Agonist-induced functional desensitization of the mu-opioid receptor is mediated by loss of membrane receptors rather than uncoupling from G protein
| Autor: | Y, Pak, A, Kouvelas, M A, Scheideler, J, Rasmussen, B F, O'Dowd, S R, George |
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| Rok vydání: | 1996 |
| Předmět: |
Analgesics
Binding Sites Narcotic Antagonists Cell Membrane Colforsin Receptors Opioid mu Down-Regulation CHO Cells Enkephalins Enkephalin Ala(2)-MePhe(4)-Gly(5) Kidney Transfection Sensitivity and Specificity Naltrexone Stimulation Chemical Cell Line Kinetics GTP-Binding Proteins Cricetinae Cyclic AMP Animals |
| Zdroj: | Molecular pharmacology. 50(5) |
| ISSN: | 0026-895X |
| Popis: | The effects of acute exposure of the opioid peptide [D-Ala2,N-MePhe4, Gly-ol5]enkephalin (DAMGO) on the mu-opioid receptor were examined in Chinese hamster ovary (CHO) K-1 and baby hamster kidney stable transfectants. In the CHO cell line, acute 1-hr treatment with DAMGO decreased the density of receptors without affecting the affinity or proportion of agonist-detected sites and attenuated the ability of the agonist to inhibit forskolin-stimulated cAMP accumulation. In contrast, similar 1-hr treatment of baby hamster kidney cells did not affect receptor density or agonist ability to inhibit cAMP accumulation, but longer duration of agonist exposure resulted in a reduction in membrane receptor, identical to the CHO cells. These results suggested that for the mu-opioid receptor, alteration in receptor density was the major determinant for the observed agonist-induced desensitization. Consistent with this notion, the ratio of the DAMGO concentration yielding half-maximal occupation of the mu receptor to that yielding half-maximal functional response was1. This suggests the necessity for a high mu receptor occupancy rate for maximal functional response, so that any loss of cell surface opioid-binding sites was a critical determinant in reducing the maximal response. This hypothesis was further supported by the observation that irreversible inactivation of fixed proportions of opioid-binding sites with beta-chlorn-altrexamine demonstrated that there were few spare receptors, which is in contrast to what has been reported for other G protein-coupled receptors, including the delta-opioid receptor. Taken together, these data suggest that the opioid agonist DAMGO has a high affinity for the mu receptor but must occupy70% of the available receptors to generate the maximal second messenger-linked response. |
| Databáze: | OpenAIRE |
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