Differential involvement of mu(1)-opioid receptors in endomorphin- and beta-endorphin-induced G-protein activation in the mouse pons/medulla

Several generic mouse models of differential sensitivity to opioids have be en used to investigate the mechanisms underlying individual variation in re sponses to opioids. The CXBK mice are inbred recombinant mice which have a lower level of mu (1)-opioid receptors than their parental strain. Endomorp hin-1 and endomorphin-2 are endogenous opioid peptides that are highly sele ctive for mu -opioid receptors, while beta -endorphin, which is also an end ogenous opioid peptide, is non-selective for mu-, delta- and putative epsil on -opioid receptors. The present study was designed to investigate the eff ects of these endogenous opioid peptides on G-protein activation by monitor ing guanosine-5'-o-(3-[S-35]thio)triphosphate binding to pons/medulla membr anes of CXBK mice and their parental strain C57BL/6ByJ mice. Endomorphin-1 (0.1-10 muM), endomorphin-2 (0.1-10 muM) and beta -endorphin (0.1-10 muM) i ncreased guanosine 5'-o-(3-[ S-35]thio)triphosphate binding to the pons/med ulla membranes from C57BL/6ByJ and CXBK mice in a concentration-dependent m anner. However, the increases of guanosine 5'-o-(3-[S-35]thio)triphosphate binding induced by either endomorphin-1 or endomorphin-2 in CXBK mice were significantly much lower than those in C57BL/6ByJ mice. However, no signifi cant difference was found in the increases of the guanosine-5'-o-(3[S-35]th io)triphosphate binding induced by beta -endorphin in C57BL/6ByJ and CXBK m ice. Moreover, whereas the increase of guanosine 5'-o-(3-[S-35]thio)triphos phate binding induced by 10 muM endomorphin-1 or endomorphin-2 were almost completely blocked by a mu -opioid receptor antagonist beta -funaltrexamine (10 muM) in both strains, the increase of guanosine-5'-o-(3-[S-35]thio)tri phosphate binding induced by 10 muM beta -endorphin was attenuated to appro ximately 70% of stimulation by co-incubation with 10 muM beta -funaltrexami ne in both strains. The residual stimulation of [S-35]guanosine-5'-o-(3-thi o)triphosphate binding by 10 muM beta -endorphin in the presence of 10 muM beta -funaltrexamine was further attenuated by the addition of putative eps ilon -opioid receptor partial agonist P-endorphin (1-27) (1 muM) in both st rains. Like the endomorphins, the synthetic mu -opioid receptor agonist [D- Ala(2),N-MePhe(4),Gly-ol(5)]enkephalin at 10 muM showed lower increases of guanosine-5'-o-(3-[S-35]thio)triphosphate binding in CXBK mice than those i n C57BL/6ByJ mice. However. there was no strain difference in the stimulati on of guanosine-5'-o-(3[S-35]thio)triphosphate binding induced by 10 muM of the selective delta (1)-opioid receptor agonist [D-Pen(2.5)]enkephalin, de lta (2)-opioid receptor agonist [D-Ala(2)]deltorphin II or kappa -opioid re ceptor agonist U50,488N. The results indicate that the G-protein activation by endomorphin-1 and end omorphin-2 in the mouse pons/medulla is mediated by both mu (1)- and mu (2) -opioid receptors. Moreover, beta -endorphin-induced G-protein activation i n the mouse pons/medulla is, in part, mediated by mu (2)- and putative epsi lon-, but not by mu (1)-opioid receptors. (C) 2000 IBRO. Published by Elsev ier Science Ltd. All rights reserved.