Figure 1.

Models of MOR signaling and regulation. The top model presents a simplistic linear progression of events wherein the agonist binds to the receptor with induces the activation of the heterotrimeric G protein and promotes dissociation of the G α protein from the βγ subunits. The receptor is then phosphorylated by a GPCR kinase (GRK) which then leads to the interactions with βarrestin proteins which prevents further interactions with the G protein. While these events can happen in this order, the simple model does not account for the interplay of variables that may differ based on where the receptor is expressed. The bottom figure represents the degree of complexity that GPCR activation may entail. All of these signaling events do not happen for all receptors in all locations, however, the diagram is meant emphasize the complexity of the system and the potential contributions that multiple effectors and regulators may have on the system. A few examples, but not comprehensive list of GPCR effectors have been included as examples. Each of these scenarios have the potential to influence the outcome of ligand-receptor-effector interactions and signaling events.