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

The brain and systemic oxidative stress and memory changes induced by inhaled paraquat in rat improved by Crocus sativus

The present study aimed to investigate the effect of Crocus sativus (Cs) on paraquat (PQ)-induced learning and memory deficits as well as brain and lung oxidative stress and systemic inflammation, and oxidative stress in rats. Rats were exposed to saline (Ctrl) or PQ (PQ groups) aerosols. PQ groups were treated with 0.03 mg/kg/day dexamethasone (Dexa), 20 and 80 mg/kg/day Cs-L and Cs-H, 5 mg/kg/day pioglitazone (Pio), and Cs-L+Pio for 16 days during PQ exposure period. Learning and memory abilities were assessed by Morris water maze (MWM) and passive avoidance tests. PQ group showed increased numbers of total and differential WBCs in blood, and increased malondialdehyde (MDA), in the serum, brain, and lung but reduced thiol, catalase (CAT), and superoxide dismutase (SOD) levels compared to the control group (for all, p < 0.001). The escape latency and traveled distance were increased in the PQ group. However, the time spent in the target quadrant in the MWM test and the latency to enter the dark room were reduced after receiving an electrical shock (p < 0.05 to P<0.001). In all treated groups, measured values were improved compared to PQ group (p < 0.05 to p < 0.001). The combination of Cs-L+Pio showed more pronounced effects compared to either treatment alone (p < 0.05 to p < 0.001). These findings suggest that Cs has neuroprotective properties and may be beneficial in the treatment of neurodegenerative diseases induced by noxious agents such as PQ.

Protective effects of amphetamine and methylphenidate against dopaminergic neurotoxicants in SH-SY5Y cells

Full treatment of the second most common neurodegenerative disorder, Parkinson's disease (PD), is still considered an unmet need. As the psychostimulants, amphetamine (AMPH) and methylphenidate (MPH), were shown to be neuroprotective against stroke and other neuronal injury diseases, this study aimed to evaluate their neuroprotective potential against two dopaminergic neurotoxicants, 6-hydroxydopamine (6-OHDA) and paraquat (PQ), in differentiated human dopaminergic SH-SY5Y cells. Neither cytotoxicity nor mitochondrial membrane potential changes were seen following a 24-hour exposure to either therapeutic concentration of AMPH or MPH (0.001-10 μM). On the other hand, a 24-hour exposure to 6-OHDA (31.25-500 μM) or PQ (100-5000 μM) induced concentration-dependent mitochondrial dysfunction, assessed by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assay, and lysosomal damage, evaluated by the neutral red uptake assay. The lethal concentrations 25 and 50 retrieved from the concentration-toxicity curves in the MTT assay were 99.9 µM and 133.6 µM for 6-OHDA, or 422 µM and 585.8 µM for PQ. Both toxicants caused mitochondrial membrane potential depolarization, but only 6-OHDA increased reactive oxygen species (ROS). Most importantly, PQ-induced toxicity was partially prevented by 1 μM of AMPH or MPH. Nonetheless, neither AMPH nor MPH could prevent 6-OHDA toxicity in this experimental model. According to these findings, AMPH and MPH may provide some neuroprotection against PQ-induced neurotoxicity, but further investigation is required to determine the exact mechanism underlying this protection.

Developmental origins of Parkinson disease: Improving the rodent models

Numerous pesticides are inhibitors of the oxidative phosphorylation system. Oxidative phosphorylation dysfunction adversely affects neurogenesis and often accompanies Parkinson disease. Since brain development occurs mainly in the prenatal period, early exposure to pesticides could alter the development of the nervous system and increase the risk of Parkinson disease. Different rodent models have been used to confirm this hypothesis. However, more precise considerations of the selected strain, the xenobiotic, its mode of administration, and the timing of animal analysis, are necessary to resemble the model to the human clinical condition and obtain more reliable results.

Mitochondrial Toxicant-Induced Neuronal Apoptosis in Parkinson's Disease: What We Know so Far

Parkinson's disease (PD) is one of the most common progressive neurodegenerative diseases caused by the loss of dopamine-producing neuronal cells in the region of substantia nigra pars compacta of the brain. During biological aging, neuronal cells slowly undergo degeneration, but the rate of cell death increases tremendously under some pathological conditions, leading to irreversible neurodegenerative diseases. By the time symptoms of PD usually appear, more than 50 to 60% of neuronal cells have already been destroyed. PD symptoms often start with tremors, followed by slow movement, stiffness, and postural imbalance. The etiology of PD is still unknown; however, besides genetics, several factors contribute to neurodegenerative disease, including exposure to pesticides, environmental chemicals, solvents, and heavy metals. Postmortem brain tissues of patients with PD show mitochondrial abnormalities, including dysfunction of the electron transport chain. Most chemicals present in our environment have been shown to target the mitochondria; remarkably, patients with PD show a mild deficiency in NADH dehydrogenase activity, signifying a possible link between PD and mitochondrial dysfunction. Inhibition of electron transport complexes generates free radicals that further attack the macromolecules leading to neuropathological conditions. Apart from that, oxidative stress also causes neuroinflammation-mediated neurodegeneration due to the activation of microglial cells. However, the mechanism that causes mitochondrial dysfunction, especially the electron transport chain, in the pathogenesis of PD remains unclear. This review discusses the recent updates and explains the possible mechanisms of mitochondrial toxicant-induced neuroinflammation and neurodegeneration in PD.

An Insight into the Molecular Mechanism of Mitochondrial Toxicant-induced Neuronal Apoptosis in Parkinson's Disease

Parkinson's disease (PD) is one of the most common progressive neurodegenerative disorders affecting approximately 1% of the world's population at the age of 50 and above. Majority of PD cases are sporadic and show symptoms after the age of 60 and above. At that time, most of the dopaminergic neurons in the region of substantia nigra pars compacta have been degenerated. Although in past decades, discoveries of genetic mutations linked to PD have significantly impacted our current understanding of the pathogenesis of this devastating disorder, it is likely that the environment also plays a critical role in the etiology of sporadic PD. Recent epidemiological and experimental studies indicate that exposure to environmental agents, including a number of agricultural and industrial chemicals, may contribute to the pathogenesis of several neurodegenerative disorders, including PD. Furthermore, there is a strong correlation between mitochondrial dysfunction and several forms of neurodegenerative disorders, including Alzheimer's disease (AD), Huntington's disease (HD), Amyotrophic lateral sclerosis (ALS) and PD. Interestingly, substantia nigra of patients with PD has been shown to have a mild deficiency in mitochondrial respiratory electron transport chain NADH dehydrogenase (Complex I) activity. This review discusses the role of mitochondrial toxicants in the selective degeneration of dopaminergic neurons targeting the electron transport system that leads to Parkinsonism.

An Investigation of the Neurotoxic Effects of Malathion, Chlorpyrifos, and Paraquat to Different Brain Regions

Acute or chronic exposures to pesticides have been linked to neurotoxicity and the potential development of neurodegenerative diseases (NDDs). This study aimed to consider the neurotoxicity of three widely utilized pesticides: malathion, chlorpyrifos, and paraquat within the hippocampus (HC), corpus striatum (CS), cerebellum (CER), and cerebral cortex (CC). Neurotoxicity was evaluated at relatively low, medium, and high pesticide dosages. All pesticides inhibited acetylcholinesterase (AChE) and neuropathy target esterase (NTE) in each of the brain regions, but esterase inhibition was greatest in the HC and CS. Each of the pesticides also induced greater disruption to cellular bioenergetics within the HC and CS, and this was monitored via inhibition of mitochondrial complex enzymes I and II, reduced ATP levels, and increased lactate production. Similarly, the HC and CS were more vulnerable to redox stress, with greater inhibition of the antioxidant enzymes catalase and superoxide dismutase and increased lipid peroxidation. All pesticides induced the production of nuclear Nrf2 in a dose-dependent manner. Collectively, these results show that pesticides disrupt cellular bioenergetics and that the HC and CS are more susceptible to pesticide effects than the CER and CC.

Role of Lung P450 Oxidoreductase in Paraquat-Induced Collagen Deposition in the Lung

Paraquat (PQ) is an agrochemical known to cause pulmonary fibrosis. PQ-induced collagen deposition in the lung is thought to require enzymatic formation of PQ radicals, but the specific enzymes responsible for this bioactivation event in vivo have not been identified. We tested the hypothesis that lung P450 oxidoreductase (POR or CPR) is important in PQ-induced lung fibrosis in mice. A lung-Cpr-null mouse model was utilized, which undergoes doxycycline-induced, Cre recombinase-mediated deletion of the Por gene specifically in airway Club cells and alveolar type 2 cells in the lung. The lungs of lung-Cpr-null mice and their wild-type littermates were collected on day 15 after a single intraperitoneal injection of saline (control) or PQ (20 mg/kg). Lung tissue sections were stained with picrosirius red for detection of collagen fibrils. Fibrotic lung areas were found to be significantly smaller (1.6-fold for males and 1.4-fold for females) in PQ-treated lung-Cpr-null mice than in sex- and treatment-matched wild-type mice. The levels of collagen in lung tissue homogenate were also lower (1.4-2.3-fold; p < 0.05) in PQ-treated lung-Cpr-null mice compared to PQ-treated wild-type mice. In contrast, plasma PQ toxicokinetic profiles were not different between sex-matched wild-type and lung-Cpr-null mice. Taken together, these results indicate that lung POR plays an important role in PQ-induced pulmonary fibrosis.

Paraquat-induced cholesterol biosynthesis proteins dysregulation in human brain microvascular endothelial cells

Despite Paraquat (PQ) being banned in several countries, it is still one of the most commonly used herbicides in agriculture. This compound is known to induce damaging effects on human and animal brain cells by generating Reactive Oxygen Species (ROS). However, there is few evidence of PQ effect on Human Brain Microvascular Endothelial Cells (HBMECs), one of the major component of the Blood–Brain Barrier (BBB). The present study aimed at unraveling biological mechanisms associated to the exposure of 1, 10 and 100 µM of PQ for 24 h on HBMECs. High-throughput mass spectrometry-based proteomics using data-independent acquisition (DIA) was applied. Biological pathway enrichment and cellular assays such as mitochondrial respiration and cholesterol level were performed to verify proteomics results. A total of 3753 proteins were quantified out of which 419 were significantly modulated by paraquat exposure. Biological pathway enrichment revealed the ubiquinone metabolism, a pathway directly linked to mitochondrial complex I proteins, confirming the well-known mechanism of PQ inducing oxidative stress. Additionally, this study also described the cholesterol biosynthesis modulation on HBMECs not yet described. In conclusion, our data indicate the toxic effect of PQ on HBMECs by downregulating proteins involved in mitochondrial complex I and cholesterol pathways.

Ubiquinone Metabolism and Transcription HIF-1 Targets Pathway Are Toxicity Signature Pathways Present in Extracellular Vesicles of Paraquat-Exposed Human Brain Microvascular Endothelial Cells

Over the last decade, the knowledge in extracellular vesicles (EVs) biogenesis and modulation has increasingly grown. As their content reflects the physiological state of their donor cells, these “intercellular messengers” progressively became a potential source of biomarker reflecting the host cell state. However, little is known about EVs released from the human brain microvascular endothelial cells (HBMECs). The current study aimed to isolate and characterize EVs from HBMECs and to analyze their EVs proteome modulation after paraquat (PQ) stimulation, a widely used herbicide known for its neurotoxic effect. Size distribution, concentration and presence of well-known EV markers were assessed. Identification and quantification of PQ-exposed EV proteins was conducted by data-independent acquisition mass spectrometry (DIA-MS). Signature pathways of PQ-treated EVs were analyzed by gene ontology terms and pathway enrichment. Results highlighted that EVs exposed to PQ have modulated pathways, namely the ubiquinone metabolism and the transcription HIF-1 targets. These pathways may be potential molecular signatures of the PQ-induced toxicity carried by EVs that are reflecting their cell of origin by transporting with them irreversible functional changes.

Application of adverse outcome pathway networks to integrate mechanistic data informing the choice of a point of departure for hydrogen sulfide exposure limits

Acute exposure to hydrogen sulfide initiates a series of hallmark biological effects that occur progressively at increasing exposure levels: odor perception, conjunctivitis, olfactory paralysis, "knockdown," pulmonary edema, and apnea. Although effects of exposure to high concentrations of hydrogen sulfide are clear, effects associated with chronic, low-level exposure in humans is under debate, leading to uncertainty in the critical effect used in regulatory risk assessments addressing low dose exposures. This study integrates experimental animal, observational epidemiology, and occupational exposure evidence by applying a pathway-based approach. A hypothesized adverse outcome pathway (AOP) network was developed from 34 studies, composed of 4 AOPs sharing 1 molecular initiating events (MIE) and culminating in 4 adverse outcomes. A comparative assessment of effect levels and weight of evidence identified an AOP leading to a biologically-plausible, low-dose outcome relative to the other outcomes (nasal lesions, 30 ppm versus olfactory paralysis, >100 ppm; neurological effects, >80 ppm; pulmonary edema, >80 ppm). This AOP (i.e. AOP1) consists of the following key events: cytochrome oxidase inhibition (>10 ppm), neuronal cell loss (>30 ppm), and olfactory nasal lesions (defined as both neuronal cell loss and basal cell hyperplasia; >30 ppm) in rodents. The key event relationships in this pathway were supported by moderate empirical evidence and have high biological plausibility due to known mechanistic understanding and consistency in observations for diverse chemicals.

Neurotoxin-Induced Rodent Models of Parkinson's Disease: Benefits and Drawbacks

Parkinson's disease (PD), the second most common neurodegenerative disorder, is characterized by cardinal motor impairments, including akinesia and tremor, as well as by a host of non-motor symptoms, including both autonomic and cognitive dysfunction. PD is associated with a death of nigral dopaminergic neurons, as well as the pathological spread of Lewy bodies, consisting predominantly of the misfolded protein alpha-synuclein. To date, only symptomatic treatments, such as levodopa, are available, and trials aiming to cure the disease, or at least halt its progression, have not been successful. Wong et al. (2019) suggested that the lack of effective therapy against neurodegeneration in PD might be attributed to the fact that the molecular mechanisms standing behind the dopaminergic neuronal vulnerability are still a major scientific challenge. Understanding these molecular mechanisms is critical for developing effective therapy. Thirty-five years ago, Calne and William Langston (1983) raised the question of whether biological or environmental factors precipitate the development of PD. In spite of great advances in technology and medicine, this question still lacks a clear answer. Only 5-15% of PD cases are attributed to a genetic mutation, with the majority of cases classified as idiopathic, which could be linked to exposure to environmental contaminants. Rodent models play a crucial role in understanding the risk factors and pathogenesis of PD. Additionally, well-validated rodent models are critical for driving the preclinical development of clinically translatable treatment options. In this review, we discuss the mechanisms, similarities and differences, as well as advantages and limitations of different neurotoxin-induced rat models of PD. In the second part of this review, we will discuss the potential future of neurotoxin-induced models of PD. Finally, we will briefly demonstrate the crucial role of gene-environment interactions in PD and discuss fusion or dual PD models. We argue that these models have the potential to significantly further our understanding of PD.

Association between liberal oxygen therapy and mortality in patients with paraquat poisoning: A multi-center retrospective cohort study

Paraquat (N, N'-dimethyl-4, 4'-bipyridinium dichloride, PQ) intoxication is a common cause of lethal poisoning. This study aimed to identify the risk of using liberal oxygen therapy in patients with PQ poisoning. This was a multi-center retrospective cohort study involving four medical institutions in Taiwan. Data were extracted from the Chang Gung Research Database (CGRD) from January 2004 to December 2016. Patients confirmed to have PQ intoxication with a urine PQ concentration ≥ 5 ppm were analyzed. Patients who received oxygen therapy before marked hypoxia (SpO2 ≥ 90%) were defined as receiving liberal oxygen therapy. The association between mortality and patient demographics, blood paraquat concentration (ppm), and liberal oxygen therapy were analyzed. A total of 416 patients were enrolled. The mortality rate was higher in the liberal oxygen therapy group (87.8% vs. 73.7%, P = 0.007), especially in 28-day mortality (adjusted odds ratio [aOR]: 4.71, 95% confidence interval [CI]: 1.533–14.471) and overall mortality (aOR: 5.97, 95% CI: 1.692–21.049) groups. Mortality in patients with PQ poisoning was also associated with age (aOR: 1.04, 95% CI: 1.015–1.073), blood creatinine level (aOR: 1.49, 95% CI: 1.124–1.978), and blood paraquat concentration (ppm) (aOR, 1.51; 95% CI: 1.298–1.766). Unless the evidence of hypoxia (SpO2 < 90%) is clear, oxygen therapy should be avoided because it is associated with increased mortality.

Toxicity assessment of the herbicide acetochlor in the human liver carcinoma (HepG2) cell line

Acetochlor is a high-volume herbicide used on a global scale and toxicity assessments are needed to define its potential for adverse effects in wildlife and humans. This study was conducted to determine the effects of acetochlor on human liver carcinoma cells (HepG2), a cell model widely used to assess the potential for chemical hepatotoxicity. Experiments were conducted at concentrations ranging 0-800 μM acetochlor over a 12 to 48h period to quantify underlying mechanisms of toxicity. Our data indicate that acetochlor suppressed HepG2 cell proliferation in both a concentration- and time-dependent manner. Acetochlor induced reactive oxygen species (ROS) generation more than 700% with exposure to 400 μM acetochlor, and acetochlor decreased the activities and levels of anti-oxidant responses (superoxide dismutase, glutathione) following exposure to 100 μM, 200 μM and 400 μM acetochlor. Acetochlor also (1) induced HepG2 cell damage through apoptotic-signaling pathways; (2) enhanced intracellular free Ca²⁺ concentration (>400%); (3) decreased mitochondrial transmembrane potential (∼77%), and reduced ATP levels (∼65%) following exposure to 400 μM acetochlor compared to untreated cells. Notably, cell cycle progression was blocked at G0/G1 phase in HepG2 cells when treated for 24 h with 400 μM acetochlor. Taken together, acetochlor induced significant cytotoxicity toward HepG2 cells, and the underlying toxicity mechanisms appear to be related to ROS generation, mitochondrial dysfunction and disruption in the cell cycle regulation. These data contribute to toxicity assessments for acetochlor, a high-use herbicide, to quantify risk to wildlife and human health.

Ginkgo biloba L. Leaf Extract Protects HepG2 Cells Against Paraquat-Induced Oxidative DNA Damage

Ginkgo biloba L. leaf extracts and herbal infusions are used worldwide due to the health benefits that are attributed to its use, including anti-neoplastic, anti-aging, neuro-protection, antioxidant and others. The aim of this study was to evaluate the effect of an aqueous Ginkgo biloba extract on HepG2 cell viability, genotoxicity and DNA protection against paraquat-induced oxidative damage. Exposure to paraquat (PQ), over 24 h incubation at 1.0 and 1.5 µM, did not significantly reduce cell viability but induced concentration and time-dependent oxidative DNA damage. Ginkgo biloba leaf extract produced dose-dependent cytotoxicity (IC₅₀ = 540.8 ± 40.5 µg/mL at 24 h exposure), and short incubations (1 h) produced basal and oxidative DNA damage (>750 and 1500 µg/mL, respectively). However, lower concentrations (e.g., 75 µg/mL) of Ginkgo biloba leaf extract were not cytotoxic and reduced basal DNA damage, indicating a protective effect at incubations up to 4 h. On the other hand, longer incubations (24 h) induced oxidative DNA damage. Co-incubation of HepG2 cells for 4 h, with G. biloba leaf extract (75 µg/mL) and PQ (1.0 or 1.5 µM) significantly reduced PQ-induced oxidative DNA damage. In conclusion, the consumption of Ginkgo biloba leaf extract for long periods at high doses/concentrations is potentially toxic; however, low doses protect the cells against basal oxidative damage and against environmentally derived toxicants that induce oxidative DNA damage.

Systems Biology Approaches Toward Understanding Primary Mitochondrial Diseases

Primary mitochondrial diseases form one of the most common and severe groups of genetic disease, with a birth prevalence of at least 1 in 5000. These disorders are multi-genic and multi-phenotypic (even within the same gene defect) and span the entire age range from prenatal to late adult onset. Mitochondrial disease typically affects one or multiple high-energy demanding organs, and is frequently fatal in early life. Unfortunately, to date there are no known curative therapies, mostly owing to the rarity and heterogeneity of individual mitochondrial diseases, leading to diagnostic odysseys and difficulties in clinical trial design. This review aims to discuss recent advances and challenges of systems approaches for the study of primary mitochondrial diseases. Although there has been an explosion in the generation of omics data, few studies have progressed toward the integration of multiple levels of omics. It is evident that the integration of different types of data to create a more complete representation of biology remains challenging, perhaps due to the scarcity of available integrative tools and the complexity inherent in their use. In addition, "bottom-up" systems approaches have been adopted for use in the iterative cycle of systems biology: from data generation to model prediction and validation. Primary mitochondrial diseases, owing to their complex nature, will most likely benefit from a multidisciplinary approach encompassing clinical, molecular and computational studies integrated together by systems biology to elucidate underlying pathomechanisms for better diagnostics and therapeutic discovery. Just as next generation sequencing has rapidly increased diagnostic rates from approximately 5% up to 60% over two decades, more recent advancing technologies are encouraging; the generation of multi-omics, the integration of multiple types of data, and the ability to predict perturbations will, ultimately, be translated into improved patient care.

Lipopolysaccharide-Induced Striatal Nitrosative Stress and Impaired Social Recognition Memory Are Not Magnified by Paraquat Coexposure

Systemic inflammation triggered by lipopolysaccharide (LPS) administration disrupts blood-brain barrier (BBB) homeostasis in animal models. This event leads to increased susceptibility of several encephalic structures to potential neurotoxicants present in the bloodstream. In this study, we investigated the effects of alternate intraperitoneal injections of LPS on BBB permeability, social recognition memory and biochemical parameters in the striatum 24 h and 60 days after treatments. In addition, we investigated whether the exposure to a moderate neurotoxic dose of the herbicide paraquat could potentiate LPS-induced neurotoxicity. LPS administration caused a transient disruption of BBB integrity, evidenced by increased levels of exogenously administered sodium fluorescein in the striatum. Also, LPS exposure caused delayed impairment in social recognition memory (evaluated at day 38 after treatments) and increase in the striatal levels of 3-nitrotyrosine. These events were observed in the absence of significant changes in motor coordination and in the levels of tyrosine hydroxylase (TH) in the striatum and substantia nigra. PQ exposure, which caused a long-lasting decrease of striatal mitochondrial complex I activity, did not modify LPS-induced behavioral and striatal biochemical changes. The results indicate that systemic administration of LPS causes delayed social recognition memory deficit and striatal nitrosative stress in adult mice and that the coexposure to a moderately toxic dose of PQ did not magnify these events. In addition, PQ-induced inhibition of striatal mitochondrial complex I was also not magnified by LPS exposure, indicating the absence of synergic neurotoxic effects of LPS and PQ in this experimental model.

Effects of Commonly Used Pesticides in China on the Mitochondria and Ubiquitin-Proteasome System in Parkinson's Disease

Evidence continues to accumulate that pesticides are the leading candidates of environmental toxins that may contribute to the pathogenesis of Parkinson’s disease. The mechanisms, however, remain largely unclear. According to epidemiological studies, we selected nine representative pesticides (paraquat, rotenone, chlorpyrifos, pendimethalin, endosulfan, fenpyroximate, tebufenpyrad, trichlorphon and carbaryl) which are commonly used in China and detected the effects of the pesticides on mitochondria and ubiquitin-proteasome system (UPS) function. Our results reveal that all the nine studied pesticides induce morphological changes of mitochondria at low concentrations. Paraquat, rotenone, chlorpyrifos, pendimethalin, endosulfan, fenpyroximate and tebufenpyrad induced mitochondria fragmentation. Furthermore, some of them (paraquat, rotenone, chlorpyrifos, fenpyroximate and tebufenpyrad) caused a significant dose-dependent decrease of intracellular ATP. Interestingly, these pesticides which induce mitochondria dysfunction also inhibit 26S and 20S proteasome activity. However, two out of the nine pesticides, namely trichlorphon and carbaryl, were found not to cause mitochondrial fragmentation or functional damage, nor inhibit the activity of the proteasome, which provides significant guidance for selection of pesticides in China. Moreover, our results demonstrate a potential link between inhibition of mitochondria and the UPS, and pesticide-induced Parkinsonism.

Tanshinone I Induces Mitochondrial Protection through an Nrf2-Dependent Mechanism in Paraquat-TreatedHuman Neuroblastoma SH-SY5Y Cells

Tanshinone I (T-I; 1,6-Dimethylnaphtho[1,2-g][1]benzofuran-10,11-dione; C₁₈H₁₂O₃), which may be found in Salvia miltiorrhiza Bunge (Danshen), is a potent anti-inflammatory, antioxidant, and anti-cancer agent. At least in part, T-I exerts antioxidant activity by activating signaling pathways associated with the maintenance of the redox state in mammalian cells. In this context, the upregulation of nuclear factor (erythroid-derived 2)-like 2 (Nrf2) has received attention regarding the role of this transcription factor in modulating the expression of antioxidant enzymes and the metabolism of glutathione (GSH). Even though there is a growing body of evidence suggesting that T-I mediates protection against several pro-oxidant challenges in both in vitro and in vivo experimental models, it remains to be examined whether and how T-I would modulate mitochondrial function during redox disturbances. Therefore, we aimed to reveal whether T-I would exhibit protective effects on mitochondria of SH-SY5Y cells treated with paraquat (PQ), a well-known mitochondrial toxic agent. We found that T-I pretreatment significantly protected mitochondria against PQ-induced redox impairment through an Nrf2-dependent mechanism involving upregulation of antioxidant enzymes, such as Mn-superoxide dismutase (Mn-SOD), glutathione peroxidase (GPx), and both catalytic and modifier subunits of γ-glutamate-cysteine ligase (γ-GCL). T-I prevented complex I and mitochondrial membrane potential (MMP) impairments elicited by PQ. Thus, T-I may be viewed as a new mitochondrial protective agent whose complete mechanism of action needs to be investigated, but it seems to involve mitochondriotropic aspects related to the chemistry of this molecule.

Parkinson's Disease: The Mitochondria-Iron Link

Mitochondrial dysfunction, iron accumulation, and oxidative damage are conditions often found in damaged brain areas of Parkinson's disease. We propose that a causal link exists between these three events. Mitochondrial dysfunction results not only in increased reactive oxygen species production but also in decreased iron-sulfur cluster synthesis and unorthodox activation of Iron Regulatory Protein 1 (IRP1), a key regulator of cell iron homeostasis. In turn, IRP1 activation results in iron accumulation and hydroxyl radical-mediated damage. These three occurrences-mitochondrial dysfunction, iron accumulation, and oxidative damage-generate a positive feedback loop of increased iron accumulation and oxidative stress. Here, we review the evidence that points to a link between mitochondrial dysfunction and iron accumulation as early events in the development of sporadic and genetic cases of Parkinson's disease. Finally, an attempt is done to contextualize the possible relationship between mitochondria dysfunction and iron dyshomeostasis. Based on published evidence, we propose that iron chelation-by decreasing iron-associated oxidative damage and by inducing cell survival and cell-rescue pathways-is a viable therapy for retarding this cycle.

Multifactorial theory applied to the neurotoxicity of paraquat and paraquat-induced mechanisms of developing Parkinson's disease

Laboratory studies involving repeated exposure to paraquat (PQ) in different animal models can induce many of the pathological features of Parkinson's disease (PD), such as the loss of dopaminergic neurons in the nigrostriatal dopamine system. Epidemiological studies identify an increased risk of developing PD in human populations living in areas where PQ exposure is likely to occur and among workers lacking appropriate protective equipment. The mechanisms involved in developing PD may not be due to any single cause, but rather a multifactorial situation may exist where PQ exposure may cause PD in some circumstances. Multifactorial theory is adopted into this review that includes a number of sub-cellular mechanisms to explain the pathogenesis of PD. The theory is placed into an environmental context of chronic low-dose exposure to PQ that consequently acts as an oxidative stress inducer. Oxidative stress and the metabolic processes of PQ-inducing excitotoxicity, α-synuclein aggregate formation, autophagy, alteration of dopamine catabolism, and inactivation of tyrosine hydroxylase are positioned as causes for the loss of dopaminergic cells. The environmental context and biochemistry of PQ in soils, water, and organisms is also reviewed to identify potential routes that can lead to chronic rates of low-dose exposure that would replicate the type of response that is observed in animal models, epidemiological studies, and other types of laboratory investigations involving PQ exposure. The purpose of this review is to synthesize key relations and summarize hypotheses linking PD to PQ exposure by using the multifactorial approach. Recommendations are given to integrate laboratory methods to the environmental context as a means to improve on experimental design. The multifactorial approach is necessary for conducting valid tests of causal relations, for understanding of potential relations between PD and PQ exposure, and may prevent further delay in solving what has proven to be an evasive etiological problem.

Protective effect of carnosic acid against paraquat-induced redox impairment and mitochondrial dysfunction in SH-SY5Y cells: Role for PI3K/Akt/Nrf2 pathway

Carnosic acid (CA) is a phenolic diterpene isolated from Rosmarinus officinalis and exerts anti-inflammatory, antioxidant, and anticarcinogenic activities in different cell types. It has been reported that CA is able to cause protective effects on experimental models of neurodegeneration. However, the exact mechanism by which CA prevents neuronal degeneration remains to be better studied. We investigated here whether there is a role for CA as a neuroprotective agent in a paraquat (PQ) model of Parkinson's disease (PD) regarding cellular and mitochondrial-related redox parameters. SH-SY5Y cells were treated with CA for 12h and were exposed to 100 μM PQ for 24h. It was found that CA at different concentrations prevented the effects of PQ on cell viability and redox parameters. CA alleviated reactive oxygen and nitrogen species production elicited by PQ, as well as decreased the toxic effect on mitochondrial function. Inhibition of Pi3K/Akt pathway with LY294002 or silencing of Nrf2 expression partially blocked the reversal of redox impairment induced by CA. Therefore, CA activated Nrf2 through modulation of PI3K/Akt pathway resulting in increased levels of antioxidant enzymes and consequent neuroprotection. Thus, CA may be viewed as a potential neuroprotective agent to be used in cases of Parkinson's disease (PD).

AMP-activated protein kinase deficiency rescues paraquat-induced cardiac contractile dysfunction through an autophagy-dependent mechanism

AIM

Paraquat, a quaternary nitrogen herbicide, is a highly toxic prooxidant resulting in multi-organ failure including the heart although the underlying mechanism still remains elusive. This study was designed to examine the role of the cellular fuel sensor AMP-activated protein kinase (AMPK) in paraquat-induced cardiac contractile and mitochondrial injury.

RESULTS

Wild-type and transgenic mice with overexpression of a mutant AMPK α2 subunit (kinase dead, KD), with reduced activity in both α1 and α2 subunits, were administered with paraquat (45 mg/kg) for 48 h. Paraquat elicited cardiac mechanical anomalies including compromised echocardiographic parameters (elevated left ventricular end-systolic diameter and reduced factional shortening), suppressed cardiomyocyte contractile function, intracellular Ca(2+) handling, reduced cell survival, and overt mitochondrial damage (loss in mitochondrial membrane potential). In addition, paraquat treatment promoted phosphorylation of AMPK and autophagy. Interestingly, deficiency in AMPK attenuated paraquat-induced cardiac contractile and intracellular Ca(2+) derangement. The beneficial effect of AMPK inhibition was associated with inhibition of the AMPK-TSC-mTOR-ULK1 signaling cascade. In vitro study revealed that inhibitors for AMPK and autophagy attenuated paraquat-induced cardiomyocyte contractile dysfunction.

CONCLUSION

Taken together, our findings revealed that AMPK may mediate paraquat-induced myocardial anomalies possibly by regulating the AMPK/mTOR-dependent autophagy.

New insights into antioxidant strategies against paraquat toxicity

Paraquat (PQ, 1,1'-dimethyl-4-4'-bipyridinium dichloride) is a highly toxic quaternary ammonium herbicide widely used in agriculture, it exerts its toxic effects mainly because of its redox cycle through the production of superoxide anions in organisms, leading to an imbalance in the redox state of the cell causing oxidative damage and finally cell death. The contribution of mitochondrial dysfunction including increased production of reactive oxygen species besides the reduction in oxygen consumption as well as in the activity of some respiratory complexes has emerged as a key component in the mechanisms through which PQ induces cell death. Although several aspects of PQ-mitochondria interaction remain to be clarified, recent advances have been conducted with reproducible results. Currently, there is no treatment for PQ poisoning; however, several studies taking into account oxidative stress as the main mechanism of PQ-induced toxicity suggest an antioxidant therapy as a viable alternative. In fact, it has been shown that the antioxidants naringin, sylimarin, edaravone, Bathysa cuspidata extracts, alpha-lipoic acid, pirfenidone, lysine acetylsalicylate, selenium, quercetin, C-phycocyanin, bacosides, and vitamin C may be useful in the treatment against PQ toxicity. The main mechanisms involved in the protective effect of these antioxidants include the reduction of oxidative stress and inflammation and the induction of antioxidant defenses. Interestingly, recent findings suggest that the induction of nuclear factor erythroid like-2 (Nrf2), a major regulator of the antioxidant response, by some of the above-mentioned antioxidants, has been involved in the protective effect against PQ-induced toxicity.

How Parkinsonian toxins dysregulate the autophagy machinery

Since their discovery, Parkinsonian toxins (6-hydroxydopamine, MPP+, paraquat, and rotenone) have been widely employed as in vivo and in vitro chemical models of Parkinson's disease (PD). Alterations in mitochondrial homeostasis, protein quality control pathways, and more recently, autophagy/mitophagy have been implicated in neurotoxin models of PD. Here, we highlight the molecular mechanisms by which different PD toxins dysregulate autophagy/mitophagy and how alterations of these pathways play beneficial or detrimental roles in dopamine neurons. The convergent and divergent effects of PD toxins on mitochondrial function and autophagy/mitophagy are also discussed in this review. Furthermore, we propose new diagnostic tools and discuss how pharmacological modulators of autophagy/mitophagy can be developed as disease-modifying treatments for PD. Finally, we discuss the critical need to identify endogenous and synthetic forms of PD toxins and develop efficient health preventive programs to mitigate the risk of developing PD.

Neuromelanin of the human substantia nigra: an update

Dopaminergic neurons of the substantia nigra selectively degenerate over the course of Parkinson's disease. These neurons are also the most heavily pigmented cells of the brain, accumulating the dark pigment neuromelanin over a lifetime. The massive presence of neuromelanin in these brain areas has long been suspected as a key factor involved in the selective vulnerability of neurons. The high concentration of neuromelanin in substantia nigra neurons seems to be linked to the presence of considerable amounts of cytosolic dopamine that have not been sequestered into synaptic vesicles. Over the past few years, studies have uncovered a dual nature of neuromelanin. Intraneuronal neuromelanin can be a protective factor, shielding the cells from toxic effects of redox active metals, toxins, and excess of cytosolic catecholamines. In contrast, neuromelanin released by dying neurons can contribute to the activation of neuroglia triggering the neuroinflammation that characterizes Parkinson's disease. This article reviews recent studies on the molecular aspects of neuromelanin of the human substantia nigra.

Does menaquinone participate in brain astrocyte electron transport?

Quinone compounds act as membrane resident carriers of electrons between components of the electron transport chain in the periplasmic space of prokaryotes and in the mitochondria of eukaryotes. Vitamin K is a quinone compound in the human body in a storage form as menaquinone (MK); distribution includes regulated amounts in mitochondrial membranes. The human brain, which has low amounts of typical vitamin K dependent function (e.g., gamma carboxylase) has relatively high levels of MK, and different regions of brain have different amounts. Coenzyme Q (Q), is a quinone synthesized de novo, and the levels of synthesis decline with age. The levels of MK are dependent on dietary intake and generally increase with age. MK has a characterized role in the transfer of electrons to fumarate in prokaryotes. A newly recognized fumarate cycle has been identified in brain astrocytes. The MK precursor menadione has been shown to donate electrons directly to mitochondrial complex III.

HYPOTHESIS

Vitamin K compounds function in the electron transport chain of human brain astrocytes.

An electrochemical magneto immunosensor (EMIS) for the determination of paraquat residues in potato samples

An electrochemical magneto immunosensor for the detection of low concentrations of paraquat (PQ) in food samples has been developed and its performance evaluated in a complex sample such as potato extracts. The immunosensor presented uses immunoreagents specifically developed for the recognition of paraquat, a magnetic graphite-epoxy composite (m-GEC) electrode and biofunctionalized magnetic micro-particles (PQ1-BSAMP) that allow reduction of the potential interferences caused by the matrix components. The amperometric signal is provided by an enzymatic probe prepared by covalently linking an enzyme to the specific antibodies (Ab198-cc-HRP). The use of hydroquinone, as mediator, allows recording of the signal at a low potential, which also contributes to reducing the background noise potentially caused by the sample matrix. The immunocomplexes formed on top of the modified MP are easily captured by the m-GEC, which acts simultaneously as transducer. PQ can be detected at concentrations as low as 0.18 ± 0.09 μg L(-1). Combined with an efficient extraction procedure, PQ residues can be directly detected and accurately quantified in potato extracts without additional clean-up or purification steps, with a limit of detection (90% of the maximum signal) of 2.18 ± 2.08 μg kg(-1), far below the maximum residue level (20 μg kg(-1)) established by the EC. The immunosensor presented here is suitable for on-site analysis. Combined with the use of magnetic racks, multiple samples can be run simultaneously in a reasonable time.

Expression of zinc-deficient human superoxide dismutase in Drosophila neurons produces a locomotor defect linked to mitochondrial dysfunction

More than 130 different mutations in the Cu/Zn superoxide dismutase (SOD1) gene have been associated with amyotrophic lateral sclerosis but the mechanism of this toxicity remains controversial. To gain insight into the importance of the zinc site in the pathogenesis of SOD1 in vivo, we generated a Drosophila model with transgenic expression of a zinc-deficient human SOD1. Expression of zinc-deficient SOD1 in Drosophila resulted in a progressive movement defect with associated mitochondrial cristae vacuolization and reductions in adenosine triphosphate (ATP) levels. Furthermore, these flies are sensitized to mitochondrial toxins, paraquat, and zinc. Importantly, we show that the zinc-deficient SOD1-induced motor defect can be ameliorated by supplementing the endogenous fly respiratory chain machinery with the single-subunit NADH-ubiquinone oxidoreductase from yeast (NADH is nicotinamide adenine dinucleotide, reduced form.). These results demonstrate that zinc-deficient SOD1 is neurotoxic in vivo and suggest that mitochondrial dysfunction plays a critical role in this toxicity. The robust behavioral, pathological, and biochemical phenotypes conferred by zinc-deficient SOD1 in Drosophila have general implications for the role of the zinc ion in familial and sporadic amyotrophic lateral sclerosis.

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.

Daily subacute paraquat exposure decreases muscle function and substantia nigra dopamine level

The use of the herbicide paraquat (1,1'-dimethyl-4,4'-bipyridylium dichloride; PQ) which is widely used in agriculture is known to cause dopaminergic neurotoxicity. However, the mechanisms underlying this effect are not fully understood. This present study investigated the behavioral manifestations, motor coordination, and dopaminergic neurodegeneration following exposure to PQ. Male rats were injected with PQ (10 mg/kg i.p.) daily for three weeks. Rotarod systems were used for measuring locomotor activity and motor coordination. The effects of PQ on dorsiflexor, electrophysiologically-induced muscle contraction were studied. Dopamine concentrations in the ventral mesencephalon were measured by high performance liquid chromatography and the number of dopaminergic neurons in substantia nigra pars compacta was estimated by tyrosine hydroxylase immunohistochemistry. PQ induced difficulty in movement and significant reduction in motor activity and twitch tension at the dorsiflexor skeletal muscle. The number of tyrosine hydroxylase positive neurons was significantly less in the substantia nigra pars compacta and nigral dopamine level was significantly reduced in PQ treated animals (20.4+/-3.4 pg/mg) when compared with control animals (55.0+/-2.4 pg/mg wet tissue). Daily treatment of PQ for three weeks induces selective dopaminergic neuronal loss in the substantia nigra and significant behavioral and peripheral motor deficit effects.

Mechanism of cytotoxicity of paraquat

Acute paraquat poisoning seems to be very complex because many possible mechanisms of paraquat cytotoxicity have been reported. Some may not be the cause of paraquat poisoning but the result or an accompanying phenomenon of paraquat action. The mechanism critical for cell damage is still unknown. Paraquat poisoning is probably a combination of several paraquat actions. Arguing which mechanism is more critical may not be important, and these clarified mechanisms should be connected and utilized in the development of treatment for paraquat poisoning. Many people still die of pulmonary fibrosis after paraquat exposure. The next target of study will be to verify the mechanism of pulmonary fibrosis by paraquat on the basis of the outcome of studies such as this review.

Ultrastructural modifications in the mitochondria of hypoxia-adapted Drosophila melanogaster

Chronic hypoxia (CH) occurs under certain physiological or pathological conditions, including in people who reside at high altitude or suffer chronic cardiovascular or pulmonary diseases. As mitochondria are the predominant oxygen-consuming organelles to generate ATP through oxidative phosphorylation in cells, their responses, through structural or molecular modifications, to limited oxygen supply play an important role in the overall functional adaptation to hypoxia. Here, we report the adaptive mitochondrial ultrastructural modifications and the functional impacts in a recently generated hypoxia-adapted Drosophila melanogaster strain that survives severe, otherwise lethal, hypoxic conditions. Using electron tomography, we discovered increased mitochondrial volume density and cristae abundance, yet also cristae fragmentation and a unique honeycomb-like structure in the mitochondria of hypoxia-adapted flies. The homeostatic levels of adenylate and energy charge were similar between hypoxia-adapted and naïve control flies and the hypoxia-adapted flies remained active under severe hypoxia as quantified by negative geotaxis behavior. The equilibrium ATP level was lower in hypoxia-adapted flies than those of the naïve controls tested under severe hypoxia that inhibited the motion of control flies. Our results suggest that the structural rearrangement in the mitochondria of hypoxia-adapted flies may be an important adaptive mechanism that plays a critical role in preserving adenylate homeostasis and metabolism as well as muscle function under chronic hypoxic conditions.

Medical management of paraquat ingestion

Poisoning by paraquat herbicide is a major medical problem in parts of Asia while sporadic cases occur elsewhere. The very high case fatality of paraquat is due to inherent toxicity and lack of effective treatments. We conducted a systematic search for human studies that report toxicokinetics, mechanisms, clinical features, prognosis and treatment. Paraquat is rapidly but incompletely absorbed and then largely eliminated unchanged in urine within 12-24 h. Clinical features are largely due to intracellular effects. Paraquat generates reactive oxygen species which cause cellular damage via lipid peroxidation, activation of NF-κB, mitochondrial damage and apoptosis in many organs. Kinetics of distribution into these target tissues can be described by a two-compartment model. Paraquat is actively taken up against a concentration gradient into lung tissue leading to pneumonitis and lung fibrosis. Paraquat also causes renal and liver injury. Plasma paraquat concentrations, urine and plasma dithionite tests and clinical features provide a good guide to prognosis. Activated charcoal and Fuller's earth are routinely given to minimize further absorption. Gastric lavage should not be performed. Elimination methods such as haemodialysis and haemoperfusion are unlikely to change the clinical course. Immunosuppression with dexamethasone, cyclophosphamide and methylprednisolone is widely practised, but evidence for efficacy is very weak. Antioxidants such as acetylcysteine and salicylate might be beneficial through free radical scavenging, anti-inflammatory and NF-κB inhibitory actions. However, there are no published human trials. The case fatality is very high in all centres despite large variations in treatment.

Melatonin protects against rotenone-induced cell injury via inhibition of Omi and Bax-mediated autophagy in Hela cells

Parkinson's disease is the second most common neurodegenerative disease, and environmental toxins such as rotenone play an important role in causing degeneration of dopaminergic neurons. Melatonin, a major secretory product of pineal, is recently reported to protect against rotenone-induced cell death in animal models. Yet, the mechanism involved in this protection needs to be elucidated. Here, we report that rotenone treatment (0-100 μM) decreased cell survival of Hela cells in a dose-dependent manner. At concentrations ranging from 0.1 to 100 μM, rotenone induced a dose-dependent increase in the expression of microtubule-associated protein 1 light chain 3 (LC3)-II, a protein associated with the autophagosomal membrane. Knockdown of Bax or Omi using shRNA inhibited 1 μM rotenone-induced autophagy. To determine whether melatonin would protect cells against rotenone-induced cell death and autophagy, we pretreated Hela cells with 250 μM melatonin for 24 hr in the presence of rotenone. Melatonin inhibited Bax expression and the release of the omi/HtrA2 into the cytoplasm induced by 1 μM rotenone. Melatonin 250 μM treatment also suppressed cell death induced by 0.1-100 μM rotenone and protected against the formation of LC3-II in cells exposed to 1 μM rotenone. This work demonstrates a novel role for melatonin as a neuroprotective agent against rotenone.

Molecular mechanisms of pesticide-induced neurotoxicity: Relevance to Parkinson's disease

Pesticides are widely used in agricultural and other settings, resulting in continued human exposure. Pesticide toxicity has been clearly demonstrated to alter a variety of neurological functions. Particularly, there is strong evidence suggesting that pesticide exposure predisposes to neurodegenerative diseases. Epidemiological data have suggested a relationship between pesticide exposure and brain neurodegeneration. However, an increasing debate has aroused regarding this issue. Paraquat is a highly toxic quaternary nitrogen herbicide which has been largely studied as a model for Parkinson's disease providing valuable insight into the molecular mechanisms involved in the toxic effects of pesticides and their role in the progression of neurodegenerative diseases. In this work, we review the molecular mechanisms involved in the neurotoxic action of pesticides, with emphasis on the mechanisms associated with the induction of neuronal cell death by paraquat as a model for Parkinsonian neurodegeneration.

Environmental toxins and Parkinson's disease: putative roles of impaired electron transport chain and oxidative stress

Despite recent advancements in the biomedical fields, the etiology and pathogenesis of Parkinson's disease (PD) is still poorly understood, though the crucial roles of oxidative stress and impaired mitochondrial respiration have been suggested in the development of PD. The oxidative modification of the proteins of mitochondrial electron transport chain alters their normal function leading to the state of energy crisis in neurons. Exposure of environmental chemicals such as 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and rotenone in mouse produces the symptoms akin to PD and therefore these neurotoxins are commonly used in experimental studies on PD. Another environmental toxin, paraquat (a commonly used herbicide) has also been implicated with the onset of PD. The neurotoxicity of these chemicals is accompanied by the blockade of electron flow from NADH dehydrogenase to coenzyme Q. The agents with the ability to improve mitochondrial respiration and ATP production have been shown to exert beneficial effects in PD patients as well as in the animal models of PD. This review summarizes the current research implicating the impairment of mitochondrial respiratory chain and the role of environmental toxins in the pathogenesis of PD.

Mitochondrial complex I inhibition is not required for dopaminergic neuron death induced by rotenone, MPP+, or paraquat

Inhibition of mitochondrial complex I is one of the leading hypotheses for dopaminergic neuron death associated with Parkinson's disease (PD). To test this hypothesis genetically, we used a mouse strain lacking functional Ndufs4, a gene encoding a subunit required for complete assembly and function of complex I. Deletion of the Ndufs4 gene abolished complex I activity in midbrain mesencephalic neurons cultured from embryonic day (E) 14 mice, but did not affect the survival of dopaminergic neurons in culture. Although dopaminergic neurons were more sensitive than other neurons in these cultures to cell death induced by rotenone, MPP(+), or paraquat treatments, the absence of complex I activity did not protect the dopaminergic neurons, as would be expected if these compounds act by inhibiting complex 1. In fact, the dopaminergic neurons were more sensitive to rotenone. These data suggest that dopaminergic neuron death induced by treatment with rotenone, MPP(+), or paraquat is independent of complex I inhibition.