Quantitative Imaging of Intracellular Sodium

A fundamental functional principle of animal cells is their ability to store energy across the plasma membrane in form of ion gradients. This is mainly realized by the activity of the Na/K-ATPase, which consumes ATP to establish an inwardly directed gradient for sodium ions. In the brain, the sodium gradient not only drives many transport processes across the plasma membrane, but also provides the basis for electrical signaling. Any change in intracellular sodium and any reduction in the sodium gradient will thus influence a multitude of processes, and breakdown of the sodium gradient during pathological conditions can have fatal consequences. Knowledge about intracellular sodium concentration and its alterations under different conditions is thus indispensable. The study of the temporal and spatial dynamics of [Na]i can be accomplished by fluorescence-based intracellular sodium imaging, which also enables measurements in small microdomains such as dendrites and spines or fine glial processes. In this chapter, we review the properties and application range of sodium-sensitive fluorescent indicator dyes. Furthermore, the principles of ratiometric imaging, which allows the reliable measurement of intracellular sodium concentrations, are presented. While focusing our experimental description on neurons and glial cells in brain slices, the reader will be provided with basic instructions and information on how to perform quantitative intracellular sodium imaging applicable also to other cells and tissues.