Abstract
Many neurodegenerative and neurological diseases are rooted in dysfunction of the neuroimmune system; therefore, manipulating this system has strong therapeutic potential. Prior work has shown that exposing mice to flickering lights at 40 Hz drives gamma frequency (∼40 Hz) neural activity and recruits microglia, the primary immune cells of the brain, revealing a novel method to manipulate the neuroimmune system. However, the biochemical signaling mechanisms between 40 Hz neural activity and immune recruitment remain unknown. Here, we exposed wild-type male mice to 5–60 min of 40 Hz or control flicker and assessed cytokine and phosphoprotein networks known to play a role in immune function. We found that 40 Hz flicker leads to increases in the expression of cytokines which promote microglial phagocytic states, such as IL-6 and IL-4, and increased expression of microglial chemokines, such as macrophage-colony-stimulating factor and monokine induced by interferon-γ. Interestingly, cytokine effects differed as a function of stimulation frequency, revealing a range of neuroimmune effects of stimulation. To identify possible mechanisms underlying cytokine expression, we quantified the effect of the flicker on intracellular signaling pathways known to regulate cytokine levels. We found that a 40 Hz flicker upregulates phospho-signaling within the nuclear factor κ-light-chain-enhancer of activated B cells (NF-κB) and mitogen-activated protein kinase (MAPK) pathways. While cytokine expression increased after 1 h of 40 Hz flicker stimulation, protein phosphorylation in the NF-κB pathway was upregulated within minutes. Importantly, the cytokine expression profile induced by 40 Hz flicker was different from cytokine changes in response to acute neuroinflammation induced by lipopolysaccharides. These results are the first, to our knowledge, to show how visual stimulation rapidly induces critical neuroimmune signaling in healthy animals.
SIGNIFICANCE STATEMENT Prior work has shown that exposing mice to lights flickering at 40 Hz induces neural spiking activity at 40 Hz (within the gamma frequency) and recruits microglia, the primary immune cells of the brain. However, the immediate effect of 40 Hz flicker on neuroimmune biochemical signaling was unknown. We found that 40 Hz flicker leads to significant increases in the expression of cytokines, key immune signals known to recruit microglia. Furthermore, we found that 40 Hz flicker rapidly changes the phosphorylation of proteins in the NF-κB and MAPK pathways, both known to regulate cytokine expression. Our findings are the first to delineate a specific rapid immune signaling response following 40 Hz visual stimulation, highlighting both the unique nature and therapeutic potential of this treatment.
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