Project Summary/Abstract Astrocytes comprise up to half of mammalian brain cells. Accumulating evidence from multiple brain circuits suggests that astrocytes, through their intracellular Ca2+ signaling, regulate and modulate neuronal activity on single-cell and network-wide levels and on a broad range of timescales—from milliseconds to days and weeks. Altered astrocyte Ca2+ signaling has also been implicated in a variety of brain disorders, including Huntington’s Disease, Alzheimer’s Disease, obsessive-compulsive disorder, stroke, and epilepsy. However, much remains to be done to uncover how astrocytes contribute to brain function. First, there is a shortage of experimentally based, sophisticated computational approaches for astrocyte data analysis and interpretation that could aid in guiding new, hypothesis-driven and rigorous experiments. Second, there is a dearth of data where astrocyte-neuron interactions have been explored carefully in a single model brain circuitry with behavioral readouts. I seek to make inroads in both of these topics by exploring astrocyte-neuron interactions in the striatum using experimental and computational approaches. This study proposes to elucidate the physiological interplay between astrocytes and neurons in striatal neural circuits. In Aim 1, experimental and computational approaches will be integrated to study in situ striatal astrocyte Ca2+ signaling and its effect on neuronal excitability on timescales of seconds. For this aim, astrocyte Ca2+ activity will be perturbed in situ using genetic knock-in and chemogenetic approaches. In Aim 2, an optogenetic tool will be developed and used to transiently stimulate striatal astrocyte Ca2+ responses in situ and in vivo, and investigate its effects on neuronal activity and behavior. The results from these will provide much needed insight into astrocyte-neuron communication in a well-characterized brain circuit relevant to behavior and neurological diseases. In addition, these studies will result in two sets of tools for future research: a novel optogenetic tool for astrocyte manipulation and an experimentally-verified mathematical model of astrocyte- neuron interactions.