Extracellular diffusion in laminar brain structures exemplified by hippocampus

Abstract

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.

Brain extracellular space, hyaluronan, and the prevention of epileptic seizures

“…Even under normal conditions in rat brain, the CA1 SP has significantly lower ECS volume than the two neighboring layers, stratum oriens (SO) and stratum radiatum (SR; McBain et al, 1990; Saghyan et al, 2012). Saghyan and colleagues (2012) showed that the diffusion of substances across the SP is significantly hindered because of lower ECS volume and lower diffusion permeability in this layer in comparison with adjacent SR and SO.…”

“…Saghyan and colleagues (2012) showed that the diffusion of substances across the SP is significantly hindered because of lower ECS volume and lower diffusion permeability in this layer in comparison with adjacent SR and SO. To determine the impact of HA deficiency in Has3 KO mice on ECS properties in the CA1 SP, we designed two experiments to study ECS properties in the CA1 region of brain slices taken from Has3 KO mice (Arranz et al, 2014).…”

“…To determine the impact of HA deficiency in Has3 KO mice on ECS properties in the CA1 SP, we designed two experiments to study ECS properties in the CA1 region of brain slices taken from Has3 KO mice (Arranz et al, 2014). First, we imaged spread of fluorophore-labeled dextran (which is confined to the ECS) through CA1 SP after its release in the SR. Second, we employed the RTI method (Nicholson and Phillips, 1981) with multilayer analysis (Saghyan et al, 2012) to quantify ECS volume and diffusion permeability (Fig. 2C).…”

Real-time Iontophoresis with Tetramethylammonium to Quantify Volume Fraction and Tortuosity of Brain Extracellular Space

“…During RTI data analysis, the Walter program generates average α and λ , which include the diffusion of all molecules traveling in all axes from a point source to the ISM. If the rate of diffusion is significantly different in any one of the three spatial axes (anisotropy) or if the tissue is non-homogeneous, additional data collection and data analysis are required to calculate α and λ 8,14 .…”

“…Larger r distances also minimize brain region-independent differences in the ECS milieu, ISM tip size, and brain tissue damage during ISM placement. Conversely, when r is increased beyond 150 μm, the diffusion of molecules from the iontophoretic point source is more susceptible to influence by non-isotropic, inhomogeneous elements surrounding the brain region of interest or the tissue-perfusate boundary 14 .…”