Perinatal arsenic exposure inhibits binding ability of glucocorticoid receptors to nuclear response elements altering gene expression and affecting learning behavior
Learning deficits in children following arsenic (As) exposure via drinking water have been epidemiologically described in the last decade. Arsenic is a persistent environmental toxin and exposure has been shown to perturb the hypothalamic-pituitary-adrenal (HPA) stress axis. The glucocorticoid receptors (GR) are an integral part of the HPA axis and are found throughout the central nervous system, particularly in the hippocampus, an area of the brain important in learning and memory. The mitogen-activated protein kinase (MAPK) pathway is involved in learning and two kinases of the MAPK pathway, Ras and Raf are transcriptionally regulated by GRs. In the studies detailed in this dissertation the impact of perinatal exposure to 50 parts per billion (ppb) sodium arsenate on learning behavior, GRs and MAPK pathway genes in the C57BL/6J adolescent mouse were examined. Hippocampal-sensitive learning was assessed using a novel object task and eight-way radial arm maze (RAM). Arsenic-exposed offspring showed increased latency to the novel object and increased number of entry errors in the RAM compared to controls. Immunoblotting revealed that arsenic-exposed offspring had significantly lower levels of both GR and mineralocorticoid receptors in the activated nuclear subcellular fraction than controls. As-exposed mice also showed significantly lower Ras and Raf-1 mRNA levels, assessed by real-time RT-PCR, compared to controls. Binding of the glucocorticoid receptor (GR) to glucocorticoid response elements (GREs) in Ras and Raf genes was measured by chromatin immunoprecipitation (ChIP). ChIP revealed reduced binding of the GR to GREs in target genes in the As group. Electrophoretic mobility shift assay (EMSA) revealed intact binding ability in both control and arsenic-exposed offspring to a synthetic GRE. Results suggest that moderate exposures to As can significantly reduce GR levels in the hippocampus, affecting expression of genes that are under the control of the GR and impacting learning behavior. Overall, these data suggest that moderate levels of perinatal As can have a lasting impact on the brain and HPA axis of offspring
