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

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 kainate^1,2. Recent experiments indicate that glutamate receptors also occur in astrocytes^3–6. 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 matter^8–10, where neurotransmission does not occur, their processes are closely associated with axons at nodes of Ranvier^9,10, 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 channels^11,12