Articles: hyperalgesia.
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We investigated changes in pain behavior after injection of acetic acid in the hindpaws of rats with L5 spinal nerve ligation (SNL)-induced neuropathy. We also examined immunoreactivity for acid-sensing ion channel 3 (ASIC3) in the dorsal root ganglion (DRG) of rats with L5 SNL. Two weeks after SNL, the withdrawal threshold to a mechanical stimulus was significantly lower in the SNL group than in the sham-operated group (n=9 per group, P<0.01). ⋯ In the ipsilateral L5 DRG, the proportion of ASIC3-ir neurons was not significantly affected by treatment. However, L5 SNL significantly increased (P<0.01) the proportion of small (<1200 mm(2)) ASIC3-ir neurons and significantly decreased (P<0.01) the proportion of large ASIC3-ir neurons compared to proportions in sham-operated animals. These findings suggest that ASIC3 is associated with hyperalgesia in response to a chemical stimulus in the L5 SNL rat model.
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Randomized Controlled Trial Comparative Study
Lack of analgesia by oral standardized cannabis extract on acute inflammatory pain and hyperalgesia in volunteers.
Cannabinoid-induced analgesia was shown in animal studies of acute inflammatory and neuropathic pain. In humans, controlled clinical trials with Delta-tetrahydrocannabinol or other cannabinoids demonstrated analgesic efficacy in chronic pain syndromes, whereas the data in acute pain were less conclusive. Therefore, the aim of this study was to investigate the effects of oral cannabis extract in two different human models of acute inflammatory pain and hyperalgesia. ⋯ To conclude, no analgesic or antihyperalgesic activity of cannabis extract was found in the experiments. Moreover, the results even point to the development of a hyperalgesic state under cannabinoids. Together with previous data, the current results suggest that cannabinoids are not effective analgesics for the treatment of acute nociceptive pain in humans.
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Randomized Controlled Trial Controlled Clinical Trial
Effects of NGF-induced muscle sensitization on proprioception and nociception.
Temporomandibular disorders (TMDs) are associated with perturbation of proprioceptive and nociceptive function. Recent studies have shown that injection of the neurotrophic protein nerve growth factor (NGF) into the masseter muscle causes sensitization to mechanical pressure stimuli; however, it is not clear if vibration sense and jaw stretch reflexes as measures of proprioceptive function as well as glutamate-evoked pain are also altered. We tested the hypothesis that NGF-induced mechanical sensitization would be associated with changes in vibration sense and stretch reflex sensitivity as well as facilitation of glutamate-evoked pain responses. ⋯ In conclusion, this study confirms that masseter muscle injection of NGF is associated with a distinct and prolonged sensitization to mechanical stimuli, but without an effect on large-diameter mechanoreceptive and the muscle spindle afferents. Additional challenge of the NGF pretreated muscle with glutamate did not indicate a conspicuous sensitization to noxious chemical stimuli. These findings are discussed in terms of the concept of "proprioceptive allodynia".
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Opioid-induced hyperalgesia (OIH) is most broadly defined as a state of nociceptive sensitization caused by exposure to opioids. The state is characterized by a paradoxical response whereby a patient receiving opioids for the treatment of pain may actually become more sensitive to certain painful stimuli. The type of pain experienced may or may not be different from the original underlying painful condition. ⋯ OIH seems to be a distinct, definable, and characteristic phenomenon that may explain loss of opioid efficacy in some cases. Clinicians should suspect expression of OIH when opioid treatment effect seems to wane in the absence of disease progression, particularly if found in the context of unexplained pain reports or diffuse allodynia unassociated with the pain as previously observed. This review highlights the important mechanistic underpinnings and clinical ramifications of OIH and discusses future research directions and the latest clinical evidence for modulation of this potentially troublesome clinical phenomenon.
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Phosphorylation of the N-methyl-D-aspartate (NMDA) receptor NR1 subunit (pNR1) in the spinal cord is associated with increased neuronal responsiveness, which underlies the process of central sensitization. Because of the importance of NR1 in central sensitization, the first goal of this study was to examine both time- and lamina-dependent changes in spinal NR1 and pNR1 expression in a chronic constriction injury (CCI) model of neuropathic pain. Increased excitability of capsaicin sensitive primary afferents (CSPAs), which express TRPV1 receptors, also contributes to central sensitization. ⋯ Pretreatment with RTX (0.3mg/kg, s.c. in the scruff of the neck or intraplantar) 2 days prior to CCI completely prevented induction of thermal hyperalgesia, but not mechanical allodynia in neuropathic rats. Interestingly, RTX treatment significantly attenuated the CCI-induced upregulation of NR1 and pNR1 in spinal laminae I-II and V-VI, but not laminae III-IV as compared with that of vehicle-treated CCI rats. These findings demonstrate that the increased expression of NR1 and pNR1 in spinal laminae I-II and V-VI is dependent on activation of CSPAs, which ultimately contribute to the development of thermal hyperalgesia in neuropathic rats.