A Functional Taxonomy of Cortical Astrocytes The vast majority of neural circuit studies neglect to take into account the non-neuronal cells in the brain, but in order to truly appreciate neural circuit function, we will need to monitor and manipulate activity in many cell types. Our understanding of astrocyte signaling is years behind that of neurons, because the appropriate tools have been lacking for these largely electrically silent cells. We don't know what extracellular signals astrocytes respond to, nor how they contribute to circuit function. This is due, in part, to the lack of methods that replicate the breadth of possible presynaptic activity in vivo, i.e. the release of neurotransmitter. The current proposal addresses gaps in our understanding of astrocytes in neural circuit function and harnesses the power of light-activatable tools to tackle them. We propose to apply a suite of optochemical tools that allow spatiotemporally precise and physiologically relevant release of neurotransmitter to astrocytes and neurons in cortical circuits. In Aim 1, we will test the hypothesis that astrocytes response acutely to the synaptic release of excitatory and inhibitory synaptic activity with differential and predictable activity. We will use simultaneous two-photon optochemical uncaging and calcium imaging in astrocyte branches to test their physiological response to glutamatergic and GABAergic synaptic events, and uncover the heterogeneity of molecular mechanisms that govern these responses. We will activate astrocytes in vivo using these optochemical techniques, and in Aim 2, genetically silence astrocyte-specific excitatory and inhibitory receptors during in vivo imaging and electrophysiology to determine the astrocytic signals that lead to downstream cortical state shifts. In Aim 3, we will validate novel optochemical tools to mimic the release of neuromodulators in the cortical circuit, testing their function in neurons and astrocytes. With these tools in hand, we will be able to probe the repertoire of signals to which astrocytes respond in the circuit.