Flow of cerebral interstitial fluid as indicated by the removal of extracellular markers from rat caudate nucleus

Abstract

Bulk flow of interstitial fluid (ISF) in brain was studied by following the intracerebral distribution of horseradish peroxidase (HRP) injected into the caudate nucleus of rats. Using this technique, channels of ISF flow are outlined as the extracellular pathways of protein distribution away from the injection site. Anionic isoenzymes of HRP were separated from Sigma's Type II HRP using ion-exchange chromatography. Anionic isoenzymes proved superior as markers of ISF flow, presumably because there is less cellular uptake and surface adsorption of anionic protein. Analysis of the distribution of protein 4–8 hr after injection suggests that ISF flows from narrow intercellular clefts of the neuropil along a system of extracellular pathways including perivascular and periventricular areas and between fiber tracts. From this system ISF appears to drain into cerebrospinal fluid (CSF) and possibly also into fenestrated vessels within the brain. Quantitative aspects of fluid drainage from brain were investigated by measuring the rate and route of efllux from brain of radiolabelled extracellular markers: [ 3 H]polyethylene glycol (PEG: 4000 daltons) and [ 14 C]dextran (70 000 daltons). In nine control rats, the amount of isotope remaining in brain 4 hr after injection into the caudate nucleus, as percent of the injected dose (mean± S.E.M. ), was 55·8±6·3 for PEG and 63·1±6·6 for dextran. Similarity in the rates of dextran and PEG removal from brain, despite large differences in molecular weight, is consistent with removal from brain by bulk flow of ISF. A maximum of 20% of the isotope cleared from brain could be recovered from CSF, in agreement with anatomical evidence that CSF may not be the sole route of ISF removal from brain.