Ca2+ activity maps of astrocytes tagged by axo-astrocytic AAV transfer

Astrocytes exhibit localized Ca²⁺ microdomain (MD) activity thought to be actively involved in information processing in the brain. However, functional organization of Ca²⁺ MDs in space and time in relationship to behavior and neuronal activity is poorly understood. Here, we first show that Adeno-Associated Virus (AAV) particles transfer anterogradely from axons to astrocytes. Then we use this axo-astrocytic AAV transfer to express genetically encoded Ca²⁺ indicators at high contrast circuit-specifically. In combination with two-photon microscopy and unbiased, event-based analysis we investigated cortical astrocytes embedded in the vibrissal thalamocortical circuit. We found a wide range of Ca²⁺ MD signals, some of which were ultrafast (≤300 ms). Frequency and size of signals were extensively increased by locomotion but only subtly with sensory stimulation. The overlay of these signals resulted in behavior dependent maps with characteristic Ca²⁺ activity hotspots, maybe representing memory engrams. These functional subdomains are stable over days, suggesting subcellular specialization.