Human delta opioid receptor: Functional studies on stably transfected Chinese hamster ovary cells after acute and chronic treatment with the selective nonpeptidic agonist SNC-80

The SNC-80 series of nonpeptidic agonists for the δ-opioid receptor are being developed as potential analgesic drugs. It is important to understand their acute and chronic effects at human δ-opioid receptors. Thus, we measured the ability of SNC-80 and [D-Pen²,4'-Cl-Phe⁴,D-Pen⁵]enkephalin to inhibit forskolin-stimulated adenylyl cyclase activity in recombinant Chinese hamster ovary cells stably expressing the cloned human δ-opioid receptor. The calculated EC₅₀ values for [D-Pen²,4'-Cl-Phe⁴,D-Pen⁵]enkephalin and SNC-80 were 0.6 ± 0.1 nM and 6.3 ± 0.1 nM, respectively. Pretreatment of these cells with SNC-80 (100 nM) for 24 hr produced 1) a time-dependent reduction of δ receptor density, as measured by radioligand binding studies with [³H]naltrindole; 2) a shift in the EC₅₀ value of SNC-80 from 7.7 ± 4.2 nM to 44.1 ± 12 nM, as measured by the cyclic AMP assay; 3) a reduction in the maximum inhibition of adenylyl cyclase activity from 86% to 48%; 4) a marked increase in the forskolin stimulation of basal cyclic AMP accumulation by nearly 100% (from 442 pmol/mg of protein to 824 pmol/mg of protein); and 5) a 5-fold increase in forskolin-stimulated cyclic AMP accumulation after addition of naltrindole. These studies showed that SNC-80 produced desensitization and down-regulation of human δ-opioid receptors in recombinant Chinese hamster ovary cells after chronic treatment and that this effect was associated with an increase in adenylyl cyclase activity.