Carbon monoxide poisoning–induced delayed encephalopathy accompanies decreased microglial cell numbers: Distinctive pathophysiological features from hypoxemia–induced brain damage

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Carbon monoxide (CO) causes not only acute fatal poisoning but also may cause a delayed neurologic syndrome called delayed encephalopathy (DE), which occasionally occurs after an interval of several days to several weeks post-exposure. However, the mechanisms of DE have not been fully elucidated. This study aimed to clarify the pathophysiology of CO–induced DE and its distinctive features compared with hypoxemic hypoxia. Rats were randomly assigned to three groups; the air group, the CO group (exposed to CO), and the low O2 group (exposed to low concentration of O2). Impairment of memory function was observed only in the CO group. The hippocampus tissues were collected and analyzed for assessment of CO–induced changes and microglial reaction. Demyelination was observed only in the CO group and it was more severe and persisted longer than that observed in the low O2 group. Moreover, in the CO group, decreased in microglial cell numbers were observed using flow cytometry, and microglia with detached branches were observed were observed using immunohistochemistry. Conversely, microglial cells with shortened branches and enlarged somata were observed in the low O2 group. Furthermore, mRNAs encoding several neurotrophic factors expressed by microglia were decreased in the CO group but were increased in the low O2 group. Thus, CO–induced DE displayed distinctive pathological features from those of simple hypoxic insults: prolonged demyelination accompanying a significant decrease in microglial cells. Decreased neurotrophic factor expression by microglial cells may be one of the causes of CO–induced DE.