Astrocytes are one type of glia cell, which are found in large numbers in the brain. They have dense processes that entwine neurons and reach blood vessels. Traditionally, astrocytes were thought to perform support roles only (that is, the regulation and optimization of the environment in which neurons function). However, recent groundbreaking research revealed many roles for these cells in modulating neuronal activity and even behavior. Here, we investigated whether astrocytes of the hippocampus (a brain area involved in spatial cognition) encode the external environment. Neocortical astrocytes have been shown to respond to specific sensory stimuli. However, the activity of hippocampal astrocytes during the performance of multisensory cognitive tasks had not been tested, let alone across time. Our main goal in this study was to answer this fundamental question.
To this end, we chronically imaged the activity of hippocampal astrocytes as mice ran on a linear treadmill, obtaining water rewards after advancing through either a familiar or a novel virtual environment (figure, top). We found that the astrocytes increase their activity as the mice approach the reward location in a familiar environment (figure, bottom-left), but do not do so for a novel environment (figure, bottom-right). Shifting the reward location within a familiar environment also resulted in diminished ramping activity; however, following additional familiarization training, the ramping activity was restored. Together, these findings demonstrate that hippocampal astrocytic activity is experience-dependent, and can encode the expected reward location in familiar spatial contexts.
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