Electrophysiologically complex' glial cells freshly isolated from the hippocampus are immunopositive for the chondroitin sulfate proteoglycan NG2

We have recently described a subgroup of isolated glial fibrillary acidic protein-positive (GFAP(+)) hippocampal astrocytes that predominantly express outwardly rectifying currents (which we term ORAs for outwardly rectifying astrocytes), which are similar to the currents already described for hippocampal GFAP- complex glia. We now report that post-recording staining of cells that were first selected as complex by morphology and then confirmed by their electrophysiological characteristics were NG2(+) similar to90% of the time. Also, the morphology of freshly isolated NG2(+) cells differs from that of isolated GFAP+ ORAs in having a smaller and round cell body with thinner processes, which usually are collapsed back onto the soma. Upon detailed examination, NG2(+) cells were found to differ quantitatively in some electrophysiological characteristics from GFAP(+) ORAs. The outward, transient K+ currents (/K-a) in the NG2(+) cells showed a slower decay than the /K-a in ORAs, and their density decreased in NG2(+) cells from older animals. The other two major cation currents, the voltage-activated Na+ and outwardly delayed rectifier K+ currents, were similar in NG2(+) cells and ORAs. To further distinguish isolated complex cells from outwardly rectifying GFAP(+) astrocytes, we performed immunocytochemistry for glial markers in fixed, freshly isolated rat hippocampal glia. NG2(+) cells were negative for GFAP and also for the astrocytic glutamate transporters GLT-1 and GLAST. Thus the isolated hippocampal NG2(+) glial cells, though having an electrophysiological phenotype similar to that of ORAs, are an immunologically and morphologically distinct glial cell population and most likely represent NG2(+) cells in situ. (C) 2003 Wiley-Liss, Inc.