title: Methyl 3 beta-(4-[125I]iodophenyl)tropane-2 beta-carboxylate in vitro binding to dopamine and serotonin transporters under "physiological" conditions.
abstract: Methyl 3 beta-(4-[125I]iodophenyl)tropane-2 beta-carboxylate ([123I]beta-CIT) is a single photon emission computed tomographic radiotracer for in vivo labeling of dopamine (DA) and serotonin (5-HT) transporters. Single photon emission computed tomographic experiments in nonhuman primates showed that [123I]beta-CIT in vivo binding to DA transporters had a much slower washout than binding to 5-HT transporters. This observation was not predicted from previously published in vitro studies. These studies, performed at 22 degrees C in nonphysiological buffer, reported similar affinity of [125I]beta-CIT for DA and 5-HT transporters. We now report [125I]beta-CIT binding parameters to fresh rat membranes at 22 degrees C and 37 degrees C, in a buffer mimicking the composition of cerebrospinal fluid. At both temperatures, binding to DA transporters was best fit by a two-site model, whereas binding to 5-HT transporters was compatible with one population of sites. At 22 degrees C, [125I]beta-CIT showed similar affinity to high-affinity DA (0.39 nM) and 5-HT transporter sites (0.47 nM). Increasing the incubation temperature from 22 degrees C to 37 degrees C reduced binding to DA transporters by 60%, whereas binding to 5-HT transporters was only marginally affected. In vitro kinetic experiments failed to detect significant differences in on or off rates that could explain the observed in vivo kinetics. These experiments thus failed to explain [125I]beta-CIT in vivo uptake kinetics, suggesting the existence of specific factors affecting the in vivo situation.
authors.affiliation: Department of Psychiatry, Yale University School of Medicine, West Haven, Connecticut.
authors.forename: M
authors.initials: M
authors.lastname: Laruelle
authors.forename: S S
authors.initials: SS
authors.lastname: Giddings
authors.forename: Y
authors.initials: Y
authors.lastname: Zea-Ponce
authors.forename: D S
authors.initials: DS
authors.lastname: Charney
authors.forename: J L
authors.initials: JL
authors.lastname: Neumeyer
authors.forename: R M
authors.initials: RM
authors.lastname: Baldwin
authors.forename: R B
authors.initials: RB
authors.lastname: Innis
mesh_terms: D000818:Animals; D001667:Binding, Competitive; D002352:Carrier Proteins / Q000378:metabolism*; D002540:Cerebral Cortex / Q000378:metabolism; D015283:Citalopram / Q000378:metabolism; D003042:Cocaine / Q000031:analogs & derivatives* / Q000378:metabolism / Q000493:pharmacokinetics; D003342:Corpus Striatum / Q000000981:diagnostic imaging / Q000378:metabolism; D050483:Dopamine Plasma Membrane Transport Proteins; D066298:In Vitro Techniques; D007457:Iodine Radioisotopes; D008297:Male; D008562:Membrane Glycoproteins / Q000378:metabolism*; D026901:Membrane Transport Proteins; D009419:Nerve Tissue Proteins / Q000378:metabolism; D010215:Papio; D051381:Rats; D017207:Rats, Sprague-Dawley; D050486:Serotonin Plasma Membrane Transport Proteins; D013696:Temperature; D015899:Tomography, Emission-Computed, Single-Photon
chemical_list: D002352:Carrier Proteins; D050483:Dopamine Plasma Membrane Transport Proteins; D007457:Iodine Radioisotopes; D008562:Membrane Glycoproteins; D026901:Membrane Transport Proteins; D009419:Nerve Tissue Proteins; D050486:Serotonin Plasma Membrane Transport Proteins; C493232:Slc6a4 protein, rat; D015283:Citalopram; C069723:2beta-carbomethoxy-3beta-(4-iodophenyl)tropane; D003042:Cocaine