Neurotoxicity of organophosphorus pesticides: predictions can be based on in vitro studies with hen and human enzymes

Computational Modeling Study of the Binding of Aging and Non-Aging Inhibitors with Neuropathy Target Esterase

Neuropathy target esterase (NTE) is a serine hydrolase with phospholipase B activity, which is involved in maintaining the homeostasis of phospholipids. It can be inhibited by aging inhibitors such as some organophosphorus (OP) compounds, which leads to delayed neurotoxicity with distal degeneration of axons. However, the detailed binding conformation of aging and non-aging inhibitors with NTE is not known. In this study, new computational models were constructed by using MODELLER 10.3 and AlphaFold2 to further investigate the inhibition mechanism of aging and non-aging compounds using molecular docking. The results show that the non-aging compounds bind the hydrophobic pocket much deeper than aging compounds and form the hydrophobic interaction with Phe1066. Therefore, the unique binding conformation of non-aging compounds may prevent the aging reaction. These important differences of the binding conformations of aging and non-aging inhibitors with NTE may help explain their different inhibition mechanism and the protection of non-aging NTE inhibitors against delayed neuropathy.

Neurotoxicity Assessment of Four Different Pesticides Using In Vitro Enzymatic Inhibition Assays

Pesticides affect different organs and tissues according to their bioavailability, chemical properties and further molecular interactions. In animal models exposed to several classes of pesticides, neurotoxic effects have been described, including the reduction of acetylcholinesterase activity in tissue homogenates. However, in homogenates, the reduction in enzymatic activity may also result from lower enzymatic expression and not only from enzymatic inhibition. Thus, in this work, we aimed to investigate the neurotoxic potential of four distinct pesticides: glyphosate (herbicide), imazalil (fungicide), imidacloprid (neonicotinoid insecticide) and lambda-cyhalothrin (pyrethroid insecticide), by assessing their inhibitory effect on the activity of acetylcholinesterase (AChE), butyrylcholinesterase (BChE) and tyrosinase, by using direct in vitro enzymatic inhibition methods. All pesticides dose-dependently inhibited AChE activity, with an inhibition of 11 ± 2% for glyphosate, 48 ± 2% for imidacloprid, 49 ± 3% for imazalil and 50 ± 3% for lambda-cyhalothrin, at 1 mM. Only imazalil inhibited BChE. Imazalil induced dose-dependent inhibition of BChE with identical pattern as that observed for AChE; however, for lower concentrations (up to 500 μM), imazalil showed higher specificity for AChE, and for higher concentrations, the same specificity was found. Imazalil, at 1 mM, inhibited the activity of BChE by 49 ± 1%. None of the pesticides, up to 1 mM, inhibited tyrosinase activity. In conclusion, the herbicide glyphosate shows specificity for AChE but low inhibitory capacity, the insecticides imidacloprid and λ-cyhalothrin present selective AChE inhibition, while the fungicide IMZ is a broad-spectrum cholinesterase inhibitor capable of inhibiting AChE and BChE in an equal manner. Among these pesticides, the insecticides and the fungicide are the ones with higher neurotoxic potential.

Facile Colorimetric Nanozyme Sheet for the Rapid Detection of Glyphosate in Agricultural Products Based on Inhibiting Peroxidase-Like Catalytic Activity of Porous Co3O4 Nanoplates

The rapid and onsite detection of glyphosate herbicides in agricultural products is still a challenge. Herein, a novel colorimetric nanozyme sheet for the rapid detection of glyphosate has been successfully prepared through the physical adsorption of porous Co3O4 nanoplates on a polyester fiber membrane. Glyphosate can specifically inhibit the peroxidase-mimicking catalytic activity of porous Co3O4 nanoplates, thereby the visual detection of glyphosate can be realized by distinguishing the change in the color intensity of the established nanozyme sheet. The prepared nanozyme sheet has good sensitivity and selectivity, with a detection limit of 0.175 mg·kg-1 for glyphosate detection just by the naked eyes. It can effectively detect glyphosate within 10 min, and the color spots can maintain more than 20 min. The nanozyme sheet is not easily affected by the external environment in detection and storage. The merits of the nanozyme sheet facilitate its practical application in the large-scale preliminary screening of glyphosate residues in agricultural products.

Neurite outgrowth inhibitory levels of organophosphates induce tissue transglutaminase activity in differentiating N2a cells: evidence for covalent adduct formation

Organophosphate compounds (OPs) induce both acute and delayed neurotoxic effects, the latter of which is believed to involve their interaction with proteins other than acetylcholinesterase. However, few OP-binding proteins have been identified that may have a direct role in OP-induced delayed neurotoxicity. Given their ability to disrupt Ca²⁺ homeostasis, a key aim of the current work was to investigate the effects of sub-lethal neurite outgrowth inhibitory levels of OPs on the Ca²⁺-dependent enzyme tissue transglutaminase (TG2). At 1-10 µM, the OPs phenyl saligenin phosphate (PSP) and chlorpyrifos oxon (CPO) had no effect cell viability but induced concentration-dependent decreases in neurite outgrowth in differentiating N2a neuroblastoma cells. The activity of TG2 increased in cell lysates of differentiating cells exposed for 24 h to PSP and chlorpyrifos oxon CPO (10 µM), as determined by biotin-cadaverine incorporation assays. Exposure to both OPs (3 and/or 10 µM) also enhanced in situ incorporation of the membrane permeable substrate biotin-X-cadaverine, as indicated by Western blot analysis of treated cell lysates probed with ExtrAvidin peroxidase and fluorescence microscopy of cell monolayers incubated with FITC-streptavidin. Both OPs (10 µM) stimulated the activity of human and mouse recombinant TG2 and covalent labelling of TG2 with dansylamine-labelled PSP was demonstrated by fluorescence imaging following SDS-PAGE. A number of TG2 substrates were tentatively identified by mass spectrometry, including cytoskeletal proteins, chaperones and proteins involved protein synthesis and gene regulation. We propose that the elevated TG2 activity observed is due to the formation of a novel covalent adduct between TG2 and OPs.

HINDLIMB PARALYSIS SYNDROME IN WILD CARNABY'S COCKATOOS (CALYPTORHYNCHUS LATIROSTRIS): A NEW THREAT FOR AN ENDANGERED SPECIES

Carnaby's Cockatoos (Calyptorhynchus latirostris) are in decline in SW Western Australia from several processes, including habitat loss and fragmentation. However, in recent years, a disease syndrome has also emerged as a significant population threat. Emerging diseases in wildlife have the potential for catastrophic effects on population numbers, especially if a species is experiencing existing pressure from other threatening processes. This article describes an investigation into a hindlimb paralysis syndrome that has occurred in the summer and autumn since 2012 in 84 wild Carnaby's Cockatoos. Recovery from the syndrome has been demonstrated in 21 of 33 cases when supportive therapy was applied. Although a definitive diagnosis has not been obtained, the hypothesized etiology is an organophosphate-induced delayed-onset neuropathy. The syndrome may indicate that interaction between the cockatoos and inland agricultural practices are affecting this migratory species in ways that are, so far, poorly understood.

Neuropathy target esterase (NTE/PNPLA6) and organophosphorus compound-induced delayed neurotoxicity (OPIDN)

Systemic inhibition of neuropathy target esterase (NTE) with certain organophosphorus (OP) compounds produces OP compound-induced delayed neurotoxicity (OPIDN), a distal degeneration of axons in the central nervous system (CNS) and peripheral nervous system (PNS), thereby providing a powerful model for studying a spectrum of neurodegenerative diseases. Axonopathies are important medical entities in their own right, but in addition, illnesses once considered primary neuronopathies are now thought to begin with axonal degeneration. These disorders include Alzheimer's disease, Parkinson's disease, and motor neuron diseases such as amyotrophic lateral sclerosis (ALS). Moreover, conditional knockout of NTE in the mouse CNS produces vacuolation and other degenerative changes in large neurons in the hippocampus, thalamus, and cerebellum, along with degeneration and swelling of axons in ascending and descending spinal cord tracts. In humans, NTE mutations cause a variety of neurodegenerative conditions resulting in a range of deficits including spastic paraplegia and blindness. Mutations in the Drosophila NTE orthologue SwissCheese (SWS) produce neurodegeneration characterized by vacuolization that can be partially rescued by expression of wild-type human NTE, suggesting a potential therapeutic approach for certain human neurological disorders. This chapter defines NTE and OPIDN, presents an overview of OP compounds, provides a rationale for NTE research, and traces the history of discovery of NTE and its relationship to OPIDN. It then briefly describes subsequent studies of NTE, including practical applications of the assay; aspects of its domain structure, subcellular localization, and tissue expression; abnormalities associated with NTE mutations, knockdown, and conventional or conditional knockout; and hypothetical models to help guide future research on elucidating the role of NTE in OPIDN.

Cholinesterase and phenyl valerate-esterase activities sensitive to organophosphorus compounds in membranes of chicken brain

Some effects of organophosphorus compounds (OPs) esters cannot be explained by action on currently recognized targets acetylcholinesterase or neuropathy target esterase (NTE). In previous studies, in membrane chicken brain fractions, four components (EPα, EPβ, EPγ and EPδ) of phenyl valerate esterase activity (PVase) had been kinetically discriminated combining data of several inhibitors (paraoxon, mipafox, PMSF). EPγ is belonging to NTE. The relationship of PVase components and acetylcholine-hydrolyzing activity (cholinesterase activity) is studied herein. Only EPα PVase activity showed inhibition in the presence of acetylthiocholine, similarly to a non-competitive model. EPα is highly sensitive to mipafox and paraoxon, but is resistant to PMSF, and is spontaneously reactivated when inhibited with paraoxon. In this papers we shows that cholinesterase activities showed inhibition kinetic by PV, which does not fit with a competitive inhibition model when tested for the same experimental conditions used to discriminate the PVase components. Four enzymatic components (CP1, CP2, CP3 and CP4) were discriminated in cholinesterase activity in the membrane fraction according to their sensitivity to irreversible inhibitors mipafox, paraoxon, PMSF and iso-OMPA. Components CP1 and CP2 could be related to EPα as they showed interactions between substrates and similar inhibitory kinetic properties to the tested inhibitors.

Analysis of the neurotoxic effects of neuropathic organophosphorus compounds in adult zebrafish

Inhibition and aging of neuropathy target esterase (NTE) by exposure to neuropathic organophosphorus compounds (OPs) can result in OP-induced delayed neuropathy (OPIDN). In the present study we aimed to build a model of OPIDN in adult zebrafish. First, inhibition and aging of zebrafish NTE activity were characterized in the brain by using the prototypic neuropathic compounds cresyl saligenin phosphate (CBDP) and diisopropylphosphorofluoridate (DFP). Our results show that, as in other animal models, zebrafish NTE is inhibited and aged by both neuropathic OPs. Then, a neuropathic concentration inhibiting NTE activity by at least 70% for at least 24 h was selected for each compound to analyze changes in phosphatidylcholines (PCs), lysophosphatidylcholines (LPCs) and glycerolphosphocholine (GPC) profiles. In spite to the strong inhibition of the NTE activity found for both compounds, only a mild increase in the LPCs level was found after 48 h of the exposure to DFP, and no effect were observed by CBDP. Moreover, histopathological evaluation and motor function outcome analyses failed to find any neurological abnormalities in the exposed fish. Thus, our results strongly suggest that zebrafish is not a suitable species for the development of an experimental model of human OPIDN.

In Vitro Cysteine Reactivates Organophosphate Insecticide Dichlorvos-Inhibited Human Cholinesterases

OBJECTIVES

Organophosphate (OP) pesticides inhibit both red blood cell (RBC) and plasma cholinesterases (ChEs). Oximes, especially pralidoxime (2-PAM), are widely used as antidotes to treat OP poisoning. In addition, N-acetylcysteine (NAC) is sometimes used as an adjuvant antidote. The current study aimed to assess the feasibility of using NAC as a single therapeutic agent for OP poisoning in comparison to in vitro 2-PAM.

METHODS

This study was carried out at the Razi Drug Research Center of Iran University of Medical Sciences, Tehran, Iran, between April and September 2014. A total of 22 healthy human subjects were recruited and 8 mL citrated blood samples were drawn from each subject. Dichlorvos-inhibited blood samples were separately exposed to low and high doses (final concentrations of 300 and 600 μmol.L^-1, respectively) of 2-PAM, NAC and cysteine. Plasma and RBCs were then separated by centrifugation and their ChE activity was measured using spectrophotometry.

RESULTS

Although cysteine-and not NAC-increased the ChE activity of both plasma and RBCs over those of dichlorvos, it did not increase them over those of a high dose of 2-PAM.

CONCLUSION

These results suggest that the direct reactions of 2-PAM and cysteine with dichlorvos and the reactivation of phosphorylated ChEs occurr via an associative stepwise addition-elimination process. High therapeutic blood concentrations of cysteine are needed for the elevation of ChE activity in plasma and RBCs; however, both this agent and NAC may still be effective in the reactivation of plasma and RBC ChEs.

Disturbed phospholipid homeostasis in endoplasmic reticulum initiates tri-o-cresyl phosphate-induced delayed neurotoxicity

Tri-o-cresyl phosphate (TOCP) is a widely used organophosphorus compound, which can cause a neurodegenerative disorder, i.e., organophosphate-induced delayed neurotoxicity (OPIDN). The biochemical events in the initiation of OPIDN were not fully understood except for the essential inhibition of neuropathy target esterase (NTE). NTE, located in endoplasmic reticulum (ER), catalyzes the deacylation of phosphatidylcholine (PC) and lysophosphatidylcholine (LPC) to glycerophosphocholine (GPC). The present study aims to study the changes of ER phospholipids profile as well as levels of important intermediates of phospholipid synthesis such as diacylglycerol (DAG) and phosphatidic acid (PA) at the initiation stage of OPIDN. Hens are the most commonly used animal models of OPIDN. The spinal cord phospholipidomic profiles of hens treated by TOCP were studied by using HPLC-MS-MS. The results revealed that TOCP induced an increase of PC, LPC, and sphingomyelin (SM) levels and a decrease of GPC, phosphatidylethanolamine (PE), lysophosphatidylethanolamine (LPE), lysophosphatidylserine (LPS), phosphatidylglycerol (PG), and phosphatidylinositol (PI) levels., Levels of DAG and PA were also decreased. Pretreatment with phenylmethylsulfonyl fluoride (PMSF) 24 h before TOCP administration prevented OPIDN and restored the TOCP-induced changes of phospholipids except GPC. Thus, the disruption of ER phospholipid homeostasis may contribute to the initiation of organophosphate-induced delayed neurotoxicity.

Subchronic Neurotoxicity of Oil Formulations Containing Either Tricresyl Phosphate or Tri-Orthocresyl Phosphate

This study was conducted to determine the threshold concentration of tricresyl phosphate (TCP) in aviation engine oil able to cause delayed peripheral neuropathy in adult hens after repeated exposure. The study also evaluated the predictive value of endpoints usually used to measure acute peripheral neurotoxicity (neurotoxic esterase [NTE] inhibition, ataxia, and histopathologic changes), as measures of neurotoxicity in a subchronic study. Animals that received oil containing 3% TCP showed significant neurotoxicity that could not be accounted for by the small amount of TOCP present. Oil containing 1% TCP was without neurotoxic activity. There was an excellent correlation between percentage inhibition of NTE and development of neuropathy. An association was also seen for ataxia and neuropathology. Further study is needed to determine the phosphate ester isomers responsible for the significant neurotoxic potency demonstrated by the aviation engine oil containing 3% TCP.

Acetylcholine-hydrolyzing activities in soluble brain fraction: Characterization with reversible and irreversible inhibitors

Some effects of organophosphorus compounds (OPs) esters cannot be explained through actions on currently recognized targets acetylcholinesterase or neuropathy target esterase (NTE). In soluble chicken brain fraction, three components (Eα, Eβ and Eγ) of pheny lvalerate esterase activity (PVase) were kinetically discriminated and their relationship with acetylcholine-hydrolyzing activity (cholinesterase activity) were studied in previous works. In this work, four enzymatic components (CS1, CS2, CS3 and CS4) of cholinesterase activity have been discriminated in soluble fraction, according to their sensitivity to irreversible inhibitors mipafox, paraoxon, PMSF and iso-OMPA and to reversible inhibitors ethopropazine and BW284C51. Cholinesterase component CS1 can be related to the Eα component of PVase activity and identified as butyrylcholinesterase (BuChE). No association and similarities can be stablished among the other PVase component (Eβ and Eγ) with the other cholinesterase components (CS2, CS3, CS4). The kinetic analysis has allowed us to stablish a method for discriminating the enzymatic component based on a simple test with two inhibitors. It can be used as biomarker in toxicological studies and for monitoring these cholinesterase components during isolation and molecular identification processes, which will allow OP toxicity to be understood by a multi-target approach.

Interindividual Variations in Enzymes Controlling Organophosphate Toxicity in Man

1 Interindividual variations in an unexposed population have been defined for five enzymes involved in organophosphate (OP) toxicity. The enzymes measured were: red blood cell acetylcholinesterase (AChE), lymphocyte neuropathy target esterase (NTE), serum cholinesterase (ChE), serum paraoxonase and serum arylesterase.

2 AChE and arylesterase were normally distributed in the population whilst the distribution of NTE, ChE and paraoxonase deviated significantly from normal.

3 Assay precision and intra-individual variability were measured for each of the enzymes; the effect on interindividual variation was assessed.

4 Variations in enzyme activities between individuals could have profound effects on susceptibility to OP toxicity. Prior determination of these enzymes may be predictive of susceptibility.

5 Lymphocyte NTE has some limitations as an indicator of exposure to neurotoxic OPs.

Further studies toward a mouse model for biochemical assessment of neuropathic potential of organophosphorus compounds

Inhibition and aging of neuropathy target esterase (NTE) by neuropathic organophosphorus (OP) compounds triggers OP compound-induced delayed neuropathy (OPIDN), whereas inhibition of acetylcholinesterase (AChE) produces cholinergic toxicity. The neuropathic potential of an OP compound is defined by its relative inhibitory potency toward NTE vs. AChE assessed by enzyme assays following dosing in vivo or after incubations of direct-acting compounds or active metabolites with enzymes in vitro. The standard animal model of OPIDN is the adult hen, but its large size and high husbandry costs make this species a burdensome model for assessing neuropathic potential. Although the mouse does not readily exhibit clinical signs of OPIDN, it displays axonal lesions and expresses brain AChE and NTE. Therefore, the present research was performed as a further test of the hypothesis that inhibition of mouse brain AChE and NTE could be used to assess neuropathic potential using mouse brain preparations in vitro or employing mouse brain assays following dosing of OP compounds in vivo. Excellent correlations were obtained for inhibition kinetics in vitro of mouse brain enzymes vs. hen brain and human recombinant enzymes. Furthermore, inhibition of mouse brain AChE and NTE after dosing with OP compounds afforded ED(50) ratios that agreed with relative inhibitory potencies assessed in vitro. Taken together, results with mouse brain enzymes demonstrated consistent correspondence between in vitro and in vivo predictors of neuropathic potential, thus adding to previous studies supporting the validity of a mouse model for biochemical assessment of the ability of OP compounds to produce OPIDN.

Towards a strategic approaches in alternative tests for pesticide safety

Pesticides have provided significant benefits including plant disease control and increased crop yields since people developed and utilized them. However, pesticide use is associated with many adverse effects, which necessitate precise toxicological tests and risk assessment. Most of these methods are based on animal studies, but considerations of animal welfare and ethics require the development of alternative methods for the evaluation of pesticide toxicity. Although the usage of laboratory animals is inevitable in scientific evaluation and alternative approaches have limitations in the whole coverage, continuous effort is necessary to minimize animal use and to develop reliable alternative tests for pesticide evaluation. This review discusses alternative approaches for pesticide toxicity tests and hazard evaluation that have been used in peer-reviewed reports and could be applied in future studies based on the critical animal research principles of reduction, replacement, and refinement.

Interaction between substrates suggests a relationship between organophosphorus-sensitive phenylvalerate- and acetylcholine-hydrolyzing activities in chicken brain

Organophosphorus compounds (OPs) induce neurotoxic disorders through interactions with well-known target esterases, such as acetylcholinesterase and neuropathy target esterase (NTE). However, OPs interact with other esterases of unknown biological function. In soluble chicken brain fractions, three components of enzymatic phenylvalerate esterase activity (PVase) called Eα, Eβ and Eγ, have been kinetically discriminated. These components are studied in this work for the relationship with acetylcholine-hydrolyzing activity. When Eα PVase activity (resistant PVase activity to 1500 μM PMSF for 30 min) was tested with different acetylthiocholine concentrations, inhibition was observed. The best-fitting model to the data was the non-competitive inhibition model (Km=0.12, 0.22 mM, Ki=6.6, 7.6 mM). Resistant acetylthiocholine-hydrolyzing activity to 1500 μM PMSF was inhibited by phenylvalerate showing competitive inhibition (Km=0.09, 0.11 mM; Ki=1.7, 2.2 mM). Eβ PVase activity (resistant PVase activity to 25 μM mipafox for 30 min) was not affected by the presence of acetylthiocholine, while resistant acetylthiocholine-hydrolyzing activity to 25 μM mipafox showed competitive inhibition in the presence of phenylvalerate (Km=0.05, 0.06 mM; Ki=0.44, 0.58 mM). The interactions observed between the substrates of AChE and PVase suggest that part of PVase activity might be a protein with acetylthiocholine-hydrolyzing activity.

Combined QSAR studies of inhibitor properties of O-phosphorylated oximes toward serine esterases involved in neurotoxicity, drug metabolism and Alzheimer's disease

Oxime reactivation of serine esterases (EOHs) inhibited by organophosphorus (OP) compounds can produce O-phosphorylated oximes (POXs). Such oxime derivatives are of interest, because some of them can have greater anti-EOH potencies than the OP inhibitors from which they were derived. Accordingly, inhibitor properties of 58 POXs against four EOHs, along with pair-wise selectivities between them, have been analysed using different QSAR approaches. EOHs (with their abbreviations and consequences of inhibition in parentheses) comprised acetylcholinesterase (AChE: acute neurotoxicity; cognition enhancement), butyrylcholinesterase (BChE: inhibition of drug metabolism or stoichiometric scavenging of EOH inhibitors; cognition enhancement), carboxylesterase (CaE: inhibition of drug metabolism or stoichiometric scavenging of EOH inhibitors), and neuropathy target esterase (NTE: delayed neurotoxicity). QSAR techniques encompassed linear regression and backpropagation neural networks in conjunction with fragmental descriptors containing labelled atoms, Molecular Field Topology Analysis (MFTA), Comparative Molecular Similarity Index Analysis (CoMSIA), and molecular modelling. All methods provided mostly consistent and complementary information, and they revealed structural features controlling the 'esterase profiles', i.e. patterns of anti-EOH activities and selectivities of the compounds of interest. In addition, MFTA models were used to design a library of compounds having a cognition-enhancement esterase profile suitable for potential application to the treatment of Alzheimer's disease.

Increased oxidative stress is associated with the development of organophosphate-induced delayed neuropathy

Organophosphate-induced delayed neuropathy (OPIDN) is a progressive neuropathic disorder that manifests in days to weeks following exposure to an acute dose of organophosphates. The precise mechanism involved in the development of OPIDN is not clear as it develops after many days of the cessation of cholinergic crisis. The present study has been designed to understand the role of oxidative stress in the development of OPIDN, wherein neuropathy was developed by the administration of acute dose of monocrotophos (MCP) or dichlorvos (2,2-dichlorovinyl dimethyl phosphate (DDVP)) to rats. Significant motor deficits in terms of reduced spontaneous locomotor activity and performance on narrow beam test were observed after 14 days of exposure to MCP or DDVP, which persisted even on day 28, suggesting the development of OPIDN. Rats with OPIDN also exhibited an increase in malondialdehyde levels along with a decrease in thiol content in cerebral cortex, cerebellum and brain stem. Concomitantly, the activities of antioxidant enzymes, superoxide dismutase, catalase, glutathione peroxidase and glutathione reductase were reduced in the three brain regions. The biochemical and functional changes were associated with histological alterations in the brain regions studied. The results clearly indicate that the development of OPIDN is mediated in part through an increased oxidative stress and suggest that the strategies aimed at restoration of antioxidant capacity may be beneficial for the individuals with OPIDN-like symptoms.

Constructs of human neuropathy target esterase catalytic domain containing mutations related to motor neuron disease have altered enzymatic properties

Neuropathy target esterase (NTE) is a phospholipase/lysophospholipase associated with organophosphorus (OP) compound-induced delayed neurotoxicity (OPIDN). Distal degeneration of motor axons occurs in both OPIDN and the hereditary spastic paraplegias (HSPs). Recently, mutations within the esterase domain of NTE were identified in patients with a novel type of HSP (SPG39) designated NTE-related motor neuron disease (NTE-MND). Two of these mutations, arginine 890 to histidine (R890H) and methionine 1012 to valine (M1012V), were created in human recombinant NTE catalytic domain (NEST) to measure possible changes in catalytic properties. These mutated enzymes had decreased specific activities for hydrolysis of the artificial substrate, phenyl valerate. In addition, the M1012V mutant exhibited a reduced bimolecular rate constant of inhibition (k(i)) for all three inhibitors tested: mipafox, diisopropylphosphorofluoridate, and chlorpyrifos oxon. Finally, while both mutated enzymes inhibited by OP compounds exhibited altered time-dependent loss of their ability to be reactivated by nucleophiles (aging), more pronounced effects were seen with the M1012V mutant. Taken together, the results from specific activity, inhibition, and aging experiments suggest that the mutations found in association with NTE-MND have functional correlates in altered enzymological properties of NTE.

Delayed neurotoxicity - do trichlorphon and/or dichlorvos cause delayed neuropathy in man or in test animals?

Many, but not all, reports of delayed neuropathy associated with acute poisoning by trichlorphon refer to cases in U.S.S.R. Adulteration of technical trichlorphon with the ethyl analogue would greatly increase the neurotoxic hazard but analysis of a few samples has not revealed such impurities. Simultaneous ingestion of alcohol does not appear to increase neuropathic hazard. In hens double doses of trichlorphon each exceeding unprotected LD50 can produce moderate neuropathy associated with appropriately high inhibitions of neurotoxic esterase. Similar results are obtained with 2 doses of 10 x LD50 of dichlorvos. In vitro the inhibitory power of dichlorvos against neurotoxic esterase of hen brain is 0.02 x the power against acetylcholinesterase. This ratio correlates reasonably with the ratio of LD50/neuropathic dose. The factor for human brain enzymes is 0.06 suggesting that man is more susceptible to neuropathic effects of near-lethal doses of circulating dichlorvos. It is concluded that the only neuropathic hazard to man from good quality trichlorphon arises from rapid ingestion of massive doses. To obtain critical levels of inhibition of neurotoxic esterase and to cause neuropathy in man by repeated doses would require each dose to be severely toxic. Dichlorvos ingested in large doses is likely to kill rather than to cause neuropathy.

Biosensor assay of neuropathy target esterase in whole blood as a new approach to OPIDN risk assessment: review of progress

Organophosphates (OPs) that inhibit neuropathy target esterase (NTE) with subsequent ageing can produce OP-induced delayed neuropathy (OPIDN). NTE inhibition in lymphocytes can be used as a biomarker of exposure to neuropathic OPs. An electrochemical method was developed to assay NTE in whole blood. The high sensitivity of the tyrosinase carbon-paste biosensors for the phenol produced by hydrolysis of the substrate, phenyl valerate, allowed NTE activity to be measured in diluted samples of whole blood, which cannot be done using the standard colorimetric assay. The biosensor was used to establish correlations of NTE inhibitions in blood with that in lymphocytes and brain after dosing hens with a neuropathic OP. The results of further studies demonstrated that whole blood NTE is a reliable biomarker of neuropathic OPs for up to 96 hours after exposure. These validation results suggest that the biosensor NTE assay for whole blood could be developed to measure human exposure to neuropathic OPs as a predictor of OPIDN. The small blood volume required (100 microL), simplicity of sample preparation and rapid analysis times indicate that the biosensor should be useful in biomonitoring and epidemiological studies. The present paper is an overview of our previous and ongoing work in this area.

Historical Neurotoxins: What We Have Learned from Toxins of the Past About Diseases of the Present

Throughout history, humans have fallen victim to a variety of neurotoxins, with exposures coming in the form of tainted products, industrial pollution, drugs of abuse, and even the bread and water that sustain them. Despite this long and tumultuous history, neurotoxic outbreaks still occur with regular frequency. Although many difficulties currently exist in linking many of today's unexplained neurologic disorders to toxins, the past suggests a prominent role for neurotoxins in diseases (such as amyotrophic lateral sclerosis and PD), unexplained peripheral neuropathies, neurodevelopmental disorders, and many psychiatric disturbances.

Organophosphate-Induced Delayed Polyneuropathy

Organophosphate-induced delayed polyneuropathy (OPIDP) is a rare toxicity resulting from exposure to certain organophosphorus (OP) esters. It is characterised by distal degeneration of some axons of both the peripheral and central nervous systems occurring 1-4 weeks after single or short-term exposures. Cramping muscle pain in the lower limbs, distal numbness and paraesthesiae occur, followed by progressive weakness, depression of deep tendon reflexes in the lower limbs and, in severe cases, in the upper limbs. Signs include high-stepping gait associated with bilateral foot drop and, in severe cases, quadriplegia with foot and wrist drop as well as pyramidal signs. In time, there might be significant recovery of the peripheral nerve function but, depending on the degree of pyramidal involvement, spastic ataxia may be a permanent outcome of severe OPIDP. Human and experimental data indicate that recovery is usually complete in the young. At onset, the electrophysiological changes include reduced amplitude of the compound muscle potential, increased distal latencies and normal or slightly reduced nerve conduction velocities. The progression of the disease, usually over a few days, may lead to non-excitability of the nerve with electromyographical signs of denervation. Nerve biopsies have been performed in a few cases and showed axonal degeneration with secondary demyelination. Neuropathy target esterase (NTE) is thought to be the target of OPIDP initiation. The ratio of inhibitory powers for acetylcholinesterase and NTE represents the crucial guideline for the aetiological attribution of OP-induced peripheral neuropathy. In fact, pre-marketing toxicity testing in animals selects OP insecticides with cholinergic toxicity potential much higher than that to result in OPIDP. Therefore, OPIDP may develop only after very large exposures to insecticides, causing severe cholinergic toxicity. However, this was not the case with certain triaryl phosphates that were not used as insecticides but as hydraulic fluids, lubricants and plasticisers and do not result in cholinergic toxicity. Several thousand cases of OPIDP as a result of exposure to tri-ortho-cresyl phosphate have been reported, whereas the number of cases of OPIDP as a result of OP insecticide poisoning is much lower. In this article, we mainly discuss OP pesticide poisoning, particularly when caused by chlorpyrifos, dichlorvos, isofenphos, methamidophos, mipafox, trichlorfon, trichlornat, phosphamidon/mevinphos and by certain carbamates. We also discuss case reports where neuropathies were not convincingly attributed to fenthion, malathion, omethoate/dimethoate, parathion and merphos. Finally, several observational studies on long-term, low-level exposures to OPs that sometimes reported mild, inconsistent and unexplained changes of unclear significance in peripheral nerves are briefly discussed.

Toxicological assessment of isomeric pesticides: a strategy for testing of chiral organophosphorus (OP) compounds for delayed polyneuropathy in a regulatory setting

Many compounds, including some pesticides, contain structural centres of asymmetry, which convey the property of a type of stereoisomerism known as chirality. Such compounds can exist in two or more forms, depending on the number of chiral atoms and are termed stereoisomers or enantiomers. Stereoisomers of a particular compound can have different biological properties; one such of particular importance for toxicological evaluation, is the potential for differences in metabolic disposal of and binding of stereoisomers to molecular targets in the cell. The combination of differential metabolism of chiral organophosphorus (OP) pesticides and opposing stereoselectivity of inhibition of neuropathy target esterase (NTE) and acetylcholinesterase (AChE) can affect the value of the hen test, performed to OECD guidelines, in predicting the potential to cause organophosphate-induced delayed polyneuropathy (OPIDP) in humans. This is a mixed central and sensory and motor neuropathy. The experimental data on structural analogues of the pesticide methamidophos and the evidence for stereoselective OPIDP are reviewed and a model is given demonstrating how the properties of a chiral OP can result in the neuropathic potential not being detected by the standard hen test. A strategy for the assessment of a racemic mixture comprised of two OP enantiomers for the potential to induce OPIDP is outlined. The strategy uses information from structure activity relationships (SAR), in vitro tests and in vivo tests to allow risk assessment decisions to be made. It is suggested that the potential for stereoselective toxicity of pesticides should be routinely considered in regulatory assessments.

Relative inhibitory potencies of chlorpyrifos oxon, chlorpyrifos methyl oxon, and mipafox for acetylcholinesterase versus neuropathy target esterase

The relative inhibitory potency (RIP) of an organophosphorus (OP) inhibitor against acetylcholinesterase (AChE) versus neuropathy target esterase (NTE) may be defined as the ratio [k(i)(AChE)/k(i)(NTE)], where k(i) is the bimolecular rate constant of inhibition for a given inhibitor against each enzyme. RIPs greater than 1 correlate with the inability of ageable OP inhibitors or their parent compounds to produce OP compound-induced delayed neurotoxicity (OPIDN) at doses below the LD50. The RIP for chlorpyrifos oxon (CPO) is >>1 for enzymes from hen brain homogenate, and the parent compound, chlorpyrifos (CPS), cannot produce OPIDN in hens at sublethal doses. This study was carried out to test the hypothesis that the RIP for the methyl homologue of CPO, chlorpyrifos methyl oxon (CPMO), is >>1 and greater than the RIP for CPO. Mipafox (MIP), an OP compound known to produce OPIDN, was included for comparison. Hen brain microsomes were used as the enzyme source, and k(i) values (mean +/- SE, microM(-1) min(-1)) were determined for AChE and NTE (n = 3 and 4 separate experiments, respectively). The k(i) values for CPO, CPMO, and MIP against AChE were 17.8 +/- 0.3, 10.9 +/- 0.1, and 0.00429 +/- 0.00001, respectively, and for NTE were 0.0993 +/- 0.0049, 0.0582 +/- 0.0013, and 0.00498 +/- 0.00006, respectively. Corresponding RIPs for CPO, CPMO, and MIP were 179 +/- 9, 187 +/- 4, and 0.861 +/- 0.011, respectively. The results demonstrate that RIPs for CPO and CPMO are comparable, markedly different from that for MIP, and >>1, indicating that CPS methyl, like CPS, could not cause OPIDN at sublethal doses.

Properties of phenyl valerate esterase activities from chicken serum are comparable with soluble esterases of peripheral nerves in relation with organophosphorus compounds inhibition

Chicken serum, the usual in vivo animal for testing organophosphorus delayed neuropathy, has long been reported not to contain a homologous activity of the neuronal neuropathy target esterase (NTE) activity when it is assayed according to standard methods as the phenyl valerate esterase (PVase) activity, which is resistant to paraoxon and sensitive to mipafox. However, a PVase activity (1000-1500 nmol/min/ml) can be measured in serum that is extremely sensitive to both paraoxon, a non-neuropathic organophosphorus compound and mipafox, a model neuropathy inducer. The inhibition was time progressive in both cases, suggesting a covalent phosphorilating reaction. The fixed time inhibition curves suggest at least two sensitive components. The IC50 for 30 min, at 37 degrees C are 6 and 51 nM for paraoxon and 4 and 110 nM for mipafox, for every sensitive component. When paraoxon was removed from a serum sample pretreated with the inhibitor, the paraoxon sensitive PVase activity was recovered, in spite of showing a time progressive inhibition suggesting that hydrolytic dephosphorylating reaction recovered at a significant rate. The reactivation of the phosphorylated enzyme could explain that the time progressive inhibitions curves for long time with paraoxon tend to reach a plateau depending on the inhibition concentration. However, with mipafox, the curve approached the same maximal inhibitions at all concentrations as expected for a permanent covalent irreversible phosphorylation, which is coherent with the observations that the activity remained inhibited after removing the inhibitor. Data of serum esterases described in this paper showed similar properties to those previously reported for peripheral nerve soluble phenylvalerate esterase: (1) extremely high sensitivity to paraoxon and mipafox; (2) time progressive kinetic with two sensitive components; (3) recovery of activity after removal of paraoxon; and (4) permanent inhibition with mipafox. These properties of serum esterases are very similar to those of soluble fraction of peripheral nerves. So, serum PVases could be considered as appropriate biomarkers, as a mirror for the neural soluble paraoxon and mipafox sensitive soluble esterases that could be used for biomonitoring purpose.

Biosensor detection of neuropathy target esterase in whole blood as a biomarker of exposure to neuropathic organophosphorus compounds

Neuropathy target esterase (NTE) is the target protein for neuropathic organophosphorus (OP) compounds that produce OP compound-induced delayed neurotoxicity (OPIDN). Inhibition/aging of brain NTE within hours of exposure predicts the potential for development of OPIDN in susceptible animal models. Lymphocyte NTE has also found limited use as a biomarker of human exposure to neuropathic OP compounds. Recently, a highly sensitive biosensor was developed for NTE activity using a tyrosinase carbon-paste electrode for amperometric detection of phenol produced by hydrolysis of the substrate, phenyl valerate. The I50 (20 min at 37 degrees C) for N,N'-di-2-propylphosphorodiamidofluoridate (mipafox) against hen lymphocyte NTE was 6.94 +/- 0.28 microM amperometrically and 6.02 +/- 0.71 microM colorimetrically. For O,O-di1-propyl O-2,2-dichlorvinyl phosphate (PrDChVP), the I50 against hen brain NTE was 39 +/- 8 nM amperometrically and 42 +/- 2 nM colorimetrically. The biosensor enables NTE to be assayed in whole blood, whereas this cannot be done with the usual colorimetric method. Amperometrically, I50 values for PrDChVP against hen and human blood NTE were 66 +/- 3 and 70 +/- 14 nM, respectively. To study the possibility of using blood NTE inhibition as a biochemical marker of neuropathic OP compound exposure, NTE activities in brain and lymphocytes as well in brain and blood were measured 24 h after dosing hens with PrDChVP. Brain, lymphocyte, and blood NTE were inhibited in a dose-responsive manner, and NTE inhibition was highly correlated between brain and lymphocyte (r = .994) and between brain and blood (r = .997). The results suggest that the biosensor NTE assay for whole blood could serve as a biomarker of exposure to neuropathic OP compounds as well as a predictor of OPIDN and an adjunct to its early diagnosis.

Quantitative structure-activity relationships predict the delayed neurotoxicity potential of a series of O-alkyl-O-methylchloroformimino phenylphosphonates

Inhibition of acetylcholinesterase (AChE) versus inhibition and aging of neuropathy target esterase (NTE) by organophosphorus (OP) compounds in vivo can give rise to distinct neurological consequences: acute cholinergic toxicity versus OP compound-induced delayed neurotoxicity (OPIDN). Previous work has shown that the relative potency of an OP compound to react with NTE versus AChE in vitro may predict its capability to produce OPIDN. The present study was conducted to evaluate further the validity of such predictions and to enhance them with quantitative structure-activity relationships (QSAR) using a homologous series of alkyl phenylphosphonates (RO)C6H5P(O)ON = CCICH3 (PhP; R = alkyl). Neuropathic potential of PhP was assessed by measuring ki(NTE)ki(AChE) ratios in vitro and comparing these with ED50 ratios in vivo. Selectivity for NTE increased with rising R-group hydrophobicity. The ki(NTE)/ki(AChE) ratios were 0.42 (methyl), 3.6 (ethyl), 15 (isopropyl), 36 (propyl), 69 (isobutyl), 105 (butyl), and 124 (pentyl). Ratios > 1 suggest the potential to produce OPIDN at doses lower than the LD50. Inhibition of NTE and AChE in hen brain in vivo was studied 24 h after i.m. injection of hens with increasing doses of methyl and butyl derivatives. Analysis of dose-response curves yielded ED50(AChE)/ED50(NTE) ratio of 0.86 for methyl PhP and 22.1 for butyl PhP. These results predict that the butyl derivative should be more neuropathic than the methyl analogue. Excellent correspondence between in vivo and in vitro predictions of neuropathic potential indicate that valid predictive QSAR models may be based on the in vitro approach. Adoption of this system would result in reducing experimental animal use, lowering costs, accelerating data production, and enabling standardization of a biochemically based risk assessment of the neuropathic potential of OP compounds.

Low-level exposures to organophosphorus esters and peripheral nerve function

This review evaluates the epidemiological and clinical evidence linking low-level and prolonged exposures to organophosphorus esters, used as insecticides or nerve agents, to peripheral nerve dysfunction. The clinical effects of large doses of these chemicals-including the cholinergic syndrome, the intermediate syndrome, and the delayed polyneuropathy-are all well established and are summarized. Based on these clinical observations and experimental studies, dose-effect relationships indicate that peripheral neuropathy always develops after cholinergic toxicity. However, several studies have suggested that this relationship may be different after low-level prolonged exposures, as, for instance, those experienced by Gulf War veterans and British sheep farmers, thereby leading to the development of peripheral neuropathy without preceding cholinergic toxicity. A critical assessment of these studies, involving subjects with either current or past exposures, indicates that changes in peripheral nerve function were mild, inconsistent, and unexplained and that most studies lack exposure data. Suggestions made about individual hypersusceptibility to delayed polyneuropathy lack support. It is concluded that there is no evidence of peripheral nerve dysfunction caused by low-level prolonged exposures to organophosphate insecticides or nerve agents.

Esterase inhibition in SH-SY5Y human neuroblastoma cells following exposure to organophosphorus compounds for 28 days

Esterase inhibition was determined in SH-SY5Y human neuroblastoma cells grown in serum-free media and exposed to 10(-11) to 10(-7) M concentrations of organophosphorus (OP) compounds for 28 days. To examine metabolic activation in these exposures, pairs of pro- and active toxicants were studied, including chlorpyrifos and its oxon, parathion and paraoxon, and tri-ortho-tolyl phosphate and phenyl saligenin phospahte. Inhibition of acetylcholinesterase was greater in cells treated for 28 days with all active organophosphorus compounds than it was in cells treated only once with the same concentration of a given OP compound. The protoxicants chlorpyrifos and parathion produced acetylcholinesterase inhibition after multiple exposures although no inhibition was seen following a single exposure to these agents. Exacerbation of neurotoxic esterase inhibition by multiple exposures to the test compounds was not as pronounced as that of acetylcholinesterase. Exposure to the test compounds for 28 days did not significantly enhance esterase inhibition produced by a subsequent exposure to 10(-9) M chlorpyrifos-oxon. The results indicate that in vitro methods can be used to study the effect of multiple OP exposures on esterase activity.

Analysis of the additivity of in vitro inhibition of cholinesterase by mixtures of chlorpyrifos-oxon and azinphos-methyl-oxon

Organophosphorus (OP) insecticides or their active metabolites act through a common mechanism of toxicity, the inhibition of cholinesterase (ChE). The effects of in vitro exposure of brain (target) and serum (biomarker) ChE to chlorpyrifos-oxon (C horizontal lineO) and azinphos-methyl-oxon (AZM horizontal lineO), the active metabolites of the insecticides chlorpyrifos and azinphos-methyl, respectively, were investigated to determine if simultaneous or sequential exposure to these two OP compounds results in purely additive effects. Additive was defined by the theoretical calculated percent inhibition (dose additivity), which takes into account the fraction of ChE molecules assumed to be available for inhibition by the second compound following inhibition by the first compound, not simple mathematical summation of percent inhibition (response additivity). Brain ChE simultaneously exposed to the two compounds resulted in additive effects, which were less than the simple mathematical summation of percent inhibition. However, serum ChE simultaneously exposed to the two compounds resulted in a nonlinear response, presumably due in part to the presence of detoxifying enzymes in the serum. Sequential exposure of both brain and serum ChE to the two compounds resulted in greater than additive effects at the higher concentrations of each compound. There was no departure from additivity at the lower concentrations of the two compounds. These data suggest that simple mathematical summation of percent inhibitions, i.e., response additivity, is not the appropriate method for describing the combined effects of C horizontal lineO and AZM horizontal lineO on ChE in vitro. In addition, there are other mechanisms involved, such as the presence of detoxication enzymes, that must be taken into account when analyzing the effects of combined exposure of ChE to these two compounds.

Bioelectrochemical analysis of neuropathy target esterase activity in blood

Bioelectrochemical analysis of neuropathy target esterase (NTE) and its inhibitors is based on the combination of the NTE-catalyzed hydrolysis of phenyl valerate and phenol detection by a tyrosinase carbon-paste electrode. The use of the tyrosinase electrode improves 10-fold the sensitivity of NTE detection in comparison with a spectrophotometric method. The tyrosinase electrode was found to be suitable for measurements in whole human blood where spectrophotometric detection is considerably restricted. The specificity of NTE in blood for mipafox and di-2-propyl phosphorofluoridate was close to that for neuronal NTE. The NTE-like activity in blood was determined to be 0.19 +/- 0.02 nmol/min/mg of protein.

A stable preparation of hen brain neuropathy target esterase for rapid biochemical assessment of neurotoxic potential of organophosphates

Neuropathy target esterase (NTE) is a molecular target for organophosphate-induced delayed neurotoxicity (OPIDN). This enzyme has proved to be an excellent tool for the assessment of neuropathic potential of organophosphates (OP), in particular by comparison of an OP inhibitory activity in vitro against NTE and acetylcholinesterase. A large-scale OP screening for delayed neurotoxicity was largely prevented by the lack of an available stable preparation of NTE. To obtain a stable NTE preparation the influence of intensive freezing and subsequent lyophilization of paraoxon-preinhibited (P2 + P3) hen brain membrane fraction on NTE properties has been studied using two neuropathic OP: mipafox and O,O-dipropyldichlorovinyl phosphate (PrDChVP). It was shown that lyophilization preserved a high NTE specific activity and did not alter the inhibitor characteristics of the enzyme. A long-term storage study showed that lyophilized NTE preparation exhibited inhibitory features actually identical to those of the native enzyme during 1 year and retained rather high specific activity; in this case some loss of NTE specific activity has been observed. Comparative studies of inhibition of the native and lyophilized NTE preparations by a model series of phenyl phosphonates RO(C6H5)P(O)ON=CClCH3 (R = alkyl), demonstrated a good correlation between the values pI50 obtained with both enzyme preparations as well as identical structure-activity relationships for the lyophilized and native enzymes. The results allow the conclusion that the obtained NTE preparation can be used as a standard, stable and readily available source of NTE for assessing the anti-NTE activity of OP.

Chronic effects of low level exposure to anticholinesterases--a mechanistic review

High dose exposure to anticholinesterases which results in symptomatic poisoning can have lasting consequences due to the trauma of intoxication, excitotoxicity, secondary hypoxic damage, and (for some agents) a delayed onset polyneuropathy (OPIDN). The potential effects of low level exposure are less well defined. The most reliable data comes from controlled clinical trials with specific agents. A single dose of sarin or repeated doses of metrifonate or mevinphos, have produced only transient adverse effects at doses causing substantial acetylcholinesterase inhibition. Other data comes from epidemiological surveys. These have often used more sensitive indices than the clinical studies, but are less reliable due to the difficulty of defining exposure and matching control and exposed populations. Subtle, mainly cognitive, differences between exposed and non-exposed populations are sometimes seen. Low level exposure can cause a reversible down-regulation of cholinergic systems, and a range of non-cholinesterase effects that are structure-specific, and do not always parallel acute toxicity. Novel protein targets sensitive to low level exposure to some organophosphates are known to exist in the brain, but their functional significance is not yet understood.

Delayed neuropathy and inhibition of soluble neuropathy target esterase following the administration of organophosphorus compounds to hens

Delayed neuropathy and inhibition of soluble neuropathy target esterase (NTE) and acetylcholinesterase (AChE) activities in different regions of brain and spinal cord of adult hens were studied after the intravenous administration of leptophos (30 mg/kg), tri-o-cresyl phosphate (TOCP 40 mg/kg) or dipterex (200 mg/kg). The level of NTE activity varied according to the regions of the central nervous system (CNS) of the control (normal) hen, being higher in the cerebrum (74.1 micromol of phenyl valerate hydrolyzed/10 minutes/mg protein) and in the cerebellum (68.7), and lower in the spinal cord (44.5 in cervical, 55.6 in thoracic and 50.0 in lumbar cord). Hens given leptophos and TOCP demonstrated delayed neuropathy with obvious inhibition of NTE, but the times of onset and the degrees of peak inhibition of NTE activity were different: 6-24 hours after dosing and 73-82% of normal activity for leptophos, and 24-48 hours and 45-80% for TOCP, respectively. Furthermore, the average inhibition of NTE during 6-48 hours after dosing, (called here 'period average inhibition') was also significantly different between the leptophos group (63-73%) and TOCP group (40-64%). Hens given dipterex did not demonstrate delayed neuropathy, and had the least peak inhibition and period average inhibition of NTE activity among the 3 groups. Ratios of NTE inhibition/AChE inhibition were higher in the leptophos group (0.91-1.24) and TOCP group (1.13-2.45) than in the dipterex group (0.25-0.79). These results indicate that the distribution of NTE in the soluble fraction of membrane proteins is different in different regions of the CNS, and that the degree of peak inhibition of NTE activity and the time of onset of peak inhibition induced by organophosphorus compounds (OPs) also differ for different OPs. Thus, practical and useful NTE measurements should identify the peak inhibition and period inhibition in several nervous tissue regions.

Poisoning by organophosphorus insecticides and sensory neuropathy

OBJECTIVES

Poisoning by organophosphate insecticides causes cholinergic toxicity. Organophosphate induced delayed polyneuropathy (OPIDP) is a sensory-motor distal axonopathy which usually occurs after ingestion of large doses of certain organophosphate insecticides and has so far only been reported in patients with preceding cholinergic toxicity. Surprisingly, it was recently reported by other authors that an exclusively sensory neuropathy developed in eight patients after repeated unquantified exposures to chlorpyrifos, which did not cause clear-cut cholinergic toxicity. The objective was to assess whether an exclusively sensory neuropathy develops in patients severely poisoned by various OPs.

METHODS

Toxicological studies and electrophysiological measurements were performed in peripheral motor and sensory nerves in 11 patients after acute organophosphate poisoning among which two subjects were poisoned with chlorpyrifos.

RESULTS

Three patients developed OPIDP, including one poisoned by chlorpyrifos. Exclusively sensory neuropathy was never seen after either single or repeated acute organophosphate poisoning. A mild sensory component was associated with a severe motor component in two of the three cases of OPIDP, the other was an exclusively motor polyneuropathy.

CONCLUSION

A sensory-motor polyneuropathy caused by organophosphate insecticides might occur after a severe poisoning and the sensory component, if present, is milder than the motor one. Bearing in mind the toxicological characteristics of these organophosphate insecticides, other causes should be sought for sensory peripheral neuropathies in patients who did not display severe cholinergic toxicity a few weeks before the onset of symptoms and signs.