Perivascular pumping in the mouse brain: Realistic boundary conditions reconcile theory, simulation, and experiment

Published: 1 July 2020| Version 2 | DOI: 10.17632/t7pgxnzsxj.2
Contributors:
Antonio Ládron-de-Guevara,
Jessica K. Shang,
Maiken Nedergaard,
Douglas H. Kelley

Description

We measured hydraulic resistance and compliance using bolus injection, an approach introduced by Marmarou et al. (1). We injected fluid briefly and rapidly, measuring the resulting change in intracranial pressure (ICP), to estimate an impulse response, approximating the CSF pathway as a linear RC system. 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. We used 8- to 12-week-old male C57BL/6 mice acquired from Charles River Laboratories (Wilmington, MA, USA). In all experiments, animals were anesthetized with a combination of ketamine (100 mg/kg) and xylazine (10 mg/kg) administered intraperitoneally. Depth of anesthesia was determined by the pedal reflex test. Body temperature was maintained at 37.5C with a rectal probe-controlled heated platform, and ECG and respiratory rate were monitored using a small animal physiological monitoring system (Harvard Apparatus). Anesthetized mice were fixed in a stereotaxic frame, and two cannulae were implanted into the right lateral ventricle (0.85 mm lateral, 2.10 mm ventral and 0.22 mm caudal to bregma) and the cisterna magna, as previously described (2). Using a computer-controlled syringe pump (Harvard Apparatus Pump 11 Elite), we injected V = 5 μL of artificial CSF (126 mM NaCl, 2.5 mM KCl, 1.25 mM NaH2PO4, 2 mM MgSO4, 2 mM CaCl2, 10 mM glucose, 26 mM NaHCO3; pH 7.4 when gassed with 95% O2 and 5% CO2) at 1 μL/s into the right lateral ventricle. We monitored ICP via the cisterna magna cannulation connected to a transducer attached to a pressure monitor (BP-1, World Precision Instruments Inc., Sarasota, FL). We have verified that the results do not change appreciably if we instead inject into the cisterna magna and measure ICP in the ventricle. We calculated the compliance C from the pressure-volume index (PVI): C = 0.4343·PVI/P0. The PVI is defined as the volume of fluid required to cause a tenfold pressure increase during bolus injection: PVI=V/(log10⁡[P_max/P_0]) The resistance R can be estimated as: R=(t·P_0)/(PVI·log10[P(t)·(P_max-P_0 )/(P_max·(P(t)-P_0))]) where P(t) is the pressure measured at time t. We expect R to be nearly constant, but to increase accuracy, we estimate R for each animal by averaging the results of the above equation to five evenly-spaced times during the experiment. 1. Marmarou A, Shulman K, Rosende RM (1978) A nonlinear analysis of the cerebrospinal fluid system and intracranial pressure dynamics. J Neurosurg 48(3):332–344. 2. Xavier AL, et al. (2018) Cannula implantation into the cisterna magna of rodents. JoVE (Journal of Visualized Experiments) (135):e57378.

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We calculated the compliance C from the pressure-volume index (PVI): C = 0.4343·PVI/P0. The PVI is defined as the volume of fluid required to cause a tenfold pressure increase during bolus injection: PVI=V/(log10⁡[P_max/P_0]) The resistance R can be estimated as: R=(t·P_0)/(PVI·log10[P(t)·(P_max-P_0 )/(P_max·(P(t)-P_0))]) where P(t) is the pressure measured at time t. We expect R to be nearly constant, but to increase accuracy, we estimate R for each animal by averaging the results of the above equation to five evenly-spaced times during the experiment.