[3H]substrate- and cell-specific effects of uptake inhibitors on human dopamine and serotonin transporter-mediated efflux
✍ Scribed by Robert A. Johnson; Amy J. Eshleman; Toni Meyers; Kim A. Neve; Aaron Janowsky
- Publisher
- John Wiley and Sons
- Year
- 1998
- Tongue
- English
- Weight
- 267 KB
- Volume
- 30
- Category
- Article
- ISSN
- 0887-4476
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✦ Synopsis
Drug-induced efflux of substrates was characterized in C6 rat glioma cells stably expressing a recombinant human dopamine (DA) or serotonin (5-HT) transporter (C6-hDAT and C6-hSERT, respectively). In the absence of Ca 2ϩ , these cells spontaneously and rapidly released preloaded [ 3 H]DA or [ 3 H]5-HT, respectively, but maintained constant levels of [ 3 H]N-methy-4-phenylpyridinium (MPP ϩ ) for up to 90 minutes. In C6-hSERT cells, transporter substrates such as methamphetamine, amphetamine, and dopamine induced relatively rapid release of [ 3 H]MPP ϩ , with t 1/2 values of approximately 15 minutes, while the t l/2 value for serotonin was about 30 minutes. Similar results were obtained with C6-hDAT cells. Uptake blockers that are not substrates at the transporters had considerably greater t 1/2 values, as compared to substrates, suggesting different mechanisms for altering transporter function. Doseresponse curves for each drug, conducted at each drug's t l/2 , indicated considerable differences in potency (EC 50 ) at stimulating [ 3 H]MPP ϩ release from C6-hSERT cells [3-(4-iodophenyl)tropane-2-carboxylic acid methyl ester (RTI-55) Ͼ imipramine Ͼ 1-[2-diphenylmethoxy]ethyl-4-(3-phenylpropyl)-piperazine (GBR-12935) threo-(Ϯ)methylphenidate Ͼ cocaine Ͼ mazindol Ͼ 2--carbomethoxy-3-(4-fluorophenyl)tropane (CFT) Ͼ (ϩ)methamphetamine Ͼ amphetamine Ͼ DA Ͼ fenfluramine Ͼ norepinephrine (NE) Ͼ 5-HT]. A different rank order of potency was observed for the effects of drugs on
Based on efficacies for stimulating [ 3 H]MPP ϩ release from C6-hDAT cells, drugs could be grouped into three categories, with substrates causing release of ϳ75% of loaded [ 3 H]MPP ϩ , cocaine analogues causing ϳ50% release, and other drugs causing an average release of ϳ25% of loaded [ 3 H]MPP ϩ . The results, taken together with results from previous reports, suggest that the transfected cell type contributes to the characteristics of transporter-mediated release, that drugs interact with different sites on the transporters in the uptake and release process, and that the mechanism of transporter-mediated release may not be a simple reversal of substrate uptake.