Assessment of primary neuronal culture as a model for soman-induced neurotoxicity and effectiveness of memantine as a neuroprotective drug

Treatment of cholinergic-induced status epilepticus with polytherapy targeting GABA and glutamate receptors

Despite new antiseizure medications, the development of cholinergic-induced refractory status epilepticus (RSE) continues to be a therapeutic challenge as pharmacoresistance to benzodiazepines and other antiseizure medications quickly develops. Studies conducted by Epilepsia. 2005;46:142 demonstrated that the initiation and maintenance of cholinergic-induced RSE are associated with trafficking and inactivation of gamma-aminobutyric acid A receptors (GABAA R) thought to contribute to the development of benzodiazepine pharmacoresistance. In addition, Dr. Wasterlain's laboratory reported that increased N-methyl-d-aspartate receptors (NMDAR) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPAR) contribute to enhanced glutamatergic excitation (Neurobiol Dis. 2013;54:225; Epilepsia. 2013;54:78). Thus, Dr. Wasterlain postulated that targeting both maladaptive responses of reduced inhibition and increased excitation that is associated with cholinergic-induced RSE should improve therapeutic outcome. We currently review studies in several animal models of cholinergic-induced RSE that demonstrate that benzodiazepine monotherapy has reduced efficacy when treatment is delayed and that polytherapy with drugs that include a benzodiazepine (eg midazolam and diazepam) to counter loss of inhibition, concurrent with an NMDA antagonist (eg ketamine) to reduce excitation provide improved efficacy. Improved efficacy with polytherapy against cholinergic-induced seizure is demonstrated by reduction in (1) seizure severity, (2) epileptogenesis, and (3) neurodegeneration compared with monotherapy. Animal models reviewed include pilocarpine-induced seizure in rats, organophosphorus nerve agent (OPNA)-induced seizure in rats, and OPNA-induced seizure in two mouse models: (1) carboxylesterase knockout (Es1-/- ) mice which, similarly to humans, lack plasma carboxylesterase and (2) human acetylcholinesterase knock-in carboxylesterase knockout (KIKO) mice. We also review studies showing that supplementing midazolam and ketamine with a third antiseizure medication (valproate or phenobarbital) that targets a nonbenzodiazepine site rapidly terminates RSE and provides further protection against cholinergic-induced SE. Finally, we review studies on the benefits of simultaneous compared with sequential drug treatments and the clinical implications that lead us to predict improved efficacy of early combination drug therapies. The data generated from seminal rodent studies of efficacious treatment of cholinergic-induced RSE conducted under Dr. Wasterlain's guidance suggest that future clinical trials should treat the inadequate inhibition and temper the excess excitation that characterize RSE and that early combination therapies may provide improved outcome over benzodiazepine monotherapy.

Acute and long-term effects of VX in rat brain cell aggregate culture

The contact poison VX (O-ethyl S-(2-diisopropylaminoethyl) methylphosphonothioate) is a chemical warfare agent that is one of the most toxic organophosphorus compounds known. Its primary mechanism of toxic action is through the inhibition of acetylcholinesterase and resultant respiratory paralysis. The majority of work on VX has thus concentrated on its potent anticholinesterase activity and acute toxicity, with few studies investigating potential long-term effects. In this report we describe the effects of VX in aggregating rat brain cell cultures out to 28 days post-exposure. Cholinesterase activity was rapidly inhibited (60 min IC50 = 0.73 +/- 0.27 nM), but recovered towards baseline values over the next four weeks. Apoptotic cell death, as measured using caspase-3 activity was evident only at 100 μM concentrations. Cell type specific enzymatic markers (glutamine synthase, choline acetyltransferase and 2',3'-cyclic nucleotide 3'-phosphodiesterase) showed no significant changes. Total Akt levels were unchanged, while an increased phosphorylation of this protein was noted only at the highest VX concentration on the first day post-exposure. In contrast, significant and delayed (28 days post-exposure) decreases were noted in vascular endothelial growth factor (VEGF) levels, a protein whose reduced levels are known to contribute to neurodegenerative disorders. These observations may indicate that the long-term effects noted in some survivors of nerve agent intoxication may be due to VX-induced declines in brain VEGF levels.

Effects of memantine and its metabolite Mrz 2/373 on soman-induced inhibition of acetylcholinesterase in vitro

Memantine is the non-competitive N-methyl-d-aspartate (NMDA) receptor antagonist, used in the treatment of Alzheimer's disease. It is also known that memantine pretreatment assured protection of skeletal muscles from poisoning with nerve agents and an interaction between memantine and AChE was proposed. In the study presented we examined interactions of memantine and its main metabolite (1-amino-3-hydroxymethyl-5-methyl adamantine, Mrz 2/373) with AChE in vitro as well as their effect on kinetics of the soman-induced AChE inhibition and aging. The results have shown that memantine and Mrz 2/373 exerted concentration-dependent inhibition of AChE, with Mrz 2/373 being a more potent inhibitor than the parent compound. Addition of soman 7.5 nmol/l induced gradual AChE inhibition that became almost complete after 20 min. Memantine (0.1, 0.5 and 1 mmol/l) and Mrz 2/373 (0.1, 0.5 and 1 mmol/l) concentration-dependently slowed down the AChE inhibition. After 30 min of incubation of AChE with soman, 5 min of aging and 20 min of reactivation by asoxime (HI-6 dichloride), AChE activity was 8.1% in control medium, 30.7% and 41.9% after addition of 1 and 10 mmol/l memantine, and 16.1% after addition of 1 mmol/l Mrz 2/373. It was concluded that it is possible that memantine and Mrz 2/373 can prevent AChE from inhibition by soman, which could, along with known memantine's neuroprotective activity, explain its potent antidotal effect in soman poisoning. The potential effect on aging of the soman-AChE complex warrants further studies.

Validating a model of benzodiazepine refractory nerve agent-induced status epilepticus by evaluating the anticonvulsant and neuroprotective effects of scopolamine, memantine, and phenobarbital

INTRODUCTION

Organophosphorus nerve agents (OPNAs) irreversibly block acetylcholinesterase activity, resulting in accumulation of excess acetylcholine at neural synapses, which can lead to a state of prolonged seizures known as status epilepticus (SE). Benzodiazepines, the current standard of care for SE, become less effective as latency to treatment increases. In a mass civilian OPNA exposure, concurrent trauma and limited resources would likely cause a delay in first response time. To address this issue, we have developed a rat model to test novel anticonvulsant/ neuroprotectant adjuncts at delayed time points.

METHODS

For model development, adult male rats with cortical electroencephalographic (EEG) electrodes were exposed to soman and administered saline along with atropine, 2-PAM, and midazolam 5, 20, or 40 min after SE onset. We validated our model using three drugs: scopolamine, memantine, and phenobarbital. Using the same procedure outlined above, rats were given atropine, 2-PAM, midazolam and test treatment 20 min after SE onset.

RESULTS

Using gamma power, delta power, and spike rate to quantify EEG activity, we found that scopolamine was effective, memantine was minimally effective, and phenobarbital had a delayed effect on terminating SE. Fluoro-Jade B staining was used to assess neuroprotection in five brain regions. Each treatment provided significant protection compared to saline + midazolam in at least two brain regions.

DISCUSSION

Because our data agree with previously published studies on the efficacy of these compounds, we conclude that this model is a valid way to test novel anticonvulsants/ neuroprotectants for controlling benzodiazepine-resistant OPNA-induced SE and subsequent neuropathology.

Sarin (GB, O-isopropyl methylphosphonofluoridate) neurotoxicity: critical review

Sarin (GB, O-isopropyl methylphosphonofluoridate) is a potent organophosphorus (OP) nerve agent that inhibits acetylcholinesterase (AChE) irreversibly. The subsequent build-up of acetylcholine (ACh) in the central nervous system (CNS) provokes seizures and, at sufficient doses, centrally-mediated respiratory arrest. Accumulation of ACh at peripheral autonomic synapses leads to peripheral signs of intoxication and overstimulation of the muscarinic and nicotinic receptors, which is described as "cholinergic crisis" (i.e. diarrhea, sweating, salivation, miosis, bronchoconstriction). Exposure to high doses of sarin can result in tremors, seizures, and hypothermia. More seriously, build-up of ACh at neuromuscular junctions also can cause paralysis and ultimately peripherally-mediated respiratory arrest which can lead to death via respiratory failure. In addition to its primary action on the cholinergic system, sarin possesses other indirect effects. These involve the activation of several neurotransmitters including gamma-amino-butyric acid (GABA) and the alteration of other signaling systems such as ion channels, cell adhesion molecules, and inflammatory regulators. Sarin exposure is associated with symptoms of organophosphate-induced delayed neurotoxicity (OPIDN) and organophosphate-induced chronic neurotoxicity (OPICN). Moreover, sarin has been involved in toxic and immunotoxic effects as well as organophosphate-induced endocrine disruption (OPIED). The standard treatment for sarin-like nerve agent exposure is post-exposure injection of atropine, a muscarinic receptor antagonist, accompanied by an oxime, an AChE reactivator, and diazepam.

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.

Type of cell death and the role of acetylcholinesterase activity in neurotoxicity induced by paraoxon in cultured rat hippocampal neurons

Organophosphate (Ops) neurotoxicity is attributed both to its well-known cholinergic and non-cholinergic effects. In the present study we compared enzymatic and morphologic changes in neurons exposed to paraoxon during one day and one week. The effect of exposure time is important in neurotoxicity of Ops. The longer the exposure time is the more damage is observed in neurons, although there are few investigations about the effect in the post-exposure period. Hippocampal cells were obtained from rat neonates and cultured in Neurobasal/B27. Paraoxon at 50 and 100 microM were added. Inverted microscope and electron microscope were used to study cell morphology and Neutral Red staining was used to measure viability. We also assayed caspase-3 and (acetylcholinesterase) AChE activity. Hoechst staining was utilized to determine the type of cell death. Culture medium was replaced after 24 h in one-day group, however, tests were all carried out at the end of the first week in both group. The results indicate that paraoxon reduced the viability in a dose-dependent manner. Our results do not confirm apoptosis in either group; it seems that the cell death in one-day exposure group was not AChE dependent. In conclusion, present data imply that the toxicity of paraoxon is both dose and duration dependent, which may even remain after the cessation of exposure.

PHARMACOLOGICAL TREATMENTS FOR TINNITUS: NEW AND OLD

Subjective tinnitus, the phantom ringing or buzzing sensation that occurs in the absence of sound, affects 12-14% of adults; in some cases the tinnitus is so severe or disabling that patients seek medical treatment. However, although the economic and emotional impact of tinnitus is large, there are currently no FDA-approved drugs to treat this condition. Clinical trials are now underway to evaluate the efficacy of N-methyl-d-aspartate (NMDA) and dopamine D(2) antagonists, selective serotonin reuptake inhibitors (SSRIs), γ-aminobutyric acid (GABA) agonists and zinc dietary supplements. Previous off-label clinical studies, while not definitive, suggest that patients with severe depression may experience improvement in their tinnitus after treatment with antidepressants such as nortriptyline or sertraline. A small subpopulation of patients with what has been described as "typewriter tinnitus" have been shown to gain significant relief from the anticonvulsant carbamazepine. Preliminary studies with misoprostol, a synthetic prostaglandin E1 analogue, and sulpiride, a dopamine D(2) antagonist, have shown promise. Animal behavioral studies suggest that GABA transaminase inhibitors and potassium channel modulators can suppress tinnitus. Additionally, improvements in tinnitus have also been noted in patients taking melatonin for significant sleep disturbances. Like other complex neurological disorders, one drug is unlikely to resolve tinnitus in all patients; therapies targeting specific subgroups are likely to yield the greatest success.

Neuroprotective effects of xenon: a therapeutic window of opportunity in rats subjected to transient cerebral ischemia

Brain insults are a major cause of acute mortality and chronic morbidity. Given the largely ineffective current therapeutic strategies, the development of new and efficient therapeutic interventions is clearly needed. A series of previous investigations has shown that the noble and anesthetic gas xenon, which has low-affinity antagonistic properties at the N-methyl-D-aspartate (NMDA) receptor, also exhibits potentially neuroprotective properties with no proven adverse side effects. Surprisingly and in contrast with most drugs that are being developed as therapeutic agents, the dose-response neuroprotective effect of xenon has been poorly studied, although this effect could be of major critical importance for its clinical development as a neuroprotectant. Here we show, using ex vivo and in vivo models of excitotoxic insults and transient brain ischemia, that xenon, administered at subanesthetic doses, offers global neuroprotection from reduction of neurotransmitter release induced by ischemia, a critical event known to be involved in excitotoxicity, to reduction of subsequent cell injury and neuronal death. Maximal neuroprotection was obtained with xenon at 50 vol%, a concentration at which xenon further exhibited significant neuroprotective effects in vivo even when administered up to 4 h after intrastriatal NMDA injection and up to at least 2 h after induction of transient brain ischemia.

VX-induced cell death involves activation of caspase-3 in cultured rat cortical neurons

Exposure of cell cultures to organophosphorous compounds such as VX can result in cell death. However, it is not clear whether VX-induced cell death is necrotic or involves programmed cell death mechanisms. Activation of caspases, a family of cysteine proteases, is often involved in cell death, and in particular, caspase-3 activation appears to be a key event in programmed cell death processes including apoptosis. In this study, we investigated VX-induced neuronal cell death, as well as the underlying mechanism in terms of its effect on caspase-3 activity. Primary cortical neuronal cultures were prepared from gestational days 17 to 19 Sprague Dawley rat fetuses. At maturation, the cells were treated with varying concentrations of VX and cell death was evaluated by lactate dehydrogenase (LDH) release. VX induced an increase in LDH release in a concentration-dependent manner. Morphological VX-induced cell death was also characterized by using nuclear staining with propidium iodide and Hoechst 33342. VX induced a concentration- and time-dependent increase in caspase-3 activation. Caspase-3 activation was also confirmed by the proteolytic cleavage of poly(ADP-ribose)polymerase (PARP), an endogenous caspase-3 substrate. These data suggested that in rat cortical neurons, VX-induced cell death via a programmed cell death pathway that involves changes in caspase-3 protease.

Repeated contact with subtoxic soman leads to synaptic vulnerability in hippocampus

Soman, an anticholinesterase and dangerous nerve agent, produces convulsions, memory impairment, and cell loss in the brain, especially in the hippocampus. Soman-induced accumulation of acetylcholine initiates mechanisms responsible for the development of incapacitating seizures. The prolonged epileptiform nature of these seizures causes the release of another excitatory neurotransmitter, glutamate, which has been linked to the toxic action of the nerve agent. Here, we tested whether subtoxic soman exposures influence the brain's sensitivity to glutamate-based excitotoxicity. Over a 1-week period, hippocampal slice cultures were exposed daily to a transient level of soman that produced no evidence of synaptic deterioration. After the subtoxic soman treatments, however, the tissue became vulnerable to a brief episode of glutamate receptor overstimulation that normally resulted in little or no excitotoxic damage. In those slice cultures treated with subtoxic soman, a decline in synaptic markers as well as an increase in spectrin breakdown occurred 24 hr after the mild excitotoxic event. Exposure to high soman concentrations alone produced similar synaptic degeneration, but without evident cell death, suggesting that synaptic decline is an early neurotoxicological response to the nerve agent. Interestingly, enhanced excitotoxic sensitivity caused the brain tissue to become susceptible to disparate insults initiated before or after the soman contact. These findings indicate that seemingly innocuous soman exposures leave the hippocampus sensitive to the types of insults implicated in traumatic brain injury and stroke. They also warn that asymptomatic contact with soman may lead to progressive synaptopathogenesis and that early indicators of soman exposure are critical to prevent potential brain injury.

Status epilepticus causes necrotic damage in the mediodorsal nucleus of the thalamus in immature rats

Status epilepticus (StE) in immature rats causes long-term functional impairment. Whether this is associated with structural alterations remains controversial. The present study was designed to test the hypothesis that StE at an early age results in neuronal loss. StE was induced with lithium-pilocarpine in 12-d-old rats, and the presence of neuronal damage was investigated in the brain from 12 hr up to 1 week later using silver and Fluoro-Jade B staining techniques. Analysis of the sections indicated consistent neuronal damage in the central and lateral segments of the mediodorsal nucleus of the thalamus, which was confirmed using adjacent cresyl violet-stained preparations. The mechanism of thalamic damage (necrosis vs apoptosis) was investigated further using TUNEL, immunohistochemistry for caspase-3 and cytochrome c, and electron microscopy. Activated microglia were detected using OX-42 immunohistochemistry. The presence of silver and Fluoro-Jade B-positive degenerating neurons in the mediodorsal thalamic nucleus was associated with the appearance of OX-42-immunopositive activated microglia but not with the expression of markers of programmed cell death, caspase-3, or cytochrome c. Electron microscopy revealed necrosis of the ultrastructure of damaged neurons, providing further evidence that the mechanism of StE-induced damage in the mediodorsal thalamic nucleus at postnatal day 12 is necrosis rather than apoptosis. Finally, these data together with previously described functions of the medial and lateral segments of the mediodorsal thalamic nucleus suggest that some functions, such as adaptation to novelty, might become compromised after StE early in development.

Memantine is a clinically well tolerated N-methyl-D-aspartate (NMDA) receptor antagonist--a review of preclinical data

N-methyl-D-aspartate (NMDA) receptor antagonists have therapeutic potential in numerous CNS disorders ranging from acute neurodegeneration (e.g. stroke and trauma), chronic neurodegeneration (e.g. Parkinson's disease, Alzheimer's disease, Huntington's disease, ALS) to symptomatic treatment (e.g. epilepsy, Parkinson's disease, drug dependence, depression, anxiety and chronic pain). However, many NMDA receptor antagonists also produce highly undesirable side effects at doses within their putative therapeutic range. This has unfortunately led to the conclusion that NMDA receptor antagonism is not a valid therapeutic approach. However, memantine is clearly an uncompetitive NMDA receptor antagonist at therapeutic concentrations achieved in the treatment of dementia and is essentially devoid of such side effects at doses within the therapeutic range. This has been attributed to memantine's moderate potency and associated rapid, strongly voltage-dependent blocking kinetics. The aim of this review is to summarise preclinical data on memantine supporting its mechanism of action and promising profile in animal models of chronic neurodegenerative diseases. The ultimate purpose is to provide evidence that it is indeed possible to develop clinically well tolerated NMDA receptor antagonists, a fact reflected in the recent interest of several pharmaceutical companies in developing compounds with similar properties to memantine.

Modulation of the production of reactive oxygen species by pre-activated neutrophils by aminoadamantane derivatives

Aminoadamantane derivatives (AAD) such as amantadine or memantine have been used for the treatment of Morbus Parkinson and Morbus Alzheimer. In this communication, we report on the immunomodulatory activities of AAD. Luminol-dependent chemiluminescence of zymosan-, N-formylmethionylleucylphenylalanine(FMLP)- or experimental Ca2+-ionophore(A 231879)-preactivated polymorphonuclear leukocytes (PMN) was strongly enhanced by submicromolar concentrations of AAD and inhibited at higher concentrations than 0.1 mM. Light emission by phorbol-12-myristate-acetate(PMA)-preactivated cells was not further stimulated but inhibited by the elevated concentrations, just as with the other, above-mentioned activators. Ethylene formation from alpha-keto-methylthiobutyrate (KMB) as an indicator for production of OH.-type reactive oxygen species by the NADPH-oxidase ("respiratory burst") was augmented by AAD and completely inhibited by superoxide dismutase. In contrast, ethylene release from 1-amino-cyclopropyl-l-carboxylic acid (ACC) as relatively specific indicator for the myeloperoxidase reaction after degranulation was not influenced by AAD. As documented by several model reactions, AAD per se did not act as scavengers or quenchers of activated oxygen species such as superoxide, OH.-radical, hydrogen peroxide or hypochlorite. Altogether, these results suggest that submicromolar concentrations of AAD upregulate the respiratory burst, but apparently not the degranulation of prestimulated polymorphonuclear leukocytes. At higher concentrations of AAD, both respiratory burst and degranulation are inhibited, however. These effects can also be shown in complete blood samples.

Inhibition by soman of NMDA-stimulated [3H]norepinephrine release from rat cortical slices, studies of non-cholinergic effect

Effects of soman, an irreversible cholinesterase (ChE) inhibitor, on [3H]norepinephrine (NE) release evoked by N-methyl-d-aspartate (NMDA) were studied in rat brain cortical slices. Soman inhibited NMDA-stimulated [3H]NE release in a concentration-dependent manner. This effect was neither reversed by atropine, an antagonist of the muscarinic receptor, nor by d-tubocurarine, an antagonist of the nicotinic receptor. Incubation of the slices with NMDA antagonists, AP5, MK-801, ketamine or magnesium, resulted in inhibitory effects on NMDA-stimulated [3H]NE release. Soman significantly shifted the inhibition curves downward and significant interactions between these chemicals and soman were observed. Glycine potentiated the release of [3H]NE stimulated by NMDA, and soman did not alter this effect of glycine. Soman also inhibited the release of [3H]NE evoked by K+ in a concentration-dependent manner. NMDA-stimulated [3H]NE release was inhibited by tetrodotoxin (TTX), an antagonist of voltage-dependent sodium channels, and a significant interaction between soman and TTX was observed. The [3H]NE release induced by NMDA was dependent on extracellular calcium concentrations and was inhibited by nifedipine, a selective blocker of the L-type voltage-dependent calcium channels (VDCC), or cadmium, a non-specific blocker of VDCC. However, no significant interaction between the effects of soman and calcium, nifedipine, or cadmium was observed. Taken together, the results suggested that: (1) soman has a direct action at non-cholinergic sites; (2) soman may interfere with some of the regulatory sites of the NMDA receptor-ion channel complex; and (3) the voltage-dependent sodium channel, but not VDCC, may be a site of action for soman.

Neuropharmacological mechanisms of nerve agent-induced seizure and neuropathology

This paper proposes a three phase "model" of the neuropharmacological processes responsible for the seizures and neuropathology produced by nerve agent intoxication. Initiation and early expression of the seizures are cholinergic phenomenon; anticholinergics readily terminate seizures at this stage and no neuropathology is evident. However, if not checked, a transition phase occurs during which the neuronal excitation of the seizure per se perturbs other neurotransmitter systems: excitatory amino acid (EAA) levels increase reinforcing the seizure activity; control with anticholinergics becomes less effective; mild neuropathology is occasionally observed. With prolonged epileptiform activity the seizure enters a predominantly non-cholinergic phase: it becomes refractory to some anticholinergics; benzodiazepines and N-methyl-D-aspartate (NMDA) antagonists remain effective as anticonvulsants, but require anticholinergic co-administration; mild neuropathology is evident in multiple brain regions. Excessive influx of calcium due to repeated seizure-induced depolarization and prolonged stimulation of NMDA receptors is proposed as the ultimate cause of neuropathology. The model and data indicate that rapid and aggressive management of seizures is essential to prevent neuropathology from nerve agent exposure.