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Neutral antagonist activity of naltrexone and 6β‐naltrexol in naïve and opioid‐dependent C6 cells expressing a µ‐opioid receptor
Author(s) -
Divin MF,
Bradbury FA,
Carroll FI,
Traynor JR
Publication year - 2009
Publication title -
british journal of pharmacology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.432
H-Index - 211
eISSN - 1476-5381
pISSN - 0007-1188
DOI - 10.1111/j.1476-5381.2008.00035.x
Subject(s) - damgo , inverse agonist , chemistry , agonist , naltrexone , opioid receptor , competitive antagonist , (+) naloxone , opioid , enkephalin , endocrinology , medicine , pharmacology , μ opioid receptor , receptor , biochemistry
Background and purpose: Adenylyl cyclase sensitization occurs on chronic agonist activation of µ‐opioid receptors and is manifested by an increase in cAMP levels (overshoot) on challenge with antagonist. It has been proposed that a long lasting constitutively active receptor is formed on chronic µ‐opioid exposure and that antagonists with inverse agonist activity rapidly return the receptor to a basal state causing a cAMP overshoot and a more severe withdrawal response in vivo . This hypothesis depends on an accurate characterization of neutral and inverse agonist properties of opioid antagonists. Experimental approach: C6 glioma and HEK293 cells expressing µ‐opioid receptors were used. Opioid antagonists were examined for their ability to induce a cAMP overshoot following chronic treatment with the agonist DAMGO ([D‐Ala 2 ,N‐Me‐Phe 4 ,Glyol 5 ]‐enkephalin). The compounds were also characterized as agonists, inverse agonists or neutral antagonists by using assays for competitive binding, [ 35 S]GTPγS (guanosine‐5′‐O‐(3‐[ 35 S]thio)triphosphate) binding and changes in cell surface receptor expression. Key results: Naltrexone, 6β‐naltrexol and naloxone were indistinguishable to the µ‐opioid receptor in the opioid‐naïve or dependent state and acted as neutral antagonists. The δ‐opioid receptor inverse agonist RTI‐5989‐25 [(+)‐ N ‐[ trans ‐4′‐(2‐methylphenyl)‐2′‐butenyl]‐(3 R ,4 R )‐dimethyl‐4‐(3‐hydroxyphenyl)piperidine], a 3,4‐dimethyl‐4‐(3‐hydroxyphenyl)‐piperidine, was an inverse agonist at the µ‐opioid receptor, and the peptide antagonist CTAP (H‐D‐Phe‐Cys‐Tyr‐D‐Trp‐Arg‐Thr‐Pen‐Thr‐NH 2 ) showed variable, assay‐dependent properties. All the antagonists precipitated the same degree of cAMP overshoot in opioid‐dependent cells. Conclusions and implications: Antagonists at the µ‐opioid receptor may be neutral or show inverse agonist activity. Formation of a constitutively active µ‐opioid receptor is not a requirement for the development or expression of adenylyl cyclase sensitization.