Multifactorial Theory Applied to the Neurotoxicity of Paraquat and Paraquat-induced Mechanisms of Developing Parkinson's Disease

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