Differential kinetics of [123I]beta-CIT binding to dopamine and serotonin transporters

Iodine-123-labelled 3beta-(4-iodophenyl)tropane-2beta-carboxylic acid ([123I]beta-CIT) labels both the dopamine transporter (DAT) and the serotonin transporter (5-HTT) and this ligand is able to clarify pathological changes in both dopaminergic and serotonergic systems. However, the differential kinetics of beta-CIT binding to DAT and 5-HTT has not been clarified fully. In this study we examined time-activity curves of [123I]beta-CIT in individual regions in the rat brain. Using cerebellum as the reference region, k3 and k4 values were estimated by a two-compartment kinetic analysis. In the striatum, the kinetics was slowest among all brain areas. In this area specific binding reached its peak 4 h after the injection. In the hypothalamus, specific binding reached its peak 1 h after the injection and its amount did not change until 4 h after the injection. In the occipital cortex, the binding and washout of the ligand were fastest among all brain regions. Estimated k3 values were 0.040+/-0.003 in the striatum, 0.019+/-0.002 in the hypothalamus and 0.082+/-0.011 in the occipital cortex (min-1, mean +/-SD). Estimated k4 values were 0.0034+/-0.0005 in the striatum, 0.0071+/-0.0009 in the hypothalamus and 0.083+/-0.013 in the occipital cortex (min-1, mean +/-SD). Therefore binding kinetics of [123i]beta-cit in the region rich in dat is apparently different from that in the region rich in 5-HTT. These results will provide fundamental data to image both DAT and 5-HTT in one series of examinations with [123I]beta-CIT.