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]. ABSTRACT: 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 KD 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 approximately 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.