Real-time monitoring of endogenous noradrenaline release in rat brain slices using fast cyclic voltammetry: 3. Selective detection of noradrenaline efflux in the locus coeruleus

Abstract

Fast cyclic voltammetry (FCV) at carbon fibre microelectrodes was used to monitor ‘real time’ endogenous noradrenaline (NA) efflux in superfused slices of rat locus coeruleus (LC) following local electrical stimulation. When stimulated with a standard train (30 pulses, 100 Hz, 0.2 ms, 10 mA, every 5 min), efflux of monoamine was constant over the experimental period (2.5 h): Amine efflux declined by only 16 ± 5% while uptake half-life lengthened by only 9 ± 8%. When calibrated in solutions of NA, peak amine efflux corresponded to 0.31 ± 0.04 μM (mean±S.E.M., n = 28) and was removed by uptake with a half-life of 2.93 ± 0.28 s ( n = 16). The released compound was confirmed as NA on the basis of pharmacological and electrochemical criteria. Stimulated monoamine efflux was reversibly reduced by 78% by omission of Ca 2+ from the superfusate for 30 min ( P < 0.05). Ro 4-1284 (1 μM), a fast-acting reserpine-like drug, decreased amine efflux by 86% ( P < 0.05). The monoamine oxidase inhibitor pargyline (2 μM) increased efflux by 30% ( P < 0.05). Desipramine (0.05 μM), a selective NA uptake blocker, significantly increased amine efflux (by 96%, P < 0.05) and uptake half-life (by 314%, P < 0.05). Fluvoxamine (0.5 μM), the selective serotonin (5HT) uptake blocker, increased efflux by 59% ( P < 0.05) and the uptake half-life by 122% ( P < 0.05). Vanoxerine (GBR 12909: 0.3 μM), the dopamine (DA) uptake blocker, had no effect on amine efflux or uptake half-life. The voltammogram of the released amine had single oxidation and reduction peaks. Comparison of these redox characteristics with standards confirmed that the released substance generated voltammograms indistinguishable from those produced by the catecholamines NA and DA but distinct from that produced by the indoleamine 5HT. Taken together, the combined pharmacological and electrochemical data confirm that the amine detected in the LC was NA. This study demonstrates the ability of FCV to selectively monitor endogenous NA efflux and uptake in ‘real time’ and with high spatial resolution.