Age-related differences in sensitivity to organophosphorus pesticides

Environmental exposure to chlorpyrifos during gestation, APOE polymorphism and the risk on autistic-like behaviors

Autism spectrum disorder (ASD) encompasses several neurodevelopmental conditions characterized by communication and social impairment, as well as repetitive patterns of behavior. However, it can co-occur with other mental conditions such as anxiety. The massive use of chlorpyrifos (CPF) has been linked to the increased prevalence of developmental disorders. Likewise, ASD has also been closely linked to a wide variety of genetic factors. The aims of the present investigation are to study how gestational CPF exposure and APOE polymorphism affects communication skills, early development and mid-term anxiety-like behaviors, as well as, changes in gene expression related to the cholinergic system. C57BL/6J and humanized apoE3 and apoE4 homozygous mice were exposed to 0 or 1 mg/kg/day of CPF through the diet, from gestational day (GD) 12-18. In addition, a group of C57BL/6J females were injected subcutaneously with 300 mg/kg/day of valproic acid (VPA) on GD 12 and 13. This group was used as a positive control for studying some core and associated autism-like behaviors. Communication skills by means of ultrasonic vocalizations and physical/motor development were assessed during the preweaning period, whereas locomotor activity, anxiety-like behaviors and the gene expression of cholinergic elements were evaluated during adolescence. Our results showed that C57BL/6J mice prenatally exposed to CPF or VPA showed a decrease in body weight and a delay in eye opening. Communication and anxiety behavior were affected differently depending on treatment, while gene expression was altered by sex and treatment. In addition, none of the parameters evaluated in apoE transgenic mice exposed to CPF were affected, but there were differences between genotypes. Therefore, we suggest that prenatal CPF exposure and VPA produce divergent effects on communication and anxiety.

Sub-lethal Effects of Chlorpyrifos on Big Brown Bats (Eptesicus fuscus)

We determined dose-response curves for sublethal effects of the organophosphorus (OP) insecticide, chlorpyrifos, on bats. Big brown bats (Eptesicus fuscus, n = 64) were given a single dose of chlorpyrifos (nominal concentrations) of 0, 5, 10, 15, 20, 25, 30, or 60 µg/g body weight and examined at 12 or 24 h after dosing. A second experiment dosed 32 bats with 0 or 60 µg/g body weight and examined 1, 3, 7, or 14 days after dosing. Skin temperature and behavioral changes were recorded, and brain and plasma cholinesterase (ChE) activity were measured. The benchmark dose (BMD10) of chlorpyrifos that altered brain and plasma ChE activity at 24 h was 3.7 and 10.1 µg/g, respectively. The 95 % lower confidence limit for the BMD10 (i.e., BMDL10) was 1.6 and 7.7 µg/g. The best of five models (as determined by AIC) for impaired flight, impaired movement, or presence of tremors provided a BMD10 of 6.2, 12.9, and 7.8 µg/g body weight of chlorpyrifos, respectively. BMDL10 for impaired flight, impaired movement, or presence of tremors was 3.5, 6.6, and 5.3 µg/g body weight, respectively. In the wild, impaired ability to fly or crawl could be life-threatening. Brain and plasma ChE activity remained low for 3 days after dosing. Gradual recovery of enzyme activity was observed by 7 days in survivors. Brain and plasma ChE activity were still significantly lower than that of the control group at 14 days after dosing.

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

BACKGROUND

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

METHODS

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

FINDINGS

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

CONCLUSIONS

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

Astrocytes protect against diazinon- and diazoxon-induced inhibition of neurite outgrowth by regulating neuronal glutathione

Evidence demonstrating that human exposure to various organophosphorus insecticides (OPs) is associated with neurobehavioral deficits in children continues to emerge. The present study focused on diazinon (DZ) and its active oxygen metabolite, diazoxon (DZO), and explored their ability to impair neurite outgrowth in rat primary hippocampal neurons as a mechanism of developmental neurotoxicity. Both DZ and DZO (0.5-10 μM) significantly inhibited neurite outgrowth in hippocampal neurons, at concentrations devoid of any cyototoxicity. These effects appeared to be mediated by oxidative stress, as they were prevented by antioxidants (melatonin, N-t-butyl-alpha-phenylnitrone, and glutathione ethyl ester). Inhibition of neurite outgrowth was observed at concentrations below those required to inhibit the catalytic activity of acetylcholinesterase. The presence of astrocytes in the culture was able to provide protection against inhibition of neurite outgrowth by DZ and DZO. Astrocytes increased neuronal glutathione (GSH) in neurons, to levels comparable to those of GSH ethyl ester. Astrocytes depleted of GSH by L-buthionine-(S,R)-sulfoximine no longer conferred protection against DZ- and DZO-induced inhibition of neurite outgrowth. The findings indicate that DZ and DZO inhibit neurite outgrowth in hippocampal neurons by mechanisms involving oxidative stress, and that these effects can be modulated by astrocytes and astrocyte-derived GSH. Oxidative stress from other chemical exposures, as well as genetic abnormalities that result in deficiencies in GSH synthesis and regulation, may render individuals more susceptible to these developmental neurotoxic effects of OPs.

Diazinon and diazoxon impair the ability of astrocytes to foster neurite outgrowth in primary hippocampal neurons

Evidence from in vivo and epidemiological studies suggests that organophosphorus insecticides (OPs) are developmental neurotoxicants, but possible underlying mechanisms are still unclear. Astrocytes are increasingly recognized for their active role in normal neuronal development. This study sought to investigate whether the widely-used OP diazinon (DZ), and its oxygen metabolite diazoxon (DZO), would affect glial-neuronal interactions as a potential mechanism of developmental neurotoxicity. Specifically, we investigated the effects of DZ and DZO on the ability of astrocytes to foster neurite outgrowth in primary hippocampal neurons. The results show that both DZ and DZO adversely affect astrocyte function, resulting in inhibited neurite outgrowth in hippocampal neurons. This effect appears to be mediated by oxidative stress, as indicated by OP-induced increased reactive oxygen species production in astrocytes and prevention of neurite outgrowth inhibition by antioxidants. The concentrations of OPs were devoid of cytotoxicity, and cause limited acetylcholinesterase inhibition in astrocytes (18 and 25% for DZ and DZO, respectively). Among astrocytic neuritogenic factors, the most important one is the extracellular matrix protein fibronectin. DZ and DZO decreased levels of fibronectin in astrocytes, and this effect was also attenuated by antioxidants. Underscoring the importance of fibronectin in this context, adding exogenous fibronectin to the co-culture system successfully prevented inhibition of neurite outgrowth caused by DZ and DZO. These results indicate that DZ and DZO increase oxidative stress in astrocytes, and this in turn modulates astrocytic fibronectin, leading to impaired neurite outgrowth in hippocampal neurons.

The effect of age on the paraoxonase and arylesterase activity of pregnant mothers and their infants

AIM

The study aimed to research the effect of maternal age at pregnancy on the paraoxonase (PON) and arylesterase activity levels of mothers and infants.

METHOD

The study comprised 30 mothers aged more than 35 years and their infants, and 30 mothers aged under 25 years and their infants. All pregnancies had been of 38-42 weeks with normal spontaneous vaginal birth. PON and arylesterase activities were measured using paraoxon and phenylacetate substrates.

RESULTS

When the PON and arylesterase enzyme activities were compared, the PON and arylesterase activity of the mothers aged more than 35 years was found to be statistically significantly lower compared to that of the mothers aged under 25 years (P = 0.006 and P = 0.03, respectively), while there was no statistically significant difference in PON and arylesterase enzyme activity between the two groups of infants (P > 0.05).

CONCLUSION

While the PON and arylesterase enzyme activity of the mothers aged more than 35 years was found to be significantly lower compared to that of the mothers aged under 25 years, there was no statistical significance between the infants. This can be explained by the reduced PON and arylesterase enzyme activity during pregnancies at an older age, although this did not affect the PON and arylesterase enzyme activity of the infants.

Paraoxonase 1 (PON1) as a genetic determinant of susceptibility to organophosphate toxicity

Paraoxonase (PON1) is an A-esterase capable of hydrolyzing the active metabolites (oxons) of a number of organophosphorus (OP) insecticides such as parathion, diazinon and chlorpyrifos. PON1 activity is highest in liver and in plasma. Human PON1 displays two polymorphisms in the coding region (Q192R and L55M) and several polymorphisms in the promoter and the 3'-UTR regions. The Q192R polymorphism imparts differential catalytic activity toward some OP substrates, while the polymorphism at position -108 (C/T) is the major contributor of differences in the levels of PON1 expression. Both contribute to determining an individual's PON1 "status". Animal studies have shown that PON1 is an important determinant of OP toxicity. Administration of exogenous PON1 to rats or mice protects them from the toxicity of specific OPs. PON1 knockout mice display a high sensitivity to the toxicity of diazoxon and chlorpyrifos oxon, but not of paraoxon. In vitro catalytic efficiencies of purified PON192 alloforms for hydrolysis of specific oxon substrates accurately predict the degree of in vivo protection afforded by each isoform. Evidence is slowly emerging that a low PON1 status may increase susceptibility to OP toxicity in humans. Low PON1 activity may also contribute to the developmental toxicity and neurotoxicity of OPs, as shown by animal and human studies.

Developmental neurotoxicity: some old and new issues

The developing central nervous system is often more vulnerable to injury than the adult one. Of the almost 200 chemicals known to be neurotoxic, many are developmental neurotoxicants. Exposure to these compounds in utero or during childhood can contribute to a variety of neurodevelopmental and neurological disorders. Two established developmental neurotoxicants, methylmercury and lead, and two classes of chemicals, the polybrominated diphenyl ether flame retardants and the organophosphorus insecticides, which are emerging as potential developmental neurotoxicants, are discussed in this paper. Developmental neurotoxicants may also cause silent damage, which would manifest itself only as the individual ages, and may contribute to neurodegenerative diseases such as Parkinson's or Alzheimer's diseases. Guidelines for developmental neurotoxicity testing have been implemented, but there is still room for their improvement and for searching and validating alternative testing approaches.

Altered Muscarinic Acetylcholine Receptor Subtype Binding in Neonatal Rat Brain following Exposure to Chlorpyrifos or Methyl Parathion

The neurodevelopmental effects of two organophosphorus (OP) insecticides, chlorpyrifos (CPS) and methyl parathion (MPS), on cholinesterase (ChE) activity and muscarinic acetylcholine receptor (mAChR) binding were investigated in neonatal rat brain. Animals were orally gavaged using an incremental dosing regimen from postnatal day 1 (PND1) until PND8 with a low, medium, and high dosage for both CPS and MPS. On PND4 and PND8, ChE activity was measured in whole brain while the total and subtype densities of mAChRs were measured in three brain sections: area anterior to optic chiasma (anterior forebrain), area from the optic chiasma to the medulla/pons (posterior forebrain); and the medulla/pons excluding the cerebellum. The ligands 3H-pirenzepine, 3H-AF-DX 384, 3H-4-DAMP, and 3H-QNB were used to measure the maximal binding of the M1, M2/M4, and M3 subtypes and total mAChR receptors, respectively. In the anterior and the posterior forebrain, the levels of all mAChRs nearly doubled from PND4 to PND8, while in the medulla/pons, M1- and M3-subtype mAChR densities were low and did not increase and M2/M4 subtype and total mAChR slightly increased from PND4 to PND8. Reduction of ChE activity and mAChR binding by CPS or MPS was more evident in rats at PND8 than at PND4. With respect to mAChR binding, the greatest effects were observed in the medulla/pons and the least effects were observed in the posterior region of the forebrain. These results demonstrate that OPs exert adverse effects on rat central nervous system development through the cholinergic system in an age- and region-dependent manner.

An age-dependent physiologically based pharmacokinetic/pharmacodynamic model for the organophosphorus insecticide chlorpyrifos in the preweanling rat

Juvenile rats are more susceptible than adults to the acute toxicity of organophosphorus insecticides like chlorpyrifos (CPF). Age- and dose-dependent differences in metabolism may be responsible. Of importance are CYP450 activation and detoxification of CPF to chlorpyrifos-oxon (CPF-oxon) and trichloropyridinol (TCP), as well as B-esterase (B-est) and PON-1 (A-esterase) detoxification of CPF-oxon to TCP. In the current study, a physiologically based pharmacokinetic/pharmacodynamic (PBPK/PD) model incorporating age-dependent changes in CYP450, PON-1, and tissue B-est levels for rats was developed. In this model, age was used as a dependent function to estimate body weight which was then used to allometrically scale both metabolism and tissue cholinesterase (ChE) levels. In addition, age-dependent changes in brain, liver, and fat volumes and brain blood flow were obtained from the literature and used in the simulations. Model simulations suggest that preweanling rats are particularly sensitive to CPF toxicity, with levels of CPF-oxon in blood and brain disproportionately increasing, relative to the response in adult rats. This age-dependent nonlinear increase in CPF-oxon concentration may potentially result from both the depletion of nontarget B-est and a lower PON-1 metabolic capacity in younger animals. The PBPK/PD model behaves consistently with the general understanding of CPF toxicity, pharmacokinetics, and tissue ChE inhibition in neonatal and adult rats. Hence, this model represents an important starting point for developing a computational model to assess the neurotoxic potential of environmentally relevant organophosphate exposures in infants and children.

Pediatric susceptibility to 18 industrial chemicals: A comparative analysis of newborn with young animals

We comprehensively re-analyzed the toxicity data for 18 industrial chemicals from repeated oral exposures in newborn and young rats, which were previously published. Two new toxicity endpoints specific to this comparative analysis were identified, the first, the presumed no observed adverse effect level (pNOAEL) was estimated based on results of both main and dose-finding studies, and the second, the presumed unequivocally toxic level (pUETL) was defined as a clear toxic dose giving similar severity in both newborn and young rats. Based on the analyses of both pNOAEL and pUETL ratios between the different ages, newborn rats demonstrated greater susceptibility (at most 8-fold) to nearly two thirds of these 18 chemicals (mostly phenolic substances), and less or nearly equal sensitivity to the other chemicals. Exceptionally one chemical only showed toxicity in newborn rats. In addition, Benchmark Dose Lower Bound (BMDL) estimates were calculated as an alternative endpoint. Most BMDLs were comparable to their corresponding pNOAELs and the overall correlation coefficient was 0.904. We discussed how our results can be incorporated into chemical risk assessment approaches to protect pediatric health from direct oral exposure to chemicals.

Age-Related Brain Cholinesterase Inhibition Kinetics following In Vitro Incubation with Chlorpyrifos-Oxon and Diazinon-Oxon

Chlorpyrifos and diazinon are two commonly used organophosphorus insecticides (OPs), and their primary mechanism of action involves the inhibition of acetylcholinesterase by their metabolites chlorpyrifos-oxon (CPO) and diazinon-oxon (DZO), respectively. The study objectives were to assess the in vitro age-related inhibition kinetics of neonatal rat brain cholinesterase (ChE) for CPO and DZO by estimating the bimolecular inhibitory rate constant (k(i)) values. Brain ChE inhibition and k(i) values following CPO and DZO incubation with neonatal Sprague-Dawley rat brain homogenates were determined at postnatal day (PND) 5, 12, and 17 and compared with the corresponding inhibition and k(i) values obtained in the adult rat. A modified Ellman method was utilized for measuring the ChE activity. CPO caused a greater ChE inhibition than DZO as evidenced from the estimated k(i) values of both compounds. Neonatal brain ChE inhibition kinetics exhibited a marked age-related sensitivity to CPO, with the order of ChE inhibition being PND 5 > PND 7 > PND 17 with k(i) values of 0.95, 0.50, and 0.22 nM(-1)hr(-1), respectively. In contrast, DZO ChE inhibition was not age related in the neonatal brain, and the estimated k(i) value at all PND ages was 0.02 nM(-1)hr(-1). These results demonstrated an age- and OP-selective inhibition of rat brain ChE, which may be critically important in understanding the potential sensitivity of juveniles to specific OPs exposures.

Current issues in organophosphate toxicology

Organophosphates (OPs) are one of the main classes of insecticides, in use since the mid 1940s. OPs can exert significant adverse effects in non-target species including humans. Because of the phosphorylation of acetylcholinesterase, they exert primarily a cholinergic toxicity, however, some can also cause a delayed polyneuropathy. Currently debated and investigated issues in the toxicology of OPs are presented in this review. These include: 1) possible long-term effects of chronic low-level exposures; 2) genetic susceptibility to OP toxicity; 3) developmental toxicity and neurotoxicity; 4) common mechanism of action; 5) mechanisms of delayed neurotoxicity; and 6) possible additional OP targets. Continuing and recent debates, and molecular advances in these areas, and their contributions to our understanding of the toxicology of OPs are discussed.

Age-dependent pharmacokinetic and pharmacodynamic response in preweanling rats following oral exposure to the organophosphorus insecticide chlorpyrifos

Previous studies have indicated that juvenile rats are more susceptible than adults to the acute toxicity from exposure to the organophosphorus insecticide chlorpyrifos (CPF) and age-dependent differences in metabolism and sensitivity to cholinesterase (ChE) inhibition may be responsible. Metabolism involves CYP450 activation and detoxification of CPF to CPF-oxon and 3,5,6-trichloro-2-pyridinol (TCP), as well as cholinesterase (acetyl- and butyrylcholinesterase), carboxylesterase (CaE), and A-esterase (PON-1) detoxification of CPF-oxon to TCP. The pharmacokinetics of CPF, TCP, and the extent of blood (plasma/RBC), and brain ChE inhibition in rats were determined on postnatal days (PND)-5, -12, and -17 following oral gavage administration of 1 and 10mg CPF/kg of body weight. As has been seen in adult animals, for all preweanling ages the blood TCP exceeded the CPF concentration, and within each age group there was no evidence of non-linear kinetics over the dose range evaluated. Consistent with previous results, younger animals demonstrated a greater sensitivity to ChE inhibition as evident by the age-dependent inhibition of plasma, RBC, and brain ChE. The brain may be particularly sensitive in younger animals (i.e. PND-5) due to substantially lower levels of ChE activity relative to later preweanling stages and adults. Of particular importance was the observation that even in rats as young as PND-5, the CYP450 metabolic capacity was adequate to metabolize CPF to both TCP and CPF-oxon based on the detection of TCP in blood and extensive ChE inhibition (biomarker of CPF-oxon) at all ages. In addition, the increase in the blood TCP concentration ( approximately 3-fold) in PND-17 rats relative to the response in the younger rats, are consistent with an increase in CYP450 metabolic capacity with age. This is the first reported study that evaluated both the pharmacokinetics of the parent pesticide, the major metabolite, and the extent of ChE inhibition as a function of preweanling age. The results suggest that in the preweanling rat, CPF was rapidly absorbed and metabolized, and the extent of metabolism and ChE inhibition was age-dependent.

Comparative susceptibility of newborn and young rats to six industrial chemicals

To elucidate the comparative susceptibility of newborn rats to chemicals, newborn and young animals were administered six industrial chemicals by gavage from postnatal days (PND) 4 to 21, and for 28 days starting at 5-6 weeks of age respectively, under the same experimental conditions as far as possible. As two new toxicity endpoints specific to this comparative analysis, presumed no-observed-adverse-effect-levels (pNOAELs) were estimated based on results of both main and dose-finding studies, and presumed unequivocally toxic levels (pUETLs) were also decided. pNOAELs for newborn and young rats were 40 and 200 for 2-chlorophenol, 100 and 100 for 4-chlorophenol, 30 and 100 for p-(alpha,alpha-dimethylbenzyl) phenol, 100 and 40 for (hydroxyphenyl)methyl phenol, 60 and 12 for trityl chloride, and 100 and 300 mg/kg/day for 1,3,5-trihydroxybenezene, respectively. To determine pUETLs, dose ranges were adopted in several cases because of the limited results of experimental doses. Values for newborn and young rats were thus estimated as 200-250 and 1000 for 2-chlorophenol, 300 and 500 for 4-chlorophenol, 300 and 700-800 for p-(alpha,alpha-dimethylbenzyl) phenol, 140-160 and 1000 for (hydroxyphenyl)methyl phenol, 400-500 and 300 for trityl chloride, and 500 and 1000 mg/kg/day for 1,3,5-trihydroxybenzene, respectively. In most cases, newborn rats were 2-5 times more susceptible than young rats in terms of both the pNOAEL and the pUETL. An exception was that young rats were clearly more susceptible than their newborn counterparts for trityl chloride.

Human milk biomonitoring data: interpretation and risk assessment issues

Biomonitoring data can, under certain conditions, be used to describe potential risks to human health (for example, blood lead levels used to determine children's neurodevelopmental risk). At present, there are very few chemical exposures at low levels for which sufficient data exist to state with confidence the link between levels of environmental chemicals in a person's body and his or her risk of adverse health effects. Human milk biomonitoring presents additional complications. Human milk can be used to obtain information on both the levels of environmental chemicals in the mother and her infant's exposure to an environmental chemical. However, in terms of the health of the mother, there are little to no extant data that can be used to link levels of most environmental chemicals in human milk to a particular health outcome in the mother. This is because, traditionally, risks are estimated based on dose, rather than on levels of environmental chemicals in the body, and the relationship between dose and human tissue levels is complex. On the other hand, for the infant, some information on dose is available because the infant is exposed to environmental chemicals in milk as a "dose" from which risk estimates can be derived. However, the traditional risk assessment approach is not designed to consider the benefits to the infant associated with breastfeeding and is complicated by the relatively short-term exposures to the infant from breastfeeding. A further complexity derives from the addition of in utero exposures, which complicates interpretation of epidemiological research on health outcomes of breastfeeding infants. Thus, the concept of "risk assessment" as it applies to human milk biomonitoring is not straightforward, and methodologies for undertaking this type of assessment have not yet been fully developed. This article describes the deliberations of the panel convened for the Technical Workshop on Human Milk Surveillance and Biomonitoring for Environmental Chemicals in the United States, held at the Hershey Medical Center, Pennsylvania State College of Medicine, on several issues related to risk assessment and human milk biomonitoring. Discussion of these topics and the thoughts and conclusions of the panel are described in this article.

Air pollution and young children's inhalation exposure to organophosphorus pesticide in an agricultural community in Japan

Assessment of airborne organophosphorus pesticides in houses of young children (1-6 years old) and childcare facilities was conducted following pesticide applications in an agricultural community in Japan. Trichlorfon and fenitrothion, applied in two separate periods, were frequently detected from outdoor and indoor air. Dichlorvos, the primary degradation product of trichlorfon, was also detected after the application of trichlorfon. Both the outdoors and indoor concentration of applied pesticide were shown to increase with decreasing distance from the pesticide-applied farm. Indoor concentration of these pesticides significantly correlated with outdoor concentration (p=0.001 for trichlorfon and p=0.001 for fenitrothion), indicating infiltration of applied pesticide inside. Ratio of indoor to outdoor concentration (I/O ratio) of fenitrothion was higher for houses with windows open during the application than those with closed windows (median value: 0.74 vs. 0.16, p=0.003). However, a similar trend was not observed for trichlorfon as well as dichlorvos in the first period. Dichlorvos was found to have a higher I/O ratio than trichlorfon during the period, and clear correlation between indoor concentrations of dichlorvos and those of trichlorfon suggested increased decomposition of trichlorfon in the indoor environment. Daily inhalation exposure estimated by using the fixed measurement data and time-activity questionnaire ranged from 0 to 35 ng/kg/day for trichlorfon, from 0 to 26 ng/kg/day for dichlorvos, and from 0 to 44 ng/kg/day for fenitrothion. Median inhalation exposure from indoor air accounted for 74%, 86.3%, and 45% of the daily inhalation exposure, respectively. For kindergarteners or nursery school children, inhalation exposure at childcare facilities was comparable with or more than that at home, indicating that pollution level at childcare facilities had potential of high impact on children's exposure. Estimated daily inhalation exposures were inversely correlated to the proximity of their activity location to the pesticide-applied farm.

Dietary modulation of parathion-induced neurotoxicity in adult and juvenile rats

Previous studies indicated that dietary glucose (15% in drinking water) could markedly exacerbate the toxicity of parathion in adult rats. The present study evaluated the effect of consumption of the commonly used sweetener, high fructose corn syrup (HFCS), on parathion toxicity in adult and juvenile rats. Animals were given free access to either water or 15% HFCS in drinking water for a total of 10 days and challenged with parathion (6 or 18 mg/kg, s.c., for juveniles or adults, respectively) on the 4th day. Signs of cholinergic toxicity, body weight and chow/fluid intake were recorded daily. Acetylcholinesterase (AChE) activity and immunoreactivity (AChE-IR) in frontal cortex and diaphragm were measured at 2, 4, and 7 days after parathion. As HFCS was associated with significant reduction in chow intake, adult rats were also pair-fed to evaluate the effect of similar reduced chow intake alone on parathion toxicity. The results indicated that the cholinergic toxicity of parathion was significantly increased by HFCS feeding in both age groups. The excess sugar consumption, however, did not significantly affect parathion-induced AChE inhibition in either tissue or either age group. Enzyme immunoreactivity in frontal cortex was generally not affected in either age group while diaphragm AChE-IR was significantly reduced by parathion and HFCS alone in adult animals at 2 and 4 days timepoints, and more so by the combination of sugar feeding and parathion exposure in both age groups. Food restriction alone did not exacerbate parathion toxicity. While the mechanism(s) remains unclear, we conclude that voluntary consumption of the common sweetener HFCS can markedly amplify parathion acute toxicity in both juvenile and adult rats.

Measurement of paraoxonase (PON1) status as a potential biomarker of susceptibility to organophosphate toxicity

Organophosphorus (OP) compounds are still among the most widely used insecticides, and their main mechanism of acute toxicity is associated with inhibition of acetylcholinesterase. Measurements of urine metabolites and of blood cholinesterase activity are established biomarkers of exposure to OPs and of early biological effects. In recent years, increasing attention has been given to biomarkers of susceptibility to OP toxicity. Here we discuss the polymorphisms of paraoxonase (PON1), a liver and serum enzyme that hydrolyzes a number of OP compounds, and its role in modulating the toxicity of OPs. We stress the importance of determining PON1 status, which encompasses the PON1192Q/R polymorphism (that affects catalytic ability toward different substrates) and PON1 levels (which are modulated in part by a C-108T polymorphism) over straight genotyping. Epidemiological studies on OP-exposed workers that include assessment of PON1 status to validate in human populations the role of PON1 as a determinant of susceptibility to OPs, as indicated by animal studies, are needed. Documentation of exposure and of early health effects would be most relevant to increase the predictive value of the test.

Age dependence of organophosphate and carbamate neurotoxicity in the postnatal rat: extrapolation to the human

One important aspect of risk assessment for the organophosphate and carbamate pesticides is to determine whether their neurotoxicity occurs at lower dose levels in human infants compared to adults. Because these compounds probably exert their neurotoxic effects through the inhibition of acetylcholinesterase (AChE), the above question can be narrowed to whether the cholinesterase inhibition and neurotoxicity they produce is age-dependent, both in terms of the effects produced and potency. The rat is the animal model system most commonly used to address these issues. This paper first discusses the adequacy of the postnatal rat to serve as a model for neurodevelopment in the postnatal human, concluding that the two species share numerous pathways of postnatal neurodevelopment, and that the rat in the third postnatal week is the neurodevelopmental equivalent of the newborn human. Then, studies are discussed in which young and adult rats were dosed by identical routes with organophosphates or carbamates. Four pesticides were tested in rat pups in their third postnatal week: aldicarb, chlorpyrifos, malathion, and methamidophos. The first three, but not methamidophos, caused neurotoxicity at dose levels that ranged from 1.8- to 5.1-fold lower (mean 2.6-fold lower) in the 2- to 3-week-old rat compared to the adult. This estimate in the rat, based on a limited data set of three organophosphates and a single carbamate, probably represents the minimum difference in the neurotoxicity of an untested cholinesterase-inhibiting pesticide that should be expected between the human neonate and adult. For the organophosphates, the greater sensitivity of postnatal rats, and, by analogy, that expected for human neonates, is correlated with generally lower levels of the enzymes involved in organophosphate deactivation.

Functional genomic of the paraoxonase (PON1) polymorphisms: effects on pesticide sensitivity, cardiovascular disease, and drug metabolism

This review focuses on the functional genomics of the human paraoxonase (PON1) polymorphisms. Levels and genetic variability of the PON1 position 192 isoforms (Gln/Arg) influence sensitivity to specific insecticides or nerve agents and risk for cardiovascular disease. A more recent area of investigation, the role of PON1 in drug metabolism, is also discussed. We emphasize the importance of considering both PON1 isoforms and PON1 levels in disease/sensitivity association studies.

Paraoxonase (PON 1) as a biomarker of susceptibility for organophosphate toxicity

Paraoxonase (PON1) is an A-esterase capable of hydrolysing the active metabolites (oxons) of a number of organophosphorus (OP) insecticides such as parathion, diazinon and chlorpyrifos. PON1 activity is highest in liver and plasma, and among animal species significant differences exist, with birds and rabbits displaying very low and high activity, respectively. Human PON1 has two polymorphisms in the coding region (Q192R and L55M) and five polymorphisms in the promoter region. The Q192R polymorphism imparts different catalytic activity toward some OP substrates, while the polymorphism at position -108 (C/T) is the major contributor to differences in the level of PON1 expression. Animal studies have shown that PON1 is an important determinant of OP toxicity, with animal species with a low PON1 activity having an increased sensitivity to OPs. Administration of exogenous PON1 to rats or mice protects them from the toxicity of OPs. PON1 knockout mice display a high sensitivity to the toxicity of diazoxon and chlorpyrifos oxon, but not paraoxon. In vitro assayed catalytic efficiencies of purified PON(192) isoforms for hydrolysis of specific oxon substrates accurately predict the degree of in vivo protection afforded by each isoform. Low PON1 activity may also contribute to the higher sensitivity of newborns to OP toxicity.

Monte Carlo analysis of the human chlorpyrifos-oxonase (PON1) polymorphism using a physiologically based pharmacokinetic and pharmacodynamic (PBPK/PD) model

Susceptibility to organophosphate (OP) insecticides, like chlorpyrifos (CPF), may result from differences in the extent of metabolic detoxification of the active metabolite, CPF-oxon. A genetic polymorphism in the arylesterase (PON1; CPF-oxonase) detoxification of OPs, results in the expression of a range of enzyme activities within humans. This study utilized Monte Carlo analysis and physiologically based pharmacokinetic and pharmacodynamic (PBPK/PD) modeling to investigate the impact of human CPF-oxonase status on the theoretical concentration of CPF-oxon in the brain. At low doses ( approximately 5 microg/kg) the model is insensitive to changes in CPF-oxonase. However, with increasing dose (>0.5 mg/kg) the model suggests a dose-dependent non-linear increase in the brain CPF-oxon concentration, which is associated with CPF-oxonase activity. Following repeated high dose exposure, the model predicted brain CPF-oxon concentration was approximately 8x higher (5 mg/kg) versus a single exposure, whereas, at low doses (5 microg/kg), the brain concentrations were comparable regardless of exposure duration. This suggests that at low environmentally relevant exposures other esterase detoxification pathways may compensate for lower CPF-oxonase activity.

Differential sensitivity of children and adults to chemical toxicity. I. Biological basis

Children, particularly neonates, can be biologically more sensitive to the same toxicant exposure on a body weight basis than adults. Current understanding of the rates of maturation of metabolic capability and evidence from case examples on pharmaceuticals, drugs of abuse, environmental contaminants, and dietary and endogenous agents indicate that human infants up to approximately 6 months of age are typically--but not always--more sensitive to chemical toxicity than adults. For most chemicals, the immaturity of infant biotransformation, elimination, and other physiologic systems usually produces higher blood levels for longer periods. There is metabolic capacity for most tested substances in the newborn, although it is quite low and immature for some chemicals. For some chemicals, unique metabolic pathways not available in the adult human can also be utilized by the newborn. The newborn's metabolic capacity rapidly matures and, by about 6 months of age, children are usually not more sensitive to chemical toxicity than adults. By then, most metabolic systems are reasonably mature, becoming almost completely capable by 1 year of age. In many cases children are less sensitive than adults. Whether children are at greater risk from chemical exposures is another question. Risk depends on both inherent sensitivity and exposure conditions. If chemical exposure levels remain below those capable of overwhelming a child's metabolic detoxification systems and producing toxicity, children will be at no greater risk than are adults. Children of all ages are still developing so even if they are exposed to chemicals at levels below those of adults, they may be at greater risk than adults. However, as long as those exposure levels are still below those required to produce toxicity, children will not be at greater risk.

Inhibition of forskolin-stimulated cAMP formation in vitro by paraoxon and chlorpyrifos oxon in cortical slices from neonatal, juvenile, and adult rats

Parathion (PS) and chlorpyrifos (CPF) are organophosphorus insecticides, which elicit toxicity following biotransformation to the potent acetylcholinesterase inhibitors, paraoxon (PO) and chlorpyrifos oxon (CPO). Both oxons have also been shown to interact directly with muscarinic receptors coupled to inhibition of adenylyl cyclase. Immature animals are more sensitive than adults to the acute toxicity of PS and CPF but little is known regarding possible age-related differences in interactions between these toxicants and muscarinic receptors. We compared the inhibition of forskolin-stimulated cAMP formation by PO and CPO (1 nM-1 mM) in vitro in brain slices from 7-, 21-, and 90-day-old rats to the effects of well-known muscarinic agonists, carbachol and oxotremorine (100 microM). Both agonists inhibited cAMP formation in tissues from all age groups and both were more effective in adult and juvenile (20-26% inhibition) than in neonatal (12-13% inhibition) tissues. Atropine (10 microM) completely blocked agonist-induced inhibition in all cases. PO maximally inhibited (37-46%) cAMP formation similarly in tissues from all age groups, but atropine blocked those effects only partially and only in tissues from 7-day-old rats. CPO similarly inhibited cAMP formation across age groups (27-38%), but ATR was partially effective in tissues from all three age groups. Both oxons were markedly more potent in tissues from younger animals. We conclude that PO and CPO can directly inhibit cAMP formation through muscarinic receptor-dependent and independent mechanisms and that the developing nervous system may be more sensitive to these noncholinesterase actions.

Dose-response and time-course of neurobehavioral changes following oral chlorpyrifos in rats of different ages

Young rats have been shown in several laboratories to be more sensitive to the neurotoxic effects of acute exposure to chlorpyrifos. To examine the neurobehavioral effects of chlorpyrifos as a function of age and dose, we conducted dose-response and time-course assessments in rats of three different ages (postnatal day, or PND, 17, 27, and adults). Doses were selected to span the effective dose range in each age group: PND17 - 4, 10, 20 mg/kg; PND27 - 10, 25, 50 mg/kg; adult - 10, 50, 100 mg/kg. Rats were tested at the time of peak effect on the day of dosing, and again at 1 and 3 days, and at 1 and 2 weeks after a single oral dose. There were age- and sex-related differences in the recovery of these behavioral effects; the adult males recovered from the behavioral effects more quickly than the other age groups, and the adult females showed the slowest recovery (up to at least 3 days). Although these doses had been shown previously to produce a similar degree of cholinesterase inhibition, the neurobehavioral alterations fell into the following three patterns of effect as a function of age. (1) Some endpoints (e.g., gait abnormalities, tremor) showed a dose-response curve that was shifted to the right in the older animals. Calculated ED50 values indicated that the PND17 rats were three- to five-fold more sensitive than the adults. (2) Some measures showed less effect in the youngest rats; for example, maximal motor activity decreases were half as great as with adults. (3) A few effects that were typically observed in adults, e.g., salivation, were not seen at all in the PND17 rats. Thus, differential responses on these neurobehavioral endpoints were observed as a function of age. These data suggest that, for some endpoints, young rats are more sensitive to a range of chlorpyrifos doses; however, the magnitude of age-related differences depends on the specific endpoint and time of assessment, as well as age and sex of the test subject.

Maturation-dependent effects of chlorpyrifos and parathion and their oxygen analogs on acetylcholinesterase and neuronal and glial markers in aggregating brain cell cultures

An in vitro model, the aggregating brain cell culture of fetal rat telencephalon, has been used to study the maturation-dependent sensitivity of brain cells to two organophosphorus pesticides (OPs), chlorpyrifos and parathion, and to their oxon derivatives. Immature (DIV 5-15) or differentiated (DIV 25-35) brain cells were treated continuously for 10 days. Acetylcholinesterase (AChE) inhibitory potency for the OPs was compared to that of eserine (physostigmine), a reversible AChE inhibitor. Oxon derivatives were more potent AChE inhibitors than the parent compounds, and parathion was more potent than chlorpyrifos. No maturation-dependent differences for AChE inhibition were found for chlorpyrifos and eserine, whereas for parathion and paraoxon there was a tendency to be more effective in immature cultures, while the opposite was true for chlorpyrifos-oxon. Toxic effects, assessed by measuring protein content as an index of general cytotoxicity, and various enzyme activities as cell-type-specific neuronal and glial markers (ChAT and GAD, for cholinergic and GABAergic neurons, respectively, and GS and CNP, for astrocytes and oligodendrocytes, respectively) were only found at more than 70% of AChE inhibition. Immature compared to differentiated cholinergic neurons appeared to be more sensitive to OP treatments. The oxon derivates were found to be more toxic on neurons than the parent compounds, and chlorpyrifos was more toxic than parathion. Eserine was not neurotoxic. These results indicate that inhibition of AChE remains the most sensitive macromolecular target of OP exposure, since toxic effects were found at concentrations in which AChE was inhibited. Furthermore, the compound-specific reactions, the differential pattern of toxicity of OPs compared to eserine, and the higher sensitivity of immature brain cells suggest that the toxic effects and inhibition of AChE are unrelated.

Effect of quinalphos and cypermethrin exposure on developing blood-brain barrier: role of nitric oxide

Effect of low-level exposure of quinalphos (QP) and cypermethrin (CP) on the blood-brain barrier (BBB) permeability to macromolecular tracers, Evans blue (EB) and horseradish peroxidase (HRP) was studied in developing rat pups. Ten-day-old rat pups were daily exposed to QP and CP at a dose of approximately 1/50th of adult LD50 through oral intubation, upto postnatal day 17 (PND). Functional integrity of the BBB was assessed by measuring the brain uptake index (BUI) of HRP and by visually grading the brains of control and treated rat pups for the staining of EB. Our results have demonstrated a significant increase in the BUI for HRP (204 and 254%) and have also shown a significant amount of EB staining in QP and CP exposed brains, respectively, as compared to the age-matched controls. Studies carried out with the nitric oxide synthase (NOS) inhibitor L-NAME (30 mg/kg, i.p., on alternate days from PND 10-17) have provided significant protection against the QP-induced increase in the BBB permeability, suggesting the possible involvement of NO in the barrier disruption. Microvessel acetylcholinesterase activity was also inhibited (53%, P<0.001) in QP-exposed rat pups only, with no change observed in CP-exposed microvessels. However, membrane fluidity was found to be decreased in both QP (18%, P<0.05) and CP (15%, P<0.05) exposed microvessels compared to controls. It is evident from the study that QP and CP exposure during early postnatal period causes significant impairment in the development and maturation of the BBB that may have adverse consequences on the normal brain functioning with long-term neurotoxic effects.

Comparative neurochemical effects of repeated methyl parathion or chlorpyrifos exposures in neonatal and adult rats

Several studies have reported higher sensitivity based on lethality in young animals compared to adults following acute exposure to organophosphorus insecticides (OPs). We propose that age-related differences in sensitivity to OPs may differ qualitatively and quantitatively with different OPs and varying exposure conditions (e. g., high vs. low dose, acute vs. repeated). To test this hypothesis, we treated neonatal (7 days of age) and adult (90 days of age) rats with either methyl parathion (MPS) or chlorpyrifos (CPF) daily for 14 days and measured neurochemical endpoints {cholinesterase (ChE) inhibition, total muscarinic receptor ([(3)H]quinuclidinyl benzilate, QNB) and muscarinic M2 subtype-preferential ([(3)H]AF-DX 384) binding} in frontal cortex and striatum at timepoints both during (1 day after the 7(th) and 14(th) dose) and after (8 days after the 14(th) dose) exposures. Repeated CPF exposures were associated with relatively similar degrees of ChE inhibition between the age groups during dosing but more extensive inhibition was noted in adults after termination of exposures. Relatively similar changes in muscarinic receptor binding were also noted between age groups following CPF exposures. Moreover, the degree of muscarinic receptor binding reduction relative to ChE inhibition appeared similar in both age groups following CPF exposures. In contrast, ChE activity and muscarinic receptor binding were generally more reduced in neonatal relative to adult brain regions following repeated MPS exposures. Furthermore, the relationship between the degree of ChE inhibition and the reduction in cortical muscarinic receptor binding appeared different between the age groups, i.e., more extensive reduction was noted in neonates compared to adults with a given level of ChE inhibition. We conclude that OP-selective differences in in vivo ChE sensitivity, differential rates of enzyme recovery following inhibition, and age-dependent differences in muscarinic receptor adaptations can all influence the nature of age-related susceptibility to OPs.

Comparison of aldicarb and methamidophos neurotoxicity at different ages in the rat: behavioral and biochemical parameters

Young organisms are often more sensitive to the toxic effects of pesticides, and this finding has spurred research on further characterization of this susceptibility. The neurotoxic effects of cholinesterase (ChE)-inhibiting pesticides are of particular concern for human health risk assessment due to the widespread exposure potential in children. This study evaluated age-related differences in susceptibility for a carbamate (aldicarb) and an organophosphorus pesticide (methamidophos). Comparisons were made between preweanling (Postnatal Day 17, PND17), postweanling (PND27), and adult (approximately PND70) male and female rats. All were acute studies using oral administration. Sensitivity was quantified by (1) determination of maximally-tolerated doses (MTDs); (2) measurement of brain and blood ChE inhibition; and (3) neurobehavioral evaluation using end points known to be sensitive indicators of exposure to anticholinesterases. MTD data showed that preweanling rats were twice as sensitive as adults to aldicarb, but there was no differential sensitivity to methamidophos. The dose-response data for brain ChE inhibition followed a similar pattern of age-related differences, and similar levels of inhibition were measured at the MTD regardless of age. Dose-response and time course studies of neurobehavioral end points indicated that differential effects due to age depend on the behavioral end point examined. Following aldicarb administration, the dose-response curves for a few end points overlapped; however, the young rats otherwise showed fewer signs of toxicity than did the adults despite similar levels of brain ChE inhibition. Motor activity assessment showed that aldicarb did not produce any activity depression in PND17 rats, whereas the data for the PND27 and adult rats overlapped. With methamidophos, the dose-response curves for most end points for preweanling and adult rats were quite similar. Aldicarb-induced ChE inhibition was readily reversible in all age groups, whereas with methamidophos, enzyme activity recovered more rapidly in the young. Most behavioral alterations had recovered by 24 h with either pesticide. The results of these studies indicate that (1) ChE-inhibiting pesticides are not all the same regarding relative sensitivity of the young; (2) age-related differences were reflected in both the MTDs and degree of ChE inhibition; and (3) age-related differences in neurobehavioral measures depended on the pesticide and on the end points examined.

Human red blood cell acetylcholinesterase inhibition as the appropriate and conservative surrogate endpoint for establishing chlorpyrifos reference dose

Chlorpyrifos (O,O-diethyl O-(3,5, 6-trichloro-2-pyridinyl)- phosphorothioate) is an organophosphorus (OP) insecticide used for controlling insect pests. Currently, the reference dose (RfD) used by the Environmental Protection Agency (EPA) to establish acceptable human exposure tolerances for chlorpyrifos is based upon inhibition of blood butyrylcholinesterase (BuChE), which is not the target enzyme of chlorpyrifos, and does not play any role in cholinergic transmission. Data are presented showing that inhibition of acetylcholinesterase (AChE) associated with red blood cells (RBC), an enzyme similar to or identical with that in the nervous system, is a more appropriate endpoint on which to base the RfD. Basing an acceptable level of human exposure (e.g., RfD) on inhibition of RBC AChE provides a significant margin of safety, since it is 12- to 14-fold more sensitive as an indicator of chlorpyrifos exposure than the AChE in the most sensitive relevant neurological tissues (brain or retina). Inhibition of RBC AChE activity is consistently exhibited at lower dosages of chlorpyrifos than those required to result in clinical symptoms of OP toxicity, or alterations in cognitive functional responses. There is no unique sensitivity of the fetus or neonates to chlorpyrifos when administered by an appropriate oral dose. Thus, inhibition of RBC AChE activity is an appropriate surrogate measurement of chlorpyrifos exposure and provides a conservative endpoint for establishing appropriate margins of safety for both adults and infants.