Flow of cerebrospinal fluid is driven by arterial pulsations and is reduced in hypertension methods
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Inducing acute hypertension to alter wall motion and flow
To test whether these changes in the vessel wall dynamics affect the efficiency of the perivascular pump, we imaged CSF flow and simultaneously recorded ECG, respiration, and arterial blood pressure before, during, and after administering Ang-II (Supplementary Movie ). The average flow speed (Fig. ) was reduced by ~40% during Ang-II-induced hypertension (Fig. ). This decrease in flow speed was independent of the time after infusion (Supplementary Fig. ). Also, microsphere trajectories show more backflow during each cardiac period in acute hypertension (Fig., insets). To quantify changes in the instantaneous flow rate, we computed the downstream component of every velocity measurement (Fig. ). Hypertension increases backflow (negative downstream velocity) by 21% relative to controls (Fig. and Supplementary Fig. ). A small percentage of t...
Methods
All experiments were approved by the University Committee on Animal Resources of the University of Rochester Medical Center (Protocol No. 2011-023) and an effort was made to minimize the number of animals used. Male C57BL/6 mice, 8-12 weeks of age (Charles River), were anesthetized with ketamine/xylazine (100/10 mg kg -1, intraperitoneally). Body temperature was maintained at 37.5 °C with a rectal probe-controlled heated platform (Harvard Apparatus). A cranial window was prepared over the MCA vascular territory on the right anterolateral parietal bone leaving dura mater intact. The cranial window was sealed with agarose (0.8% at 37 °C) and a glass coverslip to prevent intracranial depressurization. For a subset of experiments, instead of a cranial window, mice received a thin-skull preparation,. Afterwards, a cannula was placed in the...
Acute arterial hypertension induction
Angiotensin-II (Tocris) was dissolved in NaCl 0.9% and infused into a femoral vein catheter at 5 ng g -1 min -1 at a volumetric rate of 1 µl min -1 using a syringe pump (Harvard Apparatus) for the duration of the experiment. Mice with less than a 20 mm Hg increase in blood pressure were excluded from the analysis. Control mice received an intravenous infusion of NaCl 0.9% at 1 µl min -1.
Calculation of Reynolds and Péclet numbers
The Reynolds number is calculated as: 1 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathrm{Re}} = \frac{{UL}}{\nu },$$\end{document} Re = U L ν, where U is the spatial mean of the magnitude of the time-averaged flow velocity (e.g., the mean of measurements shown in Fig. ), L = 4.4 × 10 -5 m is the average PVS width that has been measured (Fig. ), and ν = 0.697 × 10 -6 m 2 s -1 is the kinematic viscosity of water at 36.8 °C (a good approximation for cerebrospinal fluid). The Péclet number is calculated as: 2 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepac...
Artery diameter and wall velocity measurements
All line scans were recorded transverse to the MCA. The line scans were recorded at about 600 Hz with synchronized ECG measurements, from which we obtained about 120 artery diameter measurements per cardiac cycle (depending on the heart rate). Line scans at each location were recorded for at least 45 s. For each line scan time series, the outer edges of the artery were located with subpixel accuracy at each instant of time by performing a cubic interpolation of the pixel intensity profile onto a finer grid and identifying the two locations where the pixel intensity crossed a given threshold. For each time series, the threshold value was chosen to approximately coincide with the region of the steepest gradient in the pixel intensity profile. The artery diameter at each instant of time was calculated by measuring the distance between these two locations. Using the synchroniz...
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To test whether these changes in the vessel wall dynamics affect the efficiency of the perivascular pump, we imaged CSF flow and simultaneously recorded ECG, respiration, and arterial blood pressure before, during, and after administering Ang-II (Supplementary Movie ). The average flow speed (Fig. ) was reduced by ~40% during Ang-II-induced hypertension (Fig. ). This decrease in flow speed was independent of the time after infusion (Supplementary Fig. ). Also, microsphere trajectories show more backflow during each cardiac period in acute hypertension (Fig., insets). To quantify changes in the instantaneous flow rate, we computed the downstream component of every velocity measurement (Fig. ). Hypertension increases backflow (negative downstream velocity) by 21% relative to controls (Fig. and Supplementary Fig. ). A small percentage of the microspheres aggregate and adhere to the PVS wall, remaining in the imaging field for the duration of the experiment; however, hypertension did not increase the number of these stuck particles compared to the controls (Supplementary Fig. ). Fig. 5 Acute arterial hypertension reduces net CSF...
All experiments were approved by the University Committee on Animal Resources of the University of Rochester Medical Center (Protocol No. 2011-023) and an effort was made to minimize the number of animals used. Male C57BL/6 mice, 8-12 weeks of age (Charles River), were anesthetized with ketamine/xylazine (100/10 mg kg -1, intraperitoneally). Body temperature was maintained at 37.5 °C with a rectal probe-controlled heated platform (Harvard Apparatus). A cranial window was prepared over the MCA vascular territory on the right anterolateral parietal bone leaving dura mater intact. The cranial window was sealed with agarose (0.8% at 37 °C) and a glass coverslip to prevent intracranial depressurization. For a subset of experiments, instead of a cranial window, mice received a thin-skull preparation,. Afterwards, a cannula was placed in the cisterna magna and red fluorescent polystyrene microspheres (FluoSpheres™ 1.0 µm, 580/605 nm, 0.25% solids in artificial CSF (aCSF), Invitrogen) were briefly sonicated and infused at 2 µl min -1 for 5 min.
Angiotensin-II (Tocris) was dissolved in NaCl 0.9% and infused into a femoral vein catheter at 5 ng g -1 min -1 at a volumetric rate of 1 µl min -1 using a syringe pump (Harvard Apparatus) for the duration of the experiment. Mice with less than a 20 mm Hg increase in blood pressure were excluded from the analysis. Control mice received an intravenous infusion of NaCl 0.9% at 1 µl min -1.
The Reynolds number is calculated as: 1 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathrm{Re}} = \frac{{UL}}{\nu },$$\end{document} Re = U L ν, where U is the spatial mean of the magnitude of the time-averaged flow velocity (e.g., the mean of measurements shown in Fig. ), L = 4.4 × 10 -5 m is the average PVS width that has been measured (Fig. ), and ν = 0.697 × 10 -6 m 2 s -1 is the kinematic viscosity of water at 36.8 °C (a good approximation for cerebrospinal fluid). The Péclet number is calculated as: 2 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathrm{Pe}} = \frac{{UL}}{D},$$\end{document} Pe = U L D, where D = 6.55 × 10 -13...
All line scans were recorded transverse to the MCA. The line scans were recorded at about 600 Hz with synchronized ECG measurements, from which we obtained about 120 artery diameter measurements per cardiac cycle (depending on the heart rate). Line scans at each location were recorded for at least 45 s. For each line scan time series, the outer edges of the artery were located with subpixel accuracy at each instant of time by performing a cubic interpolation of the pixel intensity profile onto a finer grid and identifying the two locations where the pixel intensity crossed a given threshold. For each time series, the threshold value was chosen to approximately coincide with the region of the steepest gradient in the pixel intensity profile. The artery diameter at each instant of time was calculated by measuring the distance between these two locations. Using the synchronized ECG measurements, the cardiac cycle was defined by sequential peaks of the R wave. The change in artery diameter was then calculated as the difference between the instantaneous artery diameter and a two-cardiac-cycle moving average of the artery diameter. The changes in artery diameter measuremen...
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