PERSISTENT ACTIVATION OF THE DOPAMINE D-1 RECEPTOR CONTRIBUTES TO PROLONGED RECEPTOR DESENSITIZATION - STUDIES WITH A-77636

A-77636 is a dopamine (DA) D-1 receptor-selective agonist that was pre viously shown to elicit beneficial responses in animal models of Parki nson's disease (PD) (Kebabian et al.: fur. J. Pharmacol. 229: 203, 199 2). However, A-77636 is of limited potential for PD therapy because it induces rapid tolerance in vivo. To understand the basis of rapid ons et of tolerance to the compound, we conducted studies to compare the i n vitro properties of A-77636 and A-81686; the latter is a structurall y related D-1 agonist that did not induce significant tolerance in viv o under similar experimental conditions. With SK-N-MC, a neuroblastoma cell line, as an in vitro model for the D-1 receptor, significant dif ferences in D-1 receptor function were noted after pretreatment with t he two compounds. Specifically, I-hr pretreatment with A-77636 resulte d in significant residual cAMP production, even after the drug solutio n was removed and the cells were washed. The residual cAMP activity wa s selectively inhibited by SCH 23390, a selective D-1 antagonist. The residual cAMP activity declined with pretreatment time, and after 4-hr pretreatment, little residual cAMP production was observed. Cotreatme nt of SK-N-MC cells with SCH 23390 and A-77636 did not prevent residua l cAMP production by A-77636. In contrast, A-81686 did not elicit resi dual cAMP production in SK-N-MC cells. Although A-77636-treated cells were devoid of agonist response 4 hr after drug removal, A-81686-treat ed cells exhibited significant cAMP response after drug removal. Prein cubation of rat striatal membranes with A-77636 resulted in a large de crease in D-1 receptor binding, despite repeated washings, whereas A-8 1686 pretreatment caused only a small reduction in D-1 receptor bindin g. On the basis of the present data, we conclude that A-77636 dissocia tes slowly from the D-1 receptor. The continued activation of the D, r eceptor by A-77636 leads to inability of the receptor to recover its r esponsivity, which may explain its long duration of action and its abi lity to induce rapid behavioral tolerance in vivo.