AMPA-evoked glutamate release from astrocytes in culture. A, Application of 1 μm AMPA to pure astrocyte cultures failed to evoke any significant change in the bath glutamate concentration as did similar treatment 10 min after incubation with 1 μm NGX424. However, 30 μm AMPA increased extracellular glutamate concentrations by 16.2 ± 4.0%, an effect that was completely blocked by pretreatment with 1 μm NGX424. B, C, Astrocyte cultures fixed and immunostained after glutamate release experiments showed that nearly all cells were GFAP positive and a subpopulation showing GluR1-IR. Although the majority of GFAP-immunoreactive cells were also positive for GluR1, arrows show some examples in which astrocytes did not express GluR1. D, E, GluR1 immunostaining in mixed cultures showed similar staining pattern in some astrocytes and neurons (arrows indicate GFAP-negative cells that are likely neurons). DAPI, 4′,6′-Diamidino-2-phenylindole.