Air pollution & the brain: Subchronic diesel exhaust exposure causes neuroinflammation and elevates early markers of neurodegenerative disease

Background: Increasing evidence links diverse forms of air pollution to neuroinflammation and neuropathology in both human and animal models, but the effects of long-term exposures are poorly understood. Objective: We explored the central nervous system consequences of subchronic exposure to diesel exhaust (DE) and addressed the minimum levels necessary to elicit neuroinflammation and markers of early neuropathology. Methods: Male Fischer 344 rats were exposed to DE (992, 311, 100, 35 and 0 mu g PM/m(3)) by inhalation over 6 months. Results: DE exposure resulted in elevated levels of TNF alpha at high concentrations in all regions tested, with the exception of the cerebellum. The midbrain region was the most sensitive, where exposures as low as 100 mu g PM/m(3) significantly increased brain TNF alpha levels. However, this sensitivity to DE was not conferred to all markers of neuroinflammation, as the midbrain showed no increase in IL-6 expression at any concentration tested, an increase in IL-1 beta at only high concentrations, and a decrease in MIP-1 alpha expression, supporting that compensatory mechanisms may occur with subchronic exposure. A beta 42 levels were the highest in the frontal lobe of mice exposed to 992 mu g PM/m(3) and tau [pS199] levels were elevated at the higher DE concentrations (992 and 311 mu g PM/m(3)) in both the temporal lobe and frontal lobe, indicating that proteins linked to preclinical Alzheimer's disease were affected. alpha Synuclein levels were elevated in the midbrain in response to the 992 mu g PM/m(3) exposure, supporting that air pollution may be associated with early Parkinson's disease-like pathology. Conclusions: Together, the data support that the midbrain may be more sensitive to the neuroinflammatory effects of subchronic air pollution exposure. However, the DE-induced elevation of proteins associated with neurodegenerative diseases was limited to only the higher exposures, suggesting that air pollution-induced neuroinflammation may precede preclinical markers of neurodegenerative disease in the midbrain.