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Papers of the Week

Papers: 25 Feb 2023 - 3 Mar 2023

Basic Science

Animal Studies, Molecular/Cellular, Neurobiology, Pharmacology/Drug Development


2023 Mar 01

J Physiol


Morphine-induced mechanical hypersensitivity in mice requires δ receptors, β-arrestin2 and c-Src activity.


Singleton S, Hales TG


Morphine diminishes pain, but its long-term use is compromised by tolerance and hyperalgesia. Studies implicate δ receptors, β-arrestin2 and Src kinase in tolerance. We examined whether these proteins are also involved in morphine-induced hypersensitivity (MIH). A common pathway for tolerance and hypersensitivity may provide a single target to guide improved analgesic approaches. We examined mechanical sensitivity using automated von Frey in wild type (WT) and transgenic male and female C57Bl/6 mice before and after hind paw inflammation by complete Freund’s adjuvant (CFA). CFA-evoked hypersensitivity ceased on day 7 in WT but persisted for the 15-day testing period in μ . Recovery was delayed until day 13 in δ . We explored the expression of opioid genes in the spinal cord using quantitative RT-PCR. Restoration to basal sensitivity in WT occurred with increased δ expression. By contrast, κ expression was reduced, while μ remained unchanged. Daily morphine reduced hypersensitivity in WT on day 3 compared to controls, however hypersensitivity recurred on day 9 and beyond. By contrast, WT had no recurrence of hypersensitivity in the absence of daily morphine. We used β-arrestin2 , δ and Src inhibition by dasatinib in WT to establish whether these approaches, which diminish tolerance, also attenuate MIH. While none of these approaches affected CFA-evoked inflammation or acute hypersensitivity, all caused sustained morphine anti-hypersensitivity, abolishing MIH. Like morphine tolerance, MIH in this model requires δ receptors, β-arrestin2 and Src activity. Our findings suggest that MIH is caused by a tolerance-induced reduction in endogenous opioid signalling. KEY POINTS: Morphine is effective for treating severe acute pain, but tolerance and hypersensitivity often develop during its use in treating chronic pain. It is unclear whether these detrimental effects share similar mechanisms, if so, it might be possible to develop a single approach to minimise both phenomena. Mice deficient in μ receptors, δ receptors or β-arrestin2, and wild type mice treated with the Src inhibitor, dasatinib, exhibit negligible morphine tolerance. We show that these same approaches also prevent the development of morphine-induced hypersensitivity during persistent inflammation. This knowledge identifies strategies, such as the use of Src inhibitors, which may mitigate tolerance and morphine induced hyperalgesia. Abstract figure legend Administration of complete Freund’s adjuvant (CFA) into one hind paw in WT C57BL/6J mice led to transient mechanical hypersensitivity that ceased by day 7 and was associated with changes in the expression of mRNA transcripts encoding opioid peptides and opioid receptors. Spontaneous hypersensitivity was abolished in μ-/- mice, delayed in δ-/- mice and accelerated in WT mice by inhibiting the non-receptor tyrosine kinase, c-Src. Administration of morphine into WT mice initially limited hypersensitivity but this transitioned to reinstatement of mechanical hypersensitivity following repeated daily injections whereas recovery was sustained in mice receiving vehicle injections. Strategies that abolish the development of tolerance to morphine i.e., δ-/- mice, β-arrestin2-/- mice and WT mice administered with the c-Src inhibitor, dasatinib, also prevented reinstatement of mechanical hypersensitivity caused by morphine. Our data imply that morphine tolerance and morphine-induced mechanical hypersensitivity share similar mechanisms. This article is protected by copyright. All rights reserved.