mir-500-Mediated GAD67 Downregulation Contributes to Neuropathic Pain.

UNLABELLED: Neuropathic pain is a common neurobiological disease involving multifaceted maladaptations ranging from gene modulation to synaptic dysfunction, but the interactions between synaptic dysfunction and the genes that are involved in persistent pain remain elusive. In the present study, we found that neuropathic pain induced by the chemotherapeutic drug paclitaxel or L5 ventral root transection significantly impaired the function of GABAergic synapses of spinal dorsal horn neurons via the reduction of the GAD67 expression. We also found that mir-500 expression was significantly increased and involved in the modulation of GAD67 expression via targeting the specific site of Gad1 gene in the dorsal horn. In addition, knock-out of mir-500 or using mir-500 antagomir rescued the GABAergic synapses in the spinal dorsal horn neurons and attenuated the sensitized pain behavior in the rats with neuropathic pain. To our knowledge, this is the first study to investigate the function significance and the underlying molecular mechanisms of mir-500 in the process of neuropathic pain, which sheds light on the development of novel therapeutic options for neuropathic pain. SIGNIFICANCE STATEMENT: Neuropathic pain is a common neurobiological disease involving multifaceted maladaptations ranging from gene modulation to synaptic dysfunction, but the underlying molecular mechanisms remain elusive. The present study illustrates for the first time a mir-500-mediated mechanism underlying spinal GABAergic dysfunction and sensitized pain behavior in neuropathic pain induced by the chemotherapeutic drug paclitaxel or L5 ventral root transection, which sheds light on the development of novel therapeutic options for neuropathic pain.

Cerebrospinal Fluid Oxaliplatin Contributes to the Acute Pain Induced by Systemic Administration of Oxaliplatin.

BACKGROUND: Systemic administration of oxaliplatin has no effect on the tumors in the central nervous system (CNS) due to the limited concentration of oxaliplatin in the cerebrospinal fluid (CSF), while it was clinically reported that oxaliplatin can induce acute encephalopathy. Currently, the impairment of neuronal functions in the CNS after systemic administration of oxaliplatin remains uninvestigated. METHODS: The von Frey test and the plantar test were performed to evaluate neuropathic pain behavior after a single intraperitoneal administration of oxaliplatin (4 mg/kg) in rats. Inductively coupled plasma-mass spectrometry, electrophysiologic recording, real-time quantitative reverse transcription polymerase chain reaction, chromatin immunoprecipitation, Western blot, immunohistochemistry, and small interfering RNA were applied to understand the mechanisms. RESULTS: Concentration of oxaliplatin in CSF showed a time-dependent increase after a single administration of oxaliplatin. Spinal application of oxaliplatin at the detected concentration (6.6 nM) significantly increased the field potentials in the dorsal horn, induced acute mechanical allodynia (n = 12 each) and thermal hyperalgesia (n = 12 each), and enhanced the evoked excitatory postsynaptic currents and spontaneous excitatory postsynaptic currents in the projection neurokinin 1 receptor-expressing lamina I to II neurons. The authors further found that oxaliplatin significantly increased the nuclear factor-κB p65 binding and histone H4 acetylation in cx3cl1 promoter region. Thus, the upregulated spinal CX3CL1 markedly mediated the induction of central sensitization and acute pain behavior after oxaliplatin administration. CONCLUSIONS: The findings of this study suggested that oxaliplatin in CSF may directly impair the normal function of central neurons and contribute to the rapid development of CNS-related side effects during chemotherapy. This provides novel targets to prevent oxaliplatin-induced acute painful neuropathy and encephalopathy.

The inhibition of spinal synaptic plasticity mediated by activation of AMP-activated protein kinase signaling alleviates the acute pain induced by oxaliplatin.

Our recent findings demonstrated that oxaliplatin entering CNS may directly induce spinal central sensitization, and contribute to the rapid development of CNS-related side effects including acute pain during chemotherapy. However, the mechanism is largely unclear. In the current study, we found that the amplitude of C-fiber-evoked field potentials was significantly increased and the expression of phosphorylated mammalian AMP-activated protein kinase α (AMPKα) was markedly decreased following high frequency stimulation (HFS) or single intraperitoneal injection of oxaliplatin (4mg/kg). Spinal local application of AMPK agonist metformin (25µg) prevented the long term potentiation (LTP) induction and the activation of mTOR/p70S6K signal pathway, and significantly attenuated the acute thermal hyperalgesia and mechanical allodynia following single oxaliplatin treatment. Importantly, we found that incubation of low concentration oxaliplatin at dose of 6.6nM (the detected concentration in CSF following a single intraperitoneal injection of oxaliplatin) also significantly inhibited the AMPKα activation and increased the amplitude of sEPSCs, the number of action potential, and the expression of p-mTOR and p-p70S6K in spinal cord slices. Metformin (25µg) or rapamycin (2µg) inhibited the increased excitability of dorsal horn neurons and the decrease of p-AMPKα expression induced by low concentration oxaliplatin incubation. Furthermore, spinal application of AMPK inhibitor compound C (5µg) induced the spinal LTP, thermal hyperalgesia and mechanical allodynia, and rapamycin attenuated the spinal LTP, the thermal hyperalgesia and mechanical allodynia following oxaliplatin treatment (i.p.). Local application of metformin significantly decreased the mTOR and p70S6K activation induced by tetanus stimulation or oxaliplatin (i.p.). These results suggested that the decreased AMPKα activity via negatively regulating mTOR/p70S6K signal pathway enhanced the synaptic plasticity and contributed to acute pain induced by low concentration of oxaliplatin entering CNS.

Epigenetic upregulation of CXCL12 expression mediates antitubulin chemotherapeutics-induced neuropathic pain.

Clinically, Microtubule-targeted agents-induced neuropathic pain hampers chemotherapeutics for patients with cancer. Here, we found that application of paclitaxel or vincristine increased the protein and mRNA expression of CXCL12 and frequency and amplitude of miniature excitatory post synaptic currents (mEPSCs) in spinal dorsal horn neurons. Spinal local application of CXCL12 induced the long-term potentiation of nociceptive synaptic transmission and increased the amplitude of mEPSCs. Inhibition of CXCL12 using the transgenic mice (CXCL12) or neutralizing antibody or siRNA ameliorated the mEPSC's enhancement and mechanical allodynia. In addition, paclitaxel and vincristine both could increase the phosphorylation of signal transducer and activator of transcription 3 (STAT3) and the acetylation of histone H4 in the CXCL12-expressing neurons. Immunoprecipitation and chromatin immunoprecipitation assays demonstrated that antitubulin chemotherapeutics increased the binding of STAT3 to the CXCL12 gene promoter and the interaction between STAT3 and p300, and contributed to the enhanced transcription of CXCL12 by increasing the acetylation of histone H4 in CXCL12 gene promoter. Inhibition of STAT3 by intrathecal injection of adeno-associated virus encoding Cre and green fluorescent protein into STAT3 mice or inhibitor S3I-201 into rats suppressed the CXCL12 upsurge by decreasing the acetylation of histone H4. Finally, blockade of CXCR4 but not CXCR7 ameliorated the paclitaxel- or vincristine-induced mechanical allodynia. Together, these results suggested that enhanced interaction between STAT3 and p300 mediated the epigenetic upregulation of CXCL12 in dorsal horn neurons, which contributed to the antitubulin chemotherapeutics-induced persistent pain.

[Effects of intrathecal ouabain and tizanidine injection for treatment of neuropathic pain in rats].

OBJECTIVE: To investigate the effects of intrathecal ouabain and tizanidine injection for treatment of neuropathic pain in rats. METHODS: Male SD rats weighing 250-300 g were randomly divided into 5 groups (n = 6), namely the control group, ouabain group, tizanidine group, combined ouabain and tizanidine injection group, and the antagonist group. Intrathecal catheter was implanted 7 days before spinal nerve ligation to establish the neuropathic pain model. Mechanical withdrawal threshold (MWT) before and after intrathecal administration of the agents was recorded in the rats. Isobolographic analysis was performed to evaluate the interactions between the agents. RESULTS: Intrathecal injection of ouabain (0.25-5 microg) or tizanidine (0.5-5 microg) alone produced dose-dependent analgesic effect against the neuropathic pain (P < 0.05). Isobolographic analysis revealed a synergistic interaction between ouabain and tizanidine. Intrathecal pretreatment with atropine (5 microg) or yohimbine (20 microg) antagonized the effects of ouabain and tizanidine administered alone or in combination (P < 0.05). CONCLUSION: Intathecal injection of ouabain or tizanidine produces dose-dependent analgesic effects against neuropathic pain, and their synergistic effect after combined injection probably involves the cholinergic transmission and alpha2 receptor.