Antagonistic property of buprenorphine for putative epsilon-opioid receptor-mediated G-protein activation by beta-endorphin in pons/medulla of the mu-opioid receptor knockout mouse.

beta-Endorphin is a non-selective opioid peptide which binds mu-, delta- and putative epsilon (beta-endorphin-sensitive non-mu-, non-delta- and non-kappa(1)-)-opioid receptors. We have previously reported that beta-endorphin-produced G-protein activation is mediated by the stimulation of both mu- and putative epsilon-opioid receptors. The present study was designed to further characterize this putative epsilon-opioid receptor-mediated G-protein activation in the pons/medulla membrane obtained from mice lacking mu-opioid receptor, using a guanosine-5'-O-(3-[(35)S]thio)triphosphate ([(35)S]GTPgammaS)-binding assay. beta-Endorphin and the mu-opioid receptor agonist [D-Ala(2),N-MePhe(4),Gly-ol(5)]enkephalin (DAMGO) increased the [(35)S]GTPgammaS binding in a concentration-dependent manner (0.001-10 microM), and at 10 microM beta-endorphin and DAMGO produced approximately 250 and 120% increases of [(35)S]GTPgammaS binding in the pons/medulla membrane obtained from wild-type mice, respectively. In the pons/medulla membrane obtained from mu-opioid receptor knockout mice, beta-endorphin-stimulated [(35)S]GTPgammaS binding was only partially attenuated and a more than 100% increase by 10 microM beta-endorphin still remained, while DAMGO failed to produce any increase in [(35)S]GTPgammaS binding. The residual increase in [(35)S]GTPgammaS binding by 10 microM beta-endorphin in mu-opioid receptor knockout mice was partially but significantly attenuated by the putative epsilon-opioid receptor partial agonist beta-endorphin (1-27), but not by the delta-opioid receptor antagonist naltrindole or the kappa(1)-receptor antagonist norbinaltorphimine. Furthermore, buprenorphine significantly attenuated the residual increase in [(35)S]GTPgammaS binding by 10 microM beta-endorphin in mu-opioid receptor knockout mice. The present results indicate that beta-endorphin activates G-protein by stimulation of putative epsilon-opioid receptors in the condition lacking the mu-opioid receptor, and buprenorphine acts as an antagonist for putative epsilon-opioid receptors in this condition.