The journal of pain : official journal of the American Pain Society
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The aim of this study was to investigate local opioid effects in the inflamed skin of healthy human volunteers. To induce inflammation, the circular tip of a 10-mm-diameter probe was heated to 48 degrees C and applied for 120 seconds to a site on each forearm of 24 healthy participants. Thirty minutes later, 0.2 mL of normal saline was injected subcutaneously into 1 inflamed site, and the opioid antagonist naloxone hydrochloride (80 microg in 0.2 mL) was injected subcutaneously into the other inflamed site. Participants completed tests of pain sensitivity (heat pain thresholds, heat pain ratings, and mechanical pain ratings) before and after the injections. Fentanyl citrate (10 microg in 0.2 mL) was then injected into the pretreated sites, and pain sensitivity was measured again. The thermal injuries produced thermal and mechanical hyperalgesia that did not differ between the saline and naloxone sites. After the fentanyl injections, decreases in thermal and mechanical hyperalgesia were greater at the saline site than the naloxone site. These findings demonstrate that pretreatment with naloxone blocks local opioid effects produced by the subcutaneous injection of a low dose of fentanyl in the inflamed skin of healthy humans. Thus, peripheral opioid receptors could be a therapeutic target for painful cutaneous disorders. ⋯ This article demonstrates that activation of opioid receptors in the skin inhibits sensitivity to painful mechanical and thermal stimuli. Thus, local application of low-dose opioid medications could relieve painful skin disorders.
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Variable use of pain scale anchors may influence recalled pain ratings, rating consistency, and agreement between actual rating change and ratings of pain relief. This investigation examined change in events that represent maximal pain scale anchors. Participants (N = 68, 50% women) provided events for maximal anchors of 0 to 100 pain scales, and cold pressor pain was rated by using self-selected event/s and an investigator-provided event. Participants then were allowed to change their self-selected event/s. The revised event/s or original events were then used to rate a second cold pressor trial. Forty-one percent of participants changed event/s, and the new event/s was more likely to involve cold or heat, but the painfulness of events and the pain ratings of the second trial did not change. The cold pressor pain ratings were higher when rated on the basis of self-selected event/s than the investigator-provided event for intensity (mean = 80.13, SD = 19.30; mean = 60.81, SD = 27.45) and unpleasantness (mean = 80.84, SD = 19.07; mean = 59.07, SD = 27.53), which could be due to the submaximal painfulness of the investigator-provided event. Therefore, the width of numerical scales is stable with maximal events regardless of the actual events. ⋯ This report identifies change in physical events that are used by participants to represent maximal pain scale anchors and suggests that the maximal nature of the events' painfulness is more important than variability in the actual events. We conclude that the numerical pain scales we used are understandable and stable, but we suggest that instructional sets for pain measurement may be improved by evaluation of the painfulness of events that respondents use to conceptualize maximal pain scale anchors.
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Characteristics of sensory dorsal root ganglia (DRG) neurons innervating the L5 vertebral body were investigated in rats by using a retrograde neurotransport method, lectin affinity- and immuno-histochemistry to further elucidate the causes of diffuse pain suffered by some elderly patients in their back, lateral trunk, and iliac crest, after lumbar osteoporotic vertebral fracture. We used calcitonin gene-related peptide (CGRP) as a marker of small peptide-containing neurons and the glycoprotein binding the isolectin from Griffonia simplicifolia (IB4) as a marker of small non-peptide-containing neurons. Neurons innervating the L5 vertebral bodies, retrogradely labeled with fluoro-gold (FG), were distributed throughout DRGs from T13 to L6. The proportion of CGRP-immunoreactive (IR) FG-labeled neurons was 32%. The proportion of IB4-binding FG-labeled neurons was significantly smaller, at 4%. Other neurons that were non-CGRP-IR and non-IB4-binding were mostly large neurons, and they may transmit proprioception from vertebral bodies. Most neurons transmitting pain are CGRP-IR peptide-containing neurons. They may have a more significant role in pain sensation in the vertebral bodies as peptidergic DRG neurons. ⋯ This article shows that vertebral bodies are innervated by CGRP-IR neurons. CGRP-IR neurons may play a role in pain sensation through peptidergic DRG neurons. These findings contribute to an understanding of pain associated with the vertebral body such as tumor, infection, or osteoporotic fracture.