title: Endomorphin-stimulated [35S]GTPgammaS binding in rat brain: evidence for partial agonist activity at mu-opioid receptors.
abstract: Endomorphin-1 is a peptide whose binding selectivity suggests a role as an endogenous ligand at mu-opioid receptors. In the present study, the effect of endomorphin-1 on mu receptor-coupled G proteins was compared with that of the mu agonist DAMGO by using agonist-stimulated [35S]GTPgammaS binding in rat brain. [35S]GTPgammaS autoradiography revealed a similar localization of endomorphin-1- and DAMGO-stimulated [35S]GTPgammaS binding in areas including thalamus, caudate-putamen, amygdala, periaqueductal gray, parabrachial nucleus, and nucleus tractus solitarius. Naloxone blocked endomorphin-1-stimulated labeling in all regions examined. Although the distribution of endomorphin-1-stimulated [35S]GTPgammaS binding resembled that of DAMGO, the magnitude of endomorphin-1-stimulated binding was significantly lower than that produced by DAMGO. Concentration-effect curves of endomorphin-1 and DAMGO in thalamic membranes confirmed that endomorphin-1 produced only 70% of DAMGO-stimulated [35S]GTPgammaS binding. Differences in maximal stimulation of [35S]GTPgammaS binding between DAMGO and endomorphin-1 were magnified by increasing GDP concentrations, and saturation analysis of net endomorphin-1-stimulated [35S]GTPgammaS binding revealed a lower apparent Bmax value than that obtained with DAMGO. Endomorphin-1 also partially antagonized DAMGO stimulation of [35S]GTPgammaS binding. These results demonstrate that endomorphin-1 is a partial agonist for G protein activation at the mu-opioid receptor in brain.
authors.affiliation: Department of Physiology and Pharmacology, Center for Investigative Neuroscience, Bowman Gray School of Medicine, Wake Forest University, Winston-Salem, North Carolina 27157, USA.
authors.forename: L J
authors.initials: LJ
authors.lastname: Sim
authors.forename: Q
authors.initials: Q
authors.lastname: Liu
authors.forename: S R
authors.initials: SR
authors.lastname: Childers
authors.forename: D E
authors.initials: DE
authors.lastname: Selley
mesh_terms: D000818:Animals; D001345:Autoradiography; D001921:Brain / Q000378:metabolism*; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; D004745:Enkephalins / Q000494:pharmacology; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000378:metabolism*; D006153:Guanosine Diphosphate / Q000494:pharmacology; D008297:Male; D009270:Naloxone / Q000494:pharmacology; D009292:Narcotic Antagonists / Q000494:pharmacology; D009842:Oligopeptides / Q000494:pharmacology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D017450:Receptors, Opioid, mu / Q000819:agonists*; D013462:Sulfur Radioisotopes
chemical_list: D004745:Enkephalins; D009292:Narcotic Antagonists; D009842:Oligopeptides; D017450:Receptors, Opioid, mu; D013462:Sulfur Radioisotopes; C105293:endomorphin 1; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; D006153:Guanosine Diphosphate; D009270:Naloxone; D016244:Guanosine 5'-O-(3-Thiotriphosphate)