Articles: hyperalgesia.
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Nociceptive and neuropathic pain occurs as part of the disease process after traumatic brain injury (TBI) in humans. Central and peripheral inflammation, a major secondary injury process initiated by the traumatic brain injury event, has been implicated in the potentiation of peripheral nociceptive pain. We hypothesized that the inflammatory response to diffuse traumatic brain injury potentiates persistent pain through prolonged immune dysregulation. ⋯ We conclude that traumatic brain injury increased the inflammatory pain associated with cutaneous inflammation by contributing to systemic immune dysregulation. Regulatory T cells are immune suppressors and failure of T cells to differentiate into regulatory T cells leads to unregulated cytokine production which may contribute to the potentiation of peripheral pain through the excitation of peripheral sensory neurons. In addition, regulatory T cells are identified as a potential target for therapeutic rebalancing of peripheral immune homeostasis to improve functional outcome and decrease the incidence of peripheral inflammatory pain following traumatic brain injury.
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Clinical management of neuropathic pain, which is pain arising as a consequence of a lesion or a disease affecting the somatosensory system, partly relies on the use of anticonvulsant drugs such as gabapentinoids. Therapeutic action of gabapentinoids such as gabapentin and pregabalin, which act by the inhibition of calcium currents through interaction with the α2δ-1 subunit of voltage-dependent calcium channels, is well documented. However, some aspects of the downstream mechanisms are still to be uncovered. Using behavioral, genetic, and pharmacological approaches, we tested whether opioid receptors are necessary for the antiallodynic action of acute and/or long-term pregabalin treatment in the specific context of neuropathic pain. ⋯ We demonstrate that neither acute nor long-term antiallodynic effect of pregabalin in a context of neuropathic pain is mediated by the endogenous opioid system, which differs from opioid treatment of pain and antidepressant treatment of neuropathic pain. Our data are also supportive of an impact of gabapentinoid treatment on the neuroimmune aspect of neuropathic pain.
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The function of the Cannabinoid 1 receptor (CB1R) in the development of neuropathic pain is not clear. Mounting evidence suggest that CB1R expression and activation may contribute to pain. Cannabinoid 1 receptor knockout mice (CB1R-/-) generated on a C57Bl/6 background exhibit hypoalgesia in the hotplate assay and formalin test. These findings suggest that Cannabinoid 1 receptor expression mediates the responses to at least some types of painful stimuli. By using this mouse line, we sought to determine if the lack of Cannabinoid 1 receptor unveils a general hypoalgesic phenotype, including protection against the development of neuropathic pain. The acetone test was used to measure cold sensitivity, the electronic von Frey was used to measure mechanical thresholds before and after spared-nerve injury, and analysis of footprint patterns was conducted to determine if motor function is differentially affected after nerve-injury in mice with varying levels of Cannabinoid 1 receptor. ⋯ Cold allodynia and significant recovery from spared-nerve injury-induced mechanical hypersensitivity are two novel phenotypes which characterize the global CB1R-/- mice. An increase in transient receptor potential channel of melastatin 8 channel function in DRG neurons may underlie the cold phenotype. Recovery of mechanical thresholds in the CB1R knockouts was independent of motor function. These results indicate that CB1R expression contributes to the development of persistent mechanical hypersensitivity, protects against the development of robust cold allodynia but is not involved in motor impairment following spared-nerve injury in mice.
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The mechanisms driving osteoarthritic pain remain poorly understood, but there is increasing evidence for a role of the central nervous system in the chronification of pain. We used functional magnetic resonance imaging to investigate the influence of a model of unilateral knee osteoarthritis on nociceptive processing. ⋯ We provide evidence for modulation of nociceptive processing in a chronic knee osteoarthritis pain model with stronger brain activation and alteration of brain networks induced by the pro-nociceptive stimulus. We also report a shift to a medial pain activation pattern following stimulation of the hyperalgesic hindpaw. Taken together, our data support altered neural pain processing as a result of peripheral and central pain sensitization in this model.
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Pain hypoalgesia has been reported in Rett syndrome patients, a severe neurodevelopmental disorder which can be attributed to mutations in the methyl-CpG binding protein 2 (MeCP2). Here, we examined the role of MeCP2 signaling in tongue heat sensitivity in the normal and inflamed state using Mecp2 heterozygous (Mecp2(+/-)) mice. ⋯ These findings indicate that tongue heat sensitivity and hypersensitivity are dependent on the expression of transient receptor potential vanilloid 1 which is regulated via MeCP2 signaling in trigeminal ganglion neurons innervating the tongue.