Lifestyle-related risk factors for Parkinson's disease: a population-based study

Mild Traumatic Brain Injury as a Risk Factor for Parkinsonism, Tics, and Akathisia: A Systematic Review and Meta-Analysis

This meta-analysis aimed to assess the association between mild traumatic brain injury (mTBI) and the risk of developing Parkinsonism. A systematic literature review was conducted using PubMed, Embase, and Cochrane Library databases. Studies were eligible if they reported on the association between MTBI and Parkinsonism. Pooled odds ratios (ORs) were calculated using a random-effects model. Publication bias was assessed using Egger's and Begg's tests. A total of 18 studies were included in this meta-analysis, with 1,484,752 participants. The overall OR for Parkinsonism in individuals with a history of mTBI was 1.637 (95% CI, 1.203-2.230; p = 0.01), indicating a significant association. The OR for Parkinson's disease (PD) specifically was 1.717 (95% CI, 1.206-2.447; p = 0.01). However, insufficient data on tics and akathisia limited a meta-analysis. There was no evidence of publication bias according to Egger's (p = 0.8107) and Begg's (p = 0.4717) tests. This meta-analysis provides evidence that mTBI is a significant risk factor for Parkinsonism, particularly PD. However, the findings should be interpreted with caution due to the heterogeneity among the studies included and the study's limitations. Further research is needed to confirm these findings and to investigate the underlying mechanisms of the mTBI-Parkinsonism association.

State-of-the-art review of the clinical research on menopause and hormone replacement therapy association with Parkinson's disease: What meta-analysis studies cannot tell us

The menopause is a midlife endocrinological process that greatly affects women's central nervous system functions. Over the last 2 decades numerous clinical studies have addressed the influence of ovarian hormone decline on neurological disorders like Parkinson's disease and Alzheimer's disease. However, the findings in support of a role for age at menopause, type of menopause and hormone replacement therapy on Parkinson's disease onset and its core features show inconsistencies due to the heterogeneity in the study design. Here, we provide a unified overview of the clinical literature on the influence of menopause and ovarian hormones on Parkinson's disease. We highlight the possible sources of conflicting evidence and gather considerations for future observational clinical studies that aim to explore the neurological impact of menopause-related features in Parkinson's disease.

Risk Factors of Parkinson’s Disease: A Case-Control Study in Moroccan Patients

Background: Parkinson’s disease (PD) remains a significant health concern in Morocco. Multivariate analysis was not used in any study to evaluate the role of various factors that contributed to the onset of the disease. Objectives: This study investigates the role of family, environmental, and professional factors in PD development. Methods: The present study is an age-matched case-control study with risk estimation based on odds ratios (OR) with a 95% confidence interval (CI). In total, 180 cases were matched with 360 controls. Results: The average age of participants was 68.3 ± 11.2 years. Adjusted logistic regression analysis showed that the family history of PD (ORa = 7.19, CI 95% 3.41 - 15.13), male sex (ORa = 1.92, CI 95% 1.16 - 3.16), spring water consumption (ORa = 3.31, CI 95% 2.05 - 5.34), drug use (ORa = 2.12, CI 95% 1.33 - 3.38), a history of head injury (ORa = 3.38, CI 95% 1.16 - 9.83) and non - consumption of coffee (ORa = 3.04, CI 95% 1.56 - 5.90) were significantly associated with the onset of the disease. In a univariate analysis, well water consumption was observed as a significant risk factor but could not be shown to be significant in a multivariate analysis. Previous work on a farm (ORa = 0.30, CI 95% 0.16 - 0.54) and history of general anesthesia (ORa = 0.47, CI 95% 0.27 - 0.83) were inversely associated with PD risk. No statistical significance was observed in the data on occupational exposure and disease risk, although there was a 30% decrease in risk for the service occupations (ORa = 0.05, CI 95% 0.01 - 0.18). Conclusions: As a result, further research is needed to determine additional risk factors.

Cognitive Impairment in Parkinson's Disease: Epidemiology, Clinical Profile, Protective and Risk Factors

Cognitive impairment is a common non-motor symptom in Parkinson's Disease (PD) and an important source of patient disability and caregiver burden. The timing, profile and rate of cognitive decline varies widely among individuals with PD and can range from normal cognition to mild cognitive impairment (PD-MCI) and dementia (PDD). Beta-amyloid and tau brain accumulation, oxidative stress and neuroinflammation are reported risk factors for cognitive impairment. Traumatic brain injury and pesticide and tobacco exposure have also been described. Genetic risk factors including genes such as COMT, APOE, MAPT and BDNF may also play a role. Less is known about protective factors, although the Mediterranean diet and exercise may fall in this category. Nonetheless, there is conflicting evidence for most of the factors that have been studied. The use of inconsistent criteria and lack of comprehensive assessment in many studies are important methodological issues. Timing of exposure also plays a crucial role, although identification of the correct time window has been historically difficult in PD. Our understanding of the mechanism behind these factors, as well as the interactions between gene and environment as determinants of disease phenotype and the identification of modifiable risk factors will be paramount, as this will allow for potential interventions even in established PD.

Deep learning in systems medicine

Systems medicine (SM) has emerged as a powerful tool for studying the human body at the systems level with the aim of improving our understanding, prevention and treatment of complex diseases. Being able to automatically extract relevant features needed for a given task from high-dimensional, heterogeneous data, deep learning (DL) holds great promise in this endeavour. This review paper addresses the main developments of DL algorithms and a set of general topics where DL is decisive, namely, within the SM landscape. It discusses how DL can be applied to SM with an emphasis on the applications to predictive, preventive and precision medicine. Several key challenges have been highlighted including delivering clinical impact and improving interpretability. We used some prototypical examples to highlight the relevance and significance of the adoption of DL in SM, one of them is involving the creation of a model for personalized Parkinson's disease. The review offers valuable insights and informs the research in DL and SM.

Repurposing sex steroids and related drugs as potential treatment for Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disorder for which a greater prevalence and incidence is described in men. This suggests a protective effect of sex hormones in the brain. Therefore, steroids and drugs to treat endocrine conditions could have additional application for PD. Here, we review the protective effect of sex hormones, particularly estrogens, progesterone, androgens and dehydroepiandrosterone, in animal models of PD and also in human studies. Data also support that drugs affecting estrogen neurotransmission such as selective estrogen receptor modulators or affecting steroid metabolism with 5α-reductase inhibitors could be repositioned for treatment of PD. Sex steroids are also modulator of neurotransmission, thus they could repurposed to treat PD motor symptoms and to modulate the response to PD medication. No drug is yet available to limit PD progression. PD is a complex disease implicating multiple pathological processes and a therapeutic strategy using drugs with several mechanisms of action, such as sex steroids and endocrine drugs are interesting repositioning options for symptomatic treatment and disease-modifying activity for PD. This article is part of the Special Issue entitled 'Drug Repurposing: old molecules, new ways to fast track drug discovery and development for CNS disorders'.

Epidemiology of Parkinson's Disease-East Versus West

Background

The cause of PD at present remains unknown. A number of epidemiological studies have been conducted across the globe to ascertain the disease burden and the possible risk factors. In this review, we analyze the various studies from East and West with an aim to observe the important similarities and differences in the disease occurrence and risk factor profile.

Methods

A comprehensive search of descriptive and analytical epidemiological studies was undertaken. The descriptive studies and meta-analysis providing the standardised population rates were selected. The demographics, ethnicity and geographical differences between East and West were analysed. In analytical epidemiology, more established and well-studied non-genetic risk factors for PD were reviewed utilising the prospective cohort studies, case control studies and meta-analysis where available.

Results and Conclusion

PD is more common with increasing age and shows male predominance, which is more obvious in Western studies. The PD prevalence and incidence rates are slightly lower in the East compared to the West. Incidence studies on different ethnic populations in the same country have also found a lower occurrence of PD amongst Easterners compared to Westerners. Setting methodological differences aside, studies from East and West suggest a role for both environmental and genetic risk factors in PD causation. Smoking, caffeine intake and pesticide exposure are well-established risk factors across regions. There is a robust data for dairy product consumption, urate levels and physical activity in the West while studies on certain risk factors like head injury and alcohol show conflicting and mixed results.

The Impact of Traumatic Brain Injury on Later Life: Effects on Normal Aging and Neurodegenerative Diseases

The acute and chronic effects of traumatic brain injury (TBI) have been widely described; however, there is limited knowledge on how a TBI sustained during early adulthood or mid-adulthood will influence aging. Epidemiological studies have explored whether TBI poses a risk for dementia and other neurodegenerative diseases associated with aging. We will discuss the influence of TBI and resulting medical comorbidities such as endocrine, sleep, and inflammatory disturbances on age-related gray and white matter changes and cognitive decline. Post mortem studies examining amyloid, tau, and other proteins will be discussed within the context of neurodegenerative diseases and chronic traumatic encephalopathy. The data support the suggestion that pathological changes triggered by an earlier TBI will have an influence on normal aging processes and will interact with neurodegenerative disease processes rather than the development of a specific disease, such as Alzheimer's or Parkinson's. Chronic neurophysiologic change after TBI may have detrimental effects on neurodegenerative disease.

Occupational pesticide use and Parkinson's disease in the Parkinson Environment Gene (PEG) study

OBJECTIVE

To study the influence of occupational pesticide use on Parkinson's disease (PD) in a population with information on various occupational, residential, and household sources of pesticide exposure.

METHODS

In a population-based case control study in Central California, we used structured interviews to collect occupational history details including pesticide use in jobs, duration of use, product names, and personal protective equipment use from 360 PD cases and 827 controls. We linked reported products to California's pesticide product label database and identified pesticide active ingredients and occupational use by chemical class including fungicides, insecticides, and herbicides. Employing unconditional logistic regression, we estimated odds ratios and 95% confidence intervals for PD and occupational pesticide use.

RESULTS

Ever occupational use of carbamates increased risk of PD by 455%, while organophosphorus (OP) and organochlorine (OC) pesticide use doubled risk. PD risk increased 110-211% with ever occupational use of fungicides, herbicides, and insecticides. Using any pesticide occupationally for >10years doubled the risk of PD compared with no occupational pesticide use. Surprisingly, we estimated higher risks among those reporting use of personal protective equipment (PPE).

CONCLUSIONS

Our findings provide additional evidence that occupational pesticide exposures increase PD risk. This was the case even after controlling for other sources of pesticide exposure. Specifically, risk increased with occupational use of carbamates, OPs, and OCs, as well as of fungicides, herbicides, or insecticides. Interestingly, some types of PPE use may not provide adequate protection during pesticide applications.

An Evaluation of the Proposed Worker Protection Standard with Respect to Pesticide Exposure and Parkinson's Disease

Citing a lack of information, the U.S. Environmental Protection Agency prudently did not account for the benefits of averting many chronic diseases in analyzing the Worker Protection Standards (WPS) revisions. We demonstrate that sufficient information can exist, using the example of the benefits to agricultural workers of reduced Parkinson's disease (PD) due to reduced pesticide exposure. We define the benefits as the monetary value gained by improving quality of lives of people who would otherwise develop PD, plus the value of medical care cost averted and income not lost due to being healthy. For estimation, we use readily available parameters and obtain odds ratios of developing PD by conducting a meta-analysis of studies linking pesticide exposure to PD. The sensitivity analysis varies the number of agricultural workers affected by the regulation, the probability of being diagnosed with PD, the measurement and the timing of the benefits. Our initial assessment is that the reduced PD benefits would be a small fraction of the total WPS revision costs. However, if we define benefits as the common environmental economics willingness to pay to avoid PD incidence, then they become a substantial fraction of the costs. Our analysis demonstrates that the benefits of averting PD from the WPS revisions can be estimated using existing information, and that the results are most sensitive to the choice of valuation of benefits to the worker. We encourage other researchers to extend our framework to other chronic ailments.

Association between Parkinson's Disease and Cigarette Smoking, Rural Living, Well-Water Consumption, Farming and Pesticide Use: Systematic Review and Meta-Analysis

Objective

Bradford Hill’s viewpoints were used to conduct a weight-of-the-evidence assessment of the association between Parkinson’s disease (PD) and rural living, farming and pesticide use. The results were compared with an assessment based upon meta-analysis. For comparison, we also evaluated the association between PD and cigarette smoking as a “positive control” because a strong inverse association has been described consistently in the literature.

Methods

PubMed was searched systematically to identify all published epidemiological studies that evaluated associations between Parkinson’s disease (PD) and cigarette smoking, rural living, well-water consumption, farming and the use of pesticides, herbicides, insecticides, fungicides or paraquat. Studies were categorized into two study quality groups (Tier 1 or Tier 2); data were abstracted and a forest plot of relative risks (RRs) was developed for each risk factor. In addition, when available, RRs were tabulated for more highly exposed individuals compared with the unexposed. Summary RRs for each risk factor were calculated by meta-analysis of Tier 1, Tier 2 and all studies combined, with sensitivity analyses stratified by other study characteristics. Indices of between-study heterogeneity and evidence of reporting bias were assessed. Bradford Hill’s viewpoints were used to determine if a causal relationship between PD and each risk factor was supported by the weight of the evidence.

Findings

There was a consistent inverse (negative) association between current cigarette smoking and PD risk. In contrast, associations between PD and rural living, well-water consumption, farming and the use of pesticides, herbicides, insecticides, fungicides or paraquat were less consistent when assessed quantitatively or qualitatively.

Conclusion

The weight of the evidence and meta-analysis support the conclusion that there is a causal relationship between PD risk and cigarette smoking, or some unknown factor correlated with cigarette smoking. There may be risk factors associated with rural living, farming, pesticide use or well-water consumption that are causally related to PD, but the studies to date have not identified such factors. To overcome the limitations of research in this area, future studies will have to better characterize the onset of PD and its relationship to rural living, farming and exposure to pesticides.

Association of traumatic brain injury with subsequent neurological and psychiatric disease: a meta-analysis

OBJECTIVE

Mild traumatic brain injury (TBI) has been proposed as a risk factor for the development of Alzheimer's disease, Parkinson's disease, depression, and other illnesses. This study's objective was to determine the association of prior mild TBI with the subsequent diagnosis (that is, at least 1 year postinjury) of neurological or psychiatric disease.

METHODS

All studies from January 1995 to February 2012 reporting TBI as a risk factor for diagnoses of interest were identified by searching PubMed, study references, and review articles. Reviewers abstracted the data and assessed study designs and characteristics.

RESULTS

Fifty-seven studies met the inclusion criteria. A random effects meta-analysis revealed a significant association of prior TBI with subsequent neurological and psychiatric diagnoses. The pooled odds ratio (OR) for the development of any illness subsequent to prior TBI was 1.67 (95% CI 1.44-1.93, p < 0.0001). Prior TBI was independently associated with both neurological (OR 1.55, 95% CI 1.31-1.83, p < 0.0001) and psychiatric (OR 2.00, 95% CI 1.50-2.66, p < 0.0001) outcomes. Analyses of individual diagnoses revealed higher odds of Alzheimer's disease, Parkinson's disease, mild cognitive impairment, depression, mixed affective disorders, and bipolar disorder in individuals with previous TBI as compared to those without TBI. This association was present when examining only studies of mild TBI and when considering the influence of study design and characteristics. Analysis of a subset of studies demonstrated no evidence that multiple TBIs were associated with higher odds of disease than a single TBI.

CONCLUSIONS

History of TBI, including mild TBI, is associated with the development of neurological and psychiatric illness. This finding indicates that either TBI is a risk factor for heterogeneous pathological processes or that TBI may contribute to a common pathological mechanism.

Hormone replacement therapy and Parkinson's disease risk in women: a meta-analysis of 14 observational studies

BACKGROUND AND PURPOSE

Published data on the relationship of hormone replacement therapy (HRT) with Parkinson's disease (PD) were inconclusive. Thus, a systematic meta-analysis of observational studies was performed to clarify this topic.

METHODS

The databases of PubMed and EMBASE were searched for case-control or cohort studies published up till June 2, 2014. Meta-analysis of the relative risks (RRs) with 95% confidence intervals (CIs) was estimated using random-effects models.

RESULTS

A final total of ten case-control and four cohort studies were included in our meta-analysis. The overall combined RR of PD for ever users versus never users of HRT was 1.00 (95% CI: 0.84-1.20). Limited to those subjects who only use estrogen, a similar trend was detected (RR: 0.95, 95% CI: 0.69-1.30). In the subgroup analysis by study design, no significant association was observed in case-control studies (RR: 0.79, 95% CI: 0.62-1.02), whereas a positive association was found in cohort studies (RR: 1.24, 95% CI: 1.10-1.40). In further analysis according to study quality, an inverse association was found in the low-quality group (RR: 0.58, 95% CI: 0.40-0.82), whereas a positive association was found in the high-quality group (RR: 1.16, 95% CI: 1.02-1.31).

CONCLUSION

In summary, our results of meta-analysis do not support a protective role of HRT in female PD development.

Parkinson's disease and pesticide exposure--a new assessment

Parkinson's disease (PD) is an idiopathic disease and its pathological feature is a loss of pigmented neurons in the substantia nigra. Some commonly used pesticides possess neurotoxicity, and exposure to such compounds may trigger mechanisms similar to those in the development of idiopathic PD. We conducted a systematic review of epidemiological studies, aiming at a critical evaluation of the association between the development of PD and pesticide exposure. Reported effect sizes (ES) in the relevant studies were pooled into the meta-analysis to derive summary ES. The summary ES suggested a significantly positive association between PD and overall pesticide use (non-occupational and/or occupational pesticide use) [1.42; 95% confidence interval (CI) 1.32 to 1.52, the fixed-effects model], as well as between PD and occupational pesticide exposure (1.49 with a 95% CI of 1.34-1.66). Both occupational herbicide and occupational insecticide exposure showed a significant association with PD. The results of the meta-analysis reported in this study suggest the existence of a statistically positive association between PD and pesticide exposure. The majority of the studies that were pooled in the meta-analysis were case-control design with very few cohort studies and most with poor exposure characterization thus, any further case-control studies using similar methodologies are unlikely to have a significant impact or understanding on the currently-reported association between pesticide exposure and the development of idiopathic PD. Therefore, we believe that if further epidemiological studies are going to be conducted in the area, they should be prospective cohort studies that will include accurate exposure assessment.

Examining the relationship between head trauma and neurodegenerative disease: A review of epidemiology, pathology and neuroimaging techniques

Traumatic brain injuries (TBI) are induced by sudden acceleration-deceleration and/or rotational forces acting on the brain. Diffuse axonal injury (DAI) has been identified as one of the chief underlying causes of morbidity and mortality in head trauma incidents. DAIs refer to microscopic white matter (WM) injuries as a result of shearing forces that induce pathological and anatomical changes within the brain, which potentially contribute to significant impairments later in life. These microscopic injuries are often unidentifiable by the conventional computed tomography (CT) and magnetic resonance (MR) scans employed by emergency departments to initially assess head trauma patients and, as a result, TBIs are incredibly difficult to diagnose. The impairments associated with TBI may be caused by secondary mechanisms that are initiated at the moment of injury, but often have delayed clinical presentations that are difficult to assess due to the initial misdiagnosis. As a result, the true consequences of these head injuries may go unnoticed at the time of injury and for many years thereafter. The purpose of this review is to investigate these consequences of TBI and their potential link to neurodegenerative disease (ND). This review will summarize the current epidemiological findings, the pathological similarities, and new neuroimaging techniques that may help delineate the relationship between TBI and ND. Lastly, this review will discuss future directions and propose new methods to overcome the limitations that are currently impeding research progress. It is imperative that improved techniques are developed to adequately and retrospectively assess TBI history in patients that may have been previously undiagnosed in order to increase the validity and reliability across future epidemiological studies. The authors introduce a new surveillance tool (Retrospective Screening of Traumatic Brain Injury Questionnaire, RESTBI) to address this concern.

Environmental toxins and Parkinson's disease

Parkinson's disease (PD) is a chronic, progressive, disabling neurodegenerative disorder that begins in mid to late life and is characterized by motor impairment, autonomic dysfunction, and, in many, psychological and cognitive changes. Recent advances have helped delineate pathogenetic mechanisms, yet the cause of PD in most individuals is unknown. Although at least 15 genes and genetic loci have been associated with PD, identified genetic causes are responsible for only a few percent of cases. Epidemiologic studies have found increased risk of PD associated with exposure to environmental toxicants such as pesticides, solvents, metals, and other pollutants, and many of these compounds recapitulate PD pathology in animal models. This review summarizes the environmental toxicology of PD, highlighting the consistency of observations across cellular, animal, and human studies of PD pathogenesis.

Head injury and risk of Parkinson disease: a systematic review and meta-analysis

Head trauma has been implicated in the etiopathogenesis of Parkinson's disease (PD). We performed a meta-analysis to investigate the association between head trauma and the risk of developing PD. We included observational studies if they (1) clearly defined PD, (2) defined head trauma leading to concussion, and (3) presented odds ratios (ORs) and 95% confidence intervals (CIs) or provided data to compute these statistics. Random effect model was used to estimate the pooled, adjusted OR. Heterogeneity between studies was evaluated with the Q test and the I(2) statistic. We conducted a sensitivity analysis to assess the influence of each study and repeated the analysis by excluding the studies with the largest weights. We used funnel plot to assess the presence of publication bias. After reviewing more than 636 article titles, 34 articles were selected for full review. In total, 22 studies (19 case-control studies, 2 nested case-control studies, and 1 cohort study) were included in the meta-analysis. The pooled OR for the association of PD and head trauma was 1.57 (95% CI, 1.35-1.83). The results of our meta-analysis indicate that a history of head trauma that results in concussion is associated with a higher risk of developing PD. © 2013 Movement Disorder Society.

Is pesticide use related to Parkinson disease? Some clues to heterogeneity in study results

BACKGROUND

Previous systematic reviews have indicated that pesticide exposure is possibly associated with Parkinson disease (PD). However, considerable heterogeneity has been observed in study results.

OBJECTIVE

We aimed at providing an update of the literature published on PD and exposure to pesticides by performing a systematic review and meta-analysis. In addition, we investigated whether methodological differences between studies could explain the heterogeneity in study results.

METHODS

We identified studies through a systematic literature search. We calculated summary risk ratios (sRRs) for pesticide exposure and subcategories using random effects meta-analyses and investigated sources of heterogeneity by meta-regression and stratified analyses.

RESULTS

Thirty-nine case-control studies, four cohort studies, and three cross-sectional studies were identified. An sRR of 1.62 [95% confidence interval (CI): 1.40, 1.88] for pesticide exposure (ever vs. never) was found. Summary estimates for subclasses of pesticides indicated a positive association with herbicides and insecticides, but not with fungicides. Heterogeneity in individual study results was not related to study design, source of control population, adjustment of results for potential confounders, or geographical area. However, results were suggestive for heterogeneity related to differences in the exposure assessment. Job title-based exposure assignment resulted in a higher sRR (2.5; 95% CI: 1.5, 4.1) than did assignment based on self-reported exposure (e.g., for self-reported ever/never exposure, sRR = 1.5; 95% CI: 1.3, 1.8).

CONCLUSIONS

This review affirms the evidence that exposure to herbicides and insecticides increase the risk of PD. Future studies should focus on more objective and improved methods of pesticide exposure assessment.

Paraquat, but not maneb, induces synucleinopathy and tauopathy in striata of mice through inhibition of proteasomal and autophagic pathways

SNCA and MAPT genes and environmental factors are important risk factors of Parkinson's disease [PD], the second-most common neurodegenerative disease. The agrichemicals maneb and paraquat selectively target dopaminergic neurons, leading to parkinsonism, through ill-defined mechanisms. In the current studies we have analyzed the ability of maneb and paraquat, separately and together, to induce synucleinopathy and tauopathy in wild type mice. Maneb was ineffective in increasing α-synuclein [α-Syn] or p-Tau levels. By contrast, paraquat treatment of mice resulted in robust accumulation of α-Syn and hyperphosphorylation of Tau in striata, through activation of p-GSK-3β, a major Tau kinase. Co-treatment with maneb did not enhance the effects of paraquat. Increased hyperacetylation of α-tubulin was observed in paraquat-treated mice, suggesting cytoskeleton remodeling. Paraquat, but not maneb, inhibited soluble proteasomal activity on a peptide substrate but this was not associated with a decreased expression of 26S proteasome subunits. Both paraquat and maneb treatments increased levels of the autophagy inhibitor, mammalian target of rapamycin, mTOR, suggesting impaired axonal autophagy, despite increases in certain autophagic proteins, such as beclin 1 and Agt12. Autophagic flux was also impaired, as ratios of LC3 II to LC3 I were reduced in treated animals. Increased mTOR was also observed in postmortem human PD striata, where there was a reduction in the LC3 II to LC3 I ratio. Heat shock proteins were either increased or unchanged upon paraquat-treatment suggesting that chaperone-mediated autophagy is not hampered by the agrichemicals. These studies provide novel insight into the mechanisms of action of these agrichemicals, which indicate that paraquat is much more toxic than maneb, via its inhibitory effects on proteasomes and autophagy, which lead to accumulation of α-Syn and p-Tau.

Minocycline prevents paraquat-induced cell death through attenuating endoplasmic reticulum stress and mitochondrial dysfunction

Paraquat (PQ) was demonstrated to induce dopaminergic neuron death and is used as a Parkinson's disease (PD) mimetic; however, its mechanism remains contradictory. Alternatively, minocycline is a second-generation tetracycline and is undergoing clinical trials for treating PD with an unresolved mechanism. We thus investigated the molecular mechanism of minocycline in preventing PQ-induced cytotoxicity. In this study, minocycline was effective in preventing PQ-induced apoptotic cell death, which involves the cleavages of poly (ADP-ribose) polymerase (PARP) and caspase 3 and increased fluorescence intensity of annexin V-FITC. In addition, PQ also quickly induced alterations of unfolded protein responses (UPRs) and subsequently dysfunction of the mitochondria (such as the decrease in membrane potential and increase in membrane permeability and superoxide formation). Finally, the mechanism of minocycline in preventing PQ-induced apoptosis might be mediated by attenuating endoplasmic reticulum (ER) stress and mitochondrial dysfunction, which respectively results in caspase-12 activation and the release of H2O2, HtrA2/Omi, and Smac/Diablo. Thus, minocycline could possibly be used to treat other neurodegenerative disorders with similar pathologic mechanisms.

Epidemiology and etiology of Parkinson's disease: a review of the evidence

The etiology of Parkinson's disease (PD) is not well understood but likely to involve both genetic and environmental factors. Incidence and prevalence estimates vary to a large extent-at least partly due to methodological differences between studies-but are consistently higher in men than in women. Several genes that cause familial as well as sporadic PD have been identified and familial aggregation studies support a genetic component. Despite a vast literature on lifestyle and environmental possible risk or protection factors, consistent findings are few. There is compelling evidence for protective effects of smoking and coffee, but the biologic mechanisms for these possibly causal relations are poorly understood. Uric acid also seems to be associated with lower PD risk. Evidence that one or several pesticides increase PD risk is suggestive but further research is needed to identify specific compounds that may play a causal role. Evidence is limited on the role of metals, other chemicals and magnetic fields. Important methodological limitations include crude classification of exposure, low frequency and intensity of exposure, inadequate sample size, potential for confounding, retrospective study designs and lack of consistent diagnostic criteria for PD. Studies that assessed possible shared etiological components between PD and other diseases show that REM sleep behavior disorder and mental illness increase PD risk and that PD patients have lower cancer risk, but methodological concerns exist. Future epidemiologic studies of PD should be large, include detailed quantifications of exposure, and collect information on environmental exposures as well as genetic polymorphisms.

Differential contribution of the mitochondrial respiratory chain complexes to reactive oxygen species production by redox cycling agents implicated in parkinsonism

Exposure to environmental pesticides can cause significant brain damage and has been linked with an increased risk of developing neurodegenerative disorders, including Parkinson's disease. Bipyridyl herbicides, such as paraquat (PQ), diquat (DQ), and benzyl viologen (BV), are redox cycling agents known to exert cellular damage through the production of reactive oxygen species (ROS). We examined the involvement of the mitochondrial respiratory chain in ROS production by bipyridyl herbicides. In isolated rat brain mitochondria, H2O2 production occurred with the following order of potency: BV > DQ > PQ in accordance with their measured ability to redox cycle. H2O2 production was significantly attenuated in all cases by antimycin A, an inhibitor of complex III. Interestingly, at micromolar (< or = 300 microM) concentrations, PQ-induced H2O2 production was unaffected by complex I inhibition via rotenone, whereas DQ-induced H2O2 production was equally attenuated by inhibition of complex I or III. Moreover, complex I inhibition decreased BV-induced H2O2 production to a greater extent than with PQ or DQ. These data suggest that multiple sites within the respiratory chain contribute to H2O2 production by redox cycling bipyridyl herbicides. In primary midbrain cultures, H2O2 differed slightly with the following order of potency: DQ > BV > PQ. In this model, inhibition of complex III resulted in roughly equivalent inhibition of H2O2 production with all three compounds. These data identify a novel role for complex III dependence of mitochondrial ROS production by redox cycling herbicides, while emphasizing the importance of identifying mitochondrial mechanisms by which environmental agents generate oxidative stress contributing to parkinsonism.

The role of NADPH oxidase 1-derived reactive oxygen species in paraquat-mediated dopaminergic cell death

Oxidative stress is the common downstream effect of a variety of environmental neurotoxins that are strongly implicated in the pathogenesis of Parkinson's disease. We demonstrate here that the activation of NADPH oxidase 1 (Nox1), a specialized superoxide-generating enzyme complex, plays a key role in the oxidative stress and subsequent dopaminergic cell death elicited by paraquat. Paraquat increased the expression of Nox1 in a concentration-dependent manner in rat dopaminergic N27 cells. Rac1, a key component necessary for Nox1-mediated superoxide generation, also was activated by paraquat. Paraquat-induced reactive oxygen species generation and dopaminergic cell death were significantly reduced after pretreatment with apocynin, a putative NADPH oxidase inhibitor, and Nox1 knockdown with siRNA. Male C57BL/6 mice received intraperitoneal (IP) injections of paraquat (10 mg/kg) once every 3 days and showed increased Nox1 levels in the substantia nigra as well as a 35% reduction in tyrosine hydroxylase-positive dopaminergic neurons 5 days after the last injection. Preadministration of apocynin (200 mg/kg, IP) led to a significant decrease in dopaminergic neuronal loss. Our results suggest that Nox1-generated superoxide is implicated in the oxidative stress elicited by paraquat in DA cells, and it can serve as a novel target for pharmacologic intervention.

Developmental heptachlor exposure increases susceptibility of dopamine neurons to N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)in a gender-specific manner

Parkinson's disease (PD) is primarily thought of as a disease of aging. However, recent evidence points to the potential for exposure to xenobiotics during development to increase risk of PD. Here, we report that developmental exposure to the organochlorine pesticide heptachlor alters the dopamine system and increases neurotoxicity in an animal model of PD. Exposure of pregnant mice to heptachlor led to increased levels of the dopamine transporter (DAT) and vesicular monoamine transporter 2 (VMAT2) levels at both the protein and mRNA level in their offspring. Increased DAT and VMAT2 levels were accompanied by alterations of mRNA levels of nuclear transcription factors that control dopamine neuron development and regulate DAT and VMAT2 levels in adulthood. At 12 weeks of age, control and heptachlor-exposed offspring were administered a moderate dose (2 x 10mg/kg) of the parkinsonism-inducing agent MPTP. Greater neurotoxicity as evidenced by a greater loss of striatal dopamine and potentiation of increased levels of glial fibrillary acidic protein and alpha-synuclein was observed in heptachlor-exposed offspring. The neurotoxicity observed was greater in the male offspring than the female offspring, suggesting that males are more susceptible to the long-term effects of developmental heptachlor exposure. These data suggest that developmental heptachlor exposure causes long-term alterations of the dopamine system thereby rendering it more susceptible to dopaminergic damage in adulthood.

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

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

Pesticide exposure and risk of Parkinson's disease: a family-based case-control study

BACKGROUND

Pesticides and correlated lifestyle factors (e.g., exposure to well-water and farming) are repeatedly reported risk factors for Parkinson's disease (PD), but few family-based studies have examined these relationships.

METHODS

Using 319 cases and 296 relative and other controls, associations of direct pesticide application, well-water consumption, and farming residences/occupations with PD were examined using generalized estimating equations while controlling for age-at-examination, sex, cigarette smoking, and caffeine consumption.

RESULTS

Overall, individuals with PD were significantly more likely to report direct pesticide application than their unaffected relatives (odds ratio = 1.61; 95% confidence interval, 1.13-2.29). Frequency, duration, and cumulative exposure were also significantly associated with PD in a dose-response pattern (p </= 0.013). Associations of direct pesticide application did not vary by sex but were modified by family history of PD, as significant associations were restricted to individuals with no family history. When classifying pesticides by functional type, both insecticides and herbicides were found to significantly increase risk of PD. Two specific insecticide classes, organochlorines and organophosphorus compounds, were significantly associated with PD. Consuming well-water and living/working on a farm were not associated with PD.

CONCLUSION

These data corroborate positive associations of broadly defined pesticide exposure with PD in families, particularly for sporadic PD. These data also implicate a few specific classes of pesticides in PD and thus emphasize the need to consider a more narrow definition of pesticides in future studies.

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

Epidemiological studies suggest that some pesticides might constitute a risk factor for Parkinson's disease (PD). However, risk assessment cannot be performed in the current experimental animal models because they use non-natural pathways of pesticide exposure, such as intraperitoneal or intravenous injection, that might bypass body defences. A new model based on daily inoculation of neurotoxins in the nasal cavity of C57BL/6 mice for 30 days was used to evaluate risk of three complex I inhibitors, 1-methyl-4-phenyl1,2,3,6-tetrahydropyridine (MPTP), rotenone and paraquat. These compounds displayed very different effects on motor activity, striatal dopamine and dihydroxyphenylacetic acid (DOPAC) levels and loss of dopaminergic neurons. MPTP-treated mice developed motor deficits that correlated with a severe depletion of striatal dopamine levels, and loss of tyrosine hydroxylase staining in substantia nigra and striatum. By contrast, rotenone-treated mice or rats were asymptomatic. Paraquat induced severe hypokinesia and vestibular damage but did not alter the nigrostriatal system. The new animal model described here, based on chronic intranasal inoculation of neurotoxicants, provides a new tool to assess the potential danger of environmental toxins as risk factors for development of PD.

Modification of inherent and drug-induced dopaminergic activity after exposure to benzo(alpha)pyrene

The aim of this study was to investigate the effect of benzo(alpha)pyrene (B(alpha)P), a representative polycyclic aromatic hydrocarbon (PAH), on dopaminergic activity in brain. (B(alpha)P) altered dopaminergic activity in discrete regions of the rat brain, including the hippocampus, hypothalamus, caudate putamen and nucleus accumbens. Specifically, B(alpha)P increased DA levels in the hippocampus and DA turnover in the caudate putamen. In addition, B(alpha)P suppressed DA levels in the caudate putamen and DA turnover in the nucleus accumbens. B(alpha)P also altered the effect of several dopaminergic agents, L-DOPA, sulpiride and bromocriptine, on DA activity. In particular, B(alpha)P enhanced the L-DOPA-induced increase in the DA turnover ratio in the caudate putamen and increased DA levels in the nucleus accumbens. B(alpha)P also reversed the sulpiride-induced increase of DA turnover in the nucleus accumbens and the bromocriptine-induced increase of DA turnover in the hippocampus. In addition, DA turnover was increased by B(alpha)P in the nucleus accumbens and caudate putamen and DA levels were suppressed in the nucleus accumbens of bromocriptine treated rats, though the drug alone had no effect. These changes indicate that exposure to B(alpha)P and related compounds may affect dopaminergic function in discrete brain regions that are implicated in cognitive functions, psychosis, depression and Parkinson's disease, and may possibly interfere with their pharmacological intervention.

Cross-sectional study discloses a positive family history for Parkinson’s disease and male gender as epidemiological risk factors for substantia nigra hyperechogenicity

Hyperechogenicity of the substantia nigra (SN) has been proposed to be a typical finding in Parkinson's disease (PD) and a marker of vulnerability to nigrostriatal dysfunction in healthy subjects. This large cross-sectional study including 1120 subjects older than 50 years without any signs of PD was performed to evaluate the association of SN hyperechogenicity and other proposed epidemiological risk factors for PD. Among all variables assessed only family history of PD and male gender proved to be significantly associated with SN hyperechogenicity, indicating a genetic predisposition for the ultrasound marker.

Developmental exposure to the pesticide dieldrin alters the dopamine system and increases neurotoxicity in an animal model of Parkinson's disease

Exposure to pesticides has been suggested to increase the risk of Parkinson's disease (PD), but the mechanisms responsible for this association are not clear. Here, we report that perinatal exposure of mice during gestation and lactation to low levels of dieldrin (0.3, 1, or 3 mg/kg every 3 days) alters dopaminergic neurochemistry in their offspring and exacerbates MPTP toxicity. At 12 wk of age, protein and mRNA levels of the dopamine transporter (DAT) and vesicular monoamine transporter 2 (VMAT2) were increased by perinatal dieldrin exposure in a dose-related manner. We then administered MPTP (2 x 10 mg/kg s.c) at 12 wk of age and observed a greater reduction of striatal dopamine in dieldrin-exposed offspring, which was associated with a greater DAT:VMAT2 ratio. Additionally, dieldrin exposure during development potentiated the increase in GFAP and alpha-synuclein levels induced by MPTP, indicating increased neurotoxicity. In all cases there were greater effects observed in the male offspring than the female, similar to that observed in human cases of PD. These data suggest that developmental exposure to dieldrin leads to persistent alterations of the developing dopaminergic system and that these alterations induce a "silent" state of dopamine dysfunction, thereby rendering dopamine neurons more vulnerable later in life.

Parkinsonism and Parkinson's disease in the elderly: A community-based survey in Brazil (the Bambuí study)

Several community-based surveys on the prevalence of Parkinsonism and Parkinson's disease have been conducted worldwide, with variations on their methodology and results. The objective of this study is to assess the prevalence of Parkinsonism and its causes in a cohort of individuals age 64 years or older in Bambuí, a Brazilian town. In phase I, 1,186 people older than 64 years responded to a 9-question screening questionnaire for Parkinsonism. In phase II, all subjects who scored > or = 2 points on the test were examined independently by at least 2 movement disorder-trained physicians. A movement disorder senior specialist excluded or confirmed the diagnosis in all suspected cases. The response rate was high for both phases (96% for phase I and 94% for phase II). The prevalence rate per 100 population over 64 years of age in this group was 7.2% for Parkinsonism of all types (n = 86). The most frequent causes were idiopathic Parkinson's disease and drug-induced Parkinsonism, with prevalence rates of 3.3% (n = 39) and 2.7% (n = 32), respectively. The prevalence of vascular Parkinsonism was 1.1% (n = 13). We found 1 case of posttraumatic Parkinsonism and another with multiple system atrophy. In this first population-based study of Parkinsonism conducted in Brazil, the prevalence in a cohort of elderly subjects was higher than the observed in other populations worldwide, especially because of the high rates of drug-induced and vascular Parkinsonism. The prevalence of Parkinson's disease was similar to that observed in elderly people in door-to-door surveys in other American, European, and Eastern countries.

Studying Toxicants as Single Chemicals: Does this Strategy Adequately Identify Neurotoxic Risk?

Despite the fact that virtually all chemicals exposure of humans are to mixtures, and that these mixed exposures occur in the context of numerous other risk modifiers, our current understanding of human health risks is based almost entirely on the evaluation of chemicals studied in isolation. This paper describes findings from our collaborative studies that prompt questions about these approaches in the context of neurotoxicology. The first section describes studies investigating the interactions of maternal Pb exposure with maternal stress. Examined across a range of outcome measures, it shows that maternal Pb can modulate the effects of maternal stress, and, conversely, stress modifies the effects of Pb. Further, effects of Pb+stress could be detected in the absence of an effect of either risk factor alone, and, moreover, the profile of effects of Pb alone differs notably from that of Pb+stress. Collectively, interactions were not systematic, but differed by brain region, gender and outcome measure. A second section describes outcomes of studies examining combined exposures to the pesticides paraquat (PQ) and maneb (MB) during development which likewise reveal potentiated effects of combined exposures. They also demonstrate examples of both progressive and cumulative neurotoxicity, including a marked vulnerability following gestational exposure to MB, to the effects of PQ, a pesticide with no structural relationship to MB. The ability of current hazard identification and risk assessment approaches to adequately identify and encompass such effects remains an important unanswered question. One consideration proposed for further evaluating potential interactions that may be of significance for the nervous system is based on a multi-hit hypothesis. It hypothesizes that the brain may readily compensate for the effects of an individual chemical itself acting on a particular target system, but when multiple target or functional sites within that one system are attacked by different mechanisms (i.e., multiple chemical exposures or chemical exposures combined with other risk factors), homeostatic capabilities may be restricted, thereby leading to sustained or cumulative damage.

Neurodegenerative memory disorders: a potential role of environmental toxins

The hypothesis that neurotoxins may play a role in neurodegenerative disorders remains an elusive one, given that epidemiologic studies often provide conflicting results. Although these conflicting results may result from methodological differences within and between studies, the complexity of chemical disruption of the central nervous system cannot be ignored in attempts to evaluate this hypothesis in different neurodegenerative disorders. Spencer provides a detailed review of the complex processes involved in defining the neurotoxic potential of naturally occurring and synthetic agents. Even concepts such as exposure and dose, as often reported in studies attempting to evaluate the risk imparted by a potential compound, can be deceptive. For example, although dose reflects "that amount of chemical transferred to the exposed subject", factors such as time and concentration in the organism, the ability to access the central nervous system, and how a compound reaches the central nervous system (routes of administration) or secondarily affects other organ systems leading to central nervous system disruption are clearly important to the concept of neurotoxic risk in neurodegenerative disorders. These factors would appear to explain the observed disagreements between studies using animal or neuronal models of neurotoxicity and population-based studies in humans. The importance of these factors and how a potential neurotoxin is investigated are clearly seen in the data on AD and aluminum. In contrast, the impact of MTPT on the central nervous system is more direct and compelling. Added complexity in the study of neurotoxins in human neurodegeneration is derived from data showing that agents may have additive, potentiating, synergistic, or antagonistic effects. Therefore, data from studies evaluating EMF risks could be readily confounded by the presence or absence of heavy metals (eg, arc welding). Other factors that may conceal neurotoxic causes for a given disorder focus on additional features such as genetic predispositions, physiologic changes that occur in aging, and even nutritional status that can support or hinder the affect of a given agent on the central nervous system. Finally, many studies that investigate exposure risk do not readily incorporate the five criteria proposed by Schaumburg for establishing causation. For example, if we apply Schaumburg's first criterion, epidemiologic studies often determines the presence of an agent through history, yet they cannot readily confirm exposure based on environmental or clinical chemical analyses to fulfill this criterion for causation. Additional limitations in research design along with the populations and methods that are sued to study neurotoxins in human neurodegenerative disorders often fail to meet other criteria such as linking the severity and onset with duration and exposure level. Therefore, although studies of agents such as MTPT provide compelling models of neurotoxins and neurodegeneration in humans, disorders such as ALS, PD, and particularly AD will require additional effort if research is to determine the contribution (presence or absence) of neurotoxins to these neurologic disorders.

Comparison of age of onset and development of motor complications between smokers and non-smokers in Parkinson's disease

BACKGROUND

There is growing evidence from case-control and from cohort studies that smoking is inversely related to the risk of developing Parkinson's disease (PD). However, it is still controversial if PD starts at an older age in ever-smoking patients compared to never-smoking ones.

PATIENTS AND METHODS

The present retrospective study compares in a large series of 512 out-patients, collected over the last 24 years, the age of onset of the complaints, the age at which PD was diagnosed and the start of levodopa treatment between ever- and never-smokers. Also, the occurrence of long-term side-effects of the drug was evaluated. 184 PD patients with a history of smoking were compared with 328 who had never smoked. The subgroups with and without a family history of PD were analysed separately.

RESULTS

In the overall ever-smoking group, as well as in the subgroup without a family history, the onset of the disease and the time of the diagnosis of PD and the time at which levodopa was started occurred at an older age than in the never-smoking group. This difference could not be demonstrated in the patients with a family history, due to the low number of cases and the lack of statistical power. Although the follow-up period was the same in both study groups, motor fluctuations and dyskinesia were more frequent and appeared earlier after levodopa treatment in the non-smoking compared to the ever-smoking PD patients. Only for cognitive impairment there was a non-significant trend in the smoking group.

CONCLUSION

The present study confirms the protective action of smoking on PD and also suggests some modulating effect of smoking on the dopaminergic system.

Striatal trophic factor activity in aging monkeys with unilateral MPTP-induced parkinsonism

Striatal trophic activity was assessed in female rhesus monkeys of advancing age rendered hemiparkinsonian by unilateral intracarotid administration of MPTP. Three age groups were analyzed: young adults (8-9.5 years) n=4, middle-aged adults (15-17 years) n=4, and aged adults (21-31 years) n=7. Fresh frozen tissue punches of caudate nucleus and putamen were collected 3 months after MPTP treatment and assayed for combined soluble striatal trophic activity, brain-derived neurotrophic factor (BDNF) and glial cell line-derived neurotrophic factor (GDNF). This time point was chosen in an effort to assess a relatively stable phase of the dopamine (DA)-depleted state that may model the condition of Parkinson's disease (PD) patients at the time of therapeutic intervention. Analyses were conducted on striatal tissue both contralateral (aging effects) and ipsilateral to the DA-depleting lesion (lesion x aging effects). We found that combined striatal trophic activity in the contralateral hemisphere increased significantly with aging. Activity from both middle-aged and aged animals was significantly elevated as compared to young adults. Following DA depletion, young animals significantly increased combined striatal trophic activity, but middle-aged and aged animals did not exhibit further increases in activity over their elevated baselines. BDNF levels in the contralateral hemisphere were significantly reduced in aged animals as compared to young and middle-aged subjects. With DA depletion, BDNF levels declined in young and middle-aged animals but did not change from the decreased baseline level in old animals. GDNF levels were unchanged with aging and at 3 months after DA depletion. The results are consistent with several conclusions. First, by middle age combined striatal trophic activity is elevated, potentially reflecting a compensatory reaction to ongoing degenerative changes in substantia nigra DA neurons. Second, in response to DA depletion, young animals were capable of generating a significant increase in trophic activity that was sustained for at least 3 months. This capacity was either saturated or was not sustained in middle-aged and aged animals. Third, the aging-related chronic increase in combined striatal trophic activity was not attributable to BDNF or GDNF as these molecules either decreased or did not change with aging.

Association of pesticide exposure with neurologic dysfunction and disease

Poisoning by acute high-level exposure to certain pesticides has well-known neurotoxic effects, but whether chronic exposure to moderate levels of pesticides is also neurotoxic is more controversial. Most studies of moderate pesticide exposure have found increased prevalence of neurologic symptoms and changes in neurobehavioral performance, reflecting cognitive and psychomotor dysfunction. There is less evidence that moderate exposure is related to deficits in sensory or motor function or peripheral nerve conduction, but fewer studies have considered these outcomes. It is possible that the most sensitive manifestation of pesticide neurotoxicity is a general malaise lacking in specificity and related to mild cognitive dysfunction, similar to that described for Gulf War syndrome. Most studies have focused on organophosphate insecticides, but some found neurotoxic effects from other pesticides, including fungicides, fumigants, and organochlorine and carbamate insecticides. Pesticide exposure may also be associated with increased risk of Parkinson disease; several classes of pesticides, including insecticides, herbicides, and fungicides, have been implicated. Studies of other neurodegenerative diseases are limited and inconclusive. Future studies will need to improve assessment of pesticide exposure in individuals and consider the role of genetic susceptibility. More studies of pesticides other than organophosphates are needed. Major unresolved issues include the relative importance of acute and chronic exposure, the effect of moderate exposure in the absence of poisoning, and the relationship of pesticide-related neurotoxicity to neurodegenerative disease.