Characteristics of stably expressed human dopamine D 1a and D 1b receptors: Atypical behavior of the dopamine D 1b receptor

Abstract

Human dopamine D 1a and D 1b receptors were stably expressed in Baby Hamster Kidney (BHK) or Chinese Hamster Ovary (CHO) cells. [ 3H]SCH23390 saturation experiments indicated the presence of only a single binding site in the D 1a expressing cell line with a K d of 0.5 nM. In D 1b expressing cell lines, two binding sites were observed with K d values of 0.5 and 5 nM in CHO cells and 0.05 and 1.6 nM in BHK cells, respectively. Neither of the receptors affected Ca 2+ metabolism whereas they both were coupled in a stimulatory fashion to adenylyl cyclase. The pharmacological profile of both the D 1a and D 1b receptors as assessed from inhibition of specific [ 3H]SCH 23390 binding was classical D 1-like. Thus, benzazepine derivatives as well as the atypical neuroleptics, clozapine and fluperlapine, exhibited high affinity whereas D 2 selective compounds like sulpiride and spiperone had low affinity for these receptors. Besides SCH 23390, only NNC 112, fluphenazine and bulbocapnine were able to discriminate between the two states of the D 1b receptor. In case of the D 1a receptor, the K i values obtained in binding experiments were very similar to K i values obtained from inhibition of dopamine stimulated adenylyl cyclase. In the D 1b expressing cell line, the K i values obtained from inhibition of the dopamine stimulated adenylyl cyclase indicated a significantly better correlation with the state of the D 1b receptor showing high affinity for antagonists. In agreement with observations from binding experiments, dopamine was around 20 fold more potent in stimulating adenylyl cyclase via the D 1b receptor as compared to the D 1a receptor. Further, adenylyl cyclase in the D 1a expressing cell line was stimulated by the benzazepine agonists SKF 75670 and SKF 38393 with potencies and efficacies very similar to these previously observed in rat striatum. On the other hand, in the D 1b expressing cell line these benzazepine agonists exhibited potencies and efficacies different from previously obtained data from rat brain. In summary, both in their pharmacology and functional coupling, the D 1a and the D 1b receptors appear to exhibit a classical D 1-like profile with the markedly high affinity for the atypical neuroleptic clozapine and a stimulatory coupling to adenylyl cyclase. The somewhat better correlation between D 1a data than D 1b data and rat striatal D 1 receptor data supports findings from in situ hybridization studies on a relatively higher abundance of the D 1a receptor in this brain region. Thus, the D 1a and the D 1b receptors are classical D 1-like. However, the lack of coupling to phospholipase C of these two receptors may point to still another D 1-like receptor to be identified.