STUDIES OF THE BIOGENIC-AMINE TRANSPORTERS - VII - CHARACTERIZATION OF A NOVEL COCAINE BINDING-SITE IDENTIFIED WITH [I-125] RTI-55 IN MEMBRANES PREPARED FROM HUMAN, MONKEY AND GUINEA-PIG CAUDATE

[I-125]RTI-55 is a cocaine analog with high affinity for dopamine (DA) and serotonin (5-HT) transporters. Quantitative ligand binding studie s revealed a novel high affinity [I-125]RTI-55 binding site assayed un der 5-HT transporter (SERT) conditions which has low affinity for almo st all classic biogenic amine transporter ligands, including high affi nity 5-HT transporter inhibitors such as paroxetine, but which retains high affinity for cocaine analogs. This site, termed SERTsite2 for it s detection under 5-HT transporter conditions (not for an association with the SERT) occurs in monkey caudate, human caudate, and guinea pig caudate membranes, but not in rat caudate membranes. SERTsite2 is dis tinguished from the DA transporter (DAT) and SERT by several criteria, including a distinct ligand-selectivity profile, the inability to det ect SERTsite2 in cells stably expressing the cloned human DAT, and ins ensitivity to irreversible ligands which inhibit [I-125]RTI-55 binding to the DAT and SERT. Perhaps the most striking finding about SERTsite 2 is that a nide range of representative antidepressant agents have ve ry low affinity for SERTsite2. The affinity of cocaine for this site i s not very different from the concentration cocaine achieves in the br ain at pharmacological doses. Viewed collectively with the observation that ligands with high affinity for SERTsite2 are mostly cocaine anal ogs, these data lead us to speculate that actions of cocaine which dif fer from those of classic biogenic amine uptake inhibitors may be medi ated in part via SERTsite2. (C) 1998 Wiley-Liss, Inc.