Relation between activity-induced intracellular sodium transients and ATP dynamics in mouse hippocampal neurons

Key points Employing quantitative Na+-imaging and Forster resonance energy transfer-based imaging with ATeam1.03(YEMK) (ATeam), we studied the relation between activity-induced Na+ influx and intracellular ATP in CA1 pyramidal neurons of the mouse hippocampus. Calibration of ATeam in situ enabled a quantitative estimate of changes in intracellular ATP concentrations. Different paradigms of stimulation that induced global Na+ influx into the entire neuron resulted in decreases in [ATP] in the range of 0.1-0.6 mm in somata and dendrites, while Na+ influx that was locally restricted to parts of dendrites did not evoke a detectable change in dendritic [ATP]. Our data suggest that global Na+ transients require global cellular activation of the Na+/K+-ATPase resulting in a consumption of ATP that transiently overrides its production. For recovery from locally restricted Na+ influx, ATP production as well as fast intracellular diffusion of ATP and Na+ might prevent a local drop in [ATP]. Excitatory neuronal activity results in the influx of Na+ through voltage- and ligand-gated channels. Recovery from accompanying increases in intracellular Na+ concentrations ([Na+](i)) is mainly mediated by the Na+/K+-ATPase (NKA) and is one of the major energy-consuming processes in the brain. Here, we analysed the relation between different patterns of activity-induced [Na+](i) signalling and ATP in mouse hippocampal CA1 pyramidal neurons by Na+ imaging with sodium-binding benzofurane isophthalate (SBFI) and employing the genetically encoded nanosensor ATeam1.03(YEMK) (ATeam). In situ calibrations demonstrated a sigmoidal dependence of the ATeam Forster resonance energy transfer ratio on the intracellular ATP concentration ([ATP](i)) with an apparent K-D of 2.6 mm, indicating its suitability for [ATP](i) measurement. Induction of recurrent network activity resulted in global [Na+](i) oscillations with amplitudes of similar to 10 mm, encompassing somata and dendrites. These were accompanied by a steady decline in [ATP](i) by 0.3-0.4 mm in both compartments. Global [Na+](i) transients, induced by afferent fibre stimulation or bath application of glutamate, caused delayed, transient decreases in [ATP](i) as well. Brief focal glutamate application that evoked transient local Na+ influx into a dendrite, however, did not result in a measurable reduction in [ATP](i). Our results suggest that ATP consumption by the NKA following global [Na+](i) transients temporarily overrides its availability, causing a decrease in [ATP](i). Locally restricted Na+ transients, however, do not result in detectable changes in local [ATP](i), suggesting that ATP production, together with rapid intracellular diffusion of both ATP and Na+ from and to unstimulated neighbouring regions, counteracts a local energy shortage under these conditions.