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REGIONAL ANESTHESIA AND PAIN MANAGEMENT

Peripheral Antihyperalgesic Effect of Morphine to Heat, but Not Mechanical, Stimulation in Healthy Volunteers after Ultraviolet-B Irradiation

Wolfgang Koppert, MD^*, Rudolf Likar, MD^§, Gerd Geisslinger, PhD, MD, Susanne Zeck, MD^*, Martin Schmelz, MD, and Reinhard Sittl, MD^*

Departments of *Anesthesiology, Experimental and Clinical Pharmacology and Toxicology, and Physiology I, University of Erlangen-Nuremberg, Germany; and §Department of Anesthesiology, LKH Klagenfurt, Austria

Address correspondence and reprint requests to Dr. W. Koppert, Department of Anesthesiology, University of Erlangen-Nuremberg, Krankenhausstr. 12, D-91054 Erlangen, Germany. Address e-mail to koppert{at}physiologie1.uni-erlangen.de

	Abstract

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The objective of this study was to evaluate direct peripheral analgesic effects of morphine using a peripheral model of hyperalgesia and the technique of IV regional anesthesia (IVRA), thus allowing the differentiation between central and peripheral mechanisms of action. Two spots on the ventral sides of both forearms in 12 volunteers were irradiated with ultraviolet (UV)-B to induce thermal and mechanical hyperalgesia. One day after the induction of the inflammatory reaction, 40 mL of morphine hydrochloride 0.01% was administered via IVRA. Calibrated heat and phasic mechanical stimuli were applied to differentially determine impairments of tactile and nociceptive perception. Touch and phasic mechanical stimuli of noxious intensity to normal skin did not reveal altered responsiveness caused by morphine. In contrast, the administration of morphine significantly increased heat pain thresholds in the UV-B–pretreated skin areas. The peripheral antihyperalgesic effects of morphine were demonstrated only in inflamed skin areas. Direct central analgesic effects were ruled out by the lack of measurable plasma concentrations of morphine and its metabolites. Morphine 0.01% significantly diminished thermal, but not mechanical, hyperalgesia by a peripheral mode of action, which suggests inhibition of effector pathways leading to heat, but not mechanical, sensitization.

Implications: The peripheral analgesic effects of morphine were studied using modified IV regional anesthesia. When administered 1 day after the induction of dermal inflammation, morphine 0.01% diminished heat, but not primary mechanical, hyperalgesia. Therefore, suppression of mechanical hyperalgesia seen in previous studies could be predominantly due to inhibition of secondary (central) mechanical hyperalgesia.

	Introduction

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The analgesic effects of opioids through actions within the central nervous system at spinal and supraspinal levels are well known. Beside being found on central terminals of primary afferents, opioid receptors can also be identified on peripheral sensory nerve fibers and their terminal endings (1,2). There is a growing body of evidence that the antinociceptive effects of opioids play an important role in the modulation of peripheral nociception in these terminals (3).

In clinical studies, peripheral analgesic effects of opioids have been observed mainly during inflammatory-induced pain (4–6). These studies also demonstrated that opioids are effective within a few hours after the induction of inflammation. This could be due to an enhancement of the permeability of the perineurium and to activation of opioid receptors in the terminal nerve endings, which may undergo structural changes in the acidic environment of the inflammation (3,7). Several days after induction of the inflammation, opioid receptors are newly formed and expressed in peripheral nerve terminals via axonal transport, which leads to an upregulation in the nerve endings (8).

In the present study, we evaluated peripheral effects of morphine and its underlying neural mechanisms in ultraviolet (UV)-B–induced hyperalgesia. Defined doses of UV-B irradiation led to visible erythema as a local sign of inflammation, caused by formation of prostaglandins and the release of inflammatory mediators (9).

One day after induction of the inflammatory reaction, morphine was administered using the technique of IV regional anesthesia (IVRA; Bier block) to exclude possible central effects of the drug. The sensitivity of the irradiated skin patches were tested with calibrated thermal and mechanical stimuli. Central effects were definitively ruled out, and inflammatory side effects caused by a local injection of morphine were minimized.

	Methods

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Twelve healthy subjects (four women and eight men; mean age 31 yr, range 27–48 yr) participated in this randomized, double-blinded study. Each subject gave written, informed consent to take part in the study; the experimental protocol was approved by the ethics committee of the Medical Faculty of the University of Erlangen-Nuremberg.

One week before the experimental session, the individual minimal erythema dose (MED) for UV-B irradiation was established using a calibrated UV source (290–320 nm; Saalmann multitester SBB LT 400; Saalmann Medizintechnik, Herford, Germany). Five circular spots with a diameter of 1.5 cm at the ventral side of the upper leg were irradiated with increasing intensities of UV-B light (0.02–0.06 J/cm) (2). One day before the experimental session, skin areas on the ventral side of both forearms were irradiated with UV-B receiving onefold and threefold individual MED on each forearm. No spontaneous ongoing pain was reported at the irradiated spots. Within 24 h, erythema developed, as did hypersensitivity to mechanical and thermal stimuli. Because the study took place during early spring, the subjects were told to avoid additional UV exposure.

Heat pain thresholds were assessed by irradiating the skin with a feedback-controlled halogen bulb. Computer-controlled heat stimulation with a temperature increase of 0.66°C/s was applied to the skin beginning at 32°C and was interrupted by the subject's pressing a button as soon as the heat became painful. The two spots irradiated with onefold and threefold MED and a control spot on untreated skin were tested twice in random order. The two values from each stimulation site were averaged.

Impact stimuli were delivered by shooting a small plastic cylinder (0.5 g, 4 mm diameter) against the skin using a pressurized air-driven stimulator (10). The subjects were instructed to rate each stimulus separately, with 0 = "no pain sensation" and 100 = "threshold pain sensation," whereby 200 = an intensity of sensation that was twice as intense as a pain threshold stimulus. They were asked to estimate the intensity of nonpainful or "prepain" sensations by giving proportionate values <100. In previous studies, these techniques provided reliable and reproducible methods for the application and measurement of nonpainful and painful mechanical stimuli (10,11).

Control spots and spots irradiated with onefold and threefold MED were again randomly tested twice. The ratings obtained from the same spot were averaged.

All subjects were familiar with the stimulation procedures described above. Double-cuff tourniquets were placed around both upper arms. IV cannulae (22-gauge) were placed in a vein on the back of the hand. The arms were elevated, and Esmarch bandages were applied to exsanguinate the arms. The cuffs were then inflated to 250 mm Hg and kept at that pressure while the Esmarch bandages were unwound. Forty milliliters of morphine hydrochloride 0.01% was injected into one arm (4 mg of morphine; Merck, Darmstadt, Germany), and 40 mL of saline 0.9% was injected into the other arm (control). Heat and impact stimuli were performed before exsanguination and 10 and 20 min after applying the block.

To determine systemic plasma concentrations of morphine and its metabolites, venous blood samples were taken immediately before and 5 min after reestablishing circulation to the arms. Regional venous blood samples were taken directly after veins had refilled after deflation of the cuffs below the systolic pressure. Plasma was stored at -72°C until analysis.

Morphine, morphine-3-glucuronide (M3G) and morphine-6-glucuronide (M6G) concentrations were assayed by using a high-performance liquid chromatography method as described previously (12). Briefly, 100 µL of an internal standard (hydromorphone hydrochloride, 50 µg/mL H₂O) was added to 1 mL of plasma. The samples were buffered with 3 mL of 0.5 M ammonium sulfate (pH 9.3) followed by solid-phase extraction (12). Separation was achieved with a prepacked C-18 AB column (100-mm, 2-mm inner diameter; Machery-Nagel, Dueren, Germany). The mobile phase consisted of 10 mM sodium dihydrogen phosphate and 1.25 mM sodium dodecyl sulfate (pH adjusted to 2.2 with o-phosphoric acid) plus acetonitrile (82:18). The native fluorescence intensity of morphine and its metabolites were measured (excitation and emission wavelengths of 245 nm and 335 nm, respectively). The internal standard was measured at 245 nm. The system was used in an air-conditioned room (20°C). The reliable limit of quantification was 10 ng/mL for all analyses, and the coefficient of variation over the calibration range of 10–500 ng/mL was <13%.

Student's t-tests were performed to examine differences between two repetitive testings of heat pain thresholds and pain ratings at each time point; they were further evaluated by using analysis of variance (ANOVA). The design was a two-way within-subjects (repeated measures) ANOVA including the effects medication (morphine versus placebo), UV dose (control, onefold and threefold MED), and repetition (of stimuli). Post hoc Scheffé's tests were performed when significant factors were found.

Significance levels throughout this study were P < 0.05; all data are expressed as mean ± SEM. The STATISTICA software package (Statsoft, Tulsa, OK) was used for statistical analysis.

	Results

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None of the subjects reported spontaneous pain at the beginning of the experiment. Both UV-B–irradiated spots were clearly visible: irradiation with threefold MED induced more intensive skin erythema than irradiation with onefold MED in all subjects. Repetitive measurements of heat pain thresholds and impact pain ratings showed no differences at any time point (t-test was not significant [NS]), and baseline levels were not significantly different in both arms. Exsanguination of the control arm did not lead to a significant change in heat pain thresholds (ANOVA for placebo: repetition x UV dose; F[4,40] = 0.98; NS) or impact pain ratings (ANOVA for placebo: repetition x UV dose; F[4,36] = 1.57; NS) during the observation period. Systemic morphine and M3G levels were detectable in only one subject during IVRA (Table 1). The regional venous morphine level taken directly after restoration of circulation was 665.7 ± 129.0 ng/mL (range 106.9–1311.1 ng/mL). No significant correlations to age, weight, and ratings were observed.

View larger version (35K): [in this window] [in a new window]   Figure 1. Heat pain thresholds before and 10 and 20 min after start of the modified IV regional anesthesia (IVRA). A, Irradiation with ultraviolet (UV)-B led to diminished heat pain thresholds compared with the control spot. Interstimulus intervals of 10 min elapsed between the trials. MED = individual minimal erythema dosage. B, Regional administration of morphine significantly increased heat pain thresholds in the UV-B spot irradiated with threefold MED (Scheffé's test P < 0.01). temp = relative changes of heat pain thresholds during morphine treatment compared with saline.

In contrast to the heat pain thresholds, morphine treatment did not induce any significant changes in pain ratings (Fig. 2).

View larger version (34K): [in this window] [in a new window]   Figure 2. Pain ratings after impact stimulation before and 8 and 18 min after start of modified IV regional anesthesia (IVRA). A, Pain ratings increased with higher ultraviolet (UV)-B doses. Interstimulus intervals of 10 min elapsed between the trials. MED = individual minimal erythema dosage. B, No significant changes were observed after regional administration of morphine. temp = relative changes of pain ratings during morphine treatment compared with saline.

	Discussion

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During the experiments, no detectable concentrations of morphine or its metabolites were found in the circulation except in one subject (Subject 5) (Table 1). In this subject, venous morphine and M3G levels were detected systemically during the block. M3G levels were also detected inside the IVRA compartment, showing a leakage in both directions. However, we included this subject in our study because the systemic morphine levels observed during the IVRA in this subject were assumed to be too small to produce central analgesic effects. Accordingly, heat pain threshold in normal skin and pain rating after mechanical stimulation were unchanged in this subject, whereas UV-B–induced heat hyperalgesia was diminished.

Regional morphine levels in the venous blood, taken directly from the exsanguinated arms immediately after deflating the tourniquets just below systolic pressure, showed relatively large scatter. This is likely the result of inhomogeneous dilution during venous blood collection, because the amount of replenishing blood was not controlled, yet homogenous distribution patterns during IVRA have been demonstrated in previous radiographic experiments with lidocaine (14).

Systemic venous morphine levels 5 min after removing the tourniquet had no obvious central or circulatory effects in any subject.

We studied peripheral analgesic effects of morphine using an established inflammatory model. After 6–12 h, irradiation with a defined UV-B dose-rate led to visible erythema, accompanied by primary hyperalgesia to heat and mechanical stimulation. Maximal inflammatory response is reached approximately 24 h after irradiation (9). Other investigators have recommended the UV model as being suitable for the testing of peripherally acting, antiinflammatory substances (15,16). In humans, the antihyperalgesic effects of nonsteroidal analgesic drugs to heat and mechanical stimulation have been demonstrated in the UV-B model (16). Compared with other models of subacute inflammation (17), the induction of inflammation with UV-B is not painful and is better controlled.

Irradiation with UV-B causes a release of inflammatory mediators such as prostaglandins, histamine, bradykinin, and serotonin. These mediators have been extensively investigated under in vitro conditions, and although their sensitizing effects are well established, little is known about transduction mechanisms at the terminal nerve endings. Previous findings indicate that heat sensitization depends mainly on increased intracellular cAMP analogs and Ca²⁺ concentrations in polymodal nociceptors (18,19).

In contrast, the role of polymodal nociceptors in mechanical hyperalgesia is unclear. Convincing results of mechanical sensitization were only achieved in A-HTM fibers and recruitment of previously silent CM_i-fibers (20, 21). Recruitment of these units may add a component of spatial summation of nociceptive input on central neurons, contributing to primary mechanical hyperalgesia. Furthermore, these mechanisms may cause or enhance central sensitization, leading to secondary mechanical hyperalgesia.

In the UV-B model, antihyperalgesic effects of morphine were observed only in inflamed tissues. This confirms previous experimental and clinical studies in human showing antinociceptive effects of morphine in different models of inflammation (3,4,22,23). There is growing evidence that within the first few hours of inflammation, the peripheral antihyperalgesic effects of opioids are mainly based on the interaction of the opioid with preformed but inactive opioid receptors in the terminal nerve endings, which may undergo conformational changes in inflamed tissue and thus be rendered active (24). All three receptor types (µ-, -, and -opioid receptors) were functionally active in terminal nerve endings. Depending on the type of inflammation and stimulation, preferential µ-ligands were generally the most potent agonists (24).

Effector pathways are common in all three receptor types: Aside from a G-protein–mediated decrease in intracellular cAMP, the activation of opioid receptors leads to increasing intracellullar K⁺ concentrations and decreased intracellular Ca²⁺ levels via deactivation of Ca²⁺ channels (25). This is associated with hyperpolarization and decreased excitability in sensory nerve endings. Thus, the thermal antihyperalgesic effects of morphine seen in our study may be explained by a reduction of increased concentrations of cAMP and Ca²⁺ under inflammatory conditions. However, this is speculative and requires further confirmation.

Bickel et al. (16) described a lack of antihyperalgesic effects of the peripherally acting -agonists EMD 61753 in the UV-B model using the same test procedure described in this article. UV-induced mechanical and heat hyperalgesia were unchanged after oral administration of the -agonists, whereas ibuprofen significantly reduced both (16). Given that the concentration of the - agonists in inflamed skin was sufficient, this suggests a predominant role of µ- or -opioid receptors in antihyperalgesia to heat during the first days after the induction of UV-B–induced inflammation.

In contrast to previous studies in humans, no antihyperalgesic effects of morphine to mechanical stimulation were observed during the experiment. A delayed onset of morphine effects on mechanical hyperalgesia also cannot be completely excluded, although this seems unlikely because antihyperalgesic effects to mechanical stimulation were observed within 30 min after morphine administration. Moiniche et al. (22) showed that morphine given subcutaneously into a second-degree burn injury significantly increased heat pain thresholds and pressure-pain thresholds. Moreover, Kinnman et al. (23) reported that morphine given subcutaneously before an intradermal capsaicin injection attenuates mechanical hypersensitivity. However, both studies found effects mainly on secondary (central) mechanical hyperalgesia. In our study, morphine was tested in the UV-B model, which results in primary heat and mechanical hyperalgesia without signs of secondary hyperalgesia, such as allodynia or pinprick hyperalgesia. Thus, the antihyperalgesic effects of morphine on secondary mechanical hyperalgesia previously published are not contradictory to the missing effect on primary hyperalgesia described in our article.

Antihyperalgesic effects to mechanical stimulation seen after local administration of morphine during knee and thoracic surgery (4,5) are in agreement with these findings, which indicate a significant role of secondary mechanical hyperalgesia in the above-mentioned pain states. If the concentration of morphine at terminal nerve endings was capable of producing antihyperalgesic effects to mechanical and thermal stimulation, the early administration of morphine may play an important role in preventing mechanical-induced pain.

In conclusion, the present data support findings that peripheral opioid receptors mediate antinociceptive effects mainly in inflamed tissues (3,7,8). The administration of morphine significantly suppresses hyperalgesia to heat in UV-B–induced inflammation. Polymodal nociceptors are the most likely target for this effect. Because the relatively fast onset of antinociceptive effects after 1 day is incompatible with enhanced expression and axonal transport into the nerve terminals, preexisting opioid receptors in terminal nerve endings of polymodal nociceptors seem to be present. Our data support recent studies showing that µ-agonists are generally more potent than -agonists in different models of inflammation (24).

In contrast, primary mechanical hyperalgesia in our model of inflammation was not affected by morphine hydrochloride 0.01% administered via an IVRA technique. These findings confirm that the mechanism of mechanical hyperalgesia, which may involve sensitization of silent A and C nociceptors, is different from the induction of heat hyperalgesia. Morphine seems to be predominantly capable of preventing mechanical hyperalgesia when administered before or immediately after tissue injury, thus preventing secondary mechanical hyperalgesia.

	Acknowledgments

 
Supported by the Deutsche Forschungsgemeinschaft (SFB 353).

	References

Top Abstract Introduction Methods Results Discussion References

 

1. Stein C, Hassan AHS, Przewlocki R, et al. Opioids from immunocytes interact with receptors on sensory nerves to inhibit nociception in inflammation. Proc Natl Acad Sci USA 1990;87:5935–9.[Abstract/Free Full Text]
2. Coggeshall RE, Zhou S, Carlton SM. Opioid receptors on peripheral sensory axons. Brain Res 1997;764:126–32.[ISI][Medline]
3. Stein C. The control of pain in peripheral tissue by opioids. N Engl J Med 1995;332:1685–90.[Free Full Text]
4. Stein C, Comisel K, Haimerl F, et al. Analgesic effect of intraarticular morphine after arthroscopic knee surgery. Engl J Med 1991;325:1123–6.[Abstract]
5. Welte M, Haimerl E, Groh J, et al. Effect of interpleural morphine on postoperative pain and pulmonary function after thoracotomy. Br J Anaesth 1992;69:637–9.[Abstract/Free Full Text]
6. Likar R, Schäfer M, Paulak F, et al. Intraarticular morphine analgesia in chronic patients with osteoarthritis. Analg 1997;84:1313–7.[Abstract]
7. Antonijevic I, Mousa SA, Schäfer M, Stein C. Perineural defect and peripheral opioid analgesia in inflammation. Neurosci 1995;15:165–72.[Abstract]
8. Hassan AHS, Ableitner A, Stein C, Herz A. Inflammation of the rat paw enhances axonal transport of opioid receptors in the sciatic nerve and increases their density in the inflamed tissue. Neuroscience 1993;55:185–95.[ISI][Medline]
9. Hruza LL, Pentland AP. Mechanisms of UV-induced inflammation. J Invest Dermatol 1993;100:41S–53S.
10. Kohllöffel LUE, Koltzenburg M, Handwerker HO. A novel technique for the evaluation of mechanical pain and hyperalgesia. Pain 1991;46:81–7.[ISI][Medline]
11. Koltzenburg M, Handwerker HO. Differential ability of human cutaneous nociceptors to signal mechanical pain and to produce vasodilatation. J Neurosci 1994;14:1756–65.[Abstract]
12. Lötsch J, Stockmann A, Kobal G, et al. Pharmacokinetics of morphine and its glucuronides after intravenous infusion of morphine and morphine-6-glucuronide in healthy volunteers. Clin Pharmacol Ther 1996;60:316–25.[ISI][Medline]
13. Koppert W, Zeck S, Sittl R, et al. Low-dose lidocaine suppresses experimentally induced hyperalgesia in humans. Anesthesiology. In press.
14. Raj PP, Garcia CE, Burleson JW, Jenkins MT. The site of action of intravenous regional anesthesia. Anesth Analg 1972;51:776–86.[Free Full Text]
15. Woodward DF, Raval P, Pipkin MA, Owen DAA. Re-evaluation of the effect of non-steroidal antiinflammatory agents on U.V. induced cutaneous inflammation. Agents Actions 1981;11:711–7.[ISI][Medline]
16. Bickel A, Dorfs S, Schmelz M, et al. Effects of antihyperalgesic drugs on experimentally induced hyperalgesia in man. Pain 1998;76:317–25.[ISI][Medline]
17. Kilo S, Schmelz M, Koltzenburg M, Handwerker HO. Different patterns of hyperalgesia induced by experimental inflammation in human skin. Brain 1994;117:385–96.[Abstract/Free Full Text]
18. Taiwo YO, Levine JD. Further confirmation of the role of adenyl cyclase and of cAMP-dependent protein kinase in primary afferent hyperalgesia. Neuroscience 1991;44:131–5.[ISI][Medline]
19. Kress M, Günther S, Reeh PW. Capsaicin- and proton-induced heat sensitizations of nociceptors are mediated by increased [Ca²⁺]_i or [H⁺]_i [abstract]. Pflugers Arch 1998;435:R152.
20. Reeh PW, Bayer L, Kocher L, Handwerker HO. Sensitization of nociceptive cutaneous nerve fibers from the rat's tail by noxious mechanical stimulation. Exp Brain Res 1987;65:505–12.[ISI][Medline]
21. Schmidt R, Schmelz M, Forster C, et al. Novel classes of responsive and unresponsive C nociceptors in human skin. Neurosci 1995;15:333–41.[Abstract]
22. Moiniche S, Dahl JB, Kehlet H. Peripheral antinociceptive effects of morphine after burn injury. Acta Anaesthesiol Scand 1993;37:710–2.[ISI][Medline]
23. Kinnman E, Nygards EB, Hansson P. Peripherally administrated morphine attenuates capsaicin-induced mechanical hypersensitivity in humans. Anesth Analg 1997;84:595–9.[Abstract]
24. Stein C. Peripheral mechanisms of opioid analgesia. Anesth Analg 1993;76:182–91.[Abstract/Free Full Text]
25. Standifer KM, Pasternak GW. G proteins and opioid receptor-mediated signalling. Cell Signal 1997;9:237–48.[ISI][Medline]

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Anesthesia & Analgesia® is published for the International Anesthesia Research Society® by Lippincott Williams & Wilkins with the assistance of Stanford University Libraries' HighWire Press®. Copyright 2006 by the International Anesthesia Research Society. Online ISSN: 1526-7598   Print ISSN: 0003-2999