) and pharmacodynamic (PD) parameters for carbaryl in the PBPK model for this compound. Metabolic clearance and cholinesterase (ChE) interaction parameters for carbaryl were measured in rat and human tissues. These in vitro PK and PD data were extrapolated to parameters in the whole body PBPK model using biologically appropriate scaling. The PBPK model was then used to predict the kinetics and ChE inhibition dynamics of carbaryl in vivo. This case study with carbaryl provides a reasonably successful example of utilizing the in vitro to in vivo extrapolation (IVIVE) approach for PBPK model development. This approach can be applied to other carbamates with an anticholinesterase mode of action as well as to environmental chemicals in general with further refinement of the current shortcomings in the approach. It will contribute to minimizing the need for in vivo human data for PBPK model parameterization and evaluation in human risk assessments.

Adverse Impact of Insecticides on the Health of Palestinian Farm Workers in the Gaza Strip: A Hematologic Biomarker Study

To evaluate the health impact of insecticides on Palestinian farm workers in the Gaza Strip, the study assessed biomarkers in farm workers who used organophosphorus insecticides. Serum cholinesterase and complete blood count were determined before and after spraying of organophosphorus insecticides. Burning sensations in eyes/face (62.5%), itching/skin irritation (37.5%), and chest symptoms (29.2%) were reported. Serum butyrylcholinesterase (SBuChE) was significantly decreased at the end of the work day. Burning sensations in eyes/face and skin rash were significantly associated with inhibition of SBuChE activity (p < 0.05). Younger workers were more affected. Leukocyte and platelet counts were increased and hemoglobin decreased significantly, reflecting acute poisoning. Monitoring of SBuChE and hematologic parameters of farm workers could be useful to predict and prevent health hazards of pesticides.

The use of cholinesterases in ecotoxicology

Cholinesterase (ChE) is one of the most employed biomakers in environmental analysis. Among ChEs, potentially the most significant in environmental terms is acetylcholinesterase (AChE), an enzymatic form that terminates the nerve impulse . Because of its physiological role, Ache has long been considered a highly specific biomarker for organisms exposed to anticholinesterasic agents, primarily agro-chemicals (organophosphate and carbamate pesticides). The effects of these pesticides depends upon their selective inhibition of AChE. Because large amounts of such pesticides are employed, it is plausible that they exert neurotoxic effects on some non-target species. Therefore, AChE is among the most valuable of diagnostic tools that can be used to verify exposure to such chemical agents. It is well known that assays are available for use quantifying AChE in multiple tissues of several test organisms. Enzymes other than AChE (e.g., butyrylcholinesterase and carboxylesterases) have also been used as putative markers for detecting the environmental presence of contaminating compounds. Researchers must use a step-by-step approach to identify the most prominent cholinesterasic form present in a given species, so that this form can be distinguished from others that may interfere with its use. Such fundamental work must be completed prior to using ChEs for any monitoring to assess for anticholinesterasic effects. Despite massive employment in environmental analysis, using ChE inhibition as an endpoint or effect criterion has been unsettled by the discovery the ChEs may interact in the environmental in previously unknown ways. Several chemicals, in addition to anticholinesterasic pesticides, are now known to inhibit ChE activity. Such chemical include detergents, metals, and certain organic compounds such as hydrocarbons. The situation is made worse, because the literature is contradictory as to the ability of such chemicals and elements to interact with ChEs. Some results indicate that ChE inhibition by metals, detergents , and complex mixtures do not or are unlikely to occur. These problems and contradictions are addressed in this review. It is purpose in this review to address the following practical issues related to the ChEs: 1. The situation and organisms in which ChEs have been employed as biomarkers in laboratory trials, and the need to fully characterize these enzymatic forms before they are used for environmental assessment purposes. 2. The ways in which ChEs have been used in field monitoring, and the potential for use of others complimentary markers to diagnose organophosphate exposure, and how drawbacks (such as the absence of reference values) can be overcome. 3. What requirements must be satisfied prior implementing the use of ChEs as biomarkers in species not yet studied. 4. How direct linkages have been established between ChE inhibition and effects from inhibition observed at higher levels of integration (e.g., behavioral changes and population effects, or others indices of ecological relevance). 5. The potential for ChE inhibition to be applied as an effective parameter of toxicity to detect for the environmental presence of compounds other than the organo-phosphate and carbamate pesticides, and the limitations associated therewith.

Effects of exposure to oxamyl, carbofuran, dichlorvos, and lindane on acetylcholinesterase activity in the gills of the Pacific oyster Crassostrea gigas

Acetylcholinesterase (AChE) activity has been used to test the exposure of mollusk bivalves to pesticides and other pollutants. The Pacific oyster Crassostrea gigas is a species with a worldwide distribution, and it has a high commercial value. The use of this species as a bioindicator in the marine environment, and the use of measurements of AChE activity in tissues of C. gigas require prior evaluation of organisms exposed to several toxic compounds in the laboratory. In our study, the effects of pesticides on AChE activity in the gills and mantle tissues of C. gigas were analyzed by exposing animals to organophosphate (dichlorvos), carbamate (carbofuran and oxamyl), and organochlorine (lindane) pesticides. Adult Pacific oysters were exposed to several concentrations (0.1-200 microM) of dichlorvos, carbofuran, and oxamyl for 96 h, and lindane (1.0 and 2.5 microM) was applied for 12 days. In gill tissues, all pesticides analyzed caused a decrease in AChE activity when compared to the control unexposed group. The mean inhibition concentration (IC(50)) values were determined for dichlorvos, carbofuran, and oxamyl pesticides. Dichlorvos had the highest toxic effect, with an IC(50) of 1.08 microM; lesser effects were caused by oxamyl and carbofuran, with IC(50)s of 1.67 and 3.03 microM, respectively. This study reports the effects of pesticides with several chemical structures and validates measurement of AChE activity in the gill tissues of C. gigas for use in environmental evaluations or food quality tests.

Effects of paraoxon and ethylparathion on choline oxidase from Alcaligenes species: Inhibition and denaturation

The kinetics and thermodynamics of the effects of paraoxon (POX) and ethylparathion (EPA) on choline oxidase (ChOx) were studied. Lineweaver-Burk plots of initial velocity data showed a parallel pattern indicating uncompetitive inhibition versus choline. The inhibition constant (K(I)) obtained from the secondary plots for POX and EPA were 0.14+/-0.01 and 0.48+/-0.05 mM, respectively, suggesting that POX is a more potent inhibitor of ChOx than EPA. UV absorption was used to monitor the denaturation of ChOx by POX and EPA. A decrease in FAD fluorescence associated with the interaction of POX and EPA with ChOx suggested a tertiary structural change. Interaction of the enzyme molecule with POX or EPA resulted in inhibition and subsequently denaturation of the enzyme. The results indicate that inhibition and denaturation of the enzyme by POX and EPA are linked, but not parallel events, with inhibition occurring at lower concentrations with respect to denaturation. This suggests that the loss of initial velocity of the enzyme is an active site specific effect and not due to global conformational changes induced by the inhibitors.

Determination of chlorpyrifos metabolites in human urine by reversed-phase/weak anion exchange liquid chromatography–electrospray ionisation–tandem mass spectrometry

A liquid chromatography-electrospray ionisation-tandem mass spectrometry (LC-ESI-MS/MS) method for the quantification of major chlorpyrifos (CP) metabolites, i.e. diethyl thiophosphate (DETP), diethyl phosphate (DEP), and 3,5,6-trichloro-2-pyridinol (TCP), in human urine was developed. Simultaneous separation of the parent compound and its primary biotransformation products was achieved within 20 min in gradient elution mode employing a mixed-mode reversed-phase/weak anion exchange (RP/WAX) separation principle. The analytical method was developed for a toxicokinetic study of an acute poisoning incidence with a CP containing pesticide formulation. An initial mass spectrometric screening performed with unprocessed urine samples revealed that CP is not excreted unchanged by the kidney. Hence, the quantitative assay was validated for DETP (quantifier transition: m/z 169-->95, qualifier transition: m/z 169-->141), DEP (m/z 153-->79, 153-->125), and TCP (m/z 196-->35, 198-->35) taking dibutyl phosphate (DBP) (m/z 209-->79, 209-->153) as internal standard. Clean-up of urine samples prior to LC-ESI-MS/MS analysis was carried out by a liquid-liquid extraction step with a mixture of ethylacetate and acetonitrile (70:30; v/v). Linearity was observed between 0.25 and 75 mgL(-1), and the signal-to-noise ratio at 0.25 mgL(-1) was better than six for the individual analytes. Recoveries, precision, and accuracies were all adequate across the validated range of 1-75 mgL(-1) for the present toxicological case study.

Plasma acetylcholinesterase as a biomarker of triazophos neurotoxicity in young and adult rats

The organophosphate pesticides exhibit their action by inhibiting acetylcholinesterase (AChE) enzyme in central and peripheral nervous system. They are known to affect the young animals to a greater extent, as their developing brain is more susceptible to their toxic effects. Besides inactivating acetylcholine at synaptic terminals AChE also plays an important role in neuronal growth and differentiation. A reduction in AChE activity in plasma has no known physiological function in causing brain or tissue damage, but if a good correlation between brain and plasma AChE inhibition exists, then circulating plasma AChE can be used as a reliable marker for detection of cholinesterase inhibitors. Therefore, the present investigation was designed to differentiate age and gender related neurotoxicity of an organophosphate pesticide-triazophos and to explore whether plasma AChE can serve as a biomarker of its neurotoxicity in young, i.e. post natal days 20 (PND 20) and adult rats i.e. post natal days 90 (PND 90) after single intraperitoneal administration in different doses.

Structure-based thresholds of toxicological concern (TTC): guidance for application to substances present at low levels in the diet

The threshold of toxicological concern (TTC) is a pragmatic risk assessment tool that is based on the principle of establishing a human exposure threshold value for all chemicals, below which there is a very low probability of an appreciable risk to human health. The concept that there are levels of exposure that do not cause adverse effects is inherent in setting acceptable daily intakes (ADIs) for chemicals with known toxicological profiles. The TTC principle extends this concept by proposing that a de minimis value can be identified for many chemicals, in the absence of a full toxicity database, based on their chemical structures and the known toxicity of chemicals which share similar structural characteristics. The establishment and application of widely accepted TTC values would benefit consumers, industry and regulators. By avoiding unnecessary toxicity testing and safety evaluations when human intakes are below such a threshold, application of the TTC approach would focus limited resources of time, cost, animal use and expertise on the testing and evaluation of substances with the greatest potential to pose risks to human health and thereby contribute to a reduction in the use of animals. An Expert Group of the European branch of the International Life Sciences Institute-ILSI Europe-has examined the TTC principle for its wider applicability in food safety evaluation. The Expert Group examined metabolism and accumulation, structural alerts, endocrine disrupting chemicals and specific endpoints, such as neurotoxicity, teratogenicity, developmental toxicity, allergenicity and immunotoxicity, and determined whether such properties or endpoints had to be taken into consideration specifically in a step-wise approach. The Expert Group concluded that the TTC principle can be applied for low concentrations in food of chemicals that lack toxicity data, provided that there is a sound intake estimate. The use of a decision tree to apply the TTC principle is proposed, and this paper describes the step-wise process in detail. Proteins, heavy metals and polyhalogenated-dibenzodioxins and related compounds were excluded from this approach. When assessing a chemical, a review of prior knowledge and context of use should always precede the use of the TTC decision tree. The initial step is the identification and evaluation of possible genotoxic and/or high potency carcinogens. Following this step, non-genotoxic substances are evaluated in a sequence of steps related to the concerns that would be associated with increasing intakes. For organophosphates a TTC of 18microg per person per day (0.3 microg/kg bw/day) is proposed, and when the compound is not an OP, the TTC values for the Cramer structural classes III, II and I, with their respective TTC levels (e.g. 1800, 540 and 90 microg per person per day; or 30, 9 and 1.5 microg/kg bw /day), would be applied sequentially. All other endpoints or properties were shown to have a distribution of no observed effect levels (NOELs) similar to the distribution of NOELs for general toxicity endpoints in Cramer classes I, II and III. The document was discussed with a wider audience during a workshop held in March 2003 (see list of workshop participants).

Occupational exposure limits for 30 organophosphate pesticides based on inhibition of red blood cell acetylcholinesterase

Toxicity and other relevant data for 30 organophosphate pesticides were evaluated to suggest inhalation occupational exposure limits (OELs), and to support development of a risk assessment strategy for organophosphates in general. Specifically, the value of relative potency analysis and the predictability of inhalation OELs by acute toxicity measures and by repeated oral exposure NOELs was assessed. Suggested OELs are based on the prevention of red blood cell (RBC) acetylcholinesterase (AChE) inhibition and are derived using a weight-of-evidence risk assessment approach. Suggested OEL values range from 0.002 to 2 mg/m(3), and in most cases, are less than current permissible exposure levels (PELs) or threshold limit values(R) (TLVs(R)). The available data indicate that experimental data for most organophosphates evaluated are limited; most organophosphates are equally potent RBC AChE inhibitors in different mammalian species; NOELs from repeated exposure studies of variable duration are usually equivalent; and, no particular grouping based on organophosphate structure is consistently more potent than another. Further, relative potency analyses have limited usefulness in the risk assessment of organophosphates. The data also indicated that equivalent relative potency relationships do not exist across either exposure duration (acute vs. repeated) or exposure route (oral vs. inhalation). Consideration of all variable duration and exposure route studies are therefore usually desirable in the development of an OEL, especially when data are limited. Also, neither acute measures of toxicity nor repeated oral exposure NOELs are predictive of weight-of-evidence based inhalation OELs. These deviations from what is expected based on the common mechanism of action for organophosphates across exposure duration and route - AChE inhibition - is likely due to the lack of synchrony between the timing of target tissue effective dose and the experimental observation of equivalent response. Thus, comprehensive interpretation of all toxicity data in the context of available toxicokinetic, toxicodynamic and exposure information for each individual organophosphate in a weight-of-evidence based risk assessment is desirable when deriving inhalation OELs.

Alterations on AChE activity of the fish Anguilla anguilla as response to herbicide-contaminated water

The inhibition of both total and specific acetylcholinesterase activities was measured in the whole eyes of the yellow eel Anguilla anguilla after exposure to the carbamate thiobencarb. In vivo assays were conducted under a constant flow-through system of thiobencarb-contaminated water (1/60 LC(50) 96 h=0.22 ppm for 96 h) followed by a recovery period in clean water (192 h more). The results indicated a measurable level of AChE activity on eyes of control eels, which resulted in a sensitive indicator of the presence of thiobencarb in the water. The pesticide induced significant inhibitory effects on AChE activity ranging from 35% in total AChE activity to 75% in specific AChE activity. Following 1 week of recovery, AChE activity in eel eyes was still different from that of controls. Specific AChE activity remained significantly depressed (35%). On the other hand, protein content in whole eyes of pesticide-treated eels did not exhibit any significant difference from control animals. It has been accepted that a minimum of 20% reduction in brain AChE would be necessary to be indicative of exposure to anticholinergic agents; however, few data are reported about this effect on sense organs.

Self reported symptoms and inhibition of acetylcholinesterase activity among Kenyan agricultural workers

OBJECTIVES

This study was part of the East African pesticides project. The general objective was to assess health hazards posed by handling, storage, and use of pesticides, on agricultural estates and small farms with a view to developing strategies for prevention and control of pesticide poisoning. The aim of this paper is to describe the prevalence of symptoms in this population, to relate levels of inhibition to reported symptoms and evaluate at which levels of inhibition symptoms become increased.

METHODS

Complete data were available for 256 exposed subjects and 152 controls from four regions in Kenya. A structured questionnaire on symptoms experienced at the time of interview was given to all subjects and controls. Information was also obtained on sex, age, main occupation, and level of education. Symptoms reported during the high exposure period, were initially clustered in broader symptom categories from reference literature on health effects of pesticides that inhibit cholinesterase (organophosphate and carbamate). Prevalence ratios were estimated for symptoms with changes in cholinesterase activity in serum.

RESULTS

Symptom prevalence in exposed subjects was higher during the high exposure period than the low exposure period, although these differences were not significant. Interestingly, a clear and significant change in symptoms prevalence was found in the controls with a higher prevalence in the low exposure period. Analysis of the relation between cholinesterase inhibition and symptoms showed that prevalence ratios were significantly > 1 for respiratory, eye, and central nervous system symptoms for workers with > 30% inhibition. Similar results were found for analyses with the actual level of acetylcholinesterase activity.

CONCLUSION

The results suggest the presence of a relation between exposure and acetylcholinesterase inhibition, acetylcholinesterase activity, and respiratory, eye, and central nervous system symptoms. Increased symptom prevalence was found at acetylcholinesterase activities generally considered to be non-adverse.

Sub-chronic effect of neem based pesticide (Vepacide) on acetylcholinesterase and ATPases in rat

Acetylcholinesterases (AChE), Na(+)-K+, Mg2+ and Ca(2+)-ATPases were monitored in rat brain when treated orally with 80, 160 and 320 mg/kg of Vepacide, an active ingredient from neem seed oil, daily for 90 days. Brain AChE, Na(+)-K+ and Ca(2+)-ATPases were inhibited whereas Mg(2+)-ATPase levels were enhanced in both the sexes after 45 and 90 days of treatment. The relative sensitivities of these ATPases to Vepacide indicated that Ca(2+)-ATPase being more sensitive than Na(+)-K(+)-ATPase in both the sexes. The magnitude of Ca(2+)-ATPase inhibited by this compound was higher than that of brain AChE. It appears to be sexual dimorphism in the alterations of brain AChE, Na(+)-K+ and Mg(2+)-ATPases by Vepacide with females being significant when compared with males. After 28 days of post treatment the alterations observed were approached to those of controls both in male and female rats showing reversal of the toxicity. These results indicated that the ATPases were potently inhibited by Vepacide and seemed to be its precise target among the enzyme studied. This can be used as biochemical marker of exposure to this neem derived product.

Effects of cypermethrin on the electroencephalographic activity of the rat: a model of chemically induced seizures

Cypermethrin is a potent representative member of the type II pyrethroid insecticides. This pyrethroid is used worldwide and has become a part of our environment. Until the present study, little information about its toxic effects in the central nervous system (CNS) was available. The aim of this study was, then, to determine the effects of repeated exposure to cypermethrin by means of assessing the electroencephalographic (EEG) activity in the rat. Cypermethrin was administered daily in a 300 mg/kg i.p. dose, below the LD50 value. After daily administration, the EEG activity was recorded and evaluated for 30 min. Paroxysmal epileptic activity appeared after the first and second days of cypermethrin administration. Frequency and numbers of bursts of epileptic activity also increased throughout the days of exposure to cypermethrin. Some of the paroxysmal events were present with behavioral anomalies, such as generalized tonic-clonic seizures. These effects are important because they could be related to the incidence of epileptic activity in humans chronically exposed to cypermethrin.

Human red blood cell acetylcholinesterase inhibition as the appropriate and conservative surrogate endpoint for establishing chlorpyrifos reference dose

Chlorpyrifos (O,O-diethyl O-(3,5, 6-trichloro-2-pyridinyl)- phosphorothioate) is an organophosphorus (OP) insecticide used for controlling insect pests. Currently, the reference dose (RfD) used by the Environmental Protection Agency (EPA) to establish acceptable human exposure tolerances for chlorpyrifos is based upon inhibition of blood butyrylcholinesterase (BuChE), which is not the target enzyme of chlorpyrifos, and does not play any role in cholinergic transmission. Data are presented showing that inhibition of acetylcholinesterase (AChE) associated with red blood cells (RBC), an enzyme similar to or identical with that in the nervous system, is a more appropriate endpoint on which to base the RfD. Basing an acceptable level of human exposure (e.g., RfD) on inhibition of RBC AChE provides a significant margin of safety, since it is 12- to 14-fold more sensitive as an indicator of chlorpyrifos exposure than the AChE in the most sensitive relevant neurological tissues (brain or retina). Inhibition of RBC AChE activity is consistently exhibited at lower dosages of chlorpyrifos than those required to result in clinical symptoms of OP toxicity, or alterations in cognitive functional responses. There is no unique sensitivity of the fetus or neonates to chlorpyrifos when administered by an appropriate oral dose. Thus, inhibition of RBC AChE activity is an appropriate surrogate measurement of chlorpyrifos exposure and provides a conservative endpoint for establishing appropriate margins of safety for both adults and infants.

Biochemical and molecular neurotoxicology: relevance to biomarker development, neurotoxicity testing and risk assessment

Biochemical and molecular approaches are most useful to define potential mechanisms of neurotoxicity. Information on the mechanisms of action of neurotoxicants can play a key role in neurotoxicology by allowing, among others, the development of potential biomarkers of effect, the refinement of in vitro testing procedures, and the improvement of the risk assessment process. An important class of insecticides, the organophosphates, are discussed as an example of how knowledge of molecular mechanisms is useful in various aspects of neurotoxicology. The utilization of such information in the area of biomarkers of exposure and effects, and of in vitro testing is presented. Additionally, mechanistic issues related to genetic polymorphisms and risk assessment are discussed.

The relationship of oral chlorpyrifos effects on behavior, cholinesterase inhibition, and muscarinic receptor density in rat

Behavioral changes and tissue cholinesterase (ChE) inhibition were examined in animals treated with the commonly used insecticide chlorpyrifos. Adult male rats were dosed by gavage with 0, 10, 30, 60, or 100 mg/kg chlorpyrifos. Rats (n = 20/dose group) were evaluated using a functional observational battery (FOB) and an automated measure of motor activity. All rats were tested the day before dosing and at 3.5 h (the time of peak effect) after dosing; half of these (n = 10/dose) were sacrificed immediately after testing for tissue collection. The remaining rats were tested again at 24 h, followed by sacrifice. The following tissues were collected from each animal: half brain, individual brain areas from the other half of the brain (frontal cortex, hippocampus, striatum, hypothalamus, cerebellum, pons/medulla), retina, liver, heart, diaphragm, quadriceps femoris muscle, and blood (separated into whole blood, plasma, and erythrocytes). ChE activity was measured in all tissues, and muscarinic receptor density was assessed as quinuclidinyl benzilate (QNB) binding in all brain regions, heart, and retina. The lowest dose produced no behavioral effects but did produce significant ChE inhibition in most tissues at 3.5 h. Higher doses produced more ChE inhibition and cholinergic signs of toxicity. Partial recovery from behavioral effects was evident at 24 h, with little or no corresponding recovery of ChE activity. Apparent downregulation of muscarinic receptor density was noted only in striatum and pons/medulla of rats treated with the highest dose of chlorpyrifos. Correlations for behavioral and biochemical effects were generally poor because: a) the low-dose effects on ChE inhibition were not reflected in behavioral signs, and b) behavioral signs showed recovery at 24 h, whereas ChE activity did not. Examination of data for individual rats indicated that > 60% of brain ChE inhibition was reached before neurobehavioral effects were evident.