Developmental Exposure to Traffic-Related Air Pollution and Neuropathological Outcomes in Rats

Neurodevelopmental disorders (NDDs) represent a significant public health concern, and increasing evidence suggests that environmental factors, particularly air pollution, may contribute to their etiology. Traffic-related air pollution (TRAP) is a complex mixture of pollutants, including particulate matter (PM), volatile organic compounds, nitrogen oxides, and carbon monoxide. Prior studies have shown that exposure to certain TRAP components can lead to NDD-relevant outcomes in animal models, but these studies often use exposure paradigms that don't accurately reflect real-world exposure complexity. This study aimed to address this gap by developing a novel exposure paradigm that mirrors real-world TRAP exposure, preserving both gaseous and particulate components and their daily fluctuations. This approach was used to investigate the neuropathological consequences of TRAP exposure in a rat model, focusing on outcomes relevant to NDDs.

Existing literature indicates that exposure to specific components of TRAP can influence various neurodevelopmental outcomes in rodent models. Studies have shown that exposure to fine or ultrafine PM during development increases neuroinflammatory cytokines and astrocyte activation, alters microglia morphology and neurotransmitter levels, increases oligodendrogenesis, and impacts lateral ventricle size. Developmental exposure to diesel exhaust has also been linked to altered cortical volume, disrupted cortical organization, and impaired neurogenesis. However, the limitations of many previous animal studies, including exposure paradigms that do not capture the complexity or spatiotemporal dynamics of real-world TRAP exposures, have hampered the translation of these findings to humans. The variability in composition, dose, and timing of air pollution exposures further complicates the interpretation and applicability of these findings.

The study employed a novel TRAP exposure facility located adjacent to a major freeway tunnel system. Air from the tunnel, containing both light and heavy-duty vehicle emissions, was delivered directly to exposure chambers, maintaining the complex mixture of pollutants. Control animals were exposed to filtered air (FA). Pregnant Sprague-Dawley rats were exposed from gestational day (GD) 14 through postnatal day (PND) 47-51. Pups were randomly assigned to a cohort for behavioral testing or a cohort for neuropathological assessment. At approximately PND 50, animals were euthanized. Neuropathological assessments included magnetic resonance imaging (MRI) to measure brain and lateral ventricle volumes, immunohistochemistry for microglial (IBA1), astrocyte (GFAP, S100β), and neuronal markers (NeuN, DCX, Ki67), TUNEL staining for apoptosis quantification, quantitative polymerase chain reaction (qPCR) for gene expression analysis, and a Bio-Plex Pro™ assay for cytokine and chemokine quantification in hippocampal tissue.

TRAP exposure significantly increased the percentage of IBA1-positive cells (microglia) in the CA1 region of the hippocampus, indicating microgliosis. In contrast, TRAP decreased the area of GFAP immunoreactivity (astrocyte marker) in the dentate gyrus (DG). Analysis of hippocampal cytokines revealed a sex-specific effect on IL-10; TRAP-exposed females had significantly higher IL-10 levels than FA females. TRAP exposure increased hippocampal neurogenesis, indicated by increased DCX expression in males and increased Ki67+/DCX+ cells in both the SGZ and GCL of males. In females, TRAP exposure increased the granule cell layer width. PM2.5 and TSP levels were significantly higher in the TRAP group compared to the FA group.

The findings of this study demonstrate that developmental exposure to real-world TRAP can induce significant neuropathological changes in rats. The observed microgliosis in the CA1 hippocampal region suggests a neuroinflammatory response to TRAP exposure. The decrease in GFAP immunoreactivity in the DG, coupled with the increase in neurogenesis in males, suggests complex and potentially compensatory responses to TRAP. The sex-specific effect on IL-10 levels highlights the importance of considering sex as a biological variable in environmental health research. The relevance of these findings to human neurodevelopmental health requires further investigation, but these results underscore the potential impact of TRAP exposure on the developing brain.

This study provides strong evidence that developmental exposure to real-world TRAP induces significant neuropathological changes in a rat model. The findings highlight the importance of considering both the complexity of TRAP exposure and sex differences in future research on the impact of air pollution on neurodevelopment. Further research should focus on the mechanisms underlying the observed effects and explore the long-term consequences of TRAP exposure on cognitive function and behavior.

This study focused on neuropathological outcomes and did not directly assess cognitive or behavioral effects. While the exposure paradigm closely mimics real-world TRAP exposure, it is still a model and may not perfectly replicate human exposure. The study was conducted on Sprague-Dawley rats; the findings may not be generalizable to all species, including humans. Finally, the study did not investigate the potential contributions of individual pollutants within the TRAP mix.