Extracellular diffusion parameters in spinal cord and filum terminale of the frog

Extracellular space (ECS) diffusion parameters were studied in isolated frog spinal cord grey matter and filum terminale (FT), that is predominantly composed of glial cells and axons. We compared the cell swelling induced by K+ application, hypotonic stress and tetanic stimulation of afferent input. The ECS diffusion parameters, volume fraction α (α = ECS volume/total tissue volume), tortuosity λ (λ2 = free/apparent diffusion coefficient in the tissue) and non‐specific cellular uptake k′, were determined by the real‐time iontophoretic method using TMA+‐selective microelectrodes. Stimulation‐evoked changes in extracellular K+ concentration ([K+]e) were measured by K+‐selective microelectrodes. Histological analysis revealed that in the central region of the FT, the cell density was lower than in SC, neurons and oligodendrocytes were scarce, GFAP‐positive astrocytes were abundant, and they showed thicker and more densely stained processes than in spinal cord. In the FT, α was 58% higher and λ significantly lower than in the spinal cord. In 50 mM K+, α in spinal cord decreased from about 0.19 to 0.09, i.e., by 53%, whereas in FT from about 0.32 to 0.20, i.e., by only 38%; λ increased significantly more in FT than in spinal cord. Hypotonic solution (175 mmol/kg−1) resulted in similar decreases in α, and there were no changes in λ in either spinal cord or FT. Stimulation of VIII or IX dorsal root (DR) by 30 Hz evoked an increase in [K+]e from 3 to 11–12 mM in spinal cord, but to only 4–5 mM in FT. In the spinal cord this stimulation led to a 30% decrease in α and a small increase in λ whereas in the FT the decrease in α was only about 10% and no increase in λ was found. We conclude that in spinal cord, a complex tissue with a higher density of cellular elements than the FT, 50 mM K+, hypotonic stress as well as DR stimulation evoked a greater decrease in ECS volume than in FT. Nevertheless, the K+‐induced increase in tortuosity was higher in FT, suggesting that a substantial part of the K+‐evoked increase in λ was due to astrocytic swelling. J. Neurosci. Res. 62:530–538, 2000. © 2000 Wiley‐Liss, Inc.