Chronic dichlorvos exposure: microglial activation, proinflammatory cytokines and damage to nigrostriatal dopaminergic system.

Dopaminergic cells in the substantia nigra are highly vulnerable to the neurodegenerative process of Parkinson's disease. Therefore, mechanisms that enhance their susceptibility to injury bear important implications for disease pathogenesis. We have previously shown that chronic dichlorvos exposure caused nigrostriatal dopaminergic degeneration and significant behavioral impairments. In this study, we analyzed the relationship between microglial activation and dopaminergic neurodegeneration to examine the possibility that neuroinflammation may induce dopaminergic neuronal loss in the nigrostriatal system. Chronic dichlorvos exposure causes microglial activation including induction of NADPH oxidase and a selective loss of dopaminergic neurons in rat. Microglial marker expression was increased at transcription as well as translational levels in the substantia nigra (SN) and corpus striatum (CS) of rats exposed to dichlorvos. Activated microglia were seen in SN and CS of dichlorvos-treated animals but were rarely observed in controls. Immunostaining revealed lesser number of TH-positive neurons and higher number of microglia in SN and CS regions after dichlorvos treatment. The mRNA and protein levels of the NADPH oxidase main subunit gp91(phox) were significantly increased after dichlorvos administration. Dichlorvos exposure also leads to increased level of microglial noxious mediators such as IL-1β, TNF-α and IL-6 in ventral midbrain and CS at transcription as well as translational levels. Data indicate that microglial activation and consequent induction of NADPH oxidase and proinflammatory cytokines such as TNF-α, IL-1β and IL-6 may act as risk factors for Parkinson's disease by increasing the vulnerability of dopaminergic cells to dichlorvos toxic injury.