The role of dextromethorphan in pain control

Classics in Chemical Neuroscience: Dextromethorphan (DXM)

Dextromethorphan (DXM) was introduced in 1958 as the first non-opioid cough suppressant and is indicated for multiple psychiatric disorders. It has been the most used over-the-counter cough suppressant since its emergence. However, individuals quickly noticed an intoxicating and psychedelic effect if they ingested large doses. DXM's antagonism at N-methyl-d-aspartate receptors (NMDAr) is thought to underly its efficacy in treating acute cough, but supratherapeutic doses mimic the activity of dissociative hallucinogens, such as phencyclidine and ketamine. In this Review we will discuss DXM's synthesis, manufacturing information, drug metabolism, pharmacology, adverse effects, recreational use, abuse potential, and its history and importance in therapy to present DXM as a true classic in chemical neuroscience.

Dextromethorphan as a novel nonopioid adjunctive agent for pain control with medication abortion: A randomized controlled trial

OBJECTIVES

To evaluate efficacy and satisfaction of dextromethorphan as a non-narcotic adjuvant to current analgesic regimens for medication abortion.

STUDY DESIGN

We conducted a randomized, double-blinded, placebo-controlled trial. We randomized eligible participants (N = 156) 1:1 to adjunctively take dextromethorphan (loading dose 60 mg and two subsequent 30 mg doses at 2 and 5 hours after misoprostol administration) or placebo combined with usual-care nonsteroidal anti-inflammatory medications ± opioids for pain. Participants reported pain scores and satisfaction using a secure texting application at 2, 5, 8, and 24 hours after misoprostol administration. Our primary outcome was worst pain score and total analgesic use.

RESULTS

Baseline demographics of enrolled participants were similar between randomization arms. Worst pain scores for participants receiving dextromethorphan versus placebo (8.0 vs 7.0, p = 0.06) did not differ. Total milligram usage of ibuprofen (800 mg vs 610 mg, p =.62), acetaminophen (1000 mg vs 1300 mg, p = 0.62), and opioids (10 mg vs 15 mg, p = 0.51) did not differ between the randomization groups. Participants randomized to placebo were significantly more likely to be satisfied with their pain control (91% vs 75%, p = 0.02).

CONCLUSION

Dextromethorphan used adjunctively with standard analgesics did not reduce pain associated with medication abortion. Participants who received dextromethorphan reported decreased satisfaction with their pain control.

IMPLICATIONS

Dextromethorphan used adjunctively with commonly used analgesic regimens did not reduce medication abortion associated pain. Many participants did not use analgesics as counseled, and nearly 25% used no analgesia during medication abortion.

Heterogeneous network propagation with forward similarity integration to enhance drug-target association prediction

Identification of drug-target interaction (DTI) is a crucial step to reduce time and cost in the drug discovery and development process. Since various biological data are publicly available, DTIs have been identified computationally. To predict DTIs, most existing methods focus on a single similarity measure of drugs and target proteins, whereas some recent methods integrate a particular set of drug and target similarity measures by a single integration function. Therefore, many DTIs are still missing. In this study, we propose heterogeneous network propagation with the forward similarity integration (FSI) algorithm, which systematically selects the optimal integration of multiple similarity measures of drugs and target proteins. Seven drug-drug and nine target-target similarity measures are applied with four distinct integration methods to finally create an optimal heterogeneous network model. Consequently, the optimal model uses the target similarity based on protein sequences and the fused drug similarity, which combines the similarity measures based on chemical structures, the Jaccard scores of drug-disease associations, and the cosine scores of drug-drug interactions. With an accuracy of 99.8%, this model significantly outperforms others that utilize different similarity measures of drugs and target proteins. In addition, the validation of the DTI predictions of this model demonstrates the ability of our method to discover missing potential DTIs.

Preliminary Results on the Long-Term Effects of Dextromethorphan on MDMA-Mediated Serotonergic Deficiency and Volumetric Changes in Primates Based on 4-[18F]-ADAM PET/MRI

Alterations to the serotonergic system due to 3,4-methylenedioxymethamphetamine (MDMA) (ecstasy) consumption have been extensively documented. However, knowledge of the reversibility of these neurotoxic effects based on in vivo evaluations of serotonin transport (SERT) availability remains limited. This study aimed to evaluate the long-term neurotoxicity of MDMA after 66 months abstinence and explored whether Dextromethorphan, a non-competitive N-methyl-D-aspartate (NMDA) receptor, could attenuate MDMA-induced neurotoxicity using 4-[18F]-ADAM, an imaging ligand that selectively targets SERT, with positron emission tomography technology (PET). Nine monkeys (Macaca cyclopis) were used in this study: control, MDMA, and DM + MDMA. Static 4-[18F]-ADAM PET was performed at 60 and 66 months after drug treatment. Serotonin transport (SERT) availability was presented as the specific uptake ratios (SURs) of 4-[18F]-ADAM in brain regions. Voxel-based region-specific SERT availability was calculated to generate 3D PET/MR images. Structural Magnetic Resonance Imaging (MRI) volumetric analysis was also conducted at 60 months. Significantly decreased 4-[18F]-ADAM SURs were observed in the striatum and thalamus of the MDMA group at 60 and 66 months compared to controls; the midbrain and frontal cortex SURs were similar at 60 and 66 months in the MDMA and control groups. All eleven brain regions showed significantly lower (∼13%) self-recovery rates over time; the occipital cortex and cingulate recovered to baseline by 66 months. DM attenuated MDMA-induced SERT deficiency on average, by ∼8 and ∼1% at 60 and 66 months, respectively; whereas significant differences were observed between the thalamus and amygdala of the MDMA and DM + MDMA groups at 66 months. Compared to controls, the MDMA group exhibited significantly increased (∼6.6%) gray matter volumes in the frontal cortex, occipital cortex, caudate nucleus, hippocampus, midbrain, and amygdala. Moreover, the gray matter volumes of the occipital cortex, hippocampus and amygdala correlated negatively with the 4-[18F]-ADAM SURs of the same regions. DM (n = 2) did not appear to affect MDMA-induced volumetric changes. The 4-[18F]-ADAM SURs, lower self-recovery rate and increased volumetric values indicate the occipital cortex, hippocampus and amygdala still exhibit MDMA-induced neurotoxicity after 66 months’ abstinence. Moreover, DM may prevent MDMA-induced serotonergic deficiency, as indicated by increased 4-[18F]-ADAM SURs and SERT availability, but not volumetric changes.

The Effect of Intrathecal Injection of Dextromethorphan on the Experimental Neuropathic Pain Model

Background

Peripheral glucocorticoid receptors (GRs) are altered by peripheral nerve injury and may modulate the development of neuropathic pain. Two central pathogenic mechanisms underlying neuropathic pain are neuroinflammation and N-methyl-D-aspartate receptor (NMDAR)-dependent neural plasticity in the spinal cord.

Objectives

This study examined the effect of the non-competitive NMDAR antagonist dextromethorphan on partial sciatic nerve ligation (PSL)-induced neuropathic pain and the spinal expression of the glucocorticoid receptor (GR).

Methods

Male mice were randomly assigned into a sham group and two groups receiving PSL followed by intrathecal saline vehicle or dextromethorphan (iDMP). Vehicle or iDMP was administered 8 - 14 days after PSL. The hotplate paw-withdrawal latency was considered to measure thermal pain sensitivity. The spinal cord was then sectioned and immunostained for GR.

Results

Thermal hyperalgesia developed similarly in the vehicle and iDMP groups prior to the injections (P = 0.828 and 0.643); however, it was completely mitigated during the iDMP treatment (P < 0.001). GR expression was significantly higher in the vehicle group (55.64 ± 4.50) than in the other groups (P < 0.001). The iDMP group (9.99 ± 0.66) showed significantly higher GR expression than the sham group (6.30 ± 1.96) (P = 0.043).

Conclusions

The suppression of PLS-induced thermal hyperalgesia by iDMP is associated with the downregulation of GR in the spinal cord, suggesting that this analgesic effect is mediated by inhibiting GR-regulated neuroinflammation.

Long-term Dextromethorphan Use and Acute Intoxication Results in an Episode of Mania and Autoenucleation

: The pharmacologic and neuropsychiatric sequelae of long-term dextromethorphan use and acute dextromethorphan intoxication are reviewed in this case report. Dextromethorphan ingestion at the high end of toxicity, although rare, can cause violence to oneself and others, even in those previously without any history of such behaviors. In this article, the neuropsychiatric consequences of dextromethorphan toxicity are highlighted in a case report of a 37-year-old woman who had been using dextromethorphan for 5 years. She presented to a large urban emergency department in a psychotic and manic state after attempting autoenucleation. She reported to consult liaison psychiatry staff that she had taken a total of 1400 mg of dextromethorphan over the course of 3 days with intent to experience altered state of consciousness. Toxicology screens on admission did not reveal any other substances in her system. She had no formal psychiatric history and no history of mania, psychosis, or self-harm. To our knowledge, this is the first case of autoenucleation resulting from dextromethorphan-induced mania with psychotic features.

Botulinum Toxin Injection for Analgesic Effect after Hemorrhoidectomy: A Randomized Control Trial

Objectives:

Hemorrhoid is a common disease in surgical practice, but only a few numbers of patients need surgical treatment. The most common concern of patients is postoperative pain. This study aimed to evaluate the efficacy and safety of an intersphincteric injection of botulinum toxin for post-hemorrhoidectomy pain relief.

Methods:

Overall, 90 patients were enrolled, and 44 were randomized into a botulinum toxin injection group. Preoperative gradings were grade III 37 patients and grade II 2 patients. Patients received an intersphincteric injection of 0.5 ml of a solution containing 30 units of botulinum toxin (BTX). The postoperative data were collected pain score in a visual analog score (VAS), an analgesic used, hospital stay, and complication.

Results:

The VAS was lower in the BTX group at 12 hours and 24 hours postoperative phase. VAS at 12 hours 4.435 ± 2.149 vs 6.232 ± 2.307 (p < 0.001), VAS at 24 hours 2.205 ± 2.079 vs 3.744 ± 2.361(p = 0.003). The BTX group has a shorter time in defection without pain than the control group (3 vs. two days, p = 0.007). There was no difference in immediate and delay complications between the two groups.

Conclusions:

Postoperative hemorrhoidectomy needs multimodalities for pain reduction. Botulinum toxin has some benefit in postoperative pain reduction.

Dextromethorphan and bupropion reduces high level remifentanil self-administration in rats

Opiate addiction has risen substantially during the past decade. New treatments to combat opiate addiction are sorely needed. The current study was conducted to determine the acute individual and interactive effects of bupropion and dextromethorphan in a rat model of opiate self-administration using the short-acting synthetic opioid remifentanil. Both of these drugs have been found to reduce self-administration of nicotine. Bupropion and dextromethorphan and their combination had differential effects depending on whether the rats showed higher or lower baseline remifentanil self-administration. The rats with higher initial remifentanil self-administration showed a significant decrease in remifentanil self-administration with bupropion or dextromethorphan treatment, compared to the vehicle control condition. This decrease in self-remifentanil administration was most pronounced when combination of the higher doses of bupropion and dextromethorphan were administered. In contrast, the rats with lower baseline remifentanil self-administration showed the opposite effect of drug treatment with an increase in remifentanil self-administration with bupropion treatment compared to the vehicle control condition. Dextromethorphan had no significant effect inthis group. This study shows that combination bupropion and dextromethorphan affects remifentanil self-administration in a complex fashion with differential effects on low and high baseline responders. In subjects with high baseline remifentanil self-administration, bupropion and dextromethorphan treatment significantly reduced self-administration, whereas in subjects with low baseline remifentanil self-administration, bupropion increased remifentanil self-administration and dextromethorphan had no discernible effect. This finding suggests that combination bupropion-dextromethorphan should be tested in humans, with a focus on treating people with high-level opiate use.

Changes in metabolites in the brain of patients with fibromyalgia after treatment with an NMDA receptor antagonist

The aims of this work were to evaluate whether the treatment of patients with fibromyalgia with memantine is associated with significant changes in metabolite concentrations in the brain, and to explore any changes in clinical outcome measures. Magnetic resonance spectroscopy was performed of the right anterior and posterior insula, both hippocampi and the posterior cingulate cortex. Questionnaires on pain, anxiety, depression, global function, quality of life and cognitive impairment were used. Ten patients were studied at baseline and after three months of treatment with memantine. Significant increases were observed in the following areas: N-acetylaspartate (4.47 at baseline vs. 4.71 at three months, p = 0.02) and N-acetylaspartate+N-acetylaspartate glutamate in the left hippocampus (5.89 vs. 5.98; p = 0.007); N-acetylaspartate+N-acetylaspartate glutamate in the right hippocampus (5.31 vs 5.79; p = 0.01) and the anterior insula (7.56 vs. 7.70; p = 0.033); glutamate+glutamine/creatine ratio in the anterior insula (2.03 vs. 2.17; p = 0.022) and the posterior insula (1.77 vs. 2.00; p = 0.004); choline/creatine ratio in the posterior cingulate (0.18 vs. 0.19; p = 0.023); and creatine in the right hippocampus (3.60 vs. 3.85; p = 0.007). At the three-month follow-up, memantine improved cognitive function assessed by the Cognition Mini-Exam (31.50, SD = 2.95 vs. 34.40, SD = 0.6; p = 0.005), depression measured by the Hamilton Depression Scale (7.70, SD = 0.81 vs. 7.56, SD = 0.68; p = 0.042) and severity of illness measured by the Clinical Global Impression severity scale (5.79, SD = 0.96 vs. 5.31, SD = 1.12; p = 0.007). Depression, clinical global impression and cognitive function showed improvement with memantine. Magnetic resonance spectroscopy could be useful in monitoring response to the pharmacological treatment of fibromyalgia.

PKCδ Knockout Mice Are Protected from Dextromethorphan-Induced Serotonergic Behaviors in Mice: Involvements of Downregulation of 5-HT1A Receptor and Upregulation of Nrf2-Dependent GSH Synthesis

We investigated whether a specific serotonin (5-HT) receptor-mediated mechanism was involved in dextromethorphan (DM)-induced serotonergic behaviors. We firstly observed that the activation of 5-HT_1A receptor, but not 5-HT_2A receptor, contributed to DM-induced serotonergic behaviors in mice. We aimed to determine whether the upregulation of 5-HT_1A receptor induced by DM facilitates the specific induction of certain PKC isoform, because previous reports suggested that 5-HT_1A receptor activates protein kinase C (PKC). A high dose of DM (80 mg/kg, i.p.) induced a selective induction of PKCδ out of PKCα, PKCβI, PKCβII, PKCξ, and PKCδ in the hypothalamus of wild-type (WT) mice. More importantly, 5-HT_1A receptor co-immunoprecipitated PKCδ in the presence of DM. Consistently, rottlerin, a pharmacological inhibitor of PKCδ, or PKCδ knockout significantly protected against increases in 5-HT_1A receptor gene expression, 5-HT turnover rate, and serotonergic behaviors induced by DM. Treatment with DM resulted in an initial increase in nuclear factor erythroid-2-related factor 2 (Nrf2) nuclear translocation and DNA-binding activity, γ-glutamylcysteine (GCL) mRNA expression, and glutathione (GSH) level. This compensative induction was further potentiated by rottlerin or PKCδ knockout. However, GCL mRNA and GSH/GSSG levels were decreased 6 and 12 h post-DM. These decreases were attenuated by PKCδ inhibition. Our results suggest that interaction between 5-HT_1A receptor and PKCδ is critical for inducing DM-induced serotonergic behaviors and that inhibition of PKCδ attenuates the serotonergic behaviors via downregulation of 5-HT_1A receptor and upregulation of Nrf2-dependent GSH synthesis.

The landscape of pain management in people with dementia living in care homes: a mixed methods study

OBJECTIVES

The aim of this study is to explore the current landscape of pain management in people with dementia living in care home settings. Pain is extremely common in this patient group, yet there is very limited guidance for healthcare professionals.

METHODS

Triangulation of stakeholder consultation and quality review of pain management guidance were performed. A review of existing pain management guidance was conducted using published quality criteria adapted for the field. Three focus group discussions were held with care home staff and two focus group discussions and an online survey with family carers. Data were subjected to thematic analysis to identify themes and sub-themes. Outcomes were reviewed by an expert panel, which gave recommendations.

RESULTS

Fifteen existing guidelines were identified, of which three were designed for use in dementia and none were tailored for care home settings. Thematic analysis revealed six major themes in current pain management in dementia: importance of person-centredness, current lack of pain awareness in staff, communication as a core element, disparities in staff responsibility and confidence, the need for consistency of care and current lack of staff training. In addition to the needs for practice, the expert panel identified promising pharmacological treatment candidates, which warrant clinical evaluation.

CONCLUSIONS

The findings of this study clearly articulate a need for an evidence-based pain management programme for care homes, which is informed by stakeholder input and based within a conceptual framework for this setting. There are novel opportunities for clinical trials of alternative analgesics for use in this patient group. Copyright © 2016 John Wiley & Sons, Ltd.

Targeting Opioid-Induced Hyperalgesia in Clinical Treatment: Neurobiological Considerations

Opioid analgesics have become a cornerstone in the treatment of moderate to severe pain, resulting in a steady rise of opioid prescriptions. Subsequently, there has been a striking increase in the number of opioid-dependent individuals, opioid-related overdoses, and fatalities. Clinical use of opioids is further complicated by an increasingly deleterious profile of side effects beyond addiction, including tolerance and opioid-induced hyperalgesia (OIH), where OIH is defined as an increased sensitivity to already painful stimuli. This paradoxical state of increased nociception results from acute and long-term exposure to opioids, and appears to develop in a substantial subset of patients using opioids. Recently, there has been considerable interest in developing an efficacious treatment regimen for acute and chronic pain. However, there are currently no well-established treatments for OIH. Several substrates have emerged as potential modulators of OIH, including the N-methyl-D-aspartate and γ-aminobutyric acid receptors, and most notably, the innate neuroimmune system. This review summarizes the neurobiology of OIH in the context of clinical treatment; specifically, we review evidence for several pathways that show promise for the treatment of pain going forward, as prospective adjuvants to opioid analgesics. Overall, we suggest that this paradoxical state be considered an additional target of clinical treatment for chronic pain.

Dextromethorphan potentiates morphine antinociception at the spinal level in rats

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(3)ss(4) 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 microg) or MK-801 (5-15 microg) showed no significant antinociceptive effect by themselves. The antinociceptive effect of morphine (0.5 microg, i.t.) was significantly enhanced by DM and reached the maximal potentiation (43.7%-50.4%) at doses of 2 to 10 microg. Pretreatment with MK-801 (5 or 10 microg, 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.

Intravenous dextromethorphan/quinidine inhibits activity of dura-sensitive spinal trigeminal neurons in rats

BACKGROUND

Migraine is a chronic neurological disorder characterized by episodes of throbbing headaches. Practically all medications currently used in migraine prophylaxis have a number of substantial disadvantages and use limitations. Therefore, the further search for principally new prophylactic antimigraine agents remains an important task. The objective of our study was to evaluate the effects of a fixed combination of dextromethorphan hydrobromide and quinidine sulphate (DM/Q) on activity of the spinal trigeminal neurons in an electrophysiological model of trigemino-durovascular nociception.

METHODS

The study was performed in 15 male Wistar rats, which were anaesthetized with urethane/α-chloralose and paralysed using pipecuronium bromide. The effects of cumulative intravenous infusions of DM/Q (three steps performed 30 min apart, 15/7.5 mg/kg of DM/Q in 0.5 mL of isotonic saline per step) on ongoing and dural electrical stimulation-induced neuronal activities were tested in a group of eight rats over 90 min. Other seven animals received cumulative infusion of equal volumes of saline and served as control.

RESULTS

Cumulative administration of DM/Q produced steady suppression of both the ongoing activity of the spinal trigeminal neurons and their responses to electrical stimulation of the dura mater.

CONCLUSIONS

It is evident that the observed DM/Q-induced suppression of trigeminal neuron excitability can lead to a reduction in nociceptive transmission from meninges to higher centres of the brain. Since the same mechanism is believed to underlie the pharmacodynamics of many well-known antimigraine drugs, results of the present study enable us to anticipate the potential efficacy of DM/Q in migraine.

Evaluation of the efficacy of memantine in the treatment of fibromyalgia: study protocol for a doubled-blind randomized controlled trial with six-month follow-up

Background

Fibromyalgia is a prevalent chronic rheumatic disease of great clinical importance. Recent studies have found raised levels of glutamate in the insula, hippocampus and posterior cingulate cortex regions of the brains of fibromyalgia (FM) patients. This finding has led researchers to speculate about the usefulness of glutamate-blocking drugs such as memantine in the treatment of fibromyalgia. The hypothesis of this study is that the administration of memantine will reduce the glutamate levels, and futhermore, will decrease the perceived pain. The aim of this study is to evaluate the efficacy of memantine in the treatment of pain (pain perception). A secondary objective is to evaluate the efficacy of memantine in the treatment of other clinical symptoms of FM, and to evaluate the efficacy of memantine in reducing brain levels of glutamate, and its effects on the central nervous system as a whole.

Method/Design

A double-blind parallel randomized controlled trial. Participants, Seventy patients diagnosed with FM will be recruited from primary health care centers in Zaragoza, Spain. Intervention. The subjects will be randomized in two groups: A) A treatment group (n = 35), which will receive 20 mg of memantine daily; B) A control group (n = 35), to which will be administered a placebo. There will be a six-month follow-up period (including a titration period of one month). Outcomes. The main efficacy variable of this study is pain (pain perception). The secondary efficacy variables are clinical symptoms (pain threshold, cognitive function, health status, anxiety, depression, clinical impression and quality of life) and glutamate levels in different regions of the brain, which will be assessed by magnetic resonance spectroscopy. Randomization and blinding. Randomization has been computer-generated, and the random allocation sequence will be implemented by telephone. Subjects of the study and the research assistants will be blinded to group assignment.

Discussion

There is a need for the development of innovative and more effective treatments for fibromyalgia. This clinical trial will determine whether memantine can be an effective pharmacological treatment for fibromyalgia patients.

Trial registration

Current Controlled Trials http://ISRCTN45127327 EUDRACT 2011-006244-73

Memantine and dizocilpine interactions with antinociceptive or discriminative stimulus effects of morphine in rats after acute or chronic treatment with morphine

RATIONALE

Memantine is a N-methyl-D-aspartic acid receptor (NMDAR) channel blocker that binds to dizocilpine sites and appears well tolerated during chronic use. Published studies suggest NMDAR antagonists prevent development of tolerance to effects of morphine by blocking NMDAR hyperactivation.

OBJECTIVES

We sought to compare effects of memantine to those of the more frequently studied dizocilpine and to evaluate memantine as a potential adjunct to modify tolerance to mu-opioid receptor agonists.

METHODS

Sprague-Dawley rats were trained to discriminate morphine (3.2 mg/kg) and saline under fixed ratio 15 schedules of food delivery. Potency and maximal stimulus or rate-altering effects of cumulative doses of morphine were examined 30 min after pretreatment with dizocilpine (0.032-0.1 mg/kg) or memantine (5-10 mg/kg) and after chronic treatment with combinations of dizocilpine or memantine and morphine, 10 mg/kg twice daily, for 6 to 14 days. Effects of dizocilpine or memantine on morphine antinociception were examined in a 55 °C water tail-withdrawal assay with drug treatments parallel to those in discrimination studies.

RESULTS

Acutely, memantine attenuated while dizocilpine potentiated the stimulus and antinociceptive effects of morphine. Neither chronic dizocilpine nor memantine blocked tolerance to the stimulus effects of morphine. In contrast, combined treatment with dizocilpine (0.1 mg/kg) blocked tolerance to antinociceptive effects of lower (0.1~3.2 mg/kg) but not higher doses of morphine, whereas memantine did not block tolerance.

CONCLUSIONS

Memantine and dizocilpine interacted differently with morphine, possibly due to different NMDAR binding profiles. The lack of memantine-induced changes in morphine tolerance suggests that memantine may not be a useful adjunct in chronic pain management.

Effect of a single dose of dextromethorphan on psychomotor performance and working memory capacity

BACKGROUND

Previous studies show that the prolonged use of dextromethorphan produces cognitive deterioration in humans.

AIM

The aim of this study was to investigate the effect of a single dose of dextroemthrophan on psychomotor performance and working memory capacity.

MATERIALS AND METHODS

This is a randomized, double-blind, controlled, and prospective study. Thirty-six (17 women, 19 men) medical students enrolled in the study; half of them (7 women, 11 men) were given placebo, while the other half (10 women, 8 men) received dextromethorphan. The choice reaction time, critical flicker fusion threshold, and N-back working memory task were measured before and after 2 h of taking the drugs.

RESULTS

Dextromethorphan showed a significant deterioration in the 3-back working memory task (P<0.05). No significant changes were seen as regards the choice reaction time components (total, recognition, motor) and critical flicker fusion threshold (P>0.05). On the other hand, placebo showed no significant changes as regards the choice reaction time, critical flicker fusion threshold, and N-back working memory task (P>0.05).

CONCLUSION

A single dose of dextromethorphan has no effect on attention and arousal but may significantly impair the working memory capacity.

Strategies Aimed at Preventing Chronic Post-surgical Pain: Comprehensive Perioperative Pain Management after Total Joint Replacement Surgery

PURPOSE

Chronic post-surgical pain (CPSP) is a frequent outcome of musculoskeletal surgery. Physiotherapists often treat patients with pain before and after musculoskeletal surgery. The purposes of this paper are (1) to raise awareness of the nature, mechanisms, and significance of CPSP; and (2) to highlight the necessity for an inter-professional team to understand and address its complexity. Using total joint replacement surgeries as a model, we provide a review of pain mechanisms and pain management strategies.

SUMMARY OF KEY POINTS

By understanding the mechanisms by which pain alters the body's normal physiological responses to surgery, clinicians selectively target pain in post-surgical patients through the use of multi-modal management strategies. Clinicians should not assume that patients receiving multiple medications have a problem with pain. Rather, the modern-day approach is to manage pain using preventive strategies, with the aims of reducing the intensity of acute postoperative pain and minimizing the development of CPSP.

CONCLUSIONS

The roles of biological, surgical, psychosocial, and patient-related risk factors in the transition to pain chronicity require further investigation if we are to better understand their relationships with pain. Measuring pain intensity and analgesic use is not sufficient. Proper evaluation and management of risk factors for CPSP require inter-professional teams to characterize a patient's experience of postoperative pain and to examine pain arising during functional activities.

Glutamate receptor ion channels: structure, regulation, and function

The mammalian ionotropic glutamate receptor family encodes 18 gene products that coassemble to form ligand-gated ion channels containing an agonist recognition site, a transmembrane ion permeation pathway, and gating elements that couple agonist-induced conformational changes to the opening or closing of the permeation pore. Glutamate receptors mediate fast excitatory synaptic transmission in the central nervous system and are localized on neuronal and non-neuronal cells. These receptors regulate a broad spectrum of processes in the brain, spinal cord, retina, and peripheral nervous system. Glutamate receptors are postulated to play important roles in numerous neurological diseases and have attracted intense scrutiny. The description of glutamate receptor structure, including its transmembrane elements, reveals a complex assembly of multiple semiautonomous extracellular domains linked to a pore-forming element with striking resemblance to an inverted potassium channel. In this review we discuss International Union of Basic and Clinical Pharmacology glutamate receptor nomenclature, structure, assembly, accessory subunits, interacting proteins, gene expression and translation, post-translational modifications, agonist and antagonist pharmacology, allosteric modulation, mechanisms of gating and permeation, roles in normal physiological function, as well as the potential therapeutic use of pharmacological agents acting at glutamate receptors.

REPETITIVE DEXTROMETHORPHAN AT ADOLESCENCE AFFECTS WATER MAZE LEARNING IN FEMALE RATS

Effects of repetitive dextromethorphan at adolescence on a spatial learning of rats were investigated. Rats received 10 daily injections of dextromethorphan (40 mg/kg) from postnatal day 28 thru 37, and were then subjected to the Morris water maze task from day 38. Significant impairments were found in the probe trial and the reversal training of the maze learning in the female rats, but not in males, treated with dextromethorphan. This result suggests that repetitive dextromethorphan, that is, abuse of dextromethorphan, at adolescence may induce deficits in the hippocampus-based memory function, perhaps more obviously in females.

Dextromethorphan abuse: clinical effects and management

OBJECTIVE

To describe the epidemiology, patient presentation, and clinical management associated with dextromethorphan (DM) abuse.

DATA SOURCES

PubMed/Medline search using terms dextromethorphan and abuse through July 2008, bibliographies of selected publications, epidemiology tracking databases.

STUDY SELECTION

By the authors.

DATA EXTRACTION

English language-published review articles, clinical trials, and case reports that described the epidemiologic and toxicologic profile of DM were included.

DATA SYNTHESIS

DM is a relatively inexpensive and easily accessible over-the-counter (OTC) medication intended for use as an antitussive. Increasingly, illicit use of the drug has been reported. At clinical doses, the drug produces few adverse effects. However, when abused in large quantities (>2 mg/kg), the drug has been associated with a dissociative effect similar to those described by ketamine and phencyclidine abusers. Massive ingestions of the drug may be associated with untoward effects, including tachycardia, hypertension, and respiratory depression. Overdose symptoms may also be associated with coformulated products such as antihistamines and sympathomimetic amines. Management is primarily supportive. Naloxone has been used to manage DM toxicity but with conflicting reports of effectiveness.

CONCLUSION

Recent reports indicate that DM is often abused by individuals seeking its dissociative effects. Clinicians should be aware of the abuse potential of DM. Pharmacists might be particularly cognizant of the risks involved with DM abuse as they control OTC access to the drug.

Combined use of pregabalin and memantine in fibromyalgia syndrome treatment: a novel analgesic and neuroprotective strategy?

Fibromyalgia syndrome (FMS) is a chronic widespread pain syndrome that is estimated to affect 4-8 million US adults. The exact molecular mechanisms underlying this illness remain unclear, rendering most clinical treatment and management techniques relatively ineffective. It is now known that abnormalities in both nociceptive and central pain processing systems are necessary (but perhaps not sufficient) to condition the onset and maintenance of FMS. These same systemic abnormalities are thought to be responsible for the loss of cephalic gray matter density observed in all FMS patients groups studied to date. The current scope of FMS treatment focuses largely on analgesia and does not clearly address potential neuroprotective strategies. This article proposes a combined treatment of pregabalin and memantine to decrease the pain and rate of gray matter atrophy associated with FMS. This dual-drug therapy targets the voltage-gated calcium ion channel (VGCC) and the N-methyl d-aspartate receptor (NMDAR) (respectively), two primary components of the human nociceptive and pain processing systems.

Accidental dextromethorphan ingestions in children less than 5 years old

INTRODUCTION

The purpose of this study is to evaluate the clinical presentation of accidental dextromethorphan (DXM) ingestions in children <5 years old. Two consecutive years of poison center patient encounters were reviewed. Data including age, outcomes, amount of DXM ingested, co-ingestions, vital signs, clinical manifestations, hospital admissions, and mortality were abstracted. Data were analyzed using descriptive statistics.

DISCUSSION

A total of 304 cases were identified with a mean age of 28.2 months (72% were > or =23 months). All cases co-ingested other products of over-the-counter cough and cold medications (i.e., acetaminophen, pseudoephedrine, guaifenesin, ibuprofen, various H1 receptor antagonists, and very infrequently ethanol). The mean DXM dose ingested was 35.0 mg (2.64 mg/kg). Of the patients, 62 (20.4%) experienced lethargy as the sole neurological sign and no patient had any cardiovascular abnormalities. Only 1 (13-monthold) patient, who ingested 3.2 mg/kg and presented with lethargy, was hospitalized and subsequently discharged 14 hours later. No deaths were recorded.

CONCLUSION

As demonstrated in our patient population, accidental ingestions of DXM in the pediatric patient did well with supportive care alone and rarely required inpatient treatment.

Dextromethorphan poisoning: An evidence-based consensus guideline for out-of-hospital management

The objective of this guideline is to assist poison center personnel in the appropriate out-of-hospital triage and initial out-of-hospital management of patients with a suspected ingestion of dextromethorphan by 1) describing the process by which an ingestion of dextromethorphan might be managed, 2) identifying the key decision elements in managing cases of dextromethorphan ingestion, 3) providing clear and practical recommendations that reflect the current state of knowledge, and 4) identifying needs for research. This guideline applies to the ingestion of dextromethorphan alone. Co-ingestion of additional substances could require different referral and management recommendations depending on the combined toxicities of the substances. This guideline is based on an assessment of current scientific and clinical information. The expert consensus panel recognizes that specific patient care decisions might be at variance with this guideline and are the prerogative of the patient and the health professionals providing care, considering all of the circumstances involved. This guideline does not substitute for clinical judgment. The grade of recommendation is in parentheses. 1) All patients with suicidal intent, intentional abuse, or in cases in which a malicious intent is suspected (e.g., child abuse or neglect) should be referred to an emergency department (Grade D). 2) Patients who exhibit more than mild effects (e.g., infrequent vomiting or somnolence [lightly sedated and arousable with speaking voice or light touch]) after an acute dextromethorphan ingestion should be referred to an emergency department (Grade C). 3) Patients who have ingested 5-7.5 mg/kg should receive poison center-initiated follow-up approximately every 2 hours for up to 4 hours after ingestion. Refer to an emergency department if more than mild symptoms develop (Grade D). 4) Patients who have ingested more than 7.5 mg/kg should be referred to an emergency department for evaluation (Grade C). 5) If the patient is taking other medications likely to interact with dextromethorphan and cause serotonin syndrome, such as monoamine oxidase inhibitors or selective serotonin reuptake inhibitors, poison center-initiated follow-up every 2 hours for 8 hours is recommended (Grade D). 6) Patients who are asymptomatic and more than 4 hours have elapsed since the time of ingestion can be observed at home (Grade C). 7) Do not induce emesis (Grade D). 8) Do not use activated charcoal at home. Activated charcoal can be administered to asymptomatic patients who have ingested overdoses of dextromethorphan within the preceding hour. Its administration, if available, should only be carried out by health professionals and only if no contraindications are present. Do not delay transportation in order to administer activated charcoal (Grade D). 9) For patients who have ingested dextromethorphan and are sedated or comatose, naloxone, in the usual doses for treatment of opioid overdose, can be considered for prehospital administration, particularly if the patient has respiratory depression (Grade C). 10) Use intravenous benzodiazepines for seizures and benzodiazepines and external cooling measures for hyperthermia (>104 degrees F, >40 degrees C) for serotonin syndrome. This should be done in consultation with and authorized by EMS medical direction, by a written treatment protocol or policy, or with direct medical oversight (Grade C). 11) Carefully ascertain by history whether other drugs, such as acetaminophen, were involved in the incident and assess the risk for toxicity or for a drug interaction.

Life-threatening dextromethorphan intoxication associated with interaction with amitriptyline in a poor CYP2D6 metabolizer: a single case re-exposure study

We report a case of life-threatening intoxication and a controlled re-exposure study to dextromethorphan. A 60-year-old man developed postsurgical neuropathic cervical pain treated by hydromorphone, gabapentin, clonazepam, and amitriptyline. He received a dextromethorphan preparation for a catarrhal syndrome. Two days later, he was admitted into an emergency department in a profound coma. Thirty-six hours later, after withdrawal of all drugs, the situation normalized. A genotyping for UDP-glucuronyltransferase 1A1 and CYP2D6 was followed by a re-exposure study. During the three days, vital parameters and side effects of drugs were prospectively recorded. The second day, dextromethorphan was introduced. No significant impairment in parameters nor influence on analgesic efficacy were noted. Dextromethorphan concentrations suggested an accumulation without reaching any steady state. Somnolence was noted for plasma concentrations around 100ng/mL. The CYP2D6*4 variant leading to a poor metabolizer phenotype was found. Moreover, this phenotype was potentially aggravated by amitriptyline intake. This study allowed the identification and the confirmation of the cause of the coma. In conclusion, it is probably wise to recommend avoiding dextromethorphan in patients taking tricyclic antidepressants or another inhibitor of CYP2D6. Drug-drug interactions are probably underdiagnosed and underreported, and drugs considered as safe may induce serious complications.

Efficacy of pre- and postoperative oral dextromethorphan for reduction of intra- and 24-hour postoperative morphine consumption for transabdominal hysterectomy

OBJECTIVE

We studied the effect of dextromethorphan (DEX), an N-methyl-D-aspartate receptor antagonist, on analgesic consumption and pain scores after abdominal hysterectomy. We aimed to compare the analgesic effectiveness and incidence of adverse side effects of oral DEX with placebo (P).

DESIGN

This was a double-blinded, randomized, placebo-controlled study.

SETTING AND PATIENTS

One hundred patients were randomized to two groups. Group DEX was given 30-mg tablets of oral DEX with their premedication and three more times in the first 24 hours after surgery. Group P received the placebo following the same schedule. Postoperative analgesic requirements were assessed using a patient-controlled analgesia system. Pain was assessed at rest using a visual analogue scale in the post-anesthetic care unit (PACU), 6 and 24 hours after surgery.

RESULTS

Mean pain scores were significantly lower at the PACU (62.6 vs 75.7) as was the mean sum of all resting pain scores (144.5 vs 173.1). Mean morphine consumption was greatest in the DEX group (35.1 vs 33.0 mg; P < 0.05). The sedation scores, postoperative analgesic requirements, and incidence of side effects were similar between the P and DEX groups as were the mean pain scores.

CONCLUSIONS

During PACU, however, there were two differences between the DEX and placebo groups: 1) a lower pain score at PACU; and 2) a prolonged time to the first use of morphine at PACU in the DEX group as compared with the placebo group. A low-dose DEX had a weaker though still measurable effect in this clinical trial compared with trials using higher doses.

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.

Therapeutic use of dextromethorphan: Key learnings from treatment of pseudobulbar affect

A variety of neurological conditions and disease states are accompanied by pseudobulbar affect (PBA), an emotional disorder characterized by uncontrollable outbursts of laughing and crying. The causes of PBA are unclear but may involve lesions in neural circuits regulating the motor output of emotional expression. Several agents used in treating other psychiatric disorders have been applied in the treatment of PBA with some success but data are limited and these agents are associated with unpleasant side effects due to nonspecific activity in diffuse neural networks. Dextromethorphan (DM), a widely used cough suppressant, acts at receptors in the brainstem and cerebellum, brain regions implicated in the regulation of emotional output. The combination of DM and quinidine (Q), an enzyme inhibitor that blocks DM metabolism, has recently been tested in phase III clinical trials in patients with multiple sclerosis and amyotrophic lateral sclerosis and was both safe and effective in palliating PBA symptoms. In addition, clinical studies pertaining to the safety and efficacy of DM/Q in a variety of neurological disease states are ongoing.

The spinal anaesthetic effect of dextromethorphan, dextrorphan, and 3-methoxymorphinan

Dextromethorphan, an antitussive, has a complex pharmacologic profile and has not been well studied. Our aim was to evaluate whether dextromethorphan and its metabolites, dextrorphan and 3-methoxymorphinan, have a spinal anaesthetic effect. Using a method of spinal blockade in rats, we evaluated the potencies and durations of the effects of dextromethorphan and its metabolites on spinal blockades of motor function and nociception. Bupivacaine was the active control. We found that dextromethorphan and its metabolites produced a dose-related spinal blockade of motor function and nociception. On an ED(50) basis, the ranks of potencies were bupivacaine>dextrorphan>3-methoxymorphinan>dextromethorphan (p<0.05 for the differences). On an equipotent basis, dextrorphan and bupivacaine produced similarly longer nociceptive blockades than did dextromethorphan and 3-methoxymorphinan (p<0.05 for the differences). Co-administration of dextromethorphan or its metabolites with bupivacaine produced an additive effect. In conclusion, intrathecal injections of dextromethorphan or its metabolites, dextrorphan and 3-methoxymorphinan, produced dose-related spinal blockades of motor function and nociception. The suitability of these drugs as clinical spinal anaesthetics is worth further evaluation.

Dextromethorphan or dextrorphan have a local anesthetic effect on infiltrative cutaneous analgesia in rats

BACKGROUND

Dextromethorphan blocks sodium channels, the site of action of local anesthetics. In this study we evaluated whether dextromethorphan has a local anesthetic effect.

METHODS

We administered dextromethorphan and its active metabolite--dextrorphan, and lidocaine subcutaneously to rats and tested them for cutaneous anesthesia. Drug-drug interactions and systemic safety indices (LD50s/ED50s) were also evaluated.

RESULTS

Dextromethorphan and dextrorphan had a local anesthetic effect after cutaneous infiltration. The ranking of potencies was dextromethorphan > dextrorphan > lidocaine (P < 0.01 for each comparison). A combination of dextromethorphan or dextrorphan with lidocaine produced an additive effect. Dextromethorphan and dextrorphan had 2.4- and 1.9-fold higher system safety indices than did lidocaine.

CONCLUSION

Dextromethorphan and dextrorphan were more potent local anesthetics than lidocaine, but with higher systemic safety indices. Coadministration of dextromethorphan or dextrorphan with lidocaine produced an additive effect.

Impairments in water maze learning of aged rats that received dextromethorphan repeatedly during adolescent period

RATIONALE

Dextromethorphan (DM), an over-the-counter cough suppressant, has been recently used as a drug of abuse by teenage groups in some countries, such as the United States, Canada, and Korea. We previously showed that repeated administration of DM, a noncompetitive antagonist of N-methyl-D-aspartate (NMDA) receptors, impairs spatial learning performance in adolescent rats.

OBJECTIVES

In the present study, long-term adverse effects of repetitive DM use at adolescence were examined in rats.

METHODS

Male and female Sprague-Dawley rat pups received either intraperitoneal DM (40 mg/kg) or saline daily during postnatal days 28-37, and were then subjected to the Morris water maze task at the age of 18 months. Expression levels of NMDAR1, functional subunit of NMDA receptors, in the prefrontal cortex and the hippocampus were examined by Western blot analysis. Changes in plasma corticosterone levels responding to stress were determined by radioimmunoassay.

RESULTS

DM-experienced male rats exhibited deficits in the probe trial, and female rats in the initial learning and the reversal training, in water maze performance. Expression levels of NMDAR1 in the brain regions were significantly increased in DM-experienced rats, compared to control rats. Stress-induced increases in plasma corticosterone levels were blunted both in male and female DM rats.

CONCLUSIONS

The results suggest that repeated administration of DM at high doses during adolescent period may induce permanent deficits in cognitive function and that increased expression of NMDAR1 in the prefrontal cortex and the hippocampus may take a role in DM-induced memory deficits.

Glutamate receptor ligands

Glutamate acts through a variety of receptors to modulate neurotransmission and neuronal excitability. Glutamate plays a critical role in neuroplasticity as well as in nervous system dysfunctions and disorders. Hyperfunction or dysfunction of glutamatergic neurotransmission also represents a key mechanism of pain-related plastic changes in the central and peripheral nervous system. This chapter will review the classification of glutamate receptors and their role in peripheral and central nociceptive processing. Evidence from preclinical pain models and clinical studies for the therapeutic value of certain glutamate receptor ligands will be discussed.

Dextromethorphan: a review of N-methyl-d-aspartate receptor antagonist in the management of pain

Dextromethorphan (DM) is a noncompetitive N-methyl-d-aspartate (NMDA) receptor antagonist, which is widely used as an antitussive agent. DM also prevents neuronal damage and modulates pain sensation via noncompetitive antagonism of excitatory amino acids (EAAs). DM has been found to be useful in the treatment of pain in cancer patients and in the treatment of methotrexate-induced neurotoxicity. Clinical studies with DM in cancer patients are reviewed in this article.

Preoperative Cotreatment With Dextromethorphan and Ketorolac Provides an Enhancement of Pain Relief After Laparoscopic-assisted Vaginal Hysterectomy

OBJECTIVES

Both N-methyl-D-aspartate receptor antagonists and nonsteroidal anti-inflammatory drugs have been demonstrated to produce better postoperative pain relief. The concept of multimodal analgesia has also been used for clinical pain management. The aim of the present study was to examine the analgesic effect of preoperative cotreatment with dextromethorphan (DM) and ketorolac on postoperative pain management after laparoscopic-assisted vaginal hysterectomy (LAVH).

METHODS

Eighty ASA physical status I or II patients scheduled for LAVH were included and randomly assigned to 1 of 4 groups. Patients received intramuscular (IM) chorpheniramine 20 mg+ intravenous (IV) 2 mL of normal saline, IM DM 40 mg+IV 2 mL of normal saline, IM chorpheniramine 20 mg+IV 60 mg (2 mL) of ketorolac, and IM DM 40 mg+IV ketorolac 60 mg as the groups C, DM, Keto, and DM+Keto, respectively. All patients were given a patient-controlled analgesia (PCA) with morphine for pain relief postoperatively. Analgesic effects were evaluated using Visual Analog Scale pain scores at rest and during coughing, time to first PCA request for pain relief, total morphine consumption, bed rest time, and the time to first passage of flatus for 48 hours after surgery.

RESULTS

Patients in DM and Keto groups had significantly better pain relief than patients in group C. Patients in DM+Keto group exhibited the best postoperative pain relief among groups in the following several categories: time to first trigger of PCA, total morphine consumption, the worst Visual Analog Scale, bed rest time, and the time to first passage of flatus, demonstrating an enhanced effect between DM and ketorolac. Neither synergistic nor antagonistic interaction was observed between DM and ketorolac.

DISCUSSION

Preoperative treatment with both DM and ketorolac diminish postoperative pain. Our results suggest that the N-methyl-D-aspartate antagonist-DM and the nonsteroidal anti-inflammatory drugs-ketorolac cotreatment provide an enhancement of analgesia for postoperative pain management in patients after LAVH surgery.

Ovariectomy ameliorates dextromethorphan--induced memory impairment in young female rats

We have previously found that dextromethorphan (DM), over-the-counter cough suppressant, impairs memory retention in water maze task, when it is repeatedly administrated to adolescent female rats at high doses. In this study we examined first if ovariectomy ameliorates the DM-induced memory impairment in female rats, and then whether or not the DM effect is revived by estrogen replacement in ovariectomized female rats. Female rat pups received bilateral ovariectomy or sham operation on postnatal day (PND) 21, and then intraperitoneal DM (40 mg/kg) daily during PND 28-37. Rats were subjected to the Morris water maze task from PND 38, approximately 24 h after the last DM injection. In probe trial, goal quadrant dwell time was significantly reduced by DM in the sham operated group, however, the reduction by DM did not occur in the ovariectomy group. When 17beta-estradiol was supplied to ovariectomized females during DM treatment, the goal quadrant dwell time was significantly decreased, compared to the vehicle control group. Furthermore, a major effect of estrogen replacement was found in the escape latency during the last 3 days of initial learning trials. These results suggest that ovariectomy may ameliorate the adverse effect of DM treatment on memory retention in young female rats, and that estrogen replacement may revive it, i.e. estrogen may take a major role in DM-induced memory impairment in female rats.

Dextromethorphan, 3-methoxymorphinan, and dextrorphan have local anaesthetic effect on sciatic nerve blockade in rats

Dextromethorphan has been used as an antitussive for more than 40 years and is considered a drug with a good margin of safety. The aim of the study was to evaluate whether dextromethorphan and its metabolites--3-methoxymorphinan and dextrorphan--had local anaesthetic effects. Using a method of sciatic nerve blockade in rats, the potencies and durations of actions of dextromethorphan and its metabolites on sciatic nerve blockades of motor function, proprioception, and nociception were evaluated. Lidocaine was used as control. We found that dextromethorphan and its metabolites produced dose-related local anaesthetic effects on sciatic nerve blockades of motor function, proprioception, and nociception. The ranks of potencies were lidocaine>dextromethorphan>3-methoxymorphinan>dextrorphan (P<0.01 for each comparison). Under an equi-potent basis, dextrorphan and 3-methoxymorphinan had durations of actions longer than that of lidocaine (P<0.05 for each comparison). Co-administration of dextromethorphan or its metabolites with lidocaine produced an additive effect on sciatic nerve blockades. In conclusion, dextromethorphan and its metabolites - 3-methoxymorphinan and dextrorphan- had a local anaesthetic effect on sciatic nerve blockades of motor function, proprioception and nociception with durations of actions longer than that of lidocaine. Co-administration of dextromethorphan and its metabolites produced an additive effect on sciatic nerve blockades.

Measurement of CYP2D6 and CYP3A4 activity in vivo with dextromethorphan: sources of variability and predictors of adverse effects in 419 healthy subjects

OBJECTIVE

Dextromethorphan (DEM) shares part of the adverse event profile of opioids and is widely used as a probe drug for CYP2D6 phenotyping and for the assessment of CYP2D6 activity. It has also been used to assess CYP3A4 activity. This study examined the influence of anthropometric variables, oral contraceptives, smoking habits, mu-opioid receptor and MDR1 genetic polymorphisms and components of the DEM ratios on the variability of CYP2D6 and CYP3A4 metabolic ratios and on the occurrence of adverse events following DEM administration.

METHODS

This was a retrospective analysis of a database in 419 healthy subjects. CYP2D6 and CYP3A4 metabolic ratios were measured as the log of the ratios of the amount of DEM to the amount of dextrorphan (DOR) and of the amount of DEM to the amount of 3-methoxy-morphinan (MET) excreted in urine during a 12-h time period, respectively, following the oral administration of 80 mg of dextromethorphan hydrobromide. Logistic regression was performed to examine the factors associated with changes in metabolic ratios and with the occurrence of adverse events.

RESULTS

The CYP2D6 metabolic ratio allowed identification of extensive and poor metabolizers of DEM. The CYP2D6 and CYP3A4 metabolic ratios were not strictly independent one from each other. Based on multivariate analysis, the CYP2D6 metabolic ratio was a stronger independent predictor of adverse events (p<0.0001) than the CYP2D6 phenotype (p=0.05). Anthropometric variables, oral contraceptives, smoking habits, mu-opioid receptor and MDR1 genetic polymorphisms did not significantly contribute to changes in metabolic ratios or to the occurrence of adverse events.

CONCLUSIONS

Dextromethorphan can be used for CYP2D6 phenotyping, but the CYP2D6 and CYP3A4 metabolic ratios are not strictly independent one from each other. The CYP2D6 metabolic ratio predicts adverse events to DEM as does CYP2D6 phenotype, and extensive metabolizer subjects are not protected against adverse events.