Persistent cholinergic presynaptic deficits after neonatal chlorpyrifos exposure

Differences in neurotoxic outcomes of organophosphorus pesticides revealed via multi-dimensional screening in adult and regenerating planarians

Organophosphorus pesticides (OPs) are a chemically diverse class of commonly used insecticides. Epidemiological studies suggest that low dose chronic prenatal and infant exposures can lead to life-long neurological damage and behavioral disorders. While inhibition of acetylcholinesterase (AChE) is the shared mechanism of acute OP neurotoxicity, OP-induced developmental neurotoxicity (DNT) can occur independently and/or in the absence of significant AChE inhibition, implying that OPs affect alternative targets. Moreover, different OPs can cause different adverse outcomes, suggesting that different OPs act through different mechanisms. These findings emphasize the importance of comparative studies of OP toxicity. Freshwater planarians are an invertebrate system that uniquely allows for automated, rapid and inexpensive testing of adult and developing organisms in parallel to differentiate neurotoxicity from DNT. Effects found only in regenerating planarians would be indicative of DNT, whereas shared effects may represent neurotoxicity. We leverage this unique feature of planarians to investigate potential differential effects of OPs on the adult and developing brain by performing a comparative screen to test 7 OPs (acephate, chlorpyrifos, dichlorvos, diazinon, malathion, parathion and profenofos) across 10 concentrations in quarter-log steps. Neurotoxicity was evaluated using a wide range of quantitative morphological and behavioral readouts. AChE activity was measured using an Ellman assay. The toxicological profiles of the 7 OPs differed across the OPs and between adult and regenerating planarians. Toxicological profiles were not correlated with levels of AChE inhibition. Twenty-two "mechanistic control compounds" known to target pathways suggested in the literature to be affected by OPs (cholinergic neurotransmission, serotonin neurotransmission, endocannabinoid system, cytoskeleton, adenyl cyclase and oxidative stress) and 2 negative controls were also screened. When compared with the mechanistic control compounds, the phenotypic profiles of the different OPs separated into distinct clusters. The phenotypic profiles of adult vs. regenerating planarians exposed to the OPs clustered differently, suggesting some developmental-specific mechanisms. These results further support findings in other systems that OPs cause different adverse outcomes in the (developing) brain and build the foundation for future comparative studies focused on delineating the mechanisms of OP neurotoxicity in planarians.

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.

Perinatal diazinon exposure compromises the development of acetylcholine and serotonin systems

Organophosphate pesticides are developmental neurotoxicants. We gave diazinon via osmotic minipumps implanted into dams prior to conception, with exposure continued into the second postnatal week, at doses (0.5 or 1 mg/kg/day) that did not produce detectable brain cholinesterase inhibition. We evaluated the impact on acetylcholine (ACh) and serotonin (5-hydroxytryptamine, 5HT) systems in brain regions from adolescence through full adulthood. Diazinon produced deficits in presynaptic ACh activity with regional and sex selectivity: cerebrocortical regions and the hippocampus were affected to a greater extent than were the striatum, midbrain or brainstem, and females were more sensitive than males. Diazinon also reduced nicotinic ACh receptors and 5HT_1A receptors, with the same regional and sex preferences. These patterns were similar to those of diazinon given in a much more restricted period (postnatal day 1-4) but were of greater magnitude and consistency; this suggests that the brain is vulnerable to diazinon over a wide developmental window. Diazinon's effects differed from those of the related organophosphate, chlorpyrifos, with regard to regional and sex selectivity, and more importantly, to the effects on receptors: chlorpyrifos upregulates nicotinic ACh receptors and 5HT receptors, effects that compensate for the presynaptic ACh deficits. Diazinon can thus be expected to have worse neurodevelopmental outcomes than chlorpyrifos. Further, the disparities between diazinon and chlorpyrifos indicate the problems of predicting the developmental neurotoxicity of organophosphates based on a single compound, and emphasize the inadequacy of cholinesterase inhibition as an index of safety.

Sex-Specific Neurotoxic Effects of Organophosphate Pesticides Across the Life Course

PURPOSE OF REVIEW

This review discusses the sex-specific effects of exposure to various organophosphate (OP) pesticides throughout the life course and potential reasons for the differential vulnerabilities observed across sexes.

RECENT FINDINGS

Sex is a crucial factor in the response to toxicants, yet the sex-specific effects of OP exposure, particularly in juveniles and adults, remain unresolved. This is largely due to study design and inconsistencies in exposure and outcome assessments. Exposure to OPs results in multiple adverse outcomes influenced by many factors including sex. Reported sex-specific effects suggest that males are more susceptible to OPs, which reflects the sex-dependent prevalence of various neurodevelopmental and neurodegenerative disorders such as autism and amyotrophic lateral sclerosis (ALS), in which males are at greater risk. Thus, this review proposes that the biological sex-specific effects elicited by OP exposure may in part underlie the dimorphic susceptibilities observed in neurological disorders. Understanding the immediate and long-term effects of OP exposure across sexes will be critical in advancing our understanding of OP-induced neurotoxicity and disease.

Retinal changes in the Tg-SwDI mouse model of Alzheimer's disease

Alzheimer's disease (AD), a debilitating neurodegenerative illness, is characterized by neuronal cell loss, mental deficits, and abnormalities in several neurotransmitter and protein systems. AD is also associated with visual disturbances, but their causes remain unidentified. We hypothesize that the visual disturbances stem from retinal changes, particularly changes in the retinal cholinergic system, and that the etiology in the retina parallels the etiology in the rest of the brain. To test our hypothesis, quantitative polymerase chain reaction (qPCR) and immunohistochemistry (IHC) were employed to assess changes in acetylcholine receptor (AChR) gene expression, number of retinal cells, and astrocytic gliosis in the Transgenic Swedish, Dutch and Iowa (Tg-SwDI) mouse model as compared to age-matched wild-type (WT). We observed that Tg-SwDI mice showed an initial upregulation of AChR gene expression early on (young adults and middle-aged adults), but a downregulation later on (old adults). Furthermore, transgenic animals displayed significant cell loss in the photoreceptor layer and inner retina of the young adult animals, as well as specific cholinergic cell loss, and increased astrocytic gliosis in the middle-aged adult and old adult groups. Our results suggest that the changes observed in AD cerebrum are also present in the retina and may be, at least in part, responsible for the visual deficits associated with the disease.

Developmental neurotoxicity of succeeding generations of insecticides

Insecticides are by design toxic. They must be toxic to effectively kill target species of insects. Unfortunately, they also have off-target toxic effects that can harm other species, including humans. Developmental neurotoxicity is one of the most prominent off-target toxic risks of insecticides. Over the past seven decades several classes of insecticides have been developed, each with their own mechanisms of effect and toxic side effects. This review covers the developmental neurotoxicity of the succeeding generations of insecticides including organochlorines, organophosphates, pyrethroids, carbamates and neonicotinoids. The goal of new insecticide development is to more effectively kill target species with fewer toxic side effects on non-target species. From the experience with the developmental neurotoxicity caused by the generations of insecticides developed in the past advice is offered how to proceed with future insecticide development to decrease neurotoxic risk.

Mother gestational exposure to organophosphorus pesticide induces neuron and glia loss in daughter adult brain

Chlorpyrifos (CPF) is a widely used organophosphorus pesticide with developmental neurotoxicity such as morphogenesis toxicity. In the present study, we assessed the effects of prenatal CPF exposure on systemic parameters and cytoarchitecture of medial prefrontal cortex (mPFC) in adulthood. Gestational dams were exposed to 5mg/kg/d of CPF during gestational days 13-17, while body weight, organ coefficient, and neuron and glia counts of offspring were determined on postnatal day 60. Our results showed that CPF treatment induced little or no effects on body weight and organ coefficients. There were also no significant pathological changes in mPFC. However, neuron and glia count analysis showed that CPF treatment reduced neuron and glia counts in anterior cingulate, prelimbic, and infralimbic areas of mPFC. The CPF react pattern was similar in both sexes, and there was no statistical difference in most of the sub-regions. Thus, our results revealed an embryonic origin brain deficit induced by gestational mother pesticide exposure.

Neonatal chlorpyrifos exposure induces loss of dopaminergic neurons in young adult rats

Increasing epidemiological and toxicological evidence suggests that pesticides and other environmental exposures may be associated with the development of Parkinson's disease (PD). Chlorpyrifos (CPF) is a widely used organophosphorous pesticide with developmental neurotoxicity. Its neurotoxicity, notably on the monoamine system, suggests that exposure of CPF may induce dopaminergic neuronal injury. We investigated whether neonatal exposure to CPF contributes to initiation and progression of dopaminergic neurotoxicity and explored the possible underlying mechanisms. The newborn rats were administrated 5 mg/kg CPF subcutaneously from postnatal day (PND) 11 to PND 14 daily. The effect of CPF on dopaminergic neurons, microglia, astrocyte, nuclear factor-κB (NF-κB) p. 65 and p. 38 mitogen-activated protein kinase (MAPK) signaling pathways was analyzed in the substantia nigra of rats at 12h, 24h, 72 h, 16d and 46 d after exposure. CPF-treated rats exhibited significant reduction of dopaminergic neurons at 16d and 46 d after exposure, and a significant increase in the expression of microglia and astrocytes in the substantia nigra after CPF exposure. Intense activation of NF-κB p. 65 and p. 38 MAPK inflammatory signaling pathways was observed. Our findings indicate that neonatal exposure to CPF may induce long-term dopaminergic neuronal damage in the substantia nigra mediated by the activation of inflammatory response via NF-κB p. 65 and p. 38 MAPK pathways in the nigrostriatal system.

Prenatal nicotine alters the developmental neurotoxicity of postnatal chlorpyrifos directed toward cholinergic systems: better, worse, or just "different?"

This study examines whether prenatal nicotine exposure sensitizes the developing brain to subsequent developmental neurotoxicity evoked by chlorpyrifos, a commonly-used insecticide. We gave nicotine to pregnant rats throughout gestation at a dose (3mg/kg/day) producing plasma levels typical of smokers; offspring were then given chlorpyrifos on postnatal days 1-4, at a dose (1mg/kg) that produces minimally-detectable inhibition of brain cholinesterase activity. We evaluated indices for acetylcholine (ACh) synaptic function throughout adolescence, young adulthood and later adulthood, in brain regions possessing the majority of ACh projections and cell bodies; we measured nicotinic ACh receptor binding, hemicholinium-3 binding to the presynaptic choline transporter and choline acetyltransferase activity, all known targets for the adverse developmental effects of nicotine and chlorpyrifos given individually. By itself nicotine elicited overall upregulation of the ACh markers, albeit with selective differences by sex, region and age. Likewise, chlorpyrifos alone had highly sex-selective effects. Importantly, all the effects showed temporal progression between adolescence and adulthood, pointing to ongoing synaptic changes rather than just persistence after an initial injury. Prenatal nicotine administration altered the responses to chlorpyrifos in a consistent pattern for all three markers, lowering values relative to those of the individual treatments or to those expected from simple additive effects of nicotine and chlorpyrifos. The combination produced global interference with emergence of the ACh phenotype, an effect not seen with nicotine or chlorpyrifos alone. Given that human exposures to nicotine and chlorpyrifos are widespread, our results point to the creation of a subpopulation with heightened vulnerability.

Potential pharmacological strategies for the improved treatment of organophosphate-induced neurotoxicity

Organophosphates (OP) are highly toxic compounds that cause cholinergic neuronal excitotoxicity and dysfunction by irreversible inhibition of acetylcholinesterase, resulting in delayed brain damage. This delayed secondary neuronal destruction, which arises primarily in the cholinergic areas of the brain that contain dense accumulations of cholinergic neurons and the majority of cholinergic projection, could be largely responsible for persistent profound neuropsychiatric and neurological impairments such as memory, cognitive, mental, emotional, motor, and sensory deficits in the victims of OP poisoning. The therapeutic strategies for reducing neuronal brain damage must adopt a multifunctional approach to the various steps of brain deterioration: (i) standard treatment with atropine and related anticholinergic compounds; (ii) anti-excitotoxic therapies to prevent cerebral edema, blockage of calcium influx, inhibition of apoptosis, and allow for the control of seizure; (iii) neuroprotection by aid of antioxidants and N-methyl-d-aspartate (NMDA) antagonists (multifunctional drug therapy), to inhibit/limit the secondary neuronal damage; and (iv) therapies targeting chronic neuropsychiatric and neurological symptoms. These neuroprotective strategies may prevent secondary neuronal damage in both early and late stages of OP poisoning, and thus may be a beneficial approach to treating the neuropsychological and neuronal impairments resulting from OP toxicity.

Assessment of attention and inhibitory control in rodent developmental neurotoxicity studies

In designing screens to assess potential neurotoxicants, the paramount goal is that the selected assessment tools detect dysfunction if it exists. This goal is particularly challenging in the case of cognitive assessments. Cognition is not a unitary phenomenon, and indeed there is growing evidence that different aspects of cognitive functioning are subserved by distinct neural systems. As a result, if a particular neurotoxicant selectively damages certain neural systems but not others, it can impair some cognitive, sensory, or affective functions, but leave many others intact. Accordingly, studies with human subjects use batteries of cognitive tests, cognizant of the fact that no one test is capable of detecting all forms of cognitive dysfunction. In contrast, assessment of cognitive functioning in non-human animal developmental neurotoxicity (DNT) studies typically consists of a single, presumably representative, "learning and memory" task that is expected to detect all potential effects on cognitive functioning. Streamlining the cognitive assessment in these studies saves time and money, but these shortcuts can have serious consequences if the aspect of cognitive functioning that is impaired is not tapped by the single selected task. In particular, executive functioning - a constellation of cognitive functions which enables the organism to focus on multiple streams of information simultaneously, and revise plans as necessary - is poorly assessed in most animal DNT studies. The failure to adequately assess these functions - which include attention, working memory, inhibitory control, and planning - is particularly worrisome in light of evidence that the neural systems that subserve these functions may be uniquely vulnerable to early developmental insults. We illustrate the importance of tapping these areas of functioning in DNT studies by describing the pattern of effects produced by early developmental Pb exposure. Rats exposed to lead (Pb) early in development were tested on a series of automated attention tasks, as well as on a radial arm maze task. The lead-exposed rats were not impaired in this demanding radial arm maze task, despite conditions which tapped the limits of both working and long-term memory. In contrast, the automated tests designed to assess rodent executive functioning revealed selective and functionally important deficits in attention and regulation of emotion or negative affect (produced by committing an error or not receiving an expected reward). This example underscores the importance of including tasks to specifically tap executive functioning in DNT batteries. Such tasks are not only sensitive but can also shed light on the specific nature of the dysfunction, and they can implicate dysfunction of specific neural systems, information which can be used to design therapeutic interventions. Although the use of such tasks increases the time and effort needed to complete the battery, the benefits outweigh the cost, in light of the greater sensitivity of the battery and the more complete characterization of effects.

Mitochondrial dysfunction in aging rat brain regions upon chlorpyrifos toxicity and cold stress: an interactive study

Mitochondrial dysfunction and consequent energy depletion are the major causes of oxidative stress resulting to bring alterations in the ionic homeostasis causing loss of cellular integrity. Our previous studies have shown the age-associated interactive effects in rat central nervous system (CNS) upon co-exposure to chlorpyrifos (CPF) and cold stress leading to macromolecular oxidative damage. The present study elucidates a possible mechanism by which CPF and cold stress interaction cause(s) mitochondrial dysfunction in an age-related manner. In this study, the activity levels of Krebs cycle enzymes and electron transport chain (ETC) protein complexes were assessed in the isolated fraction of mitochondria. CPF and cold stress (15 and 20 °C) exposure either individually or in combination decreased the activity level of Krebs cycle enzymes and ETC protein complexes in discrete regions of rat CNS. The findings confirm that cold stress produces significant synergistic effect in CPF intoxicated aging rats. The synergism between CPF and cold stress at 15 °C caused a higher depletion of respiratory enzymes in comparison with CPF and cold stress alone and together at 20 °C indicating the extent of deleterious functional alterations in discrete regions of brain and spinal cord (SC) which may result in neurodegeneration and loss in neuronal metabolic control. Hence, co-exposure of CPF and cold stress is more dangerous than exposure of either alone. Among the discrete regions studied, the cerebellum and medulla oblongata appears to be the most susceptible regions when compared to cortex and SC. Furthermore, the study reveals a gradual decrease in sensitivity to CPF toxicity as the rat matures.

Low level chlorpyrifos exposure increases anandamide accumulation in juvenile rat brain in the absence of brain cholinesterase inhibition

The prevailing dogma is that chlorpyrifos (CPF) mediates its toxicity through inhibition of cholinesterase (ChE). However, in recent years, the toxicological effects of developmental CPF exposure have been attributed to an unknown non-cholinergic mechanism of action. We hypothesize that the endocannabinoid system may be an important target because of its vital role in nervous system development. We have previously reported that repeated exposure to CPF results in greater inhibition of fatty acid amide hydrolase (FAAH), the enzyme that metabolizes the endocannabinoid anandamide (AEA), than inhibition of either forebrain ChE or monoacylglycerol lipase (MAGL), the enzyme that metabolizes the endocannabinoid 2-arachidonylglycerol (2-AG). This exposure resulted in the accumulation of 2-AG and AEA in the forebrain of juvenile rats; however, even at the lowest dosage level used (1.0mg/kg), forebrain ChE inhibition was still present. Thus, it is not clear if FAAH activity would be inhibited at dosage levels that do not inhibit ChE. To determine this, 10 day old rat pups were exposed daily for 7 days to either corn oil or 0.5mg/kg CPF by oral gavage. At 4 and 12h post-exposure on the last day of administration, the activities of serum ChE and carboxylesterase (CES) and forebrain ChE, MAGL, and FAAH were determined as well as the forebrain AEA and 2-AG levels. Significant inhibition of serum ChE and CES was present at both 4 and 12h. There was no significant inhibition of the activities of forebrain ChE or MAGL and no significant change in the amount of 2-AG at either time point. On the other hand, while no statistically significant effects were observed at 4h, FAAH activity was significantly inhibited at 12h resulting in a significant accumulation of AEA. Although it is not clear if this level of accumulation impacts brain maturation, this study demonstrates that developmental CPF exposure at a level that does not inhibit brain ChE can alter components of endocannabinoid signaling.

Methamidophos exposure during the early postnatal period of mice: immediate and late-emergent effects on the cholinergic and serotonergic systems and behavior

Organophosphates (OPs) are among the most used pesticides. Although some OPs have had their use progressively more restricted, other OPs are being used without sufficient investigation of their effects. Here, we investigated the immediate neurochemical and delayed neurochemical and behavioral actions of the OP methamidophos to verify whether there are concerns regarding exposure during early postnatal development. From the third to the nineth postnatal day (PN), Swiss mice were sc injected with methamidophos (1mg/kg). At PN10, we assessed cholinergic and serotonergic biomarkers in the cerebral cortex and brainstem. From PN60 to PN63, mice were submitted to a battery of behavioral tests and subsequently to biochemical analyses. At PN10, the effects were restricted to females and to the cholinergic system: Methamidophos promoted increased choline transporter binding in the brainstem. At PN63, in the brainstem, there was a decrease in choline transporter, a female-only decrease in 5HT1A and a male-only increase in 5HT2 receptor binding. In the cortex, choline acetyltransferase activity was decreased and 5HT2 receptor binding was increased both in males and females. Methamidophos elicited behavioral alterations, suggestive of increased depressive-like behavior and impaired decision making. There were no significant alterations on anxiety-related measures and on memory/learning. Methamidophos elicited cholinergic and serotonergic alterations that depended on brain region, sex, and age of the animals. These outcomes, together with the behavioral effects, indicate that this OP is deleterious to the developing brain and that alterations are indeed identified long after the end of exposure.

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.

Alterations in nitric oxide synthase-expressing neurons in the forebrain regions of rats after developmental exposure to organophosphates

Several mechanisms have been addressed as contributors to the long lasting behavioral deficits after developmental exposure to organophosphate (OP) compounds. Here, the effects of developmental exposure to two common OP insecticides, chlorpyrifos (CPF) and diazinon (DZN), on nitric oxide synthase (NOS)-expressing neurons in the rat forebrain are reported. A daily dose of 1mg/kg of either CPF or DZN was administered to rats during gestational days 15-18 or postnatal days (PND) 1-4. We then assessed NADPH-diaphorase and neuronal NOS (nNOS) immunohistochemistry in forebrain sections on different postnatal days. Prenatal exposure to CPF and DZN induced a transient reduction of NADPH-d(+)/nNOS-immunoreactive (IR) neurons in most cortical regions on PND 4 but exceptionally increased them in the entorhinal/piriform cortex. On PND 15, NADPH-d(+)/nNOS-IR neurons showed morphological abnormalities within entorhinal/piriform cortex of the rats that gestationally exposed to CPF. Postnatal exposure to CPF and DZN did not induce widespread effects on the number of NADPH-d(+)/nNOS-IR neurons on PNDs 7 and 15 but significantly reduced them in most cortical regions and hippocampal subfields on PND 60. The OPs affected NADPH-d(+)/nNOS-IR neurons in a sex independent manner and apparently spared them in the striatum. While the NADPH-d reactivity of microvessels was normally diminished by age, OP treated rats evidently preserved the NADPH-d reactivity of microvessels in the cerebral cortex and hippocampus. The effects of OPs on NADPH-d(+)/nNOS-IR neurons may contribute to the long-lasting behavioral outcomes and expand the neurotransmitter system that need to be considered in OP neurotoxicity evaluations.

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.

Developmental exposure to chlorpyrifos and diazinon differentially affect passive avoidance performance and nitric oxide synthase-containing neurons in the basolateral complex of the amygdala

Chronic exposure to low doses of organophosphates during brain development can induce persistent neurochemical and behavioral effects. This study sought to determine the long-lasting effects of developmental exposure to chlorpyrifos (CPF) and diazinon (DZN) on passive avoidance (PA) performance and neuronal nitric oxide synthase (nNOS)-containing neurons in the subnuclei within basolateral complex of amygdala (BLC). Developing rats were exposed to daily dose (1mg/kg) of CPF or DZN during gestational days 15-18 and postnatal days (PND) 1-4. PA performance was assessed in young adulthood (PND 60). Brain sections were also processed by NADPH-diaphorase (NADPH-d) and nNOS immunohistochemistry. Gestational exposure to CPF increased NADPH-d(+)/nNOS-immunoreactive (IR) neurons within the basolateral nucleus (BL) and medial paracapsular intercalated cluster, which was along with PA retention impairment in both male and female rats. Prenatal exposure to DZN did not significantly change the number of NADPH-d(+)/nNOS-IR neurons in the BLC while impaired PA retention in females. Postnatal exposure to CPF decreased NADPH-d(+)/NOS-IR neurons in the BL without affecting PA performance. Exposure to DZN during early postnatal period impaired PA retention in both sexes, albeit to a lesser extent in females, and was along with a considerable sex independent reduction of NADPH-d(+)/NOS-IR neurons in all BLC subnuclei. Our data suggest that developmental exposure to apparently subtoxic dose of CPF and DZN elicit long-lasting impairment in PA retention that are associated, but not necessarily correlated with effects on NADPH-d(+)/NOS-IR neurons in BLC of the amygdala.

Brain regional heterogeneity and toxicological mechanisms of organophosphates and carbamates

The brain is a well-organized, yet highly complex, organ in the mammalian system. Most investigators use the whole brain, instead of a selected brain region(s), for biochemical analytes as toxicological endpoints. As a result, the obtained data is often of limited value, since their significance is compromised due to a reduced effect, and the investigators often arrive at an erroneous conclusion(s). By now, a plethora of knowledge reveals the brain regional variability for various biochemical/neurochemical determinants. This review describes the importance of brain regional heterogeneity in relation to cholinergic and noncholinergic determinants with particular reference to organophosphate (OP) and carbamate pesticides and OP nerve agents.

Cold stress offered modulation on chlorpyrifos toxicity in aging rat central nervous system

The adverse effects produced by chlorpyrifos (CPF) or cold stress alone in humans and animals are well documented, but there is no information available relating to the consequences of their co- exposure in an age-related manner. In this study, effects of sublethal doses of CPF were carried out in vivo, for 48 h to assess the biochemical perturbations in relation to interactions with cold stress (15°C and 20°C) in different age group rat CNS. A positive interaction of CPF with age of animal and cold exposure was observed resulting in marked decrease in the activity levels of AChE (P<0.05), ChAT (P<0.05), Na⁺, K⁺-ATPase (P<0.05), Ca²⁺-ATPase (P<0.05), and Mg²⁺-ATPase (P<0.05). The ANOVA and posthoc analysis showed that regulatory enzymes decreased significantly (P<0.05) on CPF exposure. Overall, the effect of co-exposure was appreciably different from either of the exposures. Synergistic interaction of CPF and cold stress at 15°C showed higher inhibition in comparison with CPF and cold stress alone and together at 20°C. Further, this study reveals that young animals are significantly vulnerable and sensitive than adults.

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.

Neurobehavioral and neurodevelopmental effects of pesticide exposures

The association between pesticide exposure and neurobehavioral and neurodevelopmental effects is an area of increasing concern. This symposium brought together participants to explore the neurotoxic effects of pesticides across the lifespan. Endpoints examined included neurobehavioral, affective and neurodevelopmental outcomes among occupational (both adolescent and adult workers) and non-occupational populations (children). The symposium discussion highlighted many challenges for researchers concerned with the prevention of neurotoxic illness due to pesticides and generated a number of directions for further research and policy interventions for the protection of human health, highlighting the importance of examining potential long-term effects across the lifespan arising from early adolescent, childhood or prenatal exposure.

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.

Oxidative macromolecular alterations in the rat central nervous system in response to experimentally co-induced chlorpyrifos and cold stress: a comparative assessment in aging rats

Reactive oxygen species are generated as a result of a number of physiological and pathological processes which can promote multiple forms of oxidative damage including protein oxidation, and thereby influence the function of a diverse array of cellular processes. In our previous study we have reported that co-exposure to chlorpyrifos and cold stress in aging rats markedly influence the toxic outcome as a result of oxidative stress. In the present study, key neurochemical/enzymes were measured in order to evaluate the macromolecular alterations in response to experimentally co-induced chlorpyrifos and cold stress (15 and 20°C) either concurrently or individually in vivo for 48 h in discrete regions of brain and spinal cord of different age group rats. CPF and cold stress exposure either individually or in combination substantially increased the activity/levels of protein carbonyls, AST, ALT and decreased protein thiols, DNA, RNA and total proteins in discrete regions of CNS. Overall, the effects of co-exposure were appreciably different from either of the exposures. However, synergistic-action of CPF and cold stress at 15°C showed higher dyshomeostasis in comparison with CPF and cold stress alone and together at 20°C indicating the extent of oxidative macromolecular damage in discrete regions of brain and spinal cord. Furthermore, the present study demonstrates that macromolecular oxidative damage is highly pronounced in neonates and juveniles than the young adults.

Critical duration of exposure for developmental chlorpyrifos-induced neurobehavioral toxicity

Developmental exposure of rats to the pesticide chlorpyrifos (CPF) causes persistent neurobehavioral impairment. In a parallel series of studies with zebrafish, we have also found persisting behavioral dysfunction after developmental CPF exposure. We have developed a battery of measures of zebrafish behavior, which are reliable and sensitive to toxicant-induced damage. This study determined the critical duration of developmental CPF exposure for causing persisting neurobehavioral effects. Tests of sensorimotor response (tap startle response and habituation), stress response (novel tank diving test) and learning (3-chamber tank spatial discrimination) were conducted with adult zebrafish after early developmental CPF exposure. The CPF exposure level was 100 ng/ml with durations of 0-1, 0-2, 0-3, 0-4 and 0-5 days after fertilization. Developmental CPF exposure had persisting behavioral effects in zebrafish tested as adults. In the tactile startle test, CPF exposed fish showed decreased habituation to startle and a trend toward increased overall startle response. In the novel tank exploration test, exposed fish showed decreased escape diving response and increased swimming activity. In the 3-chamber learning test, the 0-5 day CPF exposure group had a significantly lower learning rate. There was evidence for persisting declines in brain dopamine and norepinepherine levels after developmental CPF exposure. In all of the measures the clearest persistent effects were seen in fish exposed for the full duration of five days after fertilization. In a follow-up experiment there were some indications for persisting behavioral effects after exposure during only the later phase of this developmental window. This study demonstrated the selective long-term neurobehavioral alterations caused by exposure to CPF in zebrafish. The zebrafish model can facilitate the determination of the molecular mechanisms underlying long-term neurobehavioral impairment after developmental toxicant exposure.

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.

Chlorpyrifos-oxon disrupts zebrafish axonal growth and motor behavior

Axonal morphology is a critical determinant of neuronal connectivity, and perturbation of the rate or extent of axonal growth during development has been linked to neurobehavioral deficits in animal models and humans. We previously demonstrated that the organophosphorus pesticide (OP) chlorpyrifos (CPF) inhibits axonal growth in cultured neurons. In this study, we used a zebrafish model to determine whether CPF, its oxon metabolite (CPFO), or the excreted metabolite trichloro-2-pyridinol (TCPy) alter spatiotemporal patterns of axonal growth in vivo. Static waterborne exposure to CPFO, but not CPF or TCPy, at concentrations ≥ 0.03 μM from 24- to 72-h post fertilization significantly inhibited acetylcholinesterase, and high-performance liquid chromatography detected significantly more TCPy in zebrafish exposed to 0.1 μM CPFO versus 1.0 μM CPF. These data suggest that zebrafish lack the metabolic enzymes to activate CPF during these early developmental stages. Consistent with this, CPFO, but not CPF, significantly inhibited axonal growth of sensory neurons, primary motoneurons, and secondary motoneurons at concentrations ≥ 0.1 μM. Secondary motoneurons were the most sensitive to axonal growth inhibition by CPFO, which was observed at concentrations that did not cause mortality, gross developmental defects, or aberrant somatic muscle differentiation. CPFO effects on axonal growth correlated with adverse effects on touch-induced swimming behavior, suggesting the functional relevance of these structural changes. These data suggest that altered patterns of neuronal connectivity contribute to the developmental neurotoxicity of CPF and demonstrate the relevance of zebrafish as a model for studying OP developmental neurotoxicity.

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.

Options for incorporating children's inhaled dose into human health risk assessment

Increasing attention has been placed on inhalation dosimetry in children because of children's greater air intake rate and unique windows of vulnerability for various toxicants and health outcomes. However, risk assessments have not incorporated this information because dosimetric adjustments have focused upon extrapolation across species rather than across age groups within the human population. The objectives of this study were to synthesize information regarding child/adult intake and dosimetry differences for particles and gases for potential application to risk assessment. Data and models gathered at a 2006 workshop and more recent studies were reviewed to better understand lung development and inhaled dose in children. The results show that child/adult differences exist both on a chemical intake basis and on a deposited or systemic dose basis. These differences can persist for several years and are not captured by standard intraspecies uncertainty factors or by USEPA's reference concentration (RfC) methodology. Options for incorporating children's inhalation exposures into human risk assessments include (1) 3-fold default air intake adjustment for the first 3 years of life with a reduced factor for older children; (2) superseding this default via simplified dosimetry models akin to USEPA's RfC methodology modified for children; (3) utilizing more sophisticated models with better anatomical and air flow descriptions; (4) running these models with input distributions to reflect interchild variability; (5) developing more advanced approaches involving imaging techniques and computational fluid dynamic (CFD) models. These options will enable children's inhaled dose to have a quantitative role in risk assessment that has been lacking and will establish a basis for ongoing research.

Diverse neurotoxicants converge on gene expression for neuropeptides and their receptors in an in vitro model of neurodifferentiation: effects of chlorpyrifos, diazinon, dieldrin and divalent nickel in PC12 cells

Unrelated developmental neurotoxicants can produce similar neurobehavioral outcomes. We examined whether disparate agents affect neuromodulators that control numerous neurotransmitters and circuits, employing PC12 cells to explore the targeting of neuroactive peptides by organophosphates (chlorpyrifos, diazinon), an organochlorine (dieldrin) and a metal (Ni(2+)); we utilized microarrays to profile gene expression for the peptides and their receptors. Chlorpyrifos evoked robust upregulation of cholecystokinin, corticotropin releasing hormone, galanin, neuropeptide Y, neurotensin, preproenkephalin and tachykinin 1; this involved a critical period at the commencement of neurodifferentiation, since the effects were much less notable in undifferentiated PC12 cells. Diazinon targeted a similar but smaller repertoire of neuropeptide genes and the magnitude of the effects was also generally less. Surprisingly, dieldrin shared many of the same neuropeptide targets as the organophosphates and concordance analysis showed significant overlap among all three pesticides. However, dieldrin had more notable effects on neuropeptide receptors, and overlap between diazinon and dieldrin for the receptors led to a stronger resemblance of these two agents than of chlorpyrifos and dieldrin. Ni(2+) was unique, evoking upregulation of only one of the peptides affected by the other agents, while causing downregulation of several others. Nevertheless, there was still significant concordance between Ni(2+) and either diazinon or dieldrin, reflecting similarities toward the receptors. Our results show that neuropeptides are likely to be a prominent target for the developmental neurotoxicity of organophosphates and other neurotoxicants, and further, that the convergence of disparate agents on the same genes and pathways may contribute to similar neurobehavioral outcomes.

An avian model for the reversal of neurobehavioral teratogenicity with neural stem cells

A fast and simple model which uses lower animals on the evolutionary scale is beneficial for developing procedures for the reversal of neurobehavioral teratogenicity with neural stem cells. Here, we established a procedure for the derivation of chick neural stem cells, establishing embryonic day (E) 10 as optimal for progression to neuronal phenotypes. Cells were obtained from the embryonic cerebral hemispheres and incubated for 5-7 days in enriched medium containing epidermal growth factor (EGF) and basic fibroblast growth factor (FGF2) according to a procedure originally developed for mice. A small percentage of the cells survived, proliferated and formed nestin-positive neurospheres. After removal of the growth factors to allow differentiation (5 days), 74% of the cells differentiated into all major lineages of the nervous system, including neurons (Beta III tubulin-positive, 54% of the total number of differentiated cells), astrocytes (GFAP-positive, 26%), and oligodendrocytes (O4-positive, 20%). These findings demonstrate that the cells were indeed neural stem cells. Next, the cells were transplanted in two allograft chick models; (1) direct cerebral transplantation to 24-h-old chicks, followed by post-transplantation cell tracking at 24 h, 6 days and 14 days, and (2) intravenous transplantation to chick embryos on E13, followed by cell tracking on E19. With both methods, transplanted cells were found in the brain. The chick embryo provides a convenient, precisely-timed and unlimited supply of neural progenitors for therapy by transplantation, as well as constituting a fast and simple model in which to evaluate the ability of neural stem cell transplantation to repair neural damage, steps that are critical for progress toward therapeutic applications.

Disparate developmental neurotoxicants converge on the cyclic AMP signaling cascade, revealed by transcriptional profiles in vitro and in vivo

Cell-signaling cascades are convergent targets for developmental neurotoxicity of otherwise unrelated agents. We compared organophosphates (chlorpyrifos, diazinon), an organochlorine (dieldrin) and a metal (Ni(2+)) for their effects on neuronotypic PC12 cells, assessing gene transcription involved in the cyclic AMP pathway. Each agent was introduced during neurodifferentiation at a concentration of 30 microM for 24 or 72 h and we assessed 69 genes encoding adenylyl cyclase isoforms and regulators, G-protein alpha-and beta,gamma-subunits, protein kinase A subtypes and the phosphodiesterase family. We found strong concordance among the four agents across all the gene families, with the strongest relationships for the G-proteins, followed by adenylyl cyclase, and lesser concordance for protein kinase A and phosphodiesterase. Superimposed on this pattern, chlorpyrifos and diazinon were surprisingly the least alike, whereas there was strong concordance of dieldrin and Ni(2+) with each other and with each individual organophosphate. Further, the effects of chlorpyrifos differed substantially depending on whether cells were undifferentiated or differentiating. To resolve the disparities between chlorpyrifos and diazinon, we performed analyses in rat brain regions after in vivo neonatal exposures; unlike the in vitro results, there was strong concordance. Our results show that unrelated developmental neurotoxicants can nevertheless produce similar outcomes by targeting cell signaling pathways involved in neurodifferentiation during a critical developmental period of vulnerability. Nevertheless, a full evaluation of concordance between different toxicants requires evaluations of in vitro systems that detect direct effects, as well as in vivo systems that allow for more complex interactions that converge on the same pathway.

Early postnatal parathion exposure in rats causes sex-selective cognitive impairment and neurotransmitter defects which emerge in aging

Developmental exposure of rats to the organophosphate (OP) pesticides leads to altered neurobehavioral function in juvenile and young adult stages. The current study was conducted to determine whether effects of neonatal parathion exposure on cognitive performance persist in older adult and aged rats, and the relationship of behavioral changes to underlying cholinergic and serotonergic mechanisms. We administered parathion to rat pups on postnatal days 1-4, at doses spanning the threshold for the initial signs of systemic toxicity and for barely detectable cholinesterase inhibition (0.1 or 0.2 mg/kg/day). Beginning at 14 months of age and continuing until 19 months, the rats were trained in the 16-arm radial maze. Controls showed the normal sex difference in this spatial learning and memory task, with the males committing significantly fewer working memory errors than females. Neonatal parathion exposure eliminated the sex difference primarily by causing impairment in males. In association with the effects on cognitive performance, neonatal parathion exposure elicited widespread abnormalities in indices of serotonergic (5HT) and cholinergic synaptic function, characterized by upregulation of 5HT(2) receptors and the 5HT transporter, deficits in choline acetyltransferase activity and nicotinic cholinergic receptors, and increases in hemicholinium-3 binding to the presynaptic choline transporter. Within-animal correlations between behavior and neurochemistry indicated a specific correlation between working memory performance and hippocampal hemicholinium-3 binding; parathion exposure eliminated this relationship. Like the behavioral effects, males showed greater effects of parathion on neurochemical parameters. This study demonstrates the sex-selective, long-term behavioral alterations caused by otherwise nontoxic neonatal exposure to parathion, with effects increasingly expressed with aging.

Neurobehavioral teratogenicity of sarin in an avian model

Nerve gas organophosphates like sarin are likely to be used in urban terrorism, leading to widespread exposures of pregnant women and young children. Here, we established a model for sarin neurobehavioral teratogenicity in the developing chick so as to explore the consequences of apparently subtoxic sarin exposure and the mechanisms underlying synaptic and behavioral deficits. Chicken eggs were injected with sarin (2, 6 and 12 microg/kg) on incubation days 2 and 6, treatments that did not decrease hatching and did not evoke dysmorphology. After hatching the chicks were tested for filial imprinting and neurochemical markers known to be critical for imprinting. Imprinting was reduced at 2 and 6 microg/kg but not at the highest dose. Acetylcholinesterase and choline acetyltransferase were unaffected but sarin reduced the concentration of the high-affinity choline transporter, the rate-limiting factor in acetylcholine utilization. The concentration of PKC isoforms was assessed in the imprinting-related intermediate part of the medial hyperstriatum ventrale, the region most closely associated with cholinergic function in imprinting behavior. Sarin reduced the concentration of all isoforms (alpha, beta, gamma) with a similar, biphasic dose-response curve to that seen for behavioral performance, a relationship noted in previous work with organophosphate pesticides. Our results indicate that otherwise subtoxic exposures to sarin produce neurodevelopmental deficits; since we utilized a chick model, which is devoid of maternal confounds that are present in mammalian development, the adverse effects of sarin are mediated directly in the developing organism.

CHLORPYRIFOS INCREASES THE LEVELS OF HIPPOCAMPAL NMDA RECEPTOR SUBUNITS NR2A AND NR2B IN JUVENILE AND ADULT RATS

The present study investigated the effect of chlorpyrifos on NMDA receptor subunits NR2A and NR2B in juvenile and adult rats. Chlorpyrifos was administered with the dose of 40 and 70 mg/kg to juvenile and adult rats, respectively. Chlorpyrifos significantly inhibited the AChE activity in juvenile and adult rats (p < .05). NR2A and NR2B levels significantly increased in juvenile and adult rats by chlorpyrifos application (p < .05). Increased NR2A and NR2B levels may reflect increased glutaminergic activity, consequently neuronal damage. In the case of neuronal damage, learning and memory could be affected negatively even though NR2A and NR2B increased.

Consumption of a high-fat diet in adulthood ameliorates the effects of neonatal parathion exposure on acetylcholine systems in rat brain regions

BACKGROUND

Developmental exposure to a wide variety of developmental neurotoxicants, including organophosphate pesticides, evokes late-emerging and persistent abnormalities in acetylcholine (ACh) systems. We are seeking interventions that can ameliorate or reverse the effects later in life.

OBJECTIVES

We administered parathion to neonatal rats and then evaluated whether a high-fat diet begun in adulthood could reverse the effects on ACh systems.

METHODS

Neonatal rats received parathion on postnatal days 1-4 at 0.1 or 0.2 mg/kg/day, straddling the cholinesterase inhibition threshold. In adulthood, half the animals were switched to a high-fat diet for 8 weeks. We assessed three indices of ACh synaptic function: nicotinic ACh receptor binding, choline acetyltransferase activity, and hemicholinium-3 binding. Determinations were performed in brain regions comprising all the major ACh projections and cell bodies.

RESULTS

Neonatal parathion exposure evoked widespread abnormalities in ACh synaptic markers, encompassing effects in brain regions possessing ACh projections and ACh cell bodies. In general, males were affected more than females. Of 17 regional ACh marker abnormalities (10 male, 7 female), 15 were reversed by the high-fat diet.

CONCLUSIONS

A high-fat diet reverses neurodevelopmental effects of neonatal parathion exposure on ACh systems. This points to the potential for nonpharmacologic interventions to offset the effects of developmental neurotoxicants. Further, cryptic neurodevelopmental deficits evoked by environmental exposures may thus engender a later preference for a high-fat diet to maintain normal ACh function, ultimately contributing to obesity.

Sexual function in adult male rats after prenatal modulation of the cholinergic system

Prenatal administration of the n-cholinolytic ganglerone to pregnant female rats at different periods of gestation was found to lead to long-term changes in sexual behavior in pubescent offspring: there was a reduced dynamic of acquiring sexual experience and a very low level of sexual activity, with significant impairment to the motivational and ejaculatory components of sexual behavior. The number of males with reduced sexual activity in the experimental groups was significantly greater than that in control offspring. The results obtained here provide evidence that impairments of sexual function in adult offspring induced by prenatal administration of the n-cholinolytic ganglerone at 9-11 and 12-14 days of gestation and, to a lesser extent, the m-cholinolytic metamyzil at 9-11 days of gestation, were due to impairment to the central mechanisms regulating sexual function due to stable changes in neurotransmitter activity in the hippocampus and hypothalamus, along with a significant reduction in the blood testosterone level.

Developmental chlorpyrifos and methyl parathion exposure alters radial-arm maze performance in juvenile and adult rats

Although the use of organophosphate (OP) insecticides has been restricted, sufficient exposure can occur to induce detrimental neurobehavioral effects. In this study, we measured physical and reflex development and spatial learning and memory in rats repeatedly exposed to incremental doses of chlorpyrifos (CPS) and methyl parathion (MPS) from postnatal day (PND) 1 to PND21. Other than decreased body weight in the higher dosage groups, no effects on physical or reflex development were observed. Significant hippocampal cholinesterase inhibition was induced in all treatment groups for up to 19 days following exposure. Beginning on PND36, working and reference memory was tested using a 12-arm radial maze, with subject animals trained and tested 4 days a week for 4 weeks. In males, working memory was decreased with the medium and high dosage of MPS but only the high dosage of CPS; while in females, no deficits were observed. For reference memory, errors were significantly increased in males exposed to the high dosage of CPS and all dosages of MPS. In females, enhanced performance was observed within the medium and high dosages of CPS but not with MPS. These data show that repeated developmental exposure to OP insecticides can induce sex-selective alterations and long-lasting changes in spatial learning and memory formation when measured using a radial arm maze and that MPS and CPS induce different neurobehavioral outcomes.

Protein kinase C is a target for diverse developmental neurotoxicants: transcriptional responses to chlorpyrifos, diazinon, dieldrin and divalent nickel in PC12 cells

Unrelated developmental neurotoxicants can elicit similar functional outcomes, whereas agents in the same class may differ. We compared two organophosphate insecticides (chlorpyrifos, diazinon) with an organochlorine (dieldrin) and a metal (Ni(2+)) for similarities and differences in their effects on gene expression encoding subtypes of protein kinase C and their modulators, a cell signaling cascade that integrates the actions of neurotrophic factors involved in brain development. We conducted evaluations in PC12 cells, a model for neuronal development, with each agent introduced at 30 microM for 24 or 72 h, treatments devoid of cytotoxicity. Chlorpyrifos evoked by far the largest effect, with widespread upregulation of multiple genes; the effects were greater during neurodifferentiation than when cells were exposed prior to differentiation. Diazinon had smaller and less widespread effects, consistent with its lesser long-term impact on synaptic function and behavior noted for in vivo exposures in developing rats. Surprisingly, the effects of diazinon, dieldrin and Ni(2+) showed basic similarities despite the fact that all three come from different classes of toxicants. Our findings provide some of the first evidence for a specific mechanistic cascade contributing to the cholinesterase-independent developmental neurotoxicant actions of chlorpyrifos and its differences from diazinon, while at the same time identifying mechanistic convergence between otherwise unrelated toxicants that provides predictions about common neurodevelopmental outcomes. These results further show how combined use of cell cultures and microarray technology can guide future in vivo work on diverse developmental neurotoxicants.

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.

Exposure of neonatal rats to parathion elicits sex-selective impairment of acetylcholine systems in brain regions during adolescence and adulthood

BACKGROUND

Organophosphates elicit developmental neurotoxicity through multiple mechanisms other than their shared property as cholinesterase inhibitors. Accordingly, these agents may differ in their effects on specific brain circuits.

OBJECTIVES

We gave parathion to neonatal rats [postnatal days (PNDs) 1-4], at daily doses of 0.1 or 0.2 mg/kg, spanning the threshold for barely detectable cholinesterase inhibition and systemic effects.

METHODS

We assessed neurochemical indices related to the function of acetylcholine (ACh) synapses (choline acetyltransferase, presynaptic high-affinity choline transporter, nicotinic cholinergic receptors) in brain regions comprising all the major ACh projections, with determinations carried out from adolescence to adulthood (PNDs 30, 60, and 100).

RESULTS

Parathion exposure elicited lasting alterations in ACh markers in the frontal/parietal cortex, temporal/occipital cortex, midbrain, hippocampus, and striatum. In cerebrocortical areas, midbrain, and hippocampus, effects in males were generally greater than in females, whereas in the striatum, females were targeted preferentially. Superimposed on this general pattern, the cerebrocortical effects showed a nonmonotonic dose-response relationship, with regression of the defects at the higher parathion dose; this relationship has been seen also after comparable treatments with chlorpyrifos and diazinon and likely represents the involvement of cholinesterase-related actions that mask or offset the effects of lower doses.

CONCLUSIONS

Neonatal exposure to parathion, at doses straddling the threshold for cholinesterase inhibition, compromises indices of ACh synaptic function in adolescence and adulthood. Differences between the effects of parathion compared with chlorpyrifos or diazinon and the non-monotonic dose-effect relationships reinforce the conclusion that various organophosphates diverge in their effects on neurodevelopment, unrelated to their anticholinesterase actions.

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.