Differential mechanisms of morphine antinociceptive tolerance revealed in (beta)arrestin-2 knock-out mice
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Laura M Bohn1, Robert J Lefkowitz, Marc G Caron
1Howard Hughes Medical Institute Laboratories, Department of Cell Biology, Duke University Medical Center, Durham, North Carolina 27710, USA.
Abstract:
Morphine induces antinociception by activating mu opioid receptors (muORs) in spinal and supraspinal regions of the CNS. (Beta)arrestin-2 (beta)arr2), a G-protein-coupled receptor-regulating protein, regulates the muOR in vivo. We have shown previously that mice lacking (beta)arr2 experience enhanced morphine-induced analgesia and do not become tolerant to morphine as determined in the hot-plate test, a paradigm that primarily assesses supraspinal pain responsiveness. To determine the general applicability of the (beta)arr2-muOR interaction in other neuronal systems, we have, in the present study, tested (beta)arr2 knock-out ((beta)arr2-KO) mice using the warm water tail-immersion paradigm, which primarily assesses spinal reflexes to painful thermal stimuli. In this test, the (beta)arr2-KO mice have greater basal nociceptive thresholds and markedly enhanced sensitivity to morphine. Interestingly, however, after a delayed onset, they do ultimately develop morphine tolerance, although to a lesser degree than the wild-type (WT) controls. In the (beta)arr2-KO but not WT mice, morphine tolerance can be completely reversed with a low dose of the classical protein kinase C (PKC) inhibitor chelerythrine. These findings provide in vivo evidence that the muOR is differentially regulated in diverse regions of the CNS. Furthermore, although (beta)arr2 appears to be the most prominent and proximal determinant of muOR desensitization and morphine tolerance, in the absence of this mechanism, the contributions of a PKC-dependent regulatory system become readily apparent.
Insights
Mice lacking beta-arrestin-2 (betaarr2) show enhanced morphine pain relief but develop tolerance differently. Protein kinase C inhibition reverses tolerance in these mice, revealing distinct mu-opioid receptor regulation.
Area of Science:
- Neuroscience
- Pharmacology
- Molecular Biology
Background:
- Morphine's pain relief (antinociception) relies on mu-opioid receptors (muORs) in the central nervous system.
- Beta-arrestin-2 (betaarr2) is a key regulator of muOR function.
- Previous studies showed betaarr2-deficient mice have enhanced morphine analgesia and no tolerance in supraspinal tests.
Purpose of the Study:
- To investigate the role of beta-arrestin-2 (betaarr2) in morphine tolerance using spinal reflex pathways.
- To determine if betaarr2's regulation of muORs is consistent across different CNS regions.
Main Methods:
- Utilized beta-arrestin-2 knock-out ((betaarr2)-KO) and wild-type (WT) mice.
- Assessed nociceptive responses using the warm water tail-immersion test.
- Administered morphine and evaluated analgesia and tolerance development.
- Tested the effect of chelerythrine, a protein kinase C (PKC) inhibitor, on morphine tolerance.
Main Results:
- (betaarr2)-KO mice exhibited higher basal pain thresholds and increased sensitivity to morphine.
- While developing tolerance later and to a lesser extent than WT mice, (betaarr2)-KO mice did show tolerance.
- Morphine tolerance in (betaarr2)-KO mice was fully reversed by a low dose of chelerythrine.
- WT mice did not show tolerance reversal with chelerythrine.
Conclusions:
- Mu-opioid receptor (muOR) regulation differs across distinct central nervous system (CNS) regions.
- Beta-arrestin-2 (betaarr2) is a primary factor in muOR desensitization and morphine tolerance.
- In the absence of betaarr2, a protein kinase C (PKC)-dependent system becomes crucial for morphine tolerance.