Amphetamines Take Two to Tango: an Oligomer-Based Counter-Transport Model of Neurotransmitter Transport Explores the Amphetamine Action

Abstract

Amphetamine congeners [e.g., 3,4-methylenedioxymetamphetamine (MDMA), or “ecstasy”] are substrates for monoamine transporters (i.e., the transporters for serotonin, norepinephrine, and dopamine); however, their in vivo-action relies on their ability to promote monoamine efflux. The mechanistic basis for this counter transport remains enigmatic. We tested the hypothesis that outward transport is contingent on the oligomeric nature of neurotransmitter transporters by creating a concatemer of the serotonin transporter and the amphetamine-resistant GABA transporter. In cells expressing the concatemer, amphetamine analogs promoted GABA efflux and blunted GABA influx. In contrast, the natural substrates serotonin and GABA only cause mutual inhibition of influx via the other transporter moiety in the concatemer. GABA efflux through the concatemer that was promoted by amphetamine analogs was blocked by the protein kinase C inhibitors GF109203X (bisindoylmaleimide I) and Gö6983 (2-[1-(3-dimethylaminopropyl)-5-methoxyindol-3-yl]-3-(1H-indol-3-yl)maleimide). Thus, based on our observations, we propose that, in the presence of amphetamine analogs, monoamine transporters operate as counter-transporters; influx and efflux occur through separate but coupled moieties. Influx and efflux are coupled via changes in the ionic gradients, but these do not suffice to account for the action of amphetamines; the activity of a protein kinase C isoform provides a second stimulus that primes the inward facing conformation for outward transport.