STUDIES OF THE BIOGENIC-AMINE TRANSPORTERS .2. A BRIEF STUDY ON THE USE OF [H-3] DA-UPTAKE-INHIBITION TO TRANSPORTER-BINDING-INHIBITION RATIOS FOR THE IN-VITRO EVALUATION OF PUTATIVE COCAINE ANTAGONISTS

The cocaine receptor on the dopamine transporter is a logical target b inding site for the design and synthesis of novel agents for evaluatio n as possible cocaine antagonists. Although there is no widely accepte d and validated assay for detecting a cocaine antagonist, one commonly accepted strategy is to compare the IC50 value of a test agent for in hibition of [H-3]dopamine uptake and its IC50 value for inhibition of the binding of a transporter ligand such as [I-1251]RTI-55. The goal o f such a comparison is to guide the synthesis of agents which have hig h ''uptake-to-binding ratios'', i.e. agents which are much more potent in the binding assay than they are in the uptake assay. In the presen t study we tested the hypothesis that ratios different from unity can result from the fact that the two assays are conducted under markedly different conditions. The results showed that conducting the uptake an d binding assays under identical conditions reduced the GBR12935 uptak e-to-binding ratio of 6.20 (under standard assay conditions) to 0.36. These data indicate that uptake-to-binding ratios must be interpreted with caution, and emphasizes the need for simpler and less expensive m ethods than cocaine self-administration paradigms to screen compounds as modulators of cocaine reinforcement.