Ca2+‐dependent glutamate release involves two classes of endoplasmic reticulum Ca2+ stores in astrocytes

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Abstract
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. Although we find that the presence of extracellular Cd2+, a Ca2+ 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.