Abstract
BackgroundThe exchange of cerebrospinal (CSF) and interstitial fluid is believed to be vital for waste clearance in the brain. The sleep-dependent glymphatic system, which is comprised of perivascular flow of CSF and is largely dependent on arterial pulsatility and astrocytic aquaporin-4 (AQP4) expression, facilitates much of this brain clearance. During the last decade, several observations have indicated that impaired glymphatic function goes hand in hand with neurodegenerative diseases. Since pathologies of the brain carry inflammatory components, we wanted to know how acute inflammation, e.g., with lipopolysaccharide (LPS) injections, would affect the glymphatic system. In this study, we aim to measure the effect of LPS on perivascular CSF distribution as a measure of glymphatic function.MethodsThree hours after injection of LPS (1 mg/kg i.p.), C57bl/6 mice were (1) imaged for two CSF tracers, injected into cisterna magna, (2) transcardially perfused with buffer, or (3) used for physiological readouts. Tracer flow was imaged using a low magnification microscope on fixed brains, as well as using vibratome-cut slices for measuring tracer penetration in the brain. Cytokines, glial, and BBB-permeability markers were measured with ELISAs, Western blots, and immunohistochemistry. Cerebral blood flow was approximated using laser Doppler flowmetry, respiration and heart rate with a surgical monitor, and AQP4-polarization was quantified using confocal microscopy of immunolabeled brain sections.ResultsLPS-injections significantly lowered perivascular CSF tracer flow and penetration into the parenchyma. No differences in AQP4 polarization, cytokines, astroglial and BBB markers, cerebral blood flow, or respiration were detected in LPS-injected mice, although LPS did elevate cortical Iba1+ area and heart rate.ConclusionsThis study reports another physiological response after acute exposure to the bacterial endotoxin LPS, namely the statistically significant decrease in perivascular distribution of CSF. These observations may benefit our understanding of the role of systemic inflammation in brain clearance.
Highlights
Lipopolysaccharides (LPS) are molecules found in the outer membrane of Gram-negative bacteria, and are frequently used in many animal models to mimic inflammatory responses, e.g., in sepsis, depression, and neurodegeneration [1,2,3]
Imaging cerebrospinal fluid (CSF)-tracers after Cisterna magna-injections is a standard method of tracking CSF movement [6, 9, 10]. We used this method to assess the acute effect of a single systemic LPS injection on perivascular CSF dynamics in the mouse
Cisternae Magna injections LPS injection decreases distribution of CSF tracer in the brain We assessed the effect of LPS to CSF flow in the brain by injecting two tracers (Alexa-647 conjugated BSA (66kDa) and FITC-conjugated lysine-fixable dextran (3 kDa)) into the cisterna magna, from where they could distribute for 30 min before collection of brains
Summary
Lipopolysaccharides (LPS) are molecules found in the outer membrane of Gram-negative bacteria, and are frequently used in many animal models to mimic inflammatory responses, e.g., in sepsis, depression, and neurodegeneration [1,2,3]. Imaging CSF-tracers after Cisterna magna-injections is a standard method of tracking CSF movement [6, 9, 10]. We used this method to assess the acute effect of a single systemic LPS injection on perivascular CSF dynamics in the mouse. The sleep-dependent glymphatic system, which is comprised of perivascular flow of CSF and is largely dependent on arterial pulsatility and astrocytic aquaporin-4 (AQP4) expression, facilitates much of this brain clearance. Since pathologies of the brain carry inflammatory components, we wanted to know how acute inflammation, e.g., with lipopolysaccharide (LPS) injections, would affect the glymphatic system. We aim to measure the effect of LPS on perivascular CSF distribution as a measure of glymphatic function
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