Astrocytes are active: An information theoretic approach reveals differences in Ca2+ signaling patterns among distinct astrocyte subtypes

The discovery that astrocytes are an active, rather than a passive, component of the brain has ushered in a paradigm shift in thinking about how the brain processes information. Although the mechanisms by which astrocytes integrate information from neurons are still debated, such discourse should not distract from the importance of more completely understanding how astrocytes communicate via signals amongst themselves. This work aims to study how different astrocytes signal within their own networks. We investigate group calcium (Ca 2+ ) dynamics in polygonal, stellate, and reactive astrocytes. These distinct and important astrocyte subtypes are present in the brain to varying degrees at different physiological states. We use an information-theoretic framework to quantify the dynamics embedded in the Ca2+ traces within astrocyte networks; specifically, we employ the Hurst exponent, cross-correlation, mutual information, and partitioned entropy to assess differences in the astrocyte signals across subtypes. To gain insights into the ability of astrocyte networks to respond to changes in the extracellular environment, we probe the networks with perturbations affecting their cytoskeletal dynamics (Latrunculin B) and energetic levels (Adenosine triphosphate). Overall, these three classes of astrocytes behave differently and respond idiosyncratically to their extracellular environment. We find that polygonal astrocytes are not quiescent, stellate astrocytes respond most strongly to ATP, and reactive astrocytes are uniquely perturbed by Latrunculin B. Interestingly, despite these distinct differences in behaviors, we find a uniform speed of information transport regardless of subtype or perturbation; this uniformity is maintained when using both cross-correlation and mutual information to assess this speed. We conclude that the differential ways astrocytes signal within our measured framework yield important insights into how astrocytes communicate and contribute to this pressing issue of understanding astrocyte information processing.