Abstract

In this study, we explored the relationship between regulation of surface μ-opioid receptor number, ligand-induced G protein activation {measured by [³⁵S]guanosine-5′-O-(3-thio)triphosphate (GTPγS) binding} and second messenger signaling (measured by the inhibition of cAMP accumulation). Etorphine and two isomers ofcis-β-hydroxy-3-methylfentanyl (RTI-1a and RTI-1b), which were full agonists for G protein activation and signaling, caused approximately a 50% loss of surface receptors after 1 h of treatment. Fentanyl and morphine were full agonists for inhibiting cAMP accumulation and partial agonists for stimulating [³⁵S]GTPγS binding and internalization. Although both agonists were ∼80% as efficacious as etorphine in stimulating [³⁵S]GTPγS binding, fentanyl induced a 35% loss of surface receptors, whereas morphine only caused a 10% loss. Additionally, both long- and short-term treatment with the opioid antagonist naloxone caused increases in surface receptors. Unexpectedly, the weak partial agonists buprenorphine and one isomer ofcis-β-hydroxy-3-methylfentanyl (RTI-1d) also were found to cause an increase in surface receptors. Treatment with pertussis toxin (PTX) diminished agonist-induced loss of surface receptors. Furthermore, the abilities of morphine and fentanyl to cause internalization were more impaired after PTX treatment than that of etorphine. PTX treatment also significantly enhanced the increase in surface receptor number caused by 18-h treatment with naloxone and buprenorphine. The results of this study suggest that disruption of G protein coupling by PTX treatment affects ligand-regulated μ-receptor trafficking and that partial agonists for signaling can vary greatly in the ability to regulate the number of surface μ-opioid receptors.