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Eur J Pharmacol. 2004 May 25;492(2-3):131-6.

No evidence for G-protein-coupled epsilon receptor in the brain of triple opioid receptor knockout mouse.

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Institut de Génétique et de Biologie Moléculaire et Cellulaire, CNRS/INSERM/ULP Parc d'innovation, 1 rue Laurent Fries BP 10142, C.U. de Strasbourg, 67404 Illkirch Cedex, France.


Pharmacological approaches have defined the epsilon receptor as a beta-endorphin-preferring opioid receptor, described in rat vas deferens and in brain of several species. Only three opioid receptors-mu, delta and kappa-have been cloned and the existence of this additional subtype as a distinct protein remains controversial. Recently, the mouse brain epsilon receptor was detected in a G protein activation assay, as mediating residual beta-endorphin activity following pharmacological blockade of mu, delta and kappa receptors. To clarify whether this site is independent from mu, delta and kappa receptors, we performed beta-endorphin-induced [(35)S]GTPgammaS binding using mice lacking these three receptors (triple knockout mice). We tested both pons-medulla and whole brain preparations. beta-Endorphin strongly stimulated [(35)S]GTPgammaS binding in wild-type membranes but had no detectable effect in membranes from triple knockout mice. We conclude that the brain epsilon site involves mu, delta and/or kappa receptors, possibly coupled to nonclassical G proteins.

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