Pain
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The interaction between electroacupuncture and an N-methyl-D-aspartic acid (NMDA) receptor antagonist, (DL-2-amino-5-phosphonopentanoic acid; AP5), or an (+/-)-alpha-Amino-3-hydroxy-5-methylisoxazole-4-propionic acid/kainite (AMPA/KA) receptor antagonist, (6,7-dinitroquinoxaline-2,3 (1H,4H); DNQX) administered intrathecally on carrageenan-induced thermal hyperalgesia and spinal c-Fos expression was investigated. The latency of paw withdrawal (PWL) from a thermal stimulus was used as a measure of hyperalgesia in awake rats. Intrathecal (i.t.) injection of 1 and 10 nmol AP5, but not DNQX, markedly increased the PWL of the carrageenan-injected paw. ⋯ When a combination of electroacupuncture with 10 nmol AP5 or 100 nmol DNQX was used, the level of Fos expression in the spinal cord induced by carrageenan was significantly lower than electroacupuncture or i.t. injection of AP5 or DNQX alone. These results demonstrate that electroacupuncture and NMDA or AMPA/KA receptor antagonists have a synergetic anti-nociceptive action against inflammatory pain. Furthermore, this study supports the idea that both NMDA and AMPA/KA receptors are involved in spinal nociceptive transmission in carrageenan-inflamed rats, with the former more preferentially mediating transmission of nociceptive information from cutaneous tissue.
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Clinical Trial
Assessment of the reproducibility of intradermal administration of capsaicin as a model for inducing human pain.
The reproducibility and tolerability of intradermal (i.d.) administration of capsaicin as a method for eliciting human pain was assessed in healthy male volunteers (n = 12). The primary endpoints for assessing pain were spontaneous pain response and areas of allodynia, pinprick hyperalgesia and neurogenic inflammation. These were recorded before, immediately after, and at regular intervals following each of four doses (250 microg) of capsaicin (two per trial day). ⋯ A positive correlation was found between the area of allodynia and area of pinprick hyperalgesia (r(2) = 0.835). Overall, the model was well tolerated with no reports of adverse events. We conclude that the tolerability profile, and variability of i.d. capsaicin-induced pain is acceptable for pharmacological profiling of novel anti-nociceptive agents, with limited number of subjects.
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Facial expression of pain has rarely been researched in the context of facial expression of negative emotions with which it may occur. The main aim of the study was to investigate how pain expression resembled or differed from that of other negative emotions (fear, anger, sadness, surprise, disgust and embarrassment), using multidimensional scaling, a dimensional approach to understanding relationships among emotions. As possible misidentification of facial expressions by participants could distort those results, a judgement study as a categorical approach was conducted to examine the accuracy of identification of pain and negative emotion facial expressions. ⋯ Confidence in ratings approximated accuracy of identification. Multidimensional scaling revealed two dimensions: the first distinguished embarrassment from all other emotion expressions; the second separated pain, sadness and anger from fear, surprise and disgust. Possible explanations for these findings were sought in patterns of facial action units, and in the messages conveyed by the expressions according to Fridlund's Behavioural Ecology View.
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The aim of the present study was to examine the association between pain catastrophizing and general health status in a Dutch adult community sample, including various subgroups of people with musculoskeletal pain in the analyses. For exploratory reasons this study partly replicated previous studies of the factor structure, reliability, and validity of the Pain Catastrophizing Scale (PCS). Results demonstrated that across different pain subgroups, catastrophizing uniquely contributed variance to the prediction of the various aspects of general health status beyond the variance explained by pain intensity, age, gender, and chronicity. ⋯ This implies that a one-factor model might be justifiable as well, at least in the general community. Across various pain subgroups the reliability of the PCS total and subscales was adequate. Additional evidence for the concurrent validity of the PCS was found as well.
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The pain modulatory role of dopamine D2 receptors of the human forebrain was studied by determining the association between dopamine D2 receptor binding potential and the response to experimental pain. Nineteen healthy male volunteers participated in a dopamine D2 receptor positron emission tomography study. The extrastriatal regions of interest studied with [11C]FLB 457 as radioligand (n = 11) were the anterior cingulum, the medial and lateral thalamus, the medial and lateral frontal cortex, and the medial and lateral temporal cortex. ⋯ A psychophysical control study (n = 10) showed that cold pain responses were identical in the right and left hand. The results indicate that dopamine D2 receptor binding potential in the human forebrain, particularly in the striatum, may be an important parameter in determining the individual cold pain response and the potential for central pain modulation. Accordingly, an individual with only few available D2 receptors in the forebrain is likely to have a high tonic level of pain suppression, combined with a low capacity to recruit more (dopaminergic) central pain inhibition by noxious conditioning stimulation.