Dopamine Receptor Agonists: Selectivity and Dopamine D1 Receptor Efficacy

Dopamine receptor selectivity was investigated for a number of dopamine receptor agonists. In vitro, the benzazepine derivatives, e.g., SKF 38393 and SKF 75670 as well as the isoquinoline derivatives, SKF 89626 and SKF 89615, were D1 receptor-selective. All other compounds like apomorphine, CY 208-243, 6,7-ADTN and 3-PPP were either D2-selective or did not discriminate between subtypes. In general, the same receptor profile seen in vitro was observed in vivo. The exceptions to this pattern were: compounds which did not cross the blood-brain barrier, like 6,7-ADTN and SKF 89626, and compounds which appeared nonselective in vitro but demonstrated D2 selectivity in vivo like apomorphine, CI 201-678 and CY 208-243. A number of compounds were characterized in detail with respect to a GTP-induced affinity shift in inhibition of [3H]SCH 23390 binding, and potency and efficacy in stimulating adenylate cyclase from rat striatum. Inhibition of specific [3H]SCH 23390 binding by these agonists in the absence of GTP occurred with Hill slopes below unity and could best be explained by a two-site model with a high (KH)- and low-affinity (KL) component. Inhibition of [3H]SCH 23390 binding in the presence of 15 microM GTP occurred with Hill slopes of unity. The KI values obtained in the presence of 15 microM GTP were similar to the KL values, the low-affinity component observed in the absence of GTP. The capability of the agonists to stimulate the adenylate cyclase was analyzed in relation to dopamine (efficacy = 100%). The efficacy of the benzazepine derivatives varied from 24 (SKF 75670) to 100% (SKF 83189), dependent on the substituents on the benzazepine core. The isoquinolines, SKF 89626 and SKF 89615 had full efficacy, whereas most other agonists tested appeared to have only partial efficacy. In summary, the present paper presents data on dopamine receptor selectivity and efficacy in stimulating adenylate cyclase for a number of dopaminergic agonists. These data may create a basis for selection of agonists in future characterizations of dopaminergic-mediated events.