Neuroimmune signaling and microglia in chronic ethanol consumption

Alcohol use disorder (AUD) is a pervasive and debilitating condition characterized by the inability to stop consuming alcohol. Studies suggest that chronic alcohol use damages the prefrontal cortex (PFC), the brain region important for executive control, further driving alcohol consumption. Recent work has revealed that alcohol use alters immune signaling in the PFC, perturbing cortical function and increasing consumption. Although it is known that chronic alcohol use leads to neuroimmune changes in the brain, it remains unclear which specific innate immune signaling pathways are responsible. In addition, it is unknown how microglia, the immune cells of the brain, contribute to the ethanol-induced immune response in vivo. Thus, I sought to elucidate the specific immune pathways and microglial changes that occur in response to chronic voluntary ethanol. Using a voluntary chronic drinking paradigm in mice, I evaluated gene expression changes in the Toll-like receptor (TLR) signaling pathways immediately following consumption and 24-hours after ethanol removal. I discovered that the primary TLR signaling pathway (the MyD88-dependent pathway) remained relatively unchanged, while Tlr3 and components of its pathway (the TRIF-dependent pathway) were increased 24-hours after ethanol consumption in the PFC. I also looked at the nucleus accumbens (NAc) and amygdala (AMY) and found that the NAc showed similar changes to the PFC but weaker, while the AMY showed changes in the opposite direction. Furthermore, administration of a TRIF-pathway inhibitor decreased ethanol consumption, suggesting a role for this pathway in regulating drinking behavior. Then, using isolated glial cells, I sought to determine which cell types TLR genes were localized in and which cell types showed changes following immune induction. I discovered that most TLR pathway genes are enriched and changed in microglia, however Tlr3 is enriched and increased in astrocytes. These results suggest a potential role for TRIF-dependent signaling in astrocytes. To further investigate the microglial changes in response to ethanol, I isolated the total homogenate and microglia from the PFC of mice that had undergone chronic voluntary ethanol consumption. I discovered that microglial changes were mostly undetected in the total homogenate and that microglial changes were associated with endosomal TLR signaling and TGF-β signaling. In addition, I investigated the microglial response to ethanol to the response to lipopolysaccharide (LPS), a TLR ligand that produces an immune response often compared to ethanol. Surprisingly, I found that microglia from the two treatments showed very different gene expression changes. Together, these data suggest a role for endosomal TLR signaling, the TRIF-dependent pathway, and TGF-β in the ethanol-neuroimmune response. Future studies will aim to elucidate the mechanisms by which these signaling pathways change drinking, as well as the roles of the different cell types