Binding characteristics of [ 3 H]14‐methoxymetopon, a high affinity µ‐opioid receptor agonist

The highly potent µ‐opioid receptor agonist 14‐methoxymetopon (4,5α‐epoxy‐3‐hydroxy‐14β‐methoxy‐5β,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 [ 3 H]14‐methoxymetopon were determined using radioligand binding assay in rat brain membranes. [ 3 H]14‐Methoxymetopon specifically labelled a single class of opioid sites with affinity in low subnanomolar range ( K i  = 0.43 n m ) and maximal number of binding sites of 314 fmol/mg protein. Binding of [ 3 H]14‐methoxymetopon was inhibited by ligands selective for the µ‐opioid receptor with high potency, while selective κ‐opioids and δ‐opioids were weaker inhibitors. 14‐Methoxymetopon increased guanosine‐5′‐ O ‐(3‐[ 35 S]thio)‐triphosphate ([ 35 S]GTPγS) binding with an EC 50 of 70.9 n m , thus, providing evidence for the agonist character of this ligand. The increase of [ 35 S]GTPγS binding was inhibited by naloxone and selective µ‐opioid antagonists, indicating a µ‐opioid receptor‐mediated action. [ 3 H]14‐Methoxymetopon is one of the few nonpeptide µ‐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 µ‐opioid receptor mechanisms, as well as to promote a further understanding of the opioid system at the cellular and molecular level.