Disentangling the impact of cerebrospinal fluid formation and neuronal activity on solute clearance from the brain

Abstract

Background Despite recent attention, pathways and mechanisms of fluid transposition in the brain are still a matter of intense discussion and driving forces underlying waste clearance in the brain remain elusive. Consensus exists that net solute transport is a prerequisite for efficient clearance. The individual impact of neuronal activity and liquor production, which both vary with brain state and anesthesia, remain unclear. Methods To separate conditions with high and low neuronal activity and high and low cerebrospinal fluid (CSF) formation, different anesthetic regimens in naive rat were established, using Isoflurane (ISO), Medetomidine (MED), acetazolamide or combinations thereof. With dynamic contrast-enhanced MRI, after application of low molecular weight contrast agent (CA) Gadobutrol to cisterna magna, tracer distribution was monitored as surrogate for solute clearance. Simultaneous fiber-based Ca2+-recordings informed about the state of neuronal activity under different anesthetic regimen. T2-weighted MRI and diffusion-weighted MRI (DWI) provided size of subarachnoidal space and aqueductal flow as surrogates for CSF formation. Finally, a pathway and mechanism-independent two-compartment model was introduced to provide a measure of efficiency for solute clearance from the brain. Results Anatomical imaging, DWI and Ca2+-recordings confirmed that conditions with distinct levels of neuronal activity and CSF formation were achieved. A sleep-resembling condition, with reduced neuronal activity and enhanced CSF formation was achieved using ISO + MED and an awake-like condition with high neuronal activity using MED alone. CA distribution in the brain correlated with the rate of CSF production. The cortical brain state had major influence on tracer diffusion. Under conditions with low neuronal activity, extracellular space was enlarged and solutes permeated deeper into brain parenchyma. Under conditions with high neuronal activity, solutes mostly remained in paravascular spaces and were cleared from the brain fast. Exclusively based on the measured time signal curves, the two-compartment model provided net exchange ratios, which were significantly larger for the sleep-resembling condition than for the awake-like condition. Conclusions Efficiency of solute clearance in brain is crucially dependent on both neuronal activity and CSF formation. Our clearance pathway and mechanism agnostic kinetic model informs about net solute transport, solely based on the measured time signal curves. This rather simplifying approach largely accords with preclinical and clinical findings.