Does suppression of glymphatic flow explain why chronic neuropathic pain elevates the risk of developing Alzheimer-like dementia?

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Abstract: The glymphatic system is a network of perivascular spaces that function as a waste clearance system, analogous to the peripheral lymphatic system. Reduced glymphatic function has been a hallmark observation in aging as well as models of Alzheimer's disease, diabetes, hypertension, traumatic brain injury, excess alcohol intake, and chronic unpredictable stress. Preliminary data shows that acute and chronic pain, and one night of light all suppressed glymphatic function. This application will use the murine sparse nerve injury (SNI) model to understand how the brain responds to chronic neuropathic pain. Sleep complaints are prevalent in chronic pain patients, and chronic sleep restriction increases pain sensitivity in mice. Norepinephrine (NE), which disrupts sleep and is released in stressful conditions, suppresses glymphatic function. We hypothesize that increase NE levels in SNI reduce glymphatic function, triggering cytokine accumulation, neuronal excitability, sleep disruption and pain sensitization in a feedforward loop (Aim 1). Traditional analgesics have been shown to relieve pain in models of chronic pain. Our preliminary data show that the same agents restore glymphatic function in SNI mice with no effect on glymphatic functions in control mice. We hypothesize that reducing the severity of pain via analgesia improves glymphatic function by reducing NE levels, which in turn reduces cytokine accumulation and excitability and improves sleep quality (Aim 2.1). Yet, efficacy of modern pharmacology is variable in the patient population, suggesting that while modulation of neural pathways is partially effective, pathology remains. We hypothesize that neuropathic pain induces a CNS maladaptive response involving reduced glymphatic flow, inflammation and waste accumulation. Because both natural and mind-body interventions target multiple facets of glymphatic disruption (sleep, inflammation, cardiovascular disease), we hypothesize that natural supplements (melatonin and eicosapentaenoic acid (an ω-3 fatty acid)) and mind-body interventions (voluntary exercise, improved sleep, and acupuncture) will improve glymphatic disruption in chronic pain (Aims 2.2 and 2.3). The timing of treatment is critical, because the circadian system is integrated into every process in the body including the glymphatic system, the immune system, and chronic pain. We propose that targeting therapeutics to reinforce the rhythm in glymphatic function and clearance will optimize the effect of treatment which can be quantified as an additional decrease in cytokine accumulation and hyperalgesia in SNI (Aim 3). We will time sleep improvements via increased temperature, voluntary exercise, melatonin, and acupuncture, to the endogenous rhythm of CSF distribution - high glymphatic clearance during rest, and low during wakefulness. Aim 3 is unique in that it tests whether efficacy of mind-body therapies, in improving glymphatic function and reducing pain sensitivity, can change based on when during the day they are administered. Overall, this application aims to define whether glymphatic activity may serve as a target for complementary therapeutic approaches and also as a biomarker establishing the efficacy of treatment.