Ra. Johnson et al., [H-3]SUBSTRATE-SPECIFIC AND CELL-SPECIFIC EFFECTS OF UPTAKE INHIBITORS ON HUMAN DOPAMINE AND SEROTONIN TRANSPORTER-MEDIATED EFFLUX, Synapse, 30(1), 1998, pp. 97-106
Drug-induced efflux of substrates was characterized in C6 rat glioma c
ells stably expressing a recombinant human dopamine (DA) or serotonin
(5-HT) transporter (CG-hDAT and CG-hSERT, respectively). In the absenc
e of Ca2+, these cells spontaneously and rapidly released preloaded [H
-3]DA or [H-3]5-HT, respectively, but maintained constant levels of [H
-3]N-methy-4-phenylpyridinium (MPP+) for up to 90 minutes. In C6-hSERT
cells, transporter substrates such as methamphetamine, amphetamine, a
nd dopamine induced relatively rapid release of [H-3]MPP+, with t(1/2)
values of approximately 15 minutes, while the t(1/2) value for seroto
nin was about 30 minutes. Similar results were obtained with C6-hDAT c
ells. Uptake blockers that are not substrates at the transporters had
considerably greater t(1/2) values, as compared to substrates, suggest
ing different mechanisms for altering transporter function. Dose-respo
nse curves for each drug, conducted at each drug's t(1/2), indicated c
onsiderable differences in potency (EC50) at stimulating [H-3]MPP+ rel
ease from C6hSERT cells [3 beta-(4-iodophenyl)tropane-2 beta-carboxyli
c acid methyl ester (RTI-55) > imipramine > 1- iphenylmethoxy]ethyl-4-
(3-phenylpropyl)-piperazine (GBR-12935) threo-(+/-)methylphenidate > c
ocaine > mazindol > 2-beta-carbometl loxy-3 beta-(4-fluorophenyl)tropa
ne (CFT) > (+)methamphetamine > amphetamine > DA > fenfluramine > nore
pinephrine (NE) > 5-HT].A different rank order of potency was observed
for the effects of drugs on [H-3]MPP+ release from C6-hDAT cells [imi
pramine > RTI-55 > cocaine > mazindol > CFT > GBR-12935 > threo-(+/-)-
methylphenidate > amphetamine > (+)methamphetamine > fenfluramine > DA
> NE > 5-HT]. Based on efficacies for stimulating [H-3]MPP+ release f
rom C6-hDAT cells, drugs could be grouped into three categories, with
substrates causing release of similar to 75% of loaded [H-3]MPP+, coca
ine analogues causing -50% release, and other drugs causing an average
release of similar to 25% of loaded [H-3]MPP+. The results, taken tog
ether 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 transport
er-mediated release may not be a simple reversal of substrate uptake.
Synapse 30:97-106, 1998. (C) 1998 Wiley-Liss, Inc.