Radiolabeling of dopamine uptake sites in mouse striatum: comparison of binding sites for cocaine, mazindol, and GBR 12935.
This study addressed the possibility of a unique binding interaction between cocaine and the dopamine transporter as compared with other blockers of dopamine uptake. Cocaine binding sites in a fresh P2 fraction of mouse striatum were labeled with [3H]CFT, a phenyltropane analog of cocaine also known as WIN 35,428, and compared with sites labeled with [3H]mazindol or [3H]GBR 12935. Under the conditions used, homogeneous binding was observed that was inhibited monophasically by cocaine, CFT, and mazindol; the same potencies were observed with the three radioligands. Saturation analysis in the presence and in the absence of unlabeled inhibitor (CFT, mazindol, cocaine) indicated a change in the Kd but not the Bmax, consonant with a competitive mechanisms. Tris-HCl reduced the affinity of each radioligand and unlabeled inhibitor without changing the Bmax. N-Ethylmaleimide reduced the binding of all radioligands equally and cocaine offered protection. The dissociation rate of [3H]CFT and [3H]mazindol binding was not affected by the presence of mazindol and CFT, respectively. The Bmax of [3H]CFT and [3H]mazindol binding was the same; the relatively higher value for [3H]GBR 12935 binding in analyses involving varying tritiated GBR 12935 only, was due primarily to an underestimation of the specific activity of [3H]GBR 12935. All results are in agreement with a one-site model in which cocaine, CFT, mazindol, and GBR 12935 share a common binding site in mouse striatum.
Reith
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Division of Neurochemistry, N.S. Kline Institute for Psychiatric Research, Orangeburg, NY 10962.
Selmeci
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D000818:Animals; D001665:Binding Sites; D002021:Buffers; D003042:Cocaine / Q000031:analogs & derivatives / Q000378:metabolism*; D003342:Corpus Striatum / Q000378:metabolism*; D004298:Dopamine / Q000378:metabolism*; D004347:Drug Interactions; D005033:Ethylmaleimide / Q000494:pharmacology; D007700:Kinetics; D008297:Male; D008454:Mazindol / Q000378:metabolism*; D008566:Membranes / Q000378:metabolism; D051379:Mice; D008807:Mice, Inbred BALB C; D010710:Phosphates; D010879:Piperazines / Q000378:metabolism*; D011869:Radioligand Assay; D013696:Temperature; D014316:Tritium; D014325:Tromethamine
D002021:Buffers; D010710:Phosphates; D010879:Piperazines; D014325:Tromethamine; D014316:Tritium; C021279:(1R-(exo,exo))-3-(4-fluorophenyl)-8-methyl-8- azabicyclo(3.2.1)octane-2-carboxylic acid, methyl ester; C044630:1-(2 (diphenylmethoxy)ethyl)-4-(3-phenylpropyl)piperazine; D008454:Mazindol; D003042:Cocaine; D005033:Ethylmaleimide; C018279:sodium phosphate; D004298:Dopamine