Photothrombosis ischemia stimulates a sustained astrocytic Ca2+ signaling in vivo

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Abstract
Although there is significant information concerning the consequences of cerebral ischemia on neuronal function, relatively little is known about functional responses of astrocytes, the predominant glial‐cell type in the central nervous system. In this study, we asked whether focal ischemia would impact astrocytic Ca2+ signaling, a characteristic form of excitability in this cell type. In vivo Ca2+ imaging of cortical astrocytes was performed using two‐photon (2‐P) microscopy during the acute phase of photothrombosis‐induced ischemia initiated by green light illumination of circulating Rose Bengal. Although whisker evoked potentials were reduced by over 90% within minutes of photothrombosis, astrocytes in the ischemic core remained structurally intact for a few hours. In vivo Ca2+ imaging showed that an increase in transient Ca2+ signals in astrocytes within 20 min of ischemia. These Ca2+ signals were synchronized and propagated as waves amongst the glial network. Pharmacological manipulations demonstrated that these Ca2+ signals were dependent on activation of metabotropic glutamate receptor 5 (mGluR5) and metabotropic γ‐aminobutyric acid receptor (GABABR) but not by P2 purinergic receptor or A1 adenosine receptor. Selective inhibition of Ca2+ in astrocytes with BAPTA significantly reduced the infarct volume, demonstrating that the enhanced astrocytic Ca2+ signal contributes to neuronal damage presumably through Ca2+‐dependent release of glial glutamate. Because astrocytes offer multiple functions in close communication with neurons and vasculature, the ischemia‐induced increase in astrocytic Ca2+ signaling may represent an initial attempt for these cells to communicate with neurons or provide feed back regulation to the vasculature.