Interference by pH and Ca2+ ions during measurements of catecholamine release in slices of rat amygdala with fast-scan cyclic voltammetry

Fast-scan cyclic voltammetry (FSCV) at carbon-fiber microelectrodes was used to investigate catecholamine release and uptake induced by local electrical stimulation of rat brain slices containing the basolateral amygdaloid nucleus. The amygdala contains less catecholamine than the striatum, and the observed release is proportionately smaller. Stimulus trains of long duration were required to obtain a well-resolved concentration change in the basolateral amygdala. Voltammetric detection of catecholamines under these conditions was complicated by interference from two extracellular ions, H+ and Ca2+. Ion-selective microelectrodes were used in conjunction with carbon-fiber microelectrodes to monitor pH and Ca2+. The magnitude of the pH changes was correlated with stimulation length and followed the pattern of a brief alkaline shift followed by a longer acidic shift. Extracellular Ca2+ concentration decreased during stimulation and returned fairly rapidly to baseline after the stimulation was over. Because it was not possible to account for all of the ionic interferences using information in the voltammograms, other strategies were employed. Exposure of amygdala slices to L-DOPA or DA increased electrically evoked release of catecholamine, but the effect was transient, and uptake rates decreased during continued exposure to these agents. The most successful approach to remove the interferences was to subtract the response obtained after exposure of the slice to the catecholamine depleter, Ro 4-1284. This agent eliminates the catecholamine response but does not appear to alter the ionic changes.