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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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 3

Effect of the C-terminal truncation on morphine locomotor activity and catalepsy in the mutant mice.

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Effect of the C-terminal truncation on morphine locomotor activity and c...
(A) Morphine locomotor activity. Morphine locomotor activity of the mutant mice on B6 and 129 backgrounds was measured in an open-field chamber (Med Associates) as described in Supplemental Methods. Following morphine injection (10 mg/kg, i.p.), the total distance traveled (cm) within 60 minutes was measured. Results are shown as mean ± SEM. The number of mice used were as follows: mE3M-B6, 9 WT and 8 Mut; mE4M-B6, 11 WT and 13 Mut; mE7M-B6, 9 WT and 8 Mut; mE3M-129, 8 WT and 8 Mut; mE4M-129, 7 WT and 10 Mut; mE7M-129, 9 WT and 8 Mut. *P < 0.01; #P < 0.0001, compared with WT, 1-way ANOVA with Bonferroni’s post hoc test. Mut, homozygous mice. (B) Morphine catalepsy. Morphine catalepsy in the mutant mice was assessed by using a horizontal bar test as described in Supplemental Methods. Mice were tested 30 minutes after morphine injection (60 mg/kg, s.c.). The time (s) spent in cataleptic position was recorded. Results are shown as mean ± SEM. The number of mice used were as follows: in mE3M-B6, 6 WT-Saline, 9 WT-morphine, 6 Mut-Saline, 8 Mut-morphine; in mE4M-B6, 6 WT-Saline, 12 WT-morphine, 6 Mut-Saline, 6 Mut-morphine; in mE7M-B6, 8 WT-Saline, 10 WT-morphine, 8 Mut-Saline, 10 Mut-morphine; in mE3M-129, 6 WT-Saline, 8 WT-morphine, 6 Mut-Saline, 7 Mut-morphine; in mE4M-129, 5 WT-Saline, 10 WT-morphine, 5 Mut-Saline, 9 Mut-morphine; in mE7M-129, 5 WT-Saline, 7 WT-morphine, 6 Mut-Saline, 11 Mut-morphine. *P < 0.05; #P < 0.0001, compared with group treated with Saline; *P < 0.05; **P < 0.01; ***P < 0.001, compared with WT treated with morphine, 2-way ANOVA with Bonferroni’s post hoc test.