ION CHANNELS IN MICROGLIA (BRAIN MACROPHAGES)

Microglia are immunocompetent cells in the brain that have many simila rities with macrophages of peripheral tissues. In normal adult brain, microglial cells are in a resting state, but they become activated dur ing inflammation of the central nervous system, after neuronal injury, and in several neurological diseases. Patch-clamp studies of microgli al cells in cell culture and in tissue slices demonstrate that microgl ia express a wide variety of ion channels. Six different types of K+ c hannels have been identified in microglia, namely, inward rectifier, d elayed rectifier, HERG-like, G protein-activated, as well as voltage-d ependent and voltage-independent Ca2+-activated K+ channels. Moreover, microglia express H+ channels, Na+ channels, voltage-gated Ca2+ chann els, Ca2+-release activated Ca2+ channels, and voltage-dependent and v oltage-independent Cl- channels. With respect to their kinetic and pha rmacological properties, most microglial ion channels closely resemble ion channels characterized in other macrophage preparations. Expressi on patterns of ion channels in microglia depend on the functional stat e of the cells. Microglial ion channels can be modulated by exposure t o lipopolysaccharide or various cytokines, by activation of protein ki nase C or G proteins, by factors released from astrocytes, by changes in the concentration of internal free Ca2+, and by variations of the i nternal or external pH. There is evidence suggesting that ion channels in microglia are involved in maintaining the membrane potential and a re also involved in proliferation, ramification, and the respiratory b urst. Further possible functional roles of microglial ion channels are discussed.