DAMGO-induced μ opioid receptor internalization and recycling restore morphine sensitivity in tolerant rat.

Xiaqing Ma, Rui Chen, Min Huang, Wenying Wang, Limin Luo, Dong Kwan Kim, Wei Jiang, Tao Xu

European journal of pharmacology 2020 Jul 05

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This study investigated the effect of DAMGO-induced μ opioid receptor (MOR) internalization on morphine tolerance. Male Sprague-Dawley rats (200-250 g) aged 6-8 weeks were administered morphine via intrathecal (i.t.) injection (15 μg/10 μl twice daily for 6 days) to induce antinociceptive tolerance, which was evaluated using the tail-flick and paw-withdrawal tests. Response latency was calculated as the percentage of maximum possible effect (%MPE). A bolus of DAMGO was administered by i.t. injection on day 6, and the tail-flick and paw-withdrawal tests were carried out 24, 48, and 72 h later. Membrane and cytosolic MOR expression was assessed by western blotting. HEK293 cells were transfected with MOR-FLAG plasmid and after 6 days of morphine treatment (10 μM), the cells were treated with 1 μM DAMGO, and MOR localization was examined by immunofluorescence analysis 30 and 60 min later. Repeated morphine treatment induced tolerance after 5 days; however, i.t. DAMGO administration restored morphine sensitivity and enhanced acute morphine-induced antinociception after 24, 48, and 72 h. In HEK293 cells, DAMGO treatment stimulated MOR internalization after 30 min and MOR recycling to the membrane after 1 h. Membrane and cytoplasmic MOR expression in vivo was unchanged 24, 48, and 72 h after i.t. DAMGO injection. Morphine does not cause significant MOR internalization or downregulation, and can readily induce tolerance. DAMGO counters this effect by enhancing receptor endocytosis, thereby reversing morphine-induced antinociceptive tolerance and restoring its analgesic efficacy. Copyright © 2020 Elsevier B.V. All rights reserved.

Citation

Xiaqing Ma, Rui Chen, Min Huang, Wenying Wang, Limin Luo, Dong Kwan Kim, Wei Jiang, Tao Xu. DAMGO-induced μ opioid receptor internalization and recycling restore morphine sensitivity in tolerant rat. European journal of pharmacology. 2020 Jul 05;878:173118