A hippocampal astrocytic sequence emerges during learning and memory

The dorsal hippocampus is a heterogeneous structure with numerous cell types involved in generating and maintaining detailed representations of space and time. Prior work has established that pyramidal cells contribute to these crucial aspects of episodic memory. For example, hippocampal "time cells" encode temporal information through sequential activity. However, the role of non-neuronal cell types are less often explored. In this study, we investigated dorsal hippocampal CA1 astrocytes using one-photon calcium imaging in freely moving animals during a contextual fear conditioning paradigm. To our knowledge, this is the first time a study has successfully performed longitudinal registration of astrocytic cell population using 1p calcium imaging, thus permitting the tracking of a stable population of these cells in freely-moving mice. In response to foot shock, astrocytes generated robust calcium-event sequences with a time-compressed structure akin to canonical hippocampal time cells. Upon re-exposure to the conditioned context, these astrocytic sequences persisted in the absence of shock, maintaining their time-compressed structure. Moreover, astrocytes active on the previous day retained a preserved sequential structure, indicating memory-specific properties. This phenomenon was not observed in a context different from the initial fear conditioning chamber. Taken together, these results present a potentially paradigm-shifting notion that astrocytes play a significant role in the representation of temporal information processing across learning and memory.