High-affinity binding of [125I]RTI-55 to dopamine and serotonin transporters in rat brain.
RTI-55 (3 beta-(4-iodophenyl)tropan-2 beta-carboxylic acid methyl ester), one of the most potent inhibitors of dopamine uptake reported to date, was radioiodinated and tested as a probe for the cocaine receptor in Sprague-Dawley rat brain. Saturation and kinetic studies in the striatum revealed that [125I]RTI-55 bound to both a high- and low-affinity site. The Kd for the high-affinity site was 0.2 nM, while the Kd for the low-affinity site was 5.8 nM. The corresponding number of binding sites in the striatum was 37 and 415 pmol/g protein. The pharmacological profile of specific [125I]RTI-55 binding in the striatum was consistent with that of the dopamine transporter. Additionally, [125I]RTI-55 was found to bind with high affinity to the cerebral cortex. Scatchard analysis revealed a single high-affinity component of 0.2 nM with a density of 2.5 pmol/g protein. The pharmacological profile demonstrated by [125I]RTI-55 in the cerebral cortex matched that of the serotonin transporter. Autoradiographic analysis of sagittal brain sections with [125I]RTI-55 binding was consistent with these findings. Specific binding of [125I]RTI-55 was blocked by dopamine uptake inhibitors in areas rich in dopaminergic nerve terminals. Conversely, serotonin uptake inhibitors blocked the binding of [125I]RTI-55 in brain areas rich in serotonergic neurons. These results demonstrate that [125I]RTI-55 may be a very useful ligand for the dopamine and serotonin transporters.
Boja
J W
JW
Neuroscience Branch, Addiction Research Center, National Institute on Drug Abuse, Baltimore, Maryland 21224.
Mitchell
W M
WM
Patel
A
A
Kopajtic
T A
TA
Carroll
F I
FI
Lewin
A H
AH
Abraham
P
P
Kuhar
M J
MJ
D000818:Animals; D001345:Autoradiography; D001590:Benztropine / Q000494:pharmacology; D001667:Binding, Competitive; D001921:Brain / Q000378:metabolism*; D002352:Carrier Proteins / Q000378:metabolism*; D002462:Cell Membrane / Q000378:metabolism; D002540:Cerebral Cortex / Q000378:metabolism; D003042:Cocaine / Q000031:analogs & derivatives* / Q000378:metabolism; D003342:Corpus Striatum / Q000378:metabolism; D050483:Dopamine Plasma Membrane Transport Proteins; D007457:Iodine Radioisotopes; D007700:Kinetics; D008297:Male; D008562:Membrane Glycoproteins / Q000378:metabolism*; D026901:Membrane Transport Proteins; D009419:Nerve Tissue Proteins / Q000378:metabolism*; D017374:Paroxetine / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D012701:Serotonin / Q000378:metabolism*; D050486:Serotonin Plasma Membrane Transport Proteins; D014316:Tritium
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; D014316:Tritium; D001590:Benztropine; D012701:Serotonin; D017374:Paroxetine; C069723:2beta-carbomethoxy-3beta-(4-iodophenyl)tropane; D003042:Cocaine