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Department of Neurophysiology, Institute of Physiology, Humboldt University, D-10117 Berlin, Germany
Microglia are immunocompetent cells in the brain that have many
similarities with macrophages of peripheral tissues. In normal adult
brain, microglial cells are in a resting state, but they become
activated during inflammation of the central nervous system, after
neuronal injury, and in several neurological diseases. Patch-clamp studies of microglial cells in cell culture and in tissue slices demonstrate that microglia express a wide variety of ion channels. Six
different types of K+ channels
have been identified in microglia, namely, inward rectifier, delayed
rectifier, HERG-like, G protein-activated, as well as voltage-dependent
and voltage-independent
Ca2+-activated
K+ channels. Moreover, microglia
express H+ channels,
Na+ channels, voltage-gated
Ca2+ channels,
Ca2+-release activated
Ca2+ channels, and
voltage-dependent and voltage-independent
Cl
channels. With respect
to their kinetic and pharmacological properties, most microglial ion
channels closely resemble ion channels characterized in other
macrophage preparations. Expression patterns of ion channels in
microglia depend on the functional state of the cells. Microglial ion
channels can be modulated by exposure to lipopolysaccharide or various
cytokines, by activation of protein kinase C or G proteins, by factors
released from astrocytes, by changes in the concentration of internal
free Ca2+, and by variations of
the internal or external pH. There is evidence suggesting that ion
channels in microglia are involved in maintaining the membrane
potential and are also involved in proliferation, ramification, and the
respiratory burst. Further possible functional roles of microglial ion
channels are discussed.
voltage- and calcium-activated potassium channels; proton current; fast-inactivating sodium current; calcium channels; chloride conductances
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