Elucidating agonist-selective mechanisms of G protein-coupled receptor desensitization

In pharmacology, a central tenet of receptor theory has been that different agonists acting at a particular G protein-coupled receptor subtype produce the same profile of cellular responses. In recent years, advances in molecular pharmacology and the availability of diverse cell signaling assays have indicated that this idea is not sufficient to explain all the data obtained, and that agonists can produce different response profiles-following binding to a receptor subtype in a cell. This has been termed biased agonism or functional selectivity-, and is thought to be due to the ability of agonists to stabilize different active conformations of the receptor. Logically, there is no reason why this idea cannot also be extended to receptor regulatory mechanisms, since different receptor conformations could exhibit differential affinities for regulatory elements such as the kinases involved in receptor phosphorylation and desensitization. Nevertheless, great care must be taken when analyzing agonist response and regulatory pathways, since other factors such as differences in agonist efficacy need to be considered as contributing factors to agonist-dependent regulation. In the case of the μ-opioid receptor (MOPr), we have shown that two agonists, morphine and the peptide agonist DAMGO, can induce MOPr desensitization by different mechanisms involving largely protein kinase C (PKC) and G protein-coupled receptor kinase/arrestin respectively. This could explain why opioid agonists have variable clinical profiles and liabilities to induce tolerance and dependence. Here we describe the experimental approaches that can be used to investigate mechanisms of MOPr desensitization with a particular focus on endogenous MOPr in neurons. In addition, we discuss the role that agonist efficacy might play in desensitization and describe methods to estimate agonist efficacy for responses downstream of receptor activation, including arrestin recruitment which can be regarded as both a regulatory and a signaling mechanism.