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 + ∼90% 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 (IK a ) in the NG2 + cells showed a slower decay than the IK 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.