DISTINCT POPULATIONS OF IDENTIFIED GLIAL-CELLS IN THE DEVELOPING RAT SPINAL-CORD SLICE - ION-CHANNEL PROPERTIES AND CELL MORPHOLOGY

Four types of glial cells could be distinguished in the grey matter of rat spinal cord slices at postnatal days 1-19 (P1-P19), based on thei r pattern of membrane currents as revealed by the whole cell patch cla mp technique, and by their morphological and immunocytochemical featur es. The recorded cells were labelled with Lucifer Yellow, which allowe d the subsequent identification of cells using cell-type-specific mark ers. Astrocytes were identified by positive staining for glial fibrill ary acidic protein (GFAP). These were morphologically characterized by multiple, very fine and short processes and electrophysiologically by symmetrical, non-decaying K+ selective currents. Oligodendrocytes wer e identified by a typical oligodendrocyte-like morphology, lack of GFA P staining and positive labelling with a combination of O1 and O4 anti bodies (markers of the oligodendrocyte lineage), and their membrane wa s dominated by symmetrical, passive, decaying K+ currents. The third p opulation of glial cells was also characterized by positive staining f or O1/O4 or only for O4 antigens, lack of GFAP staining and, in some c ells, oligodendrocyte-like morphology. However, these cells could be d istinguished by the presence of inwardly rectifying (K-IR), delayed ou twardly rectifying (K-DR) and A-type K+ currents (K-A), representing t he most likely glial precursor cells of the oligodendrocyte lineage, T he fourth population of glial cells had small somata and a widespread network of long processes with no apparent orientation preference. In one case, processes were positively labelled with GFAP, while 30% were characterized by faint, diffuse staining. These cells expressed a com plex pattern of voltage-gated channels, namely Na+, K-DR, K-A and K-IR channels. In contrast to neurons, the amplitude of Na+ currents was a t least one order of magnitude smaller than the K+ currents, and none of these cells showed the ability to generate action potentials in the current clamp mode. Since none of these cells could be labelled by ol igodendrocyte markers we assume that they were either astrocytes or gl ial precursor cells of the astrocyte lineage. The four cell types were found in all regions of the grey matter. When randomly accessing the glial cells, the probability of recording from the oligodendrocyte pre cursor cells and the glial cells with Na+ currents decreased during de velopment. At P1-P3, 50% of the cells revealed the Na+ current, while at P13-P15 only 18% did. Concomitantly, the number of glial cells with astrocyte- and oligodendrocyte-like membrane currents increased from 19 and 12% to 41 and 35.5% respectively. We conclude that the glial ce lls in the spinal cord slices possess distinct morphological, immunohi stochemical and physiological properties, and that the glial populatio ns undergo changes during postnatal development.