Pain
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In the past 30years, the study of nociception has relied mostly on thermal stimulation to activate nociceptors selectively. However, thermal stimulation suffers from some important limitations. For this reason, investigators have proposed intra-epidermal electrical stimulation (IES) as an alternative method to activate nociceptors selectively. ⋯ In a second experiment, we applied a nerve pressure block to the superficial radial nerve to induce a temporally dissociated impairment of Abeta-, Adelta- and C-fibre afferents, and thereby determine the fibre populations contributing to the responses elicited by IES. We found that the time course of the blockade of the responses to IES follows closely the time course of the blockade of Adelta-fibres, but not of Abeta-fibres. Taken together, our results provide converging evidence that Adelta-nociceptors can be activated selectively using IES, provided that low intensities of stimulation are used.
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Hydrogen sulfide (H(2)S) formed from l-cysteine by multiple enzymes including cystathionine-gamma-lyase (CSE) is now considered a gasotransmitter in the mammalian body. Our previous studies have shown that H(2)S activates/sensitizes Ca(v)3.2 T-type Ca(2+) channels, leading to facilitation of somatic and visceral nociception, and that CSE-derived endogenous H(2)S participates in inflammatory pain. Here, we show novel evidence for involvement of the endogenous H(2)S-Ca(v)3.2 pathway in neuropathic pain. ⋯ Finally, silencing of Ca(v)3.2 in DRG by repeated intrathecal administration of Ca(v)3.2-targeting siRNA significantly attenuated the neuropathic hyperalgesia in the L5SNC rat. In conclusion, our data suggest that Ca(v)3.2 T-type Ca(2+) channels in sensory neurons are upregulated and activated/sensitized by CSE-derived endogenous H(2)S after spinal nerve injury, contributing to the maintenance of neuropathic pain. We thus propose that Ca(v)3.2 and CSE could be targets for the development of therapeutic drugs for the treatment of neuropathic pain.
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Noxious cutaneous contact heat stimuli (48 degrees C) are perceived as increasingly painful when the stimulus duration is extended from 5 to 10s, reflecting the temporal summation of central neuronal activity mediating heat pain. However, the sensation of increasing heat pain disappears, reaching a plateau as stimulus duration increases from 10 to 20s. We used functional magnetic resonance imaging (fMRI) in 10 healthy subjects to determine if active central mechanisms could contribute to this psychophysical plateau. ⋯ In contrast, during the psychophysical plateau, both the intensity and volume of thalamic and cortical activations in the right medial thalamus, right posterior insula, and left secondary (S2) somatosensory cortex continued to increase with stimulus duration but not with perceived stimulus intensity. Activation volumes in the left medial and right lateral thalamus, and the bilateral mid-anterior cingulate, left orbitofrontal, and right S2 cortices also increased only with stimulus duration. The increased activity of specific thalamic and cortical structures as stimulus duration, but not perceived intensity, increases is consistent with the recruitment of a thalamocortical mechanism that participates in the modulation of pain-related cortical responses and the temporal summation of heat pain.
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This study supports the hypothesis that healthy older adults exhibit decreased endogenous pain inhibition compared to younger healthy controls. Twenty-two older adults (56-77years of age) and 27 controls aged 20-49 participated in five experimental sessions following a training session. Each experimental session consisted of five 60-s trials in which the experimental heat stimulus was presented to the thenar eminence of the left palm with or without a conditioning stimulus (cold-water immersion of the foot). ⋯ A novel aspect of the study was that we recorded "pain offset" (i.e., after-sensations) and found that ratings for the older sample decreased at a slower rate than observed for the group of younger adults suggesting increased central sensitization among the older sample. Decrements in CPM could contribute to the greater prevalence of pain in older age. Since a number of neurotransmitter systems are involved in pain modulation, it is possible age-related differences in CPM are due to functional changes in these systems in a number of areas within the neuroaxis.
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As part of a comprehensive study of the natural history of herpes zoster (HZ), 57 of 94 subjects in a cohort at elevated risk for post-herpetic neuralgia (PHN) consented to collection of 3-mm skin punch biopsies from affected, mirror-image, and distant control skin at baseline and followup visits. As cutaneous innervation is reduced in longstanding severe PHN, we tested the hypothesis that development of PHN is correlated with severity of initial neural injury and/or a failure of neural recovery. Quantitative analysis using single-label PGP9.5 immunofluorescence microscopy showed epidermal profiles were reduced in zoster skin by approximately 40% at study entry compared to control and mirror skin. ⋯ Sensory abnormalities and symptom aggravation by focal capsaicin application showed partial and selective recovery over 6months. In contrast, cutaneous innervation showed no recovery at all by 6months, conclusive evidence that resolution of pain and allodynia does not require cutaneous reinnervation. A much longer period of observation is needed to determine if zoster-affected skin is ever reinnervated.