Active transport pumps of HVA and DOPAC in dopaminergic nerve terminals

The effect of the membrane potential on the efflux of HVA and DOPAC from DA neurons was studied in anesthetized (1% halothane in gas mixture of 70% N2O and 30% O2) cats. Extracellular DA, HVA and DOPAC were measured continuously from the putamen, the hypothalamus, the thalamus, the raphe nuclei and the cortex using brain microdialysis technique combined with HPLC-ED monoamine measurements. HVA and DOPAC concentrations were highest in the putamen and lowest in the cerebral cortex. Extracellular HVA levels exceed those of the DOPAC. Increases in the extracellular potassium from 4 to 120 mM invariably produced decreases of the extracellular HVA and DOPAC in all the tested brain regions. These decreases were inversely proportional to the extracellular potassium concentration. Thus, it is concluded that the HVA and the DOPAC are extruded from inside the cell to the extracellular space by active mechanisms of transport similar to that reported for 5-HIAA in serotonergic neurons.