Binding characteristics of [3H]14-methoxymetopon, a high affinity mu-opioid receptor agonist.
The highly potent micro -opioid receptor agonist 14-methoxymetopon (4,5alpha-epoxy-3-hydroxy-14beta-methoxy-5beta,17-dimethylmorphinan-6-one) was prepared in tritium labelled form by a catalytic dehalogenation method resulting in a specific radioactivity of 15.9 Ci/mmol. Opioid binding characteristics of [3H]14-methoxymetopon were determined using radioligand binding assay in rat brain membranes. [3H]14-Methoxymetopon specifically labelled a single class of opioid sites with affinity in low subnanomolar range (Ki = 0.43 nm) and maximal number of binding sites of 314 fmol/mg protein. Binding of [3H]14-methoxymetopon was inhibited by ligands selective for the micro -opioid receptor with high potency, while selective kappa-opioids and delta-opioids were weaker inhibitors. 14-Methoxymetopon increased guanosine-5'-O-(3-[35S]thio)-triphosphate ([35S]GTPgammaS) binding with an EC50 of 70.9 nm, thus, providing evidence for the agonist character of this ligand. The increase of [35S]GTPgammaS binding was inhibited by naloxone and selective micro -opioid antagonists, indicating a micro -opioid receptor-mediated action. [3H]14-Methoxymetopon is one of the few nonpeptide mu-opioid receptor agonists available in radiolabelled form up to now. Due to its high affinity and selectivity, high stability and extremely low nonspecific binding (<10%), this radioligand would be an important and useful tool in probing mu-opioid receptor mechanisms, as well as to promote a further understanding of the opioid system at the cellular and molecular level.
Spetea
Mariana
M
Division of Pharmaceutical Chemistry, Department of Pharmacy, University of Innsbruck, Innrain 52a, A-6020 Innsbruck, Austria. Mariana.Spetea@uibk.ac.at
Tóth
Fanni
F
Schütz
Johannes
J
Otvös
Ferenc
F
Tóth
Géza
G
Benyhe
Sandor
S
Borsodi
Anna
A
Schmidhammer
Helmut
H
D000818:Animals; D001667:Binding, Competitive / Q000187:drug effects; D001921:Brain / Q000378:metabolism; D002462:Cell Membrane / Q000378:metabolism; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000378:metabolism; D009022:Morphine Derivatives / Q000737:chemistry / Q000378:metabolism*; D011485:Protein Binding; D011869:Radioligand Assay; D019275:Radiopharmaceuticals; D051381:Rats; D017207:Rats, Sprague-Dawley; D017450:Receptors, Opioid, mu / Q000037:antagonists & inhibitors*; D015398:Signal Transduction / Q000187:drug effects; D014316:Tritium
D009022:Morphine Derivatives; D019275:Radiopharmaceuticals; D017450:Receptors, Opioid, mu; D014316:Tritium; C081949:14-methoxymetopon; D016244:Guanosine 5'-O-(3-Thiotriphosphate)