Abstract
Agonist-selective actions of opioids on the desensitization of μ-opioid receptors (MORs) have been well characterized, but few if any studies have examined agonist-dependent recovery from desensitization. The outward potassium current induced by several opioids was studied using whole-cell voltage-clamp recordings in locus ceruleus neurons. A brief application of the irreversible opioid antagonist β-chlornaltrexamine (β-CNA) was applied immediately after treatment of slices with saturating concentrations of opioid agonists. This approach permitted the measurement of desensitization and recovery from desensitization using multiple opioid agonists, including [Met]5enkephalin (ME), [d-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (DAMGO), etorphine, fentanyl, methadone, morphine, morphine-6-glucuronide, oxycodone, and oxymorphone. The results indicate that desensitization protects receptors from irreversible antagonism with β-CNA. The amount of desensitization was measured as the decrease in current during a 10-min application of a saturating agonist concentration and was a good predictor of the extent of receptor protection from irreversible inactivation with β-CNA. After desensitization with ME or DAMGO and treatment with β-CNA, there was an initial profound inhibition of MOR-induced current that recovered significantly after 45 min. There was, however, no recovery of MOR-mediated current with time after treatment with agonists that did not cause desensitization, such as oxycodone. These results demonstrate that desensitization prevents irreversible inactivation of receptors by β-CNA.
Footnotes
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This work was supported by National Institutes of Health grants 1F30-DA021466 (to M.S.V.) and DA08163 (to J.T.W.), and by the NARSAD-Ritter Foundation (to J.T.W.).
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ABBREVIATIONS: MOR, μ-opioid receptor; LC, locus ceruleus; β-CNA, β-chlornaltrexamine; ME, [Met]5enkephalin; DAMGO, [d-Ala2,N-Me-Phe4-Gly5-ol]-enkephalin; M-6-G, morphine-6-glucuronide; GIRK, G protein-gated inwardly rectifying potassium; UK14304, 5-bromo-6-[2-imidazolin-2-ylamino]quinoxaline; BAPTA, 1,2-bis(2-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid; HEK, human embryonic kidney.
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↵ The online version of this article (available at http://molpharm.aspetjournals.org) contains supplemental material.
- Received October 25, 2007.
- Accepted January 14, 2008.
- The American Society for Pharmacology and Experimental Therapeutics
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