The Role of Metabotropic Glutamate Receptors for the Generation of Calcium Oscillations in Rat Hippocampal Astrocytes In Situ

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Ca2+ oscillations are part of the intra- and intercellular signalling in many cell types. We have studied Ca2+ oscillations in astrocytes in acute brain slices of the hippocampus of juvenile rats (postnatal 8–14 days old), using confocal laser scanning microscopy and bulk-loading of the Ca2+-sensitive dye Fluo-4. Astrocytes were identified morphologically in the stratum radiatum, and by their Ca2+ response in the absence of external K+. Thirty-five per cent of astrocytes (43 slices) showed spontaneous Ca2+ oscillations, with a frequency of 1.26 ± 0.11 transients/min (n=366). These Ca2+ signals were unaffected by tetrodotoxin (0.5 μM) and Ni2+ (2 mM), but were sensitive to interference with the phospholipase C-mediated Ca2+ release from intracellular stores. Spontaneous Ca2+ oscillations were reduced or suppressed by antagonists of metabotropic glutamate receptors (mGluRs) of groups I and II, but not affected by antagonists of group III. Glutamate (1–100 μM) and specific agonists of mGluR groups I and II evoked concentration-dependent Ca2+ signals, which were oscillatory at intermediate concentrations (e.g. at 10 μM glutamate). Our results indicate that mGluRs of both groups I and II are involved in mediating Ca2+ oscillations in astrocytes, which might be glial responses to micromolar changes of glutamate in the extracellular spaces.

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