Ca²⁺-Dependent Glutamate Release Involves Two Classes of Endoplasmic Reticulum Ca²⁺ Stores in Astrocytes

Astrocytes can modulate synaptic transmission by releasing glutamate in a Ca²⁺-dependent manner. Although the internal Ca²⁺ stores have been implicated as the predominant source of Ca²⁺ necessary for this glutamate release, the contribution of different classes of these stores is still not well defined. To address this issue, we cultured purified solitary cortical astrocytes and monitored changes in their internal Ca²⁺ levels and glutamate release into the extracellular space. Ca²⁺ levels were monitored by using the Ca²⁺ indicator fluo-3 and quantitative fluorescence microscopy. Glutamate release was monitored by an L-glutamate dehydrogenase-linked detection system. Astrocytes were mechanically stimulated with a glass pipette, which reliably caused an increase in internal Ca²⁺ levels and glutamate release into the extracellular space. Althouqh we find that the presence of extracellular Cd²⁺, a Ca ²⁺ channel blocker, significantly reduces mechanically induced glutamate release from astrocytes, we confirm that internal Ca²⁺ stores are the predominant source of Ca²⁺ necessary for this glutamate release. To test the involvement of different classes of internal Ca²⁺ stores, we used a pharmacological approach. We found that diphenylboric acid 2-aminoethyl ester, a cell-permeable inositol 1,4,5-trisphosphate (IP₃) receptor antagonist, greatly reduced mechanically induced glutamate release. Additionally, the preincubation of astrocytes with caffeine or ryanodine also reduced glutamate release. Taken together, our data are consistent with dual IP₃- and caffeine/ ryanodine-sensitive Ca²⁺ stores functioning in the control of glutamate release from astrocytes.