Multiple binding sites for [125I]RTI-121 and other cocaine analogs in rat frontal cerebral cortex.

Multiple binding sites for [125I]RTI-121 and other cocaine analogs in rat frontal cerebral cortex.

Boja J W; Carroll F I; Vaughan R A; Kopajtic T; Kuhar M J

Synapse (New York, N.Y.)

1998

1998-09

In an effort to identify novel binding sites for cocaine and its analogs, we carried out binding studies with the high-affinity and selective ligand [125I]RTI-121 in rat frontal cortical tissue. Very low densities of binding sites were found. Saturation analysis revealed that the binding was to both high- and low-affinity sites. Pharmacological competition studies were carried out with inhibitors of the dopamine, norepinephrine, and serotonin transporters. The various transporter inhibitors inhibited the binding of 15 pM [125I]RTI-121 in a biphasic fashion following a two-site binding model. The resultant data were complex and did not suggest a simple association with any single transporter. Correlational analysis supported the following hypothesis: [125I] RTI-121 binds to known transporters and not to novel sites; these include dopamine, norepinephrine, and serotonin transporters. Immunoprecipitation of transporters photoaffinity labeled with [125]RTI-82 and subsequent analysis of SDS-page gels revealed the presence of authentic dopamine transporters in these samples; displacement of the photoaffinity label occurred with a typical dopamine transporter pharmacology. These data are compatible with the binding properties of RTI-121 and the presence of several known transporters in the tissue studied.

*Membrane Transport Proteins; *Nerve Tissue Proteins; *Symporters; Animals; Binding; Binding Sites; Carrier Proteins/*metabolism; Cocaine/*analogs & derivatives/pharmacokinetics; Competitive; Corpus Striatum/*metabolism; Dopamine Plasma Membrane Transport Proteins; Dopamine/metabolism; Frontal Lobe/*metabolism; Iodine Radioisotopes/*pharmacokinetics; Kinetics; Male; Membrane Glycoproteins/*metabolism; Norepinephrine Plasma Membrane Transport Proteins; Norepinephrine/metabolism; Rats; Regression Analysis; Serotonin Plasma Membrane Transport Proteins; Serotonin/metabolism; Sprague-Dawley

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