The opioid peptide family consists principally of endorphins, enkephalins, and dynorphins, which serve as endogenous ligands for mu, delta, and kappa opioid receptors. Dynorphins, which bind with greatest affinity to kappa opioid receptors, are derived from the precursor protein prodynorphin, which can be cleaved to form numerous active peptides including Dynorphin A (DYN-A1-8, DYN-A1-13, and DYN-A1-17), Dynorphin B (DYN-B), Big Dynorphin (a 32-amino acid peptide comprised of both DYN-A and DYN-B), and α- and β-neoendorphins (Chavkin, 2013; Schwarzer, 2009). Kappa opioid receptors (KORs) are G protein-coupled receptors with seven transmembrane domains that, when activated, inhibit adenylyl cyclase and act through a variety of signaling cascades (Bruchas and Chavkin, 2010).
