Transcriptional biomarkers distinguish between vulnerable periods for developmental neurotoxicity of chlorpyrifos: Implications for toxicogenomics

Whole transcriptome analysis in offspring whose fathers were exposed to a developmental insult: a novel avian model

Although the effects of paternal exposure to insults on the offspring received limited attention in the past, it is currently gaining interest especially after understanding the mechanisms which may mediate such exposure effects. In the current study, the well-controlled avian model (Fayoumi) was utilized to investigate the effects of paternal exposure to the developmental insult, chlorpyrifos on the offspring's gene expression via mRNA and small RNA sequencing. Numerous mRNA gene expression changes were detected in the offspring after paternal exposure to the developmental insult, especially in genes related to neurogenesis, learning and memory. qPCR analysis of several genes, that were significantly changed in mRNA sequencing, confirmed the results obtained in mRNA sequencing. On the other hand, small RNA sequencing did not identify significant microRNA genes expression changes in the offspring after paternal exposure to the developmental insult. The effects of the paternal exposure were more pronounced in the female offspring compared to the male offspring. The results identified expression alterations in major genes (some of which were pertinent to the functional changes observed in other forms of early developmental exposure) after paternal insult exposure and provided a direction for future studies involving the most affected genes.

Parental Preconception and Pre-Hatch Exposure to a Developmental Insult Alters Offspring's Gene Expression and Epigenetic Regulations: An Avian Model

Parental exposure to insults was initially considered safe if stopped before conception. In the present investigation, paternal or maternal preconception exposure to the neuroteratogen chlorpyrifos was investigated in a well-controlled avian model (Fayoumi) and compared to pre-hatch exposure focusing on molecular alterations. The investigation included the analysis of several neurogenesis, neurotransmission, epigenetic and microRNA genes. A significant decrease in the vesicular acetylcholine transporter (SLC18A3) expression was detected in the female offspring in the three investigated models: paternal (57.7%, p < 0.05), maternal (36%, p < 0.05) and pre-hatch (35.6%, p < 0.05). Paternal exposure to chlorpyrifos also led to a significant increase in brain-derived neurotrophic factor (BDNF) gene expression mainly in the female offspring (27.6%, p < 0.005), while its targeting microRNA, miR-10a, was similarly decreased in both female (50.5%, p < 0.05) and male (56%, p < 0.05) offspring. Doublecortin's (DCX) targeting microRNA, miR-29a, was decreased in the offspring after maternal preconception exposure to chlorpyrifos (39.8%, p < 0.05). Finally, pre-hatch exposure to chlorpyrifos led to a significant increase in protein kinase C beta (PKCß; 44.1%, p < 0.05), methyl-CpG-binding domain protein 2 (MBD2; 44%, p < 0.01) and 3 (MBD3; 33%, p < 0.05) genes expression in the offspring. Although extensive studies are required to establish a mechanism-phenotype relationship, it should be noted that the current investigation does not include phenotype assessment in the offspring.

Health Effects of Pesticides on Pregnant Women and Children

Pesticides, along with hybrid seeds and fertilizers, are an integral part of the green revolution and are used to control and eradicate disease vectors for the improvement of agricultural production. Pesticides is an umbrella term for insecticides, nematocides, fungicides, herbicides, fumigants, repellents, and attractants. Pesticides are used against unwanted plants and animals to control diseases and losses. Efforts at different levels may help to reduce the impact of pesticides on newborn babies and on pregnant women. Different efforts can be considered at clinical, educational, and policymaking institutes. Use of risk assessment tools, encouragement of organic diets, educating parents working in agricultural fields from hazards of pesticides particularly in pregnancy and breast feeding, implementation of integrated pest management (IPM) programs, and encouraging policies supporting IPM can help in tackling the menace of pesticide hazards.

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.

The neurotoxin diethyl dithiophosphate impairs glutamate transport in cultured Bergmann glia cells

Glutamate, the main excitatory neurotransmitter in the vertebrate Central Nervous System, is involved in almost every aspect of brain physiology, and its signaling properties are severely affected in most neurodegenerative diseases. This neurotransmitter has to be efficiently removed from the synaptic cleft in order to prevent an over-stimulation of glutamate receptors that leads to neuronal death. Specific sodium-dependent membrane transporters, highly enriched in glial cells, elicit the clearance of glutamate. Once internalized, it is metabolized to glutamine by the glia-enriched enzyme Glutamine synthetase. Accumulated glutamine is released into the extracellular space for its uptake into pre-synaptic neurons and its conversion to glutamate that is packed into synaptic vesicles completing the glutamate/glutamine cycle. Diverse chemical compounds, like organophosphates, directly affect brain chemistry by altering levels of neurotransmitters in the synaptic cleft. Organophosphate compounds are widely used as pesticides, and all living organisms are continuously exposed to these substances, either in a direct or indirect manner. Its metabolites, like the diethyl dithiophosphate, are capable of causing brain damage through diverse mechanisms including perturbation of neuronal-glial cell interactions and have been associated with attention-deficit disorders and other mental illness. In order to characterize the neurotoxic mechanisms of diethyl dithiophosphate, we took advantage of the well characterized model of chick cerebellar Bergmann glia cultures. A significant impairment of [³H] d-Aspartate transport was found upon exposure to the metabolite. These results indicate that glia cells are targets of neurotoxic substances such as pesticides and that these cells might be critically involved in the associated neuronal death.

Research Review: Environmental exposures, neurodevelopment, and child mental health - new paradigms for the study of brain and behavioral effects

BACKGROUND

Environmental exposures play a critical role in the genesis of some child mental health problems.

METHODS

We open with a discussion of children's vulnerability to neurotoxic substances, changes in the distribution of toxic exposures, and cooccurrence of social and physical exposures. We address trends in prevalence of mental health disorders, and approaches to the definition of disorders that are sensitive to the subtle effects of toxic exposures. We suggest broadening outcomes to include dimensional measures of autism spectrum disorders, attention-deficit hyperactivity disorder, and child learning capacity, as well as direct assessment of brain function.

FINDINGS

We consider the impact of two important exposures on children's mental health: lead and pesticides. We argue that longitudinal research designs may capture the cascading effects of exposures across biological systems and the full-range of neuropsychological endpoints. Neuroimaging is a valuable tool for observing brain maturation under varying environmental conditions. A dimensional approach to measurement may be sensitive to subtle subclinical toxic effects, permitting the development of exposure-related profiles and testing of complex functional relationships between brain and behavior. Questions about the neurotoxic effects of chemicals become more pressing when viewed through the lens of environmental justice.

CONCLUSIONS

Reduction in the burden of child mental health disorders will require longitudinal study of neurotoxic exposures, incorporating dimensional approaches to outcome assessment, and measures of brain function. Research that seeks to identify links between toxic exposures and mental health outcomes has enormous public health and societal value.

Role of p53, Mitochondrial DNA Deletions, and Paternal Age in Autism: A Case-Control Study

BACKGROUND

The tumor suppressor p53 responds to a variety of environmental stressors by regulating cell cycle arrest, apoptosis, senescence, DNA repair, bioenergetics and mitochondrial DNA (mtDNA) copy number maintenance. Developmental abnormalities have been reported in p53-deficient mice, and altered p53 and p53-associated pathways in autism (AU). Furthermore, via the Pten-p53 crosstalk, Pten haploinsufficient-mice have autisticlike behavior accompanied by brain mitochondrial dysfunction with accumulation of mtDNA deletions.

METHODS

mtDNA copy number and deletions, and p53 gene copy ratios were evaluated in peripheral blood monocytic cells from children aged 2-5 years with AU (n = 66), race-, gender-, and age-matched typically neurodeveloping children (n = 46), and both parents from each diagnostic group, recruited by the Childhood Autism Risk from Genes and Environment study at the University of California, Davis.

RESULTS

mtDNA deletions and higher p53 gene copy ratios were more common in children with AU and their fathers. The incidence of mtDNA deletions in fathers of children with AU was increased 1.9-fold over fathers of typically neurodeveloping children, suggesting a role for deficient DNA repair capacity not driven by paternal age. Deletions in mtDNA and altered p53 gene copy ratios seem to result from genetics (children with severity scores ≥8) and/or act in concert with environmental factors (children with 6-7 severity scores).

CONCLUSIONS

Given pro- and antioxidant activities of p53, and associations of genomic instability with disorders other than AU, our study suggests a link between DNA repair capacity, genomic instability in the 17p13.1 region influenced by environmental triggers, and AU diagnosis.

Dialkyl phosphate urinary metabolites and chromosomal abnormalities in human sperm

BACKGROUND

The past decade has seen numerous human health studies seeking to characterize the impacts of environmental exposures, such as organophosphate (OP) insecticides, on male reproduction. Despite an extensive literature on OP toxicology, many hormone-mediated effects on the testes are not well understood.

OBJECTIVES

This study investigated environmental exposures to OPs and their association with the frequency of sperm chromosomal abnormalities (i.e., disomy) among adult men.

METHODS

Men (n=159) from a study assessing the impact of environmental exposures on male reproductive health were included in this investigation. Multi-probe fluorescence in situ hybridization (FISH) for chromosomes X, Y, and 18 was used to determine XX18, YY18, XY18 and total disomy in sperm nuclei. Urine was analyzed using gas chromatography coupled with mass spectrometry for concentrations of dialkyl phosphate (DAP) metabolites of OPs [dimethylphosphate (DMP); dimethylthiophosphate (DMTP); dimethyldithiophosphate (DMDTP); diethylphosphate (DEP); diethylthiophosphate (DETP); and diethyldithiophosphate (DEDTP)]. Poisson regression was used to model the association between OP exposures and disomy measures. Incidence rate ratios (IRRs) were calculated for each disomy type by exposure quartiles for most metabolites, controlling for age, race, BMI, smoking, specific gravity, total sperm concentration, motility, and morphology.

RESULTS

A significant positive trend was seen for increasing IRRs by exposure quartiles of DMTP, DMDTP, DEP and DETP in XX18, YY18, XY18 and total disomy. A significant inverse association was observed between DMP and total disomy. Findings for total sum of DAP metabolites concealed individual associations as those results differed from the patterns observed for each individual metabolite. Dose-response relationships appeared nonmonotonic, with most of the increase in disomy rates occurring between the second and third exposure quartiles and without additional increases between the third and fourth exposure quartiles.

CONCLUSIONS

This is the first epidemiologic study of this size to examine the relationship between environmental OP exposures and human sperm disomy outcomes. Our findings suggest that increased disomy rates were associated with specific DAP metabolites, suggesting that the impacts of OPs on testis function need further characterization in epidemiologic studies.

Fluorescence spectroscopy approaches for the development of a real-time organophosphate detection system using an enzymatic sensor

Organophosphates are organic substances that contain a phosphoryl or a thiophosphoryl bond. They are mainly used around the world as pesticides, but can also be used as chemical warfare agents. Their detection is normally entrusted to techniques like GC- and LC-MS that, although sensitive, do not allow their identification on site and in real time. We have approached their identification by exploiting the high-affinity binding of these compounds with the esterase 2 from Alicyclobacillus acidocaldarius. Using an in silico analysis to evaluate the binding affinities of the enzyme with organophosphate inhibitors, like paraoxon, and other organophosphate compounds, like parathion, chlorpyriphos, and other organophosphate thio-derivatives, we have designed fluorescence spectroscopy experiments to study the quenching of the tryptophan residues after esterase 2 binding with the organophosphate pesticides. The changes in the fluorescence signals permitted an immediate and quantitative identification of these compounds from nano- to picomolar concentrations. A fluorescence based polarity-sensitive probe (ANS) was also employed as a means to understand the extent of the interactions involved, as well as to explore other ways to detect organophosphate pesticides. Finally, we designed a framework for the development of a biosensor that exploits fluorescence technology in combination with a sensitive and very stable bio-receptor.

Prenatal exposure to organophosphate pesticides and reciprocal social behavior in childhood

Prenatal exposure to organophosphate pesticides (OPs) has been associated with adverse neurodevelopmental outcomes in childhood, including low IQ, pervasive developmental disorder (PDD), attention problems and ADHD. Many of these disorders involve impairments in social functioning. Thus, we investigated the relationship between biomarkers of prenatal OP exposure and impaired reciprocal social behavior in childhood, as measured by the Social Responsiveness Scale (SRS). Using a multi-ethnic urban prospective cohort of mother-infant pairs in New York City recruited between 1998 and 2002 (n=404) we examined the relation between third trimester maternal urinary levels of dialkylphosphate (ΣDAP) OP metabolites and SRS scores among 136 children who returned for the 7-9year visit. Overall, there was no association between OPs and SRS scores, although in multivariate adjusted models, associations were heterogeneous by race and by sex. Among blacks, each 10-fold increase in total diethylphosphates (ΣDEP) was associated with poorer social responsiveness (β=5.1 points, 95% confidence interval (CI) 0.8, 9.4). There was no association among whites or Hispanics, or for total ΣDAP or total dimethylphosphate (ΣDMP) biomarker levels. Additionally, stratum-specific models supported a stronger negative association among boys for ΣDEPs (β=3.5 points, 95% CI 0.2, 6.8), with no notable association among girls. Our results support an association of prenatal OP exposure with deficits in social functioning among blacks and among boys, although this may be in part reflective of differences in exposure patterns.

Induction of endocannabinoid levels in juvenile rat brain following developmental chlorpyrifos exposure

The endogenous cannabinoids 2-arachidonoylglycerol (2-AG) and anandamide (AEA) play vital roles during nervous system development. The degradation of 2-AG and AEA is mediated by monoacylglycerol lipase (MAGL) and fatty acid amide hydrolase (FAAH), respectively. These enzymes are inhibited following developmental chlorpyrifos (CPF) exposure. To investigate whether this inhibition is persistent or whether accumulation of endocannabinoids in the brain occurs, 10-day-old rat pups were orally exposed daily for 7 days to either corn oil or increasing dosages of CPF (1, 2.5, or 5mg/kg), and forebrains were collected at 4, 12, 24, and 48h following the last administration. All dosages inhibited cholinesterase (ChE), FAAH, and MAGL, and elevated AEA and 2-AG levels with the greatest effect occurring at 12h with ChE, FAAH, AEA, and 2-AG and at 4h with MAGL. With the high dosage, return to control levels occurred with 2-AG (48h) only. With the medium dosage, return to control levels occurred with MAGL, 2-AG, and AEA (48h) but not with ChE or FAAH. With the low dosage, return to control levels occurred with MAGL (12h), ChE and 2-AG (24h), and AEA (48h) but not with FAAH. With the lowest dosage, peak inhibition of FAAH (52%) is greater than that of ChE (24%) and that level of FAAH inhibition is sufficient to induce a persistent pattern of elevated AEA. It is possible that this pattern of elevation could alter the appropriate development of neuronal brain circuits.

A mechanistic overview of health associated effects of low levels of organochlorine and organophosphorous pesticides

Organochlorine and organophosphate pesticides are compounds that can be detected in human populations as a result of occupational or residential exposure. Despite their occurrence in considerably low levels in humans, their biological effects are hazardous since they interact with a plethora of enzymes, proteins, receptors and transcription factors. In this review we summarize the cell and molecular effects of organochlorine and organophosphate pesticides with respect to their toxicity, with particular emphasis on glucose and lipid metabolism, their interaction with some members of the nuclear receptor family of ligand-activated transcription factors, including the steroid and peroxisome proliferator activated receptors that changes the expression of genes involved in lipid metabolism and xenobiotic detoxification. More importantly, evidence regarding the metabolic degradation of pesticides and their accumulation in tissues is presented. Potential non-cholinergic mechanisms after long-term low-dose organophosphate exposure resulting in neurodevelopmental outcomes and neurodegeneration are also addressed. We conclude that the mechanism of pesticide-mediated toxicity is a combination of various enzyme-inhibitory, metabolic and transcriptional events acting at the cellular and molecular level.

Time course of serum S100B protein and neuron-specific enolase levels of a single dose of chlorpyrifos in rats

Organophosphate (OP) compounds are a large class of chemicals, many of which are used as pesticides. It is suggested that OPs specifically affect glia and neurons. Effects of acute exposure to chlorpyrifos (CPF), which is a common organophosphorus pesticide used worldwide, on neuron-specific enolase (NSE) and S100B levels in rat blood during 7 days were assessed. Rats were evaluated either before (0 hr) or 2, 12, 24, 48 and 168 hr (7 days) after injection of CPF (279 mg/kg, s.c.) or vehicle (peanut oil, 2 ml/kg, s.c.) for clinical signs of toxicity. Immediately after the evaluation of toxicity, blood samples were taken for biochemical assays. CPF administration produced decreases in body-weight and temperature, which were observed for first time at 12 hr after CPF administration and continued for 168 hr (p < 0.05-0.001). Serum S100B and NSE levels were acutely increased 2 hr after CPF administration and remained high at 12 hr (p < 0.01-0.001). NSE and S100B levels were not different in either CPF or vehicle groups at following time points. Serum butyrylcholinesterase (EC 3.1.1.8; BuChE) activity was dramatically reduced at 2 hr after CPF and remained low at each time points during 7 days (p < 0.01-0.001). Our results suggest that the usefulness of serum levels of these glia- and neuron-specific marker proteins in assessing OP toxicity, specifically CPF-induced toxicity.

Developmental and polyamine metabolism alterations in Rhinella arenarum embryos exposed to the organophosphate chlorpyrifos

Organophosphorus pesticides (OPs) are widely applied in the Alto Valle of Río Negro and Neuquén, Argentina, due to intensive fruit growing. Amphibians are particularly sensitive to environmental pollution, and OPs may transiently accumulate in ponds and channels of the region during their reproductive season. Organophosphorus pesticide exposure may alter amphibian embryonic development and the reproductive success of autochthonous species. In the present study, embryos of the common toad Rhinella arenarum were employed to assess developmental alterations and to study polyamine metabolism, which is essential to normal growth, as a possible target underlying the effects of the OP chlorpyrifos. As the duration of chlorpyrifos exposure increased and embryonic development progressed, the median lethal concentration (LC50) values decreased, and the percentage of malformed embryos increased. Developmental arrest was also observed and several morphological alterations were recorded, such as incomplete and abnormal closure of the neural tube, dorsal curvature of the caudal fin, reduction of body size and caudal fin length, atrophy, and edema. An early decrease in ornithine decarboxylase (ODC) activity and polyamine levels was also observed in embryos exposed to chlorpyrifos. The decrease in polyamine contents in tail bud embryos might be a consequence of the reduction in ODC activity. The alteration of polyamine metabolism occurred before embryonic growth was interrupted and embryonic malformations were observed and may be useful as a biomarker in environmental studies.

High-density real-time PCR-based in vivo toxicogenomic screen to predict organ-specific toxicity

Toxicogenomics, based on the temporal effects of drugs on gene expression, is able to predict toxic effects earlier than traditional technologies by analyzing changes in genomic biomarkers that could precede subsequent protein translation and initiation of histological organ damage. In the present study our objective was to extend in vivo toxicogenomic screening from analyzing one or a few tissues to multiple organs, including heart, kidney, brain, liver and spleen. Nanocapillary quantitative real-time PCR (QRT-PCR) was used in the study, due to its higher throughput, sensitivity and reproducibility, and larger dynamic range compared to DNA microarray technologies. Based on previous data, 56 gene markers were selected coding for proteins with different functions, such as proteins for acute phase response, inflammation, oxidative stress, metabolic processes, heat-shock response, cell cycle/apoptosis regulation and enzymes which are involved in detoxification. Some of the marker genes are specific to certain organs, and some of them are general indicators of toxicity in multiple organs. Utility of the nanocapillary QRT-PCR platform was demonstrated by screening different references, as well as discovery of drug-like compounds for their gene expression profiles in different organs of treated mice in an acute experiment. For each compound, 896 QRT-PCR were done: four organs were used from each of the treated four animals to monitor the relative expression of 56 genes. Based on expression data of the discovery gene set of toxicology biomarkers the cardio- and nephrotoxicity of doxorubicin and sulfasalazin, the hepato- and nephrotoxicity of rotenone, dihydrocoumarin and aniline, and the liver toxicity of 2,4-diaminotoluene could be confirmed. The acute heart and kidney toxicity of the active metabolite SN-38 from its less toxic prodrug, irinotecan could be differentiated, and two novel gene markers for hormone replacement therapy were identified, namely fabp4 and pparg, which were down-regulated by estradiol treatment.

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.

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.

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.

Subtoxic chlorpyrifos treatment resulted in differential expression of genes implicated in neurological functions and development

Chlorpyrifos (CPF), a commonly used organophosphorus insecticide, induces acetylcholinesterase inhibition and cholinergic toxicity. Subtoxic exposure to CPF has long-term adverse effects on synaptic function/development and behavioral performance. To gain insight into the possible mechanism(s) of these observations, this study aims to investigate gene expression changes in the forebrain of rats treated with subtoxic CPF doses using DNA microarrays. Statistical analysis revealed that CPF treatment resulted in differential expression of 277 genes. Gene ontology and pathway analyses revealed that these genes have important roles in nervous system development and functions including axon guidance, dorso-ventral axis formation, long-term potentiation, synaptic transmission, and insulin signaling. The results of biological associated network analysis showed that Gsk3b is highly connected in several of these networks suggesting its potential role in cellular response to CPF exposure/neurotoxicity. These findings might serve as the basis for future mechanistic analysis of the long-term adverse effects of subtoxic CPF exposure.

Identification and interpretation of developmental neurotoxicity effects

The reliable detection, measurement, and interpretation of treatment-related developmental neurotoxicity (DNT) effects depend on appropriate study design and execution, using scientifically established methodologies, with appropriate controls to minimize confounding factors. Appropriate statistical approaches should be optimized for the specific endpoints in advance, analyzing effects across time and functional domains as far as possible. If available, biomarkers of exposure are useful to assess the bioavailability of toxicants to the dam and offspring in utero and after birth. Finally, "weight of evidence" principles are used to aid assessment of the biological significance of differences from concurrent controls. These effects should be interpreted in light of available information from historical controls, positive controls, maternal and offspring systemic toxicity, and other relevant toxicological data. This review provides a framework for the integration of all these types of information in the interpretation of DNT studies.

Pesticides and child neurodevelopment

PURPOSE OF REVIEW

This review summarizes the recent research on pesticide exposure and child neurobehavioral development with a focus on in-utero exposure to organochlorine and organophosphate pesticides.

RECENT FINDINGS

Recent studies on in-utero exposure to the organochlorine pesticide dichlorodiphenyltrichloroethane and its breakdown product, dichlorodiphenyldichloroethene, indicate that exposure is associated with poorer infant (6 months and older) and child neurodevelopment. Yet, the studies differ on the domain of development that is affected. Research on organophosphate pesticide exposure and neurodevelopment is limited but suggests some negative association of exposure and neurodevelopment at certain ages. Two reports agree that increased levels of organophosphate exposure in utero result in greater numbers of abnormal reflexes in neonates and studies in older infants and young children also point to a negative association with development. In young children (2-3 years) two separate studies observed an increase in maternally reported pervasive developmental disorder with increased levels of organophosphate exposure.

SUMMARY

Given that the literature suggests a link between organochlorine and in-utero pesticide exposure and impaired child neurodevelopment, clinicians should educate parents about prevention of exposure, especially in populations living in agricultural areas or where household use is common.

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

Exposure to either chlorpyrifos (CPS) or methyl parathion (MPS) results in the inhibition of acetylcholinesterase and leads to altered neuronal activity which normally regulates critical genes such as the neurotrophins nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF). The effects of postnatal exposure to CPS and MPS on the expression of messenger RNA (mRNA) and protein levels for NGF and BDNF were investigated in the frontal cerebral cortex (cortex) and hippocampus of rats. Oral administration of CPS (4.0 or 6.0 mg/kg), MPS (0.6 or 0.9 mg/kg), or the safflower oil vehicle was performed daily from postnatal day 10 (PND10) through PND20. Exposure induced significant effects on growth and cholinesterase activity. Increased NGF protein levels were observed in the hippocampus but not the cortex on PND20 with some reduction occurring on PND28 in both regions. These changes did not correlate with the changes in NGF mRNA. BDNF mRNA was increased in both regions on PND20 and PND28, whereas BDNF protein levels were increased on PND20. On PND12, c-fos mRNA, a marker of neuronal activation, was increased in both regions. Total BDNF protein was increased in the hippocampus but decreased in the cortex. No changes in NGF protein were observed. These results indicate that repeated developmental OP exposure during the postnatal period alters NGF and BDNF in the cortex and the hippocampus and the patterns of these alterations differ between regions.

Comparative developmental neurotoxicity of organophosphates in vivo: transcriptional responses of pathways for brain cell development, cell signaling, cytotoxicity and neurotransmitter systems

Organophosphates affect mammalian brain development through a variety of mechanisms beyond their shared property of cholinesterase inhibition. We used microarrays to characterize similarities and differences in transcriptional responses to chlorpyrifos and diazinon, assessing defined gene groupings for the pathways known to be associated with the mechanisms and/or outcomes of chlorpyrifos-induced developmental neurotoxicity. We exposed neonatal rats to daily doses of chlorpyrifos (1mg/kg) or diazinon (1 or 2mg/kg) on postnatal days 1-4 and evaluated gene expression profiles in brainstem and forebrain on day 5; these doses produce little or no cholinesterase inhibition. We evaluated pathways for general neural cell development, cell signaling, cytotoxicity and neurotransmitter systems, and identified significant differences for >60% of 252 genes. Chlorpyrifos elicited major transcriptional changes in genes involved in neural cell growth, development of glia and myelin, transcriptional factors involved in neural cell differentiation, cAMP-related cell signaling, apoptosis, oxidative stress, excitotoxicity, and development of neurotransmitter synthesis, storage and receptors for acetylcholine, serotonin, norepinephrine and dopamine. Diazinon had similar effects on many of the same processes but also showed major differences from chlorpyrifos. Our results buttress the idea that different organophosphates target multiple pathways involved in neural cell development but also that they deviate in key aspects that may contribute to disparate neurodevelopmental outcomes. Equally important, these pathways are compromised at exposures that are unrelated to biologically significant cholinesterase inhibition and its associated signs of systemic toxicity. The approach used here demonstrates how planned comparisons with microarrays can be used to screen for developmental neurotoxicity.

Organophosphate pesticide exposure and neurodevelopment in young Mexican-American children

BACKGROUND

Organophosphate (OP) pesticides are widely used in agriculture and homes. Animal studies suggest that even moderate doses are neurodevelopmental toxicants, but there are few studies in humans.

OBJECTIVES

We investigated the relationship of prenatal and child OP urinary metabolite levels with children's neurodevelopment.

METHODS

Participating children were from a longitudinal birth cohort of primarily Latino farm-worker families in California. We measured six nonspecific dialkylphosphate (DAP) metabolites in maternal and child urine as well as metabolites specific to malathion (MDA) and chlorpyrifos (TCPy) in maternal urine. We examined their association with children's performance at 6 (n = 396), 12 (n = 395), and 24 (n = 372) months of age on the Bayley Scales of Infant Development [Mental Development (MDI) and Psychomotor Development (PDI) Indices] and mother's report on the Child Behavior Checklist (CBCL) (n = 356).

RESULTS

Generally, pregnancy DAP levels were negatively associated with MDI, but child measures were positively associated. At 24 months of age, these associations reached statistical significance [per 10-fold increase in prenatal DAPs: beta = -3.5 points; 95% confidence interval (CI), -6.6 to -0.5; child DAPs: beta = 2.4 points; 95% CI, 0.5 to 4.2]. Neither prenatal nor child DAPs were associated with PDI or CBCL attention problems, but both prenatal and postnatal DAPs were associated with risk of pervasive developmental disorder [per 10-fold increase in prenatal DAPs: odds ratio (OR) = 2.3, p = 0.05; child DAPs OR = 1.7, p = 0.04]. MDA and TCPy were not associated with any outcome.

CONCLUSIONS

We report adverse associations of prenatal DAPs with mental development and pervasive developmental problems at 24 months of age. Results should be interpreted with caution given the observed positive relationship with postnatal DAPs.

Impact of prenatal chlorpyrifos exposure on neurodevelopment in the first 3 years of life among inner-city children

OBJECTIVE

The purpose of this study was to investigate the impact of prenatal exposure to chlorpyrifos on 3-year neurodevelopment and behavior in a sample of inner-city minority children.

METHODS

As part of an ongoing prospective cohort study in an inner-city minority population, neurotoxicant effects of prenatal exposure to chlorpyrifos were evaluated in 254 children through the first 3 years of life. This report examined cognitive and motor development at 12, 24, and 36 months (measured with the Bayley Scales of Infant Development II) and child behavior at 36 months (measured with the Child Behavior Checklist) as a function of chlorpyrifos levels in umbilical cord plasma.

RESULTS

Highly exposed children (chlorpyrifos levels of >6.17 pg/g plasma) scored, on average, 6.5 points lower on the Bayley Psychomotor Development Index and 3.3 points lower on the Bayley Mental Development Index at 3 years of age compared with those with lower levels of exposure. Children exposed to higher, compared with lower, chlorpyrifos levels were also significantly more likely to experience Psychomotor Development Index and Mental Development Index delays, attention problems, attention-deficit/hyperactivity disorder problems, and pervasive developmental disorder problems at 3 years of age.

CONCLUSIONS

The adjusted mean 36-month Psychomotor Development Index and Mental Development Index scores of the highly and lower exposed groups differed by only 7.1 and 3.0 points, respectively, but the proportion of delayed children in the high-exposure group, compared with the low-exposure group, was 5 times greater for the Psychomotor Development Index and 2.4 times greater for the Mental Development Index, increasing the number of children possibly needing early intervention services.

Effect of organophosphorus insecticides and their metabolites on astroglial cell proliferation

Though little attention has been given to the possibility that glial cells may represent a target for the developmental neurotoxicity of organophosphorus (OP) insecticides, recent evidence, obtained in particular with chlorpyrifos (CP), suggests that developmental exposure to this compound may indeed target astrocytes. To substantiate and expand these observations, we carried out a series of in vitro studies utilizing fetal rat astrocytes and a human astrocytoma cell line, 1321N1 cells, to investigate the effect of the OPs CP, diazinon (DZ) and parathion (P), their oxygen analogs chlorpyrifos oxon (CPO), diazoxon (DZO) and paraoxon (PO), and their metabolites 3,5,6-trichloro-2-pyridinol (TCP), 2-isopropyl-6-methyl-4-pyrimidol (IMP) and para-nitrophenol (PNP), on cell proliferation. In fetal rat astrocytes and astrocytoma cells maintained in serum, CP, DZ, P, CPO, DZO, and PO induced a concentration-dependent inhibition in [(3)H]thymidine incorporation with a very similar potency (IC(50) between 45 and 57 microM). Among the other metabolites, PNP was the most potent (IC(50)=70-80 microM), while TCP and IMP were much less effective (IC(50)>100 microM). Cytotoxicity appears to account only for a small part of the effect on DNA synthesis. OP insecticides and their oxons were three- to six-fold more potent in inhibiting [(3)H]thymidine incorporation when cells were synchronized in the G(0)/G(1) phase of the cell cycle and re-stimulated by carbachol or epidermal growth factor. These results suggest that OP insecticides and their oxons affect astroglial cell proliferation and that the transition from the G(0)/G(1) to the S/G(2) phase of the cell cycle may be particularly sensitive to the action of these compounds.

Quantitative morphological assessment reveals neuronal and glial deficits in hippocampus after a brief subtoxic exposure to chlorpyrifos in neonatal rats

Neurochemical and behavioral studies indicate that the widely used organophosphorus insecticide, chlorpyrifos (CPF), evokes neurobehavioral teratogenicity with a wide window of vulnerability, ranging from embryonic life through postnatal development. Few studies have detailed morphological damage that corresponds to the operational deficits. We administered 5 mg/kg of CPF sc daily on postnatal days (PN) 11-14, a regimen that is devoid of systemic toxicity, but that elicits long-term cognitive impairment and disruption of cholinergic, catecholaminergic, and serotonergic synaptic function. On PN15 and 20, we conducted quantitative morphologic examinations of neurons and glia in CA1, CA3, and dentate gyrus regions of the hippocampus. Although hippocampal morphology after CPF exposure was normal on gross observation, morphometric analysis revealed a significant overall reduction in the total number of neurons and glia. Superimposed on this basic effect, CPF elicited a delayed-onset increase in the neuron/glia ratio that emerged by PN20, connoting selective gliotoxicity. The alterations in cell numbers were accompanied by significant perikaryal swelling and by enhanced development of astrocytic processes. Layer thickness also showed delayed-onset effects of CPF, with thinning of the CA1 and CA3 layers and enlargement of the dentate gyrus. Our results indicate that there are subtle morphological changes in the juvenile rat brain after neonatal CPF exposure that are detectable only with quantitative analysis and that correlate with regional and cell-specific targets identified earlier in neurochemical studies. The simultaneous targeting of neurons and glia by CPF is likely to play an important role in its developmental neurotoxicant effects.

Involvement of glial cells in the neurotoxicity of parathion and chlorpyrifos

An in vitro model, the aggregating brain cell culture of fetal rat telencephalon, has been used to investigate the influence of glial cells on the neurotoxicity of two organophosphorus pesticides (OPs), chlorpyrifos and parathion. Mixed-cell aggregate cultures were treated continuously for 10 days between DIV 5 and 15. Parathion induced astrogliosis at concentration at which MAP-2 immunostaining, found here to be more sensitive than neuron-specific enzyme activities, was not affected. In contrast, chlorpyrifos induced a comparatively weak gliotic reaction, and only at concentrations at which neurons were already affected. After similar treatments, increased neurotoxicity of parathion and chlorpyrifos was found in aggregate cultures deprived of glial cells. These results suggest that glial cells provide neuroprotection against OPs toxicity. To address the question of the difference in toxicity between parathion and chlorpyrifos, the toxic effects of their leaving groups, p-nitrophenol and trichloropyridinol, were studied in mixed-cell aggregates. General cytotoxicity was more pronounced for trichloropyridinol and both compounds had similar toxic effects on neuron-specific enzyme activities. In contrast, trichloropyridinol induced a much stronger decrease in glutamine synthetase activity, the enzymatic marker of astrocytes. Trichloropyridinol may exert a toxic effect on astrocytes, compromising their neuroprotective function, thus exacerbating the neurotoxicity of chlorpyrifos. This is in line with the suggestion that glial cells may contribute to OPs neurotoxicity, and with the view that OPs may exert their neurotoxic effects through different mechanisms.

Toxicogenomics and systems toxicology: aims and prospects

Toxicogenomics combines transcript, protein and metabolite profiling with conventional toxicology to investigate the interaction between genes and environmental stress in disease causation. The patterns of altered molecular expression that are caused by specific exposures or disease outcomes have revealed how several toxicants act and cause disease. Despite these success stories, the field faces noteworthy challenges in discriminating the molecular basis of toxicity. We argue that toxicology is gradually evolving into a systems toxicology that will eventually allow us to describe all the toxicological interactions that occur within a living system under stress and use our knowledge of toxicogenomic responses in one species to predict the modes-of-action of similar agents in other species.

Cholinergic systems in brain development and disruption by neurotoxicants: nicotine, environmental tobacco smoke, organophosphates

Acetylcholine and other neurotransmitters play unique trophic roles in brain development. Accordingly, drugs and environmental toxicants that promote or interfere with neurotransmitter function evoke neurodevelopmental abnormalities by disrupting the timing or intensity of neurotrophic actions. The current review discusses three exposure scenarios involving acetylcholine systems: nicotine from maternal smoking during pregnancy, exposure to environmental tobacco smoke (ETS), and exposure to the organophosphate insecticide, chlorpyrifos (CPF). All three have long-term, adverse effects on specific processes involved in brain cell replication and differentiation, synaptic development and function, and ultimately behavioral performance. Many of these effects can be traced to the sequence of cellular events surrounding the trophic role of acetylcholine acting on its specific cellular receptors and associated signaling cascades. However, for chlorpyrifos, additional noncholinergic mechanisms appear to be critical in establishing the period of developmental vulnerability, the sites and type of neural damage, and the eventual outcome. New findings indicate that developmental neurotoxicity extends to late phases of brain maturation including adolescence. Novel in vitro and in vivo exposure models are being developed to uncover heretofore unsuspected mechanisms and targets for developmental neurotoxicants.

Environmental contaminant-mixture effects on CNS development, plasticity, and behavior

Environmental contaminants within the polycyclic aromatic hydrocarbon (PAH) and halogenated aromatic hydrocarbon class have been shown to cross the placenta exposing the fetus to the contaminant body burden of the mother. Consequently, a gestational exposure to environmental contaminants may result in increased adverse health outcomes, possibly affecting cognitive performance. Benzo(a)pyrene [B(a)P] and 2,3,7,8, tetrachlorodibenzo-p-dioxin (TCDD) are two prototypical environmental contaminants. A systematic review of the literature suggests that there may be a relationship between vulnerability in susceptible populations and health disparities. The purpose of this mini-review is to provide a point of reference for neurotoxicological studies of environmental contaminant mixture effects on indices of development in general, and on neurodevelopment in particular. Environmental contaminant-mixture-induced decrements in (1) birth index, (2) N-methyl-D-aspartate receptor (NMDA) mRNA expression, (3) long-term potentiation (LTP), (4) fixed-ratio performance learning behavior, and (5) experience-dependent activity related cytoskeletal-associated protein (Arc) mRNA and protein expression collectively support associations between neurobehavioral deficits and gestational exposure to environmental levels of these contaminants. Collectively, data are presented in this mini-review evaluating the effect of gestational exposure to environmental contaminant-mixtures on specific indices of learning and memory, including hippocampal-based synaptic plasticity mechanisms. These indices serve as templates for learning and memory, and as such, from a vulnerability perspective, may serve as targets for dysregulation during development in susceptible populations that have been disproportionately exposed to these contaminants. Included in this review is also a discussion of the relevance of developing biomarkers for use within the framework of cumulative risk-assessment.

Morphologic effects of subtoxic neonatal chlorpyrifos exposure in developing rat brain: regionally selective alterations in neurons and glia

The widely used organophosphate insecticide, chlorpyrifos (CPF), elicits neurobehavioral teratogenesis with exposure windows ranging from the embryonic neural tube stage through postnatal development. To explore the morphologic changes occurring in late-stage exposure, newborn rats were given 5 mg/kg of CPF s.c. daily on postnatal days (PN) 11-14, a regimen that is devoid of systemic toxicity, but that elicits long-term cognitive impairment. On PN15 and 20, we examined the septal nucleus, striatum and somatosensory cortex. Across all three regions, CPF elicited a significant decrease in the number of glial cells. Superimposed on this basic pattern, there were region-specific alterations in the number and type of neurons, and neuronal perikaryal dimensions. In the septal nucleus, the CPF group exhibited an increase in the number of neurons on PN20, representing a delay in the normal maturational decline; there was a parallel decrease in the glial/neuronal ratio. In the striatum, the number of neurons per unit area was reduced in the CPF group, accompanied by perikaryal hypertrophy, as evidenced by an increase in the average neuronal cell diameter. In the somatosensory cortex, the distribution of cell sizes indicated a decrease in the proportion of small, nonpyramidal cells. Thus, there are subtle morphological changes in the juvenile rat brain after neonatal CPF exposure that are detectable with quantitative analysis and that correlate with later emergence of behavioral alterations. Furthermore, the current findings support the hypothesis that CPF interferes with gliogenesis, a relatively late event in brain development; accordingly, the vulnerable period for adverse effects of CPF is likely to extend into childhood or adolescence.

Chlorpyrifos exposure during neurulation: cholinergic synaptic dysfunction and cellular alterations in brain regions at adolescence and adulthood

The developmental neurotoxicity of chlorpyrifos (CPF) involves multiple mechanisms, thus rendering the immature brain susceptible to adverse effects over a wide window of vulnerability. Earlier work indicated that CPF exposure at the neural tube stage elicits apoptosis and disrupts mitotic patterns in the brain primordium but that rapid recovery ensues before birth. In the current study, we assessed whether defects in cholinergic synaptic activity emerge later in development. CPF was given to pregnant rats on gestational days 9-12, using regimens devoid of overt maternal or fetal toxicity. We then examined subsequent development of acetylcholine systems and compared the effects to those on general biomarkers of cell development. Choline acetyltransferase (ChAT), a constitutive marker for cholinergic nerve terminals, was increased in the hippocampus and striatum in adolescence and adulthood. In contrast, hemicholinium-3 (HC-3) binding to the presynaptic choline transporter, an index of nerve impulse activity, was markedly subnormal. Furthermore, m2-muscarinic cholinergic receptor binding was significantly reduced, instead of showing the expected compensatory upregulation for reduced neural input. CPF also elicited delayed-onset alterations in biomarkers of cell packing density, cell number, cell size and neuritic projections, involving brain regions both with and without reductions in indices of cholinergic activity. In combination with earlier results, the current findings indicate that the developing brain, and especially the hippocampus, is adversely affected by CPF regardless of whether exposure occurs early or late in brain development, and that defects emerge in adolescence or adulthood even in situations where normative values are initially restored in the immediate post-exposure period.