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Recent studies have indicated that inflammatory pathways and hypothalamic-pituitary-adrenal (HPA) axis function may be responsible for the interaction between pain and depression.
Learn More >Transcutaneous electrical nerve stimulation (TENS) is commonly used for pain control. However, the effects of TENS on osteoarthritis (OA) pain and potential underlying mechanisms remain unclear.
Learn More >Neuropathic pain is frequently driven by ectopic impulse discharge (ectopia) generated in injured peripheral afferent neurons. Observations in the spinal nerve ligation (SNL) model in rats suggest that cell bodies in the dorsal root ganglion (DRG) contribute three times more to the ectopic barrage than the site of nerve injury (neuroma). The DRG is therefore a prime interventional target for pain control. Since DRG ectopia is selectively suppressed with lidocaine at concentrations too low to block axonal impulse propagation, we asked whether targeted delivery of dilute lidocaine to the L5 DRG can relieve L5 SNL-induced tactile allodynia without blocking normal sensation or motor function. Results showed that intraforaminal injection of 10 µL bolus doses of 0.2% lidocaine suppressed allodynia transiently, while sustained infusion over 2 weeks using osmotic minipumps suppressed it for nearly 2 weeks. Bolus injections of morphine or fentanyl were ineffective. Lidocaine applied to the cut spinal nerve end or the L4 DRG did not affect allodynia suggesting that discharge originating in the neuroma and in neighboring "uninjured" afferents makes at best a minor contribution. Spike electrogenesis in the DRG is apparently the primary driver of tactile allodynia in the SNL model of neuropathic pain and it can be controlled selectively by superfusing the relevant DRG(s) with non-blocking concentrations of lidocaine. This approach has potential clinical application in conditions such as postherpetic neuralgia and phantom limb pain in which one or only a few identifiable ganglia are implicated as pain drivers.
Learn More >Accumulating evidence demonstrates the beneficial effects of physical exercise on pain conditions; however, the underlying mechanisms are not understood thoroughly. The purpose of the present study was to investigate the effects of regular swimming exercise on neuroma pain and the possible roles of adipokines (leptin and adiponectin) in the pain behaviors modulated by exercise. The results showed that 5 weeks of regular swimming exercise relieved pain behaviors in a rat model of neuroma pain and normalized the dysregulation of circulating leptin and adiponectin in plasma induced by nerve injury. Moreover, regular swimming exercise reversed the altered expressions of leptin receptor and adiponectin receptor 1 in neuroma. In addition, the administration of exogenous leptin to the neuroma site dampened the effects of regular swimming exercise on neuroma pain and adiponectin administration alleviated the neuroma pain in the non-exercised neuroma rats. These findings indicate that leptin and adiponectin might be involved in mediating the beneficial effects of exercise on neuroma pain. PERSPECTIVE: Perspective: Identifying which endogenous processes are activated by specific exercise regimes would likely reveal novel therapeutic targets for the treatment of neuropathic pain. The current study suggests that adipokines might be involved in pain behaviors modulated by exercise and thus presents them as potential targets for pain management.
Learn More >Celecoxib is an NSAID commonly used to treat pain conditions in humans. In addition to its blocking activity on COXs enzymes, several other targets could contribute to its analgesic activity. Here we explore the spinal antinociceptive actions of celecoxib and the potential implication of K7 channels in mediating its effects. Spinal cord in vitro preparations from hind paw-inflamed animals were used to assess the segmental sensory-motor and the early sensory processing of nociceptive information. Electrophysiological recordings of ventral roots and dorsal horn neurones were obtained and the effects of celecoxib and K7 modulators on responses to repetitive dorsal root stimulation at C-fibre intensity were assessed. Celecoxib applied at clinically relevant concentrations produced depressant effects on responses to dorsal root stimulation recorded from both ventral roots and individual dorsal horn neurones, in contrast the non-nociceptive monosynaptic reflex was unaffected. The NSAID indomethacin was devoid of effect on spinal reflexes, but further co-application of celecoxib still produced depressant effects. The depressant actions of celecoxib were abolished after K7 channel blockade and mimicked by its structural analogue dimethyl-celecoxib that lacks COX blocking activity. The present results identify K7 channels as novel central targets for celecoxib that may be relevant to its analgesic effect. This finding contributes to better understand the pharmacology of celecoxib, and reinforces both the role of K7 channels in modulating the excitability of central pain pathways and its validity as target for the design of analgesics. SIGNIFICANCE STATEMENT: N/A.
Learn More >Complex regional pain syndrome (CRPS) is a severely disabling disease characterized by pain, temperature changes, motor dysfunction and edema that most often occurs as an atypical response to a minor surgery or fracture. Inflammation involving activation and recruitment of innate immune cells, including both peripheral and central myeloid cells (i.e. macrophages and microglia, respectively), is a key feature of CRPS. However, the exact role and time-course of these cellular processes relative to the known acute and chronic phases of the disease are not fully understood. Positron emission tomography (PET) of translocator protein-18kDa (TSPO) is a method for non-invasively tracking these activated innate immune cells. Here, we reveal the temporal dynamics of peripheral and central inflammatory responses over 20 weeks in a tibial fracture/casting mouse model of CRPS through longitudinal TSPO-PET using [F]GE-180. PET tracer uptake quantification in the tibia revealed increased peripheral inflammation as early as 2 days post-fracture and lasting 7 weeks. Centralized inflammation was detected in the spinal cord and brain of fractured mice at 7 and 21 days post-injury. Spinal cord tissue immunofluorescent staining revealed TSPO expression in microglia (CD11b+) at 7 days, but was restricted mainly to endothelial cells (PECAM1+) at baseline and 7 weeks. Our data suggest early and persistent peripheral myeloid cell activation, and transient central microglial activation are limited to the acute phase of CRPS. Moreover, we show that TSPO-PET can be used to noninvasively monitor the spatiotemporal dynamics of myeloid cell activation in CRPS progression with potential to inform disease phase-specific therapeutics.This is an open-access article distributed under the terms of the Creative Commons Attribution-Non Commercial-No Derivatives License 4.0 (CCBY-NC-ND), where it is permissible to download and share the work provided it is properly cited. The work cannot be changed in any way or used commercially without permission from the journal.
Learn More >In this study, we focused on several itch‑related molecules and receptors in the spinal cord with the goal of clarifying the specific mediators that regulate itch sensation. We investigated the involvement of serotonin receptors, opioid receptors, glia cell markers and chemokines (ligands and receptors) in models of acetone/ether/water (AEW)‑ and diphenylcyclopropenone (DCP)‑induced chronic itch. Using reverse transcription‑quantitative polymerase chain reaction, we examined the expression profiles of these mediators in the lower cervical spinal cord (C5‑8) of two models of chronic itch. We found that the gene expression levels of opioid receptor mu 1 (Oprm1), 5‑hydroxytryptamine receptor 1A (Htr1a) and 5‑hydroxytryptamine receptor 6 (Htr6) were upregulated. Among the chemokines, the expression levels of C‑C motif chemokine ligand (Ccl)21, Cxcl3 and Cxcl16 and their receptors, Ccr7, Cxcr2 and Cxcr6, were simultaneously upregulated in the spinal cords of the mice in both models of chronic itch. By contrast, the expression levels of Ccl2, Ccl3, Ccl4 and Ccl22 were downregulated. These findings indicate that multiple mediators, such as chemokines in the spinal cord, are altered and may be central candidates in further research into the mechanisms involved in the development of chronic itch.
Learn More >α-Melanocyte-stimulating hormone (α-MSH) is an endogenous peptide hormone involved in cutaneous pigmentation in atopic dermatitis (AD) with severe itching. α-MSH elicits itch-related responses in mice. We therefore investigated whether α-MSH was involved in itching in AD. In the skin of AD patients and mice with atopy-like dermatitis, α-MSH and the prohormone convertase 2, which is the key processing enzyme for the production of α-MSH, were distributed mainly in keratinocytes. In the skin of mice with dermatitis, α-MSH receptors (MC1R and MC5R) were expressed at the mRNA level and were distributed in the dermis. In the dorsal root ganglion (DRG) of mice with dermatitis, mRNAs encoding MC1 and MC3∼5 were also expressed. MC1R antagonist agouti-signaling protein inhibited spontaneous scratching in mice with dermatitis. In healthy mice, intradermal α-MSH elicited itch-associated responses, which were inhibited by TP thromboxane (TX) receptor antagonist ONO-3708. In mouse keratinocytes, α-MSH increased the production of TXA, which was inhibited by adenylyl cyclase inhibitor SQ-22536 and Ca chelator EGTA. In mouse keratinocytes treated with siRNA for MC1R and/or MC5R, α-MSH-induced TXA production was decreased. α-MSH increased intracellular Ca ion concentration in DRG neurons and keratinocytes. These results suggest that α-MSH is involved in itching during AD and may elicit itching through the direct action of primary afferents and TXA production by keratinocytes.
Learn More >Atopic dermatitis (AD) is a chronic inflammatory skin disease characterized by cutaneous lesions and intense pruritus. The warm temperature-activated Ca2+-permeable TRPV3 channel is abundantly expressed in the keratinocytes, and gain-of-function mutations of TRPV3 cause skin lesions and pruritus in rodents and humans, suggesting an involvement of TRPV3 in the pathogenesis of AD. Here we report that pharmacological and genetic inhibition of TRPV3 attenuates skin lesions and dermatitis in mice. In mouse AD-like model induced by topical application of chemical DNFB, we found that TRPV3 proteins together with inflammatory factors TNF-α and IL-6 were upregulated in the skin detected by Western blot and immunostaining assay. Pharmacological activation of TRPV3 by channel agonist skin sensitizer carvacrol resulted in development of AD in WT mice, but not TRPV3 knockout mice. Furthermore, inhibition of TRPV3 by natural osthole reversed the severity of inflammatory dorsal skin and ear edema in dose-dependent manner, and also decreased the expression of inflammatory factors TNF-α and IL-6. Taken together, our findings demonstrate the involvement of overactive TRPV3 in the progressive pathology of AD in mice, and topical inhibition of TRPV3 channel function may represent an effective prevention and therapy for AD or inflammatory skin diseases. SIGNIFICANCE STATEMENT: Overactive TRPV3 channel is critically involved in the pathogenesis of atopic dermatitis. Inhibition of TRPV3 channel function by topical natural osthole may represent an effective therapy for management of atopic dermatitis aimed at preventing or alleviating skin lesions and severe itching.
Learn More >Operant methods that allow animals to avoid painful stimuli are interpreted to assess the aversive quality of pain; however, such measurements require investigator-initiated stimuli to animals. Here we developed a shuttle maze test to repeatedly assess activity associated nociception without forced stimulation. Rats ambulate back and forth between two treat feeders by taking either a short route with a prickly surfaced arch or a longer route with a smooth floor. L5-L6 spinal nerve ligation (SNL) reduced the preference for the short route with the arch, correlated with hypersensitivity in the hind paw. Oral gabapentin restored the short route preference and reduced hypersensitivity in SNL rats, and blockade of spinal α2-adrenoceptors reduced gabapentin's effects on hypersensitivity but not on preference index. These results suggest that SNL injury alters behavior in the shuttle maze test and that the shuttle maze test shows comparable results to reflexive hypersensitivity after SNL in magnitude and response to gabapentin.
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