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

Extracellular space (ECS) diffusion parameters were studied in isolated fro g spinal cord grey matter and filum terminale (FT), that is predominantly c omposed 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 alpha (alpha = ECS volume/to tal tissue volume), tortuosity lambda (lambda (2) = free/apparent diffusion coefficient in the tissue) and non-specific cellular uptake k', were deter mined by the real-time iontophoretic method using TMA(+)-selective microele ctrodes. Stimulation-evoked changes in extracellular K+ concentration ([K+] (e)) were measured by K+-selective microelectrodes. Histological analysis r evealed that in the central region of the FT, the cell density was lower th an in SC, neurons and oligodendrocytes were scarce, GFAP-positive astrocyte s were abundant, and they showed thicker and more densely stained processes than in spinal cord. In the FT, a was 58% higher and lambda significantly lower than in the spinal cord. In 50 mM K+, alpha in spinal cord decreased from about 0.19 to 0.09, i.e., by 53%, whereas in FT from about 0.32 to 0.2 0, i.e., by only 38%; lambda increased significantly more in FT than in spi nal cord. Hypotonic solution (175 mmol/kg(-1)) resulted in similar decrease s in alpha, and there were no changes in lambda 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 th e spinal cord this stimulation led to a 30% decrease in a and a small incre ase in lambda whereas in the FT the decrease in a was only about 10% and no increase in lambda was found. We conclude that in spinal cord, a complex t issue with a higher density of cellular elements than the FT, 50 mM K+, hyp otonic stress as well as DR stimulation evoked a greater decrease in ECS vo lume than in FT. Nevertheless, the K+-induced increase in tortuosity was hi gher in FT, suggesting that a substantial part of the K+-evoked increase in lambda was due to astrocytic swelling. (C) 2000 Wiley-Liss, Inc.