Guanine nucleotide regulation of agonist interactions at [ 3H]SCH23390-labeled D 1 dopamine receptors in rat striatum

Abstract

We report here the regulation of agonist interactions with [ 3H]SCH23390-labeled D 1 dopamine receptors in rat striatum. Scatchard analyses of [ 3H]SCH23390 saturation data revealed a single high affinity binding site (K D = 0.49 nM) with a B max of 64 pmol/g tissue. The specific binding of 0.25 nM [ 3H]SCH23390 represented 90% of total binding. Antagonist competition for [ 3H]SCH23390 binding was monophasic (i.e. pseudo-Hill slope ∼ 1) and the rank order of antagonists' affinities was consistent with the pharmacology of D 1 dopamine receptors (e.g. cis-flupentixol > haloperidol > spiperone). In contrast, agonist competition curves were shallow (pseudo-Hill slope < 1) and computer-assisted analysis indicated that, for all agonist, the data best fit a two-site model composed of a high (K H) and a low (K L) affinity component. In the presence of 0.3 mM GTP, the high affinity binding component (%R H) of various agonists was reduced by approximately 50%. No significant effect of 0.3 mM GTP on [ 3H]SCH23390 binding was observed. Additionally, it was noted that [ 3H]SCH23390 labels S 2 serotonin receptors in extrastriatal brain regions. However, [ 3H]SCH23390 apparently does not have an affinity high enough to label S 2 receptors at the concentration of [ 3H]SCH23390 employed in labeling striatal D 1 dopamine receptors. These data indicate that [ 3H]SCH23390 represents a superior radioligand for labeling the two-state striatal D 1 dopamine receptor in that its high percent specific binding makes it especially suitable for detailed mechanistic studies of this receptor.