[Effects of newly isolated opioid peptides on G-protein activation: usefulness of [35S] GTP gamma S binding study and...

Recent cloning and expression studies have revealed that the opioid mu-, delta-, kappa- and orphan receptors are seven-transmembrane domain receptors whose actions are mediated through activation of guanine nucleotide binding protein (G-protein). The activation of G-proteins by the opioid receptor can be measured by assessing agonist stimulation of membrane binding of the non-hydrolyzable analog of guanosine triphosphate (GTP), guanosine-5'-O-(3-[35S] thio) triphosphate ([35S] GTP gamma S). Our recent data suggest that 1) the level of spinal mu-, delta-, kappa- and orphan-receptor agonist-stimulated [35S] GTP gamma S binding closely parallels that of receptor binding densities, 2) the neuroanatomical distribution of opioid agonist-stimulated [35S] GTP gamma S binding relates to receptor binding distribution, 3) newly isolated opioid peptides, endomorphin-1 and -2, can activate G-proteins by specific stimulation of mu-receptors and act as partial agonists with moderate catalytic efficacies, 4) mu-receptor densities could be rate-limiting steps in the G-protein activation by mu-agonists in the spinal cord region. In conclusion, opioid agonist-stimulated [35S] GTP gamma S binding can provide a functional method to localize receptors not only by their ability to bind ligands, but also according to their ability to activate an intracellular signal transducer.