Cellular and subcellular heterogeneity of astrocytic Na⁺ homeostasis tuning astrocytes into functionally distinct subgroups in the mouse brain

Astrocytes maintain extracellular ion and transmitter homeostasis, with the Na⁺ inward gradient playing a crucial role. Earlier studies suggested a rather low, uniform Na⁺ distribution in astrocytes, consistent with the view that these basic homeostatic properties are well-protected. Here, we employed multi-photon fluorescence lifetime imaging to quantitatively determine astrocytic [Na + ] in mouse brain tissue slices and in vivo. Our data reveals a significant subcellular and cellular heterogeneity in astrocytic [Na + ], accompanied by differences in the capacity for Na + /K + -ATPase (NKA)-mediated uptake of extracellular K + . RNAscope and immunohistochemistry indicate differential spatial expression patterns of NKA ß1 and ß2 subunits in astrocytes. Biophysical modeling of differential NKA expression together with varying strength of Na + influx replicate the experimentally observed heterogeneity in astrocytic [Na + ]. Altogether, our results suggest the existence of functionally distinct astrocytes and astrocyte subdomains in which Na + homeostasis is locally adapted to the specific requirements of surrounding neural networks.