Differential involvement of μ 1-opioid receptors in endomorphin- and β-endorphin-induced G-protein activation in the mouse pons/medulla

Abstract

Several genetic mouse models of differential sensitivity to opioids have been used to investigate the mechanisms underlying individual variation in responses to opioids. The CXBK mice are inbred recombinant mice which have a lower level of μ 1-opioid receptors than their parental strain. Endomorphin-1 and endomorphin-2 are endogenous opioid peptides that are highly selective for μ-opioid receptors, while β-endorphin, which is also an endogenous opioid peptide, is non-selective for μ-, δ- and putative ε-opioid receptors. The present study was designed to investigate the effects of these endogenous opioid peptides on G-protein activation by monitoring guanosine-5′- o-(3-[ 35S]thio)triphosphate binding to pons/medulla membranes of CXBK mice and their parental strain C57BL/6ByJ mice. Endomorphin-1 (0.1–10 μM), endomorphin-2 (0.1–10 μM) and β-endorphin (0.1–10 μM) increased guanosine-5′- o-(3-[ 35S]thio)triphosphate binding to the pons/medulla membranes from C57BL/6ByJ and CXBK mice in a concentration-dependent manner. However, the increases of guanosine-5′- o-(3-[ 35S]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 significant difference was found in the increases of the guanosine-5′- o-(3-[ 35S]thio)triphosphate binding induced by β-endorphin in C57BL/6ByJ and CXBK mice. Moreover, whereas the increase of guanosine-5′- o-(3-[ 35S]thio)triphosphate binding induced by 10 μM endomorphin-1 or endomorphin-2 were almost completely blocked by a μ-opioid receptor antagonist β-funaltrexamine (10 μM) in both strains, the increase of guanosine-5′- o-(3-[ 35S]thio)triphosphate binding induced by 10 μM β-endorphin was attenuated to approximately 70% of stimulation by co-incubation with 10 μM β-funaltrexamine in both strains. The residual stimulation of [ 35S]guanosine-5′- o-(3-thio)triphosphate binding by 10 μM β-endorphin in the presence of 10 μM β-funaltrexamine was further attenuated by the addition of putative ε-opioid receptor partial agonist β-endorphin (1–27) (1 μM) in both strains. Like the endomorphins, the synthetic μ-opioid receptor agonist [ d-Ala 2, N-MePhe 4,Gly-ol 5]enkephalin at 10 μM showed lower increases of guanosine-5′- o-(3-[ 35S]thio)triphosphate binding in CXBK mice than those in C57BL/6ByJ mice. However, there was no strain difference in the stimulation of guanosine-5′- o-(3-[ 35S]thio)triphosphate binding induced by 10 μM of the selective δ 1-opioid receptor agonist [ d-Pen 2,5]enkephalin, δ 2-opioid receptor agonist [ d-Ala 2]deltorphin II or κ-opioid receptor agonist U50,488H. The results indicate that the G-protein activation by endomorphin-1 and endomorphin-2 in the mouse pons/medulla is mediated by both μ 1- and μ 2-opioid receptors. Moreover, β-endorphin-induced G-protein activation in the mouse pons/medulla is, in part, mediated by μ 2- and putative ε-, but not by μ 1-opioid receptors.