The N-methyl-D-aspartate-receptor antagonist dextromethorphan lacks analgesic effect in a human experimental ischemic pain model

Diabetic neuropathy is associated with increased pain perception, low serum beta-endorphin and increase insulin resistance among Nigerian cohorts in Ekiti State

INTRODUCTION

There has been an increase in the global prevalence of diabetic polyneuropathy and research evidence suggests that insulin resistance plays an important role in its development and prognosis. However, there seem to be a dearth of information in understanding the likely interplay between beta endorphin, insulin resistance and pain perception especially in the setting of painful diabetic neuropathy.

METHOD

This study recruited 120 volunteers divided into four groups (30 per group): group 1 healthy volunteer (control); group 2 DM type 2 without neuropathy (DM group); group 3 DM type 2 with painful neuropathy (DPN group); group 4 DM type 2 without painful neuropathy (DN). All subjects were evaluated for pain threshold and neuropathy using an ischemia-induced pain model and biothesiometer respectively. Their beta-endorphin, glycated hemoglobin, fasting plasma insulin, and HOMA values were determined and means compared using ANOVA.

RESULT

Serum beta-endorphin is significantly reduced in DN and DPN (∗p < 0.001) compared with the control and DM group. Also, DPN and DN patients have significantly increased insulin resistance compared to those without neuropathy (∗p < 0.001; ∗p < 0.0001 respectively). There is a significant positive correlation between the pain threshold and beta-endorphin in all the groups except DN group. The correlation between beta-endorphin and insulin resistance was negative and significant in control and DM groups only. Suggestive that the fact that insulin resistance plays an important role in diabetes polyneuropathy, does not alone explain the chronic pain perception noticed in the DPN patients.

CONCLUSION

The present study demonstrates that diabetic neuropathy patients have a poor endogenous opioid peptide system which is associated with increased pain perception and high insulin resistance. However, insulin resistance alone does not explain the chronic pain perception noticed in the DPN patients. Thus, further study is required.

Development of a simple radiant heat induced experimental pain model for evaluation of analgesics in normal healthy human volunteers

Objective:

Human experimental pain models help to understand the mechanism of the painful conditions and can also be adopted to test analgesic efficacy of drugs. In early phases, the clinical development of new analgesics is hindered due to the lack of reliable tests for the experimental pain models. In the present study, we have developed and validated a simple radiant heat pain model which can be used for future screening of various analgesic agents.

Materials and Methods:

We have standardized the thermal pain model by recording pain threshold and pain tolerance time in seconds at three different intensities and levels in 24 healthy subjects. Reproducibility of the test procedure was evaluated by recording the pain parameters by two observers on three consecutive days. Validity of model was further tested by evaluating the analgesic effect of tramadol.

Results and Conclusions:

Use of radiant heat pain model with high intensity and short level was found to produce low variability with coefficient of variation less than 5%. Interobserver and interperiod reproducibility was very good as shown by Bland - Altman plot; with most of the values within ± 2SD. Tramadol produced statistically significant increase in pain threshold time. The newly developed pain model produces a type of experimental pain which is responsive to analgesic effects of tramadol at clinically relevant doses.

Human experimental pain models for assessing the therapeutic efficacy of analgesic drugs

Pain models in animals have shown low predictivity for analgesic efficacy in humans, and clinical studies are often very confounded, blurring the evaluation. Human experimental pain models may therefore help to evaluate mechanisms and effect of analgesics and bridge findings from basic studies to the clinic. The present review outlines the concept and limitations of human experimental pain models and addresses analgesic efficacy in healthy volunteers and patients. Experimental models to evoke pain and hyperalgesia are available for most tissues. In healthy volunteers, the effect of acetaminophen is difficult to detect unless neurophysiological methods are used, whereas the effect of nonsteroidal anti-inflammatory drugs could be detected in most models. Anticonvulsants and antidepressants are sensitive in several models, particularly in models inducing hyperalgesia. For opioids, tonic pain with high intensity is attenuated more than short-lasting pain and nonpainful sensations. Fewer studies were performed in patients. In general, the sensitivity to analgesics is better in patients than in healthy volunteers, but the lower number of studies may bias the results. Experimental models have variable reliability, and validity shall be interpreted with caution. Models including deep, tonic pain and hyperalgesia are better to predict the effects of analgesics. Assessment with neurophysiologic methods and imaging is valuable as a supplement to psychophysical methods and can increase sensitivity. The models need to be designed with careful consideration of pharmacological mechanisms and pharmacokinetics of analgesics. Knowledge obtained from this review can help design experimental pain studies for new compounds entering phase I and II clinical trials.

Comparison of pain models to detect opioid-induced hyperalgesia

Objective

Chronic opioid therapy may be associated with hyperalgesia. Our objective was to determine if opioid-induced hyperalgesia detection sensitivity is dependent on the stimulus used to detect it.

Methods

This open design study compared the detection of hyperalgesia in opioid-dependent subjects (n = 16) and healthy control subjects (n = 16) using the following pain stimuli: cold pain, electrical stimulation, mechanical pressure, and ischemic pain. The opioid-dependent subjects were maintained on either methadone (n = 8) or buprenorphine (n = 8) for at least 3 months. None of the controls was dependent on opioids or other drugs of abuse.

Results

The opioid-dependent subjects were markedly more sensitive than controls to the cold pain test. Compared with the control group, the hazard ratio for ceasing the test due to intolerable pain was 7.7 (95% confidence interval [CI] 2.6–23.3) in the buprenorphine group and 4.5 (95% CI 1.7–15.6) in the methadone group, with similar data for the cold pain threshold. Of the remaining tests, there were differences only for the electrical pain threshold between treatment groups, with the geometric mean threshold in the buprenorphine group being 1.5 (95% CI 1.1–1.9)-fold higher (ie, less sensitive) than that of the controls; the geometric mean for the methadone group was 1.3 (95% CI 1.04–1.7)-fold higher than that of the controls. There were no significant differences between buprenorphine and methadone patients in test responses. Women were more sensitive to the cold pain (hazard ratio for tolerance, 3.1 [95% CI 1.4–7.3]) and ischemic tests (hazard ratio for tolerance, 2.7 [95% CI 1.2–6.1]). There were significant correlations between cold and ischemic tolerances (r = 0.50; P = 0.003) and between electrical and mechanical pain tolerances (r = 0.52; P = 0.002).

Conclusion

These findings indicate that cold pain is the most suitable of the methods tested to detect opioid-induced hyperalgesia. This is consistent with its sensitivity to detect opioid analgesia.

Assessing analgesic actions of opioids by experimental pain models in healthy volunteers - an updated review

AIM

Experimental pain models may help to evaluate the mechanisms of action of analgesics and target the clinical indications for their use. This review addresses how the efficacy of opioids can be assessed in human volunteers using experimental pain models. The drawback with the different study designs is also discussed.

METHOD

A literature search was completed for randomized controlled studies which included human experimental pain models, healthy volunteers and opioids.

RESULTS

Opioids with a strong affinity for the micro-opioid receptor decreased the sensation in a variety of experimental pain modalities, but strong tonic pain was attenuated more than short lasting pain and non-painful sensations. The effects of opioids with weaker affinity for the micro-opioid receptor were detected by a more narrow range of pain models, and the assessment methods needed to be more sensitive.

CONCLUSION

The way the pain is induced, assessed and summarized is very important for the sensitivity of the pain models. This review gives an overview of how different opioids perform in experimental pain models. Generally experimental pain models need to be designed with careful consideration of pharmacological mechanisms and pharmacokinetics of analgesics. This knowledge can aid the decisions needed to be taken when designing experimental pain studies for compounds entering phase 1 clinical trials.

Effects of Paracetamol Combined with Dextromethorphan in Human Experimental Muscle and Skin Pain

By combining drugs with different mechanisms of action, synergistic effects may be achieved. The aim of the present experimental pain study was to combine paracetamol with dextromethorphan for synergistic effects. Furthermore, the reproducibility of the pain assessment methods was evaluated. Eighteen volunteers completed all periods in a three-way cross-over study. Pain stimuli were assessed at baseline and 1, 2 and 3 hr after dosing. The aim was to compare the pain-alleviating effect of 1 g paracetamol, 1 g paracetamol plus 30 mg dextromethorphan and placebo in response to a number of different stimuli in a human experimental volunteer model of skin and muscle pain. Repeated electrical stimulation of skin and muscle (temporal summation) modelled central integration and intramuscular hypertonic saline mimicked musculoskeletal pain. The method provided statistically stable pain recordings between repetitions on the same day and between days (all P > 0.05). Between repetitions on the same day, all tests were reproducible within the participants (intra-class correlation > 0.60). Between days all tests, except muscular pain pressure threshold, were reproducible within the participants (intra-class correlation > 0.60). There were no statistical differences (all P > 0.05) between paracetamol compared to placebo, and between the effect of paracetamol and dextromethorphan compared to placebo. The acute pain models were not sufficiently sensitive to detect an analgesic effect of paracetamol or the combination with dextromethorphan. The selected dose of dextromethorphan was low as the aim was to use commonly used doses, and a higher dose of dextromethorphan is most likely needed to attenuate the selected pain measures.

Placebo-Controlled Comparison of a Morphine/Dextromethorphan Combination With Morphine on Experimental Pain and Hyperalgesia in Healthy Volunteers

In this double-blind, placebo-controlled, crossover study we compared the analgesic effect of a single oral dose of 30-mg dextromethorphan and 30-mg morphine combination (MS/DM) to 30 mg morphine (MS) alone and either placebo or 30 mg dextromethorphan (DM) on cutaneous sensitization induced by heat/capsaicin (topical) sensitization on the forearm and the brief thermal sensitization model on the thigh in 22 healthy volunteers. Outcome measures were areas of secondary hyperalgesia to brush and von Frey hair stimulation in both sensitization models and the painfulness of acute thermal noxious stimulation on the upper arm. Compared with placebo, both MS/DM and morphine had some effect on the secondary hyperalgesia and reduced the painfulness of a noxious thermal stimulus. The analgesic effect of MS/DM was not superior to that of morphine on any outcome measure. These results differ from preclinical studies with animal experimental pain models in which DM markedly potentiated the analgesic effects of opioids, but they are in accordance with recent clinical trials for chronic pain.

PERSPECTIVE

Adding dextromethorphan to morphine (1:1 ratio) did not enhance analgesia on measures of experimental cutaneous sensitization and acute noxious thermal stimulation in healthy volunteers. The results differ from preclinical studies but agree with clinical trials. Human experimental models of pain and neuronal sensitization, which are responsive to oral opioids, allow efficient study of opioid combination analgesics and simplify the process for determining the optimal dose and/or dose ratio.

Morphine responses and experimental pain: sex differences in side effects and cardiovascular responses but not analgesia

Sex differences in analgesic responses to mu opioid agonists have been reported, although the direction of these differences varies across studies. To further characterize sex differences in responses to mu opioids, the analgesic effects of intravenous morphine (0.08 mg/kg) were determined in healthy women (n = 61) and men (n = 39) by using 3 experimental pain models, heat pain, pressure pain, and ischemic pain. Each pain procedure was conducted before and after double-blind administration of both morphine and saline, which occurred on separate days in counterbalanced order. Although morphine produced significant analgesic effects for all pain stimuli, no significant sex differences in morphine analgesia emerged. However, morphine attenuated cardiovascular reactivity to the ischemic pain task in men but not women, and women reported significantly more drug-related adverse effects than men. These findings are in contrast with some recent clinical and experimental results suggesting more robust analgesic response to mu opioids among women compared to men, although the data indicate that sex differences in non-analgesic effects of morphine were present. These results suggest that sex differences in responses to morphine might depend on the pain model and/or drug dose as well as the specific end point assessed.

PERSPECTIVE

This study examines morphine responses in women and men by using laboratory pain measures. The results indicate no sex differences in analgesia, but women reported greater side effects, and morphine attenuated cardiovascular responses more strongly among men than women. These results add to the literature regarding sex differences in response to opioids.

Dextromethorphan and intrathecal morphine for analgesia after Caesarean section under spinal anaesthesia

BACKGROUND

Dextromethorphan is an N-methyl-D-aspartic acid antagonist which can attenuate acute pain with few side-effects. In this prospective, randomized, double-blind study of dextromethorphan and intrathecal morphine, we investigated postoperative pain, pruritus, nausea and vomiting in women undergoing Caesarean section under spinal anaesthesia.

METHODS

Women were allocated randomly to one of six groups, to receive intrathecal morphine 0.05, 0.1 or 0.2 mg plus oral dextromethorphan 60 mg or placebo.

RESULTS

The addition of dextromethorphan did not reduce postoperative pain scores (P=0.83). Compared with women receiving intrathecal morphine 0.05 mg, women receiving higher doses had a significantly higher incidence of nausea and vomiting [odds ratio for intrathecal morphine 0.1 mg, 4.0 (95% confidence interval 1.2-14.1); for intrathecal morphine 0.2 mg, 7.9 (2.3-27.1)]. Compared with women receiving intrathecal morphine 0.05 mg, women receiving higher doses also had a significantly higher incidence of pruritus [odds ratio for intrathecal morphine 0.1 mg, 3.2 (95% confidence interval 1.3-8.2); for intrathecal morphine 0.2 mg, 3.7 (1.4-9.5)]. Women receiving dextromethorphan had a lower incidence of nausea and vomiting [odds ratio 2.6 (1.1-6.3)].

CONCLUSIONS

Postoperative pain after Caesarean section under spinal anaesthesia was not reduced by the addition of oral dextromethorphan to a multimodal approach including intrathecal morphine.

Dextromethorphan and ketamine potentiate the antinociceptive effects of mu- but not delta- or kappa-opioid agonists in a mouse model of acute pain

Animal and clinical studies have reported potentiation of opioid antinociception by NMDA receptor antagonists such as ketamine and dextromethorphan. The aim of this study was to compare these clinically available NMDA antagonists in combination with classical morphine, mu-selective fentanyl-like opioids, the delta-opioid agonist SNC80 and the kappa-opioid agonist U50,488H. Using a mouse hot-plate test, dose-response relationships were first determined for all compounds individually and then for opioids co-administered with fixed doses of ketamine or dextromethorphan. All compounds were administered intraperitoneally ED(50) values were calculated from the proportion of animals failing to exhibit any response within a fixed cut-off criterion of 30 s. To varying degrees, all compounds produced increases in response latencies over time. Dextromethorphan produced lower ED(50) values for morphine, fentanyl and sufentanil but exerted no effect on the potency of SNC80 or U50,488H. Similarly, ketamine potentiated the antinociceptive potency of morphine, fentanyl and sufentanil but not SNC80 or U50,488H. In summary, these results support the use of mu-opioid agonists in combination with NMDA antagonists, but suggest that there may be no advantage in combining dextromethorphan or ketamine with delta- or kappa-opioids in the management of acute pain.

Dextromethorphan potentiates the antinociceptive effects of morphine and the delta-opioid agonist SNC80 in squirrel monkeys

Dextromethorphan (DXM) is a noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist shown to prevent the development of tolerance to the antinociceptive effects of morphine in rodents. DXM also potentiates the antinociceptive effects of the mu-opioid receptor agonist morphine under some conditions; however, the effect of DXM in combination with opioids other than morphine has not been well characterized. This study determined the antinociceptive effects of DXM administered alone or in combination with morphine or the delta-opioid receptor (DOR) agonist SNC80 using a squirrel monkey titration procedure. In this procedure, shock (delivered to the tail) increases in intensity every 15 s (0.01-2.0 mA) in 30 increments. Five lever presses during any given 15-s shock period produces a 15-s shock-free period after which shock resumes at the next lower intensity. This assay provides a measure of antinociception that is separable from motor effects [response rate (RR)]. Morphine (0.3-3.0 mg/kg i.m.) and SNC80 (1.0-10 mg/kg i.m.), but not DXM (1.0-10 mg/kg i.m.) dose- and time-dependently increased the intensity below which monkeys (n = 4) maintained shock 50% of the time [median shock level (MSL)]. Doses of morphine and SNC80 that alone did not increase MSL were potentiated by DXM. Importantly, these combinations did not significantly alter RR. These data support previous findings with other NMDA receptor antagonists and morphine using this procedure and also extend those findings to a DOR agonist.