Regulatory Modulations and Dendritic Arborization in the Mouse Hippocampus Following Gulf War Toxicant Exposure

Abstract

Gulf War illness (GWI) is a chronic multi‐symptom syndrome which affects approximately 30% of the nearly 700,000 Veterans who were deployed to the Persian Gulf from 1990 to 1991. These Veterans have reported experiencing a variety of symptoms including difficulties with learning and memory, depression and anxiety, and increased incidence of neurodegenerative diseases. Evidence suggests that combined exposure to both reversible and irreversible acetylcholinesterase (AChE) inhibitors is a likely risk factor. We modeled Gulf War exposure in male C57Bl/6J mice with three AChE inhibitors that have been implicated as causative agents for GWI: pyridostigmine bromide (PB), the anti‐sarin prophylactic; chlorpyrifos (CPF), an organophosphate insecticide; and N,N‐diethyl‐m‐toluamide (DEET), a common insect repellent. Previously, we reported acute hippocampal gene expression changes following 10 d of toxicant exposure, including significant downregulation of several neuronal immediate early genes (IEGs) such as Arc, Egr1, and Nr4a1, as well as hippocampal‐dependent memory impairment in a Y‐maze task. Arc is predominantly expressed in cortical and hippocampal neurons and is critical for long‐term potentiation (LTP) and stabilization of synaptic plasticity. IEGs such as Arc and Egr1 have also been suggested to play a role in determining the risk of developing major depressive disorder, which is often comorbid with GWI. Here, we quantified Arc protein expression in granule cells of the dentate gyrus with IHC at 2‐4 h post‐exposure and examined the effects of treatment with a neuroprotective Nrf2 activator, tert‐butylhydroquinone (tBHQ), at 14 weeks post‐exposure. Additionally, mice were either placed in a novel enriched environment for 5 min or kept in their home cage to evaluate induced Arc expression in granule cells at 4 h post‐exposure. We also assessed dendritic arborization, important in connectivity, in granule cells by measuring dendritic lengths and spine densities with Golgi staining at 12 weeks post‐exposure. We hypothesized that toxicant‐exposed mice would have fewer Arc+ granule cells in the dentate gyrus and show reductions in both basal and induced Arc expression at 4 h post‐exposure. We also hypothesized that exposed mice would display dendritic arbor reduction and loss of spines in granule cells at the chronic timepoint, and that this effect would be reversed by antioxidant treatment. Preliminary results indicate that Arc protein expression is decreased in granule cells of exposed mice at the acute but not chronic timepoint. Dendritic arbor complexity was significantly reduced at the chronic phase in exposed mice but improved when treated with 1% tBHQ formulated food pellets. Our results suggest that IEGs may only be dysregulated immediately following exposure but could contribute to long‐term detrimental effects on hippocampal neuroplasticity, which may be improved with antioxidant treatment.