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      Therapeutic effects of diclofenac, pregabalin, and duloxetine on disuse-induced chronic musculoskeletal pain in rats

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          Abstract

          The aim of this study was to clarify the mechanism of disuse-induced muscle hyperalgesia through the evaluation of the pharmacological behaviour of muscle hyperalgesia profiles in chronic post-cast pain (CPCP) rats with acute and chronic-phase mirror-image muscle hyperalgesia treated with diclofenac (NSAID), pregabalin (an inhibitor of Ca 2+ channel α2δ), and duloxetine (SNRI). After 2 weeks of cast immobilization, the peak cross-sectional area and muscle wet weight of the ipsilateral soleus and gastrocnemius muscles decreased more significantly in CPCP rats than in untreated rats. Histological findings revealed disuse-induced muscle atrophy in CPCP rats. The blood biochemical parameters of CPCP rats in acute and chronic phases did not differ significantly from those of untreated rats. The diclofenac and pregabalin-treated groups exhibited no improvement in acute or chronic muscle hyperalgesia. In contrast, the duloxetine-treated group exhibited an improvement in acute muscle hyperalgesia, but showed no apparent effect on chronic muscle hyperalgesia on ipsilateral or contralateral sides. However, the chronic muscle hyperalgesia was reversed by intrathecal administration of DAMGO (a μ-opioid receptor agonist). The results suggest that chronic muscle hyperalgesia in CPCP rats did not result from an inflammatory mechanism, and there is only a low probability that it’s caused by a neuropathic mechanism.

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          Descending control of pain.

          Upon receipt in the dorsal horn (DH) of the spinal cord, nociceptive (pain-signalling) information from the viscera, skin and other organs is subject to extensive processing by a diversity of mechanisms, certain of which enhance, and certain of which inhibit, its transfer to higher centres. In this regard, a network of descending pathways projecting from cerebral structures to the DH plays a complex and crucial role. Specific centrifugal pathways either suppress (descending inhibition) or potentiate (descending facilitation) passage of nociceptive messages to the brain. Engagement of descending inhibition by the opioid analgesic, morphine, fulfils an important role in its pain-relieving properties, while induction of analgesia by the adrenergic agonist, clonidine, reflects actions at alpha(2)-adrenoceptors (alpha(2)-ARs) in the DH normally recruited by descending pathways. However, opioids and adrenergic agents exploit but a tiny fraction of the vast panoply of mechanisms now known to be involved in the induction and/or expression of descending controls. For example, no drug interfering with descending facilitation is currently available for clinical use. The present review focuses on: (1) the organisation of descending pathways and their pathophysiological significance; (2) the role of individual transmitters and specific receptor types in the modulation and expression of mechanisms of descending inhibition and facilitation and (3) the advantages and limitations of established and innovative analgesic strategies which act by manipulation of descending controls. Knowledge of descending pathways has increased exponentially in recent years, so this is an opportune moment to survey their operation and therapeutic relevance to the improved management of pain.
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            P2X4 receptors induced in spinal microglia gate tactile allodynia after nerve injury.

            Pain after nerve damage is an expression of pathological operation of the nervous system, one hallmark of which is tactile allodynia-pain hypersensitivity evoked by innocuous stimuli. Effective therapy for this pain is lacking, and the underlying mechanisms are poorly understood. Here we report that pharmacological blockade of spinal P2X4 receptors (P2X4Rs), a subtype of ionotropic ATP receptor, reversed tactile allodynia caused by peripheral nerve injury without affecting acute pain behaviours in naive animals. After nerve injury, P2X4R expression increased strikingly in the ipsilateral spinal cord, and P2X4Rs were induced in hyperactive microglia but not in neurons or astrocytes. Intraspinal administration of P2X4R antisense oligodeoxynucleotide decreased the induction of P2X4Rs and suppressed tactile allodynia after nerve injury. Conversely, intraspinal administration of microglia in which P2X4Rs had been induced and stimulated, produced tactile allodynia in naive rats. Taken together, our results demonstrate that activation of P2X4Rs in hyperactive microglia is necessary for tactile allodynia after nerve injury and is sufficient to produce tactile allodynia in normal animals. Thus, blocking P2X4Rs in microglia might be a new therapeutic strategy for pain induced by nerve injury.
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              Algorithm for neuropathic pain treatment: an evidence based proposal.

              New studies of the treatment of neuropathic pain have increased the need for an updated review of randomized, double-blind, placebo-controlled trials to support an evidence based algorithm to treat neuropathic pain conditions. Available studies were identified using a MEDLINE and EMBASE search. One hundred and five studies were included. Numbers needed to treat (NNT) and numbers needed to harm (NNH) were used to compare efficacy and safety of the treatments in different neuropathic pain syndromes. The quality of each trial was assessed. Tricyclic antidepressants and the anticonvulsants gabapentin and pregabalin were the most frequently studied drug classes. In peripheral neuropathic pain, the lowest NNT was for tricyclic antidepressants, followed by opioids and the anticonvulsants gabapentin and pregabalin. For central neuropathic pain there is limited data. NNT and NNH are currently the best way to assess relative efficacy and safety, but the need for dichotomous data, which may have to be estimated retrospectively for old trials, and the methodological complexity of pooling data from small cross-over and large parallel group trials, remain as limitations.
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                Author and article information

                Contributors
                ohmichi@aichi-med-u.ac.jp
                Journal
                Sci Rep
                Sci Rep
                Scientific Reports
                Nature Publishing Group UK (London )
                2045-2322
                19 February 2018
                19 February 2018
                2018
                : 8
                : 3311
                Affiliations
                [1 ]ISNI 0000 0001 0727 1557, GRID grid.411234.1, Department of Anatomy, , Aichi Medical University, 1-1 Yazakokarimta, ; Nagakute, Aichi 480-1195 Japan
                [2 ]GRID grid.418042.b, Pharmacology Research Laboratories, Drug Discovery Research, Astellas Pharma Inc., 21, Miyukigaoka, Tsukuba-shi, ; Ibaraki, 305-8585 Japan
                [3 ]ISNI 0000 0001 0720 6587, GRID grid.410818.4, Department of Psychiatry, , Tokyo Women’s Medical University, Kawada-cho 8-1, Shinjuku-ku, ; Tokyo, 162-8666 Japan
                [4 ]ISNI 0000 0001 0943 978X, GRID grid.27476.30, Research Center for Next-Generation Drug Development, Research Institute of Environmental Medicine, Nagoya University, ; Nagoya, 464-8601 Japan
                Article
                21429
                10.1038/s41598-018-21429-3
                5818528
                29459641
                fdc6b510-6f96-430c-a51d-5e63fab84c14
                © The Author(s) 2018

                Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

                History
                : 14 June 2017
                : 1 February 2018
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