Pharmacological characterisation of (E)-N-(3-iodoprop-2-enyl)-2β-carbomethoxy-3β-(4′-methylphenyl)nortropane (PE2I) binding to the rat neuronal dopamine transporter expressed in COS cells

The interaction of (E)-N-(3-iodoprop-2-enyl)-2beta-Carbomethoxy-3beta-(4'-methylphenyl) nortropane (PE2I) with the rat neuronal dopamine transporter (DAT) was studied in transfected COS cells by measuring its ability to inhibit DA uptake and by measuring its affinity in radioligand binding experiments. Saturable [H-3]DA uptake was measured in COS cells transiently transfected with the cDNA sequence encoding the rat DAT. Pharmacological characterisation of this uptake revealed functional properties with a V-max,a value of 45.05 +/- 2.62 pmol/mg protein per min and a K-m value of 2.86 +/- 0.28 muM. The specific [H-3]DA uptake was fully inhibited by 1 muM PE2I. Concentration response curves revealed the high potency of PE2I in inhibiting DA uptake (pEC(50) value of 8.70 +/- 0.33), 25 times higher than that observed for the reference DAT inhibitor, GBR 12935. On crude homogenates from transfected COS cells, PE2I displaced the specific binding of [3 H]GBR 12935 with a PKi value of 7.73 +/- 0.13. Accordingly, [I-125]PE2I was found to specifically recognise a single binding site population which is almost completely displaced by GBR 12935 and nomifensine. Saturation experiments revealed the high affinity of [I-125]PE2I (K-D value of 3.8 +/- 0.63 nM) that correlates with the high potency of PEN in inhibiting the [H-3]DA uptake. This contrasts with the results obtained with GBR 12935 for which a discrepancy was found between its high affinity in binding assays (KD value of 0.43 +/- 0.04 nM) and its rather low potency in functional assays (pEC50 value of 7.30 +/- 0.05). A relatively high level of [H-3]GBR 12935 binding was detected in non transfected COS cells. Such nomifensine resistant binding is attributed to the interaction of GBR 12935 with cytochrome P-450 as it was displaced by cis-(Z)-flupentixol (an inhibitor of cytochrome P-450). Such interaction was not observed using PE2I. Taken together, these data demonstrate that PE2I was a highly potent inhibitor of cloned DAT compared with GBR 12935 and provided a useful tool for further investigations in cells transfected with cDNA encoding the DAT. (C) 2001 Elsevier Science Ltd. All rights reserved.