Postoperative delirium (POD) is an acute syndrome of altered mental status characterized by impaired consciousness and inattention, and contributes to prolonged hospitalization, decreased neurocognitive function, and readmission [1], [2]. The prevalence of POD is particularly high in elderly patients [2], [3], who are more sensitive to anesthesia and surgical stress due to senescent organ function, age-dependent blood–brain barrier (BBB) disruption, and consequent reduction of cognitive reserve [4]. BBB disruption, migration of peripheral inflammation into the central nervous system (CNS), and synaptic dysfunction are hallmarks of POD pathophysiology. Animal studies have shown that the influx of inflammatory cells and mediators into the CNS via the disrupted BBB may lead to cognitive dysfunction and POD-like behavior [5], [6]. However, the specific mechanisms by which anesthesia/surgery (A/S) disrupt the BBB remain to be fully determined.

Pericytes are contractile cells adherent to capillary endothelium, and are encapsulated within the basement membrane. Together with endothelial cells, astrocytes, neurons, and extracellular matrix, they constitute the BBB. Pericyte coverage of the vessel wall limits BBB permeability by inhibiting vesicular transport (transcellular pathway) and restricting intercellular transport of substances through the tight junction complex (paracellular pathway) [7], [8], [9]. Pericytes bind to PDGF-B secreted by brain endothelial cells through specific expression of PDGFRβ receptor, which stimulates their proliferation, migration and recruitment to the walls of newly formed blood vessels. Deficiency of PDGFRβ can result in complete absence and specific damage to pericytes, making it a specific marker of pericyte injury [10], [11], [12], [13], [14]. After subarachnoid hemorrhage, pericytes degrade tight junction and basement membranes by secreting cyclophilin A and inducing matrix metalloproteinase 9 (MMP9) expression, which promotes protein hydrolysis [15]. In addition, pericytes can also regulate the migration of peripheral inflammation into the CNS. In pericyte-deficient mice, the neurovascular unit allows leukocyte influx into the CNS, which negatively correlates with pericyte coverage, and pericyte deficiency induces intrinsic changes in the cerebral vascular system further contributing to the inflammatory cascade response, leading to neuroinflammation [16]. Therefore, we assumed that pericyte damage promoting BBB disruption is one of the possible mechanisms involved in the onset of anesthesia/surgery-induced delirium-like behavior that is worth exploring.

MMP9 is one of the most important MMPs in the gelatinase family [15]. Pericytes are the main cellular source of MMP9 release in the inflammatory state, stimulated by inflammatory cytokines [17]. Even damaged pericytes of the neurovascular unit still robustly produced MMP9 [14]. Mediators released in the early stage of inflammation, such as IL-1β, IL-6, and TNF-α, upregulate the expression of MMP9. Overexpression of MMP9 disrupts the BBB and thus promotes the transit of inflammatory cytokines into brain tissue [18], [19]. This response in turn exacerbates BBB disruption; however, the exact mechanism of MMP9-mediated BBB injury is still not well understood. Our previous study found that MMP9 overexpression impairs BBB function through the degradation of tight junctions [20]. This implies that postoperative MMP9 activation may promote vascular leakage and BBB disruption, and that MMP9 inhibition may represent a novel strategy for the prevention and/or treatment of POD.

Melatonin (MLT) is secreted primarily by the pineal gland and exerts potent anti-inflammatory activities. MLT may protect against neurodegenerative disorders such as Alzheimer's disease [21]. In addition, MLT binds directly to MMP9 and may act as an endogenous inhibitor. Rudra et al. found that MLT decreases MMP9 activity by binding to its active catalytic site [22]. Zhao et al. demonstrated that MLT inhibited MMP9 activity by decreasing the levels of IL-1β, IL-6, and TNF-α in a rat model of osteoarthritis [23]. Therefore, the present study explored whether MLT may preserve BBB structure and function and attenuate POD-like behaviors through anti-inflammatory effects or by direct inhibition of MMP9, and further confirmed the possible mechanisms.