Identification of a human delta opioid receptor: Cloning and expression

The δ opioid receptor is an important target for analgesic drug development. This report describes the identification of δ opioid receptor clones from human cDNA libraries and the preparation of a human δ receptor cDNA in the pcDNA3 expression vector for transfection studies. The cDNA encodes a 372 amino acid protein that has 93% amino acid identity to mouse and rat δ receptors. COS-7 cells transfected with this clone express over 1.0 pmole receptor/mg protein when measured by saturation binding with [³H]naltrindole. The δ receptor selective ligands NTB, BNTX, [4′-Cl-Phe⁴]DPDPE and [D-Ala², Glu⁴]deltorphin all have Ki values under 10 nM while the affinities of the μ and κ opioid receptor ligands CTAP and U-69593, respectively, are over 4.0 μM. Agonists show binding to multiple affinity states of the receptor consistent with the presence of G-protein coupled and uncoupled forms of the expressed receptor. The 8-fold higher affinity of NTB relative to BNTX suggests that the human δ receptor is of the δ₂ subtype.