Anesth Analg 2000;90:923-927
© 2000 International Anesthesia Research Society
REGIONAL ANESTHESIA AND PAIN MEDICINE
The Effect of Peripheral Opioid Block and Body Cooling on Sensitivity to Heat in Capsaicin-Treated Skin
Peter D. Drummond, PhD
School of Psychology, Murdoch University, Western Australia
Address correspondence and reprint requests to Peter Drummond, PhD, School of Psychology, Murdoch University, 6150, Western Australia. Address e-mail to drummond{at}central.murdoch.edu.au
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Abstract
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We sought to determine whether stimulation of opioid receptors during body cooling would alter sensitivity to heat in the heat-sensitized, inflamed skin of 14 healthy volunteers. To investigate the contribution of opioid receptors to nociception, the opioid antagonist naloxone was introduced into the skin by iontophoresis after the topical application of capsaicin. For comparison, the same iontophoretic dose of saline was also administered. Shortly after the iontophoreses, sensitivity to heat was greater at the naloxone and saline sites than at iontophoresis-control sites in the capsaicin-treated skin, indicating that nonspecific aspects of the iontophoreses enhanced thermal hyperalgesia. The hyperalgesic effect of saline persisted during body cooling, whereas the naloxone site was less sensitive to heat (heat pain threshold 43.6° ± 1.0°C) than either the saline site (40.8° ± 0.9°C) or iontophoresis-control sites (41.7° ± 1.0°C) (P < 0.05). We conclude that activation of opioid receptors contributed to thermal hyperalgesia in inflamed skin during body cooling.
Implications: This study shows that opiate receptor block paradoxically inhibits sensitivity to heat-pain in inflamed skin during body cooling. The findings suggest that endogenous opioids release substances from nerves or other cells during inflammation, which heighten pain. Thus, opioids may fine-tune pain and the inflammatory response while healing takes place.
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Introduction
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Local administration of opioid agonists in inflamed peripheral tissue often decreases pain (1,2), suggesting that stimulation of peripheral opioid receptors supplements spinal and supraspinal mechanisms of opioid analgesia. The local injection of the opioid antagonist naloxone increases postoperative pain after knee surgery (3), indicating that continuing pain is inhibited by the peripheral release of endogenous opioids. Peripheral opioid analgesia involves an interaction between endogenous opioids secreted from immune cells and opioid receptors on primary afferent nociceptors (1). This mechanism is more effective in inflamed than noninflamed tissue, possibly because inflammation disrupts the blood-nerve diffusion barrier that normally limits access to opioid receptors on sensory nerves (4) or activates normally quiescent opioid receptors by some other mechanism. In the long-term, inflammation stimulates the production of opioid receptors in the nerve supplying the inflamed tissue, thereby producing an accumulation of opioid binding sites in peripheral sensory nerve terminals several days after the onset of inflammation (1).
The aim of our study was to look for evidence of peripheral endogenous opioid analgesia during acute inflammation induced by topical application of the neurotoxin capsaicin to the skin of the forearm. Small doses of capsaicin sensitize cutaneous nociceptors to heat and mechanical stimulation, and induce neurogenic inflammation by releasing neuropeptides from the peripheral terminals of these fibers (5). Kinnman et al. (6) reported that subcutaneous injection of morphine (1 mg) inhibited pain induced by the mechanical stimulation of skin surrounding the site of intradermal injection of capsaicin (0.3 mg) in the forearm, and reduced the area of mechanical hyperalgesia around the injection site. Morphine acted locally, because analgesic effects were greater at the site of morphine and capsaicin injection than at the site of saline and capsaicin injection in the other arm. Similarly, injection of the µ-opioid agonists fentanyl and DAMGO into the tail of rhesus monkeys inhibited thermal hyperalgesia provoked by the subcutaneous injection of capsaicin (7). Large doses of systemically administered opioids antagonize hyperalgesia evoked by the intradermal injection of capsaicin (8), presumably by actions in the spinal cord or brainstem. However, the analgesic effect of opioids in the monkey tail was antagonized by opioid antagonists injected into the tail, not the back, indicating that the opioids acted locally in the tail (7).
In the current study, the opioid antagonist naloxone was used to investigate the presence of peripheral endogenous opioid analgesia at normal ambient temperatures and during body cooling. Because local injection of naloxone increases postoperative pain (3), it was hypothesized that local application of naloxone would increase thermal hyperalgesia in skin inflamed by the topical application of capsaicin. In laboratory animals, the stress of cold water swimming induces signs of peripheral opioid analgesia (9). Both the duration of swimming and the water temperature influence the mechanism and strength of analgesia (10). Because swimming would not be stressful for most participants, body cooling was used as the provocative stimulus. It was hypothesized that thermal hyperalgesia would intensify at the site of naloxone pretreatment during body cooling.
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Methods
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A total of 14 healthy subjects each provided informed consent for the procedures, which were approved by the Murdoch University Human Research Ethics committee. The experiments were run in a temperature-controlled laboratory, maintained at 24° ± 2°C.
Capsaicin powder (Sigma Chemical Company, St. Louis, MO) was dissolved in 50% ethanol in distilled water at a concentration of 0.02 M (0.6%). The site selected for application of capsaicin on the forearm or back of the hand was cleaned with soap and water and an alcohol swab. When necessary, hair was shaved from the site. The gauze pad of an elastic dressing (40 mm x 25 mm), containing 400 µL of the capsaicin solution, was then applied to the prepared skin. The dressing was covered with plastic tape to retard evaporation of the capsaicin solution. The dressing was removed 30 min later and the treated skin was washed with soap and water.
Naloxone hydrochloride 0.5 mM was prepared daily with distilled water from a 10 mM stock solution in distilled water. The nonspecific effects of iontophoresis were investigated with 0.9% saline. A capsule (internal diameter 0.8 cm) was attached to the site of application of capsaicin with an adhesive washer. The capsule was filled with the naloxone or saline solution, and a weak direct current (50 µA) was passed through the solution for 4 min to introduce positively charged ions into the skin. The ground electrode was a silver plate measuring 3 cm x 5 cm, covered in electrode paste and attached to the volar aspect of the forearm near the wrist. Naloxone and saline were iontophoresed one after the other, 3–4 cm apart in the capsaicin-treated skin. The order of administration of saline and naloxone was counterbalanced across subjects. Neither the participants nor the research assistant who administered the treatments was aware of the anticipated effects of naloxone.
Approximately 15 min after the second iontophoresis, freezer packs (25 cm x 35 cm) were applied for 20 min over light clothing to the subject’s chest and back. In addition, the subject’s face, trunk, and legs were sprayed periodically with cold water while air was circulated around the subject with an electric fan. Vascular responses in the capsaicin-treated skin were monitored from the integrating probe of a Moor MBF3D laser doppler flowmeter (Axminster, Devon, UK) over a surface area of approximately 7 mm2 at a depth of 1–2 mm. Blood flow was sampled once per second and was later, averaged by using Moorsoft software (Axminster) for several minutes before cooling and after 20 min of cooling. The response to cooling was expressed as the percentage change from the level at baseline. In addition, temperature of the finger tips on the experimental side was measured with a calibrated thermocouple before and after 20 min of cooling.
The radiant heat from a halogen globe was focused through a 6 mm diameter aperture placed just above the skin. Skin temperature was monitored with a thermocouple which touched the skin in the center of the aperture. To assess the heat pain threshold, skin temperature increased from 32°C at 0.5°C per sec to a maximum of 49°C or until the subject signaled the onset of stinging or burning pain by switching off the lamp. The heat pain threshold was measured at the naloxone and saline sites and at three other sites in the capsaicin-treated skin (hereafter referred to as the "iontophoresis-control" sites). The heat pain threshold at each site was calculated as the average threshold from two or three temperature ramps. Heat pain thresholds were first measured 5–20 min after the capsaicin had been washed from the skin, and 5–10 min after the iontophoreses of saline and naloxone. Heat pain thresholds were measured again 10 min later (before cooling), and after 20 min of cooling. In previous studies in our laboratory, heat pain thresholds were lower in capsaicin-treated skin than in untreated skin (11,12), and remained stable >30–40 min in untreated skin (11).
Changes in sensitivity to heat in response to the iontophoreses and in response to body cooling were investigated in separate analyses of variance. Each analysis had repeated measure factors for site (naloxone, saline, and iontophoresis-control sites) and time. The multivariate solution was used for effects that included the site factor. Significant interactions were investigated at each time point with Student-Newman-Keuls tests, to control the Type 1 error rate. Results are presented as the mean ± SE.
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Results
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The subjects were seven men and seven women, between 17 and 40 yr of age (mean age 22 ± 6 yr). Changes in sensitivity to heat after the naloxone and saline iontophoreses are shown in Figure 1. Investigation of these changes indicated that the interaction between site and time was statistically significant (Pillai’s Trace = 0.75; F [2,12] = 17.9, P < 0.001). In particular, sensitivity to heat was similar at all sites before the iontophoreses. However, the naloxone and saline sites were more sensitive to heat than the iontophoresis-control sites after the iontophoreses (P < 0.05, Student-Newman-Keuls test).
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Figure 1. Heat pain thresholds in capsaicin-treated skin before and after the iontophoresis of naloxone and saline. After the iontophoreses, sensitivity to heat was greater at the naloxone and saline sites than at the iontophoresis-control sites (*P < 0.05, Student-Newman-Keuls test). Error bars represent standard errors.
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Blood flow through the capsaicin-treated skin decreased 61% ± 9% (P < 0.001) during body cooling, and finger temperature decreased from 29.2° ± 1.0°C to 22.3° ± 0.4°C (P < 0.001). Body cooling had different effects on the experimental and iontophoresis-control sites (Figure 2). Investigation of the interaction between site and time (Pillai’s Trace = 0.76; F [2,12] = 19.4, P < 0.001) indicated that the saline site was more sensitive to heat than the iontophoresis-control sites before body cooling (P < 0.05, Student-Newman-Keuls test). At this point the naloxone site did not differ significantly from either the saline or iontophoresis-control sites. However, after body cooling the naloxone site was less sensitive to heat than either the saline or iontophoresis-control sites (P < 0.05, Student-Newman-Keuls test).
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Figure 2. Heat pain thresholds in capsaicin-treated skin before and after intense body cooling for 20 min. Before cooling, sensitivity to heat was greater at the saline site than at the iontophoresis-control sites (#P < 0.05, Student-Newman-Keuls test). However, after body cooling, sensitivity to heat was greater at the saline and iontophoresis-control sites than at the naloxone site (*P < 0.05, Student-Newman-Keuls test). Error bars represent standard errors.
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Discussion
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Our principal finding was that naloxone pretreatment inhibited thermal hyperalgesia in inflamed skin during body cooling. This result was surprising, because the naloxone pretreatment was expected to enhance sensitivity to heat during body cooling by inhibiting peripheral opioid analgesia.
Previous studies have shown that the nociceptive effects of IV administered naloxone depend on its dose. For example, Levine et al. (13) reported that small IV doses of naloxone (0.4 and 2 mg) produced analgesia in dental patients after the removal of impacted mandibular third molars, whereas larger doses (7.5 and 10 mg) increased pain. Similarly, small IV doses of naloxone (1–10 µg/kg) inhibited behavioral signs of pain in various animal models of inflammation and peripheral nerve injury, whereas larger doses (1 and 3 mg/kg) increased signs of pain (14). In addition, intraplantar and subcutaneous injection of naloxone and methylnaloxone (3–300 µg/kg) in the inflamed rat paw inhibited signs of pain (15). The development of analgesia after injection of methylnaloxone suggests a peripheral site of action, because the methylated form of naloxone does not easily penetrate the blood-brain barrier (15). Block of an autoregulatory mechanism that inhibits the release of endogenous opioids might contribute to the analgesic effect of small doses of naloxone (14).
In the current study, approximately 0.04 mg naloxone was administered iontophoretically into a small patch of skin (i.e., 0.4 to 0.7 µg/g for subjects weighing between 60 and 100 kg when the naloxone was distributed evenly throughout the body). This dose is similar to that which inhibited behavioral signs of pain in animal studies (14); however, it is unlikely that an enhanced release of opioids at spinal and supraspinal sites inhibited thermal hyperalgesia at the site of naloxone administration because this mechanism would be expected to induce analgesia at all sites in the capsaicin-treated skin. Because naloxone was administered locally, concentrations at the site of iontophoresis were probably much larger than elsewhere in the body (up to 400 mg/kg when the naloxone penetrated <2 mm into the skin, although some of the naloxone probably escaped into the general circulation). Thus, it appears unlikely that an enhanced local release of endogenous opioids inhibited sensitivity to heat-pain at the site of naloxone administration during body cooling, because a larger concentration of naloxone at this site would block opioid receptors.
Stimulation of the nociceptive afferents which signal sensations of noxious heat provokes the release of vasoactive peptides, such as substance P, neurokinin A, and calcitonin gene-related peptide from the peripheral terminals of the nociceptive afferents (16). The local neurogenic vasodilatation (flaring) evoked by these peptides is supplemented by the secondary degranulation of mast cells (17). Release of substance P from primary afferents, degranulation of mast cells, and prostaglandin release from sympathetic efferents increases local vascular permeability and contributes to wheal formation (18). The wheal and flare of neurogenic inflammation encourages the accumulation of activated leukocytes that defend the body against infection (16). Importantly, these leukocytes appear to be a major source of endogenous opioids in inflamed tissue (1,19). In skin, topically applied capsaicin provokes flaring and little or no plasma extravasation. Nevertheless, the capsaicin induces the expression of cell adhesion molecules in the microvascular endothelium, which encourage leukocyte infiltration into the inflamed tissue; this response appears to be mediated by substance P (20). Substance P also releases nociceptive mediators, such as prostanoids and cytokines from mast cells and leukocytes (16).
Intradermal injection of morphine induces local wheal and flare responses with mast cell degranulation and histamine release (21–24). Fentanyl also induces wheal and flare responses without mast cell degranulation or histamine release (22,24). Casale et al. (21) reported that the wheal size to intradermal injection of morphine (not dynorphin) was partly inhibited by naloxone; in addition, Levy et al. (22) reported that naloxone inhibited wheal and flare responses to fentanyl and flare responses to morphine. In combination, these findings indicate that stimulation of cutaneous opioid receptors induces signs of neurogenic inflammation, mediated partly by mast cell degranulation. In addition, the inflammation provoked by endogenous opioids develops, in part, by mechanisms that do not involve activation of opioid receptors (21).
The inhibitory effect of local naloxone pretreatment on sensitivity to heat-pain in inflamed skin during body cooling suggests that the peripheral release of endogenous opioids contributed to thermal hyperalgesia. Consistent with this interpretation is the observation that intradermal injection of endogenous opioid peptides provokes signs of neurogenic inflammation (21,25). Conceivably, endogenous opioids secreted from activated leukocytes (19) or other cells in the inflamed tissue (26) during body cooling induced the local release of nociceptive mediators such as prostaglandins from mast cells or leukocytes (16) perhaps mediated by neuropeptides such as substance P. The benefit of this response would be the accumulation of activated leukocytes that fight sources of infection in inflamed skin. Pain induced by the nociceptive mediators may be counterbalanced by an inhibitory prejunctional effect of endogenous opioids on nociceptor discharge (6,16). In addition, opiates appear to inhibit further leukocyte migration into inflamed tissue (27), consistent with the notion that opioid peptides fine-tune the immune response (25).
Our findings contrast with those of Stein et al. (3), who reported that postoperative pain increased for 4 hr after naloxone (0.04 mg) was injected into the knee joint. The probable reason for the discrepancy in findings is that an acute enhancement of leukocyte infiltration resulted in sufficient release of opioids to inhibit pain postoperatively, but not during body cooling. Alternatively, the method of pain assessment (hyperalgesia to heat versus ratings of postoperative pain) or the site or method of naloxone administration (transcutaneous iontophoresis versus intraarticular injection) may have contributed to the different results. It is clear that the nature and severity of stressful events influences the mechanism of centrally mediated stress-induced analgesia (10); however, further studies are needed to identify the optimal triggers for peripheral opioid analgesia.
The iontophoresis of naloxone induced thermal hyperalgesia at normal ambient temperatures; however, sensitivity to heat also increased after the iontophoresis of saline, suggesting that hyperalgesia was nonspecific. Because nonspecific current-induced vasodilatation develops in the skin during iontophoresis (28), the electric current used during iontophoresis probably provokes neurogenic inflammation and hyperalgesia (29) particularly in skin that is already inflamed. Sensitivity to heat persisted at the saline site during body cooling, presumably mediated in part by the facilitatory effect of endogenous opioids on neurogenic inflammation.
Opioid peptides appear to influence cell differentiation and immunomodulation in the epidermis (26), functions of obvious importance after injury to the skin. Endogenous opioids may also help to maintain neurogenic inflammation at an optimal level while healing takes place, by balancing the release of nociceptive mediators against a direct inhibitory influence on nociceptor discharge.
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Acknowledgments
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Supported by the National Health and Medical Research Council of Australia.
The author wishes to thank Ms. Nadene Friday for research assistance.
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References
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Accepted for publication December 10, 1999.
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Abstract
Introduction
