Down regulation of enkephalin (δ) receptors Demonstration in membrane-bound and solubilized receptors

The pentapeptide leucine enkephalin induced down-regulation of enkephalin receptors in neuroblastoma-glioma NG108-15 hybrid cells in a reversible fashion, whereas the stable enkephalin analogue, d-Ala²-Met-enkephalinamide (AMEA), and the potent opiate alkaloid, etorphine, had a prolonged effect. The opiate alkaloid, morphine, which has low affinity to δ-type enkephalin receptors of these cells did not induce down-regulation, whereas AMEA decreased the binding of both opiate agonists and antagonists but had no effect on the binding of the α₂-adrenergic ligand, ³H]yohimbine. From several experiments that were designed to remove the tightly bound AMEA, and from experiments with solubilized receptor we ruled out the possibility that the decreased binding capacity of enkephalin-treated cells reflects only receptor masking. The study suggests that down-regulation of enkephalin receptors that may also occur in vivo can account for some of the abnormal physiological responses of subjects treated chromically with opiates. However, since opiates from the morphine type can induce opiate tolerance in vivo, but not down-regulation of enkephalin receptors in the cultured cells, we suggest that down-regulation of δ-type opiate receptors may not be prerequisite for the development of the physiological tolerance/dependence on these alkaloids.