The stereoselective κ-opioid receptor antagonist Mr 2266 does not exhibit stereoselectivity as an antagonist at the orphan opioid (ORL1) receptor

Mr 2266 [(-)-(1R,5R,9R)-5,9-diethyl-2-(3-furylmethyl)-2’-hydroxy-6,7-benzomorphan] is an antagonist at κ-opioid receptors and at ORL1 receptors as well. The aim of our study was to examine whether the known stereoselective antagonism of Mr 2266 at κ-opioid receptors also extends to ORL1 receptors. In mouse brain cortex membranes, the binding of the ORL1 receptor agonist [3H]nociceptin was equipotently inhibited by Mr 2266 and its enantiomer Mr 2267 (pKi 4.82 and 5.14, respectively), whereas the binding of the κ-opioid receptor agonist [3H]U-69,593 was inhibited by Mr 2266 more potently (pKi 9.11) than by its enantiomer Mr 2267 (pKi 7.15). In mouse brain cortex slices preincubated with [3H]noradrenaline, the concentration-response curve of nociceptin for inhibition of the electrically evoked overflow of tritium was equipotently shifted to the right by Mr 2266 and Mr 2267 (pA2 5.77 and 5.64, respectively). On the other hand, the inhibitory effect of U-69,593 on the electrically evoked overflow of tritium in guinea-pig brain cortex slices preincubated with [3H]noradrenaline was more potently antagonized by Mr 2266 (pA2 8.81) than by Mr 2267 (pA2 7.15). These data show that the stereoselective antagonism of Mr 2266 at κ-opioid receptors does not extend to ORL1 receptors.