A time-course study of microglial activation and dopaminergic neuron loss in the substantia nigra of mice with paraquat-induced Parkinson's disease.

Activated microglia play an active role in the pathogenesis of PD and paraquat (PQ) induces PD. The study was to understand the time relationship between microglial activation and dopaminergic neuron loss in the substantia nigra (SN) of PQ-induced PD mice. Male C57BL/6 mice were injected intraperitoneally with PQ, twice a week for six weeks. Some mice underwent behavioral assessments each week and were sacrificed for SN tissues, in which histopathological analysis, dopaminergic neuron loss, microglial activation and phenotypic characteristics were evaluated. The results showed that motor retardation, coordination disorders and limb stiffness occurred four weeks after PQ exposure, as well as the degeneration and loss of dopaminergic neurons in the SN. Activated microglia and increased CD68 expression appeared two weeks after PQ exposure in time-dependent manners. Increased CD86 and decreased CD206 expression were observed four weeks after PQ exposure, accompanied by increased TNF-α and IL-6 levels and decreased IL-10 and TGF-β levels. These results indicate that PQ can activate microglia in vivo, and microglial activation precedes neuronal loss in the SN. Activated microglia are characterized by mixed M1/M2 polarization in the early stage and M1 polarization in the late stage of PQ-induced PD development.