Dorso-Ventral and Night-Day Regulation of Extracellular K⁺ Dynamics in Mouse Hippocampal Astrocytes

Astrocytes regulate extracellular potassium (K⁺) through multiple mechanisms operating across distinct spatiotemporal scales, yet whether this regulation exhibits regional and circadian specificity remains unclear. Using real-time K⁺ measurements combined with electrophysiology and immunohistochemistry in astrocytes, we characterize K⁺ buffering by astrocytes across the dorsoventral hippocampal axis at three circadian times. We report that hippocampal CA1 stratum radiatum astrocytes possess different functional K⁺ buffering capacities depending on both anatomical location and time of day. At the early light phase (ZT3), the ventral hippocampus (VH) exhibits faster K⁺ accumulation and greater peak amplitudes than the dorsal hippocampus (DH) in response to similar neuronal network activity. This regional divergence is driven by reduced Kir4.1 channel function in VH astrocytes, which persists across all measured time points, and which is partially compensated by enhanced Na⁺/K⁺-ATPase activity specifically at ZT3. Gap junction coupling between astrocytes shows regional and time-dependent variation, with elevated coupling in VH at ZT3 that subsequently normalizes. Kir4.1 protein expression exhibits circadian dynamics. In contrast to the steady, high levels of Kir4.1 observed in the DH, the VH shows a gradual increase in expression across the light-dark cycle. These findings establish astrocytic K⁺ buffering as a multiscale phenomenon integrating regional heterogeneity with circadian regulation, with implications for understanding regional and temporal differences in network excitability, particularly in epilepsy.