Evidence for the Sequential Formation of Two Complexes Between an Uptake Inhibitor, GBR 12783 [1-[2-(Diphenylmethoxy)ethyl]-4-(3-Phenyl-2-Propenyl)piperazine], and the Neuronal Transporter of Dopamine

Authors

  • Jean-Claude Do-Régo,

  • Hervé Hue,

    1. Laboratoire de Mathématiques, U.F.R. de Médecine et Pharmacie, Saint Etienne du Rouvray, France
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  • Jean Costentin,

  • Jean-Jacques Bonnet


  • Abbreviations used : DA, dopamine ; DAT, dopamine transporter ; GBR 12783, 1-[2-(diphenylmethoxy)ethyl]-4-(3-phenyl-2-propenyl)-piperazine ; KRM, Krebs-Ringer medium ; TI, transporter-inhibitor complex.

Address correspondence and reprint requests to Dr. J.-J. Bonnet at UPRESA C.N.R.S. 6036, U.F.R. de Médecine et Pharmacie, BP 97, 76803, Saint Etienne du Rouvray, France.

Abstract

Abstract : Incubation of a crude synaptosomal fraction from rat striatum with GBR 12783 at 37°C produced an inhibition of the specific uptake of [3H]dopamine that increased with time. The inhibition increased when GBR 12783 was present during preincubation and incubation (IC50 = 1.85 ± 0.1 nM) instead of incubation alone (IC50 = 25 ± 3.5 nM). Time-course studies of uptake inhibition demonstrated that a first collision transporter-inhibitor complex (TI) was formed immediately after addition of GBR 12783 so that the initial uptake velocity (Vo) decreased for increasing concentrations of inhibitor (Ki≥ 20 nM). TI slowly isomerized to a more stable complex TI* (K*i≤ 5 nM) with a value of t1/2 = 20-270 s. Fits of data to model 2 in which the steady-state uptake (VS) is set to zero were generally preferred, suggesting that formation of TI* could tend to irreversibility, as a consequence of a very low reverse isomerization. As expected, k, Vo, and VS tended to steady-state values in an asymptotic manner for high concentrations of GBR 12783. GBR 12783 at 2.5 nM produced a mixed inhibition of the uptake, with an increase in KM and a decrease in Vmax ; these effects were improved for 10 nM GBR 12783 and at 20°C. These results are discussed in relation to previous data concerning [3H]GBR 12783 binding. The present work gives the first experimental demonstration that dopamine uptake blockers can act according to a two-step mechanism of inhibition ; this is of great interest, because these inhibitors can oppose the effects of cocaine or amphetamine on the transporter according to a reaction that is partly nondependent on the concentration of the abused agent.

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