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ANALGESIA

The Analgesic Drug, Tramadol, Acts as an Agonist of the Transient Receptor Potential Vanilloid-1

Rita Marincsák, MD^*, Balázs I. Tóth, MSc^*, Gabriella Czifra, PhD^*, Tamás Szabó, MD, PhD, László Kovács, MD, PhD^*, and Tamás Bíró, MD, PhD^*

From the *Department of Physiology, Cell Physiology Research Group of the Hungarian Academy of Sciences, and Department of Pediatrics, University of Debrecen, Medical and Health Science Center, Research Center for Molecular Medicine, Debrecen, Hungary.

Address correspondence and reprint requests to Tamás Bíró, MD, PhD, Department of Physiology, University of Debrecen, Medical and Health Science Center, Research Center for Molecular Medicine, 4032 Debrecen, Nagyerdei krt. 98. PO Box 22, Hungary. Address e-mail to biro{at}phys.dote.hu.

Abstract

BACKGROUND: Tramadol is an effective analgesic substance widely used in medical practice. Its therapeutic action have been mainly attributed to the activation of µ-opioid receptors as well as to the inhibition of neurotransmitter reuptake mechanisms and various voltage- and ligand-gated ion channels of the nociceptive system. As transient receptor potential vanilloid-1 (TRPV1, "the capsaicin receptor") has been shown to function as a central integrator molecule of pain sensation, our aim in the current study was to define the involvement of TRPV1 in the complex mechanism of action of tramadol.

METHODS: To achieve these goals, we used single-cell Ca-imaging as well as fluorescent image plate reader assays on Chinese hamster ovary (CHO) cells heterologously over-expressing TRPV1.

RESULTS: We found that (1) tramadol, similar to the well-known TRPV1 agonist, capsaicin, significantly increased [Ca²⁺]_i of TRPV1-CHO cells in a concentration-dependent fashion; (2) its effect was reversibly prevented by the TRPV1 antagonist capsazepine; (3) repeated application of tramadol resulted in marked tachyphylaxis; and (4) tramadol did not modify [Ca²⁺]_i in control (empty vector expressing) CHO cells.

CONCLUSIONS: Collectively, these findings strongly support the intriguing and novel concept that tramadol acts as an agonist of TRPV1. Considering that activation of TRPV1 on sensory neurons is followed by a local release of vasoactive neuropeptides and a marked desensitization of the afferent fibers (hence termination of pain sensation), our findings may equally explain both the desired analgesic as well as the often-seen, yet "unexpected," local side effects (e.g., initiation of burning pain and erythema) of tramadol.

Transient receptor potential vanilloid-1 (TRPV1) is a nonselective calcium-permeable cation channel, which was originally described on nociceptive sensory afferents as a central integrator of pain sensation.1,2 TRPV1 can be activated and/or sensitized by certain exogenous agonists, such as capsaicin, a main pungent ingredient of hot chili peppers, or its ultrapotent analog, resiniferatoxin, and numerous endogenous substances, such as heat, protons, bradykinin, lipid peroxidation products, etc.3,4 The activation of TRPV1 results in depolarization of the sensory afferents, firing of action potentials and, hence, the onset of pain.1,5 Therefore, the molecule may serve as an attractive analgesic pharmacological target.6,7

Of great importance, several reports also suggest that certain analgesics and/or anesthetics, besides their previously appreciated target molecules, may also act on TRPV1. However, their action was very controversially documented by different groups. Namely, in cells heterologously expressing TRPV1, several local anesthetics, such as lidocaine, prilocaine, and procaine, were shown to inhibit the capsaicin-induced increase of intracellular calcium concentration ([Ca²⁺]_i) in a concentration-dependent manner.8 In contrast, on cultured TRPV1-expressing nociceptive neurons, tetracaine, another local anesthetic, was shown to enhance the membrane current induced by capsaicin.9 In the above study, Hirota et al. have also found that the effect of capsaicin was not modified by a wide array of IV general anesthetics (such as thiopental, ketamine, propofol). However, in the same expression system, other researchers have shown that propofol acts as a potent agonist of the receptor.10

Tramadol is an effective analgesic substance widely used in medical practice.11,12 Its therapeutic action was mainly attributed to the activation of µ-opioid receptors11,13,14 and to the inhibition of serotonin and norepinephrine reuptake by the synaptosomes.15,16 Interestingly, however, on various cultured neuronal cell populations, tramadol was also shown to inhibit the activity of voltage-dependent Na⁺ channels,17 delayed rectifier K⁺ channels18 as well as -aminobutyric acid type A and N-methyl-d-aspartate ionotropic receptors.19 These data strongly argue for a more complex mechanism of tramadol’s action.

Of further importance, data from numerous in vivo studies also suggest that tramadol may also exert a local anesthetic-like effect.20–23 In light of the above findings, in the current study, we aimed to investigate the effect of tramadol on one of the key molecules of nociception, i.e., TRPV1. Here, we report that tramadol, intriguingly, acts as an agonist of TRPV1.

METHODS

Expression System, Cell Culturing
The expression system was generated as we reported previously.24–26 Briefly, cDNA of the rat TRPV1 was subcloned into pUHG102-3 (Clontech, Palo Alto, CA) and was transfected into Chinese hamster ovary (CHO) cells carrying the pTet Off Regulator plasmid (Clontech) (TRPV1-CHO cells). In these cells, expression of the pUHG plasmid (hence TRPV1) is repressed in the presence of tetracycline and is expressed upon removal of the antibiotic. Therefore, cells were routinely cultured in Ham F-12 medium (supplemented with 10% fetal calf serum, and antibiotics, all from Sigma St. Louis, MO), which contained 500 µg/mL G418 (Geneticin) (Invitrogen, Paisley, UK), and 1 µg/mL tetracycline (Sigma). Before calcium imaging, cells were seeded on 25-mm glass coverslips or 96-well plates and were switched to tetracycline-free Ham F-12 medium. Cells were then cultured at 34.5°C rigorously for 48 h to induce similar TRPV1 expression levels in the different experiments. To evaluate the efficacy and homogeneity of the induction of TRPV1 expression, Western blot analysis was performed as described in our previous report25 (data not shown).

Measurements of [Ca²⁺]_i Using Single Cell Ca Imaging
Changes in [Ca²⁺]_i were detected as described in our earlier reports.24,25 TRPV1-CHO cells were cultured on 25-mm glass coverslips and a calcium-sensitive probe was introduced into the intracellular space by incubating the cells with 2 µM fura-2 AM (Invitrogen) for 1 h at 37°C. Before each measurement, the cells were kept at room temperature (22°C–24°C) in normal Tyrode’s solution (137 mM NaCl, 5.4 mM KCl, 0.5 mM MgCl₂, 1.8 mM CaCl₂, 11.8 mM HEPES-NaOH, 1 g/L glucose, pH 7.4, all from Sigma) for 30 min to allow de-esterification of the fluorophore. The coverslips, containing the fura-2-loaded cells, were then placed on the stage of an inverted fluorescence microscope (Diaphot, Nikon, Tokyo, Japan). Excitation was altered between 340 and 380 nm using a dual wavelength monochromator (Deltascan, Photon Technology International, New Brunswick, NJ). The emission was monitored at 510 nm with a photomultiplier at an acquisition rate of 10 Hz per ratio, and the fluorescence ratio (F₃₄₀/F₃₈₀) values were determined. Cells were continuously washed by Tyrode’s solution using a slow background perfusion system, whereas the agents investigated (capsaicin and tramadol were from Sigma; capsazepine from Alexis, San Diego, CA) were applied through a rapid perfusion system positioned in close proximity to the cell measured. In initial experiments, varying concentrations of tramadol were tested and saturating concentrations resulting in maximal responses were selected for the subsequent experiments. Analyses of the [Ca²⁺]_i transients were performed by a PTI analysis program developed by us, which measures (1) maximal amplitude of the transient above the baseline (in fluorescence ratio, F₃₄₀/F₃₈₀); (2) the time to peak value (time interval between the start of the application of the drug and the maximal value of the increase, in s); and (3) the rate of rise value (slope of the ascending phase measured between the onset and peak of the transient, in ratio per second). All data are expressed as the mean ± sem.

Microfluorimetric Measurements of [Ca²⁺]_i
Cells were seeded in 96-well black-well/clear-bottom plates (Greiner Bio-One, Frickenhausen, Germany) at a density of 40,000 cells per well in Ham F-12 medium, supplemented as above, and cultured at 34.5°C for 48 h. The cells were then incubated with Ham F-12 medium containing the cytoplasmic calcium indicator 2 µM Fluo-4 AM (Invitrogen) at 34.5°C for 40 min. The cells were washed four times with and finally cultured in Hank’s solution (136.8 mM NaCl, 5.4 mM KCl, 0.34 mM Na₂HPO₄, 0.44 mM KH₂PO₄, 0.81 mM MgSO₄, 1.26 mM CaCl₂, 5.56 mM glucose, 4.17 mM NaHCO₃, pH 7.2, all from Sigma) containing 1% bovine serum albumin (Sigma) and 2.5 mM Probenecid (Sigma) for 30 min at 34.5°C. The plates were then placed to a FlexStation II³⁸⁴ fluorimetric image plate reader (FLIPR, Molecular Devices, Sunnyvale, CA) and changes in [Ca²⁺]_i (reflected by changes fluorescence; lEX = 494 nm, lEM = 516 nm) induced by various concentrations of the drugs were recorded in each well (during the measurement, cells in a given well were exposed to only one given concentration of the agent). When calculating dose–response curves, data were fitted to the Hill equation

where B is the actual fluorescence value, B_max is the theoretical maximum of B, X is the ligand in question (tramadol), and n is the Hill coefficient. Experiments were performed in quadruplets and the averaged data (as well as sem) were used in the calculations.

Statistical Analysis
Data were analyzed using a Student’s t-test, and P < 0.05 values were regarded as significant differences.

RESULTS

Tramadol Induces Transient Increase of [Ca²⁺]_i in TRPV1-CHO Cells
We first investigated the effect of capsaicin on TRPV1-CHO cells. Confirming previous results,25 1 µM capsaicin induced a transient increase in [Ca²⁺]_i, which, upon repeated applications (in a 300-s long interval), showed no tachyphylaxis (Fig. 1A, Table 1). This effect was mediated by TRPV1 since capsaicin was unable to modify [Ca²⁺]_i on empty-vector expressing CHO cells (in contrast to adenosine triphosphate which increased [Ca²⁺]_i on 73% of the cells investigated, n = 11) (Fig. 1C) and since the TRPV1 antagonist capsazepine (5 µM) effectively abrogated the action of capsaicin on TRPV1-CHO cells (data not shown).25

View larger version (14K): [in this window] [in a new window]   Figure 1. Effect of capsaicin and tramadol on [Ca2+]i in TRPV1-CHO cells. Cells growing on glass coverslips were loaded with 2 µM fura 2-AM and fluorescence ratio (F340/F380) values of excitations at 340 and 380 nm wavelengths were recorded at an acquisition rate of 10 Hz per ratio. A and B represent the effect of 1 µM capsaicin and 1 µM tramadol, respectively, on TRPV1-CHO cells. (C) In contrast to ATP (used as a positive control), capsaicin and tramadol were unable to modify [Ca2+]i on empty-vector expressing CHO cells. Results are representative of multiple determinations.

We then intended to investigate the effect of tramadol on the capsaicin-evoked [Ca²⁺]_i transients. However, intriguingly, we observed that 1 µM tramadol alone induced a transient increase in [Ca²⁺]_i (Fig. 1B), which, again similarly to the action of capsaicin, was not observed on CHO cells lacking TRPV1 (Fig. 1C). These data strongly suggest that tramadol, surprisingly, rather acted as a TRPV1 agonist in our system.

The Tramadol-Induced [Ca²⁺]_i Increases Are Distinct from Those Evoked by Capsaicin and Exhibit Profound Tachyphylaxis upon Repeated Applications
To further assess this issue, we have characterized the effect of tramadol on a large number of TRPV1-CHO cells (Table 1). Similar to capsaicin, 1 µM tramadol was able to induce transient increases in [Ca²⁺]_i in 72% of the TRPV1-CHO cells investigated (n = 41 of 57) (the threshold was minimum 10% increase in the fluorescence ratio within 60 s after the start of the application of the drug). These transients were characterized by medium amplitudes (1.2 ± 0.1 increase in the fluorescence ratio), time to peak values of 20.9 ± 1.2 s, and rate of rise values of 0.27 ± 0.04 ratio per second (all data expressed as mean ± sem). Although these variables were comparable to those observed with the application of 1 µM capsaicin (Table 1 and Ref. 25), the maximal amplitude and rate of increase values were significantly smaller (P = 0.029 and 0.001, respectively), whereas the time to peak values were significantly greater (P = 0.0002) in the case of the tramadol-induced responses (Table 1). In addition, we found that a similar fraction of transients (76% with capsaicin, 79% with tramadol) returned to the baseline value after the cessation of administration of the drugs (data not shown).

The most striking difference was found when we compared the phenomenon of tachyphylaxis. As we have previous shown25 (and also confirmed in the current study, Fig. 1A), in TRPV1-CHO cells, the repeated application of 1 µM capsaicin resulted in an insignificant decrease in the amplitude of the subsequent [Ca²⁺]_i transients. In contrast, upon the repeated administration of 1 µM tramadol (in 300 s intervals), the amplitude of the second [Ca²⁺]_i transient was 63.4% ± 5.4% (mean ± sem, n = 41) of the first (control) one (P = 0.0003), whereas the amplitude of the third [Ca²⁺]_i transient was 46.3% ± 3.8% (mean ± sem, n = 41) of the second one (P = 0.000004) (Figs. 1B, 2A and B, Table 1).

View larger version (14K): [in this window] [in a new window]   Figure 2. Effect of repeated application of capsaicin and tramadol on [Ca2+]i in TRPV1-CHO cells. TRPV1-CHO cells growing on glass coverslips were loaded with 2 µM fura 2-am. and fluorescence ratio (F340/F380) values were recorded. (A) Effect of repeated administration of short (10 s) 1 µM tramadol "pulses" at 300 s intervals. (B) Statistical analysis of F340/F380 values upon repeated tramadol applications (as described under panel A). All values are expressed as the mean ± sem of several determinations (n = 41). Significant differences (P values) were determined by t-test.

View larger version (18K): [in this window] [in a new window]   Figure 3. Effect of the TRPV1 antagonist capsazepine on the tramadol induced [Ca2+]i increases in TRPV1-CHO cells. Cells growing on glass coverslips were loaded with 2 µM fura-2 AM and fluorescence ratio (F340/F380) values were recorded. (A) Cells were first challenged with two consecutive short (10 s) 1 µM tramadol "pulses" at 600 s intervals. Then, 300 s after the initiation of the second tramadol "pulse," cells were preincubated with 5 µM capsazepine for 300 s, and the third 1 µM tramadol challenge was administered in the presence of capsazepine. The fourth tramadol challenge was applied after washing out the antagonist. (B) Statistical analysis of F340/F380 values upon repeated tramadol applications at 600 s intervals with or without capsazepine treatment. All values are expressed as the mean ± sem of several determinations (n = 10). Significant differences (P values) were determined by t-test.

Subsequently, we repeated the above protocol; however, in this case, 300 s after the initiation of the second tramadol "pulse," cells were preincubated with 5 µM capsazepine for 300 s, and the third 1 µM tramadol challenge was administered in the presence of capsazepine. Consistent with the above findings, the presence of the TRPV1 antagonist almost fully abrogated the effect of tramadol to induce [Ca²⁺]_i increase (Fig. 3A). Statistically, the amplitude of the third tramadol-induced [Ca²⁺]_i transients in the presence of capsazepine was only 12.7% ± 2.8% (mean ± sem, n = 10) (P = 0.00008) of those (third) increases, which were recorded in the lack of the TRPV1 antagonist (Fig. 3A and B). Finally, this inhibition of the tramadol-induced responses by capsazepine was reversible, since another tramadol application (600 s after the third one) again resulted in significantly (P = 0.00003) higher [Ca²⁺]_i transients than those evoked in the presence of the antagonist.

The Effect of Tramadol Is Concentration-Dependent
Finally, we investigated the concentration-dependence of tramadol on TRPV1-CHO cells. Because of the above marked tachyphylaxis, we were unable to use the single-cell Ca-imaging technique to record the effects of various tramadol concentrations on the very same cell. Therefore, the measurement of the dose–response curve of tramadol was performed using FLIPR. As seen in Figure 4A, tramadol (similar to the single-cell data shown above) did not alter the [Ca²⁺]_i of control (empty-vector expressing) CHO cell. In contrast, on TRPV1-CHO cells, it was able to increase [Ca²⁺]_i in a concentration-dependent fashion; mathematical analysis by fitting the measured values to the Hill equation resulted in an EC₅₀ value of 0.08 ± 0.03 µM (mean ± sem for four experiments) (Fig. 4B) (and 0.04 ± 0.01 µM for capsaicin, mean ± sem for five experiments, data not shown).

View larger version (9K): [in this window] [in a new window]   Figure 4. Concentration-dependence of tramadol to elevate [Ca2+]i in TRPV1-CHO cells using FLIPR. (A) Cells were seeded into black walled clear-base 96-well plates at a density of 40,000 cells per well and were loaded with 2 µM fluo-4 AM. The plates were then placed into a FlexStation II384 to monitor cell fluorescence (lEX = 494 nM, lEM = 516 nM) before and after the addition of various concentrations of tramadol (the application time point were set to 0). Note the lack of effect of tramadol on empty vector expressing CHO cells. Results are representative of multiple determinations. (B) The theoretical curve was calculated by fitting the measured mean ± sem values of four independent determinations to the Hill equation.

DISCUSSION

In this study, we investigated the effect of an analgesic, tramadol, on the function of TRPV1. Using a heterologous expression system, we found that (1) tramadol, similar to capsaicin, significantly increased [Ca²⁺]_i of TRPV1-CHO cells in a concentration-dependent fashion; (2) its effect was reversibly prevented by the TRPV1 antagonist capsazepine; and (3) tramadol did not modify [Ca²⁺]_i in control (empty vector expressing) CHO cells. These findings strongly support the intriguing and novel concept that tramadol, surprisingly, acts as an agonist of TRPV1.

Interestingly, whereas repeated capsaicin application resulted in insignificant modification of the Ca-transients, tramadol was able to induce a marked tachyphylaxis. It was previously shown that different vanilloid agonists with different chemical features cause different patterns of calcium response (potency, maximal response, latency of response, variability of latency of response among individual cells, and tachyphylaxis) in CHO cells heterologously expressing TRPV1.26 We propose, therefore, that differences in the effects of capsaicin and tramadol to induce tachyphylaxis are also due to the structural diversities of the two agents.

Previous studies have demonstrated that 50 mg single dose of tramadol (depending on IM or IV application routes) reaches 100–300 ng/mL (i.e., 0.3–1 µM) plasma levels.27,28 It was important to observe that this plasma concentration corresponds well to the EC₅₀ value of 0.08 ± 0.03 µM measured in our current study arguing for a potential in vivo (human) relevance of our findings (see also below).

As was detailed in the Introduction, tramadol (besides stimulating µ-opioid receptors) exerts a wide-array of inhibitory actions of numerous voltage- and ligand-gated neuronal channel populations, underlying its robust effect to mitigate pain. In light of these previous reports, our data presented in the current manuscript immediately invite a key question: How would the unexpected activation of the "pain-receptor" TRPV1 "fit" to the in vivo analgesic action "pattern" of this popular therapeutic drug? One straightforward explanation could be that the activation of TRPV1 by tramadol is rapidly followed by the desensitization of the sensory afferents (a phenomenon well characterized by vanilloid administration to nociceptive neurons1,2,6), which, in turn, would lead to the cessation of action potential firing, and hence pain sensation. This idea may be supported by the fact that, in our system, tramadol induced a much stronger tachyphylaxis than capsaicin.

However, it is also well established that the activation of TRPV1 also results in the local release of various peptides (e.g., substance P, calcitonin gene-related peptide) from the sensory ending.24,29,30 These neuropeptides, in turn, act on various neighboring cell types of the given tissue (e.g., mast cells, vessels, keratinocytes) and initiate numerous local regulatory mechanisms, such as vasodilation, immunomodulation, cytokine and mediator release, etc.2,4,31 It is conceivable, therefore, that if tramadol (e.g., upon local application) stimulates TRPV1-expressing sensory afferents, the initiation of the "efferent" function of the nerve endings would result in such local responses.

As a support for this argument, in various human studies, local intradermal application of tramadol, besides inducing a local anesthetic effect similar to that of lidocaine, resulted in skin erythema, flare, and urticaria.20–23 Of further importance, in certain studies, intradermal tramadol injection also initiated burning skin sensation and pain.21,22 Likewise, when the local anesthetic effect of tramadol was investigated after short (1 min) venous retention of the drug, in 31% of the patients, transient burning pain sensation and skin erythema developed distally from the place of occlusion along the affected veins.23 These in vivo results further suggest for that tramadol may indeed activate TRPV1.

Collectively, our presented findings, along with the above in vitro and in vivo data) suggest that tramadol, besides the aforementioned multiple targets, may indeed act as a "classical" agonist of TRPV1. Namely, tramadol may first excite sensory neurons (calcium influx and transient burning pain sensation), then initiate neuropeptide release (skin erythema and flare), and finally induce desensitization (tachyphylaxis) and analgesia. Hence, although further studies (e.g., using gene-deficient mice and freshly dissected sensory neurons endogenously expressing TRPV1) are to be performed to exactly define the role of TRPV1 in mediating the action of tramadol, the presented concept of "triple response" by tramadol may equally explain both the desired analgesic as well as the "unexpected" local side effects of the drug.

ACKNOWLEDGMENTS

The technical assistance of Ms. Ibolya Varga is gratefully appreciated. Tamás Bíró is the recipient of the János Bolyai Research Scholarship of the Hungarian Academy of Sciences. The authors declare no competing financial interests.

Footnotes

Accepted for publication February 15, 2008.

Supported by Hungarian research grants: OTKA T49231, OTKA K63153, ETT 480/2006, ETT 482/2006.

REFERENCES

1. Caterina MJ, Schumacher MA, Tominaga M, Rosen TA, Levine JD, Julius D. The capsaicin receptor: a heat-activated ion channel in the pain pathway. Nature 1997;389:816–24[Medline]
2. Szallasi A, Blumberg PM. Vanilloid (capsaicin) receptors and mechanisms. Pharmacol Rev 1999;51:159–212[Abstract/Free Full Text]
3. Hwang SW, Oh U. Direct activation of capsaicin receptors by products of lipoxygenases: endogenous capsaicin-like substances. Curr Opin Pharmacol 2002;2:235–42[Web of Science][Medline]
4. Di Marzo V, Blumberg PM, Szallasi A. Endovanilloid signaling in pain. Curr Opin Neurobiol 2002;12:372–9[Web of Science][Medline]
5. Tominaga M, Caterina MJ, Malmberg AB, Rosen TA, Gilbert H, Skinner K, Raumann BE, Basbaum AI, Julius D. The cloned capsaicin receptor integrates multiple pain-producing stimuli. Neuron 1998;21:531–43[Web of Science][Medline]
6. Holzer P. Capsaicin: cellular targets, mechanism of action, and selectivity for thin sensory neurons. Pharmacol Rev 1991;43:143–201[Web of Science][Medline]
7. Veronesi B, Oortgiesen M. The TRPV1 receptor: target of toxicants and therapeutics. Toxicol Sci 2006;89:1–3[Abstract/Free Full Text]
8. Hirota K, Smart D, Lambert DG. The effects of local and intravenous anesthetics on recombinant rat VR1 vanilloid receptors. Anesth Analg 2003;96:1656–60[Abstract/Free Full Text]
9. Komai H, McDowell TS. Differential effects of bupivacaine and tetracaine on capsaicin-induced currents in dorsal root ganglion neurons. Neurosci Lett 2005;380:21–5[Web of Science][Medline]
10. Tsutsumi S, Tomioka A, Sudo M, Nakamura A, Shirakura K, Takagishi K, Kohama K. Propofol activates vanilloid receptor channels expressed in human embryonic kidney 293 cells. Neurosci Lett 2001;312:45–9[Web of Science][Medline]
11. Gibson TP. Pharmacokinetics, efficacy, and safety of analgesia with a focus on tramadol HCl. Am J Med 1996;101:47–53
12. Pyati S, Gan TJ. Perioperative pain management. CNS Drugs 2007;21:185–211[Web of Science][Medline]
13. Raffa RB, Friderichs E, Reimann W, Shank RP, Codd EE, Vaught JL. Opioid and nonopioid components independently contribute to the mechanism of action of tramadol, an ‘atypical’ opioid analgesic. J Pharmacol Exp Ther 1992;260:275–85[Abstract/Free Full Text]
14. Dayer P, Desmeules J, Collart L. Pharmacology of tramadol. Drugs 1997;53:18–24[Web of Science][Medline]
15. Berrocoso E, Mico JA, Ugedo L. In vivo effect of tramadol on locus coeruleus neurons is mediated by alpha2-adrenoceptors and modulated by serotonin. Neuropharmacology 2006;51:146–53[Web of Science][Medline]
16. Ogata J, Minami K, Uezono Y, Okamoto T, Shiraishi M, Shigematsu A, Ueta Y. The inhibitory effects of tramadol on 5-hydroxytryptamine type 2C receptors expressed in Xenopus oocytes. Anesth Analg 2004;98:1401–6[Abstract/Free Full Text]
17. Haeseler G, Foadi N, Ahrens J, Dengler R, Hecker H, Leuwer M. Tramadol, fentanyl and sufentanyl but not morphine block voltage-operated sodium channels. Pain 2006;126:234–44[Web of Science][Medline]
18. Tsai TY, Tsai YC, Wu SN, Liu YC. Tramadol-induced blockade of delayed rectifier potassium current in NG108–15 neuronal cells. Eur J Pain 2006;10:597–601[Web of Science][Medline]
19. Hara K, Minami K, Sata T. The effects of tramadol and its metabolite on glycine, gamma-aminobutyric acid, and N-methyl-d-aspartate receptors expressed in Xenopus oocytes. Anesth Analg 2005;100:1400–5[Abstract/Free Full Text]
20. Pang WW, Mok MS, Chang DP, Huang MH. Local anesthetic effect of tramadol, metoclopramide, and lidocaine following intradermal injection. Reg Anesth Pain Med 1998;23:580–3[Web of Science][Medline]
21. Acalovschi I, Cristea T, Margarit S, Gavrus R. Tramadol added to lidocaine for intravenous regional anesthesia. Anesth Analg 2001;92:209–14[Abstract/Free Full Text]
22. Altunkaya H, Ozer Y, Kargi E, Babuccu O. Comparison of local anaesthetic effects of tramadol with prilocaine for minor surgical procedures. Br J Anaesth 2003;90:320–2[Abstract/Free Full Text]
23. Pang WW, Huang PY, Chang DP, Huang MH. The peripheral analgesic effect of tramadol in reducing propofol injection pain: a comparison with lidocaine. Reg Anesth Pain Med 1999; 24:246–9[Web of Science][Medline]
24. Szallasi A, Szabó T, Bíró T, Modarres S, Blumberg PM, Krause JE, Cortright DN, Appendino G. Resiniferatoxin-type phorboid vanilloids display capsaicin-like selectivity at native vanilloid receptors on rat DRG neurons and at the cloned vanilloid receptor VR1. Br J Pharmacol 1999;128:428–34[Web of Science][Medline]
25. Lázár J, Szabó T, Kovács L, Blumberg PM, Bíró T. Distinct feature of recombinant rat vanilloid receptor-1 expressed in various expression systems. Cell Mol Life Sci 2003;60:2228–40[Web of Science][Medline]
26. Tóth A, Wang Y, Kedei N, Tran R, Pearce LV, Kang SU, Jin MK, Choi HK, Lee J, Blumberg PM. Different vanilloid agonists cause different patterns of calcium response in CHO cells heterologously expressing rat TRPV1. Life Sci 2005;76:2921–32[Web of Science][Medline]
27. Lintz W, Beier H, Gerloff J. Bioavailability of tramadol after i.m. injection in comparison to i.v. infusion. Int J Clin Pharmacol Ther 1999;37:175–83[Web of Science][Medline]
28. Lintz W, Becker R, Gerloff J, Terlinden R. Pharmacokinetics of tramadol and bioavailability of enteral tramadol formulations. Fourth communication: drops (without ethanol). Arzneimittelforschung 2000;50:99–108[Medline]
29. Holzer P. Local effector functions of capsaicin-sensitive sensory nerve endings: involvement of tachykinins, calcitonin gene-related peptide and other neuropeptides. Neuroscience 1988; 24:739–68[Web of Science][Medline]
30. Okajima K, Harada N. Regulation of inflammatory responses by sensory neurons: molecular mechanism(s) and possible therapeutic applications. Curr Med Chem 2006;13:2241–51[Web of Science][Medline]
31. Szolcsányi J. Forty years in capsaicin research for sensory pharmacology and physiology. Neuropeptides 2004;38: 377–84[Web of Science][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