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

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.

Co-administration of dextromethorphan and morphine: reduction of post-operative pain and lack of influence on morphine metabolism

We investigated co-analgesic effect of dextromethorphan in adolescent post-operative patients with idiopathic scoliosis. In a double-blind study, 60 patients with ASA physical status I-II were randomised into two groups. Group dextromethorphan (n = 30; age: 15.9 +/- 2.4 years) was given oral dextromethorphan 30 or 45 mg 1 hr before surgery and 8, 20 and 32 hr after operation. Group placebo (n = 30; age: 16.5 +/- 2.7 years) received placebo at identical times. Post-operative analgesic requirements were assessed using nurse-controlled analgesia system. Pain was assessed using numeric rating scale before first administration of morphine and at 2, 3, 4, 6, 24 and 48 hr after operation. Blood samples were taken 20 min. after the first use of morphine (within 1 hr after operation). The total use of analgesics during surgery was lower in the dextromethorphan group. The dose of morphine providing relief immediately after surgery, as well as total analgesic requirements in the first and second day after surgery did not differ between groups. Subjectively evaluated pain intensity score (numeric rating scale) was lower for the dextromethorphan patients in the first 4 hr, but not later after surgery. Plasma levels of morphine, morphine-6-glucuronide and morphine-3-glucuronide did not differ between groups. Dextromethorphan did not influence morphine glucuronidation, in terms of promotion of formation of any morphine glucuronides. In conclusion, in young patients subjected to spine surgery, addition of dextromethorphan to morphine reduced pain only in early post-operative period. In such patients, co-analgesic action of dextromethorphan was not associated with significant changes in plasma levels of morphine metabolites.

A combined effect of dextromethorphan and melatonin on neuropathic pain behavior in rats

Previous study has shown that administration of melatonin into the anterior cingulate cortex contralateral to peripheral nerve injury prevented exacerbation of mechanical allodynia with a concurrent improvement of depression-like behavior in Wistar-Kyoto (WKY) rats, a genetic variation of Wistar rats. In the present study, we examined the effect of the individual versus combined treatment of melatonin and/or dextromethorphan (DM), a clinically available N-methyl-d-aspartate (NMDA) receptor antagonist, on pain behaviors in WKY rats with chronic constriction sciatic nerve injury (CCI). Pain behaviors (thermal hyperalgesia and mechanical allodynia) were established at one week after CCI. WKY rats were then treated intraperitoneally with various doses of melatonin, DM or their combination once daily for the following week. At the end of this one-week treatment, behavioral tests were repeated in these same rats. While DM alone was effective in reducing thermal hyperalgesia at three tested doses (15, 30 or 60 mg/kg), it reduced mechanical allodynia only at high doses (30 or 60 mg/kg). By comparison, administration of melatonin alone was effective in reducing thermal hyperalgesia only at the highest dose (120 mg/kg, but not 30 or 60 mg/kg) tested in this experiment. Melatonin alone failed to reverse allodynia at all three tested doses (30, 60 and 120 mg/kg). However, the combined intraperitoneal administration of melatonin (30 mg/kg) and DM (15 mg/kg) effectively reversed both thermal hyperalgesia and mechanical allodynia although each individual dose alone did not reduce pain behaviors. These results suggest that a combination of melatonin with a clinically available NMDA receptor antagonist might be more effective than either drug alone for the treatment of neuropathic pain.

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.

NADPH oxidase as a therapeutic target in Alzheimer's disease

At present, available treatments for Alzheimer's disease (AD) are largely unable to halt disease progression. Microglia, the resident macrophages in the brain, are strongly implicated in the pathology and progressively degenerative nature of AD. Specifically, microglia are activated in response to both β amyloid (Aβ) and neuronal damage, and can become a chronic source of neurotoxic cytokines and reactive oxygen species (ROS). NADPH oxidase is a multi-subunit enzyme complex responsible for the production of both extracellular and intracellular ROS by microglia. Importantly, NADPH oxidase expression is upregulated in AD and is an essential component of microglia-mediated Aβ neurotoxicity. Activation of microglial NADPH oxidase causes neurotoxicity through two mechanisms: 1) extracellular ROS produced by microglia are directly toxic to neurons; 2) intracellular ROS function as a signaling mechanism in microglia to amplify the production of several pro-inflammatory and neurotoxic cytokines (for example, tumor necrosis factor-α, prostaglandin E2, and interleukin-1β). The following review describes how targeting NADPH oxidase can reduce a broad spectrum of toxic factors (for example, cytokines, ROS, and reactive nitrogen species) to result in inhibition of neuronal damage from two triggers of deleterious microglial activation (Aβ and neuron damage), offering hope in halting the progression of AD.

The perioperative management of pain from intracranial surgery

Analgesic therapy following intracranial procedures remains a source of concern and controversy. Although opioids are the mainstay of the "balanced" general anesthetic techniques frequently used during intracranial procedures, neurosurgeons and others have been reluctant to administer opioid analgesics to patients following such procedures. This practice is supported by the concern that the sedation and miosis associated with opioid administration could mask the early signs of intracranial catastrophe, or even exacerbate it through decreased ventilatory drive, elevated arterial carbon dioxide levels, and increased cerebral blood flow. This reluctance to use opioids following intracranial surgery is enabled by decades of training and anecdote emphasizing that pain is minimal following these procedures. However, recent data suggests otherwise, and raises the question of how to provide safe and effective analgesia for these patients. Here, this data is reviewed along with the relevant pain pathways, analgesic drugs and techniques, and the available data on their use following intracranial surgery. Although pain following intracranial surgery appears to be more intense than initially believed, it is readily treated safely and effectively with techniques that have proven useful following other types of surgery, including patient-controlled administration of opioids. The use of multimodal analgesic therapy is emphasized not only for its effectiveness, but to reduce dosages and, therefore, side effects, primarily of the opioids, that could be of legitimate concern to physicians and affect the comfort of their patients.