Proceedings of The Physiological Society
Kings College London (2005) J Physiol 565P, PC156
Evidence for bulk flow of brain interstitial fluid from 3-D image analysis of tracer-injected rat brain.
Vastani, N ; Kumaran, R ; Batchelor , P ; Felts, P ; McCormack, D ; Patel, R ; Steul, K ; Abbott, NJ ;
1. Centre for Neuroscience Research, King's College London, London, United Kingdom. 2. Radiological Imaging, King's College London, London, United Kingdom.
Brain interstitial fluid (ISF) forms the microenvironment for neurones and glia. There is accumulating evidence that like cerebrospinal fluid (CSF), ISF is flowing not static (Abbott, 2004). The dynamics of ISF flow have important implications for CNS function. This study aimed to obtain new quantitative and anatomical information on major sites for ISF flow, building on our earlier study using 20 μl tracer injection (Mihaylov et al 2002). Five adult female Wistar rats were anaesthetised (I.P.) to reach surgical anaesthesia (details in Youdim et al. 2004: 2.7ml/kg of diluted anaesthetic solution (50% water :25% Hypnorm (fentanyl citrate 0.315mg/ml + fluanisone 10mg/ml Jansen): 25% Hypnovel (midazolam 5mg/ml, Roche)). 1μl artificial CSF containing tracers Evans blue-albumin (EBA, 50mg/ml) and [3H] inulin (4μCi/μl) was stereotactically injected in the grey-white matter junction of the corpus callosum and caudate putamen, 3.3mm deep over a period of 2 minutes. Three hours later the rats were humanely killed and their brains removed, frozen, sectioned (40 μm) and prepared for image analysis (optical density for EBA, autoradiography for [3H] inulin). Modified MATLAB software was used to plot the concentration of both tracers (from optic density) as a function of distance from the point of injection (POI), in three planes (medial-lateral, dorsal-ventral, anterior-posterior). The plots were compared to theoretical curves as predicted for diffusion (Nicholson, 1984). For both tracers, deviation from the theoretical curves was evident, with spread to greater distances than predicted by diffusion, consistent with some movement by bulk flow of ISF. Asymmetries in distribution profiles were also observed, suggesting the existence of vectorial flow. Superimposition of tracer images onto the corresponding rat brain histology atlas (Paxinos and Watson, 1986) showed preferential movement of tracers along the corpus callosum (white matter tracts) with extensions into the overlying cortex, and some spread into the caudate putamen (grey matter). In one brain, a different pattern was seen, suggesting spread particularly in the perivascular space around major vessels (middle cerebral artery and branches). The pattern appeared to be critically dependent on injection site. Bulk flow of ISF has implications for non-synaptic communication, for clearance of toxic metabolites such as β-amyloid in Alzheimer’s disease, for migration of tumour and stem cells, and for drug delivery to the brain (Abbott, 2004).
Where applicable, experiments conform with Society ethical requirements