Ion channels in microglia (brain macrophages)

Microglia are immunocompetent cells in the brain that have manysimilarities with macrophages of peripheral tissues. In normal adultbrain, microglial cells are in a resting state, but they becomeactivated during inflammation of the central nervous system, afterneuronal 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. Sixdifferent types of K⁺ channelshave been identified in microglia, namely, inward rectifier, delayedrectifier, HERG-like, G protein-activated, as well as voltage-dependentand voltage-independentCa²⁺-activatedK⁺ channels. Moreover, microgliaexpress H⁺ channels,Na⁺ channels, voltage-gatedCa²⁺ channels,Ca²⁺-release activatedCa²⁺ channels, andvoltage-dependent and voltage-independentCl channels. With respectto their kinetic and pharmacological properties, most microglial ionchannels closely resemble ion channels characterized in othermacrophage preparations. Expression patterns of ion channels inmicroglia depend on the functional state of the cells. Microglial ionchannels can be modulated by exposure to lipopolysaccharide or variouscytokines, by activation of protein kinase C or G proteins, by factorsreleased from astrocytes, by changes in the concentration of internalfree Ca²⁺, and by variations ofthe internal or external pH. There is evidence suggesting that ionchannels in microglia are involved in maintaining the membranepotential and are also involved in proliferation, ramification, and therespiratory burst. Further possible functional roles of microglial ionchannels are discussed.