Release of dopamine via the human transporter

A human dopamine transporter cDNA was cloned and transfected into COS-7 cells, a cell line that lacks vesicular storage and release mechanisms. Cells expressing the dopamine transporter acquired the capacity to take up and release dopamine via the transporter. Ionic conditions that stimulate inside-out transport in vivo, such as depolarizing concentrations of K+ or low concentrations of extracellular Na+, were found to stimulate Ca(2+)-independent release of [3H]dopamine from transfected COS-7 cells. Dopamine uptake inhibitors had one of three effects on transporter-mediated efflux. Some drugs, in addition to inhibiting uptake, inhibited spontaneous release of dopamine. Drugs in this class included mazindol, GBR-12935, bupropion, nomifensine, and benztropine. All of the drugs with the potential for abuse by humans either enhanced release (methamphetamine, amphetamine, and ethanol) or had no effect on release (phencyclidine, cocaine, and WIN 35,428). The ability to define classes of uptake blockers based on their effects on human transporter-mediated dopamine efflux may lead to the identification of structural features of the transporter that differentiate abused from nonabused drugs.