Differential intracellular calcium responses to glutamate in type 1 and type 2 cultured brain astrocytes

Fluorescence image analysis using the calcium indicator fluo-3 was used to examine changes in [Ca2+]i induced by glutamate in mixed glia populations cultured from neonatal rat brains. [Ca2+]i responses were correlated with glia type by performing immunohistochemistry using markers specific for type 1 and type 2 astrocytes on the same cells used in the imaging experiments. Glutamate (30-500 microM) induced two markedly different [Ca2+]i responses in the two astrocyte types: the response in type 1 astrocytes consisted of an initial fast transient followed by varying degrees of oscillations, whereas the predominant response in type 2 astrocytes was a slow rise in [Ca2+]i to a more or less sustained and nonoscillatory level. In some type 2 astrocytes, an initial spikelike transient similar to that in type 1 astrocytes was observed; the overall size of the spike, however, was smaller than in type 1 astrocytes. Two agonists for the ionotropic glutamate receptor, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) and kainate, elicited a 6-cyano-7-dinitroquinoxaline-2,3-dione (CNQX)-sensitive, external Ca(2+)-dependent, sustained [Ca2+]i rise in type 2 but not type 1 astrocytes. The initial spike in type 2 astrocytes was less dependent on external Ca2+ and not blocked by CNQX. [Na+]i as measured by the Na(+)-fluorescence dye SBFI, was elevated by kainate in both astrocyte types, though the increase was larger in type 2 astrocytes. This increase was reduced by CNQX, suggesting this [Ca2+]i increase was mediated, at least in part, by ionotropic glutamate receptors. The results are discussed in terms of the relative distribution of two classes of glutamate receptors on these two astrocyte types: one, the ionotropic class, is linked directly to an ion channel, and the other, the metabotropic class, induces internal mobilization of Ca2+ via inositol phospholipid hydrolysis.