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Neuropathic pain is evoked by aberrant sensory processing in the peripheral or central nervous system, which is characterized by persistent pain, tactile allodynia, or hyperalgesia. Neuroinflammation is associated with the initiation and maintenance of persistent pain in both the peripheral and central nervous systems. Hydrogen sulfide plays important regulatory roles in different physiological and pathological conditions. Therefore, we investigated the effect of hydrogen sulfide on allodynia, hyperalgesia and cytokine release in rats with neuropathic pain and the related regulatory mechanism. Neuropathic pain was established by chronic constriction injury (CCI) of the sciatic nerve in rats. Nuclear factor erythroid-2 (NF-E2)-related factor 2 (Nrf2) siRNA, hemin, Sn-protoporphyrin (SnPP)-IX and/or NaHS were administered to rats with neuropathic pain, and the spinal cord was collected to detect the expression of Nrf2, hemeoxygenase-1 (HO-1), nuclear factor-kappa B (NF-κb) and the cytokines tumor necrosis factor (TNF)-α, interleukin (IL)-1β, IL-6 and high mobility group box (HMGB)-1 by Western blot (WB) analysis, reverse transcription polymerase chain reaction (RT-PCR), immunofluorescence or enzyme-linked immunosorbent assay (ELISA). Mechanical allodynia, thermal hyperalgesia and the number of paw lifts were measured at different time points after operation. In the present research, neuropathic pain induced Nrf2 and HO-1 expression in the microglial cells of the spinal cord; Nrf2 and HO-1 were necessary to alleviate the hyperalgesia of CCI-induced rats; NaHS mitigated the hyperalgesia and allodynia induced by the CCI operation; and NaHS mitigated the excessive release of the cytokines TNF-α, IL-1β, IL-6 and HMGB1 via the Nrf2/HO-1 pathway in the microglial cells of the spinal cord. These results indicated that NaHS exhibited antinociceptive and anti-inflammatory effects that were associated with the activation of the Nrf2/HO-1 pathway in the spinal cord of rats with neuropathic pain.
Learn More >The κ-opioid receptor (KOR) system has been implicated in the regulation of many behaviors including pain. While there are numerous studies suggesting KOR regulation of pain being mediated spinally, there have been reports of pain-like behaviors regulated by central KOR signaling. In particular, oxytocin-induced analgesia appears to be mediated by KOR receptors within the ventrolateral periaqueductal gray (vlPAG). We recently found that activation of dopamine (DA) neurons within the vlPAG is antinociceptive. In this study, we sought to determine the impact of KOR signaling on -GABAergic inputs onto vlPAG DA neurons, and the mechanism through which KOR impacts these inputs. We found that activation of KOR reduced GABAergic transmission onto vlPAG DA neurons. In addition, our data suggest this effect is mediated presynaptically via the G protein βγ-subunit. They raise the possibility that KOR activation disinhibits -vlPAG DA neurons, which could lead to altered regulation of pain-related behaviors.
Learn More >Injury associated pain involves subjective perception and emotional experience. The anterior cingulate cortex (ACC) is a key area involved in the affective component of pain processing. However, the neuroimmune mechanisms underlying enhanced ACC excitability following peripheral nerve injury are still not fully understood. Our previous work has shown that tumor necrosis factor-alpha (TNF-α) overexpression leads to peripheral afferent hyperexcitability and synaptic transmission potentiation in spinal cord. Here, we aimed to reveal the potential role of ACC TNF-α in ACC hyperexcitability and neuropathic pain. c-Fos, a widely used neuronal activity marker, was induced especially in contralateral ACC early [postoperative (PO) 1 h] and later (PO day 7 and 10) during the development of neuropathic pain. Spared nerve injury (SNI) elevated TNF-α level in contralateral ACC from PO day 5 to 14, delayed relative to decreased ipsilateral paw withdrawal threshold apparent from PO day 1 to 14. Microinjection of anti-TNF-α antibody into the ACC completely eliminated c-Fos overexpression and greatly attenuated pain aversion and mechanical allodynia induced by SNI, suggesting an important role of ACC TNF-α in the pain aversiveness and pain maintenance. Furthermore, modulating ACC pyramidal neurons via a Gi-coupled human M4 muscarinic receptor (hM4Di) or a Gq-coupled human M3 muscarinic receptor (hM3Dq), a type of designer receptors exclusively activated by designer drugs (DREADD), greatly changed the ACC TNF-α level and the mechanical paw withdrawal threshold. The positive interactions between TNF-α and ACC neurons might modulate the cytokine microenvironment thus contribute to the neuropathic pain.
Learn More >Spinal cord stimulation (SCS) is used clinically to limit chronic pain but questions remain on the identity of axonal populations recruited. We developed an adult mouse spinal cord preparation to assess recruitment following delivery of clinically-analogous stimuli determined by downscaling a finite element model of clinical SCS. Analogous electric field distributions were generated with 300 µm x 300 µm electrodes positioned 200 µm above the dorsal column (DC) with stimulation between 50-200 µA. We compared axonal recruitment using electrodes of comparable size and stimulus amplitudes – when contacting the caudal thoracic DC, and at 200 or 600 mm above. Antidromic responses recorded from the DC, the adjacent Lissauer tract (LT), and in dorsal roots (DRs) were found to be amplitude and site-dependent. Responses in the DC included a unique component not seen in DRs, having the lowest SCS recruitment amplitude and fastest conduction velocity. At 200 mm above, mean cathodic SCS recruitment threshold for axons in DRs and LT were 2.6 and 4.4 times higher, respectively, than DC threshold. SCS recruited primary afferents in all (up to 9) caudal segments sampled. While A and C fibers could be recruited at nearby segments, only A fiber recruitment and synaptically-mediated dorsal root reflexes were observed in more distant segments. In sum, clinically-analogous SCS led to multisegmental recruitment of several somatosensory-encoding axonal populations. Most striking is the possibility that the lowest threshold recruitment of a non-primary afferent population in the DC are postsynaptic dorsal column tract cells (PSDCs) projecting to gracile nuclei.
Learn More >The specific mechanisms underlying cyclin-dependent kinase 5 (Cdk5)-mediated neuropathic pain at the spinal cord level remain elusive. The aim of the present study was to explore the role of crosstalk between Cdk5/p35 and extracellular signal-regulated kinase 1/2 (ERK1/2) signalling in mediating spinal astrocyte activity via the PPARγ pathway in a rat model of chronic constriction injury (CCI). Here, we quantified pain behaviour after CCI; detected the localization of p35, Cdk5, phosphorylated ERK1/2 (pERK1/2), phosphorylated peroxisome proliferator-activated receptor γ (pPPARγ), neuronal nuclei (NeuN, a neuronal marker), glial fibrillary acidic protein (GFAP, an activated astrocyte marker) and ionized calcium binding adaptor molecule 1 (IBA1, a microglial marker) in the dorsal horn (DH) using immunofluorescence; measured the protein levels of Cdk5, p35, pERK1/2, pPPARγ and GFAP using Western blot analysis; and gauged the enzyme activity of Cdk5/p35 kinase by a Cdk5/p35 kinase activity assay kit. Tumour necrosis factor-α (TNF-α), interleukin (IL)-1β and IL-6 levels were measured by ELISA. Ligation of the right sciatic nerve induced mechanical allodynia; thermal hyperalgesia; and the time-dependent upregulation of p35, pERK1/2 and GFAP and downregulation of pPPARγ. p35 colocalized with Cdk5, pERK1/2, pPPARγ, neurons and astrocytes but not microglia. Meanwhile, intrathecal injection of the Cdk5 inhibitor roscovitine, the mitogen-activated ERK kinase (MEK) inhibitor U0126 and the PPARγ agonist pioglitazone prevented or reversed behavioural allodynia, increased pPPARγ expression, inhibited astrocyte activation, and alleviated proinflammatory cytokine (TNFα, IL-1β, and IL-6) release from activated astrocytes. Furthermore, crosstalk between the Cdk5/p35 and ERK1/2 pathways was observed with CCI. Blockade of either Cdk5/p35 or ERK1/2 inhibited Cdk5 activity. These findings indicate that spinal crosstalk between the Cdk5/p35 and ERK1/2 pathways mediates astrocyte activity via the PPARγ pathway in CCI rats and that targeting this crosstalk could be an effective strategy to attenuate CCI and astrocyte-derived neuroinflammation. This article is protected by copyright. All rights reserved.
Learn More >Vincristine, a widely used antineoplastic agent, is known to be neurotoxic and to lead to chemotherapy-induced peripheral neuropathy (CIPN), which is characterized by nerve damage. Growing evidence suggests that disruption of intracellular calcium homeostasis in peripheral neurons contributes largely to the pathological conditions of CIPN. Our previous study showed that forced expression of a peripheral nerve injury-induced small heat shock protein (Hsp), Hsp27, accelerates axon regeneration and functional recovery. In the current study, we examined whether neuronal expression of human Hsp27 (hHsp27) can prevent the inhibitory effects of vincristine in two mouse models of peripheral nerve injury, namely, sciatic nerve crush and CIPN.
Learn More >Botulinum neurotoxin type A, an FDA-approved prophylactic drug for chronic migraine, is thought to achieve its therapeutic effect through blocking activation of unmyelinated meningeal nociceptors and their downstream communications with myelinated nociceptors and potentially the vasculature and immune cells. Prior investigations to determine botulinum neurotoxin type A effects on meningeal nociceptors were carried out in male rats and tested with stimuli that act outside the blood brain barrier. Here, we sought to explore the effects of extracranial injections of botulinum neurotoxin type A on activation of meningeal nociceptors by cortical spreading depression, an event which occurs inside the blood brain barrier, in female rats.
Learn More >Comorbid depressive symptoms (CDS) in chronic pain are a common health problem, but the neural circuit mechanisms underlying these symptoms remain unclear. Here we identify a novel pathway involving 5-hydroxytryptamine (5-HT) projections from the dorsal raphe nucleus (5-HT) to somatostatin (SOM)-expressing and non-SOM interneurons in the central nucleus of the amygdala (CeA). The SOM neurons project directly to the lateral habenula, an area known involved in depression. Inhibition of the 5-HT→SOM pathway produced depression-like behavior in a male mouse model of chronic pain. Activation of this pathway using pharmacological or optogenetic approaches reduced depression-like behavior in these mice. Human functional magnetic resonance imaging data showed that compared to healthy controls, functional connectivity between the CeA-containing centromedial amygdala and the DRN was reduced in patients with CDS but not in patients in chronic pain without depression. These findings indicate that a novel 5-HT→SOM→lateral habenula pathway may mediate at least some aspects of CDS.
Learn More >Membrane metallo-endopeptidase (MME), also known as neprilysin (NEP), has been of interest for its role in neurodegeneration and pain due to its ability to degrade β-amyloid and substance-P, respectively. In addition to its role in the central nervous system, MME has been reported to be expressed in the peripheral system, specifically in the inner and outer border of myelinating fibers, in the Schmidt-Lantermann cleft and in the paranodes. Recently, mutations of this gene have been associated with Charcot-Marie-Tooth Type 2 (CMT2). Peripheral nerve morphometry in mice lacking MME previously showed minor abnormalities in aged animals in comparison to CMT2 patients. We found that MME expression was dysregulated after nerve injury in a Neuregulin-1 dependent fashion. We therefore explored the hypothesis that MME may have a role in remyelination. In the naïve state in adulthood we did not find any impairment in myelination in MME KO mice. After nerve injury the morphological outcome in MME KO mice was indistinguishable from WT littermates in terms of axon regeneration and remyelination. We did not find any difference in functional motor recovery. There was a significant difference in sensory function, with MME KO mice starting to recover response to mechanical stimuli earlier than WT. The epidermal reinnnervation, however, was unchanged and this altered sensitivity may relate to its known function in cleaving the peptide substance-P, known to sensitise nociceptors. In conclusion, although MME expression is dysregulated after nerve injury in a NRG1-dependent manner this gene is dispensable for axon regeneration and remyelination after injury.
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