Abstract
Gulf War Illness (GWI) is a chronic multisymptom disorder that affects approximately 25–32% of Gulf War veterans, and the combination of permethrin exposure with stress may have collectively and synergistically contributed to its development. However, the exact underlying pathogenic mechanisms within the brain that links permethrin and stress to the development of GWI remains unclear. In the current study, we investigate the effects and the associated molecular mechanisms within the microglia-neuron interplay in a mouse model of GWI, focusing on how exposure to permethrin may act as a priming agent when followed by stress. Subjecting mice to 14 days of chronic permethrin exposure followed by 7 days of stress resulted in the development of depression-like behavior. This behavioral change coincided with distinct alterations in the microglia phenotype, indicating microglial activation in the hippocampa. We revealed that blocking microglial activation through Gi inhibitory DREADD receptors in microglia effectively prevented the behavioral change associated with permethrin and stress exposure. To elucidate the transcriptional networks impacted within distinct microglia populations linked to depression-like behavior in mice exposed to both permethrin and stress, we conducted a single-cell RNA sequencing analysis using 21,566 single nuclei collected from the hippocampus of mice. For bioinformatics, UniCell Deconvolve was a pre-trained, interpretable, deep learning model used to deconvolve cell type fractions and predict cell identity across spatial datasets. Our bioinformatics analysis identified significant alterations in permethrin exposure followed by stress-associated microglia population, notably pathways related to neuronal development, neuronal communication, and neuronal morphogenesis, all of which are associated with neural synaptic plasticity. Additionally, we observed permethrin exposure followed by stress-mediated changes in signal transduction, including modulation of chemical synaptic transmission, regulation of neurotransmitter receptors, and regulation of postsynaptic neurotransmitter receptor activity, a known contributor to the pathophysiology of depression in a subset of the hippocampal pyramidal neurons in the CA3 subregions. These studies tentatively suggest that permethrin may prime toward a depression-like behavior that can be triggered by psychological stress in mice through microglial activation, resulting in alterations of neural plasticity. This new evidence underscores the significance of the synergistic role of multi-causal factors associated with GWI.