Heterogeneous and anisotropic diffusion in the developing rat spinal cord.

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.