3H]GBR-12935 binding to dopamine uptake sites in the human brain

Abstract

The binding of the selective dopamine uptake inhibitor [ 3H]GBR-12935 to post-mortem human brain membranes was studied. Competition experiments indicated the presence of multiple binding sites, but when a binding fraction that could be discriminated by either 0.3 μM mazindol or 1 mM dopamine was regarded as specific binding, a single-site binding model was obtained. The [ 3H]GBR-12935 binding was of protein nature since it was abolished after protease treatment and the binding appeared to label the dopamine uptake site. This was supported by the findings that dopamine uptake inhibitors inhibited the binding with high affinity (K i 30–130 nM), whereas substances active at dopamine D 1, D 2 or autoreceptor sites revealed much lower affinities (K i > 10 μM, or inactive). Moreover, dopamine was the neurotransmitter with the highest affinity for the [ 3H]GBR-12935 binding site (K i 30 μM). The dopaminergic nature of the [ 3H]GBR-12935 binding was further indicated by its regional distribution, which largely corresponds the known distribution of the dopamine system in the rat brain. The highest binding densities were obtained in the caudate nucleus and putamen (B max 1500–2000 fmol/mg protenin), followed by the olfactory tubercle (B max 900 fmol/mg protein) and the substantia nigra (B max 300 fmol/mg protein). The apparent binding affinity (K d) was the same in all brain regions (K d 1–1.5 nM). Detectable specific [ 3H]GBR-12395 binding was obtained also in the globus pallidus, amygdala and cortices of orbital/rectus and cingulate gyri. Drug inhibition studies with the addition of low concentrations of dopamine and mazindol produced only alterations in the apparentK d values, suggesting a competitive inhibition. It is concluded that the [ 3H]-GBR-12935 binding discriminated by 0.3 μM mazindol in the human brain reflects binding to the substrte recognition site for dopamine uptake.