Agonist-, antagonist-, and inverse agonist-regulated trafficking of the delta-opioid receptor correlates with, but does not require, G protein activation

In this study, we explored the relationship between ligand-induced regulation of surface delta opioid receptors and G protein activation. G protein activation was assessed with (35)S]guanosine-5'-O-(3-thio)triphosphate (GTP gamma S) binding assays conducted at both 37 and 0 degrees C. Ligand-independent (constitutive) activity of the delta-receptor was readily observed when the (35)S]GTP gamma S binding assay was performed at 37 degrees 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 (35)S]GTP gamma S binding at 37 degrees C showed a strong correlation with their abilities to regulate surface receptor number (r(2) = 0.86). Interestingly, the ability of fentanyl to activate G proteins was markedly temperature sensitive. Fentanyl showed no stimulation of (35)S]GTP gamma S binding at 0 degrees C but was as efficacious as etorphine, morphine, and diprenorphine at 37 degrees 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 delta-opioid receptor trafficking.