CARRIER-MEDIATED RELEASE, TRANSPORT RATES, AND CHARGE-TRANSFER INDUCED BY AMPHETAMINE, TYRAMINE, AND DOPAMINE IN MAMMALIAN-CELLS TRANSFECTED WITH THE HUMAN DOPAMINE TRANSPORTER

Amphetamine and related substances induce dopamine release. According to a traditional explanation, this dopamine release occurs in exchange for amphetamine by means of the dopamine transporter (DAT). We tested this hypothesis in human embryonic kidney 293 cells stably transfecte d with the human DAT by measuring the uptake of dopamine, tyramine, an d D- and L-amphetamine as well as substrate-induced release of preload ed N-methyl-4-[H-3]phenylpyridinium ([H-3]MPP+). The uptake of substra tes was sodium-dependent and was inhibited by ouabain and cocaine, whi ch also prevented substrate-induced release of MPP+. Patch-clamp recor dings revealed that all four substrates elicited voltage-dependent inw ard currents (on top of constitutive leak currents) that were prevente d by cocaine. Whereas individual substrates had similar affinities in release, uptake, and patch-clamp experiments, maximal effects displaye d remarkable differences. Hence, maximal effects in release and curren t induction were similar to 25% higher for D-amphetamine as compared w ith the other substrates. By contrast, dopamine was the most efficacio us substrate in uptake experiments, with its maximal initial uptake ra te exceeding those of amphetamine and tyramine by factors of 20 and 4, respectively. Our experiments indicate a poor correlation between sub strate-induced release and the transport of substrates, whereas the ab ility of substrates to induce currents correlates well with their rele asing action.