Autoradiographic identification of D 1 dopamine receptors labelled with [ 3H]dopamine: Distribution, regulation and relationship to coupling

Abstract

On the basis of experiments made on striatal membranes, Leff and Creese [ Molec. Pharmac. (1985) 27, 184–192] have proposed that tritiated dopamine binds to a high-affinity agonist state of D 1 dopamine receptors (D 1h) which adopt this conformation when they are associated with the GTP-binding protein involved in the transduction process. Quantitative autoradiography was thus used to look for the distribution of these D 1h sites in the rat brain and to compare it with that of D 1 receptors labelled with [ 3H]7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine ([ 3H]SCH23390), a D 1 antagonist. The effects of unilateral 6-hydroxydopamine lesion of the ascending dopamine pathways on the density of [ 3H]dopamine D 1h and [ 3H]SCH23390 binding sites in the striatum and the nucleus accumbens were also analysed. In the striatum, when D 2 receptors were blocked by spiroperidol (20 nM), [ 3H]dopamine was found to bind specifically to dopamine receptors of the D 1 type. Complementary experiments made with dopamine uptake blockers indicated that high-affinity dopamine uptake sites were not labelled by [ 3H]dopamine under our experimental conditions. The anatomical distribution of [ 3H]dopamine D 1h binding sites was found to be markedly different from that of [ 3H]SCH23390 binding sites. This was particularly the case in the substantia nigra, some amygdaloid nuclei and the prefrontal cortex—structures in which the ratios between [ 3H]SCH23390 and [ 3H]dopamine binding sites were more than seven-fold higher than that observed in the striatum. [ 3H]SCH23390 binding was not significantly affected in either the striatum or the nucleus accumbens six weeks after a complete unilateral destruction of ascending dopamine pathways. In contrast, a marked decrease in [ 3H]dopamine D 1h binding sites was found in both structures, but this effect was lower in the medioventral (−60%) than in the laterodorsal (− 81%) part of the striatum, even though dopamine denervation was uniform throughout the structure. Preincubation of the sections with dopamine (0.5 μ M) led to a partial recovery (+ 126%) in the lesioned striatum and an increase of [ 3H]dopamine labelling in the control striatum (+68%). This suggests that the presence of dopamine stabilizes the D 1h state of D 1 receptors. The absence or low amounts of dopamine, either due to dopamine denervation or naturally occurring (prefrontal cortex), would then impair the [ 3H]dopamine D 1h binding. In addition, a lower coupling of D 1 receptors with adenylate cyclase was observed in the substantia nigra when compared to that in the striatum: this may explain the relatively weak [ 3H]dopamine binding in the substantia nigra. Similarly, the lower loss of [ 3H]dopamine binding sites seen specifically in the medioventral part of the striatum in the 6-hydroxydopamine-lesioned rats could be related to a supersensitive dopamine-stimulated adenylate cyclase activity found in this striatal area [Herve´ et al. (1989) J. Neurosci. 9, 3699–3707], In conclusion, the D 1h state of the D 1 receptors is dependent on the endogenous dopamine levels and on the intensity of the D 1 receptor coupling. In order to explain both influences, it is proposed that the D 1h binding sites observed in brain sections are generated by the dissociation of a ternary complex in which dopamine, D 1 receptor and GTP-binding protein are associated.