Effects of gestational exposure to chlorpyrifos on postnatal central and peripheral cholinergic neurochemistry

Knockdown of Butyrylcholinesterase but Not Inhibition by Chlorpyrifos Alters Early Differentiation Mechanisms in Human Neural Stem Cells

Butyrylcholinesterase (BChE) is the evolutionary counterpart to acetylcholinesterase (AChE). Both are expressed early in nervous system development prior to cholinergic synapse formation. The organophosphate pesticide chlorpyrifos (CPF) primarily exerts toxicity through the inhibition of AChE, which results in excess cholinergic stimulation at the synapse. We hypothesized that the inhibition of AChE and BChE by CPF may impair early neurogenesis in neural stem cells (NSCs). To model neurodevelopment in vitro, we used human NSCs derived from induced pluripotent stem cells (iPSCs) with a focus on the initial differentiation mechanisms. Over the six days of NSC differentiation, the BChE activity and mRNA expression significantly increased, while the AChE activity and expression remained unchanged. The CPF treatment (10 μM) caused 82% and 92% inhibition of AChE and BChE, respectively. The CPF exposure had no effect on the cell viability or the expression of the differentiation markers HES5, DCX, or MAP2. However, the shRNA-knockdown of the BChE expression resulted in the decreased or delayed expression of the transcription factors HES5 and HES3. BChE may have a role in the differentiation of NSCs independent of, or in addition to, its enzymatic activity.

Developmental neurotoxicity of monocrotophos and lead is linked to thyroid disruption

Aim:

A role of thyroid disruption in developmental neurotoxicity of monocrotophos (MCP) and lead is studied.

Materials and Methods:

A total of 24 female rats after conception were randomized into four groups of six each and treated as follows: Group I - Sham was administered distilled water orally. Group II - A positive control was administered methyl methimazole at 0.02% orally in drinking water. Group III - MCP orally at 0.3 mg/kg and Group IV - Lead acetate at 0.2% orally in drinking water. The drug was administered from gestation day 3 through post-natal day 21 in all the groups. Acetylcholinesterase (AChE) inhibition, thyroid profile (thyroid stimulating hormone, T₃ and T₄), neurodevelopment (brain wet weights, DNA, RNA and protein), and neurobehavioral (elevated plus maze, photoactometry, and Morris water maze) parameters were assessed in pups. A histopathology of thyroid of dams and brain of progeny was conducted.

Results:

Inhibition of AChE was <20%. Thyroid profile decreased in the treatment groups. Neurodevelopmental and neurobehavioral parameters did not reveal any significant changes. Thyroid architecture was affected significantly with MCP and lead. Cortical layers too were affected. The three layers of cerebellum either had abnormal arrangement or decreased cellularity in all treated groups relating to thyroid disruption.

Conclusion:

MCP and lead might have affected the development of cerebrum and cerebellum via thyroid disruption leading to developmental neurotoxicity.

Vitamin K3 (menadione) redox cycling inhibits cytochrome P450-mediated metabolism and inhibits parathion intoxication

Parathion, a widely used organophosphate insecticide, is considered a high priority chemical threat. Parathion toxicity is dependent on its metabolism by the cytochrome P450 system to paraoxon (diethyl 4-nitrophenyl phosphate), a cytotoxic metabolite. As an effective inhibitor of cholinesterases, paraoxon causes the accumulation of acetylcholine in synapses and overstimulation of nicotinic and muscarinic cholinergic receptors, leading to characteristic signs of organophosphate poisoning. Inhibition of parathion metabolism to paraoxon represents a potential approach to counter parathion toxicity. Herein, we demonstrate that menadione (methyl-1,4-naphthoquinone, vitamin K3) is a potent inhibitor of cytochrome P450-mediated metabolism of parathion. Menadione is active in redox cycling, a reaction mediated by NADPH-cytochrome P450 reductase that preferentially uses electrons from NADPH at the expense of their supply to the P450s. Using human recombinant CYP 1A2, 2B6, 3A4 and human liver microsomes, menadione was found to inhibit the formation of paraoxon from parathion. Administration of menadione bisulfite (40mg/kg, ip) to rats also reduced parathion-induced inhibition of brain cholinesterase activity, as well as parathion-induced tremors and the progression of other signs and symptoms of parathion poisoning. These data suggest that redox cycling compounds, such as menadione, have the potential to effectively mitigate the toxicity of organophosphorus pesticides including parathion which require cytochrome P450-mediated activation.

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.

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.

Mood disorders hospitalizations, suicide attempts, and suicide mortality among agricultural workers and residents in an area with intensive use of pesticides in Brazil

As suicide rates have increased in rural areas in Brazil, it was postulated that pesticide exposure may play a role in this phenomenon. Our study compared the suicide mortality rates observed among agricultural workers from a pesticide-intensive area in Brazil to the suicide mortality frequency noted in three reference populations. In addition, hospitalization rates attributed to suicide attempts and mood disorders including depression in residents of the same agricultural area were compared to two reference populations. Finally, data on pesticide sales per agricultural worker were obtained for each city of Rio de Janeiro State and suicide mortality risk was then calculated according to the quartiles of pesticide sales per agricultural workers, using the first quartile as reference. Agricultural workers were at greater risk for lethality due to suicide when compared to all three reference populations. In addition, residents of the same study area showed higher hospitalization rates by suicide attempts and mood disorders than observed in comparison populations. Results also showed that the risk of death by suicide was significantly higher among agricultural workers who lived in areas of Rio de Janeiro State displaying higher rates of pesticide expenditure per agricultural worker. These results suggest that pesticide exposure may indeed increase the risk of suicide frequency, especially among agricultural workers.

Differential sensitivity to anticholinesterase insecticides in the juvenile rat: effects on thermoregulation

Organophosphate (OP) and carbamate (CB) insecticides inhibit cholinesterase (ChE) activity and induce acute hypothermia in adult rats. Studies showed that juveniles are generally more susceptible to neurotoxic insult than adults. However, little is known concerning the effects of OP and CB pesticides on thermoregulation in developing animals. Thus, alterations in core body temperature (Tc) in juvenile animals exposed to an OP and CB insecticide were investigated. Male rat pups were anesthetized on postnatal day (PND) 15 with metofane and a radio transmitter (Data Sciences) was implanted in the abdominal cavity to monitor Tc and motor activity (MA). Two days were allowed for recovery. The PND 17 pups were then dosed by oral gavage with the OP chlorpyrifos (CHP) (1, 5, 10, or 15 mg/kg) or the CB carbaryl (CAR) (10, 20, 80, 120, or 160 mg/kg) or the corn oil vehicle. Pups were returned to their dams and littermates immediately after dosing and monitored for the next several days. CHP doses of 10 and 15 mg/kg resulted in 1.0 degrees C and 2.4 degrees C reductions in Tc, respectively. Tc recovered to control levels by approximately 16 h after dosing. There was significant mortality in rats dosed with 15 mg/kg CHP (6 of 11). CAR doses of 10 to 80 mg/kg had little effect on Tc. The highest dose of CAR (160 mg/kg) resulted in a 1.3 degrees C reduction in Tc that recovered in 9 h. In contrast, past studies found that adult male rats become hypothermic at CHP doses of >25 mg/kg, whereas a CAR dose of 50 mg/kg is effective at inducing hypothermia. Overall, it appears that during the development from preweanling to adult rat, there is a progressive attenuation in CHP-induced hypothermia. Conversely, CAR-induced hypothermia increases as a function of development.

Pesticide exposures and developmental outcomes: the epidemiological evidence

Since the advent of DDT as an insecticide in the late 1930s, billions of kilograms of pesticide active ingredient have been sold in North America and around the world. In recent years, there has been a heightened public awareness of pesticides and child health and a number of epidemiologic studies linked pre- and postnatal exposures to pesticides to a number of adverse developmental outcomes, including fetal death, intrauterine growth restriction, preterm birth, and birth defects. Given this, it was felt prudent to critically appraise the evidence for periconceptual pesticide exposures and developmental outcomes. The epidemiological evidence for specific pesticide classes, families, and active ingredients were examined and summarized and recommendations were made for how to improve future studies in order to address the current pitfalls and gaps in the studies in this area. Many of the studies suffered from poor exposure estimation, relying on job title only and/or the exposure category "any pesticide" as a measure of exposure, and there was limited or inadequate evidence to support causality for all associations examined.

Development and application of acute exposure guideline levels (AEGLs) for chemical warfare nerve and sulfur mustard agents

Acute exposure guideline levels (AEGLs) have been developed for the chemical warfare agents GB, GA, GD, GF, VX, and sulfur mustard. These AEGLs were approved by the National Advisory Committee for Acute Exposure Guideline Levels for Hazardous Substances after Federal Register publication and comment, and judged as scientifically valid by the National Research Council Committee on Toxicology Subcommittee on AEGLs. AEGLs represent general public exposure limits for durations ranging from 10 min to 8 h, and for three levels of severity (AEGL-1, AEGL-2, AEGL-3). Mild effects are possible at concentrations greater than AEGL-1, while life-threatening effects are expected at concentrations greater than AEGL-3. AEGLs can be applied to various civilian and national defense purposes, including evacuation and shelter-in-place protocols, reentry levels, protective clothing specifications, and analytical monitoring requirements. This report documents development and derivation of AEGL values for six key chemical warfare agents, and makes recommendations for their application to various potential exposure scenarios.

A review of the reference dose for chlorpyrifos

Chlorpyrifos is an inhibitor of cholinesterase (ChE) and inhibition of ChE is believed to be the most sensitive effect in all animal species evaluated and in humans from previous evaluations. Recent literature, in particular epidemiology studies reporting associations between chlorpyrifos levels and fetal birth weight decreases, suggest the need to reevaluate the basis of the reference dose (RfD) for chlorpyrifos, however. In this paper, we evaluated newly available publications regarding chlorpyrifos toxicity and discuss the choice of critical effect--whether cholinesterase inhibition or developmental effect, the choice of appropriate species and study, the appropriate point of departure, and choice of uncertainty factors--including a discussion of the FQPA safety factor. We conclude that RBC cholinesterase inhibition is the critical effect, that human studies form the best choice of species--supported by a wealth of experimental animal data, that a NOAEL of 0.1 mg/kg/day is the most appropriate point of departure, and that a 10-fold factor for within human variability is sufficient to characterize the overall uncertainty in this rather large database. The resulting RfD is 0.01 mg/kg/day.

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.