Functionally selective dopamine D1 receptor endocytosis and signaling by catechol and non-catechol agonists

The dopamine D1 receptor (D1R) has fundamental roles in voluntary movement and memory and is a validated drug target for neurodegenerative and neuropsychiatric disorders. However, previously developed D1R selective agonists possess a catechol moiety which displays poor pharmacokinetic properties. The first selective non-catechol D1R agonists were recently discovered and unexpectedly many of these ligands showed G protein biased signaling. Here, we investigate both catechol and non-catechol D1R agonists to validate potential biased signaling and examine if this impacts agonist-induced D1R endocytosis. We determined that most, but not all, non-catechol agonists display G protein biased signaling at the D1R and have reduced or absent {beta}-arrestin recruitment. A notable exception was compound (Cmpd) 19, a non-catechol agonist with full efficacy at both D1R-G protein or D1R-{beta}-arrestin pathways. In addition, the catechol ligand A-77636 was a highly potent, super agonist for D1R-{beta}-arrestin activity. When examined for agonist-induced D1R endocytosis, balanced agonists SKF-81297 and Cmpd 19 induced robust D1R endocytosis while the G protein biased agonists did not. The {beta}-arrestin super agonist, A-77636, showed significantly increased D1R endocytosis. Moreover, {beta}-arrestin recruitment efficacy of tested agonists strongly correlated with total D1R endocytosis. Taken together, these results indicate the degree of D1R signaling functional selectivity profoundly impacts D1R endocytosis regardless of pharmacophore. The range of functional selectivity of these D1R agonists will provide valuable tools to further investigate D1R signaling, trafficking and therapeutic potential. Significance StatementThe D1R is a validated therapeutic target and the recently discovered non-catechol D1R agonists have translational potential. We have systematically characterized several structurally distinct D1R agonists including non-catechols with balanced or G protein biased activity. When examined for agonist-induced D1R endocytosis, balanced agonists induced robust D1R endocytosis while G protein biased agonists did not. These results indicate the degree of D1R signaling functional selectivity profoundly impacts receptor endocytosis. This work also independently validates agonist tools to further investigate D1R activation in basic and translational research.