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Crystal structures of the σ2 receptor template large-library docking for selective chemotypes active in vivo.

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Long-term sensitization of rat spinal neurons induced by adolescent psychophysical stress is further enhanced by a mild-nociceptive lumbar input.

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β2 adrenergic receptor mediates noradrenergic action to induce CREB phosphorylation in satellite glial cells of dorsal root ganglia to regulate visceral hypersensitivity.

Sympathoneuronal outflow into dorsal root ganglia (DRG) is suggested to be involved in sympathetically maintained chronic pain, which is mediated by norepinephrine (NE) action on DRG cells. The present study combined in vitro and in vivo approaches to identify the cell types of DRG that received NE action and examined cell-type specific expression of adrenergic receptors (ARs) in DRG. Using DRG explants, we identified that NE acted on satellite glial cells (SGCs) to induce the phosphorylation of cAMP response element-binding (CREB). Using primarily cultured SGCs, we identified that beta (β)2AR but not alpha (α)AR nor other βAR isoforms mediated NE-induced CREB phosphorylation and CRE-promoted luciferase transcriptional activity. Using fluorescence in situ hybridization and affinity purification of mRNA from specific cell types, we identified that β2AR was expressed by SGCs but not DRG neurons. We further examined β2AR expression and CREB phosphorylation in vivo in a model of colitis in which sympathetic nerve sprouting in DRG was observed. We found that β2AR expression and CREB phosphorylation were increased in SGCs of thoracolumbar DRG on day 7 following colitis induction. Inhibition but not augmentation of β2AR reduced colitis- induced calcitonin gene-related peptide (CGRP) release into the spinal cord dorsal horn and colonic pain responses to colorectal distention. Prolonged activation of β2AR in naïve DRG increased CGRP expression in DRG neurons. These findings provide molecular basis of sympathetic modulation of sensory activity and chronic pain that involves β2AR-mediated signaling in SGCs of DRG.

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Sympathectomy decreases pain behaviors and nerve regeneration by downregulating monocyte chemokine CCL2 in dorsal root ganglia in the rat tibial nerve crush model.

Peripheral nerve regeneration is associated with pain in several preclinical models of neuropathic pain. Some neuropathic pain conditions and preclinical neuropathic pain behaviors are improved by sympathetic blockade. In this study we examined the effect of a localized "microsympathectomy," i.e., cutting the gray rami containing sympathetic postganglionic axons where they enter the L4 and L5 spinal nerves, which is more analogous to clinically used sympathetic blockade compared to chemical or surgical sympathectomy. We also examined manipulations of CCL2 (monocyte chemoattractant protein 1; MCP-1), a key player in both regeneration and pain. We used rat tibial nerve crush as a neuropathic pain model in which peripheral nerve regeneration can occur successfully. CCL2 in the sensory ganglia was increased by tibial nerve crush and reduced by microsympathectomy. Microsympathectomy and localized siRNA-mediated knockdown of CCL2 in the lumbar DRG had very similar effects: partial improvement of mechanical hypersensitivity and guarding behavior; reduction of regeneration markers growth-associated protein 43 (GAP43) and activating transcription factor 3 (ATF3); and reduction of macrophage density in the sensory ganglia and regenerating nerve. Microsympathectomy reduced functional regeneration as measured by myelinated action potential propagation through the injury site and denervation-induced atrophy of the tibial-innervated gastrocnemius muscle at day 10. Microsympathectomy plus CCL2 knockdown had behavioral effects similar to microsympathectomy alone. The results show that local sympathetic effects on neuropathic pain may be mediated in large part by the effects on expression of CCL2, which in turn regulates the regeneration process.

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Spinal endomorphins attenuate burn-injury pain in male mice by inhibiting p38 MAPK signaling pathway through the mu-opioid receptor.

Burn injury is one of the main causes of mortality worldwide and frequently associated with severe and long-lasting pain that compromises the quality of patient life. Several studies have shown that the mu-opioid system plays an important role in burn pain relief. In this study, we investigated the spinal antinociception induced by the endogenous mu-opioid receptor (MOR) agonists endomorphins and explored their mechanisms of actions in burn injury-induced pain model. Our results showed that intrathecal injection of endomorphin-1 and -2 dose-dependently attenuated mechanical allodynia and thermal hyperalgesia via the mu-opioid receptor in mice on day 3 after burn injury, which was consistent with the data obtained from the mu-opioid receptor knockout mice. Western blot showed that the phosphorylation levels of extracellular signal-regulated kinase1/2 (ERK1/2) and p38 mitogen-activated protein kinase (p38 MAPK) in ipsilateral spinal cord tissues were significantly up-regulated after burn injury. Intrathecal injection of endomorphins selectively inhibited the activation of p38 MAPK on day 3 after burn injury via the mu-opioid receptor. Further studies found that repeated application of the specific p38 MAPK inhibitor SB203580 dose-dependently inhibited burn-injury pain, as well as the activation of spinal p38 MAPK. Taken together, our present study demonstrates that intrathecal injection of endomorphins attenuates burn-injury pain in male mice by affecting the spinal activation of p38 MAPK via the mu-opioid receptor.

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Up-regulating TIPE2 alleviates inflammatory pain by suppressing microglial activation-mediated inflammatory response via inhibiting Rac1/NF-κB pathway.

TNF-α-inducible protein 8-like 2 (TIPE2) is a recently discovered regulator of inflammation that can maintain immune homeostasis, exerting a significant role in the development of inflammation-related diseases. Here, we aimed to explore the role and potential regulatory mechanism of TIPE2 in the progression of inflammatory pain. In the present study, a mouse BV2 microglia cell activation-mediated inflammatory model was developed with LPS induction, and a mouse inflammatory pain model was established with complete Freund's adjuvant (CFA) injection. In vitro, the TIPE2 expression was decreased in LPS-induced BV2 cells. Overexpression of TIPE2 mitigated LPS-medicated microglial activation via decreasing nitric oxide (NO) generation and the expression of microglia marker IBA-1. Notably, increasing TIPE2 expression alleviated microglial activation-triggered expression levels and releases of proinflammatory factors such as TNF-α, IL-1β, and IL-6. Mechanism analysis verified that overexpression of TIPE2 blunted Rac1-mediated activation of NF-κB pathway following LPS stimulation. More importantly, CFA injection reduced the expression of TIPE2 in a mouse inflammatory pain model and overexpression of TIPE2 alleviated CFA-mediated pain hypersensitivity and inflammatory response, and inactivated microglia cell in vivo. Furthermore, overexpression of TIPE2 decreased Rac1 expression and suppressed the activation of NF-κB pathway in spinal cord after CFA injection. In summary, the present study revealed that overexpression of TIPE2 mitigated inflammatory pain through suppressing microglial activation-induced inflammation by inactivating Rac1/NF-κB pathway. The study provides a novel theoretical foundation for the therapy of inflammatory pain.

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Synergism between oral paracetamol and nefopam in a murine model of postoperative pain.

The use of paracetamol or nefopam for postoperative pain control is limited by the need of high doses associated with unwanted effects. Previous works suggest positive interactions between both compounds that may be exploited to obtain potentiation of antinociception.

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An orbitofrontal cortex to midbrain projection modulates hypersensitivity after peripheral nerve injury.

Neuropathic pain is a debilitating condition that is often refractory to treatment. The network of neural substrates for pain transmission and control within the brain is complex and remains poorly understood. Through a combination of neuronal tracing, optogenetics, chemogenetics, electrophysiological recordings, and behavioral assessment, we demonstrate that activation of layer 5 pyramidal neurons in the ventrolateral orbitofrontal cortex (vlOFC) attenuates mechanical and thermal hypersensitivity and cold allodynia in mice with neuropathic pain induced by spared nerve injury (SNI). These vlOFC output neurons project to the posterior ventrolateral periaqueductal gray (vlPAG) region and receive inputs from the ventromedial thalamus (VM). Specific optogenetic and chemogenetic activation of the vlOFC-vlPAG and the VM-vlOFC circuits inhibits hypersensitivity associated with neuropathy. Thus, we reveal a modulatory role of the vlOFC and its projections to the vlPAG circuit in the processing of hypersensitive nociception.

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Kappa-opioid receptor-mediated thermal analgesia evoked by the intrathecal administration of the chemokine CCL1 in mice.

The chemokine CC motif ligand 1 (CCL1) participates in immune cell recruitment and, as for other chemokines, is also involved in nociceptive processing. In contrast with previous reports indicating its participation in allodynia and cold hypernociception when spinally administered, its ability to evoke heat thermal analgesia, mediated by circulating leukocytes and endocannabinoids, after systemic administration has recently been reported. Aiming to explore the possible role played by CCL1 on spinal nociception, we study here the effect of its intrathecal administration on thermal nociception in mice. The intrathecal administration of CCL1 (0.3-30 ng) produced dose-dependent analgesia as measured by the unilateral hot plate test. This analgesia evoked by CCL1 peaked 1 h after injection, was prevented by the CCR8 antagonist R243 and was accompanied by a reduction of c-Fos protein expression in dorsal horn spinal neurons. Blood leukocyte depletion did not modify CCL1-evoked analgesic responses that, in contrast, were abolished by the microglial inhibitor minocycline, but not the astroglial inhibitor aminoadipate, suggesting the involvement of spinal microglial cells. Furthermore, analgesia remained unmodified by the coadministration of cannabinoid type 1 or 2 receptors antagonists (AM251 or SR144285). However, it was reversed by naloxone but not by cyprodime or naltrindole (selective antagonists of mu- or delta- opioid receptors). The inhibitory effect induced by the selective kappa-opioid receptor antagonist, nor-binaltorphimine, and by an anti-dynorphin A 1-17 antibody indicates that analgesia evoked by spinal CCL1 is mediated by endogenous dynorphins acting on kappa-opioid receptors. Endogenous dynorphin and microglia behave as key players in heat thermal analgesia evoked by spinal CCL1 in mice.

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Microglia and Inhibitory Circuitry in the Medullary Dorsal Horn: Laminar and Time-Dependent Changes in a Trigeminal Model of Neuropathic Pain.

Craniofacial neuropathic pain affects millions of people worldwide and is often difficult to treat. Two key mechanisms underlying this condition are a loss of the negative control exerted by inhibitory interneurons and an early microglial reaction. Basic features of these mechanisms, however, are still poorly understood. Using the chronic constriction injury of the infraorbital nerve (CCI-IoN) model of neuropathic pain in mice, we have examined the changes in the expression of GAD, the synthetic enzyme of GABA, and GlyT2, the membrane transporter of glycine, as well as the microgliosis that occur at early (5 days) and late (21 days) stages post-CCI in the medullary and upper spinal dorsal horn. Our results show that CCI-IoN induces a down-regulation of GAD at both postinjury survival times, uniformly across the superficial laminae. The expression of GlyT2 showed a more discrete and heterogeneous reduction due to the basal presence in lamina III of 'patches' of higher expression, interspersed within a less immunoreactive 'matrix', which showed a more substantial reduction in the expression of GlyT2. These patches coincided with foci lacking any perceptible microglial reaction, which stood out against a more diffuse area of strong microgliosis. These findings may provide clues to better understand the neural mechanisms underlying allodynia in neuropathic pain syndromes.

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