Detection of dopamine overflow and diffusion with voltammetry in slices of rat brain

Voltammetric electrodes have been used to monitor extracellular dopamine in rat brain slices. The electrode tips are small enough to be immersed inside the slice. Specificity for dopamine is increased through the use of voltammetry and a cation exchange membrane at the electrode tip. Dopamine overflow is observed in the caudate nucleus following electrical stimulation (60 Hz, 1 s, 3 V) with an adjacent bipolar electrode. The amount of overflow observed is increased when the tissue is perfused with 10 microM cocaine or nomifensine, both recognized inhibitors of dopamine uptake. The ability of dopamine in the perfusion buffer to permeate the slice was monitored with two voltammetric electrodes, one in the cerebral cortex and the other in the caudate nucleus. At a high concentration (100 microM), dopamine rapidly appeared (2.7 +/- 0.4 min) in the interior of the cortex, but dopamine was not observed in the caudate until a significantly later time (8.9 +/- 1.0 min). To examine whether this difference is a reflection of the presence of different uptake systems in the two regions, pressure ejection was employed. In this experiment a double-barrelled pipette was used to eject dopamine or DOPAC at a fixed distance (approximately 70 micron) from the voltammetric electrode. Ejection of small amounts of both substances could be detected in the cortex. When the ejector-detector assembly was moved to the caudate, dopamine could only be observed following pressure ejection after perfusion of the slice with 10 microM nomifensine. Detection of DOPAC was unaffected. All of these experiments indicate that uptake systems in the caudate keep dopamine concentrations very low in the extracellular fluid of the slice.