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Voltage-dependent sodium (Na) 1.8 channels regulate action potential generation in nociceptive neurons, identifying them as putative analgesic targets. Here, we show that Na1.8 channel plasma membrane localization, retention, and stability occur through a direct interaction with the postsynaptic density-95/discs large/zonula occludens-1-and WW domain-containing scaffold protein called membrane-associated guanylate kinase with inverted orientation (Magi)-1. The neurophysiological roles of Magi-1 are largely unknown, but we found that dorsal root ganglion (DRG)-specific knockdown of Magi-1 attenuated thermal nociception and acute inflammatory pain and produced deficits in Na1.8 protein expression. A competing cell-penetrating peptide mimetic derived from the Na1.8 WW binding motif decreased sodium currents, reduced Na1.8 protein expression, and produced hypoexcitability. Remarkably, a phosphorylated variant of the very same peptide caused an opposing increase in Na1.8 surface expression and repetitive firing. Likewise, in vivo, the peptides produced diverging effects on nocifensive behavior. Additionally, we found that Magi-1 bound to sequence like a calcium-activated potassium channel sodium-activated (Slack) potassium channels, demonstrating macrocomplexing with Na1.8 channels. Taken together, these findings emphasize Magi-1 as an essential scaffold for ion transport in DRG neurons and a central player in pain.-Pryce, K. D., Powell, R., Agwa, D., Evely, K. M., Sheehan, G. D., Nip, A., Tomasello, D. L., Gururaj, S., Bhattacharjee, A. Magi-1 scaffolds Na1.8 and Slack K channels in dorsal root ganglion neurons regulating excitability and pain.
Learn More >Intervertebral disc degeneration is a major cause of chronic low back pain, and excessive loading contributes to intervertebral disc degeneration. However, the lack of an effective bipedal in vivo animal model limits research about this condition.
Learn More >Peripheral nerve injury elicits an enduring increase in the excitability of the spinal dorsal horn. This change, which contributes to the development of neuropathic pain, is a consequence of release and prolonged exposure of dorsal horn neurons to various neurotrophins and cytokines. We have shown in rats that nerve injury increases excitatory synaptic drive to excitatory neurons but decreases drive to inhibitory neurons. Both effects, which contribute to an increase in dorsal horn excitability, appear to be mediated by microglial-derived BDNF. We have used multiphoton Ca imaging and whole-cell recording of spontaneous EPSC's in defined medium organotypic cultures of GAD67-GFP+ mice spinal cord to determine the receptor dependence of these opposing actions of BDNF. In mice, as in rats, BDNF enhances excitatory transmission onto excitatory neurons. This is mediated via presynaptic TrkB and p75 neurotrophin receptors and exclusively by postsynaptic TrkB. By contrast with findings from rats, in mice BDNF does not decrease excitation of inhibitory neurons. The cytokine, macrophage colony stimulating factor 1 (CSF-1) has also been implicated in the onset of neuropathic pain. Nerve injury provokes its synthesis in primary afferents, its release in spinal cord and activation of microglia. We now show that CSF-1 increases excitatory drive to excitatory neurons via a BDNF-dependent mechanism and decreases excitatory drive to inhibitory neurons via BDNF-independent processes. Our findings complete missing steps in the cascade of events whereby peripheral nerve injury instigates increased dorsal horn excitability in the context of central sensitization and the onset of neuropathic pain.
Learn More >Cloxyquin has been reported as a specific activator of TRESK (K2P18.1, TWIK-related spinal cord K+ channel) background potassium channel. In this study, we have synthetized chemically modified analogues of cloxyquin and tested their effects on TRESK and other K2P channels. The currents of murine K2P channels, expressed heterologously in Xenopus oocytes, were measured by two-electrode voltage clamp, whereas the native background K+ conductance of mouse dorsal root ganglion (DRG) neurons was examined by the whole-cell patch clamp method. Some of the analogues retained the activator character of the parent compound, but more interestingly, other derivatives inhibited mouse TRESK current. The inhibitor analogues (A2764 and A2793) exerted state-dependent effect. The degree of inhibition by 100 µM A2764 (77.8±1.5%, n=6) was larger in the activated state of TRESK (i.e. after calcineurin-dependent stimulation) than in the resting state of the channel (42.8±4.3% inhibition, n=7). The selectivity of the inhibitor compounds was tested on several K2P channels. A2793 inhibited TASK-1 (100 µM, 53.4±6%, n=5), while A2764 was more selective for TRESK, it only moderately influenced TREK-1 and TALK-1. The effect of A2764 was also examined on the background K+ currents of DRG neurons. A subpopulation of DRG neurons, prepared from wild-type animals, expressed background K+ currents sensitive to A2764, while the inhibitor did not affect the currents in the DRG neurons of TRESK-deficient mice. Accordingly, A2764 may prove to be useful for the identification of TRESK current in native cells, and for the investigation of the role of the channel in nociception and migraine.
Learn More >Evidence is accumulating that activation of mineralocorticoid (MR) and glucocorticoid (GR) receptors on peripheral sensory neurons modulates pain sensation. While the expression and exact anatomical localization of MR and GR in the various subpopulations of peripheral sensory neurons has been shown in animals, it is still unknown for the human skin. Therefore, we aimed to identify MR and GR mRNA and protein as well as the exact subpopulations of sensory neurons in human versus rat skin. Tissue samples from rat and human skin were subjected to RT-PCR, Western blot and double immunofluorescence confocal analysis of MR and GR with the neuronal markers calcitonin gene-related peptide (CGRP), neurofilament 200 (NF200) and tyrosine hydroxylase (TH). Using RT-PCR we were able to isolate MR as well as GR specific transcripts from human skin. Consistently, Western blot analysis identified MR- as well as GR- specific protein bands at the expected molecular weights of 110 and 87 kD, respectively. Double immunofluorescence confocal microscopy of human skin revealed that MR predominantly colocalized with calcitonin-gene-related peptide (CGRP)-immunoreactive (IR) nociceptive neurons – similar to rat skin – underscoring a pivotal role for MR in the modulation of pain. The majority of GR-immmunoreactivity was localized in peripheral peptidergic CGRP-IR sensory nerve fibers, but in addition on TH-IR sympathetic postganglionic, and NF200-IR myelinated mechanoreceptive nerve fibers, both within human and rat skin. Moreover, GR but not MR were localized in keratinocytes of the epidermal layer of human and rat skin. Overall, our results indicate considerable overlap in sensory neuron expression of MR and GR in humans and rats endorsing a common systems approach in mammals that may modulate the transmission of sensory information by MR and GR activation.
Learn More >Chronic pain is one of the most prevalent chronic diseases in the world. The plastic changes of sensory neurons in dorsal root ganglia (DRG) have been extensively studied as the underlying periphery mechanism. Recent studies revealed that satellite cells, the major glial cells in DRG, also played important roles in the development/modulation of chronic pain. Whether DRG satellite glial cells generate new neurons as their counterparts in enteric nerve ganglia and carotid body do under pathological conditions remains poorly investigated. Here, we report that chronic pain induces proliferation and upregulation of progenitor markers in the sex-determining region Y-box 2 (Sox2)- and platelet-derived growth factor receptor alpha (PDGFRα)-positive satellite glial cells. BrdU incorporation assay revealed the generation of IB4- and CGRP-positive neurons, but not NF200-positive neurons in DRG ipsilateral to injury. Genetic fate tracings showed that PDGFRα-positive cells did not generate neurons, whereas Sox2-positive cells produced both IB4- and CGRP-positive neurons. Interestingly, glial fibrillary acidic protein-positive cells, a subpopulation of Sox2-positive satellites, only gave birth to IB4-positive neurons. Local persistent delivery of tetrodotoxin to the sciatic nerve trunk significantly reduced the pain-induced neurogenesis. Furthermore, patch-clamp studies demonstrated that these glia-derived new neurons could fire action potentials and respond to capsaicin. Taken together, our data demonstrated a chronic pain-induced nociceptive neurogenesis in DRG from Sox2-positive satellite cells, indicating a possible contribution of DRG neurogenesis to the pathology of chronic pain.
Learn More >Opioid-induced respiratory depression results in part from direct activation of μ-opioid receptors expressed in the inspiratory rhythm generator located in the ventrolateral medulla, the preBötzinger ComplexRespiratory neurons within the medulla also express nicotinic acetylcholine receptors, which are made up of five subunits, arranged symmetrically around a central poreActivation of the nicotinic acetylcholine receptor α4, α7, and β2 subunits increases respiratory rhythm, whereas activation of the nicotinic acetylcholine receptor α4β2 or α7 subunits induces analgesia in multiple forms of pain WHAT THIS ARTICLE TELLS US THAT IS NEW: The nonselective nicotinic acetylcholine receptor agonist nicotine and the α4β2 nicotinic acetylcholine receptor agonist A85380, but not the α7 nicotinic acetylcholine receptor agonist PNU282987, reversed respiratory depression induced by activation of μ-opioid receptors in rats both in vitro and in vivoCoadministration of A85380 with fentanyl not only markedly reduced respiratory depression and apneas but also enhanced the fentanyl-induced analgesia BACKGROUND:: Opioid analgesics are widely used for treatment of acute, postoperative, and chronic pain. However, activation of opioid receptors can result in severe respiratory depression. There is an unmet clinical need to develop a pharmacologic therapy to counter opioid-induced respiratory depression without interfering with analgesia. Further, additional advances to confront accidental lethal overdose with the use of fentanyl and other opioids are needed. Here, the authors test the hypothesis that activation of nicotinic receptors expressed within respiratory rhythm-generating networks would counter opioid-induced respiratory depression without compromising analgesia.
Learn More >Painful peripheral neuropathy is the most dose-limiting side effect of paclitaxel (PTX), a widely used anti-cancer drug to treat solid tumours. The understanding of the mechanisms involved in this side effect is crucial to the development of new therapeutic approaches. CXCL1 chemokine and its receptor CXCR2 have been pointed as promising targets to treat chronic pain. Herein, we sought to evaluate the possible involvement of CXCL1 and CXCR2 in the pathogenesis of PTX-induced neuropathic pain in mice. PTX treatment led to increased levels of CXCL1 in both dorsal root ganglion and spinal cord samples. Systemic treatment with the anti-CXCL1 antibody (10 μg/kg, i.v.) or the selective CXCR2 antagonist (SB225002, 3 mg/kg, i.p.) had minor effect on PTX-induced mechanical hypersensitivity. On the other hand, the intrathecal (i.t.) treatment with anti-CXCL1 (1 ng/site) or SB225002 (10 μg/site) consistently inhibited the nociceptive responses of PTX-treated mice. Similar results were obtained by inhibiting the PI3Kγ enzyme a downstream pathway of CXCL1/CXCR2 signalling with either the selective AS605240 (5 μg/site, i.t.) or the non-selective wortmannin PI3K inhibitor (0.4 μg/site, i.t.). Overall, the data indicates that the up-regulation of CXCL1 is important for the development and maintenance of PTX-induced neuropathic pain in mice. Therefore, the spinal blockage of CXCL1/CXCR2 signalling might be a new innovative therapeutic approach to treat this clinical side effect of PTX.
Learn More >Non-selective glutamate AMPA receptor antagonists are efficacious in chronic pain, but have significant tolerability issues, likely arising from the ubiquitous expression of AMPA receptors in CNS. Recently, LY3130481 has been shown to selectively block AMPA receptors co-assembled with the auxiliary protein, TARP γ8, which is highly expressed in hippocampus, but also in pain pathways, including anterior cingulate (ACC) and somatosensory (SS) cortices and spinal cord, suggesting that selective blockade γ8/AMPA receptors may suppress nociceptive signaling with fewer CNS side effects. The potency of LY3130481 on recombinant γ8-containing AMPA receptors was modulated by co-expression with other TARPs; γ2 subunits affected activity more than γ3 subunits. Consistent with these findings, LY3130481 had decreasing potency on receptors from rat hippocampal, cortical, spinal cord, and cerebellar neurons that was replicated in tissue from human brain. LY3130481 partially suppressed, whereas the non-selective AMPA antagonist GYKI53784 completely blocked AMPA receptor-dependent EPSPs in ACC and spinal neurons in vitro. Similarly, LY3130481 attenuated short-term synaptic plasticity in spinal sensory neurons in vivo in response stimulation of peripheral afferents. LY3130481 also significantly reduced nocifensive behaviors after intraplantar formalin that was correlated with occupancy of CNS γ8-containing AMPA receptors. In addition, LY3130481 dose-dependently attenuated established gait impairment after joint damage and tactile allodynia after spinal nerve ligation; all in the absence of motor side effects. Collectively, these data demonstrate that LY3130481 can suppress excitatory synaptic transmission and plasticity in pain pathways containing γ8/AMPA receptors and significantly reduce nocifensive behaviors, suggesting a novel, effective and safer therapy for chronic pain conditions.
Learn More >The mechanisms responsible for the persistence of chemotherapy-induced peripheral neuropathy (CIPN) in a significant proportion of cancer survivors are still unknown. Our previous findings show that CD8 T cells are necessary for the resolution of paclitaxel-induced mechanical allodynia in male mice. In the present study, we demonstrate that CD8 T cells are not only essential for resolving cisplatin-induced mechanical allodynia, but also to normalize spontaneous pain, numbness, and the reduction in intra-epidermal nerve fiber density in male and female mice. Resolution of CIPN was not observed in Rag2 mice that lack T and B cells. Reconstitution of Rag2 mice with CD8 T cells prior to cisplatin treatment normalized the resolution of CIPN. In vivo education of CD8 T cells by cisplatin was necessary to induce resolution of CIPN in Rag2 mice because adoptive transfer of CD8 T cells from naïve WT mice to Rag2 mice after completion of chemotherapy did not promote resolution of established CIPN. The CD8 T cell-dependent resolution of CIPN does not require epitope recognition by the T cell receptor (TCR). Moreover, adoptive transfer of cisplatin-educated CD8 T cells to Rag2 mice prevented CIPN development induced by either cisplatin or paclitaxel, indicating that the activity of the educated CD8 T is not cisplatin-specific.In conclusion, resolution of CIPN requires in vivo education of CD8 T cells by exposure to cisplatin. Future studies should examine whether ex vivo CD8 T cell education could be applied as a therapeutic strategy for treating or preventing CIPN in patients.
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