Functional Selectivity at Cannabinoid Receptors

It is now clear that, in contrast to traditional descriptions of G protein-coupled receptor signaling, agonists can activate or inhibit characteristic patterns of downstream effector pathways depending on their structures and the conformational changes induced in the receptor. This is referred to as functional selectivity (also known as agonist-directed trafficking, ligand-induced differential signaling, or biased agonism). It is important because even small structural differences can result in significant variations in overall agonist effects (wanted and unwanted) depending on which postreceptor signaling systems are engaged by each agonist/receptor pairing. In addition to the canonical signaling pathways mediated by G_i/o proteins, CB₁ and CB₂ receptor agonists can have effects via differential activation not only of G_i subtypes but also of G_s and G_q/11 proteins. For example, the classical cannabinoid HU-210 produces maximal activation of both G_i and G_o proteins, while the endocannabinoid anandamide and aminoalkylindole WIN 55,212 both produce maximal activation of G_i, but submaximal activation of G_o. Cannabinoid agonists can also signal differentially via β-arrestins coupled to mitogen-activated protein kinases, subsequently promoting varying degrees of receptor internalization and agonist desensitization. A recent extensive characterization of the molecular pharmacology of CB₂ agonists (Soethoudt et al., 2017) identified marked differences (bias) in the ability of certain agonists to activate distinct signaling pathways (cAMP accumulation, ERK phosphorylation, GIRK activation, GTPγS binding, and β-arrestin recruitment) and to cause off-target effects, exemplifying the need to evaluate functional selectivity in agonist drug development.