Functional selectivity of dopamine D1 receptor agonists in regulating the fate of internalized receptors

Recently, we demonstrated that D(1) agonists can cause functionally selective effects when the endpoints of receptor internalization and adenylate cyclase activation are compared. The present study was designed to probe the phenomenon of functional selectivity at the D(1) receptor further by testing the hypothesis that structurally dissimilar agonists with efficacies at these endpoints that equal or exceed those of dopamine would differ in ability to influence receptor fate after internalization, a functional endpoint largely unexplored for the D(1) receptor. We selected two novel agonists of therapeutic interest that meet these criteria (the isochroman A-77636, and the isoquinoline dinapsoline), and compared the fates of the D(1) receptor after internalization in response to these two compounds with that of dopamine. We found that dopamine caused the receptor to be rapidly recycled to the cell surface within 1h of removal. Conversely, A-77636 caused the receptor to be retained intracellularly up to 48 h after agonist removal. Most surprisingly, the D(1) receptor recovered to the cell surface 48 h after removal of dinapsoline. Taken together, these data indicate that these agonists target the D(1) receptor to different intracellular trafficking pathways, demonstrating that the phenomenon of functional selectivity at the D(1) receptor is operative for cellular events that are temporally downstream of immediate receptor activation. We hypothesize that these differential effects result from interactions of the synthetic ligands with aspects of the D(1) receptor that are distal from the ligand binding domain.