Increased synaptosomal dopamine content and brain concentration of paraquat produced by selective dithiocarbamates

Exploring environmental exposomes and the gut-brain nexus: Unveiling the impact of pesticide exposure

This review delves into the escalating concern of environmental pollutants and their profound impact on human health in the context of the modern surge in global diseases. The utilisation of chemicals in food production, which results in residues in food, has emerged as a major concern nowadays. By exploring the intricate relationship between environmental pollutants and gut microbiota, the study reveals a dynamic bidirectional interplay, as modifying microbiota profile influences metabolic pathways and subsequent brain functions. This review will first provide an overview of potential exposomes and their effect to gut health. This paper is then emphasis the connection of gut brain function by analysing microbiome markers with neurotoxicity responses. We then take pesticide as example of exposome to elucidate their influence to biomarkers biosynthesis pathways and subsequent brain functions. The interconnection between neuroendocrine and neuromodulators elements and the gut-brain axis emerges as a pivotal factor in regulating mental health and brain development. Thus, manipulation of gut microbiota function at the onset of stress may offer a potential avenue for the prevention and treatment for mental disorder and other neurodegenerative illness.

Acute exposure to paraquat affects the phenotypic differentiation of substantia nigra microglia in rats

The toxicity of the bipyridine cationic herbicide paraquat (PQ) to the lung and kidneys has been widely documented, but the acute toxic effects of PQ on the nervous system have received little attention. This study aimed to explore the changes in the phenotypic differentiation of microglia in rats caused by acute PQ exposure. As results, acute PQ exposure induced pyknosis, edema, and apoptosis in substantia nigra neurons. Immunohistochemistry and western blotting showed that, on day 18, with the increase of exposure dose, the number of Iba-1-positive cells presented an increasing trend with no statistically significant difference among the groups (P > 0.05). Compared with the control group, the process length of Iba-1-positive cells decreased of acute 25 mg/kg PQ exposure on day 18 (P < 0.05). Compared with the control group, on day 39, the number of Iba-1-positive cells in the SN decreased of acute 25 mg/kg PQ exposure, while that increased of acute 45 mg/kg PQ exposure (P < 0.05). The number of endpoints decreased of acute 25 mg/kg PQ exposure (P < 0.05). The process length became shorter both of acute 25 mg/kg and 45 mg/kg PQ exposure (P < 0.05). On day 69, compared with the control group, the number of Iba-1-positive cells in the SN significantly increased of acute 45 mg/kg PQ exposure (P < 0.05). The number of endpoints increased and the process length became longer of acute 25 mg/kg PQ exposure (P < 0.05). Then, the mean fluorescence intensity of inducible nitric oxide synthase (iNOS) and arginine 1 (ARG1) was compared. The number of the M1 phenotype of microglia increased during the early stage of acute 25 mg/kg PQ exposure, whereas the number of the M2 phenotype of microglia increased during the early stage of acute 45 mg/kg PQ exposure (P < 0.05). On day 39, compared with the control group, the expression of iNOS in the SN of acute 45 mg/kg PQ exposure increased than of acute 25 mg/kg exposure. The expression of Arg-1 of 25 mg/kg PQ exposure was significantly increased (P < 0.05). On day 69, the expression of iNOS and ARG1 increased in the 25 and 45 mg/kg PQ exposure groups. In summary, changes in microglia phenotypic differentiation were related to exposure dose and exposure time (P < 0.05).

Prenatal exposure to the pesticide metam sodium induces sensorimotor and neurobehavioral abnormalities in mice offspring

The present study has investigated developmental neurotoxicity of Metam sodium (MS), from gestational day 6 and throughout the gestation period until delivery. Therefore, mated female mice were orally exposed on a daily basis to 0 (control), 50, 100 or 150 mg of MS/kg of body weight and their standard fertility and reproductive parameters were assessed. The offspring were examined for their sensorimotor development, depression and cognitive performance. Our results showed that MS exposure during pregnancy led to one case of mortality, two cases of abortion and disturbed fertility and reproductive parameters in pregnant dams. In offspring, MS induced an overall delay in innate reflexes and sensorimotor performances. Furthermore, all prenatally treated animals showed an increased level of depression-like behavior as well as a pronounced cognitive impairment in adulthood. These results demonstrated that prenatal exposure to MS causes a long-lasting developmental neurotoxicity and alters a wide range of behavioral functions in mice.

Nrf2-regulated miR-380-3p Blocks the Translation of Sp3 Protein and Its Mediation of Paraquat-Induced Toxicity in Mouse Neuroblastoma N2a Cells

Laboratorial and epidemiological research has established a relationship between paraquat (PQ) exposure and a risk for Parkinson's disease. Previously, we have investigated the effects of nuclear factor erythroid 2 related factor 2 (Nrf2) and microRNAs in PQ-induced neurotoxicity, addressing the function of miR-380-3p, a microRNA dysregulated by PQ, as well as Nrf2 deficiency. Nrf2 is known to mediate the expression of a variety of genes, including noncoding genes. By chromatin immunoprecipitation, we identified the relationship between Nrf2 and miR-380-3p in transcriptional regulation. qRT-PCR, Western blots, and dual-luciferase reporter gene assay showed that miR-380-3p blocked the translation of the transcription factor specificity protein-3 (Sp3) in the absence of degradation of Sp3 mRNA. Results based on cell counting analysis, annexin v-fluorescein isothiocyanate/propidium iodide double-staining assay, and propidium iodide staining showed that overexpression of miR-380-3p inhibited cell proliferation, increased the apoptotic rate, induced cell cycle arrest, and intensified the toxicity of PQ in mouse neuroblastoma (N2a [Neuro2a]) cells. Knockdown of Sp3 inhibited cell proliferation and eclipsed the alterations induced by miR-380-3p in cell proliferation. Two mediators of apoptosis and cell cycle identified in previous studies as Sp3-regulated, namely cyclin-dependent kinase inhibitor 1 (p21) and calmodulin (CaM), were dysregulated by PQ, but not Sp3 deficiency. In conclusion, Nrf2-regulated miR-380-3p inhibited cell proliferation and enhanced the PQ-induced toxicity in N2a cells potentially by blocking the translation Sp3 mRNA. We conclude that CaM and p21 were involved in PQ-induced toxicity.

Acute Maneb Exposure Significantly Alters Both Glycolysis and Mitochondrial Function in Neuroblastoma Cells

The pesticides paraquat (PQ) and maneb (MB) have been described as environmental risk factors for Parkinson's disease (PD), with mechanisms associated with mitochondrial dysfunction and reactive oxygen species generation. A combined exposure of PQ and MB in murine models and neuroblastoma cells has been utilized to further advance understanding of the PD phenotype. MB acts as a redox modulator through alkylation of protein thiols and has been previously characterized to inhibit complex III of the electron transport chain and uncouple the mitochondrial proton gradient. The purpose of this study was to analyze ATP-linked respiration and glycolysis in human neuroblastoma cells utilizing the Seahorse extracellular flux platform. Employing an acute, subtoxic exposure of MB, this investigation revealed a MB-mediated decrease in mitochondrial oxygen consumption at baseline and maximal respiration, with inhibition of ATP synthesis and coupling efficiency. Additionally, MB-treated cells showed an increase in nonmitochondrial respiration and proton leak. Further investigation into mitochondrial fuel flex revealed an elimination of fuel flexibility across all 3 major substrates, with a decrease in pyruvate capacity as well as glutamine dependency. Analyses of glycolytic function showed a substantial decrease in glycolytic acidification caused by lactic acid export. This inhibition of glycolytic parameters was also observed after titrating the MB dose as low as 6 μM, and appears to be dependent on the dithiocarbamate functional group, with manganese possibly potentiating the effect. Further studies into cellular ATP and NAD levels revealed a drastic decrease in cells treated with MB. In summary, MB significantly impacted both aerobic and anaerobic energy production; therefore, further characterization of MB's effect on cellular energetics may provide insight into the specificity of PD to dopaminergic neurons.

Paraquat and Parkinson's disease: a systematic review and meta-analysis of observational studies

This investigation aimed to conduct a systematic review of the literature and meta-analysis to determine whether exposure to the herbicide paraquat was associated with the development of Parkinson's disease (PD). Observational studies that enrolled adults exposed to paraquat with PD as the outcome of interest were searched in the PubMed, Embase, LILACS, TOXNET, and Web of Science databases up to May 2019. Two authors independently selected relevant studies, extracted data, and assessed methodological quality. The evidence certainty was assessed by the GRADE approach, which served as basis for a tentative causality assessment, supplemented by the Bradford Hill criteria when necessary. Results from nine case-control studies indicated that PD occurrence was 25% higher in participants exposed to paraquat. The only cohort investigation included demonstrated a non-significant OR of 1.08. Results from subgroup analyses also indicated higher PD frequency in participants that were exposed to paraquat for longer periods or individuals co-exposed with paraquat and any other dithiocarbamate. Data indicate apositive association between exposure to paraquat and PD occurrence, but the weight-of-evidence does not enable one to assume an indisputable cause-effect relationship between these two conditions. Better designed studies are needed to increase confidence in results. Systematic Review Registration: PROSPERO CRD42017069994.

Early Postnatal Exposure to Paraquat and Maneb in Mice Increases Nigrostriatal Dopaminergic Susceptibility to a Re-challenge with the Same Pesticides at Adulthood: Implications for Parkinson's Disease

Exposure to environmental contaminants represents an important etiological factor in sporadic Parkinson's disease (PD). It has been reported that PD could arise from events that occur early in development and that lead to delayed adverse consequences in the nigrostriatal dopaminergic system at adult life. We investigated the occurrence of late nigrostriatal dopaminergic neurotoxicity induced by exposures to the pesticides paraquat (PQ) and maneb (MB) during the early postnatal period in mice, as well as whether the exposure to pesticides during development could enhance mice vulnerability to subsequent challenges. Male Swiss mice were exposed to a combination of 0.3 mg/kg PQ and 1.0 mg/kg MB (PQ + MB) from postnatal (PN) day 5 to 19. PN exposure to pesticides neither induced mortally nor modified motor-related parameters. However, PN pesticides exposure decreased the number of tyrosine hydroxylase (TH)- and dopamine transporter (DAT)-positive neurons in the substantia nigra pars compacta (SNpc), as well as reduced TH and DAT immunoreactivity in the striatum. A parallel group of animals developmentally exposed to the pesticides was re-challenged at 3 months of age with 10 mg/kg PQ plus 30 mg/kg MB (twice a week, 6 weeks). Mice exposed to pesticides at both periods (PN + adulthood) presented motor deficits and reductions in the number of TH- and DAT-positive neurons in the SNpc. These findings indicate that the exposure to PQ + MB during the early PN period can cause neurotoxicity in the mouse nigrostriatal dopaminergic system, rendering it more susceptible to a subsequent adult re-challenge with the same pesticides.

Paraquat Exposure of Pregnant Women and Neonates in Agricultural Areas in Thailand

This study aimed to assess paraquat concentrations in the urine of women at 28 weeks of pregnancy, delivery and 2 months postpartum and in the meconium of neonates. In all, 79 pregnant women were recruited from three hospitals located in agricultural areas in Thailand. The subjects were interviewed about personal characteristics, agricultural activities and pesticide use patterns. Paraquat was analyzed in urine and meconium using high performance liquid chromatography equipped with a fluorescence detector. The geometric mean (GSD) of urinary paraquat concentrations at 28 weeks of pregnancy, delivery and 2 months postpartum were 2.04 (4.22), 2.06 (5.04) and 2.42 (5.33) ng/mL, respectively. The urinary paraquat concentrations at 28 weeks of pregnancy, delivery and 2 months postpartum between agriculturist and non-agriculturist were not significantly different (p = 0.632, p = 0.915, p = 0.57, respectively). The geometric mean (GSD) of paraquat concentration in the meconium was 33.31 (4.59) ng/g. The factors predicting paraquat exposures among pregnant women and neonates included working outside, living near farmland, having family members who work on a farm, drinking well water and using herbicides or paraquat.

Paraquat and Maneb Exposure Alters Rat Neural Stem Cell Proliferation by Inducing Oxidative Stress: New Insights on Pesticide-Induced Neurodevelopmental Toxicity

Pesticide exposure has been linked to the pathogenesis of neurodevelopmental and neurodegenerative disorders including autism spectrum disorders, attention deficit/hyperactivity, and Parkinson's disease (PD). Developmental exposure to pesticides, even at low concentrations not harmful for the adult brain, can lead to neuronal loss and functional deficits. It has been shown that prenatal or early postnatal exposure to the herbicide paraquat (PQ) and the fungicide maneb (MB), alone or in combination, causes permanent toxicity in the nigrostriatal dopamine system, supporting the idea that early exposure to these pesticides may contribute to the pathophysiology of PD. However, the mechanisms mediating PQ and MB developmental neurotoxicity are not yet understood. Therefore, we investigated the neurotoxic effect of low concentrations of PQ and MB in primary cultures of rat embryonic neural stem cells (NSCs), with particular focus on cell proliferation and oxidative stress. Exposure to PQ alone or in combination with MB (PQ + MB) led to a significant decrease in cell proliferation, while the cell death rate was not affected. Consistently, PQ + MB exposure altered the expression of major genes regulating the cell cycle, namely cyclin D1, cyclin D2, Rb1, and p19. Moreover, PQ and PQ + MB exposures increased the reactive oxygen species (ROS) production that could be neutralized upon N-acetylcysteine (NAC) treatment. Notably, in the presence of NAC, Rb1 expression was normalized and a normal cell proliferation pattern could be restored. These findings suggest that exposure to PQ + MB impairs NSCs proliferation by mechanisms involving alterations in the redox state.

Effect of Wnt1 and Wnt5a on the development of dopaminergic neurons, and toxicity induced by combined exposure to paraquat and maneb during gestation and lactation

Paraquat (PQ) and maneb (MB) are widely used herbicides. Wingless (Wnt) proteins serve a role in the development and differentiation of dopaminergic neurons. Previous studies demonstrated that combined exposure to PQ and MB damages dopaminergic neurons in the midbrain. Effects of PQ and MB exposure on midbrain Wnt proteins have also been previously reported. In the present study, from the 5th day of gestation to weaning of the offspring, pregnant Sprague‑Dawley rats were administrated saline, or PQ and MB at two different doses: high, 15 mg/kg body weight PQ + 45 mg/kg body weight MB; or low, 10 mg/kg body weight PQ + 30 mg/kg body weight MB. Dopamine content in the striatum was examined by high performance liquid chromatography with a fluorescence detector and mRNA and protein expression of Wnt1, Wnt5a, nuclear receptor related factor 1 (Nurr1) and tyrosine hydroxylase (TH) in the midbrain was examined by reverse transcription‑quantitative polymerase chain reaction and western blotting. Combined exposure to PQ and MB during development decreased mRNA and protein expression of Wnt1, TH and Nurr1 and increased expression of Wnt5a in the offspring.

Prenatal Paraquat exposure induces neurobehavioral and cognitive changes in mice offspring

In the present work, we investigated developmental toxicity of Paraquat (PQ), from the 1st or 6th day of mating and throughout the gestation period. We have examined several parameters, including toxicity indices, reproductive performance, sensorimotor development, as well as anxiety and cognitive performance of the offspring. Our results showed that exposure to 20mg/kg of Paraquat during the first days of pregnancy completely prevents pregnancy in treated mice, but from the 6th day of pregnancy, an alteration in fertility and reproductive parameters was observed. In offspring, the PQ was responsible for an overall delay of innate reflexes and a deficit in motor development. All exposed animals showed a decrease in the level of locomotor activity, increased levels of anxiety-like behavior and pronounced cognitive impairment in adulthood. These results demonstrated that Paraquat led to the onset of many behavioral changes that stem from the impairment of neuronal developmental processes in prenatally exposed mice.

Effects of decreased dopamine transporter levels on nigrostriatal neurons and paraquat/maneb toxicity in mice

How genetic variations in the dopamine transporter (DAT) combined with exposure to environmental toxins modulate the risk of Parkinson's disease remains unclear. Using unbiased stereology in DAT knock-down mice (DAT-KD) and wild-type (WT) littermates, we found that decreased DAT caused a loss of tyrosine hydroxylase-positive (dopaminergic) neurons in subregions of the substantia nigra pars compacta at 3-4 days, 5 weeks, and 18 months of age. Both genotypes lost dopaminergic neurons with age and remaining neurons at 11 months were resilient to paraquat/maneb. In 5-week-old mice, the toxins decreased substantia nigra pars compacta dopaminergic neurons in both genotypes but less in DAT-KD. Regional analysis revealed striking differences in the subsets of neurons affected by low DAT, paraquat/maneb, and aging. In particular, we show that a potentially protective effect of low DAT against toxin exposure is not sufficient to reduce death of all nigrostriatal dopaminergic neurons. Thus, different regional vulnerability of nigrostriatal dopaminergic neurons may contribute to an increased risk of developing Parkinson's disease when multiple factors are combined.

Parkinson's Disease and Pesticides Exposure: New Findings From a Comprehensive Study in Nebraska, USA

BACKGROUND

The association between exposure to agricultural pesticides and Parkinson's Disease (PD) has long been a topic of study in the field of environmental health. This research takes advantage of the unique Nebraska PD registry and state-level crop classification data to investigate the PD-pesticides exposure relationship.

METHODS

First, Geographic Information System and satellite remote sensing data were adopted to calculate exposure to different pesticides for Nebraska residents. An integrated spatial exploratory framework was then adopted to explore the association between PD incidence and exposure to specific pesticide ingredients at the county level.

RESULTS

Our results reveal similarities in geographic patterns of pesticide exposure and PD incidence. The regression analyses indicate that, for most Nebraska counties, PD incidence was significantly associated with exposure to certain pesticide ingredients such as alachlor and broxomy. However, the results also suggest that factors other than pesticide exposure may help further explain the risk of PD at the county level.

CONCLUSIONS

We found significant associations between PD incidence and exposure to different pesticide ingredients. These results have useful implications for PD prevention in Nebraska and other agricultural states in the United States.

Transcriptome-metabolome wide association study (TMWAS) of maneb and paraquat neurotoxicity reveals network level interactions in toxicologic mechanism

A combination of the herbicide paraquat (PQ) and fungicide maneb (MB) has been linked to Parkinson's disease. Previous studies show that this involves an additive toxicity with at least two different mechanisms. However, detailed understanding of mixtures is often difficult to elucidate because of the multiple ways by which toxic agents can interact. In the present study, we used a combination of transcriptomics and metabolomics to investigate mechanisms of toxicity of PQ and MB in a neuroblastoma cell line. Conditions were studied with concentrations of PQ and MB that each individually caused 20% cell death and together caused 50% cell death. Transcriptomic and metabolomic samples were collected at time points prior to significant cell death. Statistical and bioinformatic methods were applied to the resulting 30,869 transcripts and 1358 metabolites. Results showed that MB significantly changed more transcripts and metabolites than PQ, and combined PQ + MB impacted more than MB alone. Transcriptome-metabolome-wide association study (TMWAS) showed that significantly changed transcripts and metabolites mapped to two network substructures, one associating with significant effects of MB and the other included features significantly associated with PQ + MB. The latter contained 4 clusters of genes and associated metabolites, with one containing genes for two cation transporters and a cation transporter regulatory protein also recognized as a pro-apoptotic protein. Other clusters included stress response genes and transporters linked to cytoprotective mechanisms. MB also had a significant network structure linked to cell proliferation. Together, the results show that the toxicologic mechanism of the combined neurotoxicity of PQ and MB involves network level interactions and that TMWAS provides an effective approach to investigate such complex mechanisms.

Thiol-reactivity of the fungicide maneb

Maneb (MB) is a manganese-containing ethylene bis-dithiocarbamate fungicide that is implicated as an environmental risk factor for Parkinson's disease, especially in combination with paraquat (PQ). Dithiocarbamates inhibit aldehyde dehydrogenases, but the relationship of this to the combined toxicity of MB + PQ is unclear because PQ is an oxidant and MB activates Nrf2 and increases cellular GSH without apparent oxidative stress. The present research investigated the direct reactivity of MB with protein thiols using recombinant thioredoxin-1 (Trx1) as a model protein. The results show that MB causes stoichiometric loss of protein thiols, reversibly dimerizes the protein and inhibits its enzymatic activity. MB reacted at similar rates with low-molecular weight, thiol-containing chemicals. Together, the data suggest that MB can potentiate neurotoxicity of multiple agents by disrupting protein thiol functions in a manner analogous to that caused by oxidative stress, but without GSH depletion.

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Maneb adducts free thiols.

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Maneb can modify protein bound thiols.

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Maneb adduction can result in protein cross-linking.

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Slow rate of reaction with thiols may act as a cellular sink for maneb.

Bioaccumulation and neurotoxicity of dithiopyridine herbicide in the brain of freshwater fish, Cyprinus carpio

The freshwater carp, Cyprinus carpio, was exposed to 0.5 mg (30% of median lethal concentration (LC50)), 1.0 mg (60% of LC50), and 1.6 mg (LC50) of dithiopyridine herbicide per liter for acute (24 h) and 1/10 of LC50 (0.2 mg/L/day) for sublethal (1, 3, 7, 14, and 21 days) experiments. The herbicide bioaccumulation was significantly affected by the acute exposure levels and the experimental periods and was positively correlated with them. One-way analysis of variance revealed that the acute and sublethal exposure to the herbicide as well as the experimental periods caused significant reduction in the concentrations of catecholamines (dopamine (DA) and norepinephrine (NE)), elevation of acetylcholine (ACh), and was associated with a marked decrease in the activity of acetylcholinesterase (AChE). In comparison with the corresponding controls, most levels of the DA and NE and the activity of AChE were significantly decreased, whereas the concentration of ACh was markedly elevated, during acute and sublethal exposure. In the acute and sublethal experiments, the herbicide accumulated in the brain was inversely proportional to the levels of DA and NE and the activity of AChE but has a direct correlation with the concentration of ACh. In addition, the brain’s AChE activity was negatively correlated with ACh content during the acute ( r = −0.94) and sublethal ( r = −0.78) experiments.

Differential Effects of Methyl-4-Phenylpyridinium Ion, Rotenone, and Paraquat on Differentiated SH-SY5Y Cells

Paraquat (PQ), a cationic nonselective bipyridyl herbicide, has been used as neurotoxicant to modulate Parkinson's disease in laboratory settings. Other compounds like rotenone (ROT), a pesticide, and 1-methyl-4-phenylpyridinium ion (MPP(+)) have been widely used as neurotoxicants. We compared the toxicity of these three neurotoxicants using differentiated dopaminergic SH-SY5Y human cells, aiming to elucidate their differential effects. PQ-induced neurotoxicity was shown to be concentration and time dependent, being mitochondrial dysfunction followed by neuronal death. On the other hand, cells exposure to MPP(+) induced mitochondrial dysfunction, but not cellular lyses. Meanwhile, ROT promoted both mitochondrial dysfunction and neuronal death, revealing a biphasic pattern. To further elucidate PQ neurotoxic mechanism, several protective agents were used. SH-SY5Y cells pretreatment with tiron (TIR) and 2-hydroxybenzoic acid sodium salt (NaSAL), both antioxidants, and N ω -nitro-L-arginine methyl ester hydrochloride (L-NAME), a nitric oxide synthase inhibitor, partially protected against PQ-induced cell injury. Additionally, 1-(2-[bis(4-fluorophenyl)methoxy]ethyl)-4-(3-phenyl-propyl)piperazine (GBR 12909), a dopamine transporter inhibitor, and cycloheximide (CHX), a protein synthesis inhibitor, also partially protected against PQ-induced cell injury. In conclusion, we demonstrated that PQ, MPP(+), and ROT exerted differential toxic effects on dopaminergic cells. PQ neurotoxicity occurred through exacerbated oxidative stress, with involvement of uptake through the dopamine transporter and protein synthesis.

Agrochemicals, α-synuclein, and Parkinson's disease

Epidemiological, population-based case-control, and experimental studies at the molecular, cellular, and organism levels revealed that exposure to various environmental agents, including a number of structurally different agrochemicals, may contribute to the pathogenesis of Parkinson's disease (PD) and several other neurodegenerative disorders. The role of genetic predisposition in PD has also been increasingly acknowledged, driven by the identification of a number of disease-related genes [e.g., α-synuclein, parkin, DJ-1, ubiquitin C-terminal hydrolase isozyme L1 (UCH-L1), and nuclear receptor-related factor 1]. Therefore, the etiology of this multifactorial disease is likely to involve both genetic and environmental factors. Various neurotoxicants, including agrochemicals, have been shown to elevate the levels of α-synuclein expression in neurons and to promote aggregation of this protein in vivo. Many agrochemicals physically interact with α-synuclein and accelerate the fibrillation and aggregation rates of this protein in vitro. This review analyzes some of the aspects linking α-synuclein to PD, provides brief structural and functional descriptions of this important protein, and represents some data connecting exposure to agrochemicals with α-synuclein aggregation and PD pathogenesis.

Animal models of Parkinson's disease: a source of novel treatments and clues to the cause of the disease

Animal models of Parkinson's disease (PD) have proved highly effective in the discovery of novel treatments for motor symptoms of PD and in the search for clues to the underlying cause of the illness. Models based on specific pathogenic mechanisms may subsequently lead to the development of neuroprotective agents for PD that stop or slow disease progression. The array of available rodent models is large and ranges from acute pharmacological models, such as the reserpine- or haloperidol-treated rats that display one or more parkinsonian signs, to models exhibiting destruction of the dopaminergic nigro-striatal pathway, such as the classical 6-hydroxydopamine (6-OHDA) rat and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse models. All of these have provided test beds in which new molecules for treating the motor symptoms of PD can be assessed. In addition, the emergence of abnormal involuntary movements (AIMs) with repeated treatment of 6-OHDA-lesioned rats with L-DOPA has allowed for examination of the mechanisms responsible for treatment-related dyskinesia in PD, and the detection of molecules able to prevent or reverse their appearance. Other toxin-based models of nigro-striatal tract degeneration include the systemic administration of the pesticides rotenone and paraquat, but whilst providing clues to disease pathogenesis, these are not so commonly used for drug development. The MPTP-treated primate model of PD, which closely mimics the clinical features of PD and in which all currently used anti-parkinsonian medications have been shown to be effective, is undoubtedly the most clinically-relevant of all available models. The MPTP-treated primate develops clear dyskinesia when repeatedly exposed to L-DOPA, and these parkinsonian animals have shown responses to novel dopaminergic agents that are highly predictive of their effect in man. Whether non-dopaminergic drugs show the same degree of predictability of response is a matter of debate. As our understanding of the pathogenesis of PD has improved, so new rodent models produced by agents mimicking these mechanisms, including proteasome inhibitors such as PSI, lactacystin and epoximycin or inflammogens like lipopolysaccharide (LPS) have been developed. A further generation of models aimed at mimicking the genetic causes of PD has also sprung up. Whilst these newer models have provided further clues to the disease pathology, they have so far been less commonly used for drug development. There is little doubt that the availability of experimental animal models of PD has dramatically altered dopaminergic drug treatment of the illness and the prevention and reversal of drug-related side effects that emerge with disease progression and chronic medication. However, so far, we have made little progress in moving into other pharmacological areas for the treatment of PD, and we have not developed models that reflect the progressive nature of the illness and its complexity in terms of the extent of pathology and biochemical change. Only when this occurs are we likely to make progress in developing agents to stop or slow the disease progression. The overarching question that draws all of these models together in the quest for better drug treatments for PD is how well do they recapitulate the human condition and how predictive are they of successful translation of drugs into the clinic? This article aims to clarify the current position and highlight the strengths and weaknesses of available models.

Parkinson's disease risk from ambient exposure to pesticides

Due to the heavy and expanding agricultural use of neurotoxic pesticides suspected to affect dopaminergic neurons, it is imperative to closely examine the role of pesticides in the development of Parkinson's disease (PD). We focus our investigation on pesticide use in California's heavily agricultural central valley by utilizing a unique pesticide use reporting system. From 2001 to 2007, we enrolled 362 incident PD cases and 341 controls living in the Central Valley of California. Employing our geographic information system model, we estimated ambient exposures to the pesticides ziram, maneb, and paraquat at work places and residences from 1974 to 1999. At workplaces, combined exposure to ziram, maneb, and paraquat increased risk of PD three-fold (OR: 3.09; 95% CI: 1.69, 5.64) and combined exposure to ziram and paraquat, excluding maneb exposure, was associated with a 80% increase in risk (OR:1.82; 95% CI: 1.03, 3.21). Risk estimates for ambient workplace exposure were greater than for exposures at residences and were especially high for younger onset PD patients and when exposed in both locations. Our study is the first to implicate ziram in PD etiology. Combined ambient exposure to ziram and paraquat as well as combined ambient exposure to maneb and paraquat at both workplaces and residences increased PD risk substantially. Those exposed to ziram, maneb, and paraquat together experienced the greatest increase in PD risk. Our results suggest that pesticides affecting different mechanisms that contribute to dopaminergic neuron death may act together to increase the risk of PD considerably.

Maneb and paraquat-mediated neurotoxicity: involvement of peroxiredoxin/thioredoxin system

Epidemiological and in vivo studies have demonstrated that exposure to the pesticides paraquat (PQ) and maneb (MB) increase the risk of developing Parkinson's disease (PD) and cause dopaminergic cell loss, respectively. PQ is a well-recognized cause of oxidative toxicity; therefore, the purpose of this study was to determine if MB potentiates oxidative stress caused by PQ, thus providing a mechanism for enhanced neurotoxicity by the combination. The results show that PQ alone at a moderately toxic dose (20-30% cell death in 24 h) caused increased reactive oxygen species (ROS) generation, oxidation of mitochondrial thioredoxin-2 and peroxiredoxin-3, lesser oxidation of cytoplasmic thioredoxin-1 and peroxiredoxin-1, and no oxidation of cellular GSH/GSSG. In contrast, MB alone at a similar toxic dose resulted in no ROS generation, no oxidation of thioredoxin and peroxiredoxin, and an increase in cellular GSH after 24 h. Together, MB increased GSH and inhibited ROS production and thioredoxin/peroxiredoxin oxidation observed with PQ alone, yet resulted in more extensive (> 50%) cell death. MB treatment resulted in increased abundance of nuclear Nrf2 and mRNA for phase II enzymes under the control of Nrf2, indicating activation of cell protective responses. The results show that MB potentiation of PQ neurotoxicity does not occur by enhancing oxidative stress and suggests that increased toxicity occurs by a combination of divergent mechanisms, perhaps involving alkylation by MB and oxidation by PQ.

Glycogen synthase kinase 3β and its phosphorylated form (Y216) in the paraquat-induced model of parkinsonism

Parkinson's disease is a slowly progressing disease, due to a lesion of dopaminergic neurons in the substantia nigra and a dramatic loss of dopamine in the striatum. It is now accepted that several environmental agents including the herbicide paraquat (PQ) may contribute to its pathogenesis. However, till now nothing is known about the role of glycogen synthase kinase-3β (GSK-3β) in the PQ toxicity. Therefore, the aim of this study was to examine the influence of 37-week administration of PQ in rats on the immunohistochemically measured levels of the total GSK-3β and its active, tyrosine 216 (pY216)-phosphorylated form in subcellular fractions of the midbrain with pons, as well as of the striatum. The present results revealed that the long-term PQ administration increased the levels of total and active forms of GSK-3β in the midbrain with pons, whereas decreased them in the striatum. Examination of the lesion extent showed a decrease in the number of tyrosine-immunoreactive neurons in the substantia nigra pars compacta, ventral tegmental area, and locus coeruleus, as well as lower DOPAC/dopamine ratio and noradrenaline level in the striatum in rats treated with PQ. The long-term PQ administration disturbed also motor activity of rats. Summarizing, the present data indicate that the long-term exposure of rats to PQ, a commonly used herbicide, diversely alters levels of GSK-3β in different brain structures, which may be associated with their vulnerability to its toxicity.

Development and validation of a cellular biosensor detecting pesticide residues in tomatoes

Two of the most important categories of pesticides used in agricultural practice are organophosphates and dithiocarbamates. Their extensive and inappropriate use has rendered their reliable monitoring at trace levels more and more necessary. This study presents the construction of a rapid and sensitive cellular biosensor test based on the measurement of changes of the cell membrane potential of immobilized cells, according to the working principle of the Bioelectric Recognition Assay (BERA). The cells were immobilized by entrapment in a sodium alginate bead and directly applied in different pesticide dilutions and agricultural samples. The pesticides used were the organophosphate insecticide diazinon and the dithiocarbamate fungicide propineb. Two different cell types, N2a (neuroblastoma) and Vero (fibroblast) were used as the biosensory elements in order to investigate their differential response against the pesticides. In this way, we hoped to increase the selectivity of the assay. Based on the observed patterns of response, we demonstrate that the sensor can be used for the qualitative and, in some concentrations, quantitative detection of the pesticides with a high degree of reproducibility. The lowest detected concentration was 3nM. Finally, for the investigation of the effects of different pesticides on the accumulation of cytosolic Ca(2+), we conducted a fluorescent assay on N2a cells treated with tomato sample extracts, which were replicates of the E.U. proficiency test sample. The tomato samples were either organically grown or contained 14 different pesticides. The experimental results showed a higher increase of the intracellular Ca(2+) concentration in cells treated with non-organic samples compared to the cells treated with organic samples.

Toxicokinetics and toxicodynamics of paraquat accumulation in mouse brain

Paraquat (PQ) is a potential human neurotoxicant and is used in models of oxidative stress. We determined the toxicokinetics (TK) and toxicodynamics (TD) of PQ in adult mouse brain following repeated or prolonged PQ exposure. PQ accumulated in different brain regions and reached a plateau after approximately 18 i.p. (10 mg/kg) doses and resulted in modest morbidity and mortality unpredictably associated with dose interval and number. PQ had divergent effects on horizontal locomotor behavior depending on the number of doses. PQ decreased striatal dopamine levels after the 18th to 36th i.p. dose (10 mg/kg) and reduced the striatal level of tyrosine hydroxylase. Drinking water exposure to PQ (0.03- 0.05 mg/ml) did not result in any mortality and resulted in concentration and time dependent levels in the brain. The brain half-life of PQ varied with mouse strain. PQ accumulates and may saturate a site in mouse brain resulting in complex PQ level and duration-related consequences. These findings should alter our risk assessment of this compound and demonstrate a useful, but complex dynamic model for understanding the consequences of PQ in the brain.

Role of CYP2E1 in the mouse model of MPTP toxicity

It has been shown that diethyldithiocarbamate (DDC) potentiates 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) toxicity in mice as a result of increased levels of 1-methyl-4-phenylpyridinium ion (MPP(+)) in the striatum. Brain CYP2E1 inhibition by DDC in C57Bl mice was responsible for increased toxicity and striatal MPP(+) accumulation. However, CYP2E1-null mice did not show any enhanced sensitivity to MPTP or any MPP(+) accumulation. This unexpected finding suggested that the CYP2E1-null mice compensate with other isozymes as already described for acetaminophen-induced liver damage. MPP(+) intoxication of mesencephalic cell cultures from CYP2E1-null mice indicated a reduced sensitivity of dopaminergic (DA) neurons from knockout animals. Surprisingly, MPP(+) cell distribution under these conditions indicated that the toxin accumulates more intracellularly in knockout cultures, suggesting further that CYP2E1 has a role in MPP(+) storage and efflux.

Role of reactive oxygen species in the neurotoxicity of environmental agents implicated in Parkinson's disease

Among age-related neurodegenerative diseases, Parkinson's disease (PD) represents the best example for which oxidative stress has been strongly implicated. The etiology of PD remains unknown, yet recent epidemiological studies have linked exposure to environmental agents, including pesticides, with an increased risk of developing the disease. As a result, the environmental hypothesis of PD has developed, which speculates that chemical agents in the environment are capable of producing selective dopaminergic cell death, thus contributing to disease development. The use of environmental agents such as 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, rotenone, paraquat, dieldrin, and maneb in toxicant-based models of PD has become increasingly popular and provided valuable insight into the neurodegenerative process. Understanding the unique and shared mechanisms by which these environmental agents act as selective dopaminergic toxicants is critical in identifying pathways involved in PD pathogenesis. In this review, we discuss the neurotoxic properties of these compounds with specific focus on the induction of oxidative stress. We highlight landmark studies along with recent advances that support the role of reactive oxygen and reactive nitrogen species from a variety of cellular sources as potent contributors to the neurotoxicity of these environmental agents. Finally, human risk and the implications of these studies in our understanding of PD-related neurodegeneration are discussed.

Parkinson's disease and pesticides: a toxicological perspective

Environmental factors have been shown to contribute to the incidence of Parkinson's disease (PD). Pesticides, which represent one of the primary classes of environmental agents associated with PD, share the common feature of being intentionally released into the environment to control or eliminate pests. Pesticides consist of multiple classes and subclasses of insecticides, herbicides, rodenticides, fungicides, fumigants and others and exhibit a vast array of chemically diverse structures. In this review we examine the evidence regarding the ability of each of the major pesticide subclasses to increase the incidence of PD. We propose that, from a toxicological perspective, it would be beneficial to identify specific subclasses, common structural features and the propensity for widespread human exposure when considering the potential role in PD, rather than using the overly broad term of 'pesticides' to describe this diverse group of chemicals. Furthermore, these chemicals and their environmentally relevant combinations should be evaluated for their ability to promote or accelerate PD and not merely for being singular causative agents.

Paraquat induces apoptosis in human lymphocytes: protective and rescue effects of glucose, cannabinoids and insulin-like growth factor-1

In order to establish causal or protective treatments for Parkinson's disease (PD), it is necessary to identify the cascade of deleterious events that lead to the dysfunction and death of dopaminergic neurons. Paraquat (PQ) is a pesticide used as xenobiotic compound to model PD. However, the mechanism(s) of PQ-induced cell death and the mechanism(s) of cytoprotection in a single cell model are still unknown. In this study, lymphocytes were treated with (0.1-1 mM) PQ. Apoptotic morphology was assessed with acridine orange/ethidium bromide staining. Further evaluation included (i) superoxide radicals, reflected by nitroblue tetrazolium reduction to formazan, (ii) the production of hydrogen peroxide, reflected by rhodamine-positive fluorescent cells, (iii) the generation of hydroxyl radicals, reflected by dimethylsulfoxide and melatonin ( radical)OH scavengers, (iv) activation and/or translocation of NF-kappaB, p53 and c-Jun transcription factors showed by immunocytochemical staining, and by ammonium pyrrolidinedithiocarbamate, pifithrin-alpha and SP600125 inhibition and (V) caspase-3 activation, reflected by caspase Ac-DEVD-cho inhibition. To elucidate the mechanism of cytoprotection, lymphocytes were treated with PQ in the presence of cannabinoids, insulin-like growth factor-1 and glucose. We provide evidence that PQ induces apoptosis in lymphocytes in a concentration- and time-dependent fashion by an oxidative stress mechanism involving O(2)( radical - ), H(2)O(2)/(( radical)OH) generation, simultaneous activation of NF-kappaB/p53/c-Jun transcription factors, mitochondrial depolarization and caspase-3 activation leading to morphological apoptosis. Moreover, dying lymphocytes are protected and rescued from PQ noxious stimuli by direct antioxidant effect by cannabinoids, receptor mediated signaling by IGF-1, and/or energetic protection by glucose. It is concluded that PQ-induced apoptosis in lymphocytes by a mechanism involving reactive oxygen species generation, mitochondrial dysfunction, transcriptional factors and caspase-3 activation. However, this cell death routine can be reversed by the action of cannabinoids, IGF-1 and glucose. These data may provide innovating therapeutic strategies to intervene environmentally or genetically susceptible PD population to oxidative stress.

Prolonged toxicokinetics and toxicodynamics of paraquat in mouse brain

BACKGROUND

Paraquat (PQ) has been implicated as a risk factor for the Parkinson disease phenotype (PDP) in humans and mice using epidemiologic or experimental approaches. The toxicokinetics (TK) and toxicodynamics (TD) of PQ in the brain are not well understood.

OBJECTIVES

The TK and TD of PQ in brain were measured after single or repeated doses.

METHODS

Brain regions were analyzed for PQ levels, amount of lipid peroxidation, and functional activity of the 20S proteasome.

RESULTS

Paraquat (10 mg/kg, ip) was found to be persistent in mouse ventral midbrain (VM) with an apparent half-life of approximately 28 days and was cumulative with a linear pattern between one and five doses. PQ was also absorbed orally with a concentration in brain rising linearly after single doses between 10 and 50 mg/kg. The level of tissue lipid peroxides (LPO) was differentially elevated in three regions, being highest in VM, lower in striatum (STR), and least in frontal cortex (FCtx), with the earliest significant elevation detected at 1 day. An elevated level of LPO was still present in VM after 28 days. Despite the cumulative tissue levels of PQ after one, three, and five doses, the level of LPO was not further increased. The activity of the 20S proteasome in the striatum was altered after a single dose and reduced after five doses.

CONCLUSIONS

These data have implications for PQ as a risk factor in humans and in rodent models of the PDP.

Toxic influence of subchronic paraquat administration on dopaminergic neurons in rats

Paraquat is a toxin suggested to contribute to pathogenesis of Parkinson's disease. The aim of the present study was to examine toxic influence of subchronic treatment with this pesticide (5 days, one injection per day, 2-3 days of withdrawal) on dopaminergic, serotonergic, noradrenergic and GABAergic neurons. Paraquat decreased the number of tyrosine hydroxylase-immunoreactive (TH-ir) neurons in the substantia nigra by 22% (measured 3 days after withdrawal). Two days after withdrawal the levels of the dopamine metabolites and dopamine turnover in the caudate-putamen, substantia nigra and prefrontal cortex were reduced by ca. 20-60%, and the binding of [(3)H]GBR 12,935 to dopamine transporter dropped by 25-40% in the caudate-putamen. Three days after paraquat withdrawal, the level of dopamine in the caudate-putamen was significantly increased, and earlier decreases in DOPAC and HVA in the substantia nigra, as well as [(3)H]GBR 12,935 binding in the caudate-putamen were reversed. Moreover, an increase in serotonin turnover in the caudate-putamen and prefrontal cortex, and noradrenaline level in the former structure was observed 2-3 days after paraquat withdrawal. Three days after the last paraquat injection 24-35% decreases in the proenkephalin mRNA levels and 5-7% reduction in glutamic acid decarboxylase (GAD)67 mRNA were found in the caudate-putamen. The present study suggests that subchronic paraquat administration triggers processes characteristic of early stages of dopaminergic neuron degeneration, and activates compensatory mechanisms involving dopaminergic, noradrenergic, serotonergic and GABAergic transmissions.

Pesticide exposure and risk for Parkinson's disease

OBJECTIVE

Chronic, low-dose exposure to pesticides is suspected to increase the risk for Parkinson's disease (PD), but data are inconclusive.

METHODS

We prospectively examined whether individuals exposed to pesticides have higher risk for PD than those not exposed. The study population comprised participants in the Cancer Prevention Study II Nutrition Cohort, a longitudinal investigation of US men and women initiated in 1992 by the American Cancer Society. Follow-up surveys were conducted in 1997, 1999, and 2001. The 143,325 individuals who returned the 2001 survey and did not have a diagnosis or symptoms of PD at baseline (1992) were included in the analyses.

RESULTS

Exposure to pesticides was reported by 7,864 participants (5.7%), including 1,956 farmers, ranchers, or fishermen. Individuals exposed to pesticides had a 70% higher incidence of PD than those not exposed (adjusted relative risk, 1.7; 95% confidence interval, 1.2-2.3; p = 0.002). The relative risk for pesticide exposure was similar in farmers and nonfarmers. No relation was found between risk for PD and exposure to asbestos, chemical/acids/solvents, coal or stone dust, or eight other occupational exposures.

INTERPRETATION

These data support the hypothesis that exposure to pesticides may increase risk for PD. Future studies should seek to identify the specific chemicals responsible for this association.

Paraquat exposure as an etiological factor of Parkinson's disease

Parkinson's disease (PD) is a multifactorial chronic progressive neurodegenerative disease influenced by age, and by genetic and environmental factors. The role of genetic predisposition in PD has been increasingly acknowledged and a number of relevant genes have been identified (e.g., genes encoding alpha-synuclein, parkin, and dardarin), while the search for environmental factors that influence the pathogenesis of PD has only recently begun to escalate. In recent years, the investigation on paraquat (PQ) toxicity has suggested that this herbicide might be an environmental factor contributing to this neurodegenerative disorder. Although the biochemical mechanism through which PQ causes neurodegeneration in PD is not yet fully understood, PQ-induced lipid peroxidation and consequent cell death of dopaminergic neurons can be responsible for the onset of the Parkinsonian syndrome, thus indicating that this herbicide may induce PD or influence its natural course. PQ has also been recently considered as an eligible candidate for inducing the Parkinsonian syndrome in laboratory animals, and can therefore constitute an alternative tool in suitable animal models for the study of PD. In the present review, the recent evidences linking PQ exposure with PD development are discussed, with the aim of encouraging new perspectives and further investigation on the involvement of environmental agents in PD.

Degeneration of dopaminergic mesocortical neurons and activation of compensatory processes induced by a long-term paraquat administration in rats: implications for Parkinson's disease

A deficiency of the dopaminergic transmission in the mesocortical system has been suggested to contribute to cognitive disturbances in Parkinson's disease. Therefore, the aim of the present study was to examine whether the long-term administration of a commonly used herbicide, paraquat, which has already been found to induce a slowly progressing degeneration of the nigrostriatal neurons, influences mesocortical dopaminergic neurons in rats. Paraquat at a dose of 10 mg/kg i.p. was injected either acutely or once a week for 4, 8, 12 and 24 weeks. Acute treatment with this pesticide increased the level of homovanillic acid (HVA) and HVA/dopamine ratio in the prefrontal cortex. After 8 weeks of administration paraquat increased the number of stereologically counted tyrosine hydroxylase-immunoreactive (TH-ir) neurons and their staining intensity in the ventral tegmental area (VTA), which is a source of the mesocortical dopaminergic projection. At the same time, few TH-ir neurons appeared in different regions of the cerebral cortex: in the frontal, cingulate, retrosplenial and parietal cortices. Chronic paraquat administration did not influence the level of dopamine in the prefrontal cortex but increased the levels of its metabolites: 3,4-dihydroxyphenylacetic acid (after 8-12 weeks), HVA (after 4 and 12 weeks) and HVA/dopamine ratio (4 weeks). After 24 weeks this pesticide reduced the number of TH-ir neurons in the VTA by 42% and of the Nissl-stained neurons by 26%, and induced shrinkage of this structure by ca. 25%. Moreover, TH-ir neurons in the cortex were no more visible after such a long period of administration and levels of dopamine metabolites returned to control values. The present results suggest that the long-term paraquat administration destroys dopaminergic neurons of the VTA. However, compensatory activation of the VTA neurons and cortex overcomes progressing degeneration and maintains cortical dopaminergic transmission.

Pesticides and Parkinson's disease--is there a link?

Parkinson's disease (PD) is an idiopathic disease of the nervous system characterized by progressive tremor, bradykinesia, rigidity, and postural instability. It has been postulated that exogenous toxicants, including pesticides, might be involved in the etiology of PD. In this article we present a comprehensive review of the published epidemiologic and toxicologic literature and critically evaluate whether a relationship exists between pesticide exposure and PD. From the epidemiologic literature, there does appear to be a relatively consistent relationship between pesticide exposure and PD. This relationship appears strongest for exposure to herbicides and insecticides, and after long durations of exposure. Toxicologic data suggest that paraquat and rotenone may have neurotoxic actions that potentially play a role in the development of PD, with limited data for other pesticides. However, both the epidemiology and toxicology studies were limited by methodologic weaknesses. Particular issues of current and future interest include multiple exposures (both pesticides and other exogenous toxicants), developmental exposures, and gene-environment interactions. At present, the weight of evidence is sufficient to conclude that a generic association between pesticide exposure and PD exists but is insufficient for concluding that this is a causal relationship or that such a relationship exists for any particular pesticide compound or combined pesticide and other exogenous toxicant exposure.

Acute manganese administration alters dopamine transporter levels in the non-human primate striatum

We used positron emission tomography (PET) to measure non-invasively the effect of acute systemic administration to manganese sulfate (MnSO4) on dopamine transporter (DAT) levels in the living non-human primate brain. Baboons received [11C]-WIN 35,428 PET scans to measure DAT levels before and after acute MnSO4 administration. In one animal, we observed a 46% increase in DAT binding potential (BP), a measure of DAT binding site availability, 1 week after Mn administration. DAT levels returned to baseline values at 4 months and remained constant at 10 months after treatment. A subsequent single MnSO4 injection to the same animal also resulted in a 57% increase in DAT-BP, 2 days after administration. In a second animal, a 76% increase in DAT-BP relative to baseline was observed at 3 days after Mn injection. In this animal, the DAT-BP returned to baseline levels after 1 month. Using in vitro receptor binding assays, we found that Mn inhibits [3H]-WIN 35,428 binding to rat striatal DAT with an inhibitory constant (Ki) of 2.0+/-0.3mM (n=4). Saturation isotherms and Scatchard analysis of [3H]-WIN 35,428 binding to rat striatal DAT showed a significant decrease (30%, p<0.001) in the maximal number of binding sites (Bmax) in the presence of 2mM MnSO4. No significant effect of Mn was found on binding affinity (Kd). We also found that Mn inhibits [3H]-dopamine uptake with an IC50 of 11.4+/-1.5mM (n=4). Kinetic studies and Lineweaver-Burk analysis showed a significant decrease (40%, p<0.001) in the maximal velocity of uptake (Vmax) with 5mM MnSO4. No significant effect of Mn was found on Michaelis-Menten constant (Km). These in vitro findings suggest that the increase in DAT levels in vivo following acute Mn administration may be a compensatory response to its inhibitory action on DAT. These findings provide helpful insights on potential mechanisms of Mn-induced neurotoxicity and indicate that the DAT in the striatum is a target for Mn in the brain.

Effects of prenatal paraquat and mancozeb exposure on amino acid synaptic transmission in developing mouse cerebellar cortex

The goal of this study was to analyze the effects of prenatal exposure to the pesticides paraquat (PQ) and mancozeb (MZ) on the development of synaptic transmission in mouse cerebellar cortex. Pregnant NMRI mice were treated with either saline, 10 mg/kg PQ, 30 mg/kg MZ or the combination of PQ + MZ, between gestational days 12 (E12) and E20. Variation in the levels of amino acid neurotransmitters was determined by HPLC, between postnatal day 1 (P1) and P30. Motor coordination was assessed by locomotor activity evaluation of control and experimental pups at P14, P21 and P30. Significant reductions in the levels of excitatory neurotransmitters, aspartate and glutamate, were observed in PQ-, MZ- or combined PQ + MZ-exposed pups, with respect to control, during peak periods of excitatory innervation of Purkinje cells: between P2-P5 and P11-P15. However, at P30, lower aspartate contents, in contrast with increased glutamate levels, were detected in all experimental groups. During the first two postnatal weeks, delays in GABA and glycine ontogenesis were observed in PQ- and PQ + MZ-exposed pups, whereas notable decrements in GABA and glycine levels were seen in PQ + MZ-exposed animals. Decreased taurine contents were detected at P3 and P11 in PQ- and PQ + MZ-exposed mice. Pups in different experimental groups all showed hyperactivity at P14 and then exhibited reduced locomotor activity at P30. Taken together, our results indicate that prenatal exposure to either PQ or MZ or the combination of both could alter the chronology and magnitude of synaptic transmission in developing mouse cerebellar cortex.

Evaluation of Epidemiologic and Animal Data Associating Pesticides With Parkinson???s Disease

Exposure to pesticides may be a risk factor for developing Parkinson's disease (PD). To evaluate the evidence regarding this association in the scientific literature, we examined both analytic epidemiologic studies of PD cases in which exposure to pesticides was queried directly and whole-animal studies for PD-like effects after systemic pesticide exposure. Epidemiologic studies were considered according to study quality parameters, and results were found to be mixed and without consistent exposure-response or pesticide-specific patterns. These epidemiologic studies were limited by a lack of detailed and validated pesticide exposure assessment. In animal studies, no pesticide has yet demonstrated the selective set of clinical and pathologic signs that characterize human PD, particularly at levels relevant to human populations. We conclude that the animal and epidemiologic data reviewed do not provide sufficient evidence to support a causal association between pesticide exposure and PD.

A slowly developing dysfunction of dopaminergic nigrostriatal neurons induced by long-term paraquat administration in rats: an animal model of preclinical stages of Parkinson's disease?

The aim of the present study was to examine the influence of the long-term paraquat administration on the dopaminergic nigrostriatal system in rats. Paraquat was injected at a dose of 10 mg/kg i.p. for 4-24 weeks. We found that this pesticide reduced the number of tyrosine hydroxylase-immunoreactive neurons of the substantia nigra; after the 4-week treatment the reduction (17%, nonsignificant) was confined to the rostrocentral region of this structure but, after 24 weeks, had spread along its whole length and was approximately 37%. Moreover, it induced a biphasic effect on dopaminergic transmission. First, levels of dopamine, its metabolites and turnover were elevated (4-8 weeks) in the caudate-putamen, then all these parameters returned to control values (12 weeks) and dropped by 25-30% after 24 weeks. The binding of [3H]GBR 12,935 to dopamine transporter in the caudate-putamen was decreased after 4-8 weeks, then returned to control values after 12 weeks but was again decreased after 24 weeks. Twenty-four-week paraquat administration also decreased the level of tyrosine hydroxylase (Western blot) in the caudate-putamen. In addition, paraquat activated serotonin and noradrenaline transmission during the first 12 weeks of treatment but no decreases in levels of these neurotransmitters were observed after 24 weeks. The above results seem to suggest that long-term paraquat administration produces a slowly progressing degeneration of nigrostriatal neurons, leading to delayed deficits in dopaminergic transmission, which may resemble early, presymptomatic, stages of Parkinson's disease.

Paraquat neurotoxicity is distinct from that of MPTP and rotenone

Paraquat, MPTP, and rotenone reproduce features of Parkinson's disease (PD) in experimental animals. The exact mechanisms by which these compounds damage the dopamine system are not firmly established, but selective damage to dopamine neurons and inhibition of complex I are thought to be involved. We and others have previously documented that the toxic metabolite of MPTP, MPP+, is transported into dopamine neurons through the dopamine transporter (DAT), while rotenone is not transported by DAT. We have also demonstrated the requirement for complex I inhibition and oxidative damage in the dopaminergic neurodegeneration produced by rotenone. Based on structural similarity to MPP+, it has been proposed that paraquat exerts selective dopaminergic toxicity through transport by the DAT and subsequent inhibition of mitochondrial complex I. In this study we report that paraquat is neither a substrate nor inhibitor of DAT. We also demonstrate that in vivo exposure to MPTP and rotenone, but not paraquat, inhibits binding of 3H-dihydrorotenone to complex I in brain mitochondria. Rotenone and MPP+ were both effective inhibitors of complex I activity in isolated brain mitochondria, while paraquat exhibited weak inhibitory effects only at millimolar concentrations. These data indicate that, despite the apparent structural similarity to MPP+, paraquat exerts its deleterious effects on dopamine neurons in a manner that is unique from rotenone and MPTP.

Developmental pesticide models of the Parkinson disease phenotype

It has been hypothesized that developmental insults could contribute to Parkinson disease (PD), a neurodegenerative disorder resulting from the loss of the dopamine neurons of the nigrostriatal pathway. Two models of developmental pesticide exposures in mice are presented here that yield PD phenotypes consistent with this possibility. Combined exposures to the herbicide paraquat (PQ) and the fungicide maneb (MB), both of which adversely affect dopamine systems, administered from postnatal days 5-19, produced selective losses of dopamine and metabolites and reduced numbers of dopamine neurons in the substantia nigra. Effects were greater than those produced by adult-only exposures. Moreover, developmental PQ + MB exposures enhanced vulnerability to this pesticide regimen when administered subsequently in adulthood. In a second model, exposure to MB from gestational days 10-17 markedly increased vulnerability to PQ exposures during adulthood, with reductions in dopamine and metabolites and numbers of dopamine neurons in the substantia nigra. Females evidenced protection in both models. Collectively, these models demonstrate that developmental exposures can produce progressive, permanent, and cumulative neurotoxicity of the nigrostriatal dopamine system and enhance vulnerability to subsequent environmental insults. Finally, effects of PQ + MB were greater than those of either pesticide alone in the postnatal model. This is consistent with a multiple-hit hypothesis predicting that multiple concurrent insults occurring at different target sites within a system (here nigrostriatal dopamine) may constrict the range and flexibility of compensatory mechanisms, thereby compromising the integrity and viability of the system. As such, this hypothesis presents a biologic strategy for identifying potentially significant neurotoxic mixtures for hazard identification in future studies.

Role of oxidative stress in paraquat-induced dopaminergic cell degeneration

Systemic treatment of mice with the herbicide paraquat causes the selective loss of nigrostriatal dopaminergic neurons, reproducing the primary neurodegenerative feature of Parkinson's disease. To elucidate the role of oxidative damage in paraquat neurotoxicity, the time-course of neurodegeneration was correlated to changes in 4-hydroxy-2-nonenal (4-HNE), a lipid peroxidation marker. When mice were exposed to three weekly injections of paraquat, no nigral dopaminergic cell loss was observed after the first administration, whereas a significant reduction of neurons followed the second exposure. Changes in the number of nigral 4-HNE-positive neurons suggest a relationship between lipid peroxidation and neuronal death, since a dramatic increase in this number coincided with the onset and development of neurodegeneration after the second toxicant injection. Interestingly, the third paraquat administration did not cause any increase in 4-HNE-immunoreactive cells, nor did it produce any additional dopaminergic cell loss. Further evidence of paraquat-induced oxidative injury derives from the observation of nitrotyrosine immunoreactivity in the substantia nigra of paraquat-treated animals and from experiments with ferritin transgenic mice. These mice, which are characterized by a decreased susceptibility to oxidative stress, were completely resistant to the increase in 4-HNE-positive neurons and the cell death caused by paraquat. Thus, paraquat exposure yields a model that emphasizes the susceptibility of dopaminergic neurons to oxidative damage.

[WITHDRAWN] Overexpression of superoxide dismutase or glutathione peroxidase protects against the paraquat + maneb-induced Parkinson disease phenotype

Oxidative stress has been implicated in the pathogenesis of Parkinson disease based on its role in the cascade of biochemical changes that lead to dopaminergic neuronal death. This study analyzed the role of oxidative stress as a mechanism of the dopaminergic neurotoxicity produced by the combined paraquat and maneb model of the Parkinson disease phenotype. Transgenic mice overexpressing either Cu,Zn superoxide dismutase or intracellular glutathione peroxidase and non-transgenic mice were exposed to saline, paraquat, or the combination of paraquat + maneb twice a week for 9 weeks. Non-transgenic mice chronically exposed to paraquat + maneb exhibited significant reductions in locomotor activity, levels of striatal dopamine and metabolites, and dopaminergic neurons in the substantia nigra pars compacta. In contrast, no corresponding effects were observed in either Cu,Zn superoxide dismutase or glutathione peroxidase transgenic mice. Similarly, the increase in levels of lipid hydroperoxides in the midbrain and striatum of paraquat + maneb-treated non-transgenic mice was not detected in either Cu,Zn superoxide dismutase or glutathione peroxidase transgenic mice. To begin to determine critical pathways of paraquat + maneb neurotoxicity, the functions of cell death-inducing and protective mechanisms were analyzed. Even a single injection of paraquat + maneb in the non-transgenic treated group modulated several key pro- and anti-apoptotic proteins, including Bax, Bad, Bcl-xL, and upstream stress-induced cascade. Collectively, these findings support the assertion that protective mechanisms against paraquat + maneb-induced neurodegeneration could involve modulation of the level of reactive oxygen species and alterations of the functions of specific signaling cascades.

The role of microglia in paraquat-induced dopaminergic neurotoxicity

The herbicide paraquat (PQ) has been implicated as a potential risk factor for the development of Parkinson's disease. In this study, PQ (0.5-1 microM) was shown to be selectively toxic to dopaminergic (DA) neurons through the activation of microglial NADPH oxidase and the generation of superoxide. Neuron-glia cultures exposed to PQ exhibited a decrease in DA uptake and a decline in the number of tyrosine hydroxylase-immunoreactive cells. The selectivity of PQ for DA neurons was confirmed when PQ failed to alter gamma-aminobutyric acid uptake in neuron-glia cultures. Microglia-depleted cultures exposed to 1 microM PQ failed to demonstrate a reduction in DA uptake, identifying microglia as the critical cell type mediating PQ neurotoxicity. Neuron-glia cultures treated with PQ failed to generate tumor necrosis factor-alpha and nitric oxide. However, microglia-enriched cultures exposed to PQ produced extracellular superoxide, supporting the notion that microglia are a source of PQ-derived oxidative stress. Neuron-glia cultures from NADPH oxidase-deficient (PHOX-/-) mice, which lack the functional catalytic subunit of NADPH oxidase and are unable to produce the respiratory burst, failed to show neurotoxicity in response to PQ, in contrast to PHOX+/+ mice. Here we report a novel mechanism of PQinduced oxidative stress, where at lower doses, the indirect insult generated from microglial NADPH oxidase is the essential factor mediating DA neurotoxicity.