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Background: Severe postoperative pain remains a clinical problem that impacts patient's rehabilitation. The present work aims to investigate the role of Toll-like receptor-4 (TLR4) activation in wounded plantar tissue and dorsal root ganglion (DRG) in the genesis of postoperative pain and its underlying mechanisms. Results: Postoperative pain was induced by plantar incision in rat hind paw. Plantar incision led to increased expression of TLR4 in ipsilateral lumbar 4-5 (L4/L5) DRGs, which occurred at 2 h and was persistent to the third day after surgery. Similar to the change in TLR4 expression, there was also significant increase in phosphorylated nuclear factor-kappa B p65 (p-p65) in DRGs after surgery. Immunofluorescence staining revealed that the increased expressions of TLR4 and p-p65 not only in neuronal cells but also in satellite glial cells in DRG. Furthermore, the enhanced expressions of TLR4 and p-p65 were also detected in plantar tissues around the incision, which was observed starting at 2 h and lasting until the third day after surgery. Prior intrathecal (i.t.) injections of TAK-242 (a TLR4-specific antagonist) or 4',6-diamidino-2-phenylindole-dihydrochloride (PDTC, a nuclear factor-kappa B activation inhibitor) dose dependently alleviated plantar incision-induced mechanical allodynia and thermal hyperalgesia and inhibited the increased expressions of p-p65, tumor necrosis factor-alpha, and interleukin-1 beta in DRG. Prior subcutaneous (s.c.) plantar injection of TAK-242 or PDTC also ameliorated pain-related hypersensitivity following plantar incision. Moreover, the plantar s.c. injection of TAK-242 or PDTC inhibited the increased expressions of p-p65, tumor necrosis factor-alpha, and interleukin-1 beta not only in local wounded plantar tissue but also dramatically in ipsilateral lumbar 4-5 DRGs. Conclusion: TLR4/ nuclear factor-kappa B signaling activation in local injured tissue and DRG contribute to the development of postoperative pain via regulating pro-inflammatory cytokines release. Targeting TLR4/ nuclear factor-kappa B signaling in local tissue at early stage of surgery may be an effective strategy for the treatment of postoperative pain. [ABSTRACT FROM AUTHOR]

Peripheral nerve injury-caused hyperexcitability and abnormal ectopic discharges in the primary sensory neurons of dorsal root ganglion (DRG) play a key role in neuropathic pain development and maintenance. The two-pore domain background potassium (K2P) channels have been identified as key determinants of the resting membrane potential and neuronal excitability. However, whether K2P channels contribute to neuropathic pain is still elusive. We reported here that K2P1.1, the first identified mammalian K2P channel, was highly expressed in mouse DRG and distributed in small-, medium-, and large-sized DRG neurons. Unilateral lumbar (L) 4 spinal nerve ligation led to a significant and time-dependent reduction of K2P1.1 mRNA and protein in the ipsilateral L4 DRG, but not in the contralateral L4 or ipsilateral L3 DRG. Rescuing this reduction through microinjection of adeno-associated virus-DJ expressing full-length K2P1.1 mRNA into the ipsilateral L4 DRG blocked spinal nerve ligation-induced mechanical, thermal, and cold pain hypersensitivities during the development and maintenance periods. This DRG viral microinjection did not affect acute pain and locomotor function. Our findings suggest that K2P1.1 participates in neuropathic pain development and maintenance and may be a potential target in the management of this disorder. [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]

Background: Peripheral nerve injury leads to changes in gene expression in primary sensory neurons of the injured dorsal root ganglia. These changes are believed to be involved in neuropathic pain genesis. Previously, these changes have been identified using gene microarrays or next generation RNA sequencing with poly-A tail selection, but these approaches cannot provide a more thorough analysis of gene expression alterations after nerve injury. Methods: The present study chose to eliminate mRNA poly-A tail selection and perform strand-specific next generation RNA sequencing to analyze whole transcriptomes in the injured dorsal root ganglia following spinal nerve ligation. Quantitative real-time reverse transcriptase polymerase chain reaction assay was carried out to verify the changes of some differentially expressed RNAs in the injured dorsal root ganglia after spinal nerve ligation. Results: Our results showed that more than 50 million (M) paired mapped sequences with strand information were yielded in each group (51.87 M-56.12M in sham vs. 51.08 M-57.99M in spinal nerve ligation). Six days after spinal nerve ligation, expression levels of 11,163 out of a total of 27,463 identified genes in the injured dorsal root ganglia significantly changed, of which 52.14% were upregulated and 47.86% downregulated. The largest transcriptional changes were observed in proteincoding genes (91.5%) followed by noncoding RNAs. Within 944 differentially expressed noncoding RNAs, the most significant changes were seen in long interspersed noncoding RNAs followed by antisense RNAs, processed transcripts, and pseudogenes.We observed a notable proportion of reads aligning to intronic regions in both groups (44.0% in sham vs. 49.6% in spinal nerve ligation). Using quantitative real-time polymerase chain reaction, we confirmed consistent differential expression of selected genes including Kcna2, Oprm1 as well as lncRNAs Gm21781 and 4732491K20Rik following spinal nerve ligation. Conclusion: Our findings suggest that next generation RNA sequencing can be used as a promising approach to analyze the changes of whole transcriptomes in dorsal root ganglia following nerve injury and to possibly identify new targets for prevention and treatment of neuropathic pain. [ABSTRACT FROM AUTHOR]

Voltage-gated potassium (Kv) channels are critical in controlling neuronal excitability and are involved in the induction of neuropathic pain. Therefore, Kv channels might be potential targets for prevention and/or treatment of this disorder. We reported here that a majority of dorsal root ganglion (DRG) neurons were positive for Kv channel alpha subunit Kv1.2. Most of them were large and medium, although there was a variety of sizes. Peripheral nerve injury caused by lumbar (L)5 spinal nerve ligation (SNL) produced a time-dependent reduction in the number of Kv1.2-positive neurons in the ipsilateral L5 DRG, but not in the contralateral L5 DRG. Such reduction was also observed in the ipsilateral L5 DRG on day 7 after sciatic nerve axotomy. Rescuing nerve injury-induced reduction of Kv1.2 in the injured L5 DRG attenuated the development and maintenance of SNL-induced pain hypersensitivity without affecting acute pain and locomotor function. This effect might be attributed to the prevention of SNL-induced upregulation of endogenous Kv1.2 antisense RNA, in addition to the increase in Kv1.2 protein expression, in the injured DRG. Our findings suggest that Kv1.2 may be a novel potential target for preventing and/or treating neuropathic pain. [ABSTRACT FROM AUTHOR]