Elsevier

Neuroscience

Volume 138, Issue 2, 2006, Pages 609-619
Neuroscience

Pain mechanism
μ-Opioid receptor internalization-dependent and -independent mechanisms of the development of tolerance to μ-opioid receptor agonists: Comparison between etorphine and morphine

https://doi.org/10.1016/j.neuroscience.2005.11.046Get rights and content

Abstract

A growing body of evidences suggests that receptor desensitization is implicated in the development of tolerance to opioids, which is generally regulated by protein kinases and receptor trafficking proteins. In the present study, we demonstrated that repeated s.c. treatment with etorphine, but not morphine, produced a significant increase in protein levels of G protein-coupled receptor kinase 2, dynamin II, β-arrestin 2 and phosphorylated-conventional protein kinase C in membranes of the mouse spinal cord, suggesting that the etorphine-induced μ-opioid receptor desensitization may result from G protein-coupled receptor kinase 2/dynaminII/β-arrestin2-dependent phosphorylation of μ-opioid receptors. Unlike etorphine, morphine failed to change the levels of these trafficking proteins. Furthermore, we found that the level of glial fibrillary acidic protein in the mouse spinal cord was clearly increased by chronic in vivo and in vitro treatment with morphine, whereas no such effect was noted by etorphine. In the behavioral study, intraperitoneal pretreatment with the glial-modulating agent propentofylline suppressed the development of tolerance to morphine-induced antinociception. In addition, intrathecal injection of astrocytes and astrocyte-conditioned medium mixture, which were obtained from cultured astrocytes of the newborn mouse spinal cord, aggravated the development of tolerance to morphine. In contrast, these agents failed to affect the development of tolerance induced by etorphine. These findings provide direct evidence for the distinct mechanisms between etorphine and morphine on the development of tolerance to spinal antinociception. These findings raise the possibility that the increased astroglia response produced by chronic morphine could be associated with the lack of μ-opioid receptor internalization.

Section snippets

Animals

The present study was conducted in accordance with the guidelines established by the Institutional Animal Investigation Committee at Hoshi University and the U.S. National Institutes of Health Guide for the Care and Use of Laboratory Animals. All efforts were made to minimize the use of laboratory animals and to optimize their comfort. Male ICR mice weighing 25–30g (Tokyo Laboratory Animals Science Co., Ltd., Tokyo, Japan) were used. Animals were kept in a room with an ambient temperature of

Development of tolerance to etorphine- or morphine-induced antinociception

We first investigated the development of tolerance to etorphine- or morphine-induced antinociception using hot-plate test. Groups of mice were repeatedly s.c. injected with etorphine (10 μg/kg), morphine (10mg/kg) or saline once a day for seven consecutive days. The injection of etorphine or morphine produced about 70–90% antinociceptive effect on the first day. However, this antinociception was significantly decreased during consecutive exposure to etorphine or morphine, indicating the

Discussion

Following the binding of the agonist to MORs, the receptor signals via activation of heterotrimeric G proteins of the Gi/Go family. With continued exposure to agonist, MORs are rapidly phosphorylated by GRK, and this phosphorylation stimulates the binding of arrestins to the receptor. The MOR/arrestin complex is then recruited to a constitutive pathway that utilized clathrin-coated pits to endocytose a wide variety of cell surface proteins in a dynamin-dependent manner (Keith et al 1996, Chu et

Conclusion

In conclusion, the present data provide direct evidence for the distinct mechanisms between etorphine and morphine on the development of tolerance to antinociception. These findings raise the possibility that the increased astroglia response due to chronic morphine treatment may result in the morphine-specific receptor modulating profile, which could be related to the down-regulation of MOR function without receptor internalization and may actively participate in the development of morphine

Acknowledgments

This work was supported in part by grants from the Ministry of Health, Labor and Welfare, and the Ministry of Education, Culture, Sports, Science and Technology of Japan.

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