SciELO - Scientific Electronic Library Online

vol.65 issue3Bacterial colonization due to increased nurse workload in an intensive care unitOpioid-free total intravenous anesthesia with propofol, dexmedetomidine and lidocaine infusions for laparoscopic cholecystectomy: a prospective, randomized, double-blinded study author indexsubject indexarticles search
Home Pagealphabetic serial listing  

Services on Demand




Related links


Revista Brasileira de Anestesiologia

Print version ISSN 0034-7094On-line version ISSN 1806-907X

Rev. Bras. Anestesiol. vol.65 no.3 Campinas May/June 2015 

Scientific Articles

Local analgesic effect of tramadol is not mediated by opioid receptors in early postoperative pain in rats

Angela Maria Sousa1 

Hazem Adel Ashmawi1  * 

1Faculdade de Medicina da USP, Laboratório de Investigação Médica - LIM-08, São Paulo, SP, Brazil



Tramadol is known as a central acting analgesic drug, used for the treatment of moderate to severe pain. Local analgesic effect has been demonstrated, in part due to local anesthetic-like effect, but other mechanisms remain unclear. The role of peripheral opioid receptors in the local analgesic effect is not known. In this study, we examined role of peripheral opioid receptors in the local analgesic effect of tramadol in the plantar incision model.


Young male Wistar rats were divided into seven groups: control, intraplantar tramadol, intravenous tramadol, intravenous naloxone-intraplantar tramadol, intraplantar naloxone-intraplantar tramadol, intravenous naloxone-intravenous tramadol, and intravenous naloxone. After receiving the assigned drugs (tramadol 5 mg, naloxone 200 µg or 0.9% NaCl), rats were submitted to plantar incision, and withdrawal thresholds after mechanical stimuli with von Frey filaments were assessed at baseline, 10, 15, 30, 45 and 60 min after incision.


Plantar incision led to marked mechanical hyperalgesia during the whole period of observation in the control group, no mechanical hyperalgesia were observed in intraplantar tramadol group, intraplantar naloxone-intraplantar tramadol group and intravenous naloxone-intraplantar tramadol. In the intravenous tramadol group a late increase in withdrawal thresholds (after 45 min) was observed, the intravenous naloxone-intravenous tramadol group and intravenous naloxone remained hyperalgesic during the whole period.


Tramadol presented an early local analgesic effect decreasing mechanical hyperalgesia induced by plantar incision. This analgesic effect was not mediated by peripheral opioid receptors.

Key words: Tramadol; Postoperative pain; Opioid receptors; Wistar rats


Tramadol, (1-RS,2RS)-2-[(dimethyl-amino)-methyl]-1-(3-methoxyphenyl)-cyclohexanol hydrochloride, is an analgesic drug used mainly for treatment of moderate to severe, as well as acute and chronic pain.1 and 2 It presents a weak opioid effect and has other different mechanism of action through the decrease of the reuptake of monoaminergic neurotransmitters (5-hydroxy-tryptamine and noradrenaline).3 Additionally it presents an analgesic effect in peripheral nerves, which is, in part, local anesthetic-like.4 , 5 and 6

The peripheral analgesic effect of tramadol has been also showed in humans after infiltration for third molar extraction, minor surgical procedures in children and as adjuvant to local anesthetics to reduce postoperative pain.7 , 8 , 9 , 10 , 11 and 12 Peripheral opioid receptors have long been described and their role in analgesia in animals and humans demonstrated,13 , 14 , 15 , 16 , 17 , 18 , 19 and 20 but little is known about peripheral opioid mediated effect of tramadol.

This study aimed to evaluate the role of opioid receptors in the analgesic effects of intraplantar tramadol in a model of postoperative pain.

Materials and methods


The experiments were performed after the approval of the Bioethics Committee of the Hospital of Clinics of The University of São Paulo Faculty of Medicine and according to the Committee for Research and Ethical Issues of the IASP.21 All experiments were performed on Wistar male rats, weighing 250 g, supplied by own breeding facilities of the University of São Paulo Faculty of Medicine. The total number of animals used in the study was 35 rats. All behavioral experiments were conducted between 9:00 am and 12:00 pm. All animals were housed in pairs, in cages with bedding and free access to food and water.

Plantar incision

The rat hind paw plantar incision model was performed as previously described.22 Briefly, rats were anesthetized with 2% to 3% isoflurane delivered via nose cone. The plantar aspect of the right hind paw was prepared in a sterile manner with a 10% povidone-iodine solution and draped. A 1 cm longitudinal incision was made with a number 11 blade through skin and fascia of the plantar aspect of the paw of the rat, starting 0.5 cm from the proximal edge of the heel and extending toward the toes. The flexor muscle was elevated and incised longitudinally and the muscle origin and insertion remain intact. After hemostasis with gentle pressure, the skin was opposed with two simple sutures of 5-0 mononylon.

Mechanical hyperalgesia

Rats were placed on an elevated plastic mesh floor covered with a clear plastic cage top. The animals were allowed to ambulate, explore, and eventually rest lying on the mesh. For testing pain behaviors, calibrated von Frey filaments (thin plastic filaments with calibrated forces) were applied underneath the cage adjacent to the wound. Each filament was applied once and continuing until a withdrawal occurred, which was considered as the withdrawal threshold. Filaments with bending forces of 5.9; 9.8; 13.7; 19.7; 39.2; 58.8; 78.4; 98 147, 255, 588 and 980 mN were used in the study.

Experimental design

After assessment of the pre incision withdrawal thresholds for mechanical hyperalgesia, animals were divided into seven experimental groups of five animals each: 1 - Control, 50 µL of 0.9% NaCl solution were injected in the plantar aspect of the right hindpaw, 5 min later another 50 µL of 0.9% NaCl solution were reinjected; 2 - Intraplantar tramadol (Ipl-tramadol), 50 µL of 0.9% NaCl solution were injected in the plantar aspect of the right hindpaw and 5 min later 5 mg (20 mg/kg) of tramadol dissolved in 50 µL of 0.9% NaCl were administered in the plantar aspect of the right hindpaw; 3 - Intravenous tramadol (Iv-tramadol), 50 µL of 0.9% NaCl solution were injected in the plantar aspect of the right hindpaw and 5 min later 5 mg of tramadol were injected in the dorsal vein of the penis; 4 - Intravenous naloxone-intraplantar tramadol (Iv-naloxone + Ipl-tramadol), 200 µg naloxone injected in the dorsal vein of the penis 5 min before 5 mg of tramadol dissolved in 50 µL of 0.9% NaCl in the plantar aspect of the right hindpaw; 5 - Intraplantar naloxone-intraplantar tramadol (Ipl-naloxone + Ipl-tramadol), 200 µg naloxone was injected in the right hindpaw 5 min before intraplantar tramadol; 6 - Intravenous naloxone-intravenous tramadol (Iv-naloxone + Iv-tramadol); 200 µg naloxone injected in the dorsal vein of the penis 5 min before 5 mg of tramadol dissolved in 50 µL of 0.9% NaCl injected in the dorsal vein of the penis; 7 - Naloxone (Iv-naloxone), 200 µg naloxone injected in the dorsal vein of the penis and 50 µL of 0.9% NaCl solution injected in the paw. All injections were done under general anesthesia with 2%-3% isoflurane. Animals were kept anesthetized and 5 min after second injection they were submitted to plantar incision. As soon as the animals recovered, the tests were started and performed 10, 15, 30, 45 and 60 min after recovery from anesthesia. At the end of the experiments the animals were submitted to euthanasia with lethal dose of sodium thiopental.


Tramadol hydrochloride and naloxone hydrochloride were provided by Cristália Prod. Quim. Farm, São Paulo, SP, Brazil.

Statistical analysis

All data were analyzed by Prism 5.0 software (GraphPad Software, Inc., USA). Withdrawal threshold to mechanical stimuli are non-continuous and were analyzed with nonparametric tests. The data were expressed as median and interquartile range. Differences were determined by Kruskal-Wallis test followed by Dunn's post hoc test for comparing paw withdrawal threshold.


Plantar incision led to mechanical hyperalgesia during the whole period of observation. In the control group the median withdrawal threshold to von Frey filament decreased from 980 to 39 mN after 10 min and 98 mN after 60 min, in the intraplantar tramadol group the median withdrawal threshold was 980 mN during the whole period, in the intravenous tramadol group median decreased from 980 to 59 mN after 10 min and increased after 45 min to 255 mN and in the intraplantar naloxone-intraplantar tramadol the median withdrawal threshold was 980 mN throughout the 60 min of observation (Fig. 1).

Figure 1 - Withdrawal thresholds to mechanical stimulus after intraplantar or intravenous tramadol. Withdrawal thresholds were higher in the Ipl-tramadol and Ipl-naloxone + Ipl-tramadol groups, Iv-tramadol presented a late analgesia after 45 min of injection. Data presented as median ± interquartile interval (25th-75th percentile); p < 0.05. *Ipl-tramadol and Ipl-naloxone + Ipl-tramadol groups different from control, +Iv-tramadol different from control. 

In the intravenous naloxone-intraplantar tramadol group the median withdrawal threshold ranged from 588 to 980 mN throughout all period of observation, in the intravenous naloxone-intravenous tramadol group, median went from 980 to 59 mN, staying hyperalgesic at 60 min and in the intravenous naloxone, median went from 980 to 20 mN at 60 min of injection, the median was different from control from 15 min to 60 min (Fig. 2).

Figure 2 - Withdrawal thresholds to mechanical stimulus after naloxone with or without tramadol. Withdrawal thresholds were higher in the Iv-naloxone + Ipl-tramadol group, Iv-naloxone + Iv-tramadol was not different from control and withdrawal thresholds were lower in naloxone group than control after 25 min of injection. Data presented as median ± interquartile interval (25th-75th percentile); p < 0.05. *Iv-naloxone + Ipl-tramadol different from control, +naloxone different from control. 


The use of experimental pain models is under scrutiny due to the partial failure of translation of results obtained experimentally to clinical and real situations.23 and 24 The development and search for models that mimic clinical scenarios is very important. In this study, we used the plantar incision model, a postoperative pain model, being used for the last two decades and that brings to the experimental environment a relevant clinical situation, the postoperative pain. The prominent clinical characteristic of postoperative pain is mechanical hyperalgesia or mechanical alodinia, and we decided to observe this pain modality.

Present results showed an analgesic effect of intraplantar tramadol in the postoperative pain model used. The analgesic effect lasted throughout the whole period of observation and led to higher paw withdrawal thresholds than intravenous tramadol. Tramadol is known to be an atypical drug, presenting different mechanisms of action to achieve analgesia. When used systemically, tramadol is metabolized by cytochrome P450 (CYP) 2D6, and its major metabolite is mono-o-desmethyl-tramadol (M1), responsible for the systemic analgesic effect.3 and 25 Its analgesic effect is observed 20-40 min after oral intake.26 In the present study, intravenous tramadol decreased hyperalgesia 45 min after injection, the expected time for tramadol systemic analgesia. However, intraplantar tramadol in the same dose blocked hyperalgesia from the beginning throughout all observation period. These results indicate that the parental drug, tramadol, presents an early analgesic effect expressed right after its injection, effect that seems to be local, since the analgesic effect observed after intravenous route did not induce early analgesia. The observed local effect seems not to be mediated by opioid receptors, no reduction in analgesia was observed after naloxone neither by intraplantar nor intravenous routes. In another study analgesic effect of tramadol in the same post-operative pain model was antagonized with naloxone, but they used the intraperitoneal and intrathecal routes, and analgesic effects in the central nervous system is due to the classic decrease in monoamines reuptake and opioid mediated effects.3 , 26 and 27 Although our results did not find peripheral opioid action, we cannot rule out a local opioid analgesic effect of tramadol occurring later. It has been demonstrated that there is an increase in the expression of µ opioid receptor (MOR) synthesized in the dorsal root ganglia, then trafficked to nerve endings after intraplantar carrageenan.28 Up regulation of MOR in dorsal root ganglia was higher one day after injection and decreased at day three. Intraplantar carrageen an induces pain behavior that subsides four hours after injection. Postoperative pain in the plantar incision model peaks at day one and lasts up to five days, when thermal and mechanical thresholds are similar to baseline.20 MOR expression may occur later in this pain model what can affect peripheral opioid receptor role. An interesting data observed occurred in the intravenous naloxone group, where thresholds where lower than in control group, indicating that naloxone antagonized endogenous opioids in the first hour after incision, leading to an increase in the pain behavior after mechanical stimulus.

Local analgesic effect of tramadol has been shown to be similar to local anesthetic.4 , 5 , 6 , 29 and 30 The mechanisms involved in this peripheral action are still not well understood. Weak anti-inflammatory effect has been proposed for tramadol anlagesia and calcium dependent effect.31 and 32 Interestingly tramadol also acts as an agonist of the transient receptor potential vanilloid-1 (TRPV1) channel, maybe presenting, at some extent, a pronociceptive effect.33

The results of this study indicate that tramadol presents an early local analgesic effect decreasing mechanical hyperalgesia induced by plantar incision. This effect is not mediated, in the first hour, by peripheral opioid receptors.


Dayer P, Desmeules J, Collart L. Pharmacologie du tramadol. Drugs. 1997; Suppl. 2:18-24. [ Links ]

Lewis KS, Han NH. Tramadol: a new centrally acting analgesic. Am J Health Syst Pharm. 1997;54:643-52. [ Links ]

Raffa RB, Friderichs E, Reimann W, et al. Opioid and nonopi- oid components independently contribute to the mechanism of action of tramadol, an 'atypical' opioid analgesic. J Pharmacol Exp Ther. 1992;260:275-85. [ Links ]

Tsai YC, Chang PJ, Jou IM. Direct tramadol application on sciatic nerve inhibits spinal somatosensory evoked potentials in rats. Anesth Analg. 2001;92:1547-51. [ Links ]

Haeseler G, Foadi N, Ahrens J, et al. Tramadol, fentanyl and sufentanil but not morphine block voltage-operated sodium channels. Pain. 2006;126:234-44. [ Links ]

Sousa AM, Ashmawi HA, Costa LS, et al. Percutaneous sci- atic nerve block with tramadol induces analgesia and motor blockade in two animal pain models. Braz J Med Biol Res. 2012;45:147-52. [ Links ]

Al-Haideri Y. Comparison of local anesthetic efficacy of tra- madol hydrochloride (with adrenaline) versus plain tramadol hydrochloride in the extraction of upper molar teeth. J Oral Maxillofac Surg. 2013;71:2035-8. [ Links ]

Polat F, Tuncel A, Balci M, et al. Comparison of local anes- thetic effects of lidocaine versus tramadol and effect of child anxiety on pain level in circumcision procedure. J Pediatr Urol. 2013;9:670-4. [ Links ]

Kargi E, I¸sikdemir A, Tokgöz H, et al. Comparison of local anes- thetic effects of tramadol with prilocaine during circumcision procedure. Urology. 2010;75:672-5. [ Links ]

Prakash S, Tyagi R, Gogia A, et al. Efficacy of three doses of tramadol with bupivacaine for caudal analgesia in paediatric inguinal herniotomy. Br J Anaesth. 2006;97:385-8. [ Links ]

Kaabachi O, Ouezini R, Koubaa W, et al. Tramadol as an adjuvant to lidocaine for axillary brachial plexus block. Anesth Analg. 2009;108:367-70. [ Links ]

Demiraran Y, Albayrak M, Yorulmaz IS, et al. Tramadol and levobupivacaine wound infiltration at Cesarean delivery for postoperative analgesia. J Anesth. 2013;27:175-9. [ Links ]

Ferreira SH. Peripheral analgesia: mechanism of the analgesic action of aspirin-like drugs and opiate-antagonists. Br J Clin Pharmacol. 1980;10:237S-45S. [ Links ]

Stein C, Yassouridis A. Peripheral morphine analgesia. Pain. 1997;71:119-21. [ Links ]

Hervera A, Negrete R, Leánez S, et al. Peripheral effects of morphine and expression of µ-opioid receptors in the dorsal root ganglia during neuropathic pain: nitric oxide signaling. Mol Pain. 2011;7:25. [ Links ]

Saloman JL, Niu KY, Ro JY. Activation of peripheral delta- opioid receptors leads to anti-hyperalgesic responses in the masseter muscle of male and female rats. Neuroscience. 2011;190:379-85. [ Links ]

Auh QS, Ro JY. Effects of peripheral K opioid receptor activation on inflammatory mechanical hyperalgesia in male and female rats. Neurosci Lett. 2012;524:111-5. [ Links ]

Khalefa BI, Shaqura M, Al-Khrasani M, et al. Relative con- tributions of peripheral versus supraspinal or spinal opioid receptors to the antinociception of systemic opioids. Eur J Pain. 2012;16:690-705. [ Links ]

Katsuyama S, Mizoguchi H, Kuwahata H, et al. Involve- ment of peripheral cannabinoid and opioid receptors in β-caryophyllene-induced antinociception. Eur J Pain. 2013;17:664-75. [ Links ]

Likar R, Schäfer M, Paulak F, et al. Intraarticular morphine anal- gesia in chronic pain patients with osteoarthritis. Anesth Analg. 1997;84:1313-7. [ Links ]

Zimmermann M. Ethical guidelines for investigations of experi- mental pain in conscious animals. Pain. 1983;16:109-10. [ Links ]

Brennan TJ, Vandermeulen EP, Gebhart GF. Characterization of a rat model of incisional pain. Pain. 1996;64:493-501. [ Links ]

Mogil JS. Animal models of pain: progress and challenges. Nat Rev Neurosci. 2009;10:283-94. [ Links ]

Mogil JS, Davis KD, Derbyshire SW. The necessity of animal models in pain research. Pain. 2010;151:12-7. [ Links ]

Stamer UM, Stuber F. Genetic factors in pain and its treatment. Curr Opin Anaesthesiol. 2007;20:478-84. [ Links ]

Gentilli ME, Chauvin M. Tramadol et prise en charge de la douleur. In: Conférences d'Actualisation. 2004. p. 715-22. [ Links ]

Kimura M, Obata H, Saito S. Antihypersensitivity effects of tramadol hydrochloride in a rat model of postoperative pain. Anesth Analg. 2012;115:443-9. [ Links ]

Ji RR, Zhang Q, Law PY, et al. Expression of mu-, delta-, and kappa-opioid receptor-like immunoreactivities in rat dorsal root ganglia after carrageenan induced inflammation. J Neurosci. 1995;15:8156-66. [ Links ]

Mert T, Gunes Y, Guven M, et al. Comparison of nerve conduction blocks by an opioid and a local anesthetic. Eur J Pharmacol. 2002;439:77-81. [ Links ]

Mert T, Gunes Y, Ozcengiz D, et al. Comparative effects of lidocaine and tramadol on injured peripheral nerves. Eur J Phar- macol. 2006;543:54-62. [ Links ]

Bianchi M, Rossoni G, Sacerdote P, et al. Effects of tra- madol on experimental inflammation. Fundam Clin Pharmacol. 1999;13:220-5. [ Links ]

Mert T, Gunes Y, Gunay I. Local analgesic efficacy of tra- madol following intraplantar injection. Eur J Pharmacol. 2007;858:68-72. [ Links ]

Marincsák R, Tóth BI, Czifra G, et al. The analgesic drug, tra- madol. Acts as an agonist of the transient receptor potential vanilloid-1. Anesth Analg. 2008;106:1890-6. [ Links ]

Received: May 24, 2014; Accepted: June 26, 2014

* Corresponding author. E-mail: (H.A. Ashmawi).

Conflicts of interest The authors declare no conflicts of interest.

Creative Commons License This is an Open Access article distributed under the terms of the Creative Commons Attribution-Noncommercial No Derivative License, which permits unrestricted non-commercial use, distribution, and reproduction in any medium provided the original work is properly cited and the work is not changed in any way.