MULTIPLE BINDING-SITES FOR [I-125] RTI-121 AND OTHER COCAINE ANALOGS IN RAT FRONTAL CEREBRAL-CORTEX

In an effort to identify novel binding sites for cocaine and its analo gs, we carried out binding studies with the high-affinity and selectiv e ligand [I-125]RTI-121 in rat frontal cortical tissue. Very low densi ties of binding sites were found. Saturation analysis revealed that th e binding was to both high- and low-affinity sites. Pharmacological co mpetition studies were carried out with inhibitors of the dopamine, no repinephrine, and serotonin transporters. The various transporter inhi bitors inhibited the binding of 15 pM [I-125]RTI-121 in a biphasic fas hion following a two-site binding model. The resultant data were compl ex and did not suggest a simple association with any single transporte r. Correlational analysis supported the following hypothesis: [I-125] RTI-121 binds to known transporters and not to novel sites; these incl ude dopamine, norepinephrine, and serotonin transporters. Immunoprecip itation of transporters photoaffinity labeled with [(125)] RTI-82 and subsequent analysis of SDS-page gels revealed the presence of authenti c dopamine transporters in these samples; displacement of the photoaff inity label occurred with a typical dopamine transporter pharmacology. These data are compatible with the binding properties of RTI-181 and the presence of several known transporters in the tissue studied. Syna pse 30:9-17, 1998. (C) 1998 Wiley-Liss, Inc.dagger