Oral administration of dextromethorphan prevents the development of morphine tolerance and dependence in rats

Ketamine and Hydroxynorketamine as Novel Pharmacotherapies for the Treatment of Opioid Use Disorders

Opioid use disorder (OUD) has reached epidemic proportions, with many countries facing high levels of opioid use and related fatalities. Although currently prescribed medications for OUD are considered lifesaving, they inadequately address negative affect and cognitive impairment, resulting in high relapse rates to nonmedical opioid use even years after drug cessation (protracted abstinence). Evidence supports the notion that ketamine, an anesthetic and rapid-acting antidepressant drug, holds promise as a candidate for OUD treatment, including the management of acute withdrawal somatic symptoms, negative affect during protracted opioid abstinence, and prevention of retaking nonmedical opioids. In this review, we comprehensively discuss preclinical and clinical research that has evaluated ketamine and its metabolites as potential novel therapeutic strategies for treating OUD. Furthermore, we examine evidence that supports the relevance of the molecular targets of ketamine and its metabolites in relation to their potential effects and therapeutic outcomes in OUD. Overall, existing evidence demonstrates that ketamine and its metabolites can effectively modulate pathophysiological processes affected in OUD, suggesting a promising therapeutic role in the treatment of OUD and the prevention of return to opioid use during abstinence.

Novel GluN2B-Selective NMDA Receptor Negative Allosteric Modulator Possesses Intrinsic Analgesic Properties and Enhances Analgesia of Morphine in a Rodent Tail Flick Pain Model

Many cases of accidental death associated with drug overdose are due to chronic opioid use, tolerance, and addiction. Analgesic tolerance is characterized by a decreased response to the analgesic effects of opioids, requiring increasingly higher doses to maintain the desired level of pain relief. Overactivation of GluN2B-containing N-methyl-d-Aspartate receptors is thought to play a key role in mechanisms underlying cellular adaptation that takes place in the development of analgesic tolerance. Herein, we describe a novel GluN2B-selective negative allosteric modulator, EU93-108, that shows high potency and brain penetrance. We describe the structural basis for binding at atomic resolution. This compound possesses intrinsic analgesic properties in the rodent tail immersion test. EU93-108 has an acute and significant anodyne effect, whereby morphine when combined with EU93-108 produces a higher tail flick latency compared to that of morphine alone. These data suggest that engagement of GluN2B as a target has utility in the treatment of pain, and EU93-108 could serve as an appropriate tool compound to interrogate this hypothesis. Future structure-activity relationship work around this scaffold could give rise to compounds that can be co-administered with opioids to diminish the onset of tolerance due to chronic opioid use, thereby modifying their utility.

Complex Regional Pain Syndrome: Practical Diagnostic and Treatment Guidelines, 5th Edition

There have been some modest recent advancements in the research of Complex Regional Pain Syndrome, yet the amount and quality of the work in this complicated multifactorial disease remains low (with some notable exceptions; e.g., the recent work on the dorsal root ganglion stimulation). The semi-systematic (though in some cases narrative) approach to review is necessary so that we might treat our patients while waiting for "better research." This semi-systematic review was conducted by experts in the field, (deliberately) some of whom are promising young researchers supplemented by the experience of "elder statesman" researchers, who all mention the system they have used to examine the literature. What we found is generally low- to medium-quality research with small numbers of subjects; however, there are some recent exceptions to this. The primary reason for this paucity of research is the fact that this is a rare disease, and it is very difficult to acquire a sufficient sample size for statistical significance using traditional statistical approaches. Several larger trials have failed, probably due to using the broad general diagnostic criteria (the "Budapest" criteria) in a multifactorial/multi-mechanism disease. Responsive subsets can often be identified in these larger trials, but not sufficient to achieve statistically significant results in the general diagnostic grouping. This being the case the authors have necessarily included data from less compelling protocols, including trials such as case series and even in some instances case reports/empirical information. In the humanitarian spirit of treating our often desperate patients with this rare syndrome, without great evidence, we must take what data we can find (as in this work) and tailor a treatment regime for each patient.

Generational Effects of Opioid Exposure

The inheritance of substance abuse, including opioid abuse, may be influenced by genetic and non-genetic factors related to the environment, such as stress and socioeconomic status. These non-genetic influences on the heritability of a trait can be attributed to epigenetics. Epigenetic inheritance can result from modifications passed down from the mother, father, or both, resulting in either maternal, paternal, or parental epigenetic inheritance, respectively. These epigenetic modifications can be passed to the offspring to result in multigenerational, intergenerational, or transgenerational inheritance. Human and animal models of opioid exposure have shown generational effects that result in molecular, developmental, and behavioral alterations in future generations.

Spinal Serum- and Glucocorticoid-Regulated Kinase 1 (SGK1) Signaling Contributes to Morphine-Induced Analgesic Tolerance in Rats

Accumulating evidence indicates that phosphorylated serum- and glucocorticoid-regulated kinase 1 (SGK1) is associated with spinal nociceptive sensitization by modulating glutamatergic N-methyl-D-aspartate receptors (NMDARs). In this study, we determined whether spinal SGK1 signaling contributes to the development of morphine analgesic tolerance. Chronic morphine administration markedly induced phosphorylation of SGK1 in the spinal dorsal horn neurons. Intrathecal injection of SGK1 inhibitor GSK-650394 reduced the development of morphine tolerance with a significant leftward shift in morphine dose-effect curve. Furthermore, spinal inhibition of SGK1 suppressed morphine-induced phosphorylation of nuclear factor kappa B (NF-κB) p65 and upregulation of NMDAR NR1 and NR2B expression in the spinal dorsal horn. In contrast, intrathecal administration of NMDAR antagonist MK-801 had no effect on the phosphorylation of SGK1 in morphine-treated rats. In addition, morphine-induced upregulation of NR2B, but not NR1, was significantly abolished by intrathecal pretreatment with PDTC, a specific NF-κB activation inhibitor. Finally, spinal delivery of SGK1 small interfering RNA exhibited similar inhibitory effects on morphine-induced tolerance, phosphorylation of NF-κB p65, as well as upregulation of NR1 and NR2B expression. Our findings demonstrate that spinal SGK1 contributes to the development of morphine tolerance by enhancing NF-κB p65/NMDAR signaling. Interfering spinal SGK1 signaling pathway could be a potential strategy for prevention of morphine tolerance in chronic pain management.

Neurokinin-1 receptor-based bivalent drugs in pain management: The journey to nowhere?

Hybrid compounds (also known as chimeras, designed multiple ligands, bivalent compounds) are chemical units where two active components, usually possessing affinity and selectivity for distinct molecular targets, are combined as a single chemical entity. The rationale for using a chimeric approach is well documented as such novel drugs are characterized by their enhanced enzymatic stability and biological activity. This allows their use at lower concentrations, increasing their safety profile, particularly when considering undesirable side effects. In the group of synthetic bivalent compounds, drugs combining pharmacophores having affinities toward opioid and neurokinin-1 receptors have been extensively studied as potential analgesic drugs. Indeed, substance P is known as a major endogenous modulator of nociception both in the peripheral and central nervous systems. Hence, synthetic peptide fragments showing either agonism or antagonism at neurokinin 1 receptor were both assigned with analgesic properties. However, even though preclinical studies designated neurokinin-1 receptor antagonists as promising analgesics, early clinical studies revealed a lack of efficacy in human. Nevertheless, their molecular combination with enkephalin/endomorphin fragments has been considered as a valuable approach to design putatively promising ligands for the treatment of pain. This paper is aimed at summarizing a 20-year journey to the development of potent analgesic hybrid compounds involving an opioid pharmacophore and devoid of unwanted side effects. Additionally, the legitimacy of considering neurokinin-1 receptor ligands in the design of chimeric drugs is discussed.

Effect of N-Methyl-D-Aspartate Receptor Antagonist Dextromethorphan on Opioid Analgesia in Pediatric Intensive Care Unit

Objective. Pain control is an essential goal in the management of critical children. Narcotics are the mainstay for pain control. Patients frequently need escalating doses of narcotics. In such cases an adjunctive therapy may be beneficial. Dextromethorphan (DM) is NMDA receptor antagonist and may prevent tolerance to narcotics; however, its definitive role is still unclear. We sought whether dextromethorphan addition could decrease the requirements of fentanyl to control pain in critical children. Design. Double-blind, randomized control trial (RCT). Setting. Pediatric multidisciplinary ICU in tertiary care center. Patients. Thirty-six pediatric patients 2–14 years of age in a multidisciplinary PICU requiring analgesia were randomized into dextromethorphan and placebo. The subjects in both groups showed similarity in most of the characteristics. Interventions. Subjects while receiving fentanyl for pain control received dextromethorphan or placebo through nasogastric/orogastric tubes for 96 hours. Pain was assessed using FLACC and faces scales. Measurements and Main Results. This study found no statistical significant difference in fentanyl requirements between subjects receiving dextromethorphan and those receiving placebo (p = 0.127). Conclusions. Dextromethorphan has no effect on opioid requirement for control of acute pain in children admitted with acute critical care illness in PICU. The registration number for this trial is NCT01553435.

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.

Complex regional pain syndrome: practical diagnostic and treatment guidelines, 4th edition

OBJECTIVE

This is the fourth edition of diagnostic and treatment guidelines for complex regional pain syndrome (CRPS; aka reflex sympathetic dystrophy).

METHODS

Expert practitioners in each discipline traditionally utilized in the treatment of CRPS systematically reviewed the available and relevant literature; due to the paucity of levels 1 and 2 studies, less rigorous, preliminary research reports were included. The literature review was supplemented with knowledge gained from extensive empirical clinical experience, particularly in areas where high-quality evidence to guide therapy is lacking.

RESULTS

The research quality, clinical relevance, and "state of the art" of diagnostic criteria or treatment modalities are discussed, sometimes in considerable detail with an eye to the expert practitioner in each therapeutic area. Levels of evidence are mentioned when available, so that the practitioner can better assess and analyze the modality under discussion, and if desired, to personally consider the citations. Tables provide details on characteristics of studies in different subject domains described in the literature.

CONCLUSIONS

In the humanitarian spirit of making the most of all current thinking in the area, balanced by a careful case-by-case analysis of the risk/cost vs benefit analysis, the authors offer these "practical" guidelines.

Beneficial effect of amantadine on postoperative pain reduction and consumption of morphine in patients subjected to elective spine surgery

OBJECTIVE

To analyze the effect of coadministration of morphine and amantadine on postoperative pain reduction and morphine consumption in patients after elective spine surgery.

METHODS

In double-blinded study, 60 patients (ASA physical status I-II) were randomized into two groups. Group A was given oral amantadine 50 or 100 mg 1 hour before surgery and 8, 20, 32 hours after operation. Group P received placebo at identical times. Pain was assessed using numerical rating scale before first administration of morphine and in 2, 3, 4, 6, 24, and 48 hours after operation. The amounts of morphine consumed were recorded up to 48 hours after surgery. Blood samples were taken twice in 2 hours after surgery and plasma levels of morphine and its main metabolites were measured.

RESULTS

As compared with placebo, amantadine significantly reduced intra-operative Fentanyl use and sensation of postoperative pain. Up to 48 hours after operation, the cumulative consumption of morphine was 25% lower in the amantadine group. Moreover, intensity of nausea and vomiting tended to be lower in A group. Starting from 12th hour after surgery, the level of postoperative sedation was lower in patients who received amantadine, as compared with placebo group. No significant differences in plasma levels of morphine ant its metabolites were observed between A and P groups.

CONCLUSIONS

Pre- and postoperative administration of amantadine significantly reduced fentanyl use during operation, as well as reduced the postoperative pain and decreased morphine consumption in young patients undergoing orthopedic surgery.

Recent advances in understanding nicotinic receptor signaling mechanisms that regulate drug self-administration behavior

Tobacco smoking is one of the leading causes of disease and premature death in the United States. Nicotine is considered the major reinforcing component in tobacco smoke responsible for tobacco addiction. Nicotine acts in the brain through the neuronal nicotinic acetylcholine receptors (nAChRs). The predominant nAChR subtypes in mammalian brain are those containing α4 and β2 subunits. The α4β2 nAChRs, particularly those located in the mesoaccumbens dopamine pathway, play a key role in regulating the reinforcing properties of nicotine. Considering that twelve mammalian nAChR subunits have been cloned, it is likely that nAChRs containing subunits in addition to, or other than, α4 and β2 also play a role in the tobacco smoking habit. Consistent with this possibility, human genome-wide association studies have shown that genetic variation in the CHRNA5-CHRNA3-CHRNB4 gene cluster located in chromosome region 15q25, which encode the α5, α3 and β4 nAChR subunits, respectively, increases vulnerability to tobacco addiction and smoking-related diseases. Most recently, α5-containing nAChRs located in the habenulo-interpeduncular tract were shown to limit intravenous nicotine self-administration behavior in rats and mice, suggesting that deficits in α5-containing nAChR signaling in the habenulo-interpeduncular tract increases vulnerability to the motivational properties of nicotine. Finally, evidence suggests that nAChRs may also play a prominent role in controlling consumption of addictive drugs other than nicotine, including cocaine, alcohol, opiates and cannabinoids. The aim of the present review is to discuss recent preclinical findings concerning the identity of the nAChR subtypes that regulate self-administration of nicotine and other drugs of abuse.

Dextromethorphan attenuated the higher vulnerability to inflammatory thermal hyperalgesia caused by prenatal morphine exposure in rat offspring

Background

Co-administration of dextromethorphan (DM) with morphine during pregnancy and throughout lactation has been found to reduce morphine physical dependence and tolerance in rat offspring. No evidence was presented, however, for the effect of DM co-administered with morphine during pregnancy on inflammatory hyperalgesia in morphine-exposed offspring. Therefore, we attempt to investigate the possible effect of prenatal morphine exposure on the vulnerability to hyperalgesia and the possible therapeutic effect of DM in the present study.

Methods

Fifty μl of carrageenan (20 mg/ml) was injected subcutaneously into the plantar surface of the right hind paw in p18 rats to induce hyperalgesia. Mean paw withdrawal latency was measured in the plantar test to index the severity of hyperalgesia. Using Western blotting and RT-PCR, the quantitative analyses of NMDA receptor NR1 and NR2B subunits were performed in spinal cords from different groups of animals.

Results

In the carrageenan-induced hyperalgesia model, rat offspring passively exposed to morphine developed a severe hyperalgesia on postnatal day 18 (p18), which also had a more rapid time course than those in the controls. Co-administration of DM with morphine in the dams prevented this adverse effect of morphine in the offspring rats. Western blot and RT-PCR analysis showed that the levels of protein and mRNA of NMDA receptor NR1 and NR2B subunits were significantly higher in the lumbar spinal cords of rats (p14) exposed to prenatal morphine; the co-administration of DM could reverse the effect of morphine on NR1 and attenuate the effect on NR2B.

Conclusions

Thus, DM may have a great potential in the prevention of higher vulnerability to inflammatory thermal hyperalgesia in the offspring of morphine-addicted mothers.

Pharmacological treatment of neuropathic cancer pain: a comprehensive review of the current literature

Neuropathic cancer pain (NCP), commonly encountered in clinical practice, may be cancer-related, namely resulting from nervous system tumor invasion, surgical nerve damage during tumor removal, radiation-induced nerve damage and chemotherapy-related neuropathy, or may be of benign origin, unrelated to cancer. A neuropathic component is evident in about 1/3 of cancer pain cases. Although from a pathophysiological perspective NCP may differ from chronic neuropathic pain (NP), such as noncancer-related pain, clinical practice, and limited publications have shown that these two pain entities may share some treatment modalities. For example, co-analgesics have been well integrated into cancer pain-management strategies and are often used as First-Line options for the treatment of NCP. These drugs, including antidepressants and anticonvulsants, are recommended by evidence-based guidelines, whereas, others such as lidocaine patch 5%, are supported by randomized, controlled, clinical data and are included in guidelines for restricted conditions treatment. The vast majority of these drugs have already been proven useful in the management of benign NP syndromes. Treatment decisions for patients with NP can be difficult. The intrinsic difficulties in performing randomized controlled trials in cancer pain have traditionally justified the acceptance of drugs already known to be effective in benign NP for the management of malignant NP, despite the lack of relevant high quality data. Interest in NCP mechanisms and pharmacotherapy has increased, resulting in significant mechanism-based treatment advances for the future. In this comprehensive review, we present the latest knowledge regarding NCP pharmacological management.

Negligible analgesic tolerance seen with extended release oxymorphone: a post hoc analysis of open-label longitudinal data

OBJECTIVE

To examine the development of analgesic tolerance in patients on oxymorphone extended-release (OxymER).

DESIGN

Post hoc analysis of data from a previously conducted prospective 1 year multi-center open-label extension study in which patients were able to titrate as needed.

PATIENTS

Sample of 153 hip and knee osteoarthritis (OA) subjects on OxymER. Primary analyses were limited to study completers (n = 62) due to the large amount of missing data for the noncompleters (n = 91).

OUTCOME MEASURES

Main outcome measures included OxymER doses (pill counts) and pain intensity ratings using a visual analog scale at monthly visits.

RESULTS

There were significant dose increases from weeks 1 to 2 and 2 to 6 (P < 0.05). Doses stabilized around week 6, suggesting the completion of what we defined as "titration." Both doses and pain ratings were stable when this titration phase was excluded from the analysis (P = 0.751; P = 0.056, respectively). Only 28% of the patients had any dose changes following this titration. While there was a significantly greater dose at week 52 compared with week 10 (P = 0.010), the increase in dose became insignificant after excluding four subjects who required two dose increases (P = 0.103).

CONCLUSIONS

The results showed that most of the titration/dose stabilization changes occurred within the first 10 weeks. A minority (28%) of subjects required dosage increases after this (defined) titration period. Pain reports stabilized statistically after 2 weeks. The findings of this post hoc analysis suggest a lack of opioid tolerance in the majority (72%) of these OA patients who completed this study following a defined titration period on OxymER.

SUMMARY

This post hoc analysis of oxymorphone ER consumption in osteoarthritis pain vs pain report showed that most dose changes occurred during an initial "titration period" as defined. Following this titration few subjects increased dose and analgesia remained stable. These findings suggest a lack of longitudinal opioid tolerance in the majority of those OA subjects who completed the trial.

Effects of repeated morphine treatment on the antinociceptive effects, intestinal absorption, and efflux from intestinal epithelial cells of morphine

The present study was conducted to investigate the effects of repeated treatment with morphine on the drug's antinociceptive effects, intestinal absorption, and transepithelial transport. The antinociceptive effects of morphine in rats were markedly decreased after repeated oral administration of the drug for 5 d, indicating the development of tolerance. In the morphine-tolerant rats, intestinal absorption of morphine was determined using the in situ loop method. Absorption of morphine from the jejunum was significantly decreased after repeated administration. The permeability of human intestinal epithelial Caco-2 cells was increased in the efflux direction after repeated treatment. The repeated administration of morphine also reduced the cellular accumulation and efflux of P-glycoprotein substrates ([(3)H]vincristine and rhodamine123) from Caco-2 cells, suggesting that it enhances P-glycoprotein-mediated efflux in Caco-2 cells. These results suggest that repeated use enhances the efflux of morphine in the epithelial cells of the small intestine, subsequently decreasing its intestinal absorption.

Social isolation increases morphine intake: behavioral and psychopharmacological aspects

Environmental and situational factors are important determinants of recreational drug use in humans. We aimed to develop a reliable animal model for studying the effects of environmental variables on drug-seeking behavior using the 'social isolation/social restriction' paradigm. Adult Wistar rats housed in short-term isolation (21 days) consumed significantly more morphine solution (0.5 mg/ml) than rats living in pairs, both in one-bottle and in two-bottle tests. No differences were found in their water consumption. This effect was observed in both males and females and the results were also replicated after reversal of housing conditions. We also found that as little as 60-min of daily social-physical interaction with another rat was sufficient to completely abolish the increase in morphine consumption in socially restricted animals. We discuss some possible interpretations for these effects. These results indicate that environmental and situational factors influence drug intake in laboratory rats as they do in humans, and thus may be of interest in studying drug-seeking behavior in humans.

Glutamate modulation of antinociception, but not tolerance, produced by morphine microinjection into the periaqueductal gray of the rat

The periaqueductal gray (PAG) plays an important role in morphine antinociception and tolerance. Co-localization of mu-opioid and NMDA receptors on dendrites in the PAG suggests that glutamate may modulate morphine antinociception. Moreover, the involvement of glutamate in spinally mediated tolerance to morphine suggests that glutamate receptors may contribute to PAG mediated tolerance. These hypotheses were tested by microinjecting glutamate receptor antagonists and morphine into the ventrolateral PAG (vPAG) of the rat. Microinjection of the non-specific glutamate receptor antagonist kynurenic acid or the NMDA receptor antagonist MK-801 into the vPAG did not affect nociception. However, co-administration of these antagonists with morphine into the vPAG enhanced the acute antinociceptive effects of morphine as measured by a leftward shift in the morphine dose-response curves. Repeated microinjections of morphine into the vPAG caused a rightward shift in the dose-response curve for antinociception whether the glutamate receptor antagonists kynurenic acid or MK-801 were co-administered or not. The lack of effect of microinjecting glutamate receptor antagonists into the vPAG indicates that tonic glutamate release in the PAG does not contribute to nociceptive tone. That these antagonists enhance morphine antinociception indicates that endogenous glutamate counteracts the antinociceptive effect of morphine in the vPAG. However, this compensatory glutamate release does not contribute to tolerance to the antinociceptive effects of microinjecting morphine into the vPAG. Previous research showing that glutamate contributes to spinal mechanisms of tolerance indicate that different tolerance mechanisms are engaged in the vPAG and spinal cord.

Enhancement by grapefruit juice of morphine antinociception

The aim of this study was to investigate the effect of grapefruit juice intake on the antinociception of morphine in rats. The antinociception of morphine (30 mg/kg, per os (p.o.)) was significantly enhanced by the oral administration of grapefruit juice (2 ml/rat). Further, the effect of grapefruit juice was examined in morphine-tolerant rats. The repeated administration of morphine (100 mg/kg p.o.) for 5 d caused a marked decrease in the antinociception, indicating the development of morphine-tolerance. In the morphine-tolerant rats, oral administration of grapefruit juice potentiated significantly the antinociceptive effect of morphine. To examine the pharmacokinetics of morphine after the repeated treatment with morphine for 5 d, microdialysis probes were implanted into the jugular vein and spinal intrathecal space in rats. The morphine concentrations in the blood and intrathecal cerebrospinal fluid (CSF) were gradually decreased by the repeated treatment with morphine. The grapefruit juice treatment significantly increased the blood concentration of morphine in morphine-tolerant rats. These results suggest that oral administration of grapefruit juice enhances the morphine antinociception by increasing the intestinal absorption of this agent.

Pharmacological validation of a chronic social stress model of depression in rats: effects of reboxetine, haloperidol and diazepam

Chronic social stress is one of the most important factors responsible for precipitation of depressive disorder in humans. In recent years, the impact of social stress on the development of psychopathologies has been thoroughly investigated in preclinical animal studies. We have shown recently that behavioural effects of chronic social stress in rats can be reversed by citalopram and fluoxetine. This study has been designed for further pharmacological validation of the chronic social stress paradigm as a model of depressive symptoms in rats. For this, rats were subjected to 5 weeks of daily social defeat and were in parallel treatment for a clinically relevant period of 4 weeks with the antidepressant drug reboxetine (40 mg/kg/day) and the neuroleptic drug haloperidol (2 mg/kg/day). The anxiolytic diazepam (1 mg/kg) was administered acutely at the end of the stress period. Stress caused decreased locomotor and exploratory behaviours, decreased sucrose preference and increased immobility in the forced swim test, but did not affect behaviour in the elevated plus maze. Four weeks of oral treatment with reboxetine ameliorated the adverse effects of social stress and normalized behaviours related to motivation and reward sensitivity. The treatment with haloperidol worsened the adverse effects of chronic social stress having effects similar to stress on reward and motivation-related behaviours. Diazepam reduced anxiety-related behaviours as measured in elevated plus maze in control animals having no effects on socially stressed individuals. Neither sucrose preference nor performance in forced swim test was affected by diazepam. The effectiveness and selectivity of the treatment with the antidepressant reboxetine in ameliorating socially induced behavioural disturbances supports the validity of the chronic social stress as a model of depressive-like symptoms in rats.

Evaluation the effects of dextromethorphan and midazolam on morphine induced tolerance and dependence in mice

The main aim of this study was to evaluate the effects of dextromethorphan and midazolam and their combination on morphine tolerance and dependence in mice. In the present study, different groups of mice were rendered randomly and received morphine (50 mg kg(-1), s.c.), morphine (50 mg kg(-1), s.c.) + Dextromethorphan (25, 50 and 75 mg kg(-1), i.p.), morphine (50 mg kg(-1), s.c.) + midazolam (0.5, 1 and 2 mg kg(-1), i.p.), morphine (50 mg kg(-1), s.c.) + [Dextromethorphan (25 mg kg(-1), i.p. ) + midazolam (0.5 mg kg(-1), i.p.)] once a day for four days. Tolerance was assessed by administration of morphine (9 mg kg(-1), i.p.) on fifth day. Withdrawal symptoms (markers for dependence) was assessed by administration of naloxone (4 mg kg(-1), i.p.) 2 h after co-administration of morphine with either Dextromethorphan or midazolam or their combination. Results showed that pretreatment with Dextromethorphan or midazolam decreased the degree of tolerance and withdrawal symptoms significantly. Additionally co-administration ofDextromethorphan and midazolam couldn't decreased the tolerance and dependence significantly. From these results it may concluded that Dextromethorphan and midazolam alone or in combination could prevent the development of morphine induced tolerance and dependence. These effects can be related to the N-Methyl-D-Aspartate (NMDA) receptor antagonist behavior of Dextromethorphan and GABA-receptor agonist property of midazolam.

Memantine produces modest reductions in heroin-induced subjective responses in human research volunteers

RATIONALE

Previous studies have demonstrated an interaction between opioids and noncompetitive antagonists at N-methyl-D: -aspartate (NMDA) receptors, but few studies have examined the utility of these medications for treating opioid dependence.

OBJECTIVE

In this 8-week inpatient study, participants were maintained on the low-affinity, noncompetitive NMDA receptor antagonist memantine (0, 30, and 60 mg per day, PO) and under each maintenance dose condition, the effects of intranasal heroin (0, 12.5, and 50 mg, IN) were examined.

METHODS

During the first week after admission to the hospital, participants were detoxified from heroin. All of the volunteers received all of the memantine and heroin dose combinations. Participants (N = 8) first sampled a dose of heroin and $20. During a subsequent choice session, participants could self-administer heroin and/or money. Responses, which consisted of finger presses on a computer mouse, were made under a modified progressive ratio schedule (PR 50, 100, 200, 400, 800, 1,200, 1,600, 2,000, 2,400, and 2,800) during a ten-trial self-administration task. Subjective, performance, and physiological effects were measured repeatedly during laboratory sessions.

RESULTS

Memantine produced modest reductions in subjective ratings of drug quality, liking, willingness to pay for the drug, and craving for heroin. However, memantine produced few changes in the reinforcing effects of heroin.

CONCLUSIONS

These data demonstrate that memantine was well tolerated and modestly effective in reducing the subjective but not the reinforcing effects of heroin. Although it is unlikely that memantine will be useful as a stand-alone maintenance medication for opioid dependence, it may have some utility as an adjunct treatment medication.

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.

Central Nicotinic Receptors: Structure, Function, Ligands, and Therapeutic Potential

The growing interest in nicotinic receptors, because of their wide expression in neuronal and non-neuronal tissues and their involvement in several important CNS pathologies, has stimulated the synthesis of a high number of ligands able to modulate their function. These membrane proteins appear to be highly heterogeneous, and still only incomplete information is available on their structure, subunit composition, and stoichiometry. This is due to the lack of selective ligands to study the role of nAChR under physiological or pathological conditions; so far, only compounds showing selectivity between alpha4beta2 and alpha7 receptors have been obtained. The nicotinic receptor ligands have been designed starting from lead compounds from natural sources such as nicotine, cytisine, or epibatidine, and, more recently, through the high-throughput screening of chemical libraries. This review focuses on the structure of the new agonists, antagonists, and allosteric ligands of nicotinic receptors, it highlights the current knowledge on the binding site models as a molecular modeling approach to design new compounds, and it discusses the nAChR modulators which have entered clinical trials.

Effects of dextrorotatory morphinans on brain Na+ channels expressed in Xenopus oocytes

We previously demonstrated that dextromethorphan (DM; 3-methoxy-17-methylmorphinan) analogs have neuroprotective effects. Here, we investigated the effects of DM, three of its analogs (DF, 3-methyl-17-methylmorphinan; AM, 3-allyloxy-17-methoxymorphian; and CM, 3-cyclopropyl-17-methoxymorphinan) and one of its metabolites (HM; 3-methoxymorphinan), on Na(+) channel activity. We used the two-microelectrode voltage-clamp technique to test the effects of DM, DF, AM, CM and HM on Na(+) currents (I(Na)) in Xenopus oocytes expressing cRNAs encoding rat brain Nav1.2 alpha and beta1 or beta2 subunits. In oocytes expressing Na(+) channels, DM, DF, AM and CM, but not HM, induced tonic and use-dependent inhibitions of peak I(Na) following low- and high-frequency stimulations. The order of potency for the inhibition of peak I(Na) was AM-CM > DM=DF. The DM, DF, AM and CM-induced tonic inhibitions of peak I(Na) were voltage-dependent, dose-dependent and reversible. The IC(50) values for DM, DF, AM and CM were 116.7+/-14.9, 175.8+/-16.9, 38.6+/-15.5, and 42.5+/-8.5 microM, respectively. DM and its analogs did not affect the steady-state activation and inactivation voltages. AM and CM, but not DM and DF, inhibited the plateau I(Na) more effectively than the peak I(Na) in oocytes expressing inactivation-deficient I1485Q-F1486Q-M1487Q (IFMQ3) mutant channels; the IC(50) values for AM and CM in this system were 8.4+/-1.3 and 8.7+/-1.3 microM, respectively, for the plateau I(Na) and 43.7+/-5.9 and 32.6+/-7.8 microM, respectively, for the peak I(Na). These results collectively indicate that DM and its analogs could be novel Na(+) channel blockers acting on the resting and open states of brain Na(+) channels.

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.

Muscle pain syndromes

This article summarizes the evidence for two major clinical syndromes of muscle pain: fibromyalgia and myofascial pain syndrome. The evidence for diagnostic and treatment approaches is reviewed.

A phase III randomized, double-blind, placebo-controlled study evaluating dextromethorphan plus slow-release morphine for chronic cancer pain relief in terminally ill patients

This multicenter trial examined the efficacy and safety of dextromethorphan (DM) as an enhancer of analgesia and modulator of opioid tolerance in cancer patients with pain. Eligible patients were randomized to slow-release morphine plus DM or slow-release morphine plus placebo. The initial DM dose was 60 mg four times daily for seven days, with an increase to 120 mg four times daily, if tolerated, for another seven days. During the study, patients recorded medications and scores for pain, nausea, drowsiness, and insomnia. Sixty-five patients were randomized. Although average pain scores (12.6 vs. 15.8), number of breakthrough doses (9 vs. 11.3), and change in total morphine consumption (550.9 mg vs. 597.1mg) were less in the DM group than placebo group, the differences were not statistically significant (P=0.31-0.33). Side-effect scores were not statistically significantly different. Dizziness was greater in the DM (58%) than placebo (36%) group. This study showed a statistically nonsignificant enhancement of analgesia or modulation of opioid tolerance in cancer patients with pain when DM was added to morphine. Participants receiving the DM also had more toxicity, particularly dizziness. This toxicity and the limited evidence of effect do not support the use of DM to enhance opioid analgesia or to modulate opioid tolerance in cancer patients.

The impact of opioid-induced hyperalgesia for postoperative pain

Clinical evidence suggests that--besides their well known analgesic activity - opioids can increase rather than decrease sensitivity to noxious stimuli. Based on the observation that opioids can activate pain inhibitory and pain facilitatory systems, this pain hypersensitivity has been attributed to a relative predominance of pronociceptive mechanisms. Acute receptor desensitization via uncoupling of the receptor from G-proteins, upregulation of the cAMP pathway, activation of the N-methyl-D-aspartate (NMDA)-receptor system, as well as descending facilitation, have been proposed as potential mechanisms underlying opioid-induced hyperalgesia. Numerous reports exist demonstrating that opioid-induced hyperalgesia is observed both in animal and human experimental models. Brief exposures to micro-receptor agonists induce long-lasting hyperalgesic effects for days in rodents, and also in humans large-doses of intraoperative micro-receptor agonists were found to increase postoperative pain and morphine consumption. Furthermore, the prolonged use of opioids in patients is often associated with a requirement for increasing doses and the development of abnormal pain. Successful strategies that may decrease or prevent opioid-induced hyperalgesia include the concomitant administration of drugs like NMDA-antagonists, alpha2-agonists, or non-steroidal anti-inflammatory drugs (NSAIDs), opioid rotation or combinations of opioids with different receptor/selectivity.

Post-treatment of dextromethorphan reverses morphine effect on conditioned place preference in rats

Previously we showed that coadministration of dextromethorphan (DM) with morphine attenuates morphine-rewarding effect. Here we further investigated if DM is effective in reversing or treating drug-seeking effect when given after subchronic morphine treatment. The conditioned place preference (CPP) test was used to investigate the rewarding and drug-seeking effects of morphine. Rats were injected and conditioned with morphine for 6 days and then withdrawn from morphine for 4 days and treated with saline or DM during this period. Subchronic morphine induced a significant place preference for the drug-paired compartment. DM treatment during the morphine withdrawal period significantly reversed the preference from the drug-paired compartment to saline-paired compartment. Both nucleus accumbens (NAc) and VTA were proven to be the sites involved in the action of DM. Behavioral sensitization occurred in both morphine group and DM treatment group determined by the locomotor activity before and after subchronic morphine treatment. The dopamine (DA) turnover rate in the NAc, dorsal striatum (DS) and medial prefrontal cortex (mPFC) was increased after subchronic morphine treatment. DM treatment reversed the increase of DA turnover rate in the NAc but not in the DS or mPFC. These data suggest that DM might have potential in the treatment of morphine addiction.

Alterations of postsynaptic density proteins in the hippocampus of rat offspring from the morphine-addicted mother: Beneficial effect of dextromethorphan

Infants passively exposed to morphine or heroin through their addicted mothers usually develop characteristic withdrawal syndrome of morphine after birth. In such early life, the central nervous system exhibits significant plasticity and can be altered by various prenatal influences, including prenatal morphine exposure. Here we studied the effects of prenatal morphine exposure on postsynaptic density protein 95 (PSD-95), an important cytoskeletal specialization involved in the anchoring of the NMDAR and neuronal nitric oxide synthase (nNOS), of the hippocampal CA1 subregion from young offspring at postnatal day 14 (P14). We also evaluated the therapeutic efficacy of dextromethorphan, a widely used antitussive drug with noncompetitive antagonistic effects on NMDARs, for such offspring. The results revealed that prenatal morphine exposure caused a maximal decrease in PSD-95 expression at P14 followed by an age-dependent improvement. In addition, prenatal morphine exposure reduced not only the expression of nNOS and the phosphorylation of cAMP responsive element-binding protein at serine 133 (CREB(Serine-133)), but also the magnitude of long-term depression (LTD) at P14. Subsequently, the morphine-treated offspring exhibited impaired performance in long-term learning and memory at later ages (P28-29). Prenatal coadministration of dextromethorphan with morphine during pregnancy and throughout lactation could significantly attenuate the adverse effects as described above. Collectively, the study demonstrates that maternal exposure to morphine decreases the magnitude of PSD-95, nNOS, the phosphorylation of CREB(Serine-133), and LTD expression in hippocampal CA1 subregion of young offspring (e.g., P14). Such alterations within the developing brain may play a role for subsequent neurological impairments (e.g., impaired performance of long-term learning and memory). The results raise a possibility that postsynaptic density proteins could serve an important role, at least in part, for the neurobiological pathogenesis in offspring from the morphine-addicted mother and provide tentative therapeutic strategy.

Co-administration of dextromethorphan with methamphetamine attenuates methamphetamine-induced rewarding and behavioral sensitization

Methamphetamine (MA) is well known as a potent CNS stimulant, which produces strong rewarding and behavioral sensitization after repeated administration. In the present study, we investigated whether co-administration of dextromethorphan (DM) with MA could suppress these effects induced by acute and chronic MA treatment. The conditioned place preference (CPP) test was used to examine the rewarding/drug seeking effects and locomotor and stereotypic activities were measured to investigate behavioral sensitization induced by chronic MA. Our results revealed that co-administration of DM (20 mg/kg, ip) with MA (2 mg/kg, ip) almost completely abolished the MA-induced CPP and behavioral sensitization. Furthermore, both of the acute and chronic MA could result in an increase of dopamine (DA) turnover rate in the NAc and mPFC. The acute effects of MA on DA turnover rate could be attenuated by the co-administration of DM in both regions. The chronic effect of MA on DA turnover rate in the mPFC was also attenuated by the co-administration of DM. These results suggest that the effect of DM on blocking MA-induced rewarding and behavioral sensitization may be related to its effect on inhibiting the activity of DA neurons projected to mPFC and/or NAc.

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.

S(+)-ketamine attenuates increase in electroencephalograph activity and amplitude height of sensory-evoked potentials during rapid opioid detoxification

Anesthesia-assisted opioid detoxification offers an opportunity for patients who have undergone unsuccessful conventional detoxifications. Little is known of excitatory effects taking place in the central nervous system during this procedure. Because acute withdrawal is accompanied by N-methyl-d-aspartic acid (NMDA)-receptor activation we tested whether the administration of the nonspecific N-methyl-d-aspartic acid antagonist S(+)-ketamine results in a reduction of hyperactivity in the central nervous system. Thirty-one patients with a long history of opioid abuse were acutely withdrawn with naltrexone during propofol/clonidine anesthesia and mechanical ventilation. Electroencephalogram (EEG) power spectra as well as median nerve-evoked somatosensory potentials (SSEP) were determined at the following times: evening before detoxification (control), steady-state propofol/clonidine-anesthesia, 30 min after naltrexone administration, and 5 min and 60 min after additional S(+)-ketamine (1.5 mg/kg). Compared to steady-state anesthesia, naltrexone induced a decrease by 270% in the low delta (0.5-3 Hz) and an increase by 110% in the fast beta (13-30 Hz) domain of the EEG with only minor changes in the theta-(3-7 Hz) and alpha-(7-13 Hz) band. Simultaneously, mean amplitude of the late N(100) peak of the SSEP increased from 3.3 muV to 10.5 muV. The changes could be attenuated by the additional administration of S(+)-ketamine, 5 min and 60 min after injection. Cardiovascular changes were not a reliable index for monitoring acute withdrawal symptoms and determining termination of rapid opioid detoxification. In this regard, EEG power spectra and SSEP were more consistent and clinically useful variables. S(+)-ketamine is a valuable adjunct in the anesthetic regimen, since it attenuates hyperactivity of the central nervous system during rapid opioid detoxification.

Effects of dextrorotatory morphinans on α3β4 nicotinic acetylcholine receptors expressed in Xenopus oocytes

We previously demonstrated that dextromethorphan (DM; 3-methoxy-17-methylmorphinan) analogs have neuroprotective effects, and a recent report showed that DM reduces the adverse effects of morphine and blocks alpha3beta4 nicotinic acetylcholine receptors, a major target of anti-addictive agents. Here, we investigated the effects of DM, three of its analogs (DF, 3-methyl-17-methylmorphinan; AM, 3-allyloxy-17-methoxymorphian; and CM, 3-cyclopropyl-17-methoxymorphinan) and one of its metabolites (HM; 3-methoxymorphinan), on neuronal alpha3beta4 nicotinic acetylcholine receptor channel activity expressed in Xenopus laevis oocytes, using the two-microelectrode voltage clamp technique. We found that intraoocyte injection of neuronal alpha3 and beta4 nicotinic acetylcholine receptor subunit cRNAs elicited an inward current (IACh) in the presence of acetylcholine. Co-treatment with DM, DF, AM, CM or HM inhibited IACh in a dose-dependent, voltage-independent and reversible manner. The IC50 values for DM, DF, AM, CM and HM were 19.5+/-5.2, 15.8+/-4.5, 16.3+/-1.7, 10.1+/-2.8, and 13.5+/-4.0 microM, respectively. The order of potency for the inhibition of IACh was CM>HM>DF=AM>DM in oocytes expressing alpha3beta4 nicotinic acetylcholine receptors. The inhibitions of (IACh) by DM, DF and HM, AM and CM were non-competitive. These results indicate that AM, CM and HM could be novel non-competitive agents regulating alpha3beta4 nicotinic acetylcholine receptor channel activity.

Citalopram counteracts depressive-like symptoms evoked by chronic social stress in rats

Recently, we have described a new model of chronic social stress in rats, based on the resident-intruder paradigm. In this model, rats show behavioural changes that may be considered correlates of depressive symptoms, such as anhedonia and motivational deficits. The present study was designed for pharmacological validation of this model. Animals were socially stressed for 5 weeks and, in parallel, after the first week of stress, they were subjected to chronic (4 weeks) treatment with the antidepressant drug citalopram. The drug was administered via drinking water (30 mg/kg). The optimal dose of citalopram was determined in a pilot study. After 4 weeks of treatment, plasma levels of citalopram and its metabolite were found to be within the human therapeutic range. The effects of social stress and citalopram treatment were assessed by behavioural tests. Chronically stressed rats showed reduced locomotor and exploratory activity, reduced sucrose preference and increased immobility time in the forced swimming test. Chronic oral administration of citalopram abolished those effects and normalized behaviours related to motivation and reward sensitivity. These observations provide evidence for the predictive validity of the chronic social stress paradigm as a model of depressive symptoms in rats.

Phantom Limb Pain Treatment with Mirtazapine: A Case Series

OBJECTIVES

To pilot the efficacy of mirtazapine for relief of phantom limb pain (PLP); to correlate the putative drug mechanism with theoretical PLP mechanisms; and to develop a rationale for further study of mirtazapine in this population.

DESIGN

Open-label case series.

SUBJECTS/PATIENTS

Four individuals with PLP for at least 3 months after amputation.

METHODS

All subjects received oral mirtazapine between 7.5 and 30 mg/day. An 11-point numeric rating scale (0 to 10) measured pain intensity and relief during monitored outpatient follow-up visits.

RESULTS/DISCUSSION

Mirtazapine use improved the PLP experienced by these subjects by at least 50%. Subjects with PLP-related sleeping difficulties reported the greatest pain relief concomitant with improved sleep quality. One subject was able to eliminate the use of a selective serotonin reuptake inhibitor antidepressant while using mirtazapine for PLP and depression without change in mood or affect. Mirtazapine enhances noradrenergic and serotonergic activity and may modulate PLP by central mechanisms. Current concepts of the proposed pathophysiology of PLP and the hypothetical impact of mirtazapine are discussed.

CONCLUSION

Mirtazapine may be an effective treatment for PLP that can also potentially enhance sleep and mood. This information provides preliminary reinforcement for more formal, controlled studies concerning mirtazapine use in PLP.

Opioidvermittelte Analgesie und Hyperalgesie

Opioids are frequently used for the treatment of moderate to severe acute and chronic pain. However, clinical evidence suggests that opioids can elicit increased sensitivity to noxious stimuli suggesting that administration of opioids can activate both, pain inhibitory and pain facilitatory systems. Acute receptor desensitization via uncoupling of the receptor from G proteins, upregulation of the cAMP pathway, activation of the N-methyl-D-aspartate (NMDA) receptor system and descending facilitation have been proposed as potential mechanisms underlying opioid-induced hyperalgesia. The tolerance results from a pain sensitization process more than from a decrease in the opioid effectiveness. Opioid-induced hyperalgesia and tolerance are observed both in animal and human experimental models. Brief exposures to mu-receptor agonists induce long-lasting hyperalgesic effects for days. Furthermore, the prolonged use of opioids in patients often requires increasing doses and may be accompanied by the development of abnormal pain. Successful strategies that may decrease or prevent opioid-induced hyperalgesia include the concomitant administration of drugs such as NMDA antagonists, alpha(2)-agonists, or nonsteroidal anti-inflammatory drugs (NSAID), opioid rotation, or combinations of opioids with different receptor selectivity.

Dextromethorphan differentially affects opioid antinociception in rats

Opioid drugs such as morphine and meperidine are widely used in clinical pain management, although they can cause some adverse effects. A number of studies indicate that N-methyl-D-aspartate (NMDA) receptors may play a role in the mechanism of morphine analgesia, tolerance and dependence. Being an antitussive with NMDA antagonist properties, dextromethorphan (DM) may have some therapeutic benefits when coadministered with morphine. In the present study, we investigated the effects of DM on the antinociceptive effects of different opioids. We also investigated the possible pharmacokinetic mechanisms involved. The antinociceptive effects of the mu-opioid receptor agonists morphine (5 mg kg(-1), s.c.), meperidine (25 mg kg(-1), s.c.) and codeine (25 mg kg(-1), s.c.), and the kappa-opioid agonists nalbuphine (8 mg kg(-1), s.c.) and U-50,488H (20 mg kg(-1), s.c.) were studied using the tail-flick test in male Sprague-Dawley rats. Coadministration of DM (20 mg kg(-1), i.p.) with these opioids was also performed and investigated. The pharmacokinetic effects of DM on morphine and codeine were examined, and the free concentration of morphine or codeine in serum was determined by HPLC.It was found that DM potentiated the antinociceptive effects of some mu-opioid agonists but not codeine or kappa-opioid agonists in rats. DM potentiated morphine's antinociceptive effect, and acutely increased the serum concentration of morphine. In contrast, DM attenuated the antinociceptive effect of codeine and decreased the serum concentration of its active metabolite (morphine). The pharmacokinetic interactions between DM and opioids may partially explain the differential effects of DM on the antinociception caused by opioids.