Functionally Selective Dopamine D1 Receptor Endocytosis and Signaling by Catechol and Noncatechol 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 noncatechol D1R agonists were recently discovered and unexpectedly many of these ligands showed G protein biased signaling. Here, we investigate both catechol and noncatechol D1R agonists to validate potential biased signaling and examine if this impacts agonist-induced D1R endocytosis. We determined that most, but not all, noncatechol agonists display G protein biased signaling at the D1R and have reduced or absent β-arrestin2 recruitment. A notable exception was compound (Cmpd) 19, a noncatechol agonist with full efficacy at both D1R-G protein and D1R-β-arrestin2 pathways. In addition, the catechol ligand A-77636 was a highly potent, super agonist for D1R-β-arrestin2 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 β-arrestin2 super agonist, A-77636, showed statistically significant increases in D1R endocytosis. Moreover, β-arrestin2 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.