CHARACTERISTICS OF STABLY EXPRESSED HUMAN DOPAMINE-D(1A) AND DOPAMINE-D(1B) RECEPTORS - ATYPICAL BEHAVIOR OF THE DOPAMINE-D(1B) RECEPTOR

Human dopamine D1a and D1b receptors were stably expressed in Baby Ham ster Kidney (BHK) or Chinese Hamster Ovary (CHO) cells. [H-3]SCH23390 saturation experiments indicated the presence of only a single binding site in the D1a expressing cell line with a K(d) of 0.5 nM. In D1b ex pressing 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, respect ively. Neither of the receptors affected Ca2+ metabolism whereas they both were coupled in a stimulatory fashion to adenylyl cyclase. The ph armacological profile of both the D1a and D1b receptors as assessed fr om inhibition of specific [H-3]SCH 23390 binding was classical D1-like . Thus, benzazepine derivatives as well as the atypical neuroleptics, clozapine and fluperlapine, exhibited high affinity whereas D2 selecti ve compounds like sulpiride and spiperone had low affinity for these r eceptors. Besides SCH 23390, only NNC 112, fluphenazine and bulbocapni ne were able to discriminate between the two states of the D1b recepto r. In case of the D1a receptor, the K(i) values obtained in binding ex periments were very similar to K(i) values obtained from inhibition of dopamine stimulated adenylyl cyclase. In the D1b 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 D1b receptor showing high affinity for antagonists. In a greement with observations from binding experiments, dopamine was arou nd 20 fold more potent in stimulating adenylyl cyclase via the D1b rec eptor as compared to the D1a receptor. Further,adenylyl cyclase in the D1a expressing cell line was stimulated by the benzazepine agonists S KF 75670 and SKF 38393 with potencies and efficacies very similar to t hese previously observed in rat striatum. On the other hand, in the D1 b 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 D1b receptors appear to exhibit a classical D1-like profile with the markedly high affinity for the atypical neuroleptic clozapin e and a stimulatory coupling to adenylyl cyclase. The somewhat better correlation between D1a data than D1b data and rat striatal D1 recepto r data supports findings from in situ hybridization studies on a relat ively higher abundance of the D1a receptor in this brain region. Thus, the D1a and the D1b receptors are classical D1-like. However, the lac k of coupling to phospholipase C of these two receptors may point to s till another D1-like receptor to be identified.