@article {Zaki1015, author = {Paulette A. Zaki and Duane E. Keith, Jr. and James B. Thomas and F. I. Carroll and Christopher J. Evans}, title = {Agonist-, Antagonist-, and Inverse Agonist-Regulated Trafficking of the δ-Opioid Receptor Correlates with, but Does Not Require, G Protein Activation}, volume = {298}, number = {3}, pages = {1015--1020}, year = {2001}, publisher = {American Society for Pharmacology and Experimental Therapeutics}, abstract = {In this study, we explored the relationship between ligand-induced regulation of surface δ opioid receptors and G protein activation. G protein activation was assessed with [35S]guanosine-5'-O-(3-thio)triphosphate (GTPγS) binding assays conducted at both 37 and 0{\textdegree}C. Ligand-independent (constitutive) activity of the δ-receptor was readily observed when the [35S]GTPγS binding assay was performed at 37{\textdegree}C. We identified a new class of alkaloid inverse agonists (RTI-5989-1, RTI-5989-23, RTI-5989-25), which are more potent than the previously described peptide inverse agonist ICI-174864 (N,N-diallyl-Tyr-Aib-Aib-Phe-Leu). Treatment with these inverse agonists for 18 h caused up-regulation of surface receptors. Eighteen-hour treatment with etorphine resulted in approximately 90\% loss of surface receptor, whereas fentanyl, diprenorphine, and morphine caused between 20 and 50\% loss. The abilities of ligands to modulate [35S]GTPγS binding at 37{\textdegree}C showed a strong correlation with their abilities to regulate surface receptor number (r2 = 0.86). Interestingly, the ability of fentanyl to activate G proteins was markedly temperature sensitive. Fentanyl showed no stimulation of [35S]GTPγS binding at 0{\textdegree}C but was as efficacious as etorphine, morphine, and diprenorphine at 37{\textdegree}C. Neither the ligand-induced receptor increases nor decreases were perturbed by pertussis toxin pretreatment, suggesting that functional G proteins are not required for ligand-regulated δ-opioid receptor trafficking. The American Society for Pharmacology and Experimental Therapeutics}, issn = {0022-3565}, URL = {https://jpet.aspetjournals.org/content/298/3/1015}, eprint = {https://jpet.aspetjournals.org/content/298/3/1015.full.pdf}, journal = {Journal of Pharmacology and Experimental Therapeutics} }