Pesticides On the Brain: The Environmental Factors Behind Parkinson's Disease

Parkinson’s disease (PD) is characterized by the progressive degeneration of dopaminergic neurons within the substantia nigra, a region in the midbrain critical for motor planning and reward-seeking behavior.  The substantia nigra interacts directly with the putamen and caudate nucleus via the release of dopamine.  This release, in turn, activates a motor pathway within the basal ganglia involving regions of the globus pallidus, the sub-thalamic nucleus, thalamus, and eventually, motor regions of the cerebral cortex to facilitate normal movement. The many neuronal projections coursing through the basal ganglia can be classified into two primary categories: a direct pathway and an indirect pathway. Simply, the direct pathway facilitates motor output while the indirect pathway inhibits movement. The dopaminergic projections from the substantia nigra excite the direct pathway and inhibit the indirect pathway, thus appropriately promoting normal movement. Loss of these dopaminergic neurons often results in a stronger indirect pathway and weaker direct pathway, resulting in the tremors and rigidity characteristic of many Parkinson’s patients.

Despite its prevalence among the population, the underlying cause of PD is still under intensive study. Recent hypotheses have centered on the formation of “Lewy bodies” in the substantia nigra (3). In normal cells, modification of a protein with a ubiquitin “tag” earmarks the protein for degradation in the proteosome (3). These Lewy bodies, consisting of the protein alpha-synuclein bound to ubiquitin, are unable to be degraded in the proteosome and thus form dense aggregates (3). These protein aggregates are neurotoxic, resulting in the gradual loss of dopaminergic neurons in the substantia nigra (3).

However, these days, medical research seems to be headed towards a more multifaceted approach, exploring disease pathology from many angles. Recent studies have examined a possible environmental cause for the disorder, namely extensive exposure to pesticide or farm areas. Pesticides act as acetylcholinesterase inhibitors, the enzyme normally responsible for denaturing excessive acetylcholine in the synapse. Inhibition of this essential enzyme results in elevated and often neurotoxic levels of acetylcholine, subsequently influencing several key areas of synaptic transmission.

A Neurochemical Model
A study published last October at the Mississippi State Center for Environmental Health Science explores the possible effects of these pesticides on dopaminergic neurons in the substantia nigra (1). The researchers utilized a rat model involving both long-acting (chlorpyrifos) and short-acting (methyl parathion) pesticides, and subsequently measured the long term effects of pesticides when administered to young rats (1). Results were assessed through measurement of both immediate (22 days) and long term (50 days) dopamine and dopamine metabolite levels (1). Metabolite levels provide important information about the functioning of monoamine oxidase (MAO), an enzyme critical to the break down of synaptic dopamine (1). The two metabolites analyzed are (dihydroxyphenylacetic acid) DOPAC, an intermediary in the MAO metabolic pathway, and homovanillic acid (HVA), generally the final product of the pathway (1). Researchers also examined any alterations in the expression levels of Nurr1, LmxB, tyrosine hydroxylase, dopamine transporter genes, or nicotinic acetylcholine receptor subunits, all components essential to dopaminergic function (1).

Immediately following 21 days of treatment, the study found no significant differences in dopamine or other metabolite levels from either methyl parathion (MPT) or chlorpyrifos (CPS) (1). However, there was a substantial increase in DOPAC level at P50 (postnatal day 50) upon exposure to CPS, representative of a higher rate of dopamine turnover rate (1). Results also demonstrated a significant decrease in the ratio of α6 to α7 subunits of the acetylcholine receptor immediately following MPT and CPS treatment (1). This decrease was not maintained at P50; however, MPT did demonstrate a significant elevation of the α6 acetylcholine subunit (1).

Higher levels of the intermediary DOPAC following CPS treatment demonstrate an increased turnover and processing rate of dopamine within the synapse (1). Importantly, the increase presents at P50, well after pesticide treatment and following the return of acetylcholinesterase to basal levels (1). This phenomenon reveals subtle alterations in the metabolism and processing of synaptic nigrostriatal dopamine, extending beyond the initial acetylcholinesterase-inhibiting effects of pesticide (1). CPS and MPT also significantly impacted expression of post-synaptic acetylcholine receptors found on nigrostriatal dopaminergic neurons (1). Normally, binding of agonists to nicotinic acetylcholine receptors will increase dopamine neuron firing rates and consequentially, enhance dopamine release (1). Alteration of receptor subunits may affect dopamine release, but it may also negatively impact the survival capacity of these dopaminergic neurons (1). Studies have shown that an abnormal increase in the α6 subunit (a result of MPT treatment) can damage the nigrostriatal dopamine system, contributing to the neural degeneration characteristic of Parkinson’s disease (1).

A Mitochondrial Model
Following reports of Parkinsonian symptoms resulting from exposure to the mitochondrial pro-toxin N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), a group of researchers at Emory university examined the impact of chronic exposure to the mitochondrial inhibitor and common pesticide, Rotenone (4). MPTP itself is harmless; however, degradation of MPTP produces the metabolite MPP+, which inhibits complex I of the electron transport chain, an essential element of cellular respiration and energy production (4). Unfortunately, MPP+ appears to specifically target the dopaminergic system due to its high affinity for the dopamine transporter, which allows the toxin to easily access the soma (4). Rotenone is similar to MPP+ in its selective inhibition of complex I of the electron transport chain (4). However, its hydrophobicity allows it to readily diffuse across any cellular membrane, eliminating any specificity for dopaminergic neurons (4).

In the study, researchers treated 2-month old rats with varying doses of Rotenone, ranging from 1-12 mg/kg per day (4). High doses of Rotenone produced the expected systemic and non-specific toxicity due to its ability to effortlessly cross membranes. Surprisingly, however, lower doses of Rotenone (2-3 mg/kg per day) produced highly specific lesions of the dopaminergic system (4). Of the 25 rats treated within this “optimal” dose range, 12 of the rats presented significant lesions in the dopaminergic system of the substantia nigra (4). Resulting pathologies included significant degeneration of dopaminergic neurons in the substantia nigra and striatum, the depletion of tyrosine hydroxylase, a key enzyme in the formation of L-DOPA (dopamine precursor), and the development of cytoplasmic aggregates structurally similar to Lewy bodies (4). Behaviorally, rats treated with Rotenone presented many of the hypokinesic symptoms of human Parkinson’s patients, and 7 of the treated rats even developed a phenotype suggestive of a resting tremor (4).

Clearly, it appears that the nigrostriatal dopaminergic system has an underlying vulnerability to complex I inhibitors, even to the non-specific Rotenone (4). Though the electron transport chain was inhibited, researchers hypothesize that ATP deficiency is not a sufficient explanation for the neurodegeneration due to the unusually low concentration of Rotenone required to produce a Parkinsonian phenotype (4). Rather, the authors theorize that inhibition of complex I results in “production of reactive oxygen species,” presumably due to a downstream reduction in cytochrome oxidase’s ability to convert free oxygen into water (4). Reactive oxygen proceeds to damage proteins and DNA, eventually triggering cellular apoptosis and neurodegeneration via release of cytochrome C from the mitochondria (4).

Epidemiology
Last March, a unique epidemiological study examining the relationship between pesticide exposure and Parkinson’s disease was conducted by a team of researchers at Duke University, the Miami Institute for Human Genomics, and the University of Miami School of Medicine. Unlike previous studies, it compared the effect of pesticide exposure on patients with a familial history of Parkinson’s versus the impact on patients without such a history (2). The authors utilized questionnaires and telephone interviews to establish a medical history and evaluate the level of pesticide exposure for each patient (2). The study only examined white families and excluded patients with multiple symptoms to eliminate potential confounds (2).

The researchers found a highly significant association between pesticide and Parkinson’s disease in patients without a familial history of PD; however, little to no correlation was found in patients with a familial history (2). The authors theorize that negative history patients may be possess some genetic susceptibility, which requires an environmental trigger, such as pesticide exposure, to activate the disorder (2). On the other hand, despite the genetic vulnerability of positive history patients, statistics show little pesticide influence due to the limited number of patients with familial history available for the study; in other words, an association for positive history patients cannot be ruled out (2).  The study also found organochlorines and organophosphates to be particularly potent in triggering PD symptoms; however, living on a farm and drinking well-water had little correlation with PD incidence (2).

Conclusion
Parkinson’s disease affects more than 50,000 Americans each year, resulting in muscle rigidity and even complete loss of physical movement. Though certain clinical features of the disorder are discernible, there is no current underlying explanation for the rapid destruction of nigrostriatal dopaminergic neurons. There are various methods of symptom relief including L-DOPA (dopamine precursor) treatment and monoamine oxidase inhibitors, but no cure has been identified. Hopefully, new studies can help shed light on the disorder and lead the way to permanent relief for Parkinson’s patient.

References
1. J. B. Eells, T. Brown. Neurotoxicology and Teratology 31 (2008).
2. D. B. Hancock et al. BMC Neurology 8 (2008)
M. H. Polymeropoulus, et al. Science 276, 2045-2047 (1997).
3. R. Betarbet. Nature Neuroscience 3, 1301-1306 (2000).