Novel approach to demonstrate high efficacy of μ opioids in the rat vas deferens: A simple model of predictive value

14-O-Methyloxymorphone and 14-methoxymetopon were reported as highly selective and potent μ opioid receptor agonists. The aim of this study was to demonstrate the opioid activity of these compounds in vitro and in vivo in comparison to oxymorphone, morphine and DAMGO. The μ opioid receptor efficacy, full or partial agonist nature of opioids was analyzed in the rat vas deferens (RVD) bioassay. Compared to oxymorphone, 14-O-methyloxymorphone and 14-methoxymetopon showed greater affinities to the rodent brain μ opioid receptors in receptor binding assays. In isolated organs 14-O-methyloxymorphone and 14-methoxymetopon were 3–10-fold more potent than the μ agonist opioid peptide, DAMGO. All tested compounds reached at least 70% maximum inhibition in mouse vas deferens (MVD) except morphine and oxymorphone. In the RVD, morphine could not exceed 50% inhibition of the twitches while 14-O-methyloxymorphone and 14-methoxymetopon showed inhibitory effects more than 70%. Oxymorphone reached only 4% maximal agonist effect and antagonized the inhibitory effect of DAMGO. The investigated morphinans produced dose-dependent antinociceptive activities in mice and rats. Both, 14-O-methyloxymorphone and 14-methoxymetopon are highly efficacious μ opioid receptor agonists in the RVD exhibiting full μ agonist properties. The RVD tissue contains μ receptors indicated by the comparable Ke values of the μ antagonist naltrexone against DAMGO in the MVD. RVD may be a good alternative to assess the μ receptor efficacy of opioid agonists providing a more physiological environment for the ligand–receptor interaction than other efficacy measuring methods such as the [35S]GTPγS binding assay.