Interactions of agonists with D-2 dopamine receptors: Evidence for a single receptor population existing in multiple agonist affinity-states in rat striatal membranes

Abstract

Several lines of evidence previously indicated that [ 3H]spiroperidol (SPIRO) or [ 3H]domperidone (DOMP) might label heterogenous populations of striatal dopamine receptors in radioligand binding studies. We have examined this possibility in rat striatum using computerized non-linear curve fitting and a method to block the unwanted [ 3H]SPIRO binding to S-2 serotonergic receptors. In the absence of a serotonergic antagonist, [ 3H]SPIRO saturation data produce curved Scatchard plots which are best computer fit by assuming the presence of two sites of different [ 3H]SPIRO affinities. In the presence of appropriate concentrations of ketanserin to block S-2 serotonergic binding, Scatchard plots are linear, with data modeling best to a single population of homogenous binding sites. The D-2 dopamine receptor B max and [ 3H]SPIRO K D determined in this fashion are indistinguishable from that obtained for the higher affinity binding site by computer analysis of data obtained in the absence of ketanserin. [ 3H]DOMP produced indistinguishable values for D-2 receptor B max as well. Competitions by (-)sulpiride, metoclopramide, and DOMP for [ 3H]SPIRO binding sites in the presence of ketanserin are steep (Hill slope approximately 1), demonstrating that the previously observed heterogeneity of these sites is due entirely to serotonergic [ 3H]SPIRO binding. In contrast, agonist/ 3H-antagonist competition curves in the presence of ketanserin are best computer fit by assuming two independent receptor sites of high ( R H ) and low ( R L ) agonist affinity. With the addition of 5′-guanylylimidodiphosphate [Gpp(NH)p] computer analyses of agonist/ 3H-antagonist competition curves show an increased ratio R L R H concomitant with apparent decreases in agonist affinities for both sites. Under some conditions, some agonist/ 3H-antagonist competition curves are best fit by a single site model in which agonist affinity is indistinguishable from the agonist's affinity at R L determined in the absence of Gpp(NH)p. These data are consistent with the presence of a single dopaminergic 3H-butyrophenone binding site, representing the D-2 receptor, which exists in two interconverting states differing in agonist affinity.