Network-level encoding of local neurotransmitters in cortical astrocytes

Astrocytes-the most abundant non-neuronal cell type in the mammalian brain-are crucial circuit components that respond to and modulate neuronal activity via calcium (Ca ²⁺ ) signaling ^1-8 . Astrocyte Ca ²⁺ activity is highly heterogeneous and occurs across multiple spatiotemporal scales: from fast, subcellular activity ^3,4 to slow, synchronized activity that travels across connected astrocyte networks ^9-11 . Furthermore, astrocyte network activity has been shown to influence a wide range of processes ^5,8,12 . While astrocyte network activity has important implications for neuronal circuit function, the inputs that drive astrocyte network dynamics remain unclear. Here we used ex vivo and in vivo two-photon Ca ²⁺ imaging of astrocytes while mimicking neuronal neurotransmitter inputs at multiple spatiotemporal scales. We find that brief, subcellular inputs of GABA and glutamate lead to widespread, long-lasting astrocyte Ca ²⁺ responses beyond an individual stimulated cell. Further, we find that a key subset of Ca ²⁺ activity-propagative events-differentiates astrocyte network responses to these two major neurotransmitters, and gates responses to future inputs. Together, our results demonstrate that local, transient neurotransmitter inputs are encoded by broad cortical astrocyte networks over the course of minutes, contributing to accumulating evidence across multiple model organisms that significant astrocyte-neuron communication occurs across slow, network-level spatiotemporal scales ^13-15 . We anticipate that this study will be a starting point for future studies investigating the link between specific astrocyte Ca ²⁺ activity and specific astrocyte functional outputs, which could build a consistent framework for astrocytic modulation of neuronal activity.