Multiple conductance channels in type-2 cerebellar astrocytes activated by excitatory amino acids

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Abstract
L-GLUTAMATE and L-aspartate are thought to have a widespread function as synaptic transmitters in the mammalian central nervous system and there are at least three types of neuronal glutamate receptors, which can be activated by the selective agonists N-methyl-D-aspartate (NMDA), quisqualate and kainate1,2. Recent experiments indicate that glutamate receptors also occur in astrocytes3–6. We have used patch-clamp methods7 to determine whether one type of macroglial cell, the type-2 astrocyte, possesses glutamate receptors, as previously proposed from neurochemical studies5. We find that glutamate and related amino acids can evoke whole-cell and single-channel currents in type-2 astrocytes from rat cerebellum. Although these cells are found mainly in white matter8–10, where neurotransmission does not occur, their processes are closely associated with axons at nodes of Ranvier9,10, suggesting that such receptors are involved in neuronal–glial signalling at the node. Our experiments show that glial cells possess quisqualate- and kainate-receptor channels but lack receptors for NMDA. Interestingly, these glutamate channels exhibit multiple conductance levels that are similar in amplitude to the neuronal glutamate channels11,12