Biochemical and Pharmacological Characterization of [3H]GBR 12935 Binding in Vitro to Rat Striatal Membranes: Labeling of the Dopamine Uptake Complex

Binding of the selective dopamine (DA) uptake inhibitor [3H]GBR 12935 to rat striatal membranes was characterized biochemically and pharmacologically. [3H]GBR 12935 binding at 0 degree C was reversible and saturable and Scatchard analysis indicated a single binding site with a KD of 5.5 nM and a Bmax of 760 pmol/mg tissue. [3H]GBR 12935 labeled two binding sites. One binding site was identified as the classic DA uptake site, since methylphenidate, cocaine, diclofensine, and Lu 19-005 potently inhibited [3H]GBR 12935 binding to it. Binding to the second site was inhibited by high concentrations of the above compounds. IC50 values for inhibition of [3H]GBR 12935 binding to the DA uptake site were proportional to IC50 values for inhibition of DA uptake. However, substrates of DA uptake, e.g., DA and 1-methyl-4-phenylpyridine, and DA releasers, e.g., the amphetamines, inhibited [3H]GBR 12935 binding less than DA uptake. Rate experiments excluded the possibility that these "weak" inhibitors affected the binding by allosteric coupled binding sites. The second binding site was not a noradrenergic, serotonergic, or GABAergic uptake site. Neither was it a dopaminergic, acetylcholinergic, histaminic, serotonergic, or adrenergic receptor. However, [3H]GBR 12935 was potently displaced from it by disubstituted piperazine derivatives, i.e., flupentixol and piflutixol. DA uptake and the DA uptake binding site of [3H]GBR 12935 were located primarily in the striatum, but the piperazine acceptor site was distributed uniformly throughout the brain. Also only the DA uptake binding site was destroyed by 6-OH-DA. Thus, [3H]GBR 12935 labels the classic DA uptake site in rat striatum and also a piperazine acceptor site. Substrates for DA uptake and releasers of DA inhibited [3H]GBR 12935 binding with low potency, but did not alter the rate constants for [3H]GBR 12935 binding. Therefore inhibitors of DA uptake label the carrier site and prevent the carrier process.