HIGH-AFFINITY BINDING OF [I-125] 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 [I-125]RTI-55 bound to both a high- and low-affinity site. The K(d) for the high-affinity site was 0.2 nM, while the K(d) 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 [I-125]RTI-55 binding in the striatum was consistent with that of the dopamine transporter. Additionally, [I-125]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 [I-125]RTI-55 in the cerebral cortex matched that of the serotonin transporter. Autoradiographic analysis of sagittal brain sections with [I-125]RTI-55 binding was consistent with these findings. Specific binding of [I-125]RTI-55 was blocked by dopamine uptake inhibitors in areas rich in dopaminergic nerve terminals. Conversely, serotonin uptake inhibitors blocked the binding of [I-125]RTI-55 in brain areas rich in serotonergic neurons. These results demonstrate that [I-125]RTI-55 may be a very useful ligand for the dopamine and serotonin transporters.