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

α-Linolenic Acid, A Nutraceutical with Pleiotropic Properties That Targets Endogenous Neuroprotective Pathways to Protect against Organophosphate Nerve Agent-Induced Neuropathology

α-Linolenic acid (ALA) is a nutraceutical found in vegetable products such as flax and walnuts. The pleiotropic properties of ALA target endogenous neuroprotective and neurorestorative pathways in brain and involve the transcription factor nuclear factor kappa B (NF-κB), brain-derived neurotrophic factor (BDNF), a major neuroprotective protein in brain, and downstream signaling pathways likely mediated via activation of TrkB, the cognate receptor of BDNF. In this review, we discuss possible mechanisms of ALA efficacy against the highly toxic OP nerve agent soman. Organophosphate (OP) nerve agents are highly toxic chemical warfare agents and a threat to military and civilian populations. Once considered only for battlefield use, these agents are now used by terrorists to inflict mass casualties. OP nerve agents inhibit the critical enzyme acetylcholinesterase (AChE) that rapidly leads to a cholinergic crisis involving multiple organs. Status epilepticus results from the excessive accumulation of synaptic acetylcholine which in turn leads to the overactivation of muscarinic receptors; prolonged seizures cause the neuropathology and long-term consequences in survivors. Current countermeasures mitigate symptoms and signs as well as reduce brain damage, but must be given within minutes after exposure to OP nerve agents supporting interest in newer and more effective therapies. The pleiotropic properties of ALA result in a coordinated molecular and cellular program to restore neuronal networks and improve cognitive function in soman-exposed animals. Collectively, ALA should be brought to the clinic to treat the long-term consequences of nerve agents in survivors. ALA may be an effective therapy for other acute and chronic neurodegenerative disorders.

Blood glutamate scavenging as a novel neuroprotective treatment for paraoxon intoxication

Organophosphate-induced brain damage is an irreversible neuronal injury, likely because there is no pharmacological treatment to prevent or block secondary damage processes. The presence of free glutamate (Glu) in the brain has a substantial role in the propagation and maintenance of organophosphate-induced seizures, thus contributing to the secondary brain damage. This report describes for the first time the ability of blood glutamate scavengers (BGS) oxaloacetic acid in combination with glutamate oxaloacetate transaminase to reduce the neuronal damage in an animal model of paraoxon (PO) intoxication. Our method causes a rapid decrease of blood Glu levels and creates a gradient that leads to the efflux of the excess brain Glu into the blood, thus reducing neurotoxicity. We demonstrated that BGS treatment significantly prevented the peripheral benzodiazepine receptor (PBR) density elevation, after PO exposure. Furthermore, we showed that BGS was able to rescue neurons in the piriform cortex of the treated rats. In conclusion, these results suggest that treatment with BGS has a neuroprotective effect in the PO intoxication. This is the first time that this approach is used in PO intoxication and it may be of high clinical significance for the future treatment of the secondary neurologic damage post organophosphates exposure.

Dextromethorphan as a Potential Neuroprotective Agent With Unique Mechanisms of Action

BACKGROUND

Dextromethorphan (DM) is a widely-used antitussive. DM's complex central nervous system (CNS) pharmacology became of interest when it was discovered to be neuroprotective due to its low-affinity, uncompetitive N-methyl-D-aspartate (NMDA) receptor antagonism.

REVIEW SUMMARY

Mounting preclinical evidence has proven that DM has important neuroprotective properties in various CNS injury models, including focal and global ischemia, seizure, and traumatic brain injury paradigms. Many of these protective actions seem functionally related to its inhibitory effects on glutamate-induced neurotoxicity via NMDA receptor antagonist, sigma-1 receptor agonist, and voltage-gated calcium channel antagonist actions. DM's protection of dopamine neurons in parkinsonian models may be due to inhibition of neurodegenerative inflammatory responses. Clinical findings are limited, with preliminary evidence indicating that DM protects against neuronal damage. Negative findings seem to relate to attainment of inadequate DM brain concentrations. Small studies have shown some promise for treatment of perioperative brain injury, amyotrophic lateral sclerosis, and symptoms of methotrexate neurotoxicity. DM safety/tolerability trials in stroke, neurosurgery, and amyotrophic lateral sclerosis patients demonstrated a favorable safety profile. DM's limited clinical benefit is proposed to be associated with its rapid metabolism to dextrorphan, which restricts its central bioavailability and therapeutic utility. Systemic concentrations of DM can be increased via coadministration of low-dose quinidine (Q), which reversibly inhibits its first-pass elimination. Potential drug interactions with DM/Q are discussed.

CONCLUSIONS

Given the compelling preclinical evidence for neuroprotective properties of DM, initial clinical neuroprotective findings, and clinical demonstrations that the DM/Q combination is well tolerated, this strategy may hold promise for the treatment of various acute and degenerative neurologic disorders.

Future considerations for the medical management of nerve-agent intoxication

The use of chemical warfare agents against civilians and unprotected troops in international conflicts or by terrorists against civilians is considered to be a real threat, particularly following the terrorist attacks on 11 September 2001 against the World Trade Center in New York and against the Pentagon in Washington, DC. Over the past 10 years, terrorists have been planning to use or have used chemical warfare agents on several occasions around the world, and the attacks in 2001 illustrate their willingness to use any means of warfare to cause death and destruction among civilians. In spite of new international treaties with strong verification measures and with an aim to prohibit and prevent the use of weapons of mass destruction, nevertheless, some countries and terrorist groups have been able to develop, produce, and use such weapons, particularly nerve agents, in domestic terrorist attacks or during warfare in international conflicts. This article reviews current medical therapy for nerve-agent intoxication and discusses possible future improvement of medical therapies. Present medical counter-measures against nerve agents are not sufficiently effective particularly in protecting the brain. Therefore, new and more effective countermeasures must be developed to enable better medical treatment of civilians and military personnel following exposure to nerve agents. Therefore, it is important with an enhanced effort by all countries, to improve and increase research in medical countermeasures, in the development of protective equipment, and in carrying out regular training of medical and emergency personnel as well as of military nuclear, biological, or chemical (NBC) units. Only then will nations be able to reduce the risk from and prevent the use of such weapons of mass destruction (WMD).

Neurotoxicity of dextrorphan

BACKGROUND

The noncompetitive NMDA antagonists phencyclidine (PCP) and dizocilpine (MK-801) have been considered for use as neuroprotective therapeutic agents, although both produce injury in neurons of cingulate and retrosplenial cortices in rodents. The low-affinity, noncompetitive NMDA antagonist dextrorphan has been considered for use as a neuroprotective therapeutic drug. The aim of the present work was to evaluate the neurotoxicity of dextrorphan.

METHODS

Sprague-Dawley male rats were used and injected with either saline or dextrorphan (30 mg/kg i.p.). The animals were sacrificed 30 min later, and the brain was examined for histopathological changes.

RESULTS

After systemic administration of the drug, hyperchromatic and shrunken nuclei with chromatin condensation and disruption were observed. Also, granular and vacuolated cytoplasm was apparent in pyramidal neurons in the retrosplenial (posterior cingulate) cortex. Status spongiosus (spongy degeneration) of the neuropil was also detected.

CONCLUSIONS

Morphological changes are similar to those described previously, which are induced by high-affinity, noncompetitive NMDA antagonists, such as MK-801.

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.