Microglia were enriched in brain cell cultures from newborn mice as a result of supplementation with the growth factors macrophage colony-stimulating factor or granulocyte/macrophage colony-stimulating factor. When separately administered these two cytokines promote the outgrowth of loosely adherent cells with similar morphology which stained positive for CD11b and nonspecific esterase. Microglial cells isolated from both types of culture were electrophysiologically characterized using the whole cell configuration of the patch-clamp technique. Different resting membrane potentials were measured. In response to hyperpolarizing and depolarizing voltage commands 68 of 91 macrophage colony-stimulating factor-cultured microglial cells exhibited only an inward rectifying potassium current. By contrast, an outward potassium current was observed on 71 of 95 granulocyte/macrophage colony-stimulating factor-grown cells. Parallel testing of their capability for antigen presentation proved the activated functional state of these microglial cells. They induce antigen-specific T cell response without prior stimulus. In comparison, cells developed with macrophage colony-stimulating factor failed to present antigen. In such resting microglia a short-term treatment with granulocyte/macrophage colony-stimulating factor or interferon-γ provoked a strong appearance of outward potassium currents, however, only the interferon-γ-trigger resulted in efficient antigen presentation.The differential induction of both functional parameters suggests the detection of outward potassium currents to provide an electrophysiological activation marker of microglia which is subjected to cytokine regulation but not compellingly linked to antigen presentation.
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