Delta and mu opioid receptors from the brain of a urodele amphibian, the rough-skinned newt Taricha granulosa: Cloning, heterologous expression, and pharmacological characterization

Two full-length cDNAs, encoding delta (δ) and mu (μ) opioid receptors, were cloned from the brain of the rough-skinned newt Taricha granulosa, complementing previous work from our laboratory describing the cloning of newt brain kappa (κ) and ORL1 opioid receptors. The newt δ receptor shares 82% amino acid sequence identity with a frog δ receptor and lower (68-70%) identity with orthologous receptors cloned from mammals and zebrafish. The newt μ receptor shares 79% sequence identity with a frog μ receptor, 72% identity with mammalian μ receptors, and 66-69% identity with μ receptors cloned from teleost fishes. Membranes isolated from COS-7 cells transiently expressing the newt δ receptor possessed a single, high-affinity (Kd = 2.4 nM) binding site for the nonselective opioid antagonist [3H]naloxone. In competition binding assays, the newt δ receptor displayed highest affinity for Met-enkephalin, relatively low affinity for Leu-enkephalin, β-endorphin, and [d-penicillamine, d-penicillamine] enkephalin (DPDPE) (a δ-selective agonist in mammals), and very low affinity for μ-, κ-, or ORL1-selective agonists. COS-7 cells expressing the newt μ receptor also possessed a high-affinity (Kd = 0.44 nM) naloxone-binding site that showed highest affinity for β-endorphin, moderate-to-low affinity for Met-enkephalin and Leu-enkephalin and DAMGO (a μ-selective agonist in mammals), and very low affinity for DPDPE and κ- or ORL1-selective agonists. COS-7 cells expressing either receptor type (δ or μ) showed very high affinity (Kd = 0.1-0.3 nM) for the nonselective opioid antagonist diprenorphine. Taricha granulosa expresses the same four subtypes (δ, μ, κ, and ORL1) of opioid receptors found in other vertebrate classes, but ligand selectivity appears less stringent in the newt than has been documented in mammals.