Quantification of noradrenaline iontophoresis.

Several problems are encountered when iontophoresis1,2 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 numbers3,4, the use of ion-sensitive microelectrodes5,6 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.