Ion-selective microelectrodes and diffusion measurements as tools to explore the brain cell microenvironment.

The construction and application of liquid-membrane ion-selective microelectrodes (ISM) are described. Recommendations are provided for the selection of appropriate cocktails containing neutral carriers to form the liquid membrane to sense K+, Ca2+, H+ and Na+. The use of charged carriers to sense Cl- and the cation tetramethylammonium (TMA+) is discussed. A detailed protocol is given for constructing double-barreled electrodes (ion-sensor and reference barrel) with tips of 1 micron diameter or more for extracellular ion measurements. The primary results obtained with ISMs in the brain cell microenvironment are briefly surveyed. The theoretical basis for measuring diffusion properties of extracellular space is described. Such measurements enable the estimation of volume fraction (proportion of tissue that is extracellular space) and tortuosity (hindrance of diffusion due to cellular obstructions). A method is given for using TMA+ ISMs in combination with iontophoresis or pressure ejection of TMA+ from a nearby micropipette to measure diffusion properties.