Sodium dependent [3H]cocaine binding associated with dopamine uptake sites in the rat striatum and human putamen decrease after dopaminergic denervation and in Parkinsons disease

Abstract

The binding of radiolabelled cocaine, an inhibitor of dopamine uptake, to the post-mortem human putamen was studied and compared to that in the rat striatum. Saturation analysis of [³H]cocaine binding to the human putamen revealed the presence of a high affinity component of binding with a K _d of 0.21 μmol/l and a B _max of 1.47 pmol/mg protein. In addition a low affinity component (K _d=26.4 μmol/l) was demonstrated, having a B _max of 42.2 pmol/mg protein. Also in the rat striatum [³H]cocaine binding was both of high affinity (K _d=0.36 μmol/l, B _max=5.56 pmol/mg protein) and low affinity (K _d=25.9 μmol/l, B _max=35.6 pmol/mg protein). A pharmacological characterisation of high affinity [³H]cocaine binding to rat striatal membranes clearly indicates an association with the neuronal dopamine transporter. The IC₅₀ values of 8 selected drugs for inhibition of [³H]cocaine binding in the rat striatum were highly significantly correlated with their potency to inhibit [³H]dopamine uptake into slices of the rat striatum. [³H]Cocaine binding was stereospecifically inhibited by (+)nomifensine and (+)diclofensine which were 50–80-fold more active than their respective (-)isomers. Drugs with dopamine releasing activity were more potent at inhibiting [³H]dopamine uptake than at competing for the high affinity site of [³H]cocaine binding. A highly significant correlation was found between IC₅₀ values for [³H]cocaine binding in the rat striatum and the human putamen. Further evidence in support of an association of [³H]cocaine binding in the rat striatum with the dopamine transporter was obtained from lesion studies. Thus, intranigral 6-hydroxydopamine administration produced a marked (67%) decrease in striatal [³H]cocaine binding. Also in the human putamen high affinity [³H]cocaine binding sites appear localized on dopaminergic nerve terminals as evidenced by a prominent decrease in binding in the putamen obtained from subjects with Parkinsons disease. It is concluded that [³H]cocaine may be a useful ligand to examine the dopamine transporter in the rat striatum and the human putamen. Therefore it offers a new and valuable approach in the study of drug effects and neuropsychiatric diseases.