Optogenetic Interneuron Stimulation and Calcium Imaging in Astrocytes.

In brain networks, neurons are constantly involved in a dynamic interaction with the other cell populations and, particularly, with astrocytes, the most abundant glial cells in the brain. Astrocytes respond to neurotransmitters with Ca2+ elevations which represent a key event in the modulation of local brain circuits played by these glial cells. Due to technical limitations, the study of Ca2+ signal dynamics in astrocytes has focused for decades almost exclusively on somatic and perisomatic regions. Accordingly, Ca2+ signal in astrocytic fine protrusions, which are in close contact with the synapse, has been poorly investigated. Over the last years, the diffusion of novel tools such as the viral vector gene delivery of genetically encoded Ca2+ indicators (GECI), the optogenetics, and multiphoton laser scanning microscopy has boosted significantly our capability to study astrocytic Ca2+ signals in the different subcellular compartments. Here we report a protocol that combines these techniques to study astrocyte Ca2+ signaling in response to somatostatin (SST)-expressing interneurons, one of the main classes of GABAergic inhibitory interneurons.