Chronic inhalation of rotenone or paraquat does not induce Parkinson's disease symptoms in mice or rats

Aqueous leaf extract of Phyllanthus amarus protects against oxidative stress and misfiring of dopaminergic neurons in Paraquat-induced Parkinson's disease-like model of adult Wistar rats

BACKGROUND OF THE STUDY

Phyllanthus amarus has high nutritional value and is beneficial in managing and treating diverse ailments. This study assessed the role of aqueous leaf extract of Phyllanthus amarus on Paraquat (PQ) induced neurotoxicity in the substantia nigra of Wistar rats.

MATERIALS AND METHODS

The role of aqueous leaves extract of Phyllanthus amarus was assessed using an open field test (OFT) for motor activity, oxidative stress biomarkers [Catalase (CAT), and Superoxide Dismutase (SOD)], histological examination (H and E stained) for cytoarchitectural changes and immunohistochemical studies using tyrosine hydroxylase (TH) as a marker for dopaminergic neurons. Forty-two (42) rats were categorized into six groups (n = 7); group 1: control was administered 0.5 ml/kg distilled water, group 2: received 10 mg/kg PQ + 10 mg/kg L-dopa as reference drug, group 3; received 10 mg/kg PQ, while group 4: received 10 mg/kg PQ + 200 mg/kg P. amarus, group 5: received 10 mg/kg PQ + 300 mg/kg P. amarus, and group 6: received 10 mg/kg PQ + 400 mg/kg P. amarus respectively, for 14 days. All administrations were done orally; a significant difference was set at p < 0.05.

RESULTS AND DISCUSSION

The study's open field test (OFT) revealed no motor activity deficit with Paraquat (PQ) exposure. Also, cytoarchitectural distortions were not observed with Paraquat (PQ) only treatment group compared to the control and other groups pretreated with P. amarus and L-dopa. Moreover, the Paraquat (PQ) only treatment group showed oxidative stress by significantly decreasing the antioxidant enzyme (SOD) compared to the control and L-dopa pretreated group. A significant decrease in tyrosine hydroxylase (TH) expressing dopaminergic neurons was also observed in Paraquat (PQ) only treatment. However, P. amarus treatment showed therapeutic properties by significantly increasing tyrosine hydroxylase (TH) expressing dopaminergic neuron levels relative to control.

CONCLUSION

Aqueous leaf extract of Phyllanthus amarus possesses therapeutic properties against Paraquat (PQ) induced changes in the substantia nigra of Wistar rats.

Experimental models of gut-first Parkinson's disease: A systematic review

BACKGROUND

There is strong support from studies in humans and in animal models that Parkinson's disease (PD) may begin in the gut. This brings about a unique opportunity for researchers in the field of neurogastroenterology to contribute to advancing the field and making contributions that could lead to the ability to diagnose and treat PD in the premotor stages. Lack of familiarity with some of the aspects of the experimental approaches used in these studies may present a barrier for neurogastroenterology researchers to enter the field. Much remains to be understood about intestinal-specific components of gut-first PD pathogenesis and the field would benefit from contributions of enteric and central nervous system neuroscientists.

PURPOSE

To address these issues, we have conducted a systematic review of the two most frequently used experimental models of gut-first PD: transneuronal propagation of α-synuclein preformed fibrils and oral exposure to environmental toxins. We have reviewed the details of these studies and present methodological considerations for the use of these models. Our aim is that this review will serve as a framework and useful reference for neuroscientists, gastroenterologists, and neurologists interested in applying their expertise to advancing our understanding of gut-first PD.

Paraquat-induced systemic inflammation and oxidative stress in rats improved by Curcuma longa ethanolic extract, curcumin and a PPAR agonist

The effect of Curcuma longa (Cl) ethanolic extract, nano-curcumin (Cu) and a PPARγ activator, pioglitazone on inhaled paraquat (PQ)-induced systemic inflammation and oxidative stress was examined in the present study. Control rats were exposed to normal saline and PQ groups to 27 and 54 mg/m3 (PQ-L and PQ-H) aerosols. Nine other PQ-H groups were treated with Curcuma longa (Cl, 150 and 600 mg/kg/day), nano-curcumin (Cu, 2 and 8 mg/kg/day), pioglitazone (Pio, 5 and 10 mg/kg), low dose of Pio + Cl and Cu and dexamethasone (0.03 mg/kg/day) for 16 days after PQ exposure period (n = 8). Total and differential WBC counts, malondialdehyde (MDA) and TNF-α levels were increased but thiol, catalase (CAT), superoxide dismutase (SOD), IL-10 and IFN-γ levels were decreased in the blood in the both PQ groups (p < 0.05 to p < 0.001). Treatment with Dexa and both doses of Cl, Cu, and Pio improved all measured variables compared to the PQ-H group (p < 0.05 to p < 0.001). The improvements of most variables in the treated group with low dose of Pio + Cl and Cu were higher than the effects of three agents alone. Systemic inflammation and oxidative stress induced by inhaled PQ were improved by Cl, Cu and Pio. In addition, a synergic effect between Pio with those of Cl and Cu was shown, suggesting PPARγ mediated effects of the plant and its derivative Cu.

Non-Reproducibility of Oral Rotenone as a Model for Parkinson's Disease in Mice

Oral rotenone has been proposed as a model for Parkinson’s disease (PD) in mice. To establish the model in our lab and study complex behavior we followed a published treatment regimen. C57BL/6 mice received 30 mg/kg body weight of rotenone once daily via oral administration for 4 and 8 weeks. Motor functions were assessed by RotaRod running. Immunofluorescence studies were used to analyze the morphology of dopaminergic neurons, the expression of alpha-Synuclein (α-Syn), and inflammatory gliosis or infiltration in the substantia nigra. Rotenone-treated mice did not gain body weight during treatment compared with about 4 g in vehicle-treated mice, which was however the only robust manifestation of drug treatment and suggested local gut damage. Rotenone-treated mice had no deficits in motor behavior, no loss or sign of degeneration of dopaminergic neurons, no α-Syn accumulation, and only mild microgliosis, the latter likely an indirect remote effect of rotenone-evoked gut dysbiosis. Searching for explanations for the model failure, we analyzed rotenone plasma concentrations via LC-MS/MS 2 h after administration of the last dose to assess bioavailability. Rotenone was not detectable in plasma at a lower limit of quantification of 2 ng/mL (5 nM), showing that oral rotenone had insufficient bioavailability to achieve sustained systemic drug levels in mice. Hence, oral rotenone caused local gastrointestinal toxicity evident as lack of weight gain but failed to evoke behavioral or biological correlates of PD within 8 weeks.

Taste Impairments in a Parkinson’s Disease Model Featuring Intranasal Rotenone Administration in Mice

Background: Taste impairments are often accompanied by olfactory impairments in the early stage of Parkinson’s disease (PD). The development of animal models is required to elucidate the mechanisms underlying taste impairments in PD. Objective: This study was conducted to clarify whether the intranasal administration of rotenone causes taste impairments prior to motor deficits in mice. Methods: Rotenone was administrated to the right nose of mice once a day for 1 or 4 week(s). In the 1-week group, taste, olfactory, and motor function was assessed before and after a 1-week recovery period following the rotenone administration. Motor function was also continuously examined in the 4-weeks group from 0 to 5 weeks. After a behavioral test, the number of catecholamine neurons (CA-Nos) was counted in the regions responsible for taste, olfactory, and motor function. Results: taste and olfactory impairments were simultaneously observed without locomotor impairments in the 1-week group. The CA-Nos was significantly reduced in the olfactory bulb and nucleus of the solitary tract. In the 4-week group, locomotor impairments were observed from the third week, and a significant reduction in the CA-Nos was observed in the substantia nigra (SN) and ventral tegmental area (VTA) at the fifth week along with the weight loss. Conclusion: The intranasal administration of rotenone caused chemosensory and motor impairments in an administration time-period dependent manner. Since chemosensory impairments were expressed prior to the locomotor impairments followed by SN/VTA CA neurons loss, this rotenone administration model may contribute to the clarification of the prodromal symptoms of PD.

Rotenone induces regionally distinct α-synuclein protein aggregation and activation of glia prior to loss of dopaminergic neurons in C57Bl/6 mice

Rotenone is a naturally occurring insecticide that inhibits mitochondrial complex I and leads to neurochemical and neuropathological deficits closely resembling those in Parkinson's disease (PD). Deficits include loss of dopaminergic neurons (DAn) in the substantia nigra pars compacta (SNpc), decreased dopamine levels and aggregation of misfolded alpha-synuclein (p129). In rat models of rotenone-induced parkinsonism, the progression of neuronal injury has been associated with activation of microglia and astrocytes. However, these neuroinflammatory changes have been challenging to study in mice, in part because the systemic rotenone exposure model utilized in rats is more toxic to mice. To establish a reproducible murine model of rotenone-induced PD, we therefore investigated the progression of neuroinflammation, protein aggregation and DAn loss in C57Bl/6 mice by exposing animals to 2.5 mg/kg/day rotenone for 14 days, followed by a two-week period where neuroinflammation is allowed to progress. Our results indicate that initial cellular dysfunction leads to increased formation of proteinase K-resistant p129 aggregates in the caudate-putamen and SNpc. Clearance of these aggregates was region- and cell type-specific, with the early appearance of reactive astrocytes coinciding with accumulation of p129 in the SNpc. Phagocytic microglial cells containing p129 aggregates were observed proximal to p129+ DAn in the SNpc. The majority of neuronal loss in the SNpc occurred during the two-week period after rotenone exposure, subsequent to the peak of microglia and astrocyte activation, as well as the peak of p129 aggregation. A secondary peak of p129 coincided with neurodegeneration at later timepoints. These data indicate that systemic exposure to rotenone in C57Bl/6 mice causes progressive accumulation and regional spread of p129 aggregates that precede maximal loss of DAn. Thus, activation of glial cells and aggregation of p129 appear to drive neuronal loss following neurotoxic stress imposed by exposure to rotenone.

Role of OCT3 and DRP1 in the Transport of Paraquat in Astrocytes: A Mouse Study

BACKGROUND

Paraquat (PQ) is a pesticide, exposure to which has been associated with an increased risk of Parkinson's disease; however, PQ transport mechanisms in the brain are still unclear. Our previous studies indicated that the organic cation transporter 3 (OCT3) expressed on astrocytes could uptake PQ and protect the dopaminergic (DA) neurons from a higher level of extracellular PQ. At present, it is unknown how OCT3 levels are altered during chronic PQ exposure or aging, nor is it clear how the compensatory mechanisms are triggered by OCT3 deficiency. Dynamic related protein 1 (DRP1) was previously reported to ameliorate the loss of neurons during Parkinson's disease. Nowadays, mounting studies have revealed the functions of astrocyte DRP1, prompting us to hypothesize that DRP1 could regulate the PQ transport capacity of astrocytes.

OBJECTIVES

The present study aimed to further explore PQ transport mechanisms in the nigrostriatal system and identify pathways involved in extracellular PQ clearance.

METHODS

Models of PQ-induced neurodegeneration were established by intraperitoneal (i.p.) injection of PQ in wild-type (WT) and organic cation transporter-3-deficient (Oct3-/-) mice. DRP1 knockdown was achieved by viral tools in vivo and small interfering RNA (siRNA) in vitro. Extracellular PQ was detected by in vivo microdialysis. In vitro transport assays were used to directly observe the functions of different transporters. PQ-induced neurotoxicity was evaluated by tyrosine hydroxylase immunohistochemistry, in vivo microdialysis for striatal DA and behavior tests. Western blotting analysis or immunofluorescence was used to evaluate the expression levels and locations of proteins in vitro or in vivo.

RESULTS

Older mice and those chronically exposed to PQ had a lower expression of brain OCT3 and, following exposure to a 10-mg/kg i.p. PQ2+ loading dose, a higher concentration of extracellular PQ. DRP1 levels were higher in astrocytes and neurons of WT and Oct3-/- mice after chronic exposure to PQ; this was supported by finding higher levels of DRP1 after PQ treatment of dopamine transporter-expressing neurons with and without OCT3 inhibition and in primary astrocytes of WT and Oct3-/- mice. Selective astrocyte DRP1 knockdown ameliorated the PQ2+-induced neurotoxicity in Oct3-/- mice but not in WT mice. GL261 astrocytes with siRNA-mediated DRP1 knockdown had a higher expression of alanine-serine-cysteine transporter 2 (ASCT2), and transport studies suggest that extracellular PQ was transported into astrocytes by ASCT2 when OCT3 was absent.

DISCUSSION

The present study mainly focused on the transport mechanisms of PQ between the dopaminergic neurons and astrocytes. Lower OCT3 levels were found in the older or chronically PQ-treated mice. Astrocytes with DRP1 inhibition (by viral tools or mitochondrial division inhibitor-1) had higher levels of ASCT2, which we hypothesize served as an alternative transporter to remove extracellular PQ when OCT3 was deficient. In summary, our data suggest that OCT3, ASCT2 located on astrocytes and the dopamine transporter located on DA terminals may function in a concerted manner to mediate striatal DA terminal damage in PQ-induced neurotoxicity. https://doi.org/10.1289/EHP9505.

Time Course of Neurobehavioral Disruptions and Regional Brain Metabolism Changes in the Rotenone Mice Model of Parkinson’s Disease

Parkinson’s disease (PD) is characterized by slow progression with a long prodromal stage and the gradual evolution of both neuropsychological symptoms and subtle motor changes, preceding motor dysfunction. Thus, in order for animal models of PD to be valid, they should reproduce these characteristics of the disease. One of such models, in which neuropathology is induced by chronic injections of low doses of mitochondrial toxin rotenone, is well established in rats. However, data on this model adapted to mice remain controversial. We have designed the study to describe the timecourse of motor and non-motor symptoms during chronic subcutaneous administration of rotenone (4 mg/kg daily for 35 days) in C57BL/6 mice. We characterize the underlying neuropathological processes (dopaminergic neuron degeneration, regional brain metabolism, monoamine neurotransmitter and lipid peroxidation changes) at different timepoints: 1 day, 2 weeks and 5 weeks of daily rotenone exposure. Based on the behavioral data, we can describe three stages of pathology: cognitive changes from week 2 of rotenone exposure, subtle motor changes in week 3–4 and motor dysfunction starting roughly from week 4. Neuropathological changes in this model include a general decrease in COX activity in different areas of the brain (acute effect of rotenone) and a more specific decrease in midbrain (chronic effect), followed by significant neurodegeneration in SNpc but not VTA by the 5th week of rotenone exposure. However, we were unable to find changes in the level of monoamine neurotransmitters neither in the striatum nor in the cortex, nor in the level of lipid peroxidation in the brainstem. Thus, the gradual progression of pathology in this model is linked with metabolic changes, rather than with oxidative stress or tonic neurotransmitter release levels. Overall, this study supports the idea that a low-dose rotenone mouse model can also reproduce different stages of PD as well as rats.

Environmental triggers of Parkinson's disease - Implications of the Braak and dual-hit hypotheses

Idiopathic Parkinson's disease (PD) may take decades to develop, during which many risk or protective factors may come into play to initiate the pathogenesis or modify its progression to clinical PD. The lack of understanding of this prodromal phase of PD and the factors involved has been a major hurdle in the study of PD etiology and preventive strategies. Although still controversial, the Braak and dual-hit hypotheses that PD may start peripherally in the olfactory structures and/or the gut provides a theoretical platform to identify the triggers and modifiers of PD prodromal development and progression. This is particularly true for the search of environmental causes of PD as the olfactory structures and gut are the major human mucosal interfaces with the environment. In this review, we lay out our personal views about how the Braak and dual-hit hypotheses may help us search for the environmental triggers and modifiers for PD, summarize available experimental and epidemiological evidence, and discuss research gaps and strategies.

Chemically induced models of Parkinson's disease

Environmental toxins are harmful substances detrimental to humans. Constant exposure to these fatal neurotoxins can cause various neurodegenerative disorders. Although poisonous, specific neurotoxins at optimal concentrations mimic the clinical features of neurodegenerative diseases in several animal models. Such chemically-induced model systems are beneficial in deciphering the molecular mechanisms of neurodegeneration and drug screening for these disorders. One such neurotoxin is 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), a widely used chemical that recapitulates Parkinsonian features in various animal models. Apart from MPTP, other neurotoxins like 6-hydroxydopamine (6-OHDA), paraquat, rotenone also induce specific clinical features of Parkinson's disease in animal models. These chemically-induced Parkinson's disease models are playing a crucial role in understanding Parkinson's disease onset, pathology, and novel therapeutics. In this review, we provide a concise overview of various neurotoxins that can recapitulate Parkinsonian features in different in vivo and in vitro model systems specifically focusing on the different treatment methodologies of neurotoxins.

Intranasal Exposure to Low-Dose Rotenone Induced Alpha-Synuclein Accumulation and Parkinson's Like Symptoms Without Loss of Dopaminergic Neurons

Epidemiologically Parkinson's disease (PD) is associated with chronic ingestion or inhalation of environmental toxins leading to the development of motor symptoms. Though neurotoxin-based animal models played a major role in understanding diverse pathogenesis, they failed to identify the risk assessment due to uncommon route of toxin exposure. Towards this, the available neurotoxin-based intranasal (i.n.) PD models targeting olfactory bulb (OB) have demonstrated the dopaminergic (DAergic) neurodegeneration in both OB and substantia nigra (SN). Despite that, the studies detecting the alpha-synuclein (α-syn) accumulation in OB and its progression to other brain regions due to inhalation of environmental toxins are still lacking. Herein, we developed oil in water microemulsion of rotenone administered intranasally to the mice at a dose which is not detectable in blood, brain, and olfactory bulb by LCMS method. Our data reveals that 9 weeks of rotenone exposure did not induce olfactory and motor dysfunction. Conversely, after 16 weeks of washout period, rotenone treated mice showed both olfactory and motor impairment, along with α-syn accumulation in the OB and striatum without glial cell activation and loss of dopaminergic neurons. The results depict the progressive nature of the developed model and highlight the role of α-syn in PD like pathology or symptoms. Together, our findings suggest the adverse consequences of early exposure to the environmental toxins on the olfactory system for a shorter period with relevance to the development of synucleinopathy or Parkinson's disease in its later stage.

Using Rotenone to Model Parkinson's Disease in Mice: A Review of the Role of Pharmacokinetics

Rotenone is a naturally occurring toxin that inhibits complex I of the mitochondrial electron transport chain. Several epidemiological studies have shown an increased risk of Parkinson's disease (PD) in individuals exposed chronically to rotenone, and it has received great attention for its ability to reproduce many critical features of PD in animal models. Laboratory studies of rotenone have repeatedly shown that it induces in vivo substantia nigra dopaminergic cell loss, a hallmark of PD neuropathology. Additionally, rotenone induces in vivo aggregation of α-synuclein, the major component of Lewy bodies and Lewy neurites found in the brain of PD patients and another hallmark of PD neuropathology. Some in vivo rotenone models also reproduce peripheral signs of PD, such as reduced intestinal motility and peripheral α-synuclein aggregation, both of which are thought to precede classical signs of PD in humans, such as cogwheel rigidity, bradykinesia, and resting tremor. Nevertheless, variability has been noted in cohorts of animals exposed to the same rotenone exposure regimen and also between cohorts exposed to similar doses of rotenone. Low doses, administered chronically, may reproduce PD symptoms and neuropathology more faithfully than excessively high doses, but overlap between toxicity and parkinsonian motor phenotypes makes it difficult to separate if behavior is examined in isolation. Rotenone degrades when exposed to light or water, and choice of vehicle may affect outcome. Rotenone is metabolized extensively in vivo, and choice of route of exposure influences greatly the dose used. However, male rodents may be capable of greater metabolism of rotenone, which could therefore reduce their total body exposure when compared with female rodents. The pharmacokinetics of rotenone has been studied extensively, over many decades. Here, we review these pharmacokinetics and models of PD using this important piscicide.

C/EBPβ/δ-secretase signaling mediates Parkinson's disease pathogenesis via regulating transcription and proteolytic cleavage of α-synuclein and MAOB

Parkinson's disease (PD) is characterized by dopaminergic neuronal loss and the presence of intra-neuronal Lewy body (LB) inclusions with aggregated α-synuclein (α-Syn) as the major component. MAOB, a crucial monoamine oxidase for dopamine metabolism, triggers oxidative stress in dopaminergic neurons and α-Syn aggregation. However, the key molecular mechanism that mediates PD pathogenesis remains elusive. Here we show that C/EBPβ acts as an age-dependent transcription factor for both α-Syn and MAOB, and initiates the PD pathologies by upregulating these two pivotal players, in addition to escalating δ-secretase activity to cleave α-Syn and promotes its neurotoxicity. Overexpression of C/EBPβ in human wild-type α-Syn transgenic mice facilitates PD pathologies and elicits motor disorders associated with augmentation of δ-secretase, α-Syn, and MAOB. In contrast, depletion of C/EBPβ from human α-Syn Tg mice abolishes rotenone-elicited PD pathologies and motor impairments via downregulating the expression of these key factors. Hence, our study supports that C/EBPβ/δ-secretase signaling mediates PD pathogenesis via regulating the expression and cleavage of α-Syn and MAOB.

A novel idea for establishing Parkinson's disease mouse model by intranasal administration of paraquat

Background: In this study, we sought to provide an idea for establishing a novel mouse model for Parkinson's disease (PD) through intranasal administration of paraquat instead of the conventional method of intraperitoneal injection. Intranasal administration has the potential to lower mortality caused by intraperitoneal paraquat administration.Methods: A paraquat-loaded thermosensitive hydrogel composed of poloxamer 407 and poloxamer 188 was prepared. The survival rate of the animals was monitored upon paraquat administration nasally and intraperitoneally. The animals' behavior was also observed. Immunofluorescence staining of tyrosine hydroxylase (TH) - positive cells and western blotting of α-synuclein (α-syn)in striatum were performed. HPLC method with electrochemical detection was used to quantify monoamine neurotransmitters in striatum. Real-time RT-PCR analysis of type 1 collagen, type 3 collagen and fibronectin expression was used to evaluate pulmonary fibrosis in mice after paraquat administration.Results: The results indicated that intranasal administration of paraquat-loaded thermosensitive hydrogel can elicit Parkinsonism-like symptoms in mice. Relative to the conventional intraperitoneal injection, this strategy significantly improves survival when modeling PD and resulted in a higher loss of TH positive neurons in substantia nigra pars compacta (SNpc) and more aggregation of α-syn in striatum. Moreover, animals receiving paraquat hydrogel nasally exhibited motor disorder as well as lower levels of dopamine and dopamine metabolites in striatum when compared to those receiving paraquat intraperitoneally. The mRNA expression of collagen and fibronectinindicated that intranasal administration of paraquat was not associated with lung fibrosis.Conclusion: This strategy provides a new idea and more convenient operation for the future study of mouse model of PD.

Adult Endogenous Dopaminergic Neuroregeneration Against Parkinson's Disease: Ideal Animal Models?

Parkinson's disease (PD) is the second most common neurodegenerative disease. The etiology of PD remains an enigma with no available disease modifying treatment or cure. Pharmacological compensation is the only quality of life improving treatments available. Endogenous dopaminergic neuroregeneration has recently been considered a plausible therapeutic strategy for PD. However, researchers have to first decipher the complexity of adult endogenous neuroregeneration. This raises the need of animal models to understand the underlying molecular basis. Mammalian models with highly conserved genetic homology might aid researchers to identify specific molecular mechanisms. However, the scarcity of adult neuroregeneration potential in mammals obfuscates such investigations. Nowadays, non-mammalian models are gaining popularity due to their explicit ability to neuroregenerate naturally without the need of external enhancements, yet these non-mammals have a much diverse gene homology that critical molecular signals might not be conserved across species. The present review highlights the advantages and disadvantages of both mammalian and non-mammalian animal models that can be essentially used to study the potential of endogenous DpN regeneration against PD.

Exposure to paraquat associated with periodontal disease causes motor damage and neurochemical changes in rats

Exposure to paraquat is possibly involved with the development of several conditions, including neurodegenerative diseases, such as Parkinson's disease (PD). This condition is mainly characterized by the loss of dopaminergic neurons in the nigrostriatal pathway and the development of classical motor symptoms. Etiology includes exposure to environmental factors, such as the paraquat exposure, and inflammatory diseases may exacerbate paraquat neurotoxicity. The aim of the study was to investigate whether the exposure to paraquat associated with the presence of periodontal disease is able to induce motor and biochemical changes in rats similar to that observed in PD. Adult male Wistar rats were sent to ligature. After 48 h, they were sent to daily treatment paraquat (1 mg/kg/day; 2 mL/kg; intragastric) or vehicle for 4 weeks. Twenty-four hours after the last administration, the open field test was performed. The rats were euthanized and the left hemimandibles and striatum were dissected for the analysis of dopaminergic and inflammatory markers. Only the combination of periodontal disease model plus paraquat exposure induced motor impairments. Remarkably, the paraquat exposure increased the ligature-induced alveolar bone loss in hemimandibles. Moreover, only the combination of periodontal disease and paraquat exposure induced the loss of dopaminergic neurons and astrocyte activation in the striatum.

Chronic Systemic Exposure to Low-Dose Rotenone Induced Central and Peripheral Neuropathology and Motor Deficits in Mice: Reproducible Animal Model of Parkinson's Disease

Epidemiological studies demonstrated that pesticide exposure, such as rotenone and paraquat, increases the risk of Parkinson’s disease (PD). Chronic systemic exposure to rotenone, a mitochondrial complex I inhibitor, could reproduce many features of PD. However, the adoption of the models is limiting because of variability in animal sensitivity and the inability of other investigators to consistently reproduce the PD neuropathology. In addition, most of rotenone models were produced in rats. Here, we tried to establish a high-reproducible rotenone model using C57BL/6J mice. The rotenone mouse model was produced by chronic systemic exposure to a low dose of rotenone (2.5 mg/kg/day) for 4 weeks by subcutaneous implantation of rotenone-filled osmotic mini pump. The rotenone-treated mice exhibited motor deficits assessed by open field, rotarod and cylinder test and gastrointestinal dysfunction. Rotenone treatment decreased the number of dopaminergic neuronal cells in the substantia nigra pars compacta (SNpc) and lesioned nerve terminal in the striatum. In addition, we observed significant reduction of cholinergic neurons in the dorsal motor nucleus of the vagus (DMV) and the intestinal myenteric plexus. Moreover, α-synuclein was accumulated in neuronal soma in the SNpc, DMV and intestinal myenteric plexus in rotenone-treated mice. These data suggest that the low-dose rotenone mouse model could reproduce behavioral and central and peripheral neurodegenerative features of PD and be a useful model for investigation of PD pathogenesis.

Intranasal glyphosate-based herbicide administration alters the redox balance and the cholinergic system in the mouse brain

Pesticide exposure is associated with cognitive and psychomotor disorders. Glyphosate-based herbicides (GlyBH) are among the most used agrochemicals, and inhalation of GlyBH sprays may arise from frequent aerial pulverizations. Previously, we described that intranasal (IN) administration of GlyBH in mice decreases locomotor activity, increases anxiety, and impairs recognition memory. Then, the aim of the present study was to investigate the mechanisms involved in GlyBH neurotoxicity after IN administration. Adult male CF-1 mice were exposed to GlyBH IN administration (equivalent to 50 mg/kg/day of Gly acid, 3 days a week, during 4 weeks). Total thiol content and the activity of the enzymes catalase, acetylcholinesterase and transaminases were evaluated in different brain areas. In addition, markers of the cholinergic and the nigrostriatal pathways, as well as of astrocytes were evaluated by fluorescence microscopy in coronal brain sections. The brain areas chosen for analysis were those seen to be affected in our previous study. GlyBH IN administration impaired the redox balance of the brain and modified the activities of enzymes involved in cholinergic and glutamatergic pathways. Moreover, GlyBH treatment decreased the number of cholinergic neurons in the medial septum as well as the expression of the α7-acetylcholine receptor in the hippocampus. Also, the number of astrocytes increased in the anterior olfactory nucleus of the exposed mice. Taken together, these disturbances may contribute to the neurobehavioural impairments reported previously by us after IN GlyBH administration in mice.

Animal models of olfactory dysfunction in neurodegenerative diseases

Olfactory dysfunction seems to occur earlier than classic motor and cognitive symptoms in many neurodegenerative diseases, including Parkinson's disease (PD) and Alzheimer's disease (AD). Thus, the use of the olfactory system as a clinical marker for neurodegenerative diseases is helpful in the characterization of prodromal stages of these diseases, early diagnostic strategies, differential diagnosis, and, potentially, prediction of treatment success. The use of genetic and neurotoxin animal models has contributed to the understanding of the mechanisms underlying olfactory dysfunction in a number of neurodegenerative diseases. In this chapter, we provide an overview of behavioral and neurochemical alterations observed in animal models of different neurodegenerative diseases (such as genetic and Aβ infusion models for AD and neurotoxins and genetic models of PD), in which olfactory dysfunction has been described.

Intranasal Administration of Rotenone to Mice Induces Dopaminergic Neurite Degeneration of Dopaminergic Neurons in the Substantia Nigra

Exposure to environmental neurotoxins is suspected to be a risk factor for sporadic progressive neurodegenerative diseases. Parkinson's disease has been associated with exposure to the pesticide rotenone, a mitochondrial respiration inhibitor. We previously reported that intranasal administration of rotenone in mice induced dopaminergic (DA) neurodegeneration in the olfactory bulb (OB) and reduced olfactory functions. In the present study, we investigated the DA neurons in the brains of mice that were administered rotenone intranasally for an extended period. We found that the olfactory function of mice was attenuated by rotenone administration. Electrophysiological analysis of the mitral cells, which are output neurons in the OB, revealed that the inhibitory input into the mitral cells was retarded. In the immunohistochemical analysis, neurite degeneration of DA neurons in the substantia nigra was observed in rotenone-administered mice, indicating that rotenone progressively initiated the degeneration of cerebral DA neurons via the nasal route.

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.

Extrapyramidal system neurotoxicity: animal models

The central nervous system's extrapyramidal system provides involuntary motor control to the muscles of the head, neck, and limbs. Toxicants that affect the extrapyramidal system are generally clinically characterized by impaired motor control, which is usually the result of basal ganglionic dysfunction. A variety of extrapyramidal syndromes are recognized in humans and include Parkinson's disease, secondary parkinsonism, other degenerative diseases of the basal ganglia, and clinical syndromes that result in dystonia, dyskinesia, essential tremor, and other forms of tremor and chorea. This chapter briefly reviews the anatomy of the extrapyramidal system and discusses several naturally occurring and experimental models that target the mammalian (nonhuman) extrapyramidal system. Topics discussed include extrapyramidal syndromes associated with antipsychotic drugs, carbon monoxide, reserpine, cyanide, rotenone, paraquat, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), and manganese. In most cases, animals are used as experimental models to improve our understanding of the toxicity and pathogenesis of these agents. Another agent discussed in this chapter, yellowstar thistle poisoning in horses, however, represents an important spontaneous cause of parkinsonism that naturally occurs in animals. The central focus of the chapter is on animal models, especially the concordance between clinical signs, neurochemical changes, and neuropathology between animals and people.

An update on the rotenone models of Parkinson's disease: their ability to reproduce the features of clinical disease and model gene-environment interactions

Parkinson's disease (PD) is the second most common neurodegenerative disorder that is characterized by two major neuropathological hallmarks: the degeneration of dopaminergic neurons in the substantia nigra (SN) and the presence of Lewy bodies in the surviving SN neurons, as well as other regions of the central and peripheral nervous system. Animal models have been invaluable tools for investigating the underlying mechanisms of the pathogenesis of PD and testing new potential symptomatic, neuroprotective and neurorestorative therapies. However, the usefulness of these models is dependent on how precisely they replicate the features of clinical PD with some studies now employing combined gene-environment models to replicate more of the affected pathways. The rotenone model of PD has become of great interest following the seminal paper by the Greenamyre group in 2000 (Betarbet et al., 2000). This paper reported for the first time that systemic rotenone was able to reproduce the two pathological hallmarks of PD as well as certain parkinsonian motor deficits. Since 2000, many research groups have actively used the rotenone model worldwide. This paper will review rotenone models, focusing upon their ability to reproduce the two pathological hallmarks of PD, motor deficits, extranigral pathology and non-motor symptoms. We will also summarize the recent advances in neuroprotective therapies, focusing on those that investigated non-motor symptoms and review rotenone models used in combination with PD genetic models to investigate gene-environment interactions.

Neurotranmission systems as targets for toxicants: a review

Neurotransmitters are chemicals that transmit impulses from one nerve to another or from nerves to effector organs. Numerous neurotransmitters have been described in mammals, amongst them acetylcholine, amino acids, amines, peptides and gases. Toxicants may interact with various parts of neurotransmission systems, including synthetic and degradative enzymes, presynaptic vesicles and the specialized receptors that characterize neurotransmission systems. Important toxicants acting on the cholinergic system include the anticholinesterases (organophosphates and carbamates) and substances that act on receptors such as nicotine and the neonicotinoid insecticides, including imidacloprid. An important substance acting on the glutamatergic system is domoic acid, responsible for amnesic shellfish poisoning. 4-Aminobutyric acid (GABA) and glycine are inhibitory neurotransmitters and their antagonists, fipronil (an insecticide) and strychnine respectively, are excitatory. Abnormalities of dopamine neurotransmission occur in Parkinson's disease, and a number of substances that interfere with this system produce Parkinsonian symptoms and clinical signs, including notably 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, which is the precursor of 1-methyl-4-phenylpyridinium. Fewer substances are known that interfere with adrenergic, histaminergic or seroninergic neurotransmission, but there are some examples. Among peptide neurotransmission systems, agonists of opioids are the only well-known toxic compounds.

Cognitive deficits in animal models of basal ganglia disorders

The two most common neurological disorders of the basal ganglia are Parkinson's disease (PD) and Huntington's disease (HD). The most overt symptoms of these diseases are motoric, reflecting the loss of the striatal medium spiny neurons in HD and ascending substantia nigra dopaminergic cells in PD. However, both disease processes induce insidious psychiatric and cognitive syndromes that can manifest well in advance of the onset of motor deficits. These early deficits provide an opportunity for prophylactic therapeutic intervention in order to retard disease progression from the earliest possible point. In order to exploit this opportunity, animal models of HD and PD are being probed for the specific cognitive deficits represented in the disease states. At the neuronal level, these deficits are typically, but not exclusively, mediated by disruption of parallel corticostriatal loops that integrate motor information with sensory and higher order, "executive" cognitive functions. Dysfunction in these systems can be probed with sensitive behavioural tests that selectively probe these cognitive functions in mouse models with focal lesions of striatal or cortical regions, or of specific neurotransmitter systems. Typically these tests were designed and validated in rats. With the advent of genetically modified mouse models of disease, validated tests provide an opportunity to screen mouse models of disease for early onset cognitive deficits. This review seeks to draw together the literature on cognitive deficits in HD and PD, to determine the extent to which these deficits are represented in the current animal models of disease, and to evaluate the viability of selecting cognitive deficits as potential therapeutic targets. This article is part of a Special Issue entitled 'Animal Models'.

Mitochondrial complex I inhibitor rotenone-induced toxicity and its potential mechanisms in Parkinson's disease models

The etiology of Parkinson's disease (PD) is attributed to both environmental and genetic factors. The development of PD reportedly involves mitochondrial impairment, oxidative stress, α-synuclein aggregation, dysfunctional protein degradation, glutamate toxicity, calcium overloading, inflammation and loss of neurotrophic factors. Based on a link between mitochondrial dysfunction and pesticide exposure, many laboratories, including ours, have recently developed parkinsonian models by utilization of rotenone, a well-known mitochondrial complex I inhibitor. Rotenone models for PD appear to mimic most clinical features of idiopathic PD and recapitulate the slow and progressive loss of dopaminergic (DA) neurons and the Lewy body formation in the nigral-striatal system. Notably, potential human parkinsonian pathogenetic and pathophysiological mechanisms have been revealed through these models. In this review, we summarized various rotenone-based models for PD and discussed the implied etiology of and treatment for PD.

Insecticides of Natural Origin, Other than Pyrethrum and Nicotine

This chapter gives a detailed overview on the history, use and toxicology of substances of biological origin such as plant extracts (rotenone, azadirachtin, quassin, anabasin) and fermentation products from soil microorganisms (avermectins and spinosins) that are more or less widely used as insecticides in plant protection products or biocides but may also play a role in public health to prevent spread of infectious diseases or even to cure them.

Intranasal administration of neurotoxicants in animals: support for the olfactory vector hypothesis of Parkinson's disease

The causes of Parkinson's disease (PD) are unknown, but there is evidence that exposure to environmental agents, including a number of viruses, toxins, agricultural chemicals, dietary nutrients, and metals, is associated with its development in some cases. The presence of smell loss and the pathological involvement of the olfactory pathways in the early stages of PD are in accord with the tenants of the olfactory vector hypothesis. This hypothesis postulates that some forms of PD may be caused or catalyzed by environmental agents that enter the brain via the olfactory mucosa. In this article, we provide an overview of evidence implicating xenobiotics agents in the etiology of PD and review animal, mostly rodent, studies in which toxicants have been introduced into the nose in an attempt to induce behavioral or neurochemical changes similar to those seen in PD. The available data suggest that this route of exposure results in highly variable outcomes, depending upon the involved xenobiotic, exposure history, and the age and species of the animals tested. Some compounds, such as rotenone, paraquat, and 6-hydroxydopamine, have limited capacity to reach and damage the nigrostriatal dopaminergic system via the intranasal route. Others, such as 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), readily enter the brain via this route in some species and influence the function of the nigrostriatal pathway. Intranasal infusion of MPTP in some rodents elicits a developmental sequence of behavioral and neurochemical changes that closely mimics that seen in PD. For this reason, such an MPTP rodent model appears to be an ecologically valid means for assessing novel palliative treatments for both the motor and non-motor symptoms of PD. More research is needed, however, on this and other ecologically valid models.

Ceruloplasmin protects against rotenone-induced oxidative stress and neurotoxicity

To clarify the neuroprotective property of ceruloplasmin and the pathogenesis of aceruloplasminemia, we generated ceruloplasmin-deficient (CP⁻/⁻) mice on the C57BL/10 genetic background and further treated them with a mitochondrial complex I inhibitor, rotenone. There was no iron accumulation in the brains of CP⁻/⁻ mice at least up to 60 weeks of age. Without rotenone treatment, CP⁻/⁻ mice showed slight motor dysfunction compared with CP⁺/⁺ mice, but there were no detectable differences in the levels of oxidative stress markers between these two groups. A low dose of rotenone did not affect the mitochondrial complex I activity in our mice, however, it caused a significant change in motor behavior, neuropathology, or the levels of oxidative stress markers in CP⁻/⁻ mice, but not in CP⁺/⁺ mice. Our data support that ceruloplasmin protects against rotenone-induced oxidative stress and neurotoxicity, probably through its antioxidant properties independently of its function of iron metabolism.

Pharmacological targeting of the transcription factor Nrf2 at the basal ganglia provides disease modifying therapy for experimental parkinsonism

Current therapies for motor symptoms of Parkinson's disease (PD) are based on dopamine replacement. However, the disease progression remains unaffected, because of continuous dopaminergic neuron loss. Since oxidative stress is actively involved in neuronal death in PD, pharmacological targeting of the antioxidant machinery may have therapeutic value. Here, we analyzed the relevance of the antioxidant phase II response mediated by the transcription factor NF-E2-related factor 2 (Nrf2) on brain protection against the parkinsonian toxin methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Intraperitoneal administration of the potent Nrf2 activator sulforaphane (SFN) increased Nrf2 protein levels in the basal ganglia and led to upregulation of phase II antioxidant enzymes heme oxygenase-1 (HO-1) and NAD(P)H quinone oxidoreductase (NQO1). In wild-type mice, but not in Nrf2-knockout mice, SFN protected against MPTP-induced death of nigral dopaminergic neurons. The neuroprotective effects were accompanied by a decrease in astrogliosis, microgliosis, and release of pro-inflammatory cytokines. These results provide strong pharmacokinetic and biochemical evidence for activation of Nrf2 and phase II genes in the brain and also offer a neuroprotective strategy that may have clinical relevance for PD therapy.

Neurotoxic in vivo models of Parkinson's disease recent advances

Animal models have been invaluable to Parkinson's disease (PD) research. Of these, neurotoxin models have historically been the most widely utilized. The goal of this chapter is to give a brief historical description of classic PD models and then to identify the most recent important advances in modeling human PD in animals. Indeed, significant advances in modeling additional features of PD and expansion to new species have occurred in both older and newer models. The roles these new advances in modeling may have in future PD research are examined in this chapter.

Inflammatory mechanisms of neurodegeneration in toxin-based models of Parkinson's disease

Parkinson's disease (PD) has been associated with exposure to a variety of environmental agents, including pesticides, heavy metals, and organic pollutants; and inflammatory processes appear to constitute a common mechanistic link among these insults. Indeed, toxin exposure has been repeatedly demonstrated to induce the release of oxidative and inflammatory factors from immunocompetent microglia, leading to damage and death of midbrain dopamine (DA) neurons. In particular, proinflammatory cytokines such as tumor necrosis factor-α and interferon-γ, which are produced locally within the brain by microglia, have been implicated in the loss of DA neurons in toxin-based models of PD; and mounting evidence suggests a contributory role of the inflammatory enzyme, cyclooxygenase-2. Likewise, immune-activating bacterial and viral agents were reported to have neurodegenerative effects themselves and to augment the deleterious impact of chemical toxins upon DA neurons. The present paper will focus upon the evidence linking microglia and their inflammatory processes to the death of DA neurons following toxin exposure. Particular attention will be devoted to the possibility that environmental toxins can activate microglia, resulting in these cells adopting a “sensitized” state that favors the production of proinflammatory cytokines and damaging oxidative radicals.

Behavioral phenotyping of mouse models of Parkinson's disease

Parkinson's disease (PD) is a common neurodegenerative movement disorder afflicting millions of people in the United States. The advent of transgenic technologies has contributed to the development of several new mouse models, many of which recapitulate some aspects of the disease; however, no model has been demonstrated to faithfully reproduce the full constellation of symptoms seen in human PD. This may be due in part to the narrow focus on the dopamine-mediated motor deficits. As current research continues to unmask PD as a multi-system disorder, animal models should similarly evolve to include the non-motor features of the disease. This requires that typically cited behavioral test batteries be expanded. The major non-motor symptoms observed in PD patients include hyposmia, sleep disturbances, gastrointestinal dysfunction, autonomic dysfunction, anxiety, depression, and cognitive decline. Mouse behavioral tests exist for all of these symptoms and while some models have begun to be reassessed for the prevalence of this broader behavioral phenotype, the majority has not. Moreover, all behavioral paradigms should be tested for their responsiveness to L-DOPA so these data can be compared to patient response and help elucidate which symptoms are likely not dopamine-mediated. Here, we suggest an extensive, yet feasible, battery of behavioral tests for mouse models of PD aimed to better assess both non-motor and motor deficits associated with the disease.

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.

Progression of Parkinson's disease pathology is reproduced by intragastric administration of rotenone in mice

In patients with Parkinson's disease (PD), the associated pathology follows a characteristic pattern involving inter alia the enteric nervous system (ENS), the dorsal motor nucleus of the vagus (DMV), the intermediolateral nucleus of the spinal cord and the substantia nigra, providing the basis for the neuropathological staging of the disease. Here we report that intragastrically administered rotenone, a commonly used pesticide that inhibits Complex I of the mitochondrial respiratory chain, is able to reproduce PD pathological staging as found in patients. Our results show that low doses of chronically and intragastrically administered rotenone induce alpha-synuclein accumulation in all the above-mentioned nervous system structures of wild-type mice. Moreover, we also observed inflammation and alpha-synuclein phosphorylation in the ENS and DMV. HPLC analysis showed no rotenone levels in the systemic blood or the central nervous system (detection limit [rotenone]<20 nM) and mitochondrial Complex I measurements showed no systemic Complex I inhibition after 1.5 months of treatment. These alterations are sequential, appearing only in synaptically connected nervous structures, treatment time-dependent and accompanied by inflammatory signs and motor dysfunctions. These results strongly suggest that the local effect of pesticides on the ENS might be sufficient to induce PD-like progression and to reproduce the neuroanatomical and neurochemical features of PD staging. It provides new insight into how environmental factors could trigger PD and suggests a transsynaptic mechanism by which PD might spread throughout the central nervous system.

Interferon-gamma deficiency modifies the motor and co-morbid behavioral pathology and neurochemical changes provoked by the pesticide paraquat

In addition to nigrostriatal pathology and corresponding motor disturbances, Parkinson's disease (PD) is often characterized by co-morbid neuropsychiatric symptoms, most notably anxiety and depression. Separate lines of evidence indicate that inflammatory processes associated with microglial activation and cytokine release may be fundamental to the progression of both PD and its co-morbid psychiatric pathology. Accordingly, we assessed the contribution of the pro-inflammatory cytokine, interferon-gamma (IFN-gamma), to a range of PD-like pathology provoked by the ecologically relevant herbicide and dopamine (DA) toxin, paraquat. To this end, paraquat provoked overt motor impairment (reduced home-cage activity and impaired vertical climbing) and signs of anxiety-like behavior (reduced open field exploration) in wild-type but not IFN-gamma-deficient mice. Correspondingly, paraquat promoted somewhat divergent variations in neurochemical activity among wild-type and IFN-gamma null mice at brain sites important for both motor (striatum) and co-morbid affective pathologies (dorsal hippocampus, medial prefrontal cortex, and locus coeruleus). Specifically, the herbicide provoked a dosing regimen-dependent reduction in striatal DA levels that was prevented by IFN-gamma deficiency. In addition, the herbicide influenced serotonergic and noradrenergic activity within the dorsal hippocampus and medial prefrontal cortex; and elevated noradrenergic activity within the locus coeruleus. Although genetic ablation of IFN-gamma had relatively few effects on monoamine variations within the locus coeruleus and prefrontal cortex, loss of the pro-inflammatory cytokine did normalize the paraquat-induced noradrenergic alterations within the hippocampus. These findings further elucidate the functional implications of paraquat intoxication and suggest an important role for IFN-gamma in the striatal and motor pathology, as well as the co-morbid behavioral and hippocampal changes induced by paraquat.

Downstream mechanisms triggered by mitochondrial dysfunction in the basal ganglia: from experimental models to neurodegenerative diseases

Mitochondrial dysfunctions have been implicated in the cellular processes underlying several neurodegenerative disorders affecting the basal ganglia. These include Huntington's chorea and Parkinson's disease, two highly debilitating motor disorders for which recent research has also involved gene mutation linked to mitochondrial deficits. Experimental models of basal ganglia diseases have been developed by using toxins able to disrupt mitochondrial function: these molecules act by selectively inhibiting mitochondrial respiratory complexes, uncoupling cellular respiration. This in turn leads to oxidative stress and energy deficit that trigger critical downstream mechanisms, ultimately resulting in neuronal vulnerability and loss. Here we review the molecular and cellular downstream effects triggered by mitochondrial dysfunction, and the different experimental models that are obtained by the administration of selective mitochondrial toxins or by the expression of mutant genes.

Natural lipophilic inhibitors of mitochondrial complex I are candidate toxins for sporadic neurodegenerative tau pathologies

Annonacin, a natural lipophilic inhibitor of mitochondrial complex I has been implicated in the etiology of a sporadic neurodegenerative tauopathy in Guadeloupe. We therefore studied further compounds representing the broad biochemical spectrum of complex I inhibitors to which humans are potentially exposed. We determined their lipophilicity, their effect on complex I activity in submitochondrial particles, and their effect on cellular ATP levels, neuronal cell death and somatodendritic redistribution of phosphorylated tau protein (AD2 antibody against pS396/pS404-tau) in primary cultures of fetal rat striatum. The 24 compounds tested were lipophilic (logP range 0.9-8.5; exception: MPP(+) logP=-1.35) and potent complex I inhibitors (IC(50) range 0.9 nM-2.6 mM). They all decreased ATP levels (EC(50) range 1.9 nM-54.2 microM), induced neuronal cell death (EC(50) range 1.1 nM-54.5 microM) and caused the redistribution of AD2(+) tau from axons to the cell body (EC(5) range 0.6 nM-33.3 microM). The potency of the compounds to inhibit complex I correlated with their potency to induce tau redistribution (r=0.80, p<0.001). In conclusion, we propose that the widely distributed lipophilic complex I inhibitors studied here might be implicated in the induction of tauopathies with global prevalence.

Paraquat induces alternation of the dopamine catabolic pathways and glutathione levels in the substantia nigra of mice

The herbicide paraquat (PQ) is a strong redox agent that participates in the formation of reactive oxygen species (ROS) and induces toxicity in the nigrostriatal dopaminergic system. In this study, we investigated the effect of PQ on dopamine (DA) and glutathione levels in the substantia nigra (SN) of mice. Male C57BL/6 mice (aged 7 weeks and 23-25 g) were used for this study. The mice were treated with normal saline (vehicle) and PQ (10 mg/kg, i.p.) twice weekly for three consecutive weeks. We measured changes in tyrosine hydroxylase (TH) immunoreactivity, DA and its metabolites, and glutathione (reduced and oxidized) in the SN. After repeated PQ administration, the density of TH-positive neurons in the substantia nigra pars compacta (SNpc) decreased as compared to the control. Levels of DA and homovanillic acid (HVA) decreased significantly in the PQ-treated mice (p<0.05), but levels of 3,4-dihydroxyphenylacetic acid (DOPAC) and 3-methoxytyramine (3-MT) did not change. The rate of DA oxidation increased significantly in the SNpc, whereas the O-methylation pathway remained unchanged. Levels of reduced glutathione (GSH) in the SNpc decreased more in the PQ group than in the control group, while levels of oxidized glutathione (GSSG) increased in same region. We propose that repeated PQ injection induces dopaminergic neurotoxicity through generation of oxidative stress, and that this toxicity is related to the decline of GSH in the SNpc. The neurotoxic mechanism may specifically involve enhancement of the oxidative pathway of DA metabolism through coupling with the antioxidant GSH system of the SN.

Paraquat is excluded by the blood brain barrier in rhesus macaque: An in vivo pet study

Environmental factors have long been thought to have a role in the etiology of idiopathic Parkinson's disease (PD). Since the discovery of the selective neurotoxicity of MPTP to dopamine cells, suspicion has focused on paraquat, a common herbicide with chemical structure similar to 1-methyl-4-phenylpyridinium (MPP+), the MPTP metabolite responsible for its neurotoxicity. Although in vitro evidence for paraquat neurotoxicity to dopamine cells is well established, its in vivo effects have been ambiguous because paraquat is di-cationic in plasma, which raises questions about its ability to cross the blood brain barrier. This study assessed the brain uptake of [(11)C]-paraquat in adult male rhesus macaques using quantitative PET imaging. Results showed minimal uptake of [(11)C]-paraquat in the macaque brain. The highest concentrations of paraquat were seen in the pineal gland and the lateral ventricles. Global brain concentrations including those in known dopamine areas were consistent with the blood volume in those structures. This acute exposure study found that paraquat is excluded from the brain by the blood brain barrier and thus does not readily support the causative role of paraquat exposure in idiopathic Parkinson's disease.