Agonist-specific down-regulation of the human δ-opioid receptor

Down-regulation of the δ-opioid receptor contributes to the development of tolerance to δ-opioid receptor agonists. The involvement of the carboxy terminus of the mouse δ-opioid receptor in peptide agonist-mediated down-regulation has been established. In the present study, we examined the down-regulation of the truncated human δ-opioid receptor by structurally distinct δ-opioid receptor agonists. Chinese hamster ovary (CHO) cells, expressing the full-length or truncated epitope-tagged human δ-opioid receptors were incubated with various δ-opioid receptor agonists (100 nM, 24 h), and membrane receptor levels were determined by [ 3H]naltrindole saturation binding. Each δ-opioid receptor agonist tested down-regulated the full-length receptor. Truncation of the carboxy terminus abolished down-regulation by all δ-opioid receptor agonists, except SNC80 ((+)-4-[(α R)-α-((2 S,5 R)-4-allyl-2,5-dimethyl-1-piperazinyl)-3-methoxybenzyl] N, N-diethylbenzamide). In addition, truncation of the C-terminus completely attenuated [ d-Pen 2- d-Pen 5]enkephalin (DPDPE), but not SNC80-mediated [ 32P] incorporation into the protein immunoreactive with an anti-epitope-tagged antibody. These findings suggest that SNC80-mediated phosphorylation and down-regulation of the human δ-opioid receptor involves other receptor domains in addition to the carboxy terminus. Pertussis toxin treatment did not block SNC80-mediated down-regulation of the truncated Et-hDOR, indicating that the down-regulation is independent of G i/o protein activation and subsequent downstream signaling.