INTERPLAY BETWEEN ASTROCYTIC MORPHOLOGY AND SODIUM SIGNALLING IN SYNAPTIC SIGNALLING

The efficiency of astroglial glutamate uptake at synapses depends on the electrochemical driving forces at perisynaptic astrocytic processes (PAPs). Na⁺-coupled uptake of glutamate evokes astrocytic sodium transients ([Na⁺]_A). Depending on their amplitude and kinetics, which are likely determined by astrocyte morphology, these transients could lower the driving force for further uptake. However, the quantitative relationship between PAPs [Na⁺]_A transients and astrocyte morphology remains unclear. To investigate this, we used fluorescence lifetime imaging of whole-cell patch clamped astrocytes filled with the Na⁺ indicator ING-2 in acute hippocampal slices. First, a cuvette calibration confirmed the sensitivity of the ING-2 lifetime to changes of [Na⁺]. The dye was then calibrated in situ by patching astrocytes with internal solutions containing different [Na⁺]. This provided a direct, quantitative readout of [Na⁺] from the fluorescence lifetime.
We then assessed activity-dependent [Na⁺]_A dynamics across distinct subcellular compartments. High-frequency stimulation (HFS) of Schaffer collaterals elicited robust, long-lasting [Na⁺]_A increases in the PAPs, but not in the soma, despite membrane depolarization. These signals were abolished by the Na⁺ channel inhibitor TTX. To probe the capacity for transporter-mediated Na⁺ entry, we used pressure application of D-aspartate, which is taken up by glutamate transporters with Na⁺. Similar to HFS, D-aspartate triggered [Na⁺]_A in the PAPs but not the soma, while aCSF applications elicited no responses. Our data indicates that glutamatergic synaptic transmission and glutamate uptake drive substantial Na⁺ increases in the astrocyte periphery. Current experiments investigate how induced morphological changes impact these synaptically driven transients.