OPIOID EFFICACY IN A C6 GLIOMA CELL-LINE STABLY EXPRESSING THE DELTA-OPIOID RECEPTOR

A C6 glioma cell line stably transfected with the rat delta opioid rec eptor (C6 delta) was used to characterize receptor binding and G prote in activation by both peptide and nonpeptide delta opioid ligands. The ligand binding affinities for [H-3]naltrindole and [H-3]Cl-[D-Pen(2), D-Pen(5)]enkephalin (DPDPE) were similar to those observed in monkey b rain membranes. The nonpeptide agonists, BW373U86 and SNC80, as well a s peptide agonist [D-Ser(2),L-Leu(5)]enkephalyl-Thr maximally stimulat ed [S-35]GTP gamma S binding by 640, 654 and 576%, respectively, over basal. The peptide agonists, DPDPE and deltorphin II, both stimulated [S-35]GTP gamma S binding by 375%. Etorphine, diprenorphine, oxymorphi ndole and 7-spiroindanyloxymorphone were also partial agonists in this assay, although they were less efficacious than deltorphin II. Stimul ation of [S-35]GTP gamma S binding by agonists was blocked completely by pertussis toxin pretreatment. Both delta-1 and delta-2 selective an tagonists 7-benzylidenenaltrexone and a benzofuran analog of naltrindo le displayed high affinity for the cloned receptor (0.04 and 0.08 nM) and antagonized the stimulation of [S-35]GTP gamma S binding by BW373U 86 and DPDPE with similar potencies. Other evidence suggesting the lac k of receptor subtypes includes the finding that stimulation of [S-35] GTP gamma S binding by receptor subtype selective ligands DPDPE and de ltorphin II was not additive. BW373U86, SNC80 and DPDPE maximally inhi bited forskolin-stimulated adenylyl cyclase. These cells highly expres s a homogeneous population of delta opioid receptor that couple to inh ibitory G(o)/G(i) proteins. Ligand affinity for the delta opioid recep tor correlates with ligand EC50 values for stimulation of [S-35]GTP ga mma S binding.