Antagonistic property of buprenorphine for putative ϵ-opioid receptor-mediated G-protein activation by β-endorphin in pons/medulla of the μ-opioid receptor knockout mouse

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