Diffusion of flexible random-coil dextran polymers measured in anisotropic brain extracellular space by integrative optical imaging

¹ SUNY Downstate Medical Center
² NYU School of Medicine
³ Nathan S. Kline Institute for Psychiatric Research

^* To whom correspondence should be addressed. E-mail: sabina.hrabetova{at}downstate.edu.

Submitted on October 29, 2007
Revised on December 3, 2007
Accepted on 7 April 2008

	Abstract

There are a limited number of methods available to quantify the extracellular diffusion of macromolecules in an anisotropic brain region, e.g., an area containing numerous aligned fibers where diffusion is faster along the fibers than across. We applied the integrative optical imaging (IOI) method to measure diffusion of the fluorophore Alexa Fluor 488 (AF, MW 0.547 kDa) and fluorophore-labeled flexible random-coil dextran polymers (dexMW; MW 3, 75, 282, 525 kDa) in the extracellular space (ECS) of the anisotropic molecular layer of the isolated turtle cerebellum. For all molecules, two-dimensional (2-D) images acquired an elliptical shape with major and minor axes oriented along and across, respectively, the unmyelinated parallel fibers. The effective diffusion coefficients, D^*_major and D^*_minor, decreased with molecular size. The diffusion anisotropy ratio (DAR = D^*_major/D^*_minor) increased for AF through dex75 but then, unexpectedly, reached a plateau. We argue that dex282 and dex525 approach the ECS width and deform in order to diffuse. In support of this concept, scaling theory shows the diffusion behavior of dex282 and dex525 to be consistent with transition to a reptation regime, and estimates the average ECS width at about 31 nm. These findings have implications for the interstitial transport of molecules and drugs, and for modeling neurotransmitter diffusion during ectopic release and spillover.

Key Words: dextran, diffusion anisotropy ratio (DAR), diffusion coefficient, drug delivery, reptation, tortuosity