Long-Term Neuronal Effects Following Developmental Exposure to Low Levels of Persistent Organic Pollutants

Abstract

Introduction: Persistent organic pollutants (POPs) methylmercury (MeHg), polychlorinated biphenyls (PCBs), and organochlorines (OCs) are a significant problem in Arctic regions of Canada. The long-term impact of low-level developmental exposure to POPs on offspring is not clearly understood. The objective of the present thesis was to examine the long-term effects of low-level developmental exposure to POPs, alone or in mixtures, on selected markers of brain structure and function in the adult central nervous system (CNS). Method: Pregnant Sprague Dawley rats were exposed to one of four toxicant exposure conditions (MeHg, PCB, MeHg+PCB, or MeHg+PCB+OCs) at levels scaled to produce comparable maternal blood levels reported in epidemiological studies (or corn oil (control). Offspring were exposed to the toxicants from gestational day (GD) 1 to postnatal day (PND) 21 and thereafter with no further toxicant exposures aged to late adulthood (PND 450). The brain of adult offspring was fixed, frozen, and slide mounted for processing using immunohistochemistry and immunofluorescence techniques. A battery of markers of brain structure and function were assessed (glutamic acid decarboxylase (GAD)-67, glial cells (microglia, Bergmann glia), cleaved caspase-3, cresyl violet, doublecortin, double-stranded DNA (Hoechst), tyrosine hydroxylase, rat-endothelial cell antigen-1, lipofuscin). Eleven neuronal regions were included in the analysis. Digital image analysis was performed (ImageJ). Independent Samples T-tests compared toxicant-exposed animals with controls. Results: Most statistically significant observations were reported in neuronal regions localized in the brainstem. Across all four toxicant exposure groups (MeHg, PCB, MeHg+PCB, and MeHg+PCB+OC), a statistically significant increase in GAD67 was observed in the cerebellum, in comparison with controls. Likewise, a consistent decrease in lipofuscin autofluorescence was observed in the locus coeruleus in all toxicant-exposed offspring. Endothelial cell and capillary morphological marker RECA-1 was observed to be altered in a temporal manner with respect to neuronal development that corresponds to peak exposures to MeHg and PCBs. Toxicant mixtures (MeHg+PCB, MeHg+PCB+OC) resulted in comparatively fewer statistically significant changes in markers of brain structure and function than individual toxicant exposures (MeHg, PCBs). Conclusion: Altogether, the present study suggests that prenatal and early post-natal exposures to low levels of POPs may have lasting, selective, region-specific effects upon the CNS.