Heterogeneous and anisotropic diffusion in the developing rat spinal cord.
Prokopova S, Vargova L, Sykova E
Department of Neuroscience, Institute of Experimental Medicine AS CR, Prague,
Czech Republic.
Extrasynaptic transmission is mediated by the diffusion of transmitters,
through the extracellular space (ECS) to receptors on neurons and glia.
The three-dimensional diffusion of tetramethylammonium (mol. wt 74.1 kDa)
was investigated in the isolated rat spinal cord at postnatal days 4-20.
The diffusion parameters of the ECS, volume fraction alpha, tortuosity
lambda (lambda2 = free/apparent diffusion coefficient in tissue) and nonspecific
uptake k', were different in gray and white matter. In both gray and white
matter, alpha decreased with neuronal development and gliogenesis by about
15% while lambda significantly increased. Diffusion in gray matter remained
isotropic (lambda = 1.65), while in white matter it became anisotropic,
i.e. easier along the fibers (lambda = 1.38) than across the fibers (lambda
= 1.80). Anisotropy increased in the second postnatal week, during pronounced
myelination. In myelinated tissue, preferential diffusion of neuroactive
substances occurs along the axons.