Critical periods for the role of oxidative stress in the developmental neurotoxicity of chlorpyrifos and terbutaline, alone or in combination

Exposure to organophosphate, pyrethroid, and neonicotinoid insecticides and dyslexia: Association with oxidative stress

Organophosphates (OPPs), pyrethroids (PYRs), and neonicotinoids (NNIs) are three major classes of insecticides used worldwide. They might compromise child neurodevelopment. However, few studies have explored the association between exposure to them and dyslexia. The present study aimed to investigate the association between dyslexia and exposure to the three classes of insecticides, as well as explore the potential role of oxidative stress in the association. A total of 355 dyslexic children and 390 controls were included in this study. The exposure biomarkers were determined by liquid chromatography-tandem mass spectrometry. Specifically, the exposure biomarkers included three typical metabolites of OPPs, three of PYRs, and nine of NNIs. Additionally, three typical oxidative stress biomarkers, namely, 8-hydroxy-2'-deoxyguanosine (8-OHdG) for DNA damage, 8-hydroxyguanosine (8-OHG) for RNA damage, and 4-hydroxy-2-nonenal-mercapturic acid (HNEMA) for lipid peroxidation were measured. The detection frequencies of the urinary biomarkers ranged from 83.9% to 100%. Among the target metabolites of the insecticides, a significant association was observed between urinary 3,5,6-trichloro-2-pyridinol (TCPy, the metabolite of chlorpyrifos, an OPP insecticide) and dyslexia. After adjusting for potential confounding variables, children in the highest quartile of TCPy levels had an increased odds of dyslexia (odds ratio [OR], 1.68; 95% confidence interval [CI]: 1.03, 2.75] in comparison to those in the lowest quartile. Among the three oxidative stress biomarkers, urinary HNEMA concentration showed a significant relationship with dyslexia. Children in the highest quartile of HNEMA levels demonstrated an increased dyslexic odds in comparison to those in the lowest quartile after multiple adjustments (OR, 1.64; 95% CI: 1.01, 2.65). Mediation analysis indicated a significant effect of HNEMA in the association between urinary TCPy and dyslexia, with an estimate of 17.2% (P < 0.01). In conclusion, this study suggested the association between urinary TCPy and dyslexia. The association could be attributed to lipid peroxidation partially.

The susceptibility of humans to neurodegenerative and neurodevelopmental toxicities caused by organophosphorus pesticides

The toxicology field is concerned with the impact of organophosphorus (OP) compounds on human health. These compounds have been linked to an increased risk of neurological disorders, including neurodegenerative and neurodevelopmental diseases. This article aims to review studies on the role of OP compounds in developing these neurological disorders and explore how genetic variations can affect susceptibility to the neurotoxicity of these pesticides. Studies have shown that exposure to OP compounds can lead to the development of various neurological disorders, such as Alzheimer's disease (AD), Parkinson's disease (PD), attention deficit hyperactivity disorder (ADHD), autism, intellectual disability, and other developmental neurotoxicities. Apart from inhibiting the cholinesterase enzyme, OP compounds are believed to cause other pathological mechanisms at both the extracellular level (cholinergic, serotonergic, dopaminergic, glutamatergic, and GABAergic synapses) and the intracellular level (oxidative stress, mitochondrial dysfunction, inflammation, autophagy, and apoptosis) that contribute to these disorders. Specific genetic polymorphisms, including PON1, ABCB1, NOS, DRD4, GST, CYP, and APOE, have increased the risk of developing OP-related neurological disorders.

Programming of intestinal homeostasis in male rat offspring after maternal exposure to chlorpyrifos and/or to a high fat diet

Alteration of programming of the intestinal wall maturation may be responsible for non-communicable chronic diseases in adulthood. It may originate from prenatal exposure of mothers to deleterious environmental factors such as pesticides or western diet. This work was undertaken to determine whether disturbances of the digestive tract function and of innate immunity of offspring at adulthood could be due to maternal exposure to a pesticide, chlorpyrifos (CPF) and a High Fat Diet (HFD) starting 4 months before gestation and lasting until weaning of offspring. Fifty-one male Wistar rats coming from 4 groups of dams exposed to CPF, HFD, both and control were followed from birth to 8 weeks of age. They were fed standard chow and received no treatment. The maternal pesticide exposure slows down fetal and postnatal weight gain without histological injuries of the gut mucosa. CPF or HFD both induced modifications of tight junctions and mucins genes expressions without inducing an increase in epithelial permeability or an inflammatory state. Co-exposure to both CPF and HFD did not exacerbate the effects observed with each factor separately. Despite the lack of direct contact except through breast milk until weaning, CPF or HFD maternal exposure have demonstrated preliminary gut barrier impacts on offspring.

Perinatal exposure to chlorpyrifos and/or a high-fat diet is associated with liver damage in male rat offspring

Metabolic impairments in childhood are known to promote the development of type 2 diabetes and/or obesity in adulthood. These impairments may result from perinatal exposure to harmful environmental factors, such as pesticide residues or the consumption of a "western" diet. In the present study, we sought to determine whether an obesogenic profile, metabolic disorders and liver damage in offspring (observed during young adulthood) were related to maternal exposure to the pesticide chlorpyrifos (CPF) and/or a high-fat diet (HFD) starting 4 months before conception and ending at weaning. After the end of exposure, 51 male rat pups were left to develop under normal conditions and were studied in young adulthood. Despite the absence of direct exposure to harmful factors (other than through the dam's milk), maternal exposure to CPF or an HFD was associated with changes in the offspring's metabolic activity in the liver in the offspring. This indirect exposure to CPF was associated with a relative reduction in the expression of genes coding for enzymes involved in lipid or glucose metabolism but did induce histopathological changes in the offspring at adulthood. Maternal exposure to an HFD alone or to CPF alone gave similar results in offspring, changes in the same direction. Exposure of the mother to HFD did not exacerbate CPF effects. Co-exposure to both CPF and HFD did not increase the observed effects compared to each factor taken separately.

Organophosphate toxicity: updates of malathion potential toxic effects in mammals and potential treatments

Organophosphorus insecticides toxicity is still considered a major global health problem. Malathion is one of the most commonly used organophosphates nowadays, as being considered to possess relatively low toxicity compared with other organophosphates. However, widespread use may lead to excessive exposure from multiple sources. Mechanisms of MAL toxicity include inhibition of acetylcholinesterase enzyme, change of oxidants/antioxidants balance, DNA damage, and facilitation of apoptotic cell damage. Exposure to malathion has been associated with different toxicities that nearly affect every single organ in our bodies, with CNS toxicity being the most well documented. Malathion toxic effects on liver, kidney, testis, ovaries, lung, pancreas, and blood were also reported. Moreover, malathion was considered as a genotoxic and carcinogenic chemical compound. Evidence exists for adverse effects associated with prenatal and postnatal exposure in both animals and humans. This review summarizes the toxic data available about malathion in mammals and discusses new potential therapeutic modalities, with the aim to highlight the importance of increasing awareness about its potential risk and reevaluation of the allowed daily exposure level.

Environmental Mechanisms of Neurodevelopmental Toxicity

PURPOSE OF REVIEW

With the incidence of neurodevelopmental disorders on the rise, it is imperative to identify and understand the mechanisms by which environmental contaminants can impact the developing brain and heighten risk. Here, we report on recent findings regarding novel mechanisms of developmental neurotoxicity and highlight chemicals of concern, beyond traditionally defined neurotoxicants.

RECENT FINDINGS

The perinatal window represents a critical and extremely vulnerable period of time during which chemical insult can alter the morphological and functional trajectory of the developing brain. Numerous chemical classes have been associated with alterations in neurodevelopment including metals, solvents, pesticides, and, more recently, endocrine-disrupting compounds. Although mechanisms of neurotoxicity have traditionally been identified as pathways leading to neuronal cell death, neuropathology, or severe neural injury, recent research highlights alternative mechanisms that result in more subtle but consequential changes in the brain and behavior. These emerging areas of interest include neuroendocrine and immune disruption, as well as indirect toxicity via actions on other organs such as the gut and placenta. Understanding of the myriad ways in which the developing brain is vulnerable to chemical exposures has grown tremendously over the past decade. Further progress and implementation in risk assessment is critical to reducing risk of neurodevelopmental disorders.

A neuroprotective role of kaempferol against chlorpyrifos-induced oxidative stress and memory deficits in rats via GSK3β-Nrf2 signaling pathway

Chlorpyrifos (CPF) is an agricultural pesticide and a potential food contaminant, which causes neurotoxicity. Here, we aimed at exploring the link between the repeated exposure to CPF and memory dysfunction in rats and the possible protective effect of kaempferol, a flavonoid with appreciable antioxidant and anti-inflammatory activities. Rats were divided into: Control group (received drug vehicles for 14 days); CPF-treated group (received subcutaneous 18 mg/kg BW of CPF daily for 14 days and CPF + Kaempferol treated group (received the same CPF dose +21 mg/kg BW of Kaempferol intraperitoneally for 14 days. On the 14^th day, Y-maze and novel target recognition behavioral tests were employed to evaluate memory deficits. 24 h after the last dose of CPF, animals were sacrificed, and brain tissues were used for the determination of oxidative stress biomarkers and gene expression levels of GSK3β and Nrf2. The results revealed that CPF-treated rats suffered from severe deterioration of spatial and non-spatial memory functions with low activities of antioxidant enzymes and acetylcholinesterase (AChE). The administration of kaempferol significantly protected against CPF-induced neuronal damage, increased the activities of antioxidant enzymes and AChE and induced a better performance in the behavioral tests. The protective effect of kaempferol was mediated through the inhibition of GSK3β gene expression and the induction of Nrf2 expression in the brain tissues. In conclusion, the repeated exposure to CPF is associated with oxidative stress and memory deficits in rats. However, kaempferol administration effectively alleviated CPF- induced brain toxicity, possibly through the modulation of GSK3β-Nrf2 signaling pathway.

Developmental neurotoxicity resulting from pharmacotherapy of preterm labor, modeled in vitro: Terbutaline and dexamethasone, separately and together

Terbutaline and dexamethasone are used in the management of preterm labor, often for durations of treatment exceeding those recommended, and both have been implicated in increased risk of neurodevelopmental disorders. We used a variety of cell models to establish the critical stages at which neurodifferentiation is vulnerable to these agents and to determine whether combined exposures produce a worsened outcome. Terbutaline selectively promoted the initial emergence of glia from embryonic neural stem cells (NSCs). The target for terbutaline shifted with developmental stage: at later developmental stages modeled with C6 and PC12 cells, terbutaline had little effect on glial differentiation (C6 cells) but impaired the differentiation of neuronotypic PC12 cells into neurotransmitter phenotypes. In contrast to the specificity shown by terbutaline, dexamethasone affected both neuronal and glial differentiation at all stages, impairing the emergence of both cell types in NSCs but with a much greater impairment for glia. At later stages, dexamethasone promoted glial cell differentiation (C6 cells), while shifting neuronal cell differentiation so as to distort the balance of neurotransmitter phenotypes (PC12 cells). Finally, terbutaline and dexamethasone interacted synergistically at the level of late stage glial cell differentiation, with dexamethasone boosting the ability of terbutaline to enhance indices of glial cell growth and neurite formation while producing further decrements in glial cell numbers. Our results support the conclusion that terbutaline and dexamethasone are directly-acting neuroteratogens, and further indicate the potential for their combined use in preterm labor to worsen neurodevelopmental outcomes.

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.

Polyphenol-rich fraction of Parquetina nigrescens mitigates dichlorvos-induced neurotoxicity and apoptosis

BACKGROUND

Parquetina nigrescens (Afzel.) Bullock of the family Asclepiadaceae is known for its antioxidant effects with wide range of uses in Southwestern Nigeria especially in traditional medicine. This study was undertaken to explore if polyphenol-rich fraction (prf) from P. nigrescens will ameliorate dichlorvos-induced neurotoxicity and apoptosis. The exploration utilized evaluation of markers of oxidative stress, apoptosis and serum acetylcholinesterase (AchE) levels.

METHODS

Forty Wistar rats randomly placed in four groups were utilized for the study. Animals in Group A received corn oil, group B- dichlorvos (16 mg/kg), groups C and D- dichlorvos + 100 and 200 mg/kg prf of P. nigrescens respectively. Markers of oxidative stress, antioxidants and apoptosis were assessed in the serum and brain tissues using biochemical assay and immunohistochemistry.

RESULTS

Exposure to dichlorvos caused significant decreases in AchE, catalase, superoxide dismutase, glutathione peroxidase (GPx) and increases in hydrogen peroxide (H2O2) generation and malondialdehyde levels. Histopathology and immunohistochemistry of the cerebellum and cerebrum of rats exposed to dichlorvos revealed greater neurotoxic effects in the cerebellum as well as decreased expressions of AchE with a concomitant increase in Bax (proapototic) compared to prf of P. nigrescens treated rats.

CONCLUSION

This study showed that dichlorvos caused cellular and tissue neurotoxicity by inhibiting AchE activity, induced oxidative stress and apoptosis in rats with prominent effects on the cerebellum than cerebrum. The prf of P. nigrescens showed amelioration of neurotoxicity by its antioxidative and antiapoptotic properties in rats exposed to dichlorvos.

Single dose and repeated administrations of liraglutide alter energy metabolism in the brains of young and adult rats

Liraglutide is a human glucagon-like peptide-1 (GLP-1) analogue that was recently approved to treat obesity in some countries. Considering that liraglutide effects on brain energy metabolism are little known, we evaluated the effects of liraglutide on the energy metabolism. Animals received a single or daily injection of saline or liraglutide during 7 days (25, 50, 100, or 300 μg/kg i.p.). Twenty-four hours after the single or last injection, the rats were euthanized and the hypothalamus, prefrontal cortex, cerebellum, hippocampus, striatum, and posterior cortex were isolated. Our results demonstrated that a single dose of liraglutide in young rats increased the activity of complexes and inhibited creatine kinase activity. Repeated administrations of liraglutide in young rats reduced the activity of complexes and activated creatine kinase activity. In adult rats, a single dose of liraglutide reduced the activity of complex I and creatine kinase and increased the activity of complexes II and IV. Repeated administrations of liraglutide in adult rats increased the activity of complexes I and IV and reduced the activity of complex II and creatine kinase. We concluded that liraglutide may interfere in energy metabolism, because analysis of different times of administrations, concentrations, and level of brain development leads to divergent results.

The role of oxidative stress in organophosphate and nerve agent toxicity

Organophosphate (OP) nerve agents exert their toxicity through inhibition of acetylcholinesterase. The excessive stimulation of cholinergic receptors rapidly causes neuronal damage, seizures, death, and long-term neurological impairment in those that survive. Owing to the lethality of organophosphorus agents and the growing risk they pose, medical interventions that prevent OP toxicity and the delayed injury response are much needed. Studies have shown that oxidative stress occurs in models of subacute, acute, and chronic exposure to OP agents. Key findings of these studies include alterations in mitochondrial function and increased free radical-mediated injury, such as lipid peroxidation. This review focuses on the role of reactive oxygen species in OP neurotoxicity and its dependence on seizure activity. Understanding the sources, mechanisms, and pathological consequences of OP-induced oxidative stress can lead to the development of rational therapies for treating toxic exposures.

Oxidative stress, unfolded protein response, and apoptosis in developmental toxicity

Physiological development requires precise spatiotemporal regulation of cellular and molecular processes. Disruption of these key events can generate developmental toxicity in the form of teratogenesis or mortality. The mechanism behind many developmental toxicants remains unknown. While recent work has focused on the unfolded protein response (UPR), oxidative stress, and apoptosis in the pathogenesis of disease, few studies have addressed their relationship in developmental toxicity. Redox regulation, UPR, and apoptosis are essential for physiological development and can be disturbed by a variety of endogenous and exogenous toxicants to generate lethality and diverse malformations. This review examines the current knowledge of the role of oxidative stress, UPR, and apoptosis in physiological development as well as in developmental toxicity, focusing on studies and advances in vertebrates model systems.

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.

Active and peripheral anionic sites of acetylcholinesterase have differential modulation effects on cell proliferation, adhesion and neuritogenesis in the NG108-15 cell line

The classical enzymatic role of acetylcholinesterase (AChE) is to terminate impulse transmission at cholinergic synapses through rapid hydrolysis of acetylcholine (ACh). Inactivation of this enzyme's catalytic site is the primary mechanism of acute toxicity of OP insecticides (e.g. parathion, chlorpyrifos). There is now sufficient evidence to suggest that AChE has a neurotrophic function that may be altered by organophosphate (OP) exposure, resulting in defects of neuronal growth and development, though the clarification of the mechanisms involved require further in vitro investigation. In the present study, the mouse neuroblastoma×rat glioma hybrid NG108-15 cell line was used to investigate the differential effects between inhibition of the catalytic site and peripheral anionic site (PAS) of acetylcholinesterase (AChE) on cell adhesion, proliferation and neuritogenesis, in the presence and absence of human red blood cell (hRBC) AChE (ED3.1.1.7). AChE active-site inhibitor paraoxon (PO; 0.1-1.0μM), when added to NG108-15 cells grown on AChE-coated plates, had no effect on cell proliferation, but exerted a significant reduction in strongly adherent viable cells accompanied by mostly short process formations, with 18% of cells considered to be neuritogenic, similar to that observed on uncoated plates. In contrast, PO had no significant effect on cell adhesion and proliferation of NG108-15 cells on uncoated plates. The PAS-ligand thioflavin-T (Th-T; 0.5-25μM), however, decreased cell adhesion and proliferation, on both uncoated and ACh-E coated plates, with less magnitude on AChE-coated plates. Taken together, these results suggest that strong cell adherence and neuritogenesis are sensitive to PO in this cell culture model, with no impact on proliferation, in the presence of membrane bound AChE-coating, while there is no sensitivity to PO on uncoated plates. On the other hand, binding of Th-T directly to the PAS affects both cell adherence and proliferation, with less magnitude in the presence of membrane-bound AChE. The current study indicates that PO is deleterious in neural development during critical periods of strong cell adhesion and differentiation, interfering with AChE trophic function.

4-Hydroxy TEMPO attenuates dichlorvos induced microglial activation and apoptosis

Microglial cells have been implicated in various neurodegenerative diseases. Previous studies from our lab have shown that dichlorvos (an organophosphate) could induce Parkinson's like features in rats. Recently, we have shown that dichlorvos can induce microglial activation, and if not checked in time could ultimately induce neuronal apoptosis. However, this activation does not always pose a threat to the neurons. Activated microglia also secrete various neuronal growth factors, suggesting that they have beneficial roles in CNS repair. Therefore, it is essential to control their detrimental functions selectively. Here, we tried to find out how microglial cells behave when exposed to dichlorvos in either the presence or absence of potent nitric oxide scavenger and superoxide dismutase mimetic, 4-hydroxy TEMPO (4-HT). Wistar rat pups (1 day) were used to isolate and culture primary microglial cells. We found 4-HT pretreatment successfully attenuated the dichlorvos mediated microglial activation. Moreover, 4-HT pretreatment decreased the up-regulated levels of p53 and its downstream effector, p21. The expression of various cell cycle regulators such as Chk2, CDC25a, and cyclin A remained close to their basal levels when 4-HT pretreatment was given. DNA fragmentation analysis showed significant reduction in the DNA damage of 4-HT pretreated microglia as compared to dichlorvos treated cells. In addition to this, we found 4-HT pretreatment prevented the microglial cells from undergoing apoptotic cell death even after 48 h of dichlorvos exposure. Taken together, our results showed 4-HT pretreatment could successfully ameliorate the dichlorvos induced microglial cell damage.

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.

Effect of acute administration of L-tyrosine on oxidative stress parameters in brain of young rats

Tyrosinemia type II, also known as Richner-Hanhart syndrome, is an autosomal recessive inborn error of metabolism caused by a deficiency of hepatic cytosolic tyrosine aminotransferase, and is associated with neurologic and development difficulties in numerous patients. Considering that the mechanisms underlying the neurological dysfunction in hypertyrosinemic patients are poorly known and that studies demonstrated that high concentrations of tyrosine provoke oxidative stress in vitro and in vivo in the cerebral cortex of rats, in the present study we investigate the oxidative stress parameters (enzymatic antioxidant defenses, thiobarbituric acid-reactive substances and protein carbonyl content) in cerebellum, hippocampus and striatum of 30-old-day rats after acute administration of L-tyrosine. Our results demonstrated that the acute administration of L-tyrosine increased the thiobarbituric acid reactive species levels in hippocampus and the carbonyl levels in cerebellum, hippocampus and striatum. In addition, acute administration of L-tyrosine significantly decreased superoxide dismutase activity in cerebellum, hippocampus and striatum, while catalase was increased in striatum. In conclusion, the oxidative stress may contribute, along with other mechanisms, to the neurological dysfunction characteristic of hypertyrosinemia and the administration of antioxidants may be considered as a potential adjuvant therapy for tyrosinemia, especially type II.

Monocrotophos induced oxidative stress and alterations in brain dopamine and serotonin receptors in young rats

Human exposure to monocrotophos, an organophosphate pesticide, could occur due to its high use in agriculture to protect crops. Recently, we found that postlactational exposure to monocrotophos impaired cholinergic mechanisms in young rats and such changes persisted even after withdrawal of monocrotophos exposure. In continuation to this, the effect of monocrotophos on noncholinergic targets and role of oxidative stress in its neurotoxicity has been studied. Exposure of rats from postnatal day (PD)22 to PD49 to monocrotophos (0.50 or 1.0 mg kg−1 body weight, perorally) significantly impaired motor activity and motor coordination on PD50 as compared to controls. A significant decrease in the binding of 3H-spiperone to striatal membrane (26%, p < 0.01; 30%, p < 0.05) in rats exposed to monocrotophos at both the doses and increase in the binding of 3H-ketanserin to frontocortical membrane (14%, p > 0.05; 37%, p < 0.05) in those exposed at a higher dose, respectively, was observed on PD50 compared with the controls. Alterations in the binding persisted even after withdrawal of monocrotophos exposure on PD65. Increased oxidative stress in brain regions following exposure of rats to monocrotophos was also observed on PD50 that persisted 15 days after withdrawal of exposure on PD65. The results suggest that monocrotophos exerts its neurobehavioral toxicity by affecting noncholinergic functions involving dopaminergic and serotonergic systems associated with enhanced oxidative stress. The results also exhibit vulnerability of developing brain to monocrotophos as most of the changes persisted even after withdrawal of its exposure.

An evaluation of the effects of acute and chronic L-tyrosine administration on BDNF levels and BDNF mRNA expression in the rat brain

Tyrosinemia type II, which is also known as Richner-Hanhart syndrome, is an inborn error of metabolism that is due to a block in the transamination reaction that converts tyrosine to p-hydroxyphenylpyruvate. Because the mechanisms of neurological dysfunction in hypertyrosinemic patients are poorly known and the symptoms of these patients are related to the central nervous system, the present study evaluated brain-derived neurotrophic factor (BDNF) levels and bdnf mRNA expression in young rats and during growth. In our acute protocol, Wistar rats (10 and 30 days old) were killed 1 h after a single intraperitoneal L-tyrosine injection (500 mg/kg) or saline. Chronic administration consisted of L-tyrosine (500 mg/kg) or saline injections 12 h apart for 24 days in Wistar rats (7 days old), and the rats were killed 12 h after the last injection. The brains were rapidly removed, and we evaluated the BDNF levels and bdnf mRNA expression. The present results showed that the acute administration of L-tyrosine decreased both BDNF and bdnf mRNA levels in the striatum of 10-day-old rats. In the 30-day-old rats, we observed decreased BDNF levels without modifications in bdnf transcript level in the hippocampus and striatum. Chronic administration of L-tyrosine increased the BDNF levels in the striatum of rats during their growth, whereas bdnf mRNA expression was not altered. We hypothesize that oxidative stress can interact with the BDNF system to modulate synaptic plasticity and cognitive function. The present results enhance our knowledge of the pathophysiology of hypertyrosinemia.

The effect of chlorpyrifos on isolated thoracic aorta in rats

This study investigated the effect of chlorpyrifos on thoracic aorta and on the level of NO in plasma and aorta. The effect of chlorpyrifos on thoracic aorta in organ bath was determined in 10 rats. Another 45 rats were assigned to 3 groups with 15 rats each: control group 1 received distilled water, control group 2 was given corn oil, and the last group was given 13.5 mg/kg chlorpyrifos dissolved in corn oil every other day for 8 weeks orally. Chlorpyrifos (10⁻¹⁰ M–10⁻⁵ M) showed no effect on isolated thoracic aorta. Plasma AChE activity was decreased, while LDH, ALT, GGT, and AST activities were increased in chlorpyrifos group compared to control groups. Plasma NO level was increased in chlorpyrifos group compared to control groups. iNOS expression was present in all groups in the cytoplasm of the endothelia and in the smooth muscle cells of aorta. According to semiquantitative histomorphological analysis, iNOS immunopositive reactions were seen in the decreasing order in chlorpyrifos, control 2, and control 1 groups. eNOS immunopositive reactions were observed in the endothelial cell cytoplasm, rarely in the subintimal layer, and the smooth muscle cells of aorta. There were no differences among the groups in terms of eNOS immunostaining. In conclusion, chlorpyrifos induced NO production in aorta following an increase in NOS expression.

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.

L-tyrosine administration increases acetylcholinesterase activity in rats

Tyrosinemia is a rare genetic disease caused by mutations on genes that codify enzymes responsible for tyrosine metabolism. Considering that tyrosinemics patients usually present symptoms associated with central nervous system alterations that ranges from slight decreases in intelligence to severe mental retardation, we decided to investigate whether acute and chronic administration of L-tyrosine in rats would affect acetylcholinesterase mRNA expression and enzymatic activity during their development. In our acute protocol, Wistar rats (10 and 30 days old) were killed one hour after a single intraperitoneal L-tyrosine injection (500 mg/kg) or saline. Chronic administration consisted of L-tyrosine (500 mg/kg) or saline injections 12 h apart for 24 days in Wistar rats (7 days old) and rats were killed 12 h after last injection. Acetylcholinesterase activity was measured by Ellman's method and acetylcholinesterase expression was carried out by a semi-quantitative reverse transcriptase polymerase chain reaction (RT-PCR) assay. We observed that acute (10 and 30 days old rats) and chronic L-tyrosine administration increased acetylcholinesterase activity in serum and all tested brain areas (hippocampus, striatum and cerebral cortex) when compared to control group. Moreover, there was a significant decrease in mRNA levels of acetylcholinesterase in hippocampus was observed after acute protocol (10 and 30 days old rats) and in striatum after chronic protocol. In case these alterations also occur in the brain of the patients, our results may explain, at least in part, the neurological sequelae associated with high plasma concentrations of tyrosine seen in patients affected by tyrosinemia type II.

Tyrosine inhibits creatine kinase activity in cerebral cortex of young rats

Tyrosine accumulates in inborn errors of tyrosine catabolism, especially in tyrosinemia type II, where tyrosine levels are highly elevated in tissues and physiological fluids of affected patients. Tyrosinemia type II is a disorder of autosomal recessive inheritance characterized by neurological symptoms similar to those observed in patients with creatine deficiency syndromes. Considering that the mechanisms of brain damage in these disorders are poorly known, in the present study our main objective was to investigate the in vivo and in vitro effects of different concentrations and preincubation times of tyrosine on cytosolic and mitochondrial creatine kinase activities of the cerebral cortex from 14-day-old Wistar rats. The cytosolic CK was reduced by 15% at 1 mM and 32% at 2 mM tyrosine. Similarly, the mitochondrial CK was inhibited by 15% at 1 mM and 22% at 2 mM tyrosine. We observed that the inhibition caused by tyrosine was concentration-dependent and was prevented by reduced glutathione. Results also indicated that mitochondrial, but not cytosolic creatine kinase activity was inhibited by tyrosine in a time-dependent way. Finally, a single injection of L-Tyrosine methyl ester administered i.p. decreased cytosolic (31%) and mitochondrial (18%) creatine kinase activities of brain cortex from rats. Considering that creatine kinase is an enzyme dependent of thiol residues for its function and tyrosine induces oxidative stress, the results suggest that the inhibition caused by tyrosine might occur by oxidation of essential sulfhydryl groups of the enzyme. In case this also occurs in patients with tyrosinemia, it is possible that creatine kinase inhibition may contribute to the neurological dysfunction characteristic of tyrosinemia.

Effects of chlorpyrifos and chlorpyrifos-oxon on the dynamics and movement of mitochondria in rat cortical neurons

Organophosphate (OP)-based pesticides have been used extensively for decades, and as a result, they have become almost ubiquitous in our environment. There is clinical and animal evidence to suggest that chronic exposures to OPs can lead to cognitive dysfunction and other neurological abnormalities, although the mechanism for these effects is unknown. We previously reported that repeated, subthreshold exposures (defined as doses not associated with signs of acute toxicity) to the commonly used OP chlorpyrifos (CPF) resulted in protracted impairments in the performance of attention and memory-related tasks in rodents as well as deficits in axonal transport ex vivo (in the sciatic nerve). Here, we investigated the effects of CPF and its active metabolite CPF oxon (CPO) on the dynamics and movement of mitochondria in rat primary cortical neurons using time-lapse imaging techniques. Exposure to CPF (1.0-20.0 μM) or CPO (5.0 nM-20.0 μM) for 1 or 24 h resulted in a concentration-dependent increase in mitochondrial length, a decrease in mitochondrial number (indicative of increased fusion events), and a decrease in their movement in axons. The changes occurred at concentrations of CPF and CPO that did not inhibit acetylcholinesterase activity (the commonly cited mechanism of acute OP toxicity), and they were not blocked by cholinergic receptor antagonists. Furthermore, the changes did not seem to be associated with direct (OP-related) effects on mitochondrial viability or function (i.e., mitochondrial membrane potential or ATP production). The results suggest that an underlying mechanism of organophosphate-based deficits in cognitive function might involve alterations in mitochondrial dynamics and/or their transport in axons.

Ameliorative effects of vitamin C on short-term sensorimotor and cognitive changes induced by acute chlorpyrifos exposure in Wistar rats

Human and experimental animal studies have shown long- and short-term neurological sequelae following acute organophosphate (OP) exposure. Although the main molecular mechanism of OP neurotoxiicty involves acetylcholinesterase (AChE) inhibition, studies have also implicated the induction of oxidative stress. The present study was therefore aimed at evaluating the effect of acute chlorpyrifos (CPF) exposure on short-term sensorimotor and cognitive changes in Wistar rats, the role of brain lipoperoxidative changes and the effect of pretreatment with vitamin C. Twenty-eight rats divided into four groups of seven rats in each group served as subjects for this study. Rats in group I were given soya oil (2 ml/kg) while those in group II were dosed vitamin C (100 mg/kg). Group III were administered CPF only (42.5 mg/kg ∼50% of LD50), while group IV were pretreated with vitamin C (100 mg/kg) and then exposed to CPF (42.5 mg/kg), 30 min later. The regimens were administered once orally and the animals were examined for clinical signs, death and subjected to periodic neurobehavioral evaluation for motor strength, coordinated gait, neuromuscular coordination, learning and memory. At the end of 8 weeks of evaluation, the rats were sacrificed and the brain tissue evaluated for AChE activity and malonaldehyde (MDA) concentration, as an index of lipoperoxidative changes. The rats administered with CPF showed deficits in motor strength, coordinated gaits, neuromuscular coordination, learning and memory, slight decrease in AChE activity and an increase in brain MDA concentration. Pretreatment with vitamin C improved the neurobehavioral deficits and AChE activity, and caused a significant reduction in MDA concentration. In conclusion, the study has demonstrated that acute CPF exposure in Wistar rats caused short-term impairment in sensorimotor and cognitive functions partly due to brain lipoperoxidative changes, which were ameliorated by pretreatment with vitamin C.

Neonatal parathion exposure and interactions with a high-fat diet in adulthood: Adenylyl cyclase-mediated cell signaling in heart, liver and cerebellum

Organophosphates are developmental neurotoxicants but recent evidence points to additional adverse effects on metabolism and cardiovascular function. One common mechanism is disrupted cell signaling mediated through cyclic AMP, targeting neurohumoral receptors, G-proteins and adenylyl cyclase (AC) itself. Earlier, we showed that neonatal parathion evokes later upregulation of the hepatic AC pathway in adolescence but that the effect wanes by young adulthood; nevertheless metabolic changes resembling prediabetes persist. Here, we administered parathion to neonatal rats (postnatal days 1-4, 0.1 or 0.2 mg/kg/day), straddling the threshold for cholinesterase inhibition, but we extended the studies to much later, 5 months of age. In addition, we investigated whether metabolic challenge imposed by consuming a high-fat diet for 7 weeks would exacerbate neonatal parathion's effects. Parathion alone increased the expression or function of G(i), thus reducing AC responses to fluoride. Receptors controlling AC activity were also affected: beta-adrenergic receptors (betaARs) in skeletal muscle were increased, whereas those in the heart were decreased, and the latter also showed an elevation of m(2)-muscarinic acetylcholine receptors, which inhibit AC. The high-fat diet also induced changes in AC signaling, enhancing the hepatic AC response to glucagon while impairing the cardiac response to fluoride or forskolin, and suppressing betaARs and m(2)-muscarinic receptors; the only change in the cerebellum was a decrease in betaARs. Although there were no significant interactions between neonatal parathion exposure and a high-fat diet, their convergent effects on the same signaling cascade indicate that early OP exposure, separately or combination with dietary factors, may contribute to the worldwide increase in the incidence of obesity and diabetes.

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.

Postnatal age influences hypoglycemia-induced poly(ADP-ribose) polymerase-1 activation in the brain regions of rats

Poly(ADP-ribose) polymerase-1 (PARP-1) overactivation plays a significant role in hypoglycemia-induced brain injury in adult rats. To determine the influence of postnatal age on PARP-1 activation, developing and adult male rats were subjected to acute hypoglycemia of equivalent severity and duration. The expression of PARP-1 and its downstream effectors, apoptosis-inducing factor (Aifm1), caspase 3 (Casp3), NF-kappaB (Nfkb1) and bcl-2 (Bcl2), and cellular poly(ADP-ribose) (PAR) polymer expression were assessed in the cerebral cortex, hippocampus, striatum, and hypothalamus at 0 h and 24 h posthypoglycemia. Compared with the control group, PARP-1 expression increased in the cerebral cortex of adult rats 24 h posthypoglycemia, but not at 0 h, and it was accompanied by increased number of PAR-positive cells. The expression was not altered in other brain regions. Aifm1, Nfkb1, Casp3, and Bcl2 expressions also increased in the cerebral cortex of adult rats 24 h posthypoglycemia. Conversely, hypoglycemia did not alter PARP-1 expression and its downstream effectors in any brain region in developing rats. These data parallel the previously demonstrated pattern of hypoglycemia-induced brain injury and suggest that PARP-1 overactivation may determine age- and region-specific vulnerability during hypoglycemia.

Do terbutaline- and mold-associated impairments of the brain and lung relate to autism?

Increased prevalence of the autism spectrum disorders (ASD) and the failure to find genetic explanations has pushed the hunt for environmental causes. These disorders are defined clinically but lack objective characterization. To meet this need, we measured neurobehavioral and pulmonary functions in eight ASD boys aged 8 to 19 years diagnosed clinically and compared them to 145 unaffected children from a community with no known chemical exposures. As 6 of 35 consecutive mold/mycotoxin (mold)-exposed children aged 5 to 13 years had ASD, we compared them to the 29 non-ASD mold-exposed children, and to the eight ASD boys. Comparisons were adjusted for age, height, weight, and grade attained in school. The eight ASD boys averaged 6.8 abnormalities compared to 1.0 in community control boys. The six mold-exposed ASD children averaged 12.2 abnormalities. The most frequent abnormality in both groups was balance, followed by visual field quadrants, and then prolonged blink reflex latency. Neuropsychological abnormalities were more frequent in mold-exposed than in terbutaline-exposed children and included digit symbol substitution, peg placement, fingertip number writing errors, and picture completion. Profile of mood status scores averaged 26.8 in terbutaline-exposed, 52 in mold exposed, and 26 in unexposed. The mean frequencies of 35 symptoms were 4.7 in terbutaline, 5.4 in mold/mycotoxins exposed and 1.7 in community controls.

Oxidative and excitatory mechanisms of developmental neurotoxicity: transcriptional profiles for chlorpyrifos, diazinon, dieldrin, and divalent nickel in PC12 cells

BACKGROUND

Oxidative stress and excitotoxicity underlie the developmental neurotoxicity of numerous chemicals.

OBJECTIVES

We compared the effects of organophosphates (chlorpyrifos and diazinon), an organo-chlorine (dieldrin), and a metal [divalent nickel (Ni2+)] to determine how these mechanisms contribute to similar or dissimilar neurotoxic outcomes.

METHODS

We used PC12 cells as a model of developing neurons and evaluated transcriptional profiles for genes for oxidative stress responses and glutamate receptors.

RESULTS

Chlorpyrifos had a greater effect on oxidative-stress-related genes in differentiating cells compared with the undifferentiated state. Chlorpyrifos and diazinon showed significant concordance in their effects on glutathione-related genes, but they were negatively correlated for effects on catalase and superoxide dismutase isoforms and had no concordance for effects on ionotropic glutamate receptors. Surprisingly, the correlations were stronger between diazinon and dieldrin than between the two organophosphates. The effects of Ni2+ were the least similar for genes related to oxidative stress but had significant concordance with dieldrin for effects on glutamate receptors.

CONCLUSIONS

Our results point to underlying mechanisms by which different organophosphates produce disparate neurotoxic outcomes despite their shared property as cholinesterase inhibitors. Further, apparently unrelated neurotoxicants may produce similar outcomes because of convergence on oxidative stress and excitotoxicity. The combined use of cell cultures and microarrays points to specific end points that can distinguish similarities and disparities in the effects of diverse developmental neurotoxicants.

Characterization of chlorpyrifos-induced apoptosis in placental cells

The mechanism by which chlorpyrifos exerts its toxicity in fetal and perinatal animals has yet to be elucidated. Since the placenta is responsible for transport of nutrients and is a major supplier hormone to the fetus, exposure to xenobiotics that alter the function or viability of placenta cells could ostensibly alter the development of the fetus. In this study, JAR cells were used to determine if CPF and the metabolites 3,5,6-trichloro-2-pyridinol (TCP) and chlorpyrifos-oxon (CPO) are toxic to the placenta. Our results indicate that chlorpyrifos (CPF), and its metabolite chlorpyrifos-oxon (CPO) caused a dose-dependent reduction in cellular viability with CPF being more toxic than its metabolites. Chlorpyrifos-induced toxicity was characterized by the loss of mitochondrial potential, the appearance of nuclear condensation and fragmentation, down-regulation of Bcl-2 as well as up-regulation of TNFalpha and FAS mRNA. Pharmacological inhibition of FAS, nicotinic and TNF-alpha receptors did not attenuate CPF-induced toxicity. Atropine exhibited minimal ability to reverse toxicity. Furthermore, signal transduction inhibitors PD98059, SP600125, LY294002 and U0126 failed to attenuate toxicity; however, SB202190 (inhibitor of p38alpha and p38beta MAPK) sensitized cells to CPF-induced toxicity. Pan-caspase inhibitor Q-VD-OPh produced a slight but significant reversal of CPF-induced toxicity indicating that the major caspase pathways are not integral to CPF-induced toxicity. Taken collectively, these results suggest that chlorpyrifos induces apoptosis in placental cells through pathways not dependent on FAS/TNF signaling, activation of caspases or inhibition of cholinesterase. In addition, our data further indicates that activation of p38 MAPK is integral to the protection cells against CPF-induced injury.

Ameliorating the developmental neurotoxicity of chlorpyrifos: a mechanisms-based approach in PC12 cells

BACKGROUND

Organophosphate developmental neurotoxicity involves multiple mechanisms converging on neural cell replication and differentiation.

OBJECTIVES

We evaluated mechanisms contributing to the adverse effects of chlorpyrifos (CPF) on DNA synthesis, cell number and size, and cell signaling mediated by adenylyl cyclase (AC) in PC12 cells, a neuronotypic cell line that recapitulates the essential features of developing mammalian neurons.

RESULTS

In undifferentiated cells, cholinergic receptor antagonists had little or no protective effect against the antimitotic actions of CPF; however, when nerve growth factor was used to evoke differentiation, the antagonists showed partial protection against deficits in cell loss and alteration in cell size elicited by CPF, but were ineffective in preventing the deterioration of AC signaling. Nicotine, which stimulates nicotinic acetylcholine receptors but also possesses a mixture of prooxidant/antioxidant activity, had adverse effects by itself but also protected undifferentiated cells from the actions of CPF and had mixed additive/protective effects on cell number in differentiating cells. The antioxidant vitamin E also protected both undifferentiated and differentiating cells from many of the adverse effects of CPF but worsened the impact on AC signaling. Theophylline, which prevents the breakdown of cyclic AMP, was the only agent that restored AC signaling to normal or supranormal levels but did so at further cost to cell replication.

CONCLUSIONS

Our results show definitive contributions of cholinergic hyperstimulation, oxidative stress, and interference with AC signaling in the developmental neurotoxicity of CPF and point to the potential use of this information to design treatments to ameliorate these adverse effects.

Exposure to organophosphates reduces the expression of neurotrophic factors in neonatal rat brain regions: similarities and differences in the effects of chlorpyrifos and diazinon on the fibroblast growth factor superfamily

BACKGROUND

The fibroblast growth factor (FGF) superfamily of neurotrophic factors plays critical roles in neural cell development, brain assembly, and recovery from neuronal injury.

OBJECTIVES

We administered two organophosphate pesticides, chlorpyrifos and diazinon, to neonatal rats on postnatal days 1-4, using doses below the threshold for systemic toxicity or growth impairment, and spanning the threshold for barely detectable cholinesterase inhibition: 1 mg/kg/day chlorpyrifos and 1 or 2 mg/kg/day diazinon.

METHODS

Using microarrays, we then examined the regional expression of mRNAs encoding the FGFs and their receptors (FGFRs) in the forebrain and brain stem.

RESULTS

Chlorpyrifos and diazinon both markedly suppressed fgf20 expression in the forebrain and fgf2 in the brain stem, while elevating brain stem fgfr4 and evoking a small deficit in brain stem fgf22. However, they differed in that the effects on fgf2 and fgfr4 were significantly larger for diazinon, and the two agents also showed dissimilar, smaller effects on fgf11, fgf14, and fgfr1.

CONCLUSIONS

The fact that there are similarities but also notable disparities in the responses to chlorpyrifos and diazinon, and that robust effects were seen even at doses that do not inhibit cholinesterase, supports the idea that organophosphates differ in their propensity to elicit developmental neurotoxicity, unrelated to their anticholinesterase activity. Effects on neurotrophic factors provide a mechanistic link between organophosphate injury to developing neurons and the eventual, adverse neurodevelopmental outcomes.

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

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

Developmental exposure to terbutaline and chlorpyrifos, separately or sequentially, elicits presynaptic serotonergic hyperactivity in juvenile and adolescent rats

Developmental exposure to unrelated neurotoxicants can nevertheless converge on common final targets so as to exacerbate damage or functional deficits. We examined the effects of developmental exposure to terbutaline, a beta2-adrenergic receptor agonist used to arrest preterm labor, and chlorpyrifos, a widely used organophosphate pesticide, on serotonin (5HT) systems. Treatments were chosen to parallel periods typical of human developmental exposures, terbutaline (10 mg/kg) on postnatal days (PN) 2-5 and chlorpyrifos (5 mg/kg) on PN11-14, with assessments conducted in juvenile and adolescent stages (PN21, PN30 and PN45), comparing each agent alone as well as sequential administration of both. By itself, terbutaline produced persistent 5HT presynaptic hyperactivity as evidenced by increased 5HT turnover in brain regions containing 5HT terminal zones; this effect was similar to that seen in earlier studies with chlorpyrifos administration during the same early postnatal period. Later administration of chlorpyrifos (PN11-14) produced a transient increase in 5HT turnover during the juvenile stage, and the sequential exposure paradigm, terbutaline followed by chlorpyrifos, showed a corresponding increase in effect over either agent alone. In combination with our earlier work on 5HT receptors, these results indicate that terbutaline is a developmental neurotoxicant that targets the 5HT system, findings that lend a mechanistic underpinning to clinical indications of elevated childhood psychiatric disorders in the offspring of women treated with beta-agonist tocolytics. Equally importantly, the interaction between terbutaline and chlorpyrifos suggests that tocolytic therapy may alter the subsequent susceptibility to common environmental toxicants.

beta2-Adrenergic receptor gene variants and risk for autism in the AGRE cohort

The beta2-adrenergic receptor is part of the catecholamine system, and variants at two polymorphic sites in the gene coding for the receptor (ADRB2) confer increased activity. Overstimulation of this receptor may alter brain development, and has been linked to autism in non-identical twins. The objective of this study was to determine whether alleles in ADRB2 are associated with diagnosis of autism in the Autism Genetic Resource Exchange (AGRE) population. Three hundred and thirty-one independent autism case-parent trios were included in the analysis. Subjects were genotyped at activity-related polymorphisms rs1042713 (codon 16) and rs1042714 (codon 27). Association between autism and genotypes at each polymorphic site was tested using genotype-based transmission disequilibrium tests, and effect modification by family and pregnancy characteristics was evaluated. Sensitivity to designation of the proband in each family was assessed by performing 1000 repeats of the analysis selecting affected children randomly. A statistically significant OR of 1.66 for the Glu27 homozygous genotype was observed. Increased associations with this genotype were observed among a subset of Autism Diagnostic Observation Schedule confirmed cases and a subset reporting experience of pregnancy-related stressors. In conclusion, the Glu27 allele of the ADRB2 gene may confer increased risk of autism and shows increased strength with exposure to pregnancy related stress.

Maternal effects and the evolution of brain size in birds: overlooked developmental constraints

A central dogma for the evolution of brain size posits that the maintenance of large brains incurs developmental costs, because they need prolonged periods to grow during the early ontogeny. Such constraints are supported by the interspecific relationship between ontological differences and relative brain size in birds and mammals. Given that mothers can strongly influence the development of the offspring via maternal effects that potentially involve substances essential for growing brains, we argue that such effects may represent an important but overlooked component of developmental constraints on brain size. To demonstrate the importance of maternal effect on the evolution of brains, we investigated the interspecific relationship between relative brain size and maternal effects, as reflected by yolk testosterone, carotenoids, and vitamins A and E in a phylogenetic study of birds. Females of species with relatively large brains invested more in eggs in terms of testosterone and vitamin E than females of species with small brains. The effects of carotenoid and vitamin A levels on the evolution of relative brain size were weaker and non-significant. The association between relative brain size and yolk testosterone was curvilinear, suggesting that very high testosterone levels can be suppressive. However, at least in moderate physiological ranges, the positive relationship between components of maternal effects and relative brain size may imply one aspect of developmental costs of large brains. The relationship between vitamin E and relative brain size was weakened when we controlled for developmental mode, and thus the effect of this antioxidant may be indirect. Testosterone-enhanced neurogenesis and vitamin E-mediated defence against oxidative stress may have key functions when the brain of the embryo develops, with evolutionary consequences for relative brain size.

Oxidative stress as a mechanism of teratogenesis

Emerging evidence shows that redox-sensitive signal transduction pathways are critical for developmental processes, including proliferation, differentiation, and apoptosis. As a consequence, teratogens that induce oxidative stress (OS) may induce teratogenesis via the misregulation of these same pathways. Many of these pathways are regulated by cellular thiol redox couples, namely glutathione/glutathione disulfide, thioredoxinred/thioredoinox, and cysteine/cystine. This review outlines oxidative stress as a mechanism of teratogenesis through the disruption of thiol-mediated redox signaling. Due to the ability of many known and suspected teratogens to induce oxidative stress and the many signaling pathways that have redox-sensitive components, further research is warranted to fully understand these mechanisms.

Lactational exposure to malathion inhibits brain acetylcholinesterase in mice

The organophosphorus (OP) pesticide malathion is a highly neurotoxic compound. Although some studies have reported neurotoxicity signs after the in utero exposure to OP pesticides, there is no evidence of the exclusive contribution of the lactational exposure to malathion as a possible cause of neurotoxicity in the offspring. In this study, we investigated the exclusive contribution of malathion exposure through maternal milk on the activity of acetylcholinesterase (AChE), as well as on biochemical parameters related to the oxidative stress (glutathione levels, lipid peroxidation and glutathione reductase and glutathione peroxidase activities) in the brain of suckling mice. The same parameters were also evaluated in the brains of the respective mothers, which where directly exposed to malathion during the lactational period (daily s.c. injections; doses of 20, 60 and 200mg/kg of body weight). Our results showed that the lactational exposure to malathion caused a high inhibitory effect of the brain AChE activity in the offspring, even when dams were exposed to the lowest malathion dose (20mg/kg). Brain AChE activity was also inhibited in mothers; however, only at the highest malathion dose (200mg/kg). No changes were observed in the biochemical parameters related to the oxidative stress for both dams and pups brains. The present study shows, for the first time, that the exposure of neonatal mice to malathion via lactation inhibits the activity of brain AChE in the offspring. These data, summed to the fact that OP pesticides are excreted in human milk, makes relevant the lactational exposure to these xenobiotics in terms of human health concerns.

Anomalous regulation of β-adrenoceptor signaling in brain regions of the newborn rat

Desensitization, an essential homeostatic response to excessive or continued beta-adrenoceptor (betaAR) stimulation, is deficient in immature cells. To determine the mechanisms underlying anomalous betaAR responses in newborn rats, we administered terbutaline, a beta2AR agonist, on postnatal day 2 and evaluated signaling through adenylyl cyclase (AC) in cell membrane preparations 4 h later. Although a small decrement in isoproterenol-stimulated AC was obtained in the forebrain, robust sensitization was seen in the brainstem and cerebellum, in association with heterologous increases in AC catalytic activity: increased basal, dopamine-stimulated and forskolin-stimulated AC. Superimposed on this global increase, there was a small degree of betaAR and dopamine receptor desensitization, characterized by decreases in the isoproterenol/forskolin and dopamine/forskolin AC activity ratios. Our results indicate that, in some immature brain regions, betaAR desensitization is masked by more substantial increases in the activity of signaling elements downstream from the receptors, resulting in sustained responses in the face of continued receptor stimulation. These effects are likely responsible for the maintenance of betaAR activity associated with neurotrophic input during synaptogenesis but may also contribute to adverse effects of betaAR agonists used in preterm labor.

Imbalances emerge in cardiac autonomic cell signaling after neonatal exposure to terbutaline or chlorpyrifos, alone or in combination

During early neonatal development, the future reactivity of the heart to cardiac autonomic stimulation is programmed by the timing and intensity of the arrival of parasympathetic and sympathetic inputs. In neonatal rats, we examined the effects of exposure to terbutaline, a beta-adrenoceptor (betaAR) agonist used to arrest preterm labor, and chlorpyrifos (CPF), a widely used organophosphate pesticide that acts in part through inhibition of cholinesterase, using scenarios mimicking the likely developmental stages corresponding to peak human exposures: postnatal days (PN) 2-5 for terbutaline and PN11-14 for CPF. Terbutaline evoked a progressive deficit in cardiac betaAR binding but did not interfere with the ability of the receptors to stimulate adenylyl cyclase (AC). Terbutaline also reduced expression of m2 muscarinic acetylcholine receptors and suppressed their ability to inhibit AC. Surprisingly, CPF produced similar actions, a decrement in betaAR and m2 muscarinic receptor binding and a loss of the cholinergic AC response, and also augmented the ability of betaARs to stimulate AC. The effects of CPF are thus unlikely to reside in cholinergic hyperstimulation resulting from cholinesterase inhibition but instead involve other actions converging on receptors and cell signaling. Exposure to both agents, terbutaline followed by CPF, produced a summation of the two individual effects. Our findings at the level of cell signaling thus indicate that neonatal exposure to terbutaline or CPF, or sequentially to both agents, results in an imbalance of cardiac autonomic inputs favoring increased excitability, an outcome that may have an impact on cardiovascular responses.