SPECIFIC BINDING OF [H-3] GBR-12783 TO THE DOPAMINE NEURONAL CARRIER INCLUDED IN POLARIZED MEMBRANES

We have compared the properties of the binding to the neuronal dopamine carrier located either in polarized membranes of synaptosomes or in non polarized, classical membranes. Non-polarized membranes were prepared by sonication of the partially purified synaptosomal fraction obtained from rat striatum which was used as the source of polarized membranes. Binding experiments were carried out at 37-degrees-C in Krebs Ringer related media. [H-3]GBR 12783 (1-[2-(diphenylmethoxy)ethyl]4-(3-phenyl-2-[1-H-3]propenyl)piperazine) specifically bound with a nanomolar affinity to a homogeneous population of site (maximal binding site concentration: 8-10 pmol/mg protein). Pure uptake inhibitors, but not substrates, competed for the [H-3]GBR 12783 binding site located in polarized membranes of synaptosomes at concentrations effective against dopamine neuronal transport. Except for [H-3]GBR 12783, the replacement of Cl- by isethionate- did not result in significant change in the ability of pure uptake inhibitors to compete for the specific binding site. A reduction in the Na+ concentration from 135 to 10 mM induced a significant decrease in the inhibitory potency of GBR 12783, mazindol, nomifensine and methylphenidate. This decrease was likely to result from the presence of K+, Mg2+ and Ca2+, whose inhibitory effects were modified and/or increased by decreasing the Na+ concentration. These data indicate that the membrane polarity is not clearly involved in the binding of pure uptake inhibitors to the dopamine neuronal carrier; furthermore they underline the critical role of Na+ and K+ transmembrane gradients in both the recognition of the carrier by dopamine and its inward transport.