Quantification of noradrenaline iontophoresis

Abstract

Several problems are encountered when iontophoresis^1,2 is used to study the effects of putative neurotransmitters. The most significant is that it is not usually practical to estimate the concentration of drug obtained at the tip of the microelectrode by a current of a given strength. The usual methods, albeit rarely used, include measurement of transport numbers^3,4, the use of ion-sensitive microelectrodes^5,6 and quantitative fluorescent microscopy⁷. With the exception of the ion-sensitive microelectrodes developed for acetylcholine⁵, these techniques are elaborate and time consuming, and cannot be routinely applied to every electrode used. Furthermore, conventional multibarrel microelectrodes have high-impedance recording barrels and thus often display low signal-to-noise ratios when recording single-cell activity, the noise being increased during iontophoresis. We describe here a technique which largely overcomes the problem of low spike signal-to-noise ratio in conventional multibarrel electrodes, and which, unlike the latter, also allows precise determination of the concentration of noradrenaline in the environment of the cell, which affects its excitability. The recording and iontophoretic properties of these electrodes have been described previously⁸. The use of these electrodes to quantify precisely iontophoresed noradrenaline by adapting polarographic techniques is described.