ABSTRACT
BACKGROUND: The analgesic potency of opioids is reduced in neuropathic pain. However, the molecular mechanism is not well understood. RESULTS: The present study demonstrated that increased methylation of the Mu opioid receptor (MOR) gene proximal promoter (PP) in dorsal root ganglion (DRG) plays a crucial role in the decreased morphine analgesia. Subcutaneous (s.c.), intrathecal (i.t.) and intraplantar (i.pl.), not intracerebroventricular (i.c.v.) injection of morphine, the potency of morphine analgesia was significantly reduced in nerve-injured mice compared with control sham-operated mice. After peripheral nerve injury, we observed a decreased expression of MOR protein and mRNA, accompanied by an increased methylation status of MOR gene PP, in DRG. However, peripheral nerve injury could not induce a decreased expression of MOR mRNA in the spinal cord. Treatment with 5-aza-2'-deoxycytidine (5-aza-dC), inhibited the increased methylation of MOR gene PP and prevented the decreased expression of MOR in DRG, thereby improved systemic, spinal and periphery morphine analgesia. CONCLUSIONS: Altogether, our results demonstrate that increased methylation of the MOR gene PP in DRG is required for the decreased morphine analgesia in neuropathic pain.
Subject(s)
Morphine/administration & dosage , Neuralgia , Promoter Regions, Genetic/physiology , Receptors, Opioid, mu/genetics , Receptors, Opioid, mu/metabolism , Sensory Receptor Cells/metabolism , Animals , Azacitidine/analogs & derivatives , Azacitidine/pharmacology , Cell Line, Tumor , CpG Islands/drug effects , Decitabine , Disease Models, Animal , Dose-Response Relationship, Drug , Enzyme Inhibitors/pharmacology , Ganglia, Spinal/cytology , Gene Expression Regulation/drug effects , Hyperalgesia/drug therapy , Methylation , Mice , Neuralgia/drug therapy , Neuralgia/metabolism , Neuralgia/pathology , Neuroblastoma/metabolism , Neuroblastoma/pathology , Pain Measurement , Pain Threshold/drug effects , Promoter Regions, Genetic/drug effects , Sensory Receptor Cells/drug effects , Spinal Cord/drug effects , Spinal Cord/metabolismABSTRACT
There is accumulating evidence to implicate the importance of EphBs receptors and ephrinBs ligands were involved in modulation of spinal nociceptive information. However, the downstream mechanisms that control this process are not well understood. In the present study, we investigated whether phosphatidylinositol 3-kinase (PI3K), as the downstream effectors, participates in modulation of spinal nociceptive information related to ephrinBs/EphBs. Intrathecal injection of ephrinB1-Fc produced a dose- and time-dependent thermal and mechanical hyperalgesia, accompanied by the increase of spinal PI3K-p110γ, phosphorylation of AKT (p-AKT) and c-Fos expression. Pre-treatment with PI3K inhibitor wortmannin or LY294002 prevented activation of spinal AKT induced by ephrinB1-Fc. Inhibition of spinal PI3K signaling dose-dependently prevented and reversed pain behaviors and spinal c-Fos protein expression induced by intrathecal injection of ephrinB1-Fc. Inhibition of EphBs receptors by intrathecal injection of EphB1-Fc reduced formalin-induced inflammation and chronic constrictive injury-induced neuropathic pain behaviors accompanied by decreased expression of spinal PI3K,p-AKT and c-Fos protein. Furthermore, pre-treatment with PI3K inhibitor wortmannin or LY294002 prevented ephrinB1-Fc-induced ERK activation in spinal. These data demonstrated that PI3K and PI3K crosstalk to ERK signaling contributed to modulation of spinal nociceptive information related to ephrinBs/EphBs.
