Articles: hyperalgesia.
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Eighteen patients with peripheral neuropathic pain (PNeP) and seven patients with central post-stroke pain (CPSP) all suffering from dynamic mechanical allodynia (DMA) in a limb were studied. From recent research it is reasonable to suggest that A-beta fibres constitute the peripheral substrate for DMA in patients with PNeP. The pathophysiological basis for DMA in patients with CPSP is unknown. ⋯ The rest of the patients lost DMA without transition to DMD. The transition or loss of DMA without transition occurred early and concurrently in time during the block and was paralleled by a continuous impairment of mainly A-beta fibre function. We therefore suggest DMA to be the hyperbole of DMD, the difference being the number of mechanoreceptive fibres having access to the nociceptive system.
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Despite the impact of chronic pain on the quality of life in patients, including changes to affective state and daily life activities, rodent preclinical models rarely address this aspect of chronic pain. To better understand the behavioral consequences of the tissue and nerve injuries typically used to model neuropathic and inflammatory pain in mice, we measured home cage and affective state behaviors in animals with spared nerve injury, chronic constriction injury (CCI), or intraplantar complete Freund's adjuvant. Mechanical hypersensitivity is prominent in each of these conditions and persists for many weeks. ⋯ Animals with CCI were initially less active, but the difference between CCI and controls disappeared by 2 weeks after injury. Further, in all pain models, there was no change in any measure of affective state. We conclude that in these standard models of persistent pain, despite the development of prolonged hypersensitivity, the mice do not have significantly altered "quality of life." As alteration in daily life activities is the feature that is so disrupted in patients with chronic pain, our results suggest that the models used here do not fully reflect the human conditions and point to a need for development of a murine chronic pain model in which lifestyle changes are manifest.
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Intense noxious stimuli impair GABAergic inhibition in spinal dorsal horn, which has been proposed as a critical contributor to pathological pain. However, how the reduced inhibition exacerbates the transfer of nociceptive information at excitatory glutamatergic synapses is still poorly understood. The present study demonstrated that one of the striking consequences of GABAergic disinhibition was to enhance the function of N-methyl-D-aspartate subtype glutamate receptors (NMDARs), a well-characterized player in central sensitization. ⋯ When PKA inhibitor H-89 was intrathecally applied, it totally eliminated bicuculline-induced NMDARs phosphorylation, synaptic redistribution as well as pain sensitization. Importantly, the reduced inhibition also operated to enhance NMDARs functions after peripheral inflammation, because spinal injection of diazepam to rescue the inhibition in inflamed mice greatly depressed PKA phosphorylation of NR1-S897, reduced the synaptic concentration of NR1/NR2B and meanwhile, alleviated the inflammatory pain. These data suggested that removal of GABAergic inhibition allowed for PKA-mediated NMDARs phosphorylation and synaptic accumulation, thus exaggerating NMDARs-dependent nociceptive transmission and behavioral sensitization.
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It has been reported that the whisker pad (WP) area, which is innervated by the second branch of the trigeminal nerve, shows allodynia/hyperalgesia following transection of the mental nerve (MN: the third branch of the trigeminal nerve). However, the mechanisms of this extra-territorial pain induction still remain unclear. Glia and cytokines are known to facilitate perception of noxious input, raising a possibility that these non-neuronal elements are involved in the induction and spread of allodynia/hyperalgesia at non-injured skin territory. ⋯ Administration of a noncompetitive antagonist of NMDA receptors MK-801 (i.t., 5 μg/rat) reversed allodynia/hyperalgesia. IL-1 receptor type I (IL-1RI) was localized in Fos- and phospho NR1-immunoreactive neurons. These results suggest that IL-1beta in the Vc plays an important role in the development of extra-territorial tactile allodynia/hyperalgesia after MN transection.
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Neuroscience research · May 2011
A subset of μ-opioid receptor-expressing cells in the rostral ventromedial medulla contribute to thermal hyperalgesia in experimental neuropathic pain.
The rostral ventromedial medulla (RVM) is a major region for the descending modulation of pain at the spinal cord level, and neurons in the RVM have been implicated in the inhibition and facilitation of spinal nociceptive transmission. Although recent studies have established that the RVM facilitation of nociceptive transmission in the spinal cord contributes to neuropathic pain, the underlying mechanisms remain largely unknown. In the present study, we investigated the effects of kainic acid (KA)-induced RVM damage on neuropathic pain behavior and the expression of molecules implicated in pain modulation. ⋯ KA injection alone did not affect the nocifensive responses to mechanical and thermal stimuli on the intact side. Immunohistochemical analysis revealed that KA injection into the RVM significantly reduced the number of immunoreactive neurons for μ-opioid receptors, but not tryptophan hydroxylase, in association with the analgesic effect. These results suggest that a subset of RVM neurons expressing μ-opioid receptors contribute to the maintenance of thermal hyperalgesia in neuropathic pain.