Pain
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Comparative Study
Refractoriness cannot explain why C-fiber laser-evoked brain potentials are recorded only if concomitant Adelta-fiber activation is avoided.
Co-activation of Adelta- and C-fiber nociceptors by brief cutaneous laser heat stimuli may induce a dual sensation composed of first and second pain but evokes only a single, Adelta-fiber related, late laser-evoked potential (LEP). Yet, when concomitant activation of Adelta-nociceptors is avoided, C-nociceptor activation evokes an ultra-late LEP. As cumulating evidence indicates that late and ultra-late LEPs may share common generators, investigators have hypothesized that when Adelta-fibers trigger a late LEP, the later arriving C-fiber afferent volley cannot trigger an ultra-late LEP because underlying generators are in a 'refractory state'. ⋯ Studies have shown that this component is probably related to the P3b component described in other sensory modalities. This result provides support to the 'context closure' model hypothesizing that this component reflects the closure of information processing occurring when expectations are terminated. Altogether, these results suggest that late and ultra-late LEPs reflect very general processes, which are mainly related to detection and orientation and constitute only a fraction of the central processing of both nociceptive inputs.
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Comparative Study
Cognitive modulation of pain-related brain responses depends on behavioral strategy.
Interactions of pain and cognition have been studied in humans and animals previously, but the relationship between such behavioral interactions and brain activity is unknown. We aimed to show using functional MRI (fMRI) how a cognitively demanding task (Stroop) modulates pain-related brain activations and conversely, how pain modulates attention-related activity. Reaction time data indicated two types of pain responders: subjects in the A group had a faster Stroop reaction time when pain was concomitant to the attention task, while those in the P group had a slower Stroop performance during painful stimulation. fMRI data obtained during Stroop performance with and without noxious stimulation were subjected to region of interest analyses. ⋯ None of the areas showing attention-related responses, including bilateral dorsolateral prefrontal and posterior parietal cortices, were modulated by pain. These findings suggest that cortical regions associated with pain can be modulated by cognitive strategies. Furthermore, the distinction of behavioral subgroups may relate to cognitive coping strategies taken by patients with chronic pain.
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Comparative Study
Involvement of cholecystokinin in the opioid tolerance induced by dipyrone (metamizol) microinjections into the periaqueductal gray matter of rats.
The analgesic effect of non-steroidal anti-inflammatory drugs (NSAIDs) is partly due to an action upon the periaqueductal gray matter (PAG), which triggers the descending pain control system and thus inhibits nociceptive transmission. This action of NSAIDs engages endogenous opioids at the PAG, the nucleus raphe magnus and the spinal cord. Repeated administration of NSAIDs such as dipyrone (metamizol) and acetylsalicylate thus induces tolerance to these compounds and cross-tolerance to morphine. ⋯ In rats tolerant to PAG dipyrone, a PAG microinjection of proglumide restored the antinociceptive effect of a subsequent microinjection of dipyrone or morphine. These results suggest that PAG-microinjected dipyrone triggers and/or potentiates local opioidergic circuits leading to descending inhibition of nociception, on the one hand, and to a local antiopioid action by cholecystokinin, on the other. Reiteration of these events would then result in an enhancement of cholecystokinin's antiopioid action and thus tolerance to opioids and dipyrone in the PAG.
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Comparative Study
Spontaneous discharge and increased heat sensitivity of rat C-fiber nociceptors are present in vitro after plantar incision.
Postoperative pain is characterized by spontaneous pain at the surgical site and increased pain due to movements. To study postoperative pain mechanisms, we investigated discharge properties of mechano-heat sensitive C-fiber afferents innervating the rat glabrous hindpaw skin 1 day after plantar incision. Behaviors indicating spontaneous pain, heat and mechanical hyperalgesia were present 1 day after incision. ⋯ The mean mechanical response thresholds, measured by a servo force-controlled stimulator, were not different between groups. The total spikes evoked at supra-threshold mechanical stimulation were not increased in afferents from the incision. In conclusion, 1 day after incision, when behaviors indicating spontaneous pain, heat and mechanical hyperalgesia are present, C-fibers close to incision showed spontaneous discharge and sensitization to heat but not to mechanical stimuli, in vitro.
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Comparative Study
Perception and modulation of pain in waking and hypnosis: functional significance of phase-ordered gamma oscillations.
Somatosensory event-related phase-ordered gamma oscillations (40-Hz) to electric painful standard stimuli under an odd-ball paradigm were analyzed in 13 high, 13 medium, and 12 low hypnotizable subjects during waking, hypnosis, and post-hypnosis conditions. During these conditions, subjects received a suggestion of Focused Analgesia to produce an obstructive hallucination of stimulus perception; a No-Analgesia treatment served as a control. After hypnosis, a post-hypnotic suggestion was given to draw waking subjects into a deep hypnosis with opened eyes. ⋯ Phase-ordered gamma scores over central scalp site predicted subject pain ratings across Waking-Pain and Waking-Analgesia conditions, while phase-ordered gamma scores over frontal scalp site predicted pain ratings during post-hypnosis analgesia condition. During waking conditions, this relationship was present in high, low and medium hypnotizable subjects and was independent of stimulus intensity measures. This relationship was unchanged by hypnosis induction in the low hypnotizable subjects, but not present in the high and medium ones during hypnosis, suggesting that hypnosis interferes with phase-ordered gamma and pain relationship.