Effects of paraquat on mitochondrial electron transport system and catecholamine contents in rat brain

Paraquat-induced apoptosis in human neuroblastoma SH-SY5Y cells: involvement of p53 and mitochondria

The herbicide paraquat is a suspected etiologic factor in the development of Parkinson's disease (PD). Paraquat was therefore used to reproduce Parkinsonian syndromes in lab animals, in which it produces dopaminergic pathogenesis. However, the factors or mechanisms by which paraquat kills dopaminergic neurons are not fully understood. Based on reported evidence that paraquat increases p53 protein levels and inhibits mitochondrial function, it was hypothesized that paraquat induces cell death in dopaminergic neurons through a mechanism in which p53 and mitochondrial apoptotic pathway are linked. To explore this possibility, dopaminergic SY5Y cells were treated with paraquat for 48 h and p53 responses were investigated, as well as biomarkers of the mitochondrial intrinsic pathway of apoptosis. Paraquat significantly increased protein levels of p53 and one of its target genes, Bax. By 24 h, paraquat decreased mitochondrial complex I activity and mitochondrial transmembrane potential and induced the release of cytochrome c from mitochondria. In addition, paraquat increased the activities of caspases 9 and 3. Finally, nuclear condensation and DNA fragmentation occurred 48 h after treatment. The decrease of mitochondrial functions, the release of cytochrome c, the increase of caspase 9 and 3 activities, and DNA damage that were produced by paraquat were inhibited by a specific p53 inhibitor, pifithrin-alpha. These findings support the conclusion that paraquat produced apoptosis in SY5Y cells through the mitochondrial intrinsic pathway associated with p53.

The bipyridyl herbicide paraquat induces proteasome dysfunction in human neuroblastoma SH-SY5Y cells

Paraquat (PQ) is suspected to be an environmental risk factor for Parkinson's disease (PD). A strong correlation between exposure to paraquat and the occurrence of PD was reported in Canada, Taiwan, and the United States. This correlation is supported by in vivo work showing that paraquat produces dopaminergic pathogenesis. In particular, paraquat forms abnormal protein aggregates in dopaminergic neurons of mice. However, it is not clear how paraquat produces this pathology. Given that proteasome dysfunction induces aberrant protein aggregation, it was hypothesized that paraquat induces proteasome dysfunction. To explore this possibility, proteasome activity and some factors possibly contributing to proteasome dysfunction were investigated in dopaminergic SY5Y cells treated with paraquat. Furthermore, levels of alpha-synuclein and ubiquitin-conjugated proteins were measured to test whether paraquat induces protein accumulation in SY5Y cells. Results showed that at a concentration of paraquat that reduced viability by about 60% at 48 h (0.5 mM) loss of proteasome activity occurred. In addition, the cells showed decreased ATP levels and reduced mitochondrial complex V activity. These changes were significant 24 h after treatment with paraquat. Furthermore, paraquat-treated cells showed decreased protein levels of proteasome 19S subunits, but not 20S alpha or beta subunits, suggesting that the effects observed were not the result of general cytotoxicity. Paraquat also increased levels of alpha-synuclein and ubiquitinated proteins, suggesting that paraquat-induced proteasome dysfunction leads to aberrant protein accumulation. Taken together, these findings support the hypothesis that paraquat impairs proteasome function in SY5Y cells.

Divergent Mechanisms of Paraquat, MPP+, and Rotenone Toxicity: Oxidation of Thioredoxin and Caspase-3 Activation

Paraquat, N-methyl-4-phenyl-1,2,3,6 tetrahydropyridine, and rotenone have been shown to reproduce several features of Parkinson's disease in animal and cell culture models. Although these chemicals are known to perturb dopamine homeostasis and induce dopaminergic cell death, their molecular mechanisms of action are not well defined. We have previously shown that paraquat does not require functional dopamine transporter and does not inhibit mitochondrial complex I in order to mediate its toxic action (Richardson et al., 2005). In this study, we show that paraquat specifically oxidized the cytosolic form of thioredoxin and activated Jun N-terminal kinase (JNK), followed by caspase-3 activation. Conversely, 1-methyl-4-phenylpyridinium (MPP(+)) and rotenone oxidized the mitochondrial form of thioredoxin but did not activate JNK-mitogen-activated protein kinase and caspase-3. Loading cells with exogenous dopamine did not exacerbate the toxicity of any of these compounds. These data suggest that oxidative modification of cytosolic proteins is critical to paraquat toxicity, while oxidation of mitochondrial proteins is important for MPP(+) and rotenone toxicity. In addition, intracellular dopamine does not seem to exacerbate the toxicity of these dopaminergic neurotoxicants in this model.

Temporal effects of paraquat/maneb on microglial activation and dopamine neuronal loss in older rats

We investigated the effects of combined systemic exposure to the herbicide paraquat (PQ) and the fungicide maneb (MB) in 6-month-old rats, an animal model of Parkinson's disease resulting from environmental toxin exposure. Following two doses of PQ (10 mg/kg) and MB (30 mg/kg), 52% of animals developed fatal lung injury. Examination of the remaining animals showed degeneration of dopaminergic (DA) neurons in the substantia nigra pars compacta 6 weeks, but not 4 weeks, following PQ/MB. In contrast, microglial activation was observed at 4 weeks, but had abated by 6 weeks. Compared with our previous findings in younger rats, these results suggest increased susceptibility of older animals to lung and brain toxicity from PQ/MB exposure. Microglial activation preceded, and therefore likely contributed to, DA neurodegeneration. Further, electron microscopy revealed an abnormal appearance of the Golgi apparatus at 4 weeks that was confirmed using double immunostaining for tyrosine hydroxylase and Golgi. This suggests that PQ/MB causes protein processing dysfunction in nigral DA neurons that may be either a direct effect of PQ/MB or the result of microglial activation.

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.

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.

The bipyridyl herbicide paraquat produces oxidative stress-mediated toxicity in human neuroblastoma SH-SY5Y cells: relevance to the dopaminergic pathogenesis

Paraquat (PQ) is a cationic nonselective bipyridyl herbicide widely used to control weeds and grasses in agriculture. Epidemiologic studies indicate that exposure to pesticides can be a risk factor in the incidence of Parkinson's disease (PD). A strong correlation has been reported between exposure to paraquat and PD incidence in Canada, Taiwan, and the United States. This correlation is supported by animal studies showing that paraquat produces toxicity in dopaminergic neurons of the rat and mouse brain. However, it is unclear how paraquat triggers toxicity in dopaminergic neurons. Based on the prooxidant properties of paraquat, it was hypothesized that paraquat may induce oxidative stress-mediated toxicity in dopaminergic neurons. To explore this possibility, dopaminergic SH-SY5Y cells were treated with paraquat, and several biomarkers of oxidativestress were measured. First, a specific dopamine transporter inhibitor GBR12909 significantly protected SY5Y cells against the toxicity of paraquat, indicating that paraquat exerts its toxicity by a mechanism involving the dopamine transporter (DAT). Second, paraquat increased intracellular levels of reactive oxygen species (ROS), but decreased the levels of glutathione. Third, paraquat inhibited glutathione peroxidase activity, but did not affect glutathione reductase activity. On the other hand, paraquat increased GST activity by 24 h, after which GST activity returned to the control value at 48 h. Fourth, paraquat dissipated mitochondrial transmembrane potential (MTP). Fifth, paraquat produced increases of malondialdehyde (MDA) and protein carbonyls, as well as DNA fragmentation, indicating oxidative damage to major cellular components. Sixth, paraquat increased the protein level of heme oxygenase-1 (HO-1). Taken together, these findings verify our hypothesis that paraquat produces oxidative stress-mediated toxicity in SH-SY5Y cells. Thus, current findings suggest that paraquat may induce the pathogenesis of dopaminergic neurons through oxidative stress.

Neuroprotective effects of Polygonum multiflorum on nigrostriatal dopaminergic degeneration induced by paraquat and maneb in mice

The neuroprotective effects of Polygonum multiflorum extract (PME) and its two fractions, ethanol-soluble PME (PME-I) and -insoluble PME (PME-II), on the degeneration of nigrostriatal dopaminergic neurons induced by a combination of paraquat and maneb (PQMB) were investigated in male C57BL/6 mice. The mice were treated twice a week for 6 weeks with intraperitoneal injections of PQMB. This combination caused a reduction of spontaneous locomotor activity, motor incoordination, and declines of dopamine level in the striatum and tyrosine hydroxylase-positive neurons in the substantia nigra. Administration of PME and PME-I once daily for 47 days during 6 weeks of PQMB treatment and last 8 days after PQMB significantly attenuated the impairment of behavioral performance and the decrease in striatal dopamine level and substantia nigral tyrosine hydroxylase-positive neurons in the PQMB-treated animals, whereas the administration of PME-II had no effect on these behavioral, neurochemical and histological indices. The present findings suggest that PME has a beneficial influence on parkinsonism induced by PQMB and that the effects of PME are attributable to some substance(s) included in the ethanol-soluble fraction of PME (PME-I).

Methoxychlor inhibits brain mitochondrial respiration and increases hydrogen peroxide production and CREB phosphorylation

The organochlorine insecticide methoxychlor (mxc) is an established reproductive toxicant that affects other systems including the central nervous system (CNS), possibly by mechanisms involving oxidative stress. This study tested the hypothesis that mxc inhibits brain mitochondrial respiration, resulting in increased production of reactive oxygen species (ROS). Oxygen electrode measurements of mitochondrial respiration and Amplex Red measurements of H(2)O(2) production were performed with rat brain mitochondria exposed in vitro to mxc (0-10 microg/ml) and with brain mitochondria from mice chronically exposed in vivo to mxc (0-64 mg/kg/day) for 20 days by intraperitoneal injection. In vitro mxc exposure inhibited ADP-dependent respiration (state 3) using both complex I- and II-supported substrates. Similarly, state 3 respiration was inhibited following in vivo mxc exposure using complex I substrates. H(2)O(2) production was stimulated after in vitro mxc treatment in the presence of complex I substrates, but not in mitochondria isolated from in vivo mxc-treated mice. Because previous studies demonstrated a relationship between oxidative stress and CREB phosphorylation, we also tested the hypothesis that mxc elevates phosphorylated CREB (pCREB) in mitochondria. Enzyme-linked immunosorbent assay (ELISA) measurements demonstrated that pCREB immunoreactivity was elevated by in vitro mxc exposure in the presence or absence of respiratory substrates, indicating that stimulation of H(2)O(2) production is not necessary for this effect. These multiple effects of mxc on mitochondria may play an important role in its toxicity, particularly in the CNS.

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.

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.

Modulation of Antioxidant Defense Systems by the Environmental Pesticide Maneb in Dopaminergic Cells

A lack of evidence supporting a role of heritability in the development of idiopathic Parkinson's disease (PD) has implicated exposures to environmental contaminants in the disease etiology. Epidemiological and clinical studies, as well as animal models of the PD phenotype, have consistently linked agrichemical exposure with dopaminergic (DAergic) damage, particularly through oxidative stress mechanisms. Maneb (MB) is a dithiocarbamate (DTC) fungicide that has specifically been implicated to have adverse effects on dopamine (DA) systems, but the role MB plays in modulating the oxidative state of DAergic cells has not previously been described. Since glutathione (GSH) is a major cellular antioxidant, it was hypothesized that exposure to MB would disrupt this system. The current study primarily utilized the PC12 cell line, which displays a catecholaminergic phenotype. Low concentrations of MB (50-1000 ng/ml) had little effect on cell viability, as measured by LDH release. These same concentrations, however, led to increases in GSH and its oxidized form, GSSG. Effects on viability and GSH were correlated to a primary mesencephalic culture system. Furthermore, these effects were markedly different from those observed with the classical oxidative stressor and pesticide, paraquat (PQ). To determine how MB would affect cells in which antioxidant systems were compromised, PC12 cells were treated with L-buthionine-(S,R)-sulfoximine (BSO) to deplete cellular GSH, followed by treatment with MB. Results suggest that following an insult to the GSH antioxidant system, MB can act as an additional insult to the system and prevent the normal recovery of those defenses. Altered protein levels of heme oxygenase-1 (HO-1) further indicated an oxidative stress response elicited by MB in PC12 cells. DAergic neurons, as a population, are inherently vulnerable to oxidative stress, and the disruption of antioxidant systems by the fungicide MB may contribute to the neurodegeneration of these cells, especially with concurrent exposures to other environmentally relevant oxidative stressors, such as PQ.

A Fetal Risk Factor for Parkinson’s Disease

A lack of strong evidence for genetic heritability of idiopathic Parkinson's disease (PD) has focused attention on environmental toxicants in the disease etiology, particularly agrichemicals. PD is associated with advanced age, but it is unclear whether specific neuronal damage could result from insults during development. This study hypothesized that prenatal exposure to pesticides would disrupt the development of the nigrostriatal dopamine (DA) system and enhance its vulnerability to dopaminergic neurotoxicant exposures later in life. Pregnant C57BL/6J mice were treated on gestational days 10-17 with saline or the pesticides maneb (MB, 1 mg/kg) or paraquat (PQ, 0.3 mg/kg). When offspring were evaluated in adulthood, there were no significant effects of prenatal MB or PQ exposure on locomotor activity. Subsequently, offspring were treated for 8 consecutive days with saline, MB (30 mg/kg), or PQ (5 mg/kg). One week after the last exposure, only males exposed to prenatal MB and adulthood PQ showed significant reductions in locomotor activity (95%) and changes in striatal neurochemistry. Stereological assessment of the substantia nigra pars compacta (SNpc) and ventral tegmental area correspondingly confirmed selective dopaminergic-neuron loss in SNpc. The lack of changes in other exposure groups suggests a specificity to the sequence of exposures as well as gender specificity. These results suggest that prenatal exposure to MB produces selective, permanent alterations of the nigrostriatal dopaminergic system and enhances adult susceptibility to PQ exposure. This study implicates a role for developmental neurotoxicant exposure in the induction of neurodegenerative disorders such as PD.

Attenuation of paraquat-induced motor behavior and neurochemical disturbances by l-valine in vivo

Alterations of motor behavioral patterns and monoamine contents in the discrete rat brain areas after acute paraquat exposure (3, 5, 10, 20 mg/kg, s.c.) have been studied. The results showed that paraquat at the doses of 5, 10, and 20 mg/kg significantly reduced locomotive, stereotypic, and rotational behaviors. Significant decreases of norepinephrine (NE) contents in cortex and hypothalamus, as well as striatal contents of dopamine (DA) and its acidic metabolites, were detected. In addition, L-valine (200 mg/kg, i.p.) significantly attenuated paraquat-induced toxicity at moderate dose (5 mg/kg) but not at high dose (20 mg/kg). The results provide evidence that paraquat can enter the brain as illustrated by the alterations in the motor behavioral pattern and neurochemical contents. Furthermore, the attenuation effect of L-valine against systemic administration of paraquat-induced motor behaviors was detected, with a slightly protective effect on paraquat-induced neurochemical alterations.

Paraquat leads to dopaminergic neural vulnerability in organotypic midbrain culture

Paraquat (1,1'-dimethyl-4,4'-bipyridinium, PQ) is a herbicide to possibly induce Parkinson's disease (PD), since a strong correlation has been found between the incidence of the disease and the amount of PQ used. In this study, we examined PQ toxicity in rat organotypic midbrain slice cultures. PQ dose dependently reduced the number of dopaminergic neurons in cultured slices. Since this damage was prevented by GBR-12909, the dopamine transporter could be an initial step of the PQ induced dopaminergic neurotoxicity. The sequential treatments with lower PQ and 1-methyl-4-phenyl pyridinium (MPP+) doses, where each dose alone was not lethal, markedly killed dopamine neurons, suggesting that the exposure of a lower dose of PQ could lead to the vulnerability of dopaminergic neurons. This cell death was prevented by the inhibitors of NMDA, nitric oxide synthase (NOS), cycloheximide and caspase cascade. Neurons expressing NOS were identified inside and around the regions where dopamine neurons were packed. The cell death induced by the sequential treatments with PQ and MPP+ was also rescued by L-deprenyl and dopamine D2/3 agonists. These results strongly support that the constant exposure to low levels of PQ would lead to the vulnerability of dopaminergic neurons in the nigrostriatal system by the excitotoxic pathway, and might potentiate neurodegeneration caused by the exposure of other substances and aging.

Paraquat induces long-lasting dopamine overflow through the excitotoxic pathway in the striatum of freely moving rats

The herbicide paraquat is an environmental factor that could be involved in the etiology of Parkinson's disease. We have previously shown that paraquat penetrates through the blood-brain barrier and is taken up by neural cells. In this study, we examined the in vivo toxic mechanism of paraquat to dopamine neurons. GBR-12909, a selective dopamine transporter inhibitor, reduced paraquat uptake into the striatal tissue including dopaminergic terminals. The subchronic treatment with systemic paraquat significantly decreased brain dopamine content in the striatum and slightly in the midbrain and cortex, and was accompanied by the diminished level of its acidic metabolites in rats. When paraquat was administered through a microdialysis probe, a transitory increase in the extracellular levels of glutamate, followed by long-lasting elevations of the extracellular levels of NO(x)(-) (NO(2)(-) plus NO(3)(-)) and dopamine were detected in the striatum of freely moving rats. This dopamine overflow lasted for more than 24 h after the paraquat treatment. Dopamine overflow was inhibited by N(G)-nitro-L-arginine methyl ester, dizocilpine, 6,7-dinitroquinoxaline-2,3-dione and L-deprenyl. The toxic mechanism of paraquat involves glutamate induced activation of non-NMDA receptors, resulting in activation of NMDA receptor-channels. The influx of Ca(2+) into cells stimulates nitric oxide synthase. Released NO would diffuse to dopaminergic terminals and further induce mitochondrial dysfunction by the formation of peroxynitrite, resulting in continuous and long-lasting dopamine overflow. The constant exposure to low levels of paraquat may lead to the vulnerability of dopaminergic terminals in humans, and might potentiate neurodegeneration caused by the exposure of other substances, such as endogenous dopaminergic toxins.

The nigrostriatal dopaminergic system as a preferential target of repeated exposures to combined paraquat and maneb: implications for Parkinson's disease

Experimental evidence supporting 1,1'-dimethyl-4,4'-bipyridinium [paraquat (PQ)] as a risk factor for Parkinson's disease (PD) is equivocal. Other agricultural chemicals, including dithiocarbamate fungicides such as manganese ethylenebisdithiocarbamate [maneb (MB)], are widely used in the same geographical regions as paraquat and also impact dopamine systems, suggesting that mixtures may be more relevant etiological models. This study therefore proposed that combined PQ and MB exposures would produce greater effects on dopamine (DA) systems than would either compound administered alone. Male C57BL/6 mice were treated twice a week for 6 weeks with intraperitoneal saline, 10 mg/kg paraquat, 30 mg/kg maneb, or their combination (PQ + MB). MB, but not PQ, reduced motor activity immediately after treatment, and this effect was potentiated by combined PQ + MB treatment. As treatments progressed, only the combined PQ + MB group evidenced a failure of motor activity levels to recover within 24 hr. Striatal DA and dihydroxyphenylacetic acid increased 1-3 d and decreased 7 d after injections. Only PQ + MB reduced tyrosine hydroxylase (TH) and DA transporter immunoreactivity and did so in dorsal striatum but not nucleus accumbens. Correspondingly, striatal TH protein levels were decreased only by combined PQ + MB 5 d after injection. Reactive gliosis occurred only in response to combined PQ + MB in dorsal-medial but not ventral striatum. TH immunoreactivity and cell counts were reduced only by PQ + MB and in the substantia nigra but not ventral tegmental area. These synergistic effects of combined PQ + MB, preferentially expressed in the nigrostriatal DA system, suggest that such mixtures could play a role in the etiology of PD.

Paraquat elicited neurobehavioral syndrome caused by dopaminergic neuron loss

The herbicide paraquat, bearing structural similarity to the known dopaminergic neurotoxicant MPTP, has been suggested as a potential etiologic factor in Parkinson's disease. Consideration of paraquat as a candidate neurotoxicant requires demonstration that systemic delivery produces substantia nigra dopaminergic neuron loss and the attendant neurobehavioral syndrome reflecting depletion of dopamine terminals within the striatum. To address these issues paraquat was administered systemically into adult C57 bl/6 mice, ambulatory behavior monitored, substantia nigra dopamine neuron number and striatal dopamine terminal density quantified. The data indicate that paraquat like MPTP elicits a dose-dependent decrease in substantia nigra dopaminergic neurons assessed by a Fluoro-gold prelabeling method, a decline in striatal dopamine nerve terminal density assessed by measurement of tyrosine hydroxylase immunoreactivity; and neurobehavioral syndrome characterized by reduced ambulatory activity. Taken together, these data suggest that systemically absorbed paraquat crosses the blood-brain barrier to cause destruction of dopamine neurons in the substantia nigra, consequent reduction of dopaminergic innervation of the striatum and a neurobehavioral syndrome similar to the well characterized and bona fide dopaminergic toxin MPTP.

Paraquat-induced cell death in PC12 cells

Paraquat was taken up by PC12 cells in a carrier-mediated, saturable manner. When PC12 cells were permeabilized with digitonin (50 microg/ml) lipid peroxidation was observed after paraquat treatment in the presence of NADPH and chelated iron. The fact that lipid peroxidation preceded the appearance of LDH release provides positive evidence that lipid peroxidation may be one of the important factors leading to cytotoxicity of cells. Furthermore, the fact that addition of superoxide dismutase, catalase and promethazine efficiently blocked the malondialdehyde formation and attenuated the cell death indicated the involvement of reactive oxygen radicals in mediating the cytotoxicity induced by paraquat. Taken together the results present in vitro evidence that neurotoxicity of paraquat may be a consequence of cellular lipid peroxidation, which leads to cell death and may have great implications in assessing the risk of exposure to paraquat in Parkinson's disease.