Vascular pulsations, contractions of vascular smooth muscle cells and breathing have
been reported to foster movement and clearance of interstitial and cerebrospinal
fluids from the brain. The aim of this study was to estimate the contribution of
these vital functions. We compared the spread of an injected hydrophilic tracer
(Fluoro-Emerald, a 10 kDa fluorescein-coupled dextran amine) in the brains of live
anesthetized and sacrificed rats at 30 and 90 min after injection. To determine the
overall pattern of distribution of tracers, we created 3D-reconstructions of the horizontal
transections of the whole brain. Immunofluorescence staining with laminin and collagen
IV was performed to determine the pattern of distribution of tracer in relation to the
cerebrovascular basement membranes. We found that diffusion was widely restricted
to the periventricular region in sacrificed rats with no spread to the contralateral
hemisphere, while the bulk flow occurred along the vasculature and reached the
surface and the contralateral hemisphere as soon as 30 min after injection in live
anesthetized animals. The tracer appeared to be localized along the vascular basement
membranes and along fiber tracts as reported previously. Thus, our data indicate that
vital functions are essential for the remote movement of extracellular fluids within the
cerebral parenchyma