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.