Astrocytes respond to neuronal activity. However, whether astrocytic activity 
has any significance in brain function is unknown. Signaling pathway leading 
from astrocytes to neurons would be required for astrocytes to participate in 
neuronal functions and, here, we investigated the presence of such pathway. 
Optogenetics was used to manipulate astrocytic activity. A light-sensitive 
protein, channelrhodopsin-2 (ChR2), was selectively expressed in astrocytes. 
Photostimulation of these astrocytes induced glutamate release which modulated 
neuronal activity and animal behavior. Such glutamate release was triggered by 
intracellular acidification produced by ChR2 photoactivation. Astrocytic 
acidification occurs upon brain ischemia, and we found that another optogenetic 
tool, archaerhodopsin (ArchT), could counter the acidification and suppress 
astrocytic glutamate release. Controlling of astrocytic pH may become a 
therapeutic strategy upon ischemia.