VISUALIZING DOPAMINE AND SEROTONIN TRANSPORTERS IN THE HUMAN BRAIN WITH THE POTENT COCAINE ANALOG [I-125] RTI-55 - IN-VITRO BINDING AND AUTORADIOGRAPHIC CHARACTERIZATION

The cocaine analogue RTI-55 was evaluated as a probe for in vitro labe ling and localization of dopamine and serotonin transporters after dea th in the human brain. Kinetic, saturation, and competition binding ex periments indicated complex interactions of the radioligand with the i dentification of multiple recognition sites. In membrane binding assay s, the association of [I-125]RTI-55 at 25 degrees C to putamen membran es was monophasic. In contrast, dissociation of [I-125]RTI-55 occurred in two phases with t(1/2) values of 9.4 and 36.5 min, respectively. S aturation analysis of [I-125]RTI-55 binding demonstrated two binding s ites in the human putamen with K-D values of 0.10 +/- 0.02 and 1.81 +/ - 0.46 nnn. The binding of [I-125]RTI-55 was displaced by a wide range of cocaine analogues and monoamine uptake inhibitors. The rank order of potency demonstrated in competition assays with human putamen membr anes indicates that the radioligand labels cocaine recognition sites o n the dopamine transporter (mazindol > GBR 12909 > GBR 12935 > paroxet ine > nisoxetine > desipramine much greater than fluoxetine > citalopr am). In the human occipital cortex, [I-125]RTI-55 recognized multiple binding sites with K-D values of 0.02 +/- 0.01 and 4.18 +/- 0.46 nM. T he rank order of potency for inhibition of [I-125]RTI-55 binding to ce rebral cortex membranes (paroxetine > citalopram > GBR 12909 much grea ter than mazindol much greater than nisoxetine > benztropine) suggests that [I-125]RTI-55 labels the serotonin transporter in the human occi pital cortex. Autoradiographic mapping of [I-125]RTI-55 revealed very high densities of cocaine recognition sites over areas known to be ric h in dopaminergic innervation, including the caudate, putamen, and nuc leus accumbens. Moderately elevated densities of [I-125]RTI-55 binding sites were also seen throughout the thalamus, hypothalamus, and subst antia nigra. [I-125]RTI-55 binding sites were prevalent throughout the cerebral cortex and amygdala. In autoradiographic studies, the additi on of the selective serotonin transport blocker citalopram completely prevented [I-125]RTI-55 labeling in the thalamus, hypothalamus, and th roughout most of the cerebral cortex. In the presence of citalopram, [ I-125]RTI-55 binding site densities remained elevated over the striatu m and substantia nigra, with selective residual labeling also seen in the external segment of the globus pallidus and the lateral nucleus of the amygdala. These results demonstrate that in the human brain, [I-1 25]RTI-55 labels multiple recognition sites on dopamine and serotonin transporters.