title: Dopamine transport by the serotonin transporter: a mechanistically distinct mode of substrate translocation.
abstract: The serotonin transporter (SERT) is the principal mechanism for terminating serotonin (5-HT) signals in the nervous system and is a site of action for a variety of psychoactive drugs including antidepressants, amphetamines, and cocaine. Here we show that human SERTs (hSERTs) and rat SERTs are capable of robust dopamine (DA) uptake through a process that differs mechanistically from 5-HT transport in several unanticipated ways. DA transport by hSERT has a higher maximum velocity than 5-HT transport, requires significantly higher Na(+) and Cl(-) concentrations to sustain transport, is inhibited noncompetitively by 5-HT, and is more sensitive to SERT inhibitors, including selective serotonin reuptake inhibitors. We use a thiol-reactive methane thiosulfonate (MTS) reagent to modify a conformationally sensitive cysteine residue to demonstrate that hSERT spends more time in an outward facing conformation when transporting DA than when transporting 5-HT. Cotransfection of an inactive or an MTS-sensitive SERT with wild-type SERT subunits reveals an absence of cooperative interactions between subunits during DA but not 5-HT transport. To establish the physiological relevance of this mechanism for DA clearance, we show using in vivo high-speed chronoamperometry that SERT has the capacity to clear extracellularly applied DA in the hippocampal CA3 region of anesthetized rats. Together, these observations suggest the possibility that SERT serves as a DA transporter in vivo and highlight the idea that there can be distinct modes of transport of alternative physiological substrates by SERT.
authors.affiliation: Department of Neurobiology, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania 15260, USA.
authors.forename: Mads Breum
authors.initials: MB
authors.lastname: Larsen
authors.forename: Mark S
authors.initials: MS
authors.lastname: Sonders
authors.forename: Ole Valente
authors.initials: OV
authors.lastname: Mortensen
authors.forename: Gaynor A
authors.initials: GA
authors.lastname: Larson
authors.forename: Nancy R
authors.initials: NR
authors.lastname: Zahniser
authors.forename: Susan G
authors.initials: SG
authors.lastname: Amara
mesh_terms: D000704:Analysis of Variance; D000818:Animals; D056654:CA3 Region, Hippocampal / Q000166:cytology; D019556:COS Cells; D002461:Cell Line, Transformed; D002522:Chlorocebus aethiops; D015283:Citalopram / Q000494:pharmacology; D003042:Cocaine / Q000031:analogs & derivatives / Q000493:pharmacokinetics; D004298:Dopamine / Q000378:metabolism* / Q000494:pharmacology; D050483:Dopamine Plasma Membrane Transport Proteins / Q000235:genetics / Q000378:metabolism*; D004305:Dose-Response Relationship, Drug; D055664:Electrochemical Techniques; D006801:Humans; D008297:Male; D016297:Mutagenesis, Site-Directed / Q000379:methods; D011869:Radioligand Assay / Q000379:methods; D019275:Radiopharmaceuticals / Q000493:pharmacokinetics; D051381:Rats; D017207:Rats, Sprague-Dawley; D012701:Serotonin / Q000378:metabolism / Q000494:pharmacology; D050486:Serotonin Plasma Membrane Transport Proteins / Q000235:genetics / Q000502:physiology*; D017367:Selective Serotonin Reuptake Inhibitors / Q000494:pharmacology; D012964:Sodium / Q000378:metabolism; D014162:Transfection / Q000379:methods; D014316:Tritium / Q000378:metabolism
chemical_list: D015283:Citalopram; D003042:Cocaine; D004298:Dopamine; D050483:Dopamine Plasma Membrane Transport Proteins; D019275:Radiopharmaceuticals; D012701:Serotonin; D050486:Serotonin Plasma Membrane Transport Proteins; D017367:Selective Serotonin Reuptake Inhibitors; D012964:Sodium; D014316:Tritium; C069723:2beta-carbomethoxy-3beta-(4-iodophenyl)tropane