Opioid and adrenergic receptor extracellular loops (ECLs)

The extracellular loops (ECLs) of opioid and adrenergic receptors are critical structural domains that function as allosteric binding sites, facilitating functional crosstalk between these two major G protein-coupled receptor (GPCR) families (Root-Bernstein, 2018). Research, particularly involving the mu-opioid receptor (MOR) and beta-2 adrenergic receptor (B2AR), has demonstrated that ligands from one system can bind to the ECLs of the other, modulating receptor affinity and signaling efficacy (NIH, 2018). For instance, adrenergic agonists like epinephrine and dopamine can bind to the ECLs of opioid receptors to enhance their analgesic effects, while opioids like morphine and naloxone can bind to adrenergic ECLs to influence cardiovascular and sympathetic responses (MDPI, 2018). These loops, especially ECL2, are essential for initial ligand recognition and the stabilization of active receptor conformations. Targeting these extracellular regions with synthetic peptides, known as pepducins, or small molecules represents a novel therapeutic approach to achieve biased signaling and improve the safety profile of drugs used in pain management and cardiovascular therapy (Medical News Today, 2023).

Other names

GPCR extracellular loopsExtracellular loop 1 (ECL1)Extracellular loop 2 (ECL2)Extracellular loop 3 (ECL3)Allosteric binding regions of opioid and adrenergic receptorsOpioid and adrenergic receptor extracellular loop peptides

Allosteric modulation of G protein-coupled receptor (GPCR) activity through binding to extracellular loop domains, which alters the affinity and efficacy of orthosteric ligands and facilitates receptor crosstalk.