Morphological, immunophenotypical and electrophysiological properties of resting microglia in vitro

Morphological, immunophenotypical and electrophysiological properties were investigated in isolated cultured murine microglia before and after exposur e to astrocyte-conditioned medium (ACM). Following application of ACM, micr oglial cells underwent a dramatic shape transformation from an amoeboid app earance to a ramified morphology. In parallel to morphological changes, a d ownregulation of macrophage surface antigens was observed in microglia expo sed to ACM. Staining intensities for major histocompatibility complex (MHC) class II molecules and for the adhesion molecules leukocyte function-assoc iated antigen-1 (LFA-1) and intercellular adhesion molecule-1 (ICAM-1) were significantly decreased in ramified microglia 5 days after exposure to ACM . In microglial cells treated daily with ACM over a period of 5 days, the s mallest staining intensities for all surface antigens as well as the smalle st ramification index as a measure for the highest degree of ramification w ere determined. In addition, upregulation of delayed rectifier K+ currents was observed in microglia exposed to ACM for 1 day or treated daily with AC M for 5 days. In contrast, untreated amoeboid microglia or ramified microgl ia analysed 5 days after exposure to ACM did not express delayed rectifier K+ currents. Analyses of the resting membrane potential and expression leve ls and properties of inward rectifier K+ currents did not reveal any differ ences between untreated and ACM-treated microglia. It is suggested that ele ctrophysiological properties of microglia do not strongly correlate with th e morphology or the immunophenotype of microglial cells.