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Canadian Journal of Anesthesia 51:905-910 (2004)
© Canadian Anesthesiologists' Society, 2004
Regional Anesthesia and Pain

Dextromethorphan potentiates morphine antinociception at the spinal level in rats

[Le dextrométhorphane potentialise l’antinociception de la morphine au niveau rachidien chez les rats]

Lok-Hi Chow, MD*,¶, Eagle Y.-K. Huang, PhD{dagger}, Shung-Tai Ho, MD{ddagger},§, Tak-Yu Lee, MD and Pao-Luh Tao, PhD{dagger}

* From the Graduate Institute of Medical Science,
{dagger} the Departments of Pharmacology, and
{ddagger} Anesthesiology,
§ National Defense Medical Center; the Department of Anesthesiology, Tri-Service General Hospital, and
the Department of Anesthesiology, Taipei Veterans General Hospital and National Yang-Ming University School of Medicine, Taipei, Taiwan.

Address correspondence to: Dr. Pao-Luh Tao, Head of Department of Pharmacology, National Defense Medical Center, P.O. Box 90048-504, Nei-Hu, Taipei, Taiwan. Phone: 886-2-87923100, ext. 18153; Fax: 886-2-87923155; E-mail: pltao{at}ndmctsgh.edu.tw


   Abstract
TOP
 Abstract
Introduction
 Methods
 Results
 Discussion
 References
 
Purpose: Morphine is an effective analgesic, but adverse effects limit its clinical use in higher doses. The non-opioid antitussive, dextromethorphan (DM), can potentiate the analgesic effect of morphine and decrease the dose of morphine in acute postoperative pain, but the underlying mechanism remains unclear. We previously observed that DM increases the serum concentration of morphine in rats. Therefore, we investigated the effects of drugs administered at the spinal level to exclude possible pharmacokinetic interactions. As DM has widespread binding sites in the central nervous system [such as N-methyl-D-aspartate (NMDA) receptors, sigma receptors and {alpha}3ß4 nicotinic receptors], we investigated whether the potentiation of morphine antinociception by DM at the spinal level is related to NMDA receptors.

Methods: We used MK-801 as a tool to block the NMDA channel first, and then studied the interaction between intrathecal (i.t.) morphine and DM. The tail-flick test was used to examine the antinociceptive effects of different combinations of morphine and other drugs in rats.

Results: DM (2–20 µg) or MK-801 (5–15 µg) showed no significant antinociceptive effect by themselves. The antinociceptive effect of morphine (0.5 µg, i.t.) was significantly enhanced by DM and reached the maximal potentiation (43.7%–50.4%) at doses of 2 to 10 µg. Pretreatment with MK-801 (5 or 10 µg, i.t.) significantly potentiated morphine antinociception by 49.9% or 38.7%, respectively. When rats were pretreated with MK-801, DM could not further enhance morphine antinociception (45.7% vs 50.5% and 43.3%).

Conclusion: Our results suggest that spinal NMDA receptors play an important role in the effect of DM to potentiate morphine antinociception.


   Introduction
TOP
 Abstract
 Introduction
Methods
 Results
 Discussion
 References
 
MORPHINE is an effective analgesic used in the treatment of severe and chronic pain. Some adverse effects of morphine such as nausea, vomiting, and respiratory depression limit its clinical use in higher doses. The development of tolerance and dependence are also major disadvantages associated with the long-term use of morphine.1–3 In recent animal studies, N-methyl-D-aspartate (NMDA) receptor antagonists such as MK-801 and ketamine have been shown to enhance the antinociceptive effects of morphine without increasing side effects and also prevented the development of morphine tolerance.4–7 However, since MK-801 and ketamine either have a narrow therapeutic window or produce certain side effects (including hallucination and neurotoxicity), these drugs may not be the most appropriate adjuvants in acute pain management.3,8,9

Dextromethorphan (DM), a non-opioid antitussive, has widespread binding sites in the central nervous system (including NMDA receptors, sigma receptors and {alpha}3ß4 nicotinic receptors) and produces both anticonvulsant and neuroprotective effects.10–12 It is particularly attractive for clinical use as an NMDA antagonist since it has been dispensed as a non-prescription drug for 40 years and is known to have a wide margin of safety.13

Preoperative administration of DM attenuates postoperative pain and morphine consumption in humans.14–17 The co-administration of morphine and DM potentiates the antinociceptive effect of morphine18–20 and also attenuates the dependence and tolerance to morphine in rats.2,6,8 It has been speculated that DM potentiates morphine antinociception by antagonizing NMDA receptors, but the evidence is not available.10,18,21 In our preliminary study, we observed that DM increases the serum concentration of morphine (free form) in rats (data not shown). Therefore, the purpose of this study was to investigate, in rats, the effects of drugs administered at the spinal level to exclude possible pharmacokinetic interactions.


   Methods
TOP
 Abstract
 Introduction
 Methods
Results
 Discussion
 References
 
Animals
The experimental protocol was approved by the Animal Care and Use Committee of the National Defense Medical Center. The experimental animals were ten-week-old male Sprague-Dawley rats weighing between 300 and 350 g (purchased from the National Experimental Animal Center, Taipei, Taiwan). All animals were bred in the Animal Facility of the National Defense Medical Center. The animal rooms were maintained at 23 ± 2°C with a 12-hr light/dark cycle. Food and water were available ad libitum throughout the experiment. Animals were taken to the testing room in the morning of the experiment; the experiments were carried out during the light cycle.

Surgical preparations
All surgical procedures were performed under sodium pentobarbital anesthesia (50 mg·kg–1, ip) and a PE10 i.t. catheter was implanted for drug administration, as in our previous study.22 Before testing, each i.t. catheter was injected with 20 µL of 2% lidocaine. Correct position of the catheter was evidenced by prompt motor block of the lower limbs, developing in one to five minutes and lasting for ten to 20 min. Any rat showing no motor blockade of lower limbs was excluded. Location of the distal end of the catheter was verified at the end of experiment. The location was confirmed by the injection of 10 µL of methylene blue and post-mortem examination of the spinal cord. Animals were housed individually after surgery with free access to food and water and were allowed to recover at least four days before experiments. Drugs were administered in 1 to 3 µL of solution, and the drug administration was followed by flushing with 10 µL of saline.

Determination of the antinociceptive effects of drugs
Morphine- or drug-induced antinociception was evaluated by the tail-flick test.1,23 Basal latency (tail-flick latency before drug administration) was determined and found to range from 2.5 to 3.5 sec. The cut-off time was set at ten seconds to prevent injury to the tail.

For each rat, basal latency was determined at least three times, and the drug to be tested was injected intrathecally. Tail-flick latency was recorded at 30, 60, 90, 120, 150, 180, 210, and 240 min after drug administration. Analgesia was evaluated by calculating the area under the time-response curve (AUC) obtained by plotting tail-flick latency (sec) on the ordinate and time after drug administration (from 30 to 240 min at 30-min intervals) on the abscissa. The AUC was calculated using the Trapezoidal rule and regarded as an index of the antinociceptive effect of drug(s). An example is presented in Figure 1Go. In order to ensure that the doses of DM or MK-801 used had no antinociceptive effect, the tail-flick test was also performed for DM (2, 5, 10, or 20 µg, i.t.) or MK-801 alone (5, 10, 15, or 30 µg, i.t.).



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  		FIGURE 1 The antinociceptive effect of morphine (0.5 µg, i.t.) and normal saline (1 µL, i.t.) in rats. Values are presented as means ± S.D. Each group contains at least six rats. The AUC is considered an index of antinociceptive effect. i.t. = intrathecal; AUC = area under the time-response curve.

 
Determination of the doses of DM or MK-801 that maximally enhance the antinociceptive effect of morphine
Rats were randomly assigned to six groups. Randomization of animals was carried out by animal house staff who were blinded to the experiment. Each group received morphine (0.5 µg) with either saline or DM at different doses (0.5, 1, 2, 5, or 10 µg) intrathecally. The doses of DM that enhanced the effect of morphine maximally were found to be at or above 2 µg i.t. Therefore, 2 µg of DM i.t. was chosen for the subsequent experiments. In another set of experiments, rats were randomly divided into three groups and pre-treated separately with either saline or MK-801 at a dose of 5 or 10 µg i.t. 20 min before the injection of morphine (0.5 µg, i.t.), in order to determine whether these doses ensure maximal potentiation of morphine antinociception.

Determination of the effect of DM on potentiating morphine antinociception after pretreatment with a maximal dose of MK-801
To evaluate the influence of MK-801 on the antinociceptive effect of the DM-morphine combination, rats were randomly divided into four groups. Both M and DM+M groups were pretreated with saline 20 min before drug injection intrathecally. Then the M group was given 0.5 µg morphine i.t. The DM+M group received an i.t. injection of 2 µg DM and 0.5 µg morphine i.t. The MK5+DM+M group was pretreated with 5 µg MK-801 i.t. 20 min before injection of 2 µg DM and 0.5 µg morphine i.t.. The MK10+DM+M group was pretreated with 10 µg MK-801 i.t. 20 min before injection of 2 µg DM i.t. and 0.5 µg morphine i.t.

Statistical analyses
Based on our preliminary results, the mean difference in AUC among different groups was approximately 240 sec x min and the standard deviation 120 sec x min. With 80% power and a statistically significance set at 0.05, we calculated that the required sample size was six per group.

Experimental data were analyzed by analysis of variance (ANOVA), followed by Newman-Keuls post hoc test for multiple comparisons. All data are expressed as mean ± S.D.; P < 0.05 was considered to be statistically significant.


   Results
TOP
 Abstract
 Introduction
 Methods
 Results
Discussion
 References
 
Determination of the doses of DM or MK-801 which have no antinociceptive effect
DM at doses of 2, 5, 10 or 20 µg i.t. or MK-801 at doses of 5, 10 or 15 µg i.t. had no significant antinociceptive effect or overt behavioural evidence of neurotoxicity. A significant antinociceptive effect was observed with 30 µg of MK-801 i.t. as shown in Figure 2Go. An i.t. injection of 40 µg MK-801 produced significant signs of neurotoxicity, including single-side rotation, rolling, or jumping and the tail-flick test was difficult to administer at this dose. Therefore, we used DM and MK-801 at doses lower than 10 µg in subsequent experiments.



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  		FIGURE 2 The antinociceptive effects of DM (2, 5, 10, 20 µg, i.t.), MK-801 (5, 10, 15, 30 µg, i.t.), and morphine (0.5 µg, i.t.). Values are presented as means ± S.D. Each group contains at least three rats. *P < 0.05, **P < 0.01 vs control group. i.t. = intrathecal; AUC = area under the time-response curve; DM = dextromethorphan; MK = MK-801; Morp = morphine.

 
Dose-response of the antinociceptive effect of morphine
An i.t. injection of morphine (0.5, 1, or 5 µg) showed dose-dependent antinociceptive effects (548.0 ± 198.3, 1066.4 ± 346.5, and 1485.6 ± 323.3 respectively, P < 0.01; Figure 3Go). In order to maintain some capacity for enhancement of the antinociceptive effect of morphine, morphine 0.5 µg was chosen for subsequent experiments.



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  		FIGURE 3 Dose-dependent antinociceptive effects of morphine (0.5, 1, or 5 µg, i.t.). Values are presented as means ± S.D. Each group contains at least eight rats. **P < 0.01 vs control group. i.t. = intrathecal; AUC = area under the time-response curve; M = morphine.

 
Effect of DM on potentiation of morphine antinociception
As shown in Figure 4Go, the antinociceptive effects (AUC) of DM0.5+M, DM1+M, DM2+M, DM5+M, or DM10+M (625.2 ± 179.1, 719.1 ± 205.2, 768.9 ± 163.0, 774.3 ± 155.1, and 805.2 ± 233.4, respectively) indicated that DM dose-dependently potentiated morphine antinociception (535.2 ± 175.1) and reached the maximal effect at the dose of 2 µg, (143.7% of morphine group, P < 0.05). When the doses of DM were increased to 5 or 10 µg, the potentiation of morphine antinociception was not increased further (144.9% and 150.4% respectively).



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  		FIGURE 4 The potentiating effects of DM (0.5, 1, 2, 5, or 10 µg, i.t.) on the antinociceptive effect of morphine (0.5 µg, i.t.). Each group contains at least six rats. *P < 0.05 vs morphine group. i.t. = intrathecal; AUC = area under the time-response curve; DM = dextromethorphan; M = morphine).

 
Effect of MK-801 on the potentiation of morphine antinociception
The AUC for MK5+M group (879.3 ± 119.4) or MK10+M group (814.2 ± 90.5) was significantly higher than the AUC for morphine (586.5 ± 179.6, P < 0.01; Figure 5Go). Both doses of MK-801 showed a similar degree of potentiation (149.9% and 138.7%) on morphine’s antinociceptive effect.



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  		FIGURE 5 The potentiating effects of MK-801 (5 or 10 µg, i.t.) on the antinociceptive effect of morphine (0.5 µg, i.t.). Each group contains at least eight rats. **P < 0.01 vs morphine group. i.t. = intrathecal; AUC = area under the time-response curve; MK = MK-801; M = morphine.

 
Effect of DM on morphine antinociception after pre-treatment with MK-801
When rats were pretreated with either saline or MK-801 (5 µg or 10 µg, i.t.) 20 min before DM (2 µg, i.t.) and morphine (0.5 µg, i.t.) administration, the AUC reached 145.7% (DM+M), 150.5% (MK5+DM+M) and 143.3% (MK10+DM+M) (P < 0.05) of morphine (Figure 6Go).



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  		FIGURE 6 Effect of MK-801 (5 or 10 µg, i.t.) pretreatment on the antinociceptive effect of DM+M. Each group contains at least six rats. *P < 0.05 vs morphine group. No significant difference was found when MK5+DM+M or MK10+DM+M group was compared with DM+M (P = 0.50 to 0.76). i.t. = intrathecal; AUC = area under the time-response curve; DM = dextromethorphan; MK = MK-801; M = morphine).

 

   Discussion
TOP
 Abstract
 Introduction
 Methods
 Results
 Discussion
References
 
DM is a dextro- form isomer of levorphanol. Unlike opioids, DM has an established safety record. The therapeutic cough suppressant dose (1 mg·kg–1·day–1) produces no major opioid-like respiratory or hemodynamic side effects and does not induce complications related to histamine release.13,24 DM has been reported to potentiate the analgesic effects of morphine in both animal and human studies.5,8,13,15–20,25 Grass and colleagues20 showed that DM at the dose of 30 mg·kg–1 ip produced stronger and more prolonged potentiation on morphine-induced antinociception than that of MK-801 at 0.1 mg·kg–1 ip or CGS19755 at 5 mg·kg–1 ip in Sprague-Dawley rats. Hoffmann and Wiesenfeld-Hallin18 reported that DM potentiated morphine antinociception, but did not reverse tolerance in rats. In a clinical trial, Kawamata and colleagues15 examined the role of DM as an adjuvant in postoperative pain management and found that pre-medication with oral DM (45 mg) reduced postoperative pain after tonsillectomy. Wong and colleagues14 also pointed out that DM can effectively attenuate the postoperative pain and morphine requirement after modified radical mastectomy. Oral DM (30–90 mg) attenuated acute pain and reduced the amount of analgesic required in the majority of postoperative DM-treated patients in a study by Weinbroum and coworkers.17 In our previous clinical trial,16 we also found that preoperative iv administration of DM reduced postoperative morphine consumption. However, there was also a recent report showing that oral DM could not enhance i.t. morphine analgesia in patients after a Cesarean section.26 Although the reason of this conflicting result is still unknown, patients receiving DM had a lower incidence of nausea and vomiting, which may also reveal certain beneficial potentials of DM in clinical pain management.

DM was given systemically in most of the above studies. Few studies have addressed the mechanism by which DM potentiates morphine analgesia. Caruso has reported that there is no pharmacokinetic interaction between DM and morphine in a human study.27 However, our preliminary experiment has shown that DM increases the serum concentration of morphine (free form) in rats (data not shown). Therefore we designed the present study on the i.t. administration of drugs to exclude pharmacokinetic factor interactions.

In the present study, we found that DM potentiates the antinociceptive effect of morphine administered i.t. (Figure 4Go). It means that there must be a mechanism(s), other than pharmacokinetic, involved in the effect of DM on morphine antinociception at the spinal level. Second, we found that pre-treatment with MK-801 (5 or 10 µg i.t.) also maximally potentiates morphine antinociception (Figure 5Go). Since MK-801 is a selective NMDA receptor antagonist, we assume that most spinal NMDA receptors involved in the potentiation of morphine antinociception were blocked by MK-801. When we pretreated animals with these doses of MK-801 (5–10 µg i.t.), DM could not further enhance the antinociceptive effect of morphine (49.9% vs 50.5% and 38.7% vs 43.3%, Figures 5Go and 6Go). This indicates that NMDA receptors indeed play an important role in the mechanism of DM to potentiate morphine antinociception. However, it should be also noted that the tail-flick test is only one of several types of animal analgesic behavioural tests and there is no absolute correlation with pain in humans.


   Footnotes
 
This study was supported by grants from the National Health Research Institutes (NHRI-EX92-8909BP) and the Taipei Veterans General Hospital (92-V313), Taipei, Taiwan.

Accepted for publication March 11, 2004. Revision accepted August 2, 2004.


   References
TOP
 Abstract
 Introduction
 Methods
 Results
 Discussion
 References
 
1 Tao PL, Yeh GC, Su CH, Wu YH. Co-administration of dextromethorphan during pregnancy and throughout lactation significantly decreases the adverse effects associated with chronic morphine administration in rat offspring. Life Sci 2001; 69: 2439–50.[Medline]

2 Mao J, Price DD, Caruso FS, Mayer DJ. Oral administration of dextromethorphan prevents the development of morphine tolerance and dependence in rats. Pain 1996; 67: 361–8.[Medline]

3 Yeh GC, Tao PL, Chen JY, Lai MC, Gao FS, Hu CL. Dextromethorphan attenuates morphine withdrawal syndrome in neonatal rats passively exposed to morphine. Eur J Pharmacol 2002; 453: 197–202.[Medline]

4 Trujillo KA, Akil H. Inhibition of morphine tolerance and dependence by the NMDA receptor antagonist MK-801. Science 1991; 251: 85–7.[Abstract/Free Full Text]

5 Elliott K, Minami N, Kolesnikov YA, Pasternak GW, Inturrisi CE. The NMDA receptor antagonists, LY274614 and MK-801, and the nitric oxide synthase inhibitor, NG-nitro-L-arginine, attenuate analgesic tolerance to the muopioid morphine but not to kappa opioids. Pain 1994; 56: 69–75.[Medline]

6 Elliott K, Hynansky A, Inturrisi CE. Dextromethorphan attenuates and reverses analgesic tolerance to morphine. Pain 1994; 59: 361–8.[Medline]

7 Su MT, Lin WB, Lue WM, Cheng CY, Tao PL. Blockade of the development of morphine tolerance by U-50,488, an AVP antagonist or MK-801 in the rat hippocampal slice. Br J Pharmacol 1998; 123: 625–30.[Medline]

8 Manning BH, Mao J, Frenk H, Price DD, Mayer DJ. Continuous co-administration of dextromethorphan or MK-801 with morphine: attenuation of morphine dependence and naloxone-reversible attenuation of morphine tolerance. Pain 1996; 67: 79–88.[Medline]

9 Wong CS, Liaw WJ, Tung CS, Su YF, Ho ST. Ketamine potentiates analgesic effect of morphine in postoperative epidural pain control. Reg Anesth 1996; 21: 534–41.[Medline]

10 Klein M, Musacchio JM. High affinity dextromethorphan binding sites in guinea pig brain. Effect of sigma ligands and other agents. J Pharmacol Exp Ther 1989; 251: 207–15.[Abstract/Free Full Text]

11 Hernandez SC, Bertolino M, Xiao Y, Pringle KE, Caruso FS, Kellar KJ. Dextromethorphan and its metabolite dextrorphan block alpha3beta4 neuronal nicotinic receptors. J Pharmacol Exp Ther 2000; 293: 962–7.[Abstract/Free Full Text]

12 Craviso GL, Musacchio JM. High-affinity dextromethorphan binding sites in guinea pig brain. II. Competition experiments. Mol Pharmacol 1983; 23: 629–40.[Abstract]

13 Steinberg GK, Bell TE, Yenari MA. Dose escalation safety and tolerance study of the N-methyl-D-aspartate antagonist dextromethorphan in neurosurgery patients. J Neurosurg 1996; 84: 860–6.[Medline]

14 Wong CS, Wu CT, Yu JC, Yeh CC, Lee MM, Tao PL. Preincisional dextromethorphan decreases postoperative pain and opioid requirement after modified radical mastectomy. Can J Anesth 1999; 46: 1122–6.[Abstract/Free Full Text]

15 Kawamata T, Omote K, Kawamata M, Namiki A. Premedication with oral dextromethorphan reduces postoperative pain after tonsillectomy. Anesth Analg 1998; 86: 594–7.[Abstract]

16 Chia YY, Liu K, Chow LH, Lee TY. The preoperative administration of intravenous dextromethorphan reduces postoperative morphine consumption. Anesth Analg 1999; 89: 748–52.[Abstract/Free Full Text]

17 Weinbroum AA, Rudick V, Paret G, Ben-Abraham R. The role of dextromethorphan in pain control. Can J Anesth 2000; 47: 585–96.[Abstract/Free Full Text]

18 Hoffmann O, Wiesenfeld-Hallin Z. Dextromethorphan potentiates morphine antinociception, but does not reverse tolerance in rats. Neuroreport 1996; 7: 838–40.[Medline]

19 Baker AK, Hoffmann VL, Meert TF. Dextromethorphan and ketamine potentiate the antinociceptive effects of µ- but not {delta}- or {kappa}-opioid agonists in a mouse model of acute pain. Pharmacol Biochem Behav 2002; 74: 73–86.[Medline]

20 Grass S, Hoffmann O, Xu XJ, Wiesenfeld-Hallin Z. N-methyl-D-aspartate receptor antagonists potentiate morphine’s antinociceptive effect in the rat. Acta Physiol Scand 1996; 158: 269–73.[Medline]

21 Church J, Lodge D, Berry SC. Differential effects of dextrorphan and levorphanol on the excitation of rat spinal neurons by amino acids. Eur J Pharmacol 1985; 111: 185–90.[Medline]

22 Wong CS, Hsu MM, Chou YY, Tao PL, Tung CS. Morphine tolerance increases [3H]MK-801 binding affinity and constitutive neuronal nitric oxide synthase expression in rat spinal cord. Br J Anaesth 2000; 85: 587–91.[Abstract/Free Full Text]

23 D’Amour FE, Smith DL. A method for determining loss of pain sensation. J Pharmacol Exp Ther 1941; 72: 74–9.[Abstract/Free Full Text]

24 Schmitt B, Netzer R, Fanconi S, Baumann P, Boltshauser E. Drug refractory epilepsy in brain damage: effect of dextromethorphan on EEG in four patients. J Neurol Neurosurg Psychiatry 1994; 57: 333–9.[Abstract]

25 Price DD, Mao J, Frenk H, Mayer DJ. The N-methyl-D-aspartate receptor antagonist dextromethorphan selectively reduces temporal summation of second pain in man. Pain 1994; 59: 165–74.[Medline]

26 Choi DM, Kliffer AP, Douglas MJ. Dextromethorphan and intrathecal morphine for analgesia after caesarean section under spinal anaesthesia. Br J Anaesth 2003; 90: 653–8.[Abstract/Free Full Text]

27 Caruso FS. MorphiDex® pharmacokinetic studies and single-dose analgesic efficacy studies in patients with postoperative pain. J Pain Symptom Manage 2000; 19: S31–6.[Medline]





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