Chloride/anion Channels in Glial Cell Membranes

At least seven different chloride/anion currents have now been identified in astrocytes, oligodendrocytes/Schwann cells, and microglia. Only for two of these currents is the corresponding gene known. One of these genes is not encoding for a chloride channel, but for a class of mitochondria-like pores also found in cell membranes. Astrocytes and oligodendrocytes differ in their resting properties: astrocytes accumulate chloride but do not have a significant permeability. Oligodendrocytes have a close to passive distribution and a significant permeability. Under certain circumstances, astrocytes can express a resting chloride conductance. Reactive and neoplastic astrocytes as well as astrocytes with an altered shape exhibit a resting conductance. The function of these channels certainly involves volume regulation. Other possible functions are potassium homeostasis, migration, proliferation (in microglia), and involvement in spreading depression waves. Of greatest interest are two phenomena discovered in situ: The ClC-2 channel is only found in astrocytic endfeet near blood capillaries adjacent to neuronal GABA(A) receptors. In the supraoptic nucleus of the hypothalamus, there is an osmosensitive astrocytic taurine release. This released taurine interacts with glycine receptors in neighboring neurons, causing inhibition. It is assumed that with the future availability of more in situ, rather than in vitro, studies, an increased number of such complex interactions between glial cells, neurons, and blood vessels will be discovered.