Drug refractory epilepsy in brain damage: effect of dextromethorphan on EEG in four patients

Dextromethorphan moderates reward deficiency associated with central serotonin transporter availability in 3,4-methylenedioxy-methamphetamine-treated animals

BACKGROUND

The neurotoxicity of 3,4-methylenedioxy-methamphetamine (MDMA) to the serotonergic system is well-documented. Dextromethorphan (DM), an antitussive drug, decreased morphine- or methamphetamine (MA)-induced reward in rats and may prevent MDMA-induced serotonergic deficiency in primates, as indicated by increased serotonin transporter (SERT) availability. We aimed to investigate the effects of DM on reward, behavioral sensitization, and neurotoxicity associated with loss of SERT induced by chronic MDMA administration in rats.

METHODS

Conditioned place preference (CPP) and locomotor activity tests were used to evaluate drug-induced reward and behavioral sensitization; 4-[ 18 F]-ADAM/animal-PET and immunohistochemistry were used to explore the effects of DM on MDMA-induced loss of SERT.

RESULTS

MDMA significantly reduced SERT binding in the rat brain; however, co-administration of DM significantly restored SERT, enhancing the recovery rate at day 14 by an average of ~23% compared to the MDMA group. In confirmation of the PET findings, immunochemistry revealed MDMA reduced SERT immunoactivity in all brain regions, whereas DM markedly increased the serotonergic fiber density after MDMA induction.

CONCLUSION

Behavioral tests and in vivo longitudinal PET imaging demonstrated the CPP indexes and locomotor activities of the reward system correlate negatively with PET 4-[ 18 F]ADAM SERT activity in the reward system. Our findings suggest MDMA induces functional abnormalities in a network of brain regions important to decision-making processes and the motivation circuit. DM may exert neuroprotective effects to reverse MDMA-induced neurotoxicity.

The Roles of Glutamate Receptors and Their Antagonists in Status Epilepticus, Refractory Status Epilepticus, and Super-Refractory Status Epilepticus

Status epilepticus (SE) is a neurological emergency with a high mortality rate. When compared to chronic epilepsy, it is distinguished by the durability of seizures and frequent resistance to benzodiazepine (BZD). The Receptor Trafficking Hypothesis, which suggests that the downregulation of γ-Aminobutyric acid type A (GABAA) receptors, and upregulation of N-methyl-D-aspartate (NMDA) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors play major roles in the establishment of SE is the most widely accepted hypothesis underlying BZD resistance. NMDA and AMPA are ionotropic glutamate receptor families that have important excitatory roles in the central nervous system (CNS). They are both essential in maintaining the normal function of the brain and are involved in a variety of neuropsychiatric diseases, including epilepsy. Based on animal and human studies, antagonists of NMDA and AMPA receptors have a significant impact in ending SE; albeit most of them are not yet approved to be in clinically therapeutic guidelines, due to their psychomimetic adverse effects. Although there is still a dearth of randomized, prospective research, NMDA antagonists such as ketamine, magnesium sulfate, and the AMPA antagonist, perampanel, are regarded to be reasonable optional adjuvant therapies in controlling SE, refractory SE (RSE) or super-refractory SE (SRSE), though there are still a lack of randomized, prospective studies. This review seeks to summarize and update knowledge on the SE development hypothesis, as well as clinical trials using NMDA and AMPA antagonists in animal and human studies of SE investigations.

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

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

KCNT1-positive epilepsy of infancy with migrating focal seizures successfully treated with nonnarcotic antitussive drugs after treatment failure with quinidine: A case report

BACKGROUND

Epilepsy of infancy with migrating focal seizures (EIMFS) is one of the early-onset epileptic encephalopathies resistant to antiepileptic drugs, therefore carrying an extremely poor neurodevelopmental outcome. KCNT1, encoding for a sodium-activated potassium channel (K_Ca4.1 channel), has recently been reported as the major gene responsible for EIMFS. Since gain of function is the only type of mutation identified in patients with EIMFS, quinidine, a partial antagonist of K_Ca4.1 channel, is considered as a potential candidate for targeted treatment of EIMFS. However, treatment results reported so far vary from seizure-free state to no response, and cardiac side effect remains a challenge for dose titration and long-term treatment.

CASE REPORT

Our case was an infant diagnosed with EIMFS with confirmed mutation in KCNT1 gene. Quinidine therapy was started as early as 9 months old. Within the first month of treatment, the number of seizures reduced to about one third. However, seizure-free state was not obtained and his neuropsychological development remained severely delayed. After 16 months of treatment, quinidine had to be discontinued because of cardiac side effects. At 27 months of age, however, his seizures suddenly stopped and he remained seizure-free for five days. This coincided with the prescription of tipepidine, a commonly used antitussive, administered for his persistent cough. Reduction in seizure frequency was also observed with dextromethorphan, another conventional antitussive drug. Although the relation between these treatments and his symptom improvement is a matter of elucidation, there is a possibility that these nonnarcotic antitussive drugs might play a role in the treatment of EIFMS.

Randomized open-label trial of dextromethorphan in Rett syndrome

OBJECTIVE

To determine safety and perform a preliminary assessment of dose-dependent efficacy of dextromethorphan in normalizing electrographic spikes, clinical seizures, and behavioral and cognitive functions in girls with Rett syndrome.

METHODS

We used a prospective randomized, open-label trial in fast metabolizers of dextromethorphan to examine the effect of dextromethorphan on core clinical features of Rett syndrome. Interictal spike activity and clinical seizures were determined using EEG and parent reporting. Cognitive data were obtained using the Mullen Scales of Early Learning and Vineland Adaptive Behavior Scales, while behavioral data were obtained from parent-completed checklists, the Aberrant Behavior Checklist-Community Version, and the Screen for Social Interaction. Anthropometric data were obtained according to the National Health and Nutrition Examination Survey. The Rett Syndrome Severity Scale provided a clinical global impression of the effect of dextromethorphan on clinical severity.

RESULTS

Dextromethorphan is safe for use in 3- to 15-year-old girls with Rett syndrome. Thirty-five girls were treated with 1 of 3 doses of dextromethorphan over a period of 6 months. Statistically significant dose-dependent improvements were seen in clinical seizures, receptive language, and behavioral hyperactivity. There was no significant improvement in global clinical severity as measured by the Rett Syndrome Severity Scale.

CONCLUSIONS

Dextromethorphan is a potent noncompetitive antagonist of the NMDA receptor channel that is safe for use in young girls with Rett syndrome. Preliminary evidence suggests that dextromethorphan may improve some core features of Rett syndrome.

CLASSIFICATION OF EVIDENCE

This study provides Class IV evidence that dextromethorphan at various doses does not change EEG spike counts over 6 months, though precision was limited to exclude an important effect.

Dextromethorphan: An update on its utility for neurological and neuropsychiatric disorders

Dextromethorphan (DM) is a commonly used antitussive and is currently the only FDA-approved pharmaceutical treatment for pseudobulbar affect. Its safety profile and diverse pharmacologic actions in the central nervous system have stimulated new interest for repurposing it. Numerous preclinical investigations and many open-label or blinded clinical studies have demonstrated its beneficial effects across a variety of neurological and psychiatric disorders. However, the optimal dose and safety of chronic dosing are not fully known. This review summarizes the preclinical and clinical effects of DM and its putative mechanisms of action, focusing on depression, stroke, traumatic brain injury, seizure, pain, methotrexate neurotoxicity, Parkinson's disease and autism. Moreover, we offer suggestions for future research with DM to advance the treatment for these and other neurological and psychiatric disorders.

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.

Preclinical toxicity study of intrathecal administration of the pain relievers dextrorphan, dextromethorphan, and memantine in the sheep model

Objectives To determine the toxicity window for the continuous intrathecal administration of dextrorphan, dextromethorphan, and memantine via an implanted delivery pump. Materials and Methods Using 48 sheep with programmable continuous intrathecal infusion systems we determined the behavioral, motor, neurological, and histopathological changes produced by a 43-day continuous infusion study of dextrorphan, dextromethorphan, and memantine dissolved in 0.9% NaCl. Daily doses of each N-methyl-D-aspartate (NMDA) antagonist were 0.013, 0.051, 0.203, 0.510, 0.811, and 2.533 mg/kg/day, flow rates ranged from 13.25 ml/day to 0.051 ml/day at a concentration of 10 mg/ml. Control animals received saline in the range of 7.9985 ml/day to 1 ml/day. Conclusions Infusion of saline in the control animals produced no behavioral or motor changes. However, infusion of dextrorphan, dextromethorphan, and memantine at the higher doses (> 0.051 mg/kg/day) produced dose-dependent negative behavioral, motor, and histopathologic changes as indicated by a series of nonparametric statistical analyses. The minimal toxic doses were dextrorphan dose 3, dextromethorphan dose 1 and memantine dose 1. This study suggests that continuous intrathecal infusion of dextrorphan, dextromethorphan, and memantine via an implantable pump system can cause significant toxicities at the higher doses studied.

The NMDA receptor complex as a therapeutic target in epilepsy: a review

A substantial amount of research has shown that N-methyl-D-aspartate receptors (NMDARs) may play a key role in the pathophysiology of several neurological diseases, including epilepsy. Animal models of epilepsy and clinical studies demonstrate that NMDAR activity and expression can be altered in association with epilepsy and particularly in some specific seizure types. NMDAR antagonists have been shown to have antiepileptic effects in both clinical and preclinical studies. There is some evidence that conventional antiepileptic drugs may also affect NMDAR function. In this review, we describe the evidence for the involvement of NMDARs in the pathophysiology of epilepsy and provide an overview of NMDAR antagonists that have been investigated in clinical trials and animal models of epilepsy.

Nonketotic hyperglycinemia: proposal of a diagnostic and treatment strategy

Early myoclonic encephalopathy presents neonatally with fragmented myoclonus and a suppression-burst electroencephalography pattern. We describe a newborn boy with early myoclonic encephalopathy caused by nonketotic hyperglycinemia. He presented with severe hypotonia, progressive apneic episodes, and erratic myoclonus. Screening of deletions in GLDC, using the multiplex ligation-dependent probe amplification method, and a (13)C breath test confirmed the diagnosis of nonketotic hyperglycinemia. Treatment with the N-methyl-d-aspartate receptor antagonist ketamine exerted dramatic suppressive effects on his seizures, and ameliorated his clinical status.

Neuroprotective strategies to avert seizure-induced neurodegeneration in epilepsy

Neurodegeneration in limbic circuits is a hallmark feature of chronic temporal lobe epilepsy (TLE). Studies in experimental animal models and human patients indicate that seizure-induced neuronal injury involves some active, as well as passive cell death processes. Experimental approaches that inhibit active steps in cell death programs have been shown to reduce neuronal cell death and sclerosis, but not to prevent epileptogenesis in animal models of TLE. These findings suggest that we need additional research using both animal models and brain slices from human patients to understand the pathological mechanisms underlying seizure generation. Such comparative studies will also aid in evaluating the potential therapeutic value of inhibiting cell death in seizure disorders.

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.

Malignant migrating partial seizures in infancy

Malignant migrating partial seizures in infancy is a rare, age-specific epileptic encephalopathy. It is characterized by onset before age 6 months, virtually continuous multifocal seizures with ictal electrical encephalographic activity shifting from one hemisphere to the other, no identifiable immediate or remote causes, intractability to antiepileptic drugs, and developmental arrest. This report adds two patients to the 21 previously described in the literature. One infant, microcephalic at birth, developed at age 4 months clusters of nearly continuous multifocal seizures with secondary generalization, refractory to antiepileptic drugs. By age 4.5 years she was seizure-free but remains without any cognitive or motor function. Patient 2, born with a normal head circumference, began seizures at age 3 months, never became seizure-free, and died at age 18 months. Electroencephalograms of both children were characteristic, and the neuroimaging finding was one of progressive cortical and subcortical atrophy. It has been hypothesized that neurotransmitter dysfunction with persistent, pronounced excitatory or cytotoxic mechanisms may explain the continuous, erratic epileptic activity. Awareness of malignant migrating partial seizures in infancy and research focused on its pathophysiologic mechanisms may reveal innovative treatments of this devastating, age-specific disorder.

Phenobarbital and MK-801, but not phenytoin, improve the long-term outcome of status epilepticus

To examine the effect of therapy on status epilepticus (SE) acutely and on long-term outcome, we compared three drugs with three different mechanisms. Phenobarbital, MK-801, and phenytoin were administered at 1, 2, and 4 hours after initiation of limbic status epilepticus by "continuous" hippocampal stimulation in rats. We evaluated the effects of these drugs on the course of SE and the subsequent development of chronic epilepsy. Phenobarbital and MK-801 were superior to phenytoin in suppressing SE and in preventing chronic epilepsy. There was no benefit if treatment was given 2 hours after the initiation of SE. Phenobarbital was most effective in suppressing electrographic seizure activity, but MK-801 had a slightly wider window for the prevention of chronic epilepsy. Early treatment, rather than electrographic suppression of SE, correlated with prevention of chronic epilepsy. This study shows that the drugs administered, which have different mechanisms of action, have clear differences in altering the outcomes. The findings suggest that studies of SE treatment should examine the effect of therapy on SE itself, as well as the long-term benefits of each treatment. The use of N-methyl-D-aspartate receptor antagonists should be considered early in the treatment of SE.

Retrograde perfusion with a sodium channel antagonist provides ischemic spinal cord protection

BACKGROUND

Neuronal voltage-dependent sodium channel antagonists have been shown to provide neuroprotection in focal and global cerebral ischemic models. We hypothesized that retrograde spinal cord venous perfusion with phenytoin, a neuronal voltage-dependent sodium channel antagonist, would provide protection during prolonged spinal cord ischemia.

METHODS

In a rabbit model, spinal cord ischemia was induced for 45 minutes. Six groups of animals were studied. Controls (group I, n = 8) received no intervention during aortic cross-clamping. Group II (n = 8) received systemic phenytoin (100 mg). Group III (n = 4) received systemic phenytoin (200 mg). Group IV (n = 8) received retrograde infusion of room temperature saline (22 degrees C) only. Group V (n = 8) and group VI (n = 9) received retrograde infusion of 50 mg and 100 mg of phenytoin, respectively, (infusion rate: 0.8 mL x kg(-1) x min(-1) during the ischemic period). Mean arterial blood pressure was monitored continuously. Animals were allowed to recover for 24 hours before assessment of neurologic function using the Tarlov scale.

RESULTS

Tarlov scores (0 = complete paraplegia, 1 = slight lower limb movement, 2 = sits with assistance, 3 = sits alone, 4 = weak hop, 5 = normal hop) were as follows (mean +/- SEM): group I, 0.50 +/- 0.50; group II, 0.25 +/- 0.46; group IV, 1.63 +/- 0.56; group V, 4.13 +/- 0.23; and group VI, 4.22 +/- 0.22 (p < 0.0001 V, VI versus I, II, IV by analysis of variance). No differences in mean arterial blood pressure were observed. All animals in group III became profoundly hypotensive and died before the conclusion of the 45-minute ischemic time.

CONCLUSIONS

Retrograde venous perfusion of the spinal cord with phenytoin, a voltage-sensitive sodium channel blocker, is safe and provides significant protection during prolonged spinal cord ischemia.

Binding of dimemorfan to sigma-1 receptor and its anticonvulsant and locomotor effects in mice, compared with dextromethorphan and dextrorphan

Dextromethorphan ((+)-3-methoxy-N-methylmorphinan, DM) has been shown to have both anticonvulsant and neuroprotective effects. The mechanisms of these CNS effects of DM have been suggested to be associated with the low-affinity, noncompetitive, N-methyl-d-aspartate (NMDA) antagonism of DM and/or the high-affinity DM/sigma receptors. DM is largely O-demethylated into the phencyclidine (PCP)-like compound dextrorphan (DR), which may limit its therapeutic use by producing PCP-like adverse effects, such as hyperlocomotion. Dimemorfan ((+)-3-methyl-N-methylmorphinan, DF), an analog of DM, which has been safely used as an antitussive for more than 20 years, is also known not to form DR. This study therefore characterized the binding of DF to the sigma receptors and NMDA-linked PCP sites and examined the anticonvulsant as well as locomotor effects of DF in mice in comparison with those of DM and DR. We found that DF, DM, and DR were relative high-affinity ligands at sigma-1 receptors (Ki=151, 205, 144 nM, respectively) while all of them were with low affinity at sigma-2 receptors (Ki=4-11 microM). Only DR exhibited moderate affinity for PCP sites (Ki=0.9 microM), whereas DF (Ki=17 microM) and DM (Ki=7 microM) were much less active. DF, DM and DR produced prominent anticonvulsant effects in mice as measured by the supramaximal electroshock test with comparable potency (ED50 approximately 70 micromol/kg, i.p.). At the tested doses (20-260 micromol/kg, i.p.), DM and DR exhibited biphasic effects on the locomotor activity whereas DF produced a consistent dose-dependent decrease. These results revealed that, unlike DM and DR, DF did not cause a PCP-like hyperlocomotion adverse effect that is parallel with the PCP sites binding data. Furthermore, since they have equipotent anticonvulsant effects and similar binding affinities to sigma-1 receptors, the very low affinity of DF at PCP sites may suggest that acting on the PCP sites may not be the requisite for mediating the anticonvulsant activity of these DM analogs. With the history of safety and relative less adverse effects, DF appears to be worth further studying on its CNS effects other than the antitussive effect.

Dextromethorphan and dextrorphan in rats: common antitussives--different behavioural profiles

Dextromethorphan (DM), a widely used and well-tolerated centrally acting antitussive, has been tested in several clinical trials for its antiepileptic and neuroprotective properties. However, the use of DM in these new clinical indications requires higher doses than antitussive doses, which may therefore induce phencyclidine (PCP)-like side-effects (memory and psychotomimetic disturbances) through its metabolic conversion to the active metabolite dextrorphan (DX), a more potent PCP-like non-competitive antagonist at the N-methyl-D-aspartate (NMDA) receptor than DM. Thus, we compared the behavioural effects in rats of intraperitoneal administration of DM and DX on motor activity in an open field and on learning and memory in the Morris water maze. DM (20, 30, 40 mg/kg) produced a dose-dependent decrease in both locomotion and stereotyped behaviour with a slight ataxia for the highest dose. DX (20, 30, 40 mg/kg) induced a dose-dependent increase in locomotion and stereotypies (swaying, turning) with moderate ataxia. Assessments of learning and memory were performed with lower doses of DM (10, 20, 30 mg/kg) and DX (5, 10, 15 mg/kg) because of motivational deficits (40 mg/kg of DM, 20-40 mg/kg of DX) and motor disorders (30, 40 mg/kg of DX) in the cue learning procedure. DX (10, 15 mg/kg) impaired spatial learning with a long-lasting effect for the highest dose whereas 5 mg/kg of DX and DM (10-30 mg/kg) did not. Only 15 mg/kg of DX appeared to slightly impair working memory. DM (10-30 mg/kg) and DX (5-15 mg/kg) did not impair reference memory. Thus, the two antitussives DM and DX induced different behavioural effects suggesting sedative effects for DM and PCP-like effects for DX. However, PCP-like side-effects with DM remain possible through its metabolic conversion to DX, with very high doses and/or in extensive metabolizers and/or in aged subjects prone to cognitive dysfunction. Therefore, the identification of DM metabolism phenotype, an adapted prescription and a pharmacological modulation of the DM metabolism may avoid adverse effects.

Long-term use of high-dose benzoate and dextromethorphan for the treatment of nonketotic hyperglycinemia

OBJECTIVE

The objective of this study was to test the hypotheses that reduction of glycine and blocking of the N-methyl-D-aspartate receptor channel complex would be beneficial for both seizure reduction and developmental progress in patients with nonketotic hyperglycinemia.

METHODS

We administered benzoate (at doses of 500 to 750 mg/kg/day) and dextromethorphan (at doses of 3.5 to 22.5 mg/kg/day) to four infants with nonketotic hyperglycinemia with follow-up of 3 months to 6 years.

RESULTS

Benzoate reduced to normal the glycine concentration in plasma and substantially reduced but did not normalize the glycine concentration in cerebrospinal fluid. Dextromethorphan was a potent anticonvulsant in some but not all patients. There was remarkable interpatient variability in dextromethorphan metabolism. Three patients are living (ages ranging from 4 to 6 years) and are moderately to severely developmentally delayed; two are free of seizures. The third patient, with the slowest development, had intractable seizures for nearly a month before diagnosis, and although seizure-free for 30 months, now has grand-mal seizures. One patient died of intractable seizures at 3 months.

CONCLUSIONS

These outcomes suggest that benzoate and dextromethorphan are not uniformly effective in nonketotic hyperglycinemia, but for some patients they improve arousal, decrease or eliminate seizures, and allow for some developmental progress. Trials with additional patients and other receptor channel blockers are warranted.

Evaluation of dextromethorphan and dextrorphan as a preventive treatment of soman toxicity in mice

Phencyclidine-like drugs are effective against convulsions and brain lesions related to soman intoxication but induce severe side effects. The well tolerated antitussive dextromethorphan (DM) and its metabolite dextrorphan (DX) have antiepileptic and neuroprotective properties that we evaluated in mice against 2 LD50 of soman in a three-drug pretreatment (atropine sulfate and oxime HI-6 plus DM: 20-50 mg/kg or DX: 10-40 mg/kg i.p). Neuroprotection was evaluated by measurement of hippocampal omega 3 binding site density. DM and DX have weak anticonvulsant and neuroprotective activities which are counterbalanced at high doses by an increased mortality due to respiratory distress for DM and by ataxia for DX. Thus DM and DX do not appear to be appropriate for the pretreatment of soman intoxication.

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.

Induction of heat shock protein HSP-70 in rat retrosplenial cortex following administration of dextromethorphan

Dextromethorphan, a non-competitive antagonist of N-methyl-d-aspartate (NMDA) receptor, is one of the most widely used non-opioid cough suppressants, and it is generally considered to be a safe drug. In this study, we have examined whether dextromethorphan is neurotoxic to rat cerebrocortical neurons. Induction of heat shock protein HSP-70, an indicator of cellular stress, was observed in the posterior cingulate and retrosplenial cortex of rat brain after a single administration of dextromethorphan (75 mg/kg). Furthermore, administration of dextromethorphan (75 mg/kg) caused vacuolization in the same regions. These results suggest that high doses of dextromethorphan could cause neuronal injury in the cerebrocortical neurons.

Adverse effects of dextromethorphan on the spatial learning of rats in the Morris water maze

The effects of the non-competitive NMDA receptor antagonist dextromethorphan on spatial learning were assessed using the Morris water maze. Dextromethorphan was administered to 4 groups of rats in 10, 20, 30, and 40 mg/kg doses. An additional group of rats was administered saline to serve as a vehicle control group. Dextromethorphan impaired learning dose dependently in the initial training phase of the experiment. During the probe trial, dose-dependent performance deficits were noted in the first 15 s of the trial only. Search strategy differences between the lowest and highest dose groups were also observed during the probe trial. During the reversal training phase, when the platform was moved to a new location, the dose-dependent impairment was seen again, but the 40 mg/kg group perseverated to the former location longer than the other groups. A cued control trial indicated that in addition to the learning impairment produced, the highest dose of dextromethorphan may also impair sensory-motor coordination.

Neuroprotection by dextromethorphan in acute experimental subdural hematoma in the rat

Experimental acute subdural hematoma in the rat has been shown to produce a zone of apparent infarction under the clot, and excitatory amino acid toxicity appears to play a role in the damage observed. We report the effect of dextromethorphan, a commonly used antitussive and a noncompetitive NMDA-type glutamate receptor antagonist, on the volume of histologic damage seen at 72 h after acute subdural hematoma in the rat. Sixty-five Long-Evans rats underwent placement of acute subdural hematoma using the "cranial window" model. Fourteen animals received oral dextromethorphan, 10 mg/kg/dose, twice daily for 3 days, and an additional 20 animals also received a single 20 mg/kg intraperitoneal dose 15 min after clot placement in addition to the oral regimen. Control animals received equal volumes of sterile water. Brain lesions in all animals were characterized by well-circumscribed infarctions underlying the subdural hematoma. Lesion volume in control animals was 88.3 +/- 9.3 mm3 (mean +/- standard error of the mean), while animals receiving dextromethorphan had significantly smaller lesions, which was independent of dosing schedule (59.9 +/- 9.2 mm3)(p = 0.0403). Animal weight was also found to be a significant covariate (p = 0.038). Because of its safety in humans and efficacy as a neuroprotectant in a variety of models, dextromethorphan may be a promising agent for clinical use, particularly in children.

Excitatory amino acids in health and disease

PURPOSE

To review the role of excitatory neurotransmitters in normal mammalian brain function, the concept of excitotoxic neuronal death as an important final common path in a variety of diseases, and modification of excitatory synaptic transmission as an important new pharmacological principle. These principles are discussed, with special emphasis on diseases of importance to older adults.

DATA SOURCES

A MEDLINE search from 1966 to May 1995 was undertaken, as well as a manual search of current issues of clinical and basic neuroscience journals, for articles that addressed glutamate N-methyl-D-aspartate and/or excitotoxicity.

STUDY SELECTION

A total of 5398 original and 68 review articles were identified that addressed animal and human experimentation relevant to excitotoxic neuronal death. There were 364 articles with potential significance for clinical application identified; 132 of the most recent references are provided.

DATA EXTRACTION

All articles were classified into three categories: general receptor, biology pathogenesis of disease, and pharmacotherapy.

RESULTS

Glutamic and aspartic acids are the physiological mediators of most excitatory synaptic transmission. This is critical to several normal nervous system functions, including memory and long-term modification of synaptic transmission and nociception. Activation of the inotropic NMDA and non-NMDA receptors increases transmembrane calcium and sodium fluxes, and the metabotropic glutamate receptor activation results in generation of inositol triphosphate and inhibition of adenylate cyclase. Numerous modulatory sites exist, especially on the NMDA receptor. Nitric oxide, arachidonic acid, superoxide, and intracellular calcium overload are the ultimate mediators of neuronal death. Glutamate re-uptake transporters belong to a unique family of amino acid transport systems, the malfunction of which is intricately involved in disease pathogenesis. Ischemic stroke, hypoglycemia, Parkinson's disease, alcohol intoxication and withdrawal, Alzheimer's disease, epilepsy, and chronic pain syndromes are only some of the important clinical neurological disorders with a major pathogenic role for the excitatory amino acids.

CONCLUSIONS

Pharmacological manipulation of the excitatory amino acid receptors is likely to be of benefit in important and common diseases of the nervous system. Only a few of the currently available drugs that modify excitatory neurotransmission, such as remacemide, lamotrigine, and tizanidine, have an acceptable therapeutic index. The identification of numerous receptor subtypes, topographic variabilities of distribution, and multiple modulatory sites will provide a true challenge to the neuropharmacologist.

Clinical experience with excitatory amino acid antagonist drugs

BACKGROUND

Excitotoxic damage due to excess release of neuronal glutamate is hypothesized to play a pivotal role in the pathogenesis of focal cerebral ischemia. Drugs that antagonize excitatory amino acid function are consistently neuroprotective in preclinical models of stroke, and many are now entering clinical trials.

SUMMARY

Antagonists of the N-methyl-D-aspartate (NMDA) receptor are furthest advanced in clinical development for stroke. Both noncompetitive (aptiganel hydrochloride, dextrorphan) and competitive (selfotel, d-CPPene) antagonists have undergone tolerability studies in acute stroke and traumatic brain injury. These agents all cause a similar spectrum of neuropsychological symptoms, and several have important cardiovascular effects. Other modulatory sites on the NMDA receptor complex, notably the polyamine and magnesium ion sites, are also the subject of clinical trials. Glycine site antagonists are in early clinical development. Non-NMDA glutamate receptor antagonists remain in preclinical study. Neuroprotection by agents that block glutamate release in vitro may be due to sodium channel blockade in vivo, but some agents (619C89) exhibit neurological effects similar to NMDA antagonists in stroke. The therapeutic index is unknown for different drugs but may be determined by cardiovascular effects, especially hypotension, which may be detrimental after stroke.

CONCLUSIONS

Excitatory amino acid antagonists are in advanced development in the treatment of stroke and traumatic brain injury. A similar pattern of side effects is apparent with the majority of agents. However, cardiovascular effects may ultimately define therapeutic index for each drug.