Noradrenergic Slow Vasomotion: The Hidden Fluid Pump Linking Sleep, Brain Clearance, and Dementia Pathogenesis

Brain function is reliant upon maintaining a constant internal environment; however, the methods employed to maintain this environment have historically been viewed as largely passive in nature, relying on diffusion and vascular pulsations to create the conditions necessary for continued brain activity. This review seeks to provide an overview of current data suggesting that brain clearance is in fact an active process that is dependent upon both the current regulatory state of the brain and the presence of noradrenergic slow vasomotion, which is generated by rhythmic output from the locus coeruleus (LC). The LC-generated output has been found to influence the degree of contraction exhibited by pericytes, the geometric shape of astrocytic end-feet, and vascular tone, ultimately impacting the rate of exchange between cerebrospinal fluid (CSF), interstitial fluid (ISF), and the blood-brain barrier through aquaporin-4 (AQP4) channels. These LC-generated rhythmic changes are thought to provide the mechanical forces necessary for sustaining the metabolic clearance of waste products within the parenchyma. This review seeks to synthesize several recent studies which indicate that LC-generated vasomotion correlates with both the structure and progression of sleep states, neuronal oscillation patterns, and metabolic states, and that dysfunction of this LC-generated rhythm may contribute to pathological features associated with Alzheimer's disease, Parkinson's disease, and small-vessel disease. Understanding the mechanisms of clearance within the brain as a physiologically tunable system will allow researchers to view brain clearance as an adaptive neuro-modulatory function rather than merely as a passive event. Therefore, the focus of this review is on identifying the potential applications of advancements in the field of physiological imaging, molecular biomarkers, and neuro-modulatory or vascular-based therapies for early detection and therapeutic manipulation of clearance processes. Understanding these mechanisms will potentially lead to enhanced cognitive resilience and immune regulation, and promote healthy brain aging.