Structure and regulation of opioid receptors.

Significant advances have been made in understanding the structure, function, and regulation of opioid receptors and endogenous opioid peptides since their discovery approximately 25 years ago. This review summarizes recent studies aimed at identifying key amino acids that confer ligand selectivity to the opioid receptors and that are critical constituents of the ligand binding sites. A molecular model of the delta receptor based on the crystal structure of rhodopsin is presented. Agonist-induced down regulation of opioid receptors is discussed, highlighting recent evidence for the involvement of the ubiquitin/proteasome system in this process.