Activation of G-proteins in the mouse pons/medulla by β-endorphin is mediated by the stimulation of μ- and putative ε-receptors

The activation of mu-, delta- and kappa (1)-opioid receptors by their respective agonists increases the binding of the non-hydrolyzable GTP analog guanosine-5'-(gamma -thio)-briphosphate (GTP gammaS) to G proteins. beta -Endorphin is an endogenous opioid peptide which binds nonselectively to mu-, delta- and putative epsilon -opioid receptors. The present experiment was designed to determine which opioid receptors are involved in the stimulation of [S-35]GTP gammaS binding induced by beta -endorphin in the mouse pons/medulla. The mouse pons/medulla membranes were incubated in an assay buffer containing 50 pM [S-35]GTP gammaS, 30 muM GDP and various concentrations of beta -endorphin. beta -Endorphin (0.1 nM-10 muM) increased [S-35]GTP gammaS binding in a concentration-dependent manner, and 10 muM beta -endorphin produced a maximal stimulation of approximately 260% over baseline. This stimulation of [S-35]GTP gammaS binding by beta -endorphin was partially attenuated by the mu -opioid receptor antagonist beta -funaltrexamine (beta -FNA), but not by the delta -opioid receptor antagonist naltrindole (NTI) or the kappa (1)-opioid receptor antagonist nor-binaltorphimine (nor-BNI), beta -Endorphin stimulated [S-35]GTP gammaS binding by about 80% in the presence of 10 muM beta -FNA, 30 nM NTI and 100 nM nor-BNI. The same concentrations of these antagonists completely blocked the stimulation of [S-35]GTP gammaS binding induced by 10 muM [D-Ala(2), NHPhe(4), Gly-ol]enkephalin, [D-Pen(2,5)]enkephalin and U50,488H, respectively. Moreover, the residual stimulation of [S-35]GTP gammaS binding induced by beta -endorphin in the presence of the three opioid receptor antagonists was significantly attenuated by 100 nM of the putative epsilon -opioid receptor partial agonist beta -endorphin (1-27). These results indicate that the stimulation of [S-35]GTP gammaS binding induced by beta -endorphin is mediated by the stimulation of both mu- and putative epsilon -opioid receptors in the mouse pons/medulla. (C) 2000 Elsevier Science Inc. All rights reserved.