Signaling Properties of Structurally Diverse Kappa Opioid Receptor Ligands: Toward in Vitro Models of in Vivo Responses

Biased ligands preferentially activate certain signaling pathways downstream of their target receptor, leading to differential physiological or behavioral responses downstream. The kappa opioid receptor (KOR) is a drug target for diseases involving mood and reward, such as depression and addiction. Biased KOR ligands offer the potential to overcome negative side effects that have previously hampered the therapeutic development of KOR agonists by preferentially activating certain signaling pathways. Understanding relationships between ligand bias and behavior is difficult, however, because differences in cellular context and bias quantification methods lead to variation between studies. Here, a set of 21 structurally diverse KOR ligands were tested in parallel, to systematically quantify ligand bias at the KOR. Compounds included the endogenous peptide ligand Dynorphin A(1–17), two novel compounds synthesized for our research, and 18 additional compounds of different structural classes, including morphinans and the natural product Salvinorin A. Compounds were tested for their activity in early KOR signaling pathways (G-protein and β-arrestin recruitment) in KOR-expressing U2OS cells, and ligand bias was calculated. A subset of compounds was tested for sedative properties in the rotarod assay in mice. We found that rotarod sedation significantly correlated with β-arrestin signaling in this system, indicating that this in vitro system can be used to accurately describe this in vivo behavior caused by KOR agonists. Additionally, downstream signaling pathways ERK1/2 and mTOR were evaluated, and we determined that signaling via both of these pathways could diverge from KOR-mediated G-protein and arrestin signaling in this system.