Behavioral and Biochemical Consequences of Simulated Vehicle Exhaust Exposure in Rats

Air pollution is one of the most serious environmental threats to mankind. According to a report by the World Health Organization, urban air pollutants, specifically gasoline and diesel emissions from internal combustion engines of automobiles, trucks, locomotives and ships, contribute to a wide range of adverse health effects including cardiovascular, respiratory and neurological complications. While there has been a significant amount of research on the effect of vehicle exhaust exposure on heart and lungs, and some surveys have reported occurrence of mental co-morbidities with air pollution; adverse effects of exhaust emissions on the brain and its psychological impact, has been largely ignored. Interestingly, gaseous constituents of vehicle exhaust, namely, carbon dioxide (CO2), carbon monoxide (CO) and nitrogen dioxide (NO2), are pro-oxidants in nature. This is important considering the recently established causal link between oxidative stress in the brain and behavioral and cognitive impairments. Our postulation is that pro-oxidants in vehicle exhaust potentially increase oxidative stress in the brain damaging neuronal circuits and networks that modulate behavior and cognition. Therefore, we believe that it is critical to examine whether chronic exposure to vehicle exhaust emissions elevates oxidative stress and compromises our psychological health. In this project, we examined the neurobiological effect of prolonged exposure to pro-oxidant constituents of vehicle exhaust in rats using a simulated vehicle exhaust exposure (SVEE) model. We hypothesized that sub-chronic and chronic exposure to simulated exhaust leads to increased oxidative stress in the brain eventually resulting in behavioral and cognitive deficits. Our second hypothesis was that interventions that attenuate oxidative stress such as mitochondria-targeted antioxidant, Mito-Q prevent SVEE-induced increase in oxidative stress and consequently prevent occurrence of behavioral and cognitive deficits. First, we developed a model using adult male Sprague Dawley (SD) rats and utilized a simulated mixture of vehicle exhaust comprising of pro-oxidant exhaust constituents, namely, carbon dioxide (13%), carbon monoxide (0.68%) and nitrogen dioxide (1000 ppm) in air. Rats were exposed either to a high dose with brief duration (1:10 dilution {CO2: 1.3%, CO: 0.068%, NO2: 100 ppm}, 30 min/day) or low dose with prolonged duration (~1:1000 dilution {CO2: 0.04%, CO: 3 ppm, NO2: 0.9 ppm}, 5 h/day) of simulated exhaust in separate experiments for a duration of 2 weeks (sub-chronic) or 6 weeks (chronic). Durations of exposures were comparable to daily exposure of exhaust levels in areas of high traffic. Control rats were exposed to normal air for the same duration. Following SVEE, a comprehensive behavioral and cognitive analysis was performed to assess anxiety- and depression-like behavior, as well as cognitive function and intelligence quotient (IQ) in rats. Later, rats were sacrificed for biochemical analysis to determine the effect of SVEE on oxidative stress and oxidative stress-associated pathways such as mitochondrial impairment. It was revealed that at both sub-chronic and chronic level of high dose-brief and low dose-prolonged duration SVEE, exposed rats exhibited an increased anxiety-like, depression-like behavior, impaired memory and lowered IQ. At the biochemical level, SVEE led to increase in oxidative stress in specific regions of the brain including pre-frontal cortex, hippocampus and amygdala as indicated by increased level of oxidative stress markers and reduced antioxidant function. SVEE-induced oxidative stress also led to mitochondrial impairment in the form of lowered oxygen consumption, suppressed ATP synthesis and alteration in mitochondrial fission/fusion. In order to test the second hypothesis that antioxidant interventions have a protective role in preventing SVEE-induced impairments, SD rats were pre-treated with Mito-Q/ drinking water for 4 weeks prior to SVEE following which behavioral and biochemical analysis were performed. It was revealed that Mito-Q treated rats were protected from SVEE-induced increased anxiety- and depression-like behavior. Mito-Q also prevented learning-memory and IQ impairment. This protective effect of Mito-Q was extended at the molecular level where it prevented SVEE-induced increased oxidative stress levels in the three brain regions. Mito-Q pre-treated rats were also protected from SVEE-induced mitochondrial impairment. In summary, our study suggests that prolonged exposure to pro-oxidant constituents of vehicle exhaust emissions is associated with behavioral and cognitive deficits. Targeted antioxidant intervention such as Mito-Q seem to be protective against vehicle exhaust-induced deficits at behavior, cognitive as well as at molecular level. This suggests that exhaust emissions most likely cause excessive generation of reactive oxygen species (ROS) and reactive nitrogen species (RNS) in the emotion and cognition regulating brain regions. This buildup of oxidative stress adversely affects neuronal circuitry leading to the development of an altered behavioral and cognitive phenotype. This is the first preclinical evidence suggesting negative neurobiological impact of pro-oxidant constituents of vehicle exhaust emissions, a finding highly relevant to human populations exposed to high vehicular traffic. Furthermore, our study provides a novel and innovative model that can not only be used to study behavioral and psychological effects of a variety of laboratory simulations but it can also be used to probe underlying neurobiological mechanisms.