MECHANISM OF THE DOPAMINE-RELEASING ACTIONS OF AMPHETAMINE AND COCAINE - PLASMALEMMAL DOPAMINE TRANSPORTER VERSUS VESICULAR MONOAMINE TRANSPORTER

The effects of amphetamine and cocaine were studied in [H-3]-dopamine- loaded and superfused COS-7 cells transfected with either the cDNA of the plasmalemmal dopamine transporter (''DAT cells'') or the cDNA of t he vesicular amine transporter (''VAT cells''), or with both transport ers (''DAT/VAT cells''). Amphetamine (0.01-100 mu M, added for 4 min o f superfusion) led to a concentration-dependent increase in dopamine r elease in DAT cells, as well as in DAT/VAT cells. The EC(50) of the ef fect of amphetamine on DAT cells was 1.1 +/- 0.6 mu M; the effect on D AT/VAT cells did not reach a plateau in the concentration range tested . With longer exposure to amphetamine, dopamine efflux from DAT cells reached a peak and quickly returned to baseline, in spite of the conti nued presence of the drug, whereas in DAT/VAT cells and in VAT cells t he effect was sustained. Cocaine (up to 100 mu M) did not exert any ef fect of its own in DAT cells or VAT cells but inhibited the amphetamin e-induced release of dopamine from DAT cells in a competitive manner. In DAT/VAT cells cocaine and its analogue (-)-2 beta-carbomethoxy-3 be ta-(4-fluorophenyl)tropane caused an efflux of dopamine resembling tha t caused by amphetamine but quantitatively much smaller. The rank orde r of potency was the same as in uptake experiments [(-)-2 beta-carbome thoxy-3 beta-(4-fluorophenyl)tropane > cocaine]. The effect of cocaine was mimicked by the reduction of chloride. The results indicate that there is a plasmalemmal component and a vesicular component in the dop amine-releasing action of amphetamine. The releasing action of cocaine is dependent on the existence of a vesicular pool of the neurotransmi tter and seems to be linked to inhibition of the plasmalemmal dopamine transporter.