Membrane currents and morphological properties of neurons and glial cells in the spinal cord and filum terminale of the frog

Using the patch-clamp technique in the whole-cell configuration combined wi th intracellular dialysis of the fluorescent dye Lucifer yellow (LY), the m embrane properties of cells in slices of the lumbar portion of the frog spi nal cord (n = 64) and the filum terminale (FT, n = 48) have been characteri zed and correlated with their morphology. Four types of cells were found in lumbar spinal cord and FT with membrane and morphological properties simil ar to those of cells that were previously identified in the rat spinal cord (Chvatal, A., Pastor, A., Mauch, M.. Sykova, E., Kettenmann, H., 1995. Dis tinct populations of identified glial cells in the developing rat spinal co rd: Ion channel properties and cell morphology. Eur. J. Neurosci. 7, 129-14 2). Neurons, in response to a series of symmetrical voltage steps, displaye d large repetitive voltage-dependent Na+ inward currents and K+ delayed rec tifying outward currents. Three distinct types of non-neuronal cells were f ound. First, cells that exhibited passive symmetrical non-decaying currents were identified as astrocytes. These cells immunostained for GFAP and typi cally had at least one thick process and a number of fine processes. Second , cells with the characteristic properties of rat spinal cord oligodendrocy tes, with passive symmetrical decaying currents and large tail currents aft er the end of the voltage step. These cells exhibited either long parallel or short hairy processes. Third. cells that expressed small brief inward cu rrents in response to depolarizing steps, delayed rectifier outward current s and small sustained inward currents identical to rat glial precursor cell s. Morphologically. they were characterized by round cell bodies with a num ber of finely branched processes. LY dye-coupling in the frog spinal cord g ray matter and FT was observed in neurons and in all glial populations. All four cell types were found in both the spinal cord gray matter and FT. The glia/neuron ratio in the spinal cord was 0.78, while in FT it was 2.0. Mor eover, the overall cell density was less in the FT than in the spinal cord. The present study shows that the membrane and morphological properties of glial cells in the frog and rat spinal cords are similar. Such striking phy logenetic similarity suggests a significant contribution from distinct glia l cell populations to various spinal cord functions. particularly ionic and volume homeostasis in both mammals and amphibians. (C) 2001 Elsevier Scien ce Ireland Ltd and the Japan Neuroscience Society. All rights reserved.