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Opioids are the gold standard for pharmacological treatment of neuropathic pain, but their analgesic effects are unsatisfactory in part due to nerve injury-induced downregulation of opioid receptors in dorsal root ganglia (DRG) neurons. How nerve injury drives such downregulation remains elusive. DNA methyltransferase (DNMT)-triggered DNA methylation represses gene expression. We show here that blocking the nerve injury-induced increase in DRG DNMT3a (a de novo DNMT) rescued the expression of Oprm1 and Oprk1 mRNAs and their respective encoding mu-opioid receptor (MOR) and kappa-opioid receptor (KOR) proteins in the injured DRG. Blocking this increase also prevented the nerve injury-induced increase in DNA methylation in the promoter and 5'-untranslated region of the Oprm1 gene in the injured DRG, restored morphine or loperamide (a peripheral acting MOR preferring agonist) analgesic effects, and attenuated the development of their analgesic tolerance under neuropathic pain conditions. Mimicking this increase reduced the expression of Oprm1 and Oprk1 mRNAs and their coding MOR and KOR in DRG and augmented MOR-gated neurotransmitter release from the primary afferents. Mechanistically, DNMT3a regulation of Oprm1 gene expression required the methyl-CpG-binding protein 1, MBD1, as MBD1 knockout resulted in the decreased binding of DNMT3a to the Oprm1 gene promoter and blocked the DNMT3a-triggered repression of Oprm1 gene expression in DRG neurons. These data suggest that DNMT3a is required for nerve injury-induced and MBD1-mediated epigenetic silencing of the MOR and KOR in the injured DRG. DNMT3a inhibition may serve as a promising adjuvant therapy for opioid use in neuropathic pain management. [ABSTRACT FROM AUTHOR]

Neuropathic pain, a distressing and debilitating disorder, is still poorly managed in clinic. Opioids, like morphine, remain the mainstay of prescribed medications in the treatment of this disorder, but their analgesic effects are highly unsatisfactory in part due to nerve injury-induced reduction of opioid receptors in the first-order sensory neurons of dorsal root ganglia. G9a is a repressor of gene expression. We found that nerve injury-induced increases in G9a and its catalyzed repressive marker H3K9m2 are responsible for epigenetic silencing of Oprm1, Oprk1, and Oprd1 genes in the injured dorsal root ganglia. Blocking these increases rescued dorsal root ganglia Oprm1, Oprk1, and Oprd1 gene expression and morphine or loperamide analgesia and prevented the development of morphine or loperamide-induced analgesic tolerance under neuropathic pain conditions. Conversely, mimicking these increases reduced the expression of three opioid receptors and promoted the mu opioid receptor-gated release of primary afferent neurotransmitters. Mechanistically, nerve injury-induced increases in the binding activity of G9a and H3K9me2 to the Oprm1 gene were associated with the reduced binding of cyclic AMP response element binding protein to the Oprm1 gene. These findings suggest that G9a participates in the nerve injury-induced reduction of the Oprm1 gene likely through G9a-triggered blockage in the access of cyclic AMP response element binding protein to this gene. [ABSTRACT FROM AUTHOR]

The development of opioid-induced analgesic tolerance and hyperalgesia is a clinical challenge for managing chronic pain. Adaptive changes in protein translation in the nervous system are thought to promote opioid tolerance and hyperalgesia; however, how opioids drive such changes remains elusive. Here, we report that mammalian target of rapamycin (mTOR), which governs most protein transhtion, was activated in rat spina dorsal horn neurons after repeated intrathecal morphine injections. Activation was triggered through µ opioid receptor and mediated by intracellular PI3K/Akt. Spinal mTOR inhibition blocked both induction and maintenance of morphine tolerance and hyperalgesia, without affecting basal pain perception or locomotor functions. These effects were attributed to the attenuation of morphine-induced increases in translation initiation activity, nascent protein synthesis, and expression of some known key tolerance-associated proteins, including neuronal NOS (nNOS), in dorsal horn. Moreover, elevating spinal mTOR activity by knocking down the mTOR-negative regulator TSC2 reduced morphine analgesia, produced pain hypersensitivity, and increased spinal nNOS expression. Our findings implicate the µ opioid receptor-triggered PI3K/Akt/mTOR pathway in promoting morphine-induced spinal protein translation changes and associated morphine tolerance and hyperalgesia. These data suggest that mTOR inhibitors could be explored for prevention and/or reduction of opioid tolerance in chronic pain management. [ABSTRACT FROM AUTHOR]