Pharmacological modulation of cortical excitability shifts induced by transcranial direct current stimulation in humans

Transcranial direct current stimulation (tDCS) of the human motor cortex results in polarity-specific shifts of cortical excitability during and after stimulation. Anodal tDCS enhances and cathodal stimulation reduces excitability. Animal experiments have demonstrated that the effect of anodal tDCS is caused by neuronal depolarisation, while cathodal tDCS hyperpolarises cortical neurones. However, not much is known about the ion channels and receptors involved in these effects. Thus, the impact of the sodium channel blocker carbamazepine, the calcium channel blocker flunarizine and the NMDA receptor antagonist dextromethorphane on tDCS-elicited motor cortical excitability changes of healthy human subjects were tested. tDCS-protocols inducing excitability alterations (1) only during tDCS and (2) eliciting long-lasting after-effects were applied after drug administration. Carbamazepine selectively eliminated the excitability enhancement induced by anodal stimulation during and after tDCS. Flunarizine resulted in similar changes. Antagonising NMDA receptors did not alter current-generated excitability changes during a short stimulation, which elicits no after-effects, but prevented the induction of long-lasting after-effects independent of their direction. These results suggest that, like in other animals, cortical excitability shifts induced during tDCS in humans also depend on membrane polarisation, thus modulating the conductance of sodium and calcium channels. Moreover, they suggest that the after-effects may be NMDA receptor dependent. Since NMDA receptors are involved in neuroplastic changes, the results suggest a possible application of tDCS in the modulation or induction of these processes in a clinical setting. The selective elimination of tDCS-driven excitability enhancements by carbamazepine proposes a role for this drug in focussing the effects of cathodal tDCS, which may have important future clinical applications.

Pharmacological approach to the mechanisms of transcranial DC-stimulation-induced after-effects of human motor cortex excitability

Weak transcranial direct current stimulation (tDCS) induces persisting excitability changes in the human motor cortex. These plastic excitability changes are selectively controlled by the polarity, duration and current strength of stimulation. To reveal the underlying mechanisms of direct current (DC)-induced neuroplasticity, we combined tDCS of the motor cortex with the application of Na(+)-channel-blocking carbamazepine (CBZ) and the N-methyl-D-aspartate (NMDA)-receptor antagonist dextromethorphan (DMO). Monitored by transcranial magnetic stimulation (TMS), motor cortical excitability changes of up to 40% were achieved in the drug-free condition. Increase of cortical excitability could be selected by anodal stimulation, and decrease by cathodal stimulation. Both types of excitability change lasted several minutes after cessation of current stimulation. DMO suppressed the post-stimulation effects of both anodal and cathodal DC stimulation, strongly suggesting the involvement of NMDA receptors in both types of DC-induced neuroplasticity. In contrast, CBZ selectively eliminated anodal effects. Since CBZ stabilizes the membrane potential voltage-dependently, the results reveal that after-effects of anodal tDCS require a depolarization of membrane potentials. Similar to the induction of established types of short- or long-term neuroplasticity, a combination of glutamatergic and membrane mechanisms is necessary to induce the after-effects of tDCS. On the basis of these results, we suggest that polarity-driven alterations of resting membrane potentials represent the crucial mechanisms of the DC-induced after-effects, leading to both an alteration of spontaneous discharge rates and to a change in NMDA-receptor activation.

A temporally asymmetric Hebbian rule governing plasticity in the human motor cortex

Synaptic plasticity is conspicuously dependent on the temporal order of the pre- and postsynaptic activity. Human motor cortical excitability can be increased by a paired associative stimulation (PAS) protocol. Here we show that it can also be decreased by minimally changing the interval between the two associative stimuli. Corticomotor excitability of the abductor pollicis brevis (APB) representation was tested before and after repetitively pairing of single right median nerve simulation with single pulse transcranial magnetic stimulation (TMS) delivered over the optimal site for activation of the contralateral APB. Following PAS, depression of TMS-evoked motor-evoked potentials (MEPs) was induced only when the median nerve stimulation preceded the TMS pulse by 10 ms, while enhancement of cortical excitability was induced using an interstimulus interval of 25 ms, suggesting an important role of the sequence of cortical events triggered by the two stimulation modalities. Experiments using F-wave studies and electrical brain stem stimulation indicated that the site of the plastic changes underlying the decrease of MEP amplitudes following PAS (10 ms) was within the motor cortex. MEP amplitudes remained depressed for approximately 90 min. The decrease of MEP amplitudes was blocked when PAS(10 ms) was performed under the influence of dextromethorphan, an N-methyl-d-aspartate-receptor antagonist, or nimodipine, an L-type voltage-gated calcium-channel antagonist. The physiological profile of the depression of human motor cortical excitability following PAS(10 ms) suggests long-term depression of synaptic efficacy to be involved. Together with earlier findings, this study suggests that strict temporal Hebbian rules govern the induction of long-term potentiation/long-term depression-like phenomena in vivo in the human primary motor cortex.

Mechanisms of use-dependent plasticity in the human motor cortex

Practicing movements results in improvement in performance and in plasticity of the motor cortex. To identify the underlying mechanisms, we studied use-dependent plasticity in human subjects premedicated with drugs that influence synaptic plasticity. Use-dependent plasticity was reduced substantially by dextromethorphan (an N-methyl-d-aspartate receptor blocker) and by lorazepam [a gamma-aminobutyric acid (GABA) type A receptor-positive allosteric modulator]. These results identify N-methyl-d-aspartate receptor activation and GABAergic inhibition as mechanisms operating in use-dependent plasticity in intact human motor cortex and point to similarities in the mechanisms underlying this form of plasticity and long-term potentiation.

Dextromethorphan decreases the excitability of the human motor cortex

OBJECTIVE

To assess the acute effects of dextromethorphan (DM) on human motor cortical excitability.

BACKGROUND

DM, a noncompetitive N-methyl-D-aspartate receptor antagonist, has recently attracted clinical interest for its potential as a neuroprotective agent in various models of excitotoxicity. We were interested in learning whether this drug can modulate the excitability of the motor cortex in healthy subjects.

METHODS

The effects of DM on the excitability of the normal human motor cortex were studied in eight healthy volunteers by means of focal transcranial magnetic stimulation before and 1.5, 4, 6.5, and 24 hours after a single oral dose of 150 mg DM. Motor evoked potentials (MEPs) were recorded from the relaxed abductor digiti minimi muscle. Measures of motor cortical excitability were motor threshold, MEP recruitment, duration of the cortical silent period, and intracortical inhibition and facilitation. In addition, the authors explored spinal and neuromuscular excitability by means of F waves, duration of the peripheral silent period, and maximum M wave.

RESULTS

Intracortical inhibition increased temporarily, intracortical facilitation decreased, and the cortical silent period lengthened slightly. Motor threshold, MEP recruitment, and spinal and peripheral motor excitability were not affected significantly.

CONCLUSIONS

Our findings suggest that DM can exert a significant suppression of the excitatory drive in the normal human cortex, which may be relevant for its potential therapeutic use in excitotoxicity-related neurologic disease. Furthermore, the noninvasive technique described may prove useful in preclinical studies to assess the effects on motor cortical excitability induced by new modulators of glutamatergic transmission currently under development.

Modulation of human corticomotor excitability by somatosensory input

In humans, somatosensory stimulation results in increased corticomotoneuronal excitability to the stimulated body parts. The purpose of this study was to investigate the underlying mechanisms. We recorded motor evoked potentials (MEPs) to transcranial magnetic stimulation (TMS) from abductor pollicis brevis (APB), first dorsal interosseous (FDI), and abductor digiti minimi (ADM) muscles. MEP amplitudes, recruitment curves (RC), intracortical inhibition (ICI), intracortical facilitation (ICF), resting (rMT) and active motor thresholds (aMT) were recorded before and after a 2-h period of ulnar nerve electrical stimulation at the wrist. Somatosensory input was monitored by recording somatosensory evoked potentials. To differentiate excitability changes at cortical vs. subcortical sites, we recorded supramaximal peripheral M-responses and MEPs to brainstem electrical stimulation (BES). In order to investigate the involvement of GABAergic mechanisms, we studied the influence of lorazepam (LZ) (a GABA(A) receptor agonist) relative to that of dextromethorphan (DM) (an NMDA receptor antagonist) and placebo in a double-blind design. We found that somatosensory stimulation increased MEP amplitudes to TMS only in the ADM, confirming a previous report. This effect was blocked by LZ but not by either DM or placebo and lasted between 8 and 20 min in the absence of (i) changes in MEPs elicited by BES, (ii) amplitudes of early somatosensory-evoked potentials or (iii) M-responses. We conclude that somatosensory stimulation elicited a focal increase in corticomotoneuronal excitability that outlasts the stimulation period and probably occurs at cortical sites. The antagonistic effect of LZ supports the hypothesis of GABAergic involvement as an operating mechanism.

Inhibition by fluoxetine of cytochrome P450 2D6 activity

Potent inhibition of cytochrome P450 2D6 (CYP2D6) in human liver microsomes by fluoxetine and its major metabolite norfluoxetine was confirmed (apparent inhibition constant values, 0.2 mumol/L). Several other serotonergic agents were also found to be competitive inhibitors of this genetically polymorphic enzyme. The O-demethylation ratio of dextromethorphan that expressed CYP2D6 activity in 19 patients receiving fluoxetine fell in the region of the antimode separating the O-demethylation ratio values observed in 208 extensive metabolizers from 15 poor metabolizers of a control group of healthy subjects. Inhibition of CYP2D6 activity in patients undergoing treatment with fluoxetine or other serotonin uptake inhibitors could contribute to toxicity or attenuated response from concurrent medications that are substrates of this enzyme. Other in vitro studies indicated that CYP2D6 catalyzes the O-demethylation of oxycodone to form oxymorphone. This reaction was inhibited by fluoxetine and its normetabolite in liver microsomes from both extensive and poor metabolizer individuals, indicating that these compounds are not selective inhibitors of CYP2D6 activity.

Dextromethorphan and neuromodulation: old drug coughs up new activities

Dextromethorphan is one of the most widely used non-opioid cough suppressants, representing the active ingredient in several over-the-counter antitussive formulations. It does not possess the CNS pharmacology of other opiates in humans (i.e. analgesia, respiratory depression, abuse liability or psychotomimetic properties), but since the discovery in 1981 of high affinity recognition sites in brain for dextromethorphan a unique neuropharmacological profile has emerged for this relatively innocuous drug. Anticonvulsant and neuroprotective properties have been demonstrated, and treatment with dextromethorphan has been shown to improve the cerebrovascular and functional consequences of global cerebral ischemia. Frank Tortella and colleagues review the CNS pharmacology of dextromethorphan, its possible involvement with NMDA or sigma-receptors, and the potential clinical importance of this old 'new' drug.

Neuroprotective effect of dextromethorphan in the MPTP Parkinson's disease model: role of NADPH oxidase

Parkinson's disease (PD) is a neurodegenerative movement disorder characterized by a progressive loss of dopaminergic neurons in the substantia nigra and depletion of the neurotransmitter dopamine in the striatum. Progress in the search for effective therapeutic strategies that can halt this degenerative process remains limited. Mechanistic studies using animal systems such as the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) rodent PD model have revealed the involvement of the brain's immune cells and free radical-generating processes. We recently reported that dextromethorphan (DM), a widely used anti-tussive agent, attenuated endotoxin-induced dopaminergic neurodegeneration in vitro. In the current study, we investigated the potential neuroprotective effect of DM and the underlying mechanism of action in the MPTP rodent PD model. Mice (C57BL/6J) that received daily MPTP injections (15 mg free base/kg body weight, s.c.) for 6 consecutive days exhibited significant degeneration of the nigrostriatal dopaminergic pathway. However, the MPTP-induced loss of nigral dopaminergic neurons was significantly attenuated in those mice receiving DM (10 mg/kg body weight, s.c.). In mesencephalic neuron-glia cultures, DM significantly reduced the MPTP-induced production of both extracellular superoxide free radicals and intracellular reactive oxygen species (ROS). Because NADPH oxidase is the primary source of extracellular superoxide and intracellular ROS, we investigated the involvement of NADPH oxidase in the neuroprotective effect of DM. Indeed, the neuroprotective effect of DM was only observed in the wild-type but not in the NADPH oxidase-deficient mice, indicating that NADPH oxidase is a critical mediator of the neuroprotective activity of DM. More importantly, due to its proven safety record of long-term clinical use in humans, DM may be a promising agent for the treatment of degenerative neurological disorders such as PD.

Dextromethorphan protects dopaminergic neurons against inflammation-mediated degeneration through inhibition of microglial activation

Inflammation in the brain has increasingly been recognized to play an important role in the pathogenesis of several neurodegenerative disorders, including Parkinson's disease and Alzheimer's disease. Inflammation-mediated neurodegeneration involves activation of the brain's resident immune cells, the microglia, which produce proinflammatory and neurotoxic factors, including cytokines, reactive oxygen intermediates, nitric oxide, and eicosanoids that impact on neurons to induce neurodegeneration. Hence, identification of compounds that prevent microglial activation may be highly desirable in the search for therapeutic agents for inflammation-mediated neurodegenerative diseases. In this study, we report that dextromethorphan (DM), an ingredient widely used in antitussive remedies, reduced the inflammation-mediated degeneration of dopaminergic neurons through inhibition of microglial activation. Pretreatment (30 min) of rat mesencephalic neuron-glia cultures with DM (1-10 micro M) reduced, in a dose-dependent manner, the microglia-mediated degeneration of dopaminergic neurons induced by lipopolysaccharide (LPS, 10 ng/ml). Significant neuroprotection by DM was also evident when DM was applied to cultures up to 60 min after the addition of LPS. The neuroprotective effect of DM was attributed to inhibition of LPS-stimulated microglial activation because DM significantly inhibited the LPS-induced production of tumor necrosis factor-alpha, nitric oxide, and superoxide free radicals. This conclusion was further supported by the finding that DM failed to prevent 1-methyl-4-phenylpyridinium- or beta-amyloid peptide (1-42)-induced dopaminergic neurotoxicity in neuron-enriched cultures. In addition, because LPS did not produce any significant increase in the release of excitatory amino acids from neuron-glia cultures and N-methyl-D-aspartate antagonist dizocilpine maleate failed to afford significant neuroprotection, it is unlikely that the neuroprotective effect of DM is mediated through N-methyl-D-aspartate receptors. These results suggest that DM may be a promising therapeutic agent for the treatment of Parkinson's disease.

Effect of honey, dextromethorphan, and no treatment on nocturnal cough and sleep quality for coughing children and their parents

OBJECTIVES

To compare the effects of a single nocturnal dose of buckwheat honey or honey-flavored dextromethorphan (DM) with no treatment on nocturnal cough and sleep difficulty associated with childhood upper respiratory tract infections.

DESIGN

A survey was administered to parents on 2 consecutive days, first on the day of presentation when no medication had been given the prior evening and then the next day when honey, honey-flavored DM, or no treatment had been given prior to bedtime according to a partially double-blinded randomization scheme.

SETTING

A single, outpatient, general pediatric practice.

PARTICIPANTS

One hundred five children aged 2 to 18 years with upper respiratory tract infections, nocturnal symptoms, and illness duration of 7 days or less.

INTERVENTION

A single dose of buckwheat honey, honey-flavored DM, or no treatment administered 30 minutes prior to bedtime.

MAIN OUTCOME MEASURES

Cough frequency, cough severity, bothersome nature of cough, and child and parent sleep quality.

RESULTS

Significant differences in symptom improvement were detected between treatment groups, with honey consistently scoring the best and no treatment scoring the worst. In paired comparisons, honey was significantly superior to no treatment for cough frequency and the combined score, but DM was not better than no treatment for any outcome. Comparison of honey with DM revealed no significant differences.

CONCLUSIONS

In a comparison of honey, DM, and no treatment, parents rated honey most favorably for symptomatic relief of their child's nocturnal cough and sleep difficulty due to upper respiratory tract infection. Honey may be a preferable treatment for the cough and sleep difficulty associated with childhood upper respiratory tract infection.

TRIAL REGISTRATION

clinicaltrials.gov Identifier: NCT00127686.

Isobolographic analysis for combinations of a full and partial agonist: curved isoboles

Combinations of drugs are frequently used therapeutically to achieve an enhanced effect without using an excess quantity of either agent. If the drugs exert overtly similar action, e.g., two analgesics, the effect of the combination may be tested for additivity, i.e., an effect level that is achieved based on the individual drug potencies. But combinations of agonists will sometimes display either superadditive (synergistic) or subadditive responses. Whether the two agonists are both drugs, or a combination of a drug and an endogenous chemical, there is interest in characterizing the interaction to determine whether it departs from additivity because quantitative information of this kind, aside from its therapeutic importance, may also illuminate mechanism. A common method for this characterization uses the isobologram. This is a plot in rectangular coordinates of dose combinations (a,b) that produce the same effect level (often taken to be 50% of the maximum). In its usual form, this plot is constructed as a straight line (of additivity) connecting intercepts that represent the individually effective doses, e.g., ED50 values of each. This line is the reference for distinguishing additive from nonadditive interactions accordingly as the tested combination is on or off this line. Discussed here are the assumptions that underlie this linear plot. Specifically we show that a combination of drugs with a variable potency ratio, exemplified by a full and a partial agonist, lead to curvilinear isoboles of additivity that may erroneously be attributed to either synergism or subadditivity.

Glutamate neurotoxicity in spinal cord cell culture

The neurotoxicity of glutamate was investigated quantitatively in mixed neuronal and glial spinal cord cell cultures from fetal mice at 12-13 days of gestation. Five-minute exposure to 10-1000 microM glutamate produced widespread acute neuronal swelling, followed by neuronal degeneration over the next 24 h (EC50 for death about 100-200 microM); glia were not injured. Glutamate was neurotoxic in cultures as young as four days in vitro, although greater death was produced in older cultures. By 14-20 days in vitro, 80-90% of the neuronal population was destroyed by a 5-min exposure to 500 microM glutamate. Acute neuronal swelling following glutamate exposure was prevented by replacement of extracellular sodium with equimolar choline, with minimal reduction in late cell death. Removal of extracellular calcium enhanced acute neuronal swelling but attenuated late neuronal death. Both acute neuronal swelling and late degeneration were effectively blocked by the noncompetitive N-methyl-D-aspartate receptor antagonist dextrorphan and by the novel competitive antagonist CGP 37849. Ten micromolar 7-chlorokynurenate also inhibited glutamate neurotoxicity; protection was reversed by the addition of 1 mM glycine to the bathing medium. These observations suggest that glutamate is a potent and rapidly acting neurotoxin on cultured spinal cord neurons, and support involvement of excitotoxicity in acute spinal cord injury. Similar to telencephalic neurons, spinal neurons exposed briefly to glutamate degenerate in a manner dependent on extracellular Ca2+ and the activation of N-methyl-D-aspartate receptors.

Dextrorphan and dextromethorphan attenuate glutamate neurotoxicity

The dextrorotatory morphinan opioid, dextrorphan, which has recently been reported to block the excitation of cortical neurons by N-methyl-D-aspartate, was found at 10-100 microM concentrations to attenuate both morphological and chemical evidence of glutamate neurotoxicity in murine neocortical cell cultures; a similar effect was found with its methyl ester derivative, dextromethorphan. Given other data suggesting that glutamate neurotoxicity may participate in the pathogenesis of the central neuronal loss associated with certain human neurological diseases, the present observations raise the possibility that these clinically tested opioids, or related compounds, may eventually prove to have some clinical therapeutic utility.

Pharmacology of dextromethorphan: Relevance to dextromethorphan/quinidine (Nuedexta®) clinical use

Dextromethorphan (DM) has been used for more than 50years as an over-the-counter antitussive. Studies have revealed a complex pharmacology of DM with mechanisms beyond blockade of N-methyl-d-aspartate (NMDA) receptors and inhibition of glutamate excitotoxicity, likely contributing to its pharmacological activity and clinical potential. DM is rapidly metabolized to dextrorphan, which has hampered the exploration of DM therapy separate from its metabolites. Coadministration of DM with a low dose of quinidine inhibits DM metabolism, yields greater bioavailability and enables more specific testing of the therapeutic properties of DM apart from its metabolites. The development of the drug combination DM hydrobromide and quinidine sulfate (DM/Q), with subsequent approval by the US Food and Drug Administration for pseudobulbar affect, led to renewed interest in understanding DM pharmacology. This review summarizes the interactions of DM with brain receptors and transporters and also considers its metabolic and pharmacokinetic properties. To assess the potential clinical relevance of these interactions, we provide an analysis comparing DM activity from in vitro functional assays with the estimated free drug DM concentrations in the brain following oral DM/Q administration. The findings suggest that DM/Q likely inhibits serotonin and norepinephrine reuptake and also blocks NMDA receptors with rapid kinetics. Use of DM/Q may also antagonize nicotinic acetylcholine receptors, particularly those composed of α3β4 subunits, and cause agonist activity at sigma-1 receptors.

Plasma levels of TNF-alpha and IL-6 are inversely related to cytochrome P450-dependent drug metabolism in patients with congestive heart failure

BACKGROUND

Cytochrome P450 (CYP) enzymes are important mediators of drug metabolism, and activity of these enzymes is a major determinant of the duration and intensity of drug effect. Circulating plasma concentrations of pro-inflammatory cytokines (e.g., tumor necrosis factor [TNF]-alpha and interleukin [IL]-6) are elevated in patients with heart failure and these cytokines have been shown to down-regulate CYP enzyme activity. The purpose of this study was to evaluate the relationship between plasma cytokine concentrations and CYP enzyme activities in patients with heart failure.

METHODS AND RESULTS

Sixteen patients with congestive heart failure (New York Heart Association classes II-IV) received a metabolic probe cocktail consisting of caffeine, mephenytoin, dextromethorphan, and chlorzoxazone to assess the activities of the CYP enzymes 1A2, 2C19, 2D6, and 2E1. Blood and urine samples were collected for drug and metabolite determinations by high-performance liquid chromatography (HPLC); cytokine concentrations were measured by enzyme-linked immunosorbent assay (ELISA). We found a striking inverse relationship between both TNF-alpha and IL-6 plasma concentrations and the activity of CYP2C19; metabolism of caffeine (CYP1A2) also had a negative association with IL-6 plasma concentrations.

CONCLUSIONS

Cytokine-mediated decreases in drug metabolism may contribute to observed variability in drug response and augment the risk of adverse drug effects in CHF patients.

Dextromethorphan inhibits NMDA-induced convulsions

Dextromethorphan, its metabolite dextrorphan, phencyclidine, ketamine, MK-801, 3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid and DL-2-amino-7-phosphonoheptanoic acid were evaluated for potency to antagonize N-methyl-D-aspartate-induced convulsions following intraperitoneal administration using male CF-1 mice. Whereas reference anticonvulsants (e.g., phenytoin) were ineffective in this model, dextromethorphan and all competitive and noncompetitive N-methyl-D-aspartate antagonists blocked seizures. The results are consistent with the interpretation that dextromethorphan elicits some of its pharmacological responses via an interaction with receptors for excitatory amino acids.

Dextrorphan and dextromethorphan, common antitussives, are antiepileptic and antagonize N-methyl-D-aspartate in brain slices

The antitussive, dextromethorphan (DM), and its metabolite, dextrorphan (DX), were evaluated for antiepileptic properties in vitro. Interictal bursts and prolonged ictal epileptiform afterdischarges, induced by perfusion of guinea pig neocortical brain slices with Mg2+-free solution, were blocked by DX (1-250 microM) or DM (100 microM). Intracellular records showed that these agents blocked N-methyl-D-aspartate (NMDA)-induced depolarizations without altering intrinsic membrane properties. DX blocked NMDA but not quisqualate-evoked multi-unit excitatory responses. DM is a widely available, orally effective drug with low toxicity in antitussive doses, which has antiepileptic and NMDA-antagonist properties in vitro. Its toxicity and effectiveness as an anticonvulsant should be expeditiously examined in clinical trials.

Dextromethorphan blocks N-methyl-D-aspartate-induced currents and voltage-operated inward currents in cultured cortical neurons

The effect of dextromethorphan on several types of cation currents in cultured rat cortical neurons and PC12 cells was studied by using the whole-cell configuration of the patch-clamp technique. The Ba2+ current through L- and N-type Ca2+ channels was blocked with similar potencies (52-71 microM) in both types of cells. The effect was not voltage-dependent, in contrast to that of amlodipine (a dihydropyridine). Dextromethorphan was able to block the Ba2+ current completely unlike amlodipine and omega-conotoxin (an N-type channel blocker) which produced only partial inhibition. The voltage-activated Na+ and Ca2+ channels in cortical neurons were inhibited by similar concentrations of dextromethorphan (IC50 approximately 80 microM). The morphinan was at least 100 times more potent (IC50 = 0.55 microM) as a blocker of the current induced by N-methyl-D-aspartate (NMDA) in cortical neurons. Currents induced by (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid ((RS)-AMPA) or kainic acid were not significantly affected even at 1 mM. The results suggest that the neuroprotective effect of dextromethorphan, previously found to occur in a concentration range of 10-100 microM, may be due to a complete blockade of the NMDA receptor channel and a partial inhibition of voltage-dependent Ca2+ and Na+ channels.

Continuous co-administration of dextromethorphan or MK-801 with morphine: attenuation of morphine dependence and naloxone-reversible attenuation of morphine tolerance

N-Methyl-D-aspartate (NMDA) receptor antagonists have been repeatedly shown to attenuate the development of opiate tolerance and dependence in rodents. In the present experiments, continuous subcutaneous infusion of either MK-801 (0.01 mg/kg/h but not 0.005 mg/kg/h) or DM (0.133, 0.67 and 1.33 mg/kg/h) reliably prolonged the antinociceptive effect of continuous subcutaneous infusion of morphine sulfate (2.0 mg/kg/h), indicating attenuation of the development of morphine tolerance. Furthermore, this prolonged antinociception was completely reversible by naloxone (10 mg/kg, i.p.). Doses of MK-801 and DM that were equipotent in attenuating morphine tolerance (0.01 mg/kg/h and 1.33 mg/kg/h, respectively) revealed different profiles of effects, however, on locomotor activity and naloxone-precipitated abstinence/withdrawal symptoms. With regard to locomotor activity, rats having received continuous (48 h) subcutaneous infusion of morphine sulfate and MK-801, but not rats having received morphine sulfate and DM, displayed a reliable and striking increase in locomotor activity as compared with rats having received morphine alone. With regard to naloxone-precipitated withdrawal symptoms, continuous (48 h) subcutaneous co-infusion of either MK-801 (0.01 mg/kg/h) or DM (1.33 mg/kg/h) with morphine attenuated naloxone-precipitated hyperalgesia as compared with rats infused with morphine alone. MK-801 (0.01 mg/kg/h) was more effective than DM (0.133, 0.67, or 1.33 mg/kg/h), however, in reducing other naloxone-precipitated withdrawal symptoms (teeth chattering, jumping and wet dog shakes). The effects of MK-801 on all withdrawal symptoms were confounded, however, by the appearance of flaccidity following naloxone administration to rats having received MK-801 and morphine. These results extend previous observations by showing that the prolonged antinociception observed following co-administration of morphine and an NMDA antagonist is completely naloxone-reversible, supporting the notion that this antinociception reflects prolongation of an opioid receptor-mediated effect. The different profiles of side effects associated with MK-801 and DM, however, suggest that (1) attenuation of naloxone-precipitated withdrawal symptoms by MK-801 may be an artifact of toxicity, and (2) DM may prove clinically useful for the prevention of morphine tolerance, given its lack of observable side effects when administered concurrently with morphine to rodents.

MK-801 and dextromethorphan block microglial activation and protect against methamphetamine-induced neurotoxicity

Methamphetamine causes long-term toxicity to dopamine nerve endings of the striatum. Evidence is emerging that microglia can contribute to the neuronal damage associated with disease, injury, or inflammation, but their role in methamphetamine-induced neurotoxicity has received relatively little attention. Lipopolysaccharide (LPS) and the neurotoxic HIV Tat protein, which cause dopamine neuronal toxicity after direct infusion into brain, cause activation of cultured mouse microglial cells as evidenced by increased expression of intracellular cyclooxygenase-2 and elevated secretion of tumor necrosis factor-alpha. MK-801, a non-competitive NMDA receptor antagonist that is known to protect against methamphetamine neurotoxicity, prevents microglial activation by LPS and HIV Tat. Dextromethorphan, an antitussive agent with NMDA receptor blocking properties, also prevents microglial activation. In vivo, MK-801 and dextromethorphan reduce methamphetamine-induced activation of microglia in striatum and they protect dopamine nerve endings against drug-induced nerve terminal damage. The present results indicate that the ability of MK-801 and dextromethorphan to protect against methamphetamine neurotoxicity is related to their common property as blockers of microglial activation.

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

Combined oral administration of morphine sulfate (MS) and the over-the-counter antitussive drug and N-methyl-D-aspartate receptor antagonist dextromethorphan (DM) prevented the development of tolerance to the antinociceptive effects of MS (15, 24, or 32 mg/kg) in rats. This combined oral treatment regimen also attenuated signs of naloxone-precipitated physical dependence on morphine in the same rats. A wide range of ratios of MS to DM (2:1, 1:1, and 1:2) were effective for preventing the development of morphine tolerance and dependence. In addition, we provide evidence that under certain circumstances DM increases the acute antinociceptive effects of MS. All of these results indicate that oral treatment that combines DM with opiate analgesics may be a powerful approach for simultaneously preventing opiate tolerance and dependence and enhancing analgesia in humans.

The effects of St John's wort (Hypericum perforatum) on human cytochrome P450 activity

BACKGROUND

St John's wort (Hypericum perforatum) is a popular over-the-counter dietary supplement and herbal remedy that has been implicated in drug interactions with substrates of several cytochrome P450 (CYP) isozymes. The effect of St John's wort on CYP activity in vivo was examined with a probe drug cocktail.

METHODS

Twelve healthy subjects (5 female, 7 male) completed this 3-period, open-label, fixed schedule study. Tolbutamide (CYP2C9), caffeine (CYP1A2), dextromethorphan (CYP2D6), oral midazolam (intestinal wall and hepatic CYP3A), and intravenous midazolam (hepatic CYP3A) were administered before, with short-term St John's wort dosing (900 mg), and after 2 weeks of intake (300 mg 3 times a day) to determine CYP activities.

RESULTS

Short-term administration of St John's wort had no effect on CYP activities. Long-term St John's wort administration caused a significant (P <.05) increase in oral clearance of midazolam from 121.8 +/- 70.7 to 254.5 +/- 127.8 and a corresponding significant decline in oral bioavailability from 0.28 +/- 0.15 to 0.17 +/- 0.06. In contrast to the >50% decrease in the area under the plasma concentration-time curve (AUC) when midazolam was administered orally, long-term St John's wort administration caused a 20% decrease in AUC when midazolam was given intravenously. There was no change in CYP1A2, CYP2C9, or CYP2D6 activities as a result of St John's wort administration.

CONCLUSION

Long-term St John's wort administration resulted in a significant and selective induction of CYP3A activity in the intestinal wall. St John's wort did not alter the CYP2C9, CYP1A2, or CYP2D6 activities. Reduced therapeutic efficacy of drugs metabolized by CYP3A should be anticipated during long-term administration of St John's wort.

Sigma receptor-mediated neuroprotection against glutamate toxicity in primary rat neuronal cultures

The role of the putative sigma receptor in mediating neuroprotection against glutamate-induced neuronal injury was examined in mature cultured rat cortical neurons. With the exception of the selective sigma 1 ligand (+)-3-PPP, all of the sigma ligands tested were neuroprotective, preventing glutamate-induced morphological changes and increases in LDH release. Their rank order of neuroprotective potency (and EC50 values) was as follows: (+)-SKF 10,047 (0.81 microM) > (+)- cyclazocine (2.3 microM) > dextromethorphan (3.1 microM) = haloperidol (3.7 microM) > (+)-pentazocine (8.5 microM) > DTG (42.7 microM) = carbetapentane (46.3 microM). When corrected for relative sigma versus PCP binding affinity, it appears that a positive correlation exists between neuroprotective potency and sigma 1 site affinity. However, there does not appear to be a significant correlation between neuroprotective potency and the sigma 2 site. Critically, none of the sigma ligands were neurotoxic when tested alone at concentrations at least 5-30 times their respective neuroprotective EC50 values. Results from preliminary experiments with the selective sigma 1 ligand (+)-pentazocine indicated that sigma-mediated neuroprotection may involve the buffering of glutamate-induced calcium flux. Collectively, the results of these in vitro experiments demonstrate that sigma ligands are neuroprotective and therefore deserve further exploration as potential therapeutic agents in in vivo models of CNS injury and neurodegenerative disorders.