Region-specific and state-dependent astrocyte Ca²⁺ dynamics during the sleep-wake cycle in mice

Abstract

Neural activity is diverse, and varies depending on brain regions and sleep/wakefulness states. However, whether astrocyte activity differs between sleep/wakefulness states, and whether there are differences in astrocyte activity among brain regions remain poorly understood. In this study, we recorded astrocyte intracellular calcium (Ca²⁺) concentrations of mice during sleep/wakefulness states in the cortex, hippocampus, hypothalamus, cerebellum, and pons using fiber photometry. For this purpose, male transgenic mice in which their astrocytes specifically express the genetically encoded ratiometric Ca²⁺ sensor YCnano50 were used. We demonstrated that Ca²⁺ levels in astrocytes significantly decrease during Rapid Eye Movement (REM) sleep and increase after the onset of wakefulness. In contrast, differences in Ca²⁺ levels during non-Rapid Eye Movement (NREM) sleep were observed among different brain regions, and no significant decrease was observed in the hypothalamus and pons. Further analyses focusing on the transition between sleep/wakefulness states and correlation analysis with episode duration of REM showed that Ca²⁺ dynamics differed among brain regions, suggesting the existence of several clusters. To quantify region-specific Ca²⁺ dynamics, principal component analysis was performed to uncover three clusters; i.e., the first comprised the cortex and hippocampus, the second comprised the cerebellum, and the third comprised the hypothalamus and pons. Our study demonstrated that astrocyte Ca²⁺ levels change substantially according to sleep/wakefulness states. These changes were generally consistent, unlike neural activity. However, we also clarified that Ca²⁺ dynamics varies depending on the brain region, implying that astrocytes may play various physiological roles in sleep.