Alternatively spliced mu opioid receptor C termini impact the diverse actions of morphine
Jin Xu, … , Gavril W. Pasternak, Ying-Xian Pan
Jin Xu, … , Gavril W. Pasternak, Ying-Xian Pan
Published March 20, 2017
Citation Information: J Clin Invest. 2017;127(4):1561-1573. https://doi.org/10.1172/JCI88760.
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Research Article Neuroscience

Alternatively spliced mu opioid receptor C termini impact the diverse actions of morphine

Abstract

Extensive 3′ alternative splicing of the mu opioid receptor gene OPRM1 creates multiple C-terminal splice variants. However, their behavioral relevance remains unknown. The present study generated 3 mutant mouse models with truncated C termini in 2 different mouse strains, C57BL/6J (B6) and 129/SvEv (129). One mouse truncated all C termini downstream of Oprm1 exon 3 (mE3M mice), while the other two selectively truncated C-terminal tails encoded by either exon 4 (mE4M mice) or exon 7 (mE7M mice). Studies of these mice revealed divergent roles for the C termini in morphine-induced behaviors, highlighting the importance of C-terminal variants in complex morphine actions. In mE7M-B6 mice, the exon 7–associated truncation diminished morphine tolerance and reward without altering physical dependence, whereas the exon 4–associated truncation in mE4M-B6 mice facilitated morphine tolerance and reduced morphine dependence without affecting morphine reward. mE7M-B6 mutant mice lost morphine-induced receptor desensitization in the brain stem and hypothalamus, consistent with exon 7 involvement in morphine tolerance. In cell-based studies, exon 7–associated variants shifted the bias of several mu opioids toward β-arrestin 2 over G protein activation compared with the exon 4–associated variant, suggesting an interaction of exon 7–associated C-terminal tails with β-arrestin 2 in morphine-induced desensitization and tolerance. Together, the differential effects of C-terminal truncation illustrate the pharmacological importance of OPRM1 3′ alternative splicing.

Authors

Jin Xu, Zhigang Lu, Ankita Narayan, Valerie P. Le Rouzic, Mingming Xu, Amanda Hunkele, Taylor G. Brown, William F. Hoefer, Grace C. Rossi, Richard C. Rice, Arlene Martínez-Rivera, Anjali M. Rajadhyaksha, Luca Cartegni, Daniel L. Bassoni, Gavril W. Pasternak, Ying-Xian Pan

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Figure 2

Effect of the C-terminal truncation on morphine physical dependence and CPP in the mutant mice.

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Effect of the C-terminal truncation on morphine physical dependence and ...
(A) Morphine physical dependence. Morphine physical dependence was assessed by naloxone-precipitated withdrawal after chronic morphine treatment in the mutant mice on B6 or 129 backgrounds, as described in Methods. Results are shown as the number of jumps within 15 minutes immediately following naloxone injection. The number of mice used were as follows: mE3M-B6, 15 WT and 14 Mut in 2 independent experiments; mE4M-B6, 18 WT and 17 Mut in 2 independent experiments; mE7M-B6, 10 WT and 10 Mut in 1 independent experiment; mE3M-129, 15 WT and 11 Mut in 2 independent experiments; mE4M-129, 13 WT and 13 Mut in 2 independent experiments; mE7M-129, 7 WT and 7 Mut in 1 experiment. *P < 0.05; #P < 0.0001, compared with WT, 1-way ANOVA with Bonferroni’s post hoc test. Mut, homozygous mice. (B) Morphine CPP. Morphine CPP was assessed using a 3-chamber apparatus (Med Associates) with a 6-day paradigm as described in Methods. Results were calculated by different scores (seconds) in drug-paired chamber between the test day and preconditioning day and from 2 independent experiments. The number of mice used were as follows: mE3M-B6, 18 WT and 18 Mut; mE4M-B6, 11 WT and 13 Mut; mE7M-B6, 17 WT and 15 Mut. **P < 0.01, compared with WT, 1-way ANOVA with Bonferroni’s post hoc test.