Doctor of Philosophy

dissertationAstrocytes and neurons, the major cell types in the mammalian brain, communicate bidirectionally with one another. Astrocytes respond to neuronal activity primarily through G-protein coupled receptor (GPCR) activation and subsequent intracellular Ca2+ concentration elevation. These glial cells also play a major role in K+ buffering and neurotransmitter uptake, which are linked to Ca2+ elevations. These and other potential Ca2+-dependent functions (e.g. release of neuroactive compounds, known as gliotransmission), have the ability to considerably modify neuronal activity and brain function. Unsurprisingly, astrocyte Ca2+ activity is implicated in several diseases and behaviors, such as locomotion. Recent advances in Ca2+ imaging have enabled the study of astrocyte Ca2+ signaling. However, astrocyte Ca2+ activity is highly variable (e.g. between experimental trials) and has complex spatiotemporal patterns. Moreover, basic principles of astrocyte Ca2+ activity are unclear, such as the degree of its heterogeneity and its reliability in responding to ongoing stimulation. In addition, there are very few computational models of astrocyte Ca2+ activity that are based on experimental measurements, most of which do not consider Ca2+ heterogeneity. Such models would allow us to interpret experimental findings, evaluate existing hypotheses, generate new testable hypotheses, and guide future experiments in the astrocyte field. To address these issues, we integrated two-photon Ca2+ imaging experiments and computational modeling. We first developed a biophysical, mechanistic model of astrocyte Ca2+ activity based on our experimental measurements. We found that Ca2+ responses have complex, but informative, relationships with their underlying cellular mechanisms, resulting in response variability among different experimental trials, different cells, and different regions within one cell. Our model also identified important roles for various cellular mechanisms in generating a variety of Ca2+ response patterns. We also developed a data-driven probabilistic model of astrocyte Ca2+ activity and used it to find stimulation frequency-dependent response patterns in astrocytes. We characterized mechanisms that allow astrocytes to respond in two opposing manners to the same agonist, depending on the stimulation frequency. These collective findings provide new perspectives on interpreting astrocyte Ca2+ dynamics under various experimental conditions. Additionally, the biophysical and probabilistic models we developed provide valuable tools for future studies of astrocyte activity