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AJ Eshleman, RL Neve, A Janowsky and KA Neve
Research Service, Veterans Affairs Medical Center, Portland, Oregon, USA.
A 3.5-kilobase cDNA encoding the dopamine transporter was isolated from a human substantia nigra cDNA library. Sequence analysis of the coding region of the transporter identified two nucleotide differences between the cDNA and published human dopamine transporter sequences. One of the substitutions changed an amino acid conserved among previously cloned dopamine (DA) and norepinephrine transporters, Arg-344, to a methionine. C6 glioma cells or COS-7 cells transfected with the cDNA (C6-hDAT and Cos7-hDAT cells) accumulated [3H]DA with high affinity (Km = 1.2 and 1.5 microM, respectively), and DA uptake inhibitors had similar potencies in both cell lines. [3H]2 beta-carbomethoxy-3 beta-(4- fluorophenyl)tropane ([3H]CFT) bound to membranes prepared from both cell lines with high affinity (Kd = 2-6 nM), although some experiments with C6-hDAT cell membranes indicated the presence of a second class of binding sites with lower affinity for the radioligand. Using the high- affinity Kd value for [3H]CFT binding determined in Cos7-hDAT cells to calculate Ki values, drug affinity for inhibition was highly correlated (r = .92) with affinity for inhibition of [3H]DA uptake, although transporter substrates were significantly more potent inhibitors of uptake than of [3H]CFT binding. The binding of [3H]1-[2- diphenylmethoxy]ethyl-4-(3-phenylpropyl)-piperazine ([3H]GBR-12935) to C6-hDAT cells could not be characterized due to high binding to untransfected C6 cells, but on Cos7-hDAT cells the radioligand labeled a single population of binding sites (Kd = 1 nM). Inhibition of [3H]GBR- 12935 binding by drugs correlated highly with inhibition of either [3H]CFT binding (r = .98) or of [3H]DA uptake (r = .95).(ABSTRACT TRUNCATED AT 250 WORDS)
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