Local depletion of monoamines induced with in vivo voltammetry in the cat brain

A significant depletion of the electroactive monoamines and their metabolites in the vicinity of a carbon fiber microelectrode may be induced by in vivo staircase voltammetry in the brain, even if the duration of the voltammetric scans is relatively short (approximately 5 s). The variation of this depletion was determined in the extracellular fluid of the cat thalamus at different durations of the pauses separating consecutive measurements. Pauses not shorter than 5 min ensured a nearly full relaxation, so that at the beginning of each subsequent scan a virtually undisturbed environment surrounded the electrode. With pauses shorter than 5 min, it is still possible to monitor major changes in the monoamine concentration. Staircase scans separated with 45 s pauses, for example, were suitable to study the increase in monoamine levels after administration of reserpine, and release phenomena stimulated with KCl were monitored with frequently repeated voltammetric pulses. The electrochemically induced depletion, on the other hand, can be used for characterizing the dynamics of mass transport in the studied brain structure. This was demonstrated with staircase voltammetry alternated with pauses of 1-100 s, and with quasi-chronoamperometry. In vivo brain voltammetry is generally used for monitoring extracellular monoamine (including dopamine) levels. These may be significantly altered by the voltammetric measurement itself through depletion in the vicinity of the electrode. This effect can be minimized with appropriate selection of sampling intervals and other parameters of staircase voltammetry. Conversely, depletion and the following relaxation can be used for determining dynamic characteristics of the studied brain structure which would be difficult to obtain otherwise.