Chronic Morphine Treatment Inhibits Opioid Receptor Desensitization and Internalization

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Abstract
Chronic opioid receptor (OR) activation by morphine causes distinct cellular adaptations responsible for the development of tolerance. The present study examines the effect of chronic morphine exposure on the ability of high-efficacy agonists to mediate δ-OR (DOR) and μ-OR (MOR) uncoupling and internalization, two regulatory mechanisms contributing to rapid desensitization of OR function. Chronic morphine treatment (1 μm; 72 hr) of DOR carrying neuroblastoma x glioma (NG108-15) hybrid cells, a prototypical model system frequently used to study cellular aspects of opioid tolerance, completely blocked the capacity of [d-Ala2,d-Leu5]enkephalin (DADLE) and etorphine to desensitize opioid-stimulated [35S]GTPγS binding and to mediate DOR internalization. Similar findings were obtained on stably DOR- and MOR-transfected human embryonic kidney (HEK) 293 cells. Chronic morphine treatment also heterologously impaired agonist regulation of non-opioid G-protein-coupled receptors, such as the m4-muscarinic acetylcholine receptor and the brain-type cannabinoid receptor. As a possible underlying mechanism, we found that chronic morphine treatment completely blocked agonist-induced redistribution of β-arrestin1 in both NG108-15 and stably MOR-transfected HEK293 cells. Moreover, attenuation of β-arrestin1 function appears to depend on persistent stimulation of MAP kinase activity during the course of chronic morphine treatment, because coincubation of the cells together with the MAP kinase blocker PD98059 fully restored β-arrestin1 translocation and receptor internalization. These results demonstrate that chronic morphine treatment produces adaptational changes at the β-arrestin1 level, which in turn attenuates agonist-mediated desensitization and internalization of G-protein-coupled receptors.