ABSTRACT
Several non-peptidic opioids have been synthesized recently as part of a program to develop selective delta receptor agonists. In this study, the affinities of a set of compounds for cloned delta and mu opioid receptors expressed in HEK 293 cell lines were determined by competition analysis of [3H]bremazocine binding to membrane preparations. All compounds studied exhibited high affinity and selectivity, with apparent dissociation constants in the range of 0.6-1.7 nM for the delta opioid receptor and 240-1165 nM for the mu opioid receptor. We next sought to determine which domain of the delta receptor was critical for mediating the highly selective binding by analysis of ligand affinities for mu/delta receptor chimeras. Receptor binding profiles suggested that a critical site of receptor/ligand interaction was located between transmembrane domain 5 (TM5) and TM7 of the delta receptor. Substitution of tryptophan 284, located at the extracellular surface of TM6, with lysine, which is found at the equivalent position in the mu opioid receptor, led to a spectrum of effects on affinities, depending on the ligand tested. Affinities of SB 219825 and SB 222941 were particularly sensitive to the substitution, displaying a 50-fold and 70-fold decrease in affinity, respectively. Activities of the delta receptor-selective agonists were tested in two functional assays. Brief exposure of HEK 293 cells expressing delta opioid receptors with selective ligands induced phosphorylation of MAP kinase, although the non-peptidic ligands were less efficacious than the enkephalin derivative DADL (Tyr-D-Ala-Gly-Phe-D-Leu). Similarly, chronic exposure of HEK 293 cells expressing delta opioid receptors with selective, non-peptidic ligands, with the exception of SB 206848, caused receptor down-regulation, however, the SB compounds were less efficacious than DADL.
Subject(s)
Receptors, Opioid, delta , Amino Acid Sequence , Analgesics/metabolism , Analgesics/pharmacology , Analgesics, Opioid/metabolism , Analgesics, Opioid/pharmacology , Benzomorphans/metabolism , Benzomorphans/pharmacology , Binding, Competitive , Cells, Cultured , Cloning, Molecular , Down-Regulation/drug effects , Down-Regulation/physiology , Enkephalin, Leucine-2-Alanine/pharmacology , GTP-Binding Proteins/metabolism , Heterocyclic Compounds, 4 or More Rings/chemistry , Heterocyclic Compounds, 4 or More Rings/pharmacology , Humans , Indoles/chemistry , Indoles/pharmacology , Isoquinolines/chemistry , Isoquinolines/pharmacology , Kidney/cytology , Ligands , Mitogen-Activated Protein Kinases/metabolism , Molecular Sequence Data , Morphine/metabolism , Morphine/pharmacology , Mutagenesis, Site-Directed , Naloxone/pharmacology , Narcotic Antagonists/pharmacology , Quinolines/chemistry , Quinolines/metabolism , Quinolines/pharmacology , Radioligand Assay , Receptors, Opioid, delta/agonists , Receptors, Opioid, delta/antagonists & inhibitors , Receptors, Opioid, delta/genetics , Receptors, Opioid, mu/agonists , Receptors, Opioid, mu/antagonists & inhibitors , Receptors, Opioid, mu/genetics , TritiumABSTRACT
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.
