Extracellular diffusion in laminar brain structures exemplified by hippocampus

Numerous brain structures are composed of distinct layers and such stratification has a profound effect on extracellular diffusion transport in these structures. We have derived a more general form of diffusion equation incorporating inhomogeneities in both the extracellular volume fraction (α) and diffusion permeability (θ). A numerical solution of this equation for a special case of layered environment was employed to analyze diffusion in the CA1 region of hippocampus where stratum pyramidale occupied by the bodies of principal neurons is flanked by stratum radiatum and stratum oriens. Extracellular diffusion in the CA1 region was measured in vitro by real-time iontophoretic and real-time pressure methods, and numerical analysis found that stratum pyramidale had a significantly smaller extracellular volume fraction (α=0.127) and lower diffusion permeability (θ=0.327) than the other two layers (α=0.218, θ=0.447). Stratum pyramidale thus functioned as a diffusion barrier for molecules attempting to cross it. We also demonstrate that unless the detailed properties of all layers are taken into account when diffusion experiments are interpreted, the extracted apparent parameters of the extracellular space lose their physical meaning and capacity to describe any individual layer. Such apparent parameters depend on diffusion distance and direction, giving a false impression of microscopic anisotropy and non-Gaussian behavior. This finding has implications for all diffusion mediated physiological processes as well as for other diffusion methods including integrative optical imaging and diffusion-weighted magnetic resonance imaging.