Alteration of neurotrophins in the hippocampus and cerebral cortex of young rats exposed to chlorpyrifos and methyl parathion

Effects of chronic insecticide exposure on neuronal network development in vitro in rat cortical cultures

Developmental exposure to carbamates, organophosphates, and pyrethroids has been associated with impaired neurodevelopmental outcomes. Sex-specific differences following chronic insecticide exposure are rather common in vivo. Therefore, we assessed the chronic effects of in vitro exposure to different carbamates (carbaryl, methomyl and aldicarb), organophosphates [chlorpyrifos (CPF), chlorpyrifos-oxon (CPO), and 3,5,6,trichloropyridinol (TCP)], and pyrethroids [permethrin, alpha-cypermethrin and 3-phenoxy benzoic acid (3-PBA)] on neuronal network development in sex-separated rat primary cortical cultures using micro-electrode array (MEA) recordings. Our results indicate that exposure for 1 week to carbaryl inhibited neurodevelopment in male cultures, while a hyperexcitation was observed in female cultures. Methomyl and aldicarb evoked a hyperexcitation after 2 weeks of exposure, which was more pronounced in female cultures. In contrast to acute MEA results, exposure to ≥ 10 µM CPF caused hyperexcitation in both sexes after 10 days. Interestingly, exposure to 10 µM CPO induced a clear hyperexcitation after 10 days of exposure in male but not female cultures. Exposure to 100 µM CPO strongly inhibited neuronal development. Exposure to the type I pyrethroid permethrin resulted in a hyperexcitation at 10 µM and a decrease in neuronal development at 100 µM. In comparison, exposure to ≥ 10 µM of the type II pyrethroid alpha-cypermethrin decreased neuronal development. In female but not in male cultures, exposure to 1 and 10 µM permethrin changed (network) burst patterns, with female cultures having shorter (network) bursts with fewer spikes per (network) burst. Together, these results show that MEA recordings are suitable for measuring sex-specific developmental neurotoxicity in vitro. Additionally, pyrethroid exposure induced effects on neuronal network development at human-relevant concentrations. Finally, chronic exposure has different effects on neuronal functioning compared to acute exposure, highlighting the value of both exposure paradigms.

Prediction of in vivo prenatal chlorpyrifos exposure leading to developmental neurotoxicity in humans based on in vitro toxicity data by quantitative in vitro-in vivo extrapolation

Introduction: Epidemiological studies in children suggested that in utero exposure to chlorpyrifos (CPF), an organophosphate insecticide, may cause developmental neurotoxicity (DNT). We applied quantitative in vitro-in vivo extrapolation (QIVIVE) based on in vitro concentration and non-choline esterase-dependent effects data combined with Benchmark dose (BMD) modelling to predict oral maternal CPF exposure during pregnancy leading to fetal brain effect concentration. By comparing the results with data from epidemiological studies, we evaluated the contribution of the in vitro endpoints to the mode of action (MoA) for CPF-induced DNT. Methods: A maternal-fetal PBK model built in PK-Sim^® was used to perform QIVIVE predicting CPF concentrations in a pregnant women population at 15 weeks of gestation from cell lysate concentrations obtained in human induced pluripotent stem cell-derived neural stem cells undergoing differentiation towards neurons and glia exposed to CPF for 14 days. The in vitro concentration and effect data were used to perform BMD modelling. Results: The upper BMD was converted into maternal doses which ranged from 3.21 to 271 mg/kg bw/day. Maternal CPF blood levels from epidemiological studies reporting DNT findings in their children were used to estimate oral CPF exposure during pregnancy using the PBK model. It ranged from 0.11 to 140 μg/kg bw/day. Discussion: The effective daily intake doses predicted from the in vitro model were several orders of magnitude higher than exposures estimated from epidemiological studies to induce developmental non-cholinergic neurotoxic responses, which were captured by the analyzed in vitro test battery. These were also higher than the in vivo LOEC for cholinergic effects. Therefore, the quantitative predictive value of the investigated non-choline esterase-dependent effects, although possibly relevant for other chemicals, may not adequately represent potential key events in the MoA for CPF-associated DNT.

Combining in vitro assays and mathematical modelling to study developmental neurotoxicity induced by chemical mixtures

Prenatal and postnatal co-exposure to multiple chemicals at the same time may have deleterious effects on the developing nervous system. We previously showed that chemicals acting through similar mode of action (MoA) and grouped based on perturbation of brain derived neurotrophic factor (BDNF), induced greater neurotoxic effects on human induced pluripotent stem cell (hiPSC)-derived neurons and astrocytes compared to chemicals with dissimilar MoA. Here we assessed the effects of repeated dose (14 days) treatments with mixtures containing the six chemicals tested in our previous study (Bisphenol A, Chlorpyrifos, Lead(II) chloride, Methylmercury chloride, PCB138 and Valproic acid) along with 2,2'4,4'-tetrabromodiphenyl ether (BDE47), Ethanol, Vinclozolin and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)), on hiPSC-derived neural stem cells undergoing differentiation toward mixed neurons/astrocytes up to 21 days. Similar MoA chemicals in mixtures caused an increase of BDNF levels and neurite outgrowth, and a decrease of synapse formation, which led to inhibition of electrical activity. Perturbations of these endpoints are described as common key events in adverse outcome pathways (AOPs) specific for DNT. When compared with mixtures tested in our previous study, adding similarly acting chemicals (BDE47 and EtOH) to the mixture resulted in a stronger downregulation of synapses. A synergistic effect on some synaptogenesis-related features (PSD95 in particular) was hypothesized upon treatment with tested mixtures, as indicated by mathematical modelling. Our findings confirm that the use of human iPSC-derived mixed neuronal/glial models applied to a battery of in vitro assays anchored to key events in DNT AOP networks, combined with mathematical modelling, is a suitable testing strategy to assess in vitro DNT induced by chemical mixtures.

Lacto-N-fucopentaose-III ameliorates acute and persisting hippocampal synaptic plasticity and transmission deficits in a Gulf War Illness mouse model

AIMS

The present study investigated if treatment with the immunotherapeutic, lacto-N-fucopentaose-III (LNFPIII), resulted in amelioration of acute and persisting deficits in synaptic plasticity and transmission as well as trophic factor expression along the hippocampal dorsoventral axis in a mouse model of Gulf War Illness (GWI).

MAIN METHODS

Mice received either coadministered or delayed LNFPIII treatment throughout or following, respectively, exposure to a 15-day GWI induction paradigm. Subsets of animals were subsequently sacrificed 48 h, seven months, or 11 months post GWI-related (GWIR) exposure for hippocampal qPCR or in vitro electrophysiology experiments.

KEY FINDINGS

Progressively worsened impairments in hippocampal synaptic plasticity, as well as a biphasic effect on hippocampal synaptic transmission, were detected in GWIR-exposed animals. Dorsoventral-specific impairments in hippocampal synaptic responses became more pronounced over time, particularly in the dorsal hippocampus. Notably, delayed LNFPIII treatment ameliorated GWI-related aberrations in hippocampal synaptic plasticity and transmission seven and 11 months post-exposure, an effect that was consistent with enhanced hippocampal trophic factor expression and absence of increased interleukin 6 (IL-6) in animals treated with LNFPIII.

SIGNIFICANCE

Approximately a third of Gulf War Veterans have GWI; however, GWI therapeutics are presently limited to targeted and symptomatic treatments. As increasing evidence underscores the substantial role of persisting neuroimmune dysfunction in GWI, efficacious neuroactive immunotherapeutics hold substantial promise in yielding GWI remission. The findings in the present report indicate that LNFPIII may be an efficacious candidate for ameliorating persisting neurological abnormalities presented in GWI.

Integrating biokinetics and in vitro studies to evaluate developmental neurotoxicity induced by chlorpyrifos in human iPSC-derived neural stem cells undergoing differentiation towards neuronal and glial cells

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Human iPSC-derived NSCs undergoing differentiation possess some metabolic competence.

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CPF entered the cells and was biotrasformed into its two main metabolites (CPFO and TCP).

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After repeated exposure, very limited bioaccumulation of CPF was observed.

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Treatment with CPF decreased neurite outgrowth, synapse number and electrical activity.

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Treatment with CPF increased BDNF levels and the percentage of astrocytes.

For some complex toxicological endpoints, chemical safety assessment has conventionally relied on animal testing. Apart from the ethical issues, also scientific considerations have been raised concerning the traditional approach, highlighting the importance for considering real life exposure scenario. Implementation of flexible testing strategies, integrating multiple sources of information, including in vitro reliable test methods and in vitro biokinetics, would enhance the relevance of the obtained results. Such an approach could be pivotal in the evaluation of developmental neurotoxicity (DNT), especially when applied to human cell-based models, mimicking key neurodevelopmental processes, relevant to human brain development.

Here, we integrated the kinetic behaviour with the toxicodynamic alterations of chlorpyrifos (CPF), such as in vitro endpoints specific for DNT evaluation, after repeated exposure during differentiation of human neural stem cells into a mixed culture of neurons and astrocytes. The upregulation of some cytochrome P450 and glutathione S-transferase genes during neuronal differentiation and the formation of the two major CPF metabolites (due to bioactivation and detoxification) supported the metabolic competence of the used in vitro model. The alterations in the number of synapses, neurite outgrowth, brain derived neurotrophic factor, the proportion of neurons and astrocytes, as well as spontaneous electrical activity correlated well with the CPF ability to enter the cells and be bioactivated to CPF-oxon. Overall, our results confirm that combining in vitro biokinetics and assays to evaluate effects on neurodevelopmental endpoints in human cells should be regarded as a key strategy for a quantitative characterization of DNT effects.

Protective effects of curcumin and Ganoderma lucidum on hippocampal damage caused by the organophosphate insecticide chlorpyrifos in the developing rat brain: Stereological, histopathological and immunohistochemical study

The aim of this study is to draw attention to the possible effects of chlorpyrifos exposure on the developing rat hippocampus in the prenatal period and to determine whether these effects can be reduced with various antioxidant substances. Pregnant rats were divided into 7 groups.; Chlorpyrifos (CPF), Curcumin (CUR), Ganoderma lucidum (GNL), Chlorpyrifos + Curcumin (CPF + CUR), Chlorpyrifos + Ganoderma lucidum, (CPF + GNL), SHAM and Control (C). After the experiments, brain tissues were evaluated by stereological and immunohistochemical methods. As a result of the stereological analyzes, it was determined that the number of pyramidal neurons in the hippocampus of the CPF group decreased significantly from all other groups. In contrast, the number of neurons in the CPF + CUR and CPF + GNL groups was significantly higher than the CPF group. In addition, immunohistochemical analyzes showed that the density of cells stained with glial fibrillar acidic protein (GFAP) positive in all areas in the hippocampus of the rats in the CPF group was significantly higher compared to the control group, whereas in the CPF + CUR and CPF + GNL groups were less than the CPF group. As a result, the exposure of CPF in the prenatal period caused neurotoxicity in the brain hippocampus, whereas CUR and GNL reduced this toxicity caused by CPF.

Red Beetroot Extract Abrogates Chlorpyrifos-Induced Cortical Damage in Rats

Organophosphorus insecticides including chlorpyrifos (CPF) are mainly used for agriculture, household, and military purposes; their application is associated with various adverse reactions in animals and humans. This study was conducted to evaluate the potential neuroprotective effect of red beetroot methanolic extract (RBR) against CPF-induced cortical damage. Twenty-eight adult male Wistar albino rats were divided into 4 groups (n = 7 in each group): the control group was administered physiological saline (0.9% NaCl), the CPF group was administered CPF (10 mg/kg), the RBR group was administered RBR (300 mg/kg), and the RBR+CPF group was treated with RBR (300 mg/kg) 1 hr before CPF (10 mg/kg) supplementation. All groups were treated for 28 days. Rats exposed to CPF exhibited a significant decrease in cortical acetylcholinesterase activity and brain-derived neurotrophic factor and a decrease in glial fibrillary acidic protein. CPF intoxication increased lipid peroxidation, inducible nitric oxide synthase expression, and nitric oxide production. This was accompanied by a decrease in glutathione content and in the activities of glutathione peroxidase, glutathione reductase, superoxide dismutase, and catalase in the cortical tissue. Additionally, CPF enhanced inflammatory response, indicated by increased levels and expression of interleukin-1β and tumor necrosis factor-α. CPF triggered neuronal apoptosis by upregulating Bax and caspase-3 and downregulating Bcl-2. However, RBR reversed the induced neuronal alterations following CPF intoxication. Our findings suggest that RBR can minimize and prevent CPF neurotoxicity through its antioxidant, anti-inflammatory, and antiapoptotic activities.

Assessment of developmental neurotoxicity induced by chemical mixtures using an adverse outcome pathway concept

BACKGROUND

In light of the vulnerability of the developing brain, mixture risk assessment (MRA) for the evaluation of developmental neurotoxicity (DNT) should be implemented, since infants and children are co-exposed to more than one chemical at a time. One possible approach to tackle MRA could be to cluster DNT chemicals in a mixture on the basis of their mode of action (MoA) into 'similar' and 'dissimilar', but still contributing to the same adverse outcome, and anchor DNT assays to common key events (CKEs) identified in DNT-specific adverse outcome pathways (AOPs). Moreover, the use of human in vitro models, such as induced pluripotent stem cell (hiPSC)-derived neuronal and glial cultures would enable mechanistic understanding of chemically-induced adverse effects, avoiding species extrapolation.

METHODS

HiPSC-derived neural progenitors differentiated into mixed cultures of neurons and astrocytes were used to assess the effects of acute (3 days) and repeated dose (14 days) treatments with single chemicals and in mixtures belonging to different classes (i.e., lead(II) chloride and methylmercury chloride (heavy metals), chlorpyrifos (pesticide), bisphenol A (organic compound and endocrine disrupter), valproic acid (drug), and PCB138 (persistent organic pollutant and endocrine disrupter), which are associated with cognitive deficits, including learning and memory impairment in children. Selected chemicals were grouped based on their mode of action (MoA) into 'similar' and 'dissimilar' MoA compounds and their effects on synaptogenesis, neurite outgrowth, and brain derived neurotrophic factor (BDNF) protein levels, identified as CKEs in currently available AOPs relevant to DNT, were evaluated by immunocytochemistry and high content imaging analysis.

RESULTS

Chemicals working through similar MoA (i.e., alterations of BDNF levels), at non-cytotoxic (IC20/100), very low toxic (IC5), or moderately toxic (IC20) concentrations, induce DNT effects in mixtures, as shown by increased number of neurons, impairment of neurite outgrowth and synaptogenesis (the most sensitive endpoint as confirmed by mathematical modelling) and increase of BDNF levels, to a certain extent reproducing autism-like cellular changes observed in the brain of autistic children.

CONCLUSIONS

Our findings suggest that the use of human iPSC-derived mixed neuronal/glial cultures applied to a battery of assays anchored to key events of an AOP network represents a valuable approach to identify mixtures of chemicals with potential to cause learning and memory impairment in children.

Mechanisms of organophosphorus pesticide toxicity in the context of airway hyperreactivity and asthma

Numerous epidemiologic studies have identified an association between occupational exposures to organophosphorus pesticides (OPs) and asthma or asthmatic symptoms in adults. Emerging epidemiologic data suggest that environmentally relevant levels of OPs may also be linked to respiratory dysfunction in the general population and that in utero and/or early life exposures to environmental OPs may increase risk for childhood asthma. In support of a causal link between OPs and asthma, experimental evidence demonstrates that occupationally and environmentally relevant OP exposures induce bronchospasm and airway hyperreactivity in preclinical models. Mechanistic studies have identified blockade of autoinhibitory M2 muscarinic receptors on parasympathetic nerves that innervate airway smooth muscle as one mechanism by which OPs induce airway hyperreactivity, but significant questions remain regarding the mechanism(s) by which OPs cause neuronal M2 receptor dysfunction and, more generally, how OPs cause persistent asthma, especially after developmental exposures. The goals of this review are to 1) summarize current understanding of OPs in asthma; 2) discuss mechanisms of OP neurotoxicity and immunotoxicity that warrant consideration in the context of OP-induced airway hyperreactivity and asthma, specifically, inflammatory responses, oxidative stress, neural plasticity, and neurogenic inflammation; and 3) identify critical data gaps that need to be addressed in order to better protect adults and children against the harmful respiratory effects of low-level OP exposures.

Cypermethrin, chlorpyrifos, deltamethrin, and imidacloprid exposure up-regulates the mRNA and protein levels of bdnf and c-fos in the brain of adult zebrafish (Danio rerio)

The aim of the present study is to investigate the toxicity effects of frequently used pesticides, involving cypermethrin, deltamethrin, chlorpyrifos and imidacloprid, on the expression of bdnf and c-fos genes in zebrafish brain tissues. Therefore, brain tissues exposed to intoxication was primarily analyzed by indirect immunofluorescence assay. Afterwards, the mRNA transcription levels of BNDF and c-fos genes and the protein levels were measured by qRT-PCR and Western blotting, respectively. The data of the immunofluorescence assay revealed intensive immunopositivity for bdnf and c-fos genes in the tissues exposed to pesticide intoxication in comparison to the control group (p<0.05). Moreover, the transcription levels of BNDF and c-fos genes, and protein levels were elevated following the intoxication (p<0.05, p<0.01, and p<0.001, respectively). These results showed that the exposure to the acute cypermethrin, deltamethrin, chlorpyrifos and imidacloprid intoxication disrupted the normal neuronal activity, resulting in neurotoxic effect, also DNA-binding Increasing c-fos activation, an oncoprotein from the family of the Nuclear Proteins, is also true of the knowledge that these chemicals are oncogenic in zebrafish brain tissues. Thus, the use of these pesticides poses a potential neuronal and oncogenic risk to the non-target organisms.

Specific Effects of Chronic Dietary Exposure to Chlorpyrifos on Brain Gene Expression-A Mouse Study

Chlorpyrifos (CPF) is an organophosphate insecticide used to control pests on a variety of food and feed crops. In mammals, maternal exposure to CPF has been reported to induce cerebral cortex thinning, alteration of long-term brain cognitive function, and Parkinson-like symptoms, but the mechanisms of these processes are not fully understood. In this study, we aimed to gain a deeper understanding of the alterations induced in the brains of mice chronically exposed to CPF by dietary intake. For our purpose, we analysed F1 offspring (sacrificed at 3 and 8 months) of Mus musculus, treated in utero and postnatally with 3 different doses of CPF (0.1-1-10 mg/kg/day). Using RT² Profiler PCR Arrays, we evaluated the alterations in the expression of 84 genes associated with neurodegenerative diseases. In the brains of exposed mice, we evidenced a clear dose–response relationship for AChE inhibition and alterations of gene expression. Some of the genes that were steadily down-regulated, such as Pink1, Park 2, Sv2b, Gabbr2, Sept5 and Atxn2, were directly related to Parkinson’s onset. Our experimental results shed light on the possibility that long-term CPF exposure may exert membrane signalling alterations which make brain cells more susceptible to develop neurodegenerative diseases.

Malathion increases apoptotic cell death by inducing lysosomal membrane permeabilization in N2a neuroblastoma cells: a model for neurodegeneration in Alzheimer's disease

Malathion is an organophosphate with severe neurotoxic effects. Upon acute exposure, malathion initially enhances cholinergic activity by inhibition of acetylcholinesterase, which is its major pathological mechanism. Malathion also induces non-cholinergic neuronal cell death in neurodegenerative conditions; the associated molecular mechanism is not well-characterized. To investigate the molecular mechanism of malathion-induced cell death, N2a mouse neuroblastoma cells were exposed to malathion and cell death-related parameters were examined. Malathion reduced cell viability mainly by apoptosis through mitochondrial dysfunction in N2a cells, as judged by an increase in the level of the pro-apoptotic protein Bax and decrease in the levels of the anti-apoptotic proteins p-Akt and Bcl2, resulting in cytochrome c release and caspase-dependent DNA fragmentation and condensation. Malathion treatment also induced autophagy and lysosomal membrane permeabilization (LMP) in N2a cells. LMP caused a lessening of autophagic flux via inhibition of lysosomal fusion with the autophagosome. LMP-induced cathepsin B release and its proteolytic effect may intensify apoptotic insults. Moreover, malathion-exposed N2a cells showed a marked reduction in the levels of the neuronal marker proteins vascular endothelial growth factor and heart fatty acid binding protein 3, along with diminished neuritogenesis in N2a cells and nerve growth factor secretion in C6 glioma cells. Our data suggest that the non-cholinergic effect of malathion may be mediated by apoptotic cell death via LMP induction in N2a cells. Malathion-treated N2a cells can be utilized as an in vitro model system to screen natural and new chemical drug candidates for neurodegenerative diseases such as Alzheimer’s disease.

Repeated exposure to neurotoxic levels of chlorpyrifos alters hippocampal expression of neurotrophins and neuropeptides

Chlorpyrifos (CPF), an organophosphorus pesticide (OP), is one of the most widely used pesticides in the world. Subchronic exposures to CPF that do not cause cholinergic crisis are associated with problems in cognitive function (i.e., learning and memory deficits), but the biological mechanism(s) underlying this association remain speculative. To identify potential mechanisms of subchronic CPF neurotoxicity, adult male Long Evans (LE) rats were administered CPF at 3 or 10mg/kg/d (s.c.) for 21 days. We quantified mRNA and non-coding RNA (ncRNA) expression profiles by RNA-seq, microarray analysis and small ncRNA sequencing technology in the CA1 region of the hippocampus. Hippocampal slice immunohistochemistry was used to determine CPF-induced changes in protein expression and localization patterns. Neither dose of CPF caused overt clinical signs of cholinergic toxicity, although after 21 days of exposure, cholinesterase activity was decreased to 58% or 13% of control levels in the hippocampus of rats in the 3 or 10mg/kg/d groups, respectively. Differential gene expression in the CA1 region of the hippocampus was observed only in the 10mg/kg/d dose group relative to controls. Of the 1382 differentially expressed genes identified by RNA-seq and microarray analysis, 67 were common to both approaches. Differential expression of six of these genes (Bdnf, Cort, Crhbp, Nptx2, Npy and Pnoc) was verified in an independent CPF exposure study; immunohistochemistry demonstrated that CRHBP and NPY were elevated in the CA1 region of the hippocampus at 10mg/kg/d CPF. Gene ontology enrichment analysis suggested association of these genes with receptor-mediated cell survival signaling pathways. miR132/212 was also elevated in the CA1 hippocampal region, which may play a role in the disruption of neurotrophin-mediated cognitive processes after CPF administration. These findings identify potential mediators of CPF-induced neurobehavioral deficits following subchronic exposure to CPF at a level that inhibits hippocampal cholinesterase to less than 20% of control. An equally significant finding is that subchronic exposure to CPF at a level that produces more moderate inhibition of hippocampal cholinesterase (approximately 50% of control) does not produce a discernable change in gene expression.

Chlorpyrifos and malathion have opposite effects on behaviors and brain size that are not correlated to changes in AChE activity

Organophosphates, a type of neurotoxicant pesticide, are used globally for the treatment of pests on croplands and are therefore found in a large number of conventional foods. These pesticides are harmful and potentially deadly if ingested or inhaled in large quantities by causing a significant reduction in acetylcholinesterase (AChE) activity in the central and peripheral nervous system. However, much less is known about the effects of exposure to small quantities of the pesticides on neural systems and behavior during development. In the current study we used zebrafish larvae in order to determine the effects of two of the most widely used organophosphates, chlorpyrifos and malathion, on zebrafish behavior and AChE activity. Embryos and larvae were exposed to the organophosphates during different time points in development and then tested at 5 days post-fertilization for behavioral, neurodevelopmental and AChE abnormalities. The results of the study indicate that chlorpyrifos and malathion cause opposing behaviors in the larvae such as swim speed (hypoactivity vs. hyperactivity) and rest. Additionally, the pesticides affect only certain behaviors, such as thigmotaxis, during specific time points in development that are unrelated to changes in AChE activity. Larvae treated with malathion but not chlorpyrifos also had significantly smaller forebrain and hindbrain regions compared to controls by 5 days post-fertilization. We conclude that exposure to very low concentrations of organophosphate pesticides during development cause abnormalities in behavior and brain size.

Neuronal connectivity as a convergent target of gene × environment interactions that confer risk for Autism Spectrum Disorders

Evidence implicates environmental factors in the pathogenesis of Autism Spectrum Disorders (ASD). However, the identity of specific environmental chemicals that influence ASD risk, severity or treatment outcome remains elusive. The impact of any given environmental exposure likely varies across a population according to individual genetic substrates, and this increases the difficulty of identifying clear associations between exposure and ASD diagnoses. Heritable genetic vulnerabilities may amplify adverse effects triggered by environmental exposures if genetic and environmental factors converge to dysregulate the same signaling systems at critical times of development. Thus, one strategy for identifying environmental risk factors for ASD is to screen for environmental factors that modulate the same signaling pathways as ASD susceptibility genes. Recent advances in defining the molecular and cellular pathology of ASD point to altered patterns of neuronal connectivity in the developing brain as the neurobiological basis of these disorders. Studies of syndromic ASD and rare highly penetrant mutations or CNVs in ASD suggest that ASD risk genes converge on several major signaling pathways linked to altered neuronal connectivity in the developing brain. This review briefly summarizes the evidence implicating dysfunctional signaling via Ca(2+)-dependent mechanisms, extracellular signal-regulated kinases (ERK)/phosphatidylinositol-3-kinases (PI3K) and neuroligin-neurexin-SHANK as convergent molecular mechanisms in ASD, and then discusses examples of environmental chemicals for which there is emerging evidence of their potential to interfere with normal neuronal connectivity via perturbation of these signaling pathways.

Pesticide exposure and neurodevelopmental outcomes: review of the epidemiologic and animal studies

Assessment of whether pesticide exposure is associated with neurodevelopmental outcomes in children can best be addressed with a systematic review of both the human and animal peer-reviewed literature. This review analyzed epidemiologic studies testing the hypothesis that exposure to pesticides during pregnancy and/or early childhood is associated with neurodevelopmental outcomes in children. Studies that directly queried pesticide exposure (e.g., via questionnaire or interview) or measured pesticide or metabolite levels in biological specimens from study participants (e.g., blood, urine, etc.) or their immediate environment (e.g., personal air monitoring, home dust samples, etc.) were eligible for inclusion. Consistency, strength of association, and dose response were key elements of the framework utilized for evaluating epidemiologic studies. As a whole, the epidemiologic studies did not strongly implicate any particular pesticide as being causally related to adverse neurodevelopmental outcomes in infants and children. A few associations were unique for a health outcome and specific pesticide, and alternative hypotheses could not be ruled out. Our survey of the in vivo peer-reviewed published mammalian literature focused on effects of the specific active ingredient of pesticides on functional neurodevelopmental endpoints (i.e., behavior, neuropharmacology and neuropathology). In most cases, effects were noted at dose levels within the same order of magnitude or higher compared to the point of departure used for chronic risk assessments in the United States. Thus, although the published animal studies may have characterized potential neurodevelopmental outcomes using endpoints not required by guideline studies, the effects were generally observed at or above effect levels measured in repeated-dose toxicology studies submitted to the U.S. Environmental Protection Agency (EPA). Suggestions for improved exposure assessment in epidemiology studies and more effective and tiered approaches in animal testing are discussed.

Alterations of neocortical development and maturation in autism: insight from valproic acid exposure and animal models of autism

Autism spectrum disorder (ASD) is a behaviourally defined brain disorder affecting approximately 1 in 88 children. Many pathological studies have shown that ASD is frequently associated with grey and white matter changes that can be described by their deviations from the normal trajectory of cortical maturation. For example, during the early (i.e. <2 years) postnatal period there is marked and selective tissue overgrowth in the higher-order temporal and frontal networks involved in emotional, social, and communication functions. In this focused review we first summarize some basic principles of neocortical neural organization and how they are disrupted in ASD. We will then highlight some of the potential mechanisms by which the normal developmental trajectory and organization of neocortical networks can be altered based on animal studies of valproic acid, a teratogen widely used in animal models of ASD. We argue that the trajectory of postnatal cerebral neocortex development may be influenced by several cellular and molecular mechanisms that may all converge to produce a neuropathology characterized by premature or accelerated neuronal growth.

Chlorpyrifos and neurodevelopmental effects: a literature review and expert elicitation on research and policy

BACKGROUND

Organophosphate pesticides are widely used on food crops grown in the EU. While they have been banned from indoor use in the US for a decade due to adverse health effects, they are still the most prevalent pesticides in the EU, with Chlorpyrifos (CPF) being the most commonly applied. It has been suggested CPF affects neurodevelopment even at levels below toxicity guidelines. Younger individuals may be more susceptible than adults due to biological factors and exposure settings.

METHODS

A literature review was undertaken to assess the evidence for CPF contributing to neurodevelopmental disorders in infants and children. Other literature was consulted in order to formulate a causal chain diagram showing the origins, uptake, and neurological effects of animal and human exposure to CPF.The causal chain diagram and a questionnaire were distributed online to scientific experts who had published in relevant areas of research. They were asked to assess their confidence levels on whether CPF does in fact contribute to adverse neurodevelopment outcomes and rate their confidence in the scientific evidence. A second questionnaire queried experts as to which kind of policy action they consider justifiable based on current knowledge. In a special workshop session at the EuroTox congress in Dresden in 2009 the results of both questionnaires were further discussed with invited experts, as a basis for a policy brief with main messages for policy makers and stakeholders.

RESULTS

Most experts who responded to the first questionnaire felt that there was already enough evidence to support a ban on indoor uses of CPF in the EU. However, most felt additional research is still required in several areas. The responses from the first questionnaire were used to formulate the second questionnaire addressing the feasibility of government action. In turn, these expert participants were invited to attend a special session at the EuroTox congress in Dresden in 2009.

CONCLUSIONS

Some of the evidence that CPF contributes to neurodevelopmental disorders is still disputed among experts, and the overall sense is that further research and public awareness are warranted. There have been campaigns in North America making the potential exposure concerns known, but such information is not widely known in the EU. The ability of government action to produce change is strongly felt in some quarters while others believe better knowledge of consumer use trends would have a greater impact.

Steady and fluctuant methods of inhibition of acetylcholinesterase differentially regulate neurotrophic factors in the hippocampus of juvenile mice

The present study was designed to evaluate the effects of steady and fluctuant inhibition of acetylcholinesterase (AChE) activity on neurotrophic factors in the hippocampus of juvenile mice. Steady inhibition of AChE activity was induced by an intramuscular injection of huperizine A (HupA) sustained-release microspheres. Fluctuant inhibition of AChE activity was induced by an intragastric administration of HupA tablets. Six days after cessation of steady AChE inhibition, there was a significant increase in the levels of brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF). In contrast, fluctuant AChE inhibition had no effect on BDNF and NGF levels. Additionally, neither steady nor fluctuant inhibition of AChE activity altered the choline acetyltransferase activity or spatial learning in juvenile mice. These findings indicate that steady and fluctuant methods of inhibition of AChE have different effects on the levels of BDNF and NGF in the hippocampus. In addition, the effects of AChE inhibitors may not improve learning in normal juvenile animals.

Delayed reduction of hippocampal synaptic transmission and spines following exposure to repeated subclinical doses of organophosphorus pesticide in adult mice

Agricultural and household organophosphorus (OP) pesticides inhibit acetylcholinesterase (AchE), resulting in increased acetylcholine (Ach) in the central nervous system. In adults, acute and prolonged exposure to high doses of AchE inhibitors causes severe, clinically apparent symptoms, followed by lasting memory impairments and cognitive dysfunction. The neurotoxicity of repeated environmental exposure to lower, subclinical doses of OP pesticides in adults is not as well studied. However, repeated exposure to acetylcholinesterase inhibitors, such as chlorpyrifos (CPF), pyridostigmine, and sarin nerve agent, has been epidemiologically linked to delayed onset symptoms in Gulf War Illness and may be relevant to environmental exposure in farm workers among others. We treated adult mice with a subclinical dose (5 mg/kg) of CPF for 5 consecutive days and investigated hippocampal synaptic transmission and spine density early (2-7 days) and late (3 months) after CPF administration. No signs of cholinergic toxicity were observed at any time during or after treatment. At 2-7 days after the last injection, we found increased synaptic transmission in the CA3-CA1 region of the hippocampus of CPF-treated mice compared with controls. In contrast, at 3 months after CPF administration, we observed a 50% reduction in synaptic transmission likely due to a corresponding 50% decrease in CA1 pyramidal neuron synaptic spine density. This study is the first to identify a biphasic progression of synaptic abnormalities following repeated OP exposure and suggests that even in the absence of acute cholinergic toxicity, repeated exposure to CPF causes delayed persistent damage to the adult brain in vivo.

Evaluation of epidemiology and animal data for risk assessment: chlorpyrifos developmental neurobehavioral outcomes

Developmental neurobehavioral outcomes attributed to exposure to chlorpyrifos (CPF) obtained from epidemiologic and animal studies published before June 2010 were reviewed for risk assessment purposes. For epidemiological studies, this review considered (1) overall strength of study design, (2) specificity of CPF exposure biomarkers, (3) potential for bias, and (4) Hill guidelines for causal inference. In the case of animal studies, this review focused on evaluating the consistency of outcomes for developmental neurobehavioral endpoints from in vivo mammalian studies that exposed dams and/or offspring to CPF prior to weaning. Developmental neuropharmacologic and neuropathologic outcomes were also evaluated. Experimental design and methods were examined as part of the weight of evidence. There was insufficient evidence that human developmental exposures to CPF produce adverse neurobehavioral effects in infants and children across different cohort studies that may be relevant to CPF exposure. In animals, few behavioral parameters were affected following gestational exposures to 1 mg/kg-d but were not consistently reported by different laboratories. For postnatal exposures, behavioral effects found in more than one study at 1 mg/kg-d were decreased errors on a radial arm maze in female rats and increased errors in males dosed subcutaneously from postnatal day (PND) 1 to 4. A similar finding was seen in rats exposed orally from PND 1 to 21 with incremental dose levels of 1, 2, and 4 mg/kg-d, but not in rats dosed with constant dose level of 1 mg/kg-d. Neurodevelopmental behavioral, pharmacological, and morphologic effects occurred at doses that produced significant brain or red blood cell acetylcholinesterase inhibition in dams or offspring.

Effect of developmental chlorpyrifos exposure, on endocannabinoid metabolizing enzymes, in the brain of juvenile rats

The endogenous cannabinoids 2-arachidonoylglycerol (2-AG) and arachidonoyl ethanolamide (AEA or anandamide) play vital roles during nervous system development including regulating axonal guidance and synaptogenesis. The enzymatic degradation of 2-AG and AEA is highly susceptible to inhibition by organophosphate compounds in vitro. Furthermore, acute in vivo exposure of adult animals to the agricultural insecticide chlorpyrifos (CPS) caused moderate inhibition of both 2-AG and AEA hydrolysis. However, the effects of repeated exposure to lower levels of CPS, especially during development, on endocannabinoid metabolism in the brain is not known. To examine this, rat pups were orally exposed daily from postnatal days 10-16 to either 1.0, 2.5, or 5.0 mg/kg CPS. Body weight gain was reduced by 5.0 mg/kg on all days of treatment whereas 2.5 mg/kg reduced the weight gain only on the last two days of treatment. At 4-h postexposure on day 16, forebrain cholinesterase (ChE) activity and hydrolysis of 2-AG and AEA were inhibited in a dose-related manner, and the extent of inhibition from highest to lowest level was AEA hydrolysis > ChE activity > 2-AG hydrolysis. The extent of inhibition of AEA hydrolysis was approximately twice than that of ChE activity with AEA hydrolysis being virtually eliminated by 2.5 and 5.0 mg/kg and 1.0 mg/kg causing 40% inhibition. The sensitivity of AEA hydrolysis, compared with canonical targets such as ChE activity, suggests a potential alternative developmental target for CPS. Inhibition of AEA hydrolysis could result in accumulation of endocannabinoids, which could alter normal endocannabinoid transmission during brain maturation.

Dose-related gene expression changes in forebrain following acute, low-level chlorpyrifos exposure in neonatal rats

Chlorpyrifos (CPF) is a widely used organophosphorus insecticide (OP) and putative developmental neurotoxicant in humans. The acute toxicity of CPF is elicited by acetylcholinesterase (AChE) inhibition. We characterized dose-related (0.1, 0.5, 1 and 2mg/kg) gene expression profiles and changes in cell signaling pathways 24h following acute CPF exposure in 7-day-old rats. Microarray experiments indicated that approximately 9% of the 44,000 genes were differentially expressed following either one of the four CPF dosages studied (546, 505, 522, and 3,066 genes with 0.1, 0.5, 1.0 and 2.0mg/kg CPF). Genes were grouped according to dose-related expression patterns using K-means clustering while gene networks and canonical pathways were evaluated using Ingenuity Pathway Analysis®. Twenty clusters were identified and differential expression of selected genes was verified by RT-PCR. The four largest clusters (each containing from 276 to 905 genes) constituted over 50% of all differentially expressed genes and exhibited up-regulation following exposure to the highest dosage (2mg/kg CPF). The total number of gene networks affected by CPF also rose sharply with the highest dosage of CPF (18, 16, 18 and 50 with 0.1, 0.5, 1 and 2mg/kg CPF). Forebrain cholinesterase (ChE) activity was significantly reduced (26%) only in the highest dosage group. Based on magnitude of dose-related changes in differentially expressed genes, relative numbers of gene clusters and signaling networks affected, and forebrain ChE inhibition only at 2mg/kg CPF, we focused subsequent analyses on this treatment group. Six canonical pathways were identified that were significantly affected by 2mg/kg CPF (MAPK, oxidative stress, NFΚB, mitochondrial dysfunction, arylhydrocarbon receptor and adrenergic receptor signaling). Evaluation of different cellular functions of the differentially expressed genes suggested changes related to olfactory receptors, cell adhesion/migration, synapse/synaptic transmission and transcription/translation. Nine genes were differentially affected in all four CPF dosing groups. We conclude that the most robust, consistent changes in differential gene expression in neonatal forebrain across a range of acute CPF dosages occurred at an exposure level associated with the classical marker of OP toxicity, AChE inhibition. Disruption of multiple cellular pathways, in particular cell adhesion, may contribute to the developmental neurotoxicity potential of this pesticide.

Toxicogenomic profiling in maternal and fetal rodent brains following gestational exposure to chlorpyrifos

Considering the wide variety of effects that have been reported to occur in the developmental neurotoxicity of chlorpyrifos (CP) and the lack of consensus on their dependence of brain acetylcholinesterase (AChE) activity inhibition, we applied microarray technology to explore dose-dependent alterations in transcriptional response in the fetal and maternal C57BL/6 mouse brain after daily gestational exposure (days 6 to 17) to CP (2, 4, 10, 12 or 15 mg/kg, sc). We identified significantly altered genes across doses and assessed for overrepresentation of Gene Ontology (GO) biological processes and KEGG pathways. We further clustered genes based on their expression profiles across doses and repeated the GO/pathways analysis for each cluster. The dose-effect relationship of CP on gene expression, both at the gene and pathway levels was non-monotonic and not necessarily related to brain AChE inhibition. The largest impact was observed in the 10mg/kg dose group which was also the LOAEL for brain AChE inhibition. In the maternal brain, lower doses (4 mg/kg) influenced GO categories and pathways such as cell adhesion, behavior, lipid metabolism, long-term potentiation, nervous system development, neurogenesis, synaptic transmission. In the fetal brain, lower doses (2 and/or 4 mg/kg) significantly altered cell division, translation, transmission of nerve impulse, chromatin modification, long-term potentiation. In addition, some genes involved in nervous system development and signaling were shown to be specifically influenced by these lower CP doses. Our approach was sensitive and reflected the diversity of responses known to be disrupted by CP and highlighted possible additional consequences of CP neurotoxicity, such as disturbance of the ubiquitin proteasome system.

Unrelated developmental neurotoxicants elicit similar transcriptional profiles for effects on neurotrophic factors and their receptors in an in vitro model

Diverse developmental neurotoxicants can often produce similar functional and behavioral outcomes. We examined an organophosphate pesticide (diazinon), an organochlorine pesticide (dieldrin) and a metal (Ni(2+)) for effects on the expression of neurotrophic factors and their receptors and modulators in differentiating PC12 cells, an in vitro model of neuronal development. Each agent was introduced at 30 microM for 24 or 72 h, treatments devoid of cytotoxicity. Using microarrays, we examined the mRNAs encoding members of the fibroblast growth factor (fgf) family, the neurotrophins (ntfs), brain-derived neurotrophic factor (bdnf), nerve growth factor (ngf), the wnt and fzd gene families, and the receptors and modulators for each class. All three agents evoked highly concordant patterns of effects on genes encoding the fgf family, whereas the correlations were poor for the group comprising bdnf, ngf and their respective receptors. For wnt, fzd and their receptors/modulators, the relationships between diazinon and dieldrin were highly concordant, whereas the effect of Ni(2+) was less similar, albeit still significantly correlated with the others. Our results show that otherwise disparate developmental neurotoxicants converge on common sets of neurotrophic pathways known to control neuronal differentiation, likely contributing to similarities in functional outcomes. Further, cell culture models can provide a useful initial screen to identify members of a given class of compounds that may be greater or lesser risks for developmental neurotoxicity, or to provide an indication of agents in different classes that might produce similar effects.

Transcriptional profiles reveal similarities and differences in the effects of developmental neurotoxicants on differentiation into neurotransmitter phenotypes in PC12 cells

Unrelated developmental neurotoxicants nevertheless converge on common functional and behavioral outcomes. We used PC12 cells, a model of neuronal development, to explore similarities and differences for organophosphate pesticides (chlorpyrifos, diazinon), an organochlorine pesticide (dieldrin) and a metal (Ni(2+)), focusing on transcriptional profiles related to differentiation into acetylcholine, dopamine and norepinephrine phenotypes. Agents were introduced at 30 microM for 24 or 72 h, treatments devoid of cytotoxicity. Using microarrays, we examined the mRNAs encoding the proteins involved in neurotransmitter biosynthesis, storage, and degradation, along with the complete panoply of receptors for each transmitter. All three pesticides evoked concordant patterns of effects on genes involved in neural growth and neurite extension, with a distinctly different pattern for Ni(2+). All four toxicants promoted differentiation into the dopamine phenotype at the expense of the acetylcholine phenotype, involving separable effects of each agent on the various gene families; however, there were major differences in the ability of each to promote or repress the norepinephrine phenotype. Chlorpyrifos and diazinon, although displaying many similarities in their transcriptional profiles, also showed major disparities in keeping with their known differences in synaptic and behavioral outcomes after neonatal exposures to these agents in vivo. Surprisingly, there were closer similarities among diazinon, dieldrin and Ni(2+) than for each agent to chlorpyrifos. Our results illustrate how cell culture systems, combined with microarray technology, can screen for developmental neurotoxicants, serving as a model for alternative approaches to the detection and characterization of the impact of exogenous chemicals on brain development.

Effect of different administration paradigms on cholinesterase inhibition following repeated chlorpyrifos exposure in late preweanling rats

Chlorpyrifos (CPS) is widely used in agricultural settings and residue analysis has suggested that children in agricultural communities are at risk of exposure. This has resulted in a large amount of literature investigating the potential for CPS-induced developmental neurotoxic effects. Two developmental routes of administration of CPS are orally in corn oil at a rate of 0.5 ml/kg and subcutaneously in dimethyl sulfoxide (DMSO) at a rate of 1.0 ml/kg. For comparison between these methods, rat pups were exposed daily from days 10 to 16 to CPS (5 mg/kg) either orally dissolved in corn oil or subcutaneously dissolved in DMSO, both at rates of either 0.5 or 1.0 ml/kg. A representative vehicle/route group was present for each treatment. Both the low and high volume CPS in DMSO subcutaneous groups were lower than that of the low and high volume CPS in oil oral groups. At 4 h following the final administration, serum carboxylesterase was inhibited > 90% with all treatments. For cholinesterase activity in the cerebellum, medulla-pons, forebrain, and hindbrain, and serum, inhibition in the CPS-oil groups was similar and inhibition in the CPS-DMSO groups was similar. However, significantly greater inhibition was present in the high volume CPS-DMSO group as compared to the CPS-oil groups. Inhibition in the low volume CPS-DMSO group was generally between that in the CPS-oil groups and the high volume CPS-DMSO group. These data suggest that using DMSO as a vehicle for CPS may alter the level of brain ChE inhibition.

Reproductive Toxicity Evaluation of Pestban Insecticide Exposure in Male and Female Rats

Sexually mature male and female rats were orally intubated with the organophosphorus insecticide, Pestban at a daily dosage of 7.45 or 3.72 mg/kg bwt, equivalent to 1/20 and 1/40 LD50, respectively. Male rats were exposed for 70 days, while the female rats were exposed for 14 days, premating, during mating and throughout the whole length of gestation and lactation periods till weaning. The results showed depressed acetylcholinesterase (AChE) activity in the brain of parents, fetuses and their placentae in a dose-dependent manner. The fertility was significantly reduced with increasing the dose in both treated groups, with more pronounced suppressive effects in the male treated group. The number of implantation sites and viable fetuses were significantly reduced in pregnant females of both treated groups. However, the number of resorptions, dead fetuses, and pre-and postimplantation losses were significantly increased. The incidence of resorptions was more pronounced in treated female compared to male group and was dose dependant. The behavioral responses as well as fetal survival and viability indices were altered in both treated groups during the lactation period. The incidence of these effects was more pronounced in the treated female group and occurred in a dose-related manner. The recorded morphological, visceral, and skeletal anomalies were significantly increased with increasing the dose in fetuses of both treated groups, with more pronounced effects on fetuses of treated females. In conclusion, the exposure of adult male and female rats to Pestban would cause adverse effects on fertility and reproduction.