Methylene blue reduces postoperative delirium in mice through neuroinflammation suppression and blood-brain barrier repair

Postoperative delirium (POD) is a serious complication following anesthesia and surgery, with currently unclear pathogenesis and no effective treatments. In recent years, emerging evidence implicates that neuroinflammatory and blood-brain barrier (BBB) dysfunction play an important role in the pathogenesis of POD. Methylene blue (MB), an FDA-approved drug, has been reported to have anti-inflammatory and neuroprotective properties. We applied a POD model with a simple laparotomy to assess the POD-like behavior in adult mice. Behavioral tests were performed at 24 h preoperatively, 6 h and 24 h postoperatively. We evaluated neuroinflammation and the reactive states of microglia in the hippocampus and prefrontal cortex. We also evaluated the permeability of the BBB by detecting tight junction proteins, transcytosis, and the leakage of IgG and dextran. Our results found that POD-like behavior in adult mice mainly occurred at 6 h after surgery and gradually subsided at 24 h postoperatively. We found that MB (5 mg/kg) attenuated neuroinflammation and microglia activation in both the hippocampus and prefrontal cortex after anesthesia/surgery. By downregulating the protein levels of matrix metalloproteinase-inducing factor EMMPRIN and matrix metalloproteinase 9 (MMP9), up-regulating the expression of BBB tight junction proteins, and decreasing transcytosis, MB could improve the function of BBB and ameliorate the delirious-like behavior in mice after anesthesia/surgery. Our findings confirm the anti-inflammatory and BBB protective effects of MB in an inflammatory environment in vivo and provide better insights into the pathophysiology and potential treatment of POD.