Elsevier

NeuroToxicology

Volume 33, Issue 6, December 2012, Pages 1476-1490
NeuroToxicology

27th Int Neurotox Conf
Exposure to nerve agents: From status epilepticus to neuroinflammation, brain damage, neurogenesis and epilepsy

https://doi.org/10.1016/j.neuro.2012.09.001Get rights and content

Abstract

Epilepsy is a common neurological disorder characterized by an initial injury due to stroke, traumatic brain injury, brain infection, or febrile seizures causing status epilepticus (SE). This phenomenon precedes recurrent (secondary) seizures, the latent period (period without seizures) and downstream appearance of spontaneous recurrent seizures (SRS). Epilepsy inducers include the organophosphorous (OP) compounds modified as chemical warfare nerve agents, such as soman. SE induced by soman is a result of cholinergic system hyperactivity caused by the irreversible inhibition of acetylcholinesterase, and the subsequent increase in the amount of the neurotransmitter acetylcholine at central and peripheral sites. SE leads to profound, permanent, complex and widespread brain damage and associated cognitive and behavioral deficits, accompanied by impaired neurogenesis. Several anticonvulsant and neuroprotective strategies have been studied in order to avoid the epileptogenesis which occurs after SE caused by soman exposure. In recent studies, we showed that SRS occur post-soman exposure and neuropathology can be reduced with diazepam (DZP) and valproic acid (VPA) when administered in combination treatment. These effects are accompanied by neurogenesis seen 15 days post-exposure in the hippocampal dentate gyrus (DG). This review discusses several findings about epilepsy induced by soman exposure such as behavioral changes, EEG anomalies, neuropathology, neuroinflammation, neurogenesis, possible circuitry changes and current strategies for treatment. The soman seizure model is an important model of temporal lobe epilepsy (TLE) and comparable in certain respects with well studied models in the literature such as pilocarpine and kainic acid. All these models together allow for a greater understanding of the different mechanisms of seizure induction, propagation and options for treatment. These studies are very necessary for current military and civilian treatment regimens, against OP nerve agent exposure, which fail to prevent SE resulting in severe neuropathology and epilepsy.

Introduction

Exposure to organophosphorous (OP) compounds, such as those present in insecticides and chemical warfare agents, can induce severe neurological dysfunction (Carpentier et al., 1990, McDonough et al., 1986, Myhrer, 2007, Petras, 1994, Shih and McDonough, 1997). Organophosphorous insecticides can cause miosis, convulsions and changes in consciousness (Violan et al., 1985). According to Eskenazi and colleagues (1999), children can be easily exposed to OPs during exploration of their environment, either through oral or dermal contact with surfaces contaminated with OPs. Bradman and co-workers (2007) found several types of OP pesticides in food samples and urine when investigating the children of farm workers in the US. Also, according to Guodong and co-workers (2012), there are higher levels of OP urinary metabolites in children living in China than in developed countries, but without relationship with development. However, animal models indicate that repeated low-level exposure to OP pesticides reduces performance in maze, locomotion and balance, suggesting that children could also suffer similar changes (Eskenazi et al., 2008). Indeed, there is a significant association between level of exposure to OP, measured through dialkylphosphate metabolite levels, and the number of abnormal reflexes (Young et al., 2005). Rauh and colleagues (2006) reported neurodevelopmental problems in children after pre-natal exposure to OP. Also, according to (Eskenazi et al., 2008), OP dialkylphosphate metabolites levels in prenatal maternal urine were related to the number of abnormal reflexes in the neonates. Additionally, dialkylphosphate metabolite levels were associated with reduced mental developmental scores at 24 months (Eskenazi et al., 2008). Searles Nielsen and co-workers (2010) noted the presence of brain tumors in children exposed to OPs that did not have appropriate detoxification. Interestingly, a variant of the paraoxonase 1 gene results in reduced levels of paraoxonase enzyme, resulting in increased risk of severe effects of OP exposure, since this enzyme metabolizes OPs (Eskenazi et al., 2008).

During World War II, the Germans modified OP compounds and produced several chemical warfare nerve agents including, but not limited to, soman (Petras, 1994). These nerve agents (NA) are irreversible inhibitors of the enzyme acetylcholinesterase, which, when blocked, causes an increase in acetylcholine at central and peripheral sites (Shih and McDonough, 1997). As a result, muscle fasciculation and cardiac and respiratory distress occur very rapidly (McDonough et al., 1995) after exposure and can ultimately lead to death if left untreated or if treated too late (McDonough et al., 1995, Shih and McDonough, 1997).

The autonomic system is heavily affected since the transmission from pre-ganglionic to post-ganglionic neurons is dependent on acetylcholine. The levels of acetylcholinesterase remain low for several days after exposure (McDonough et al., 1986), but the range of autonomic changes is also dependent on the exposure route. If exposure to NA is via inhalation, the vapor comes into contact with the eyes and the respiratory tract, causing miosis (pupil constriction) and difficulty breathing, respectively. In one study when rats were exposed to soman vapor and received oxotremorine (a cholinomimetic that acts as a non-selective muscarinic receptor agonist), the miotic response was decreased, indicating that soman likely has a downregulatory effect on muscarinic receptor function (Dabisch et al., 2007). In this same study, pupil diameter returned to baseline pre-exposure measurements before acetylcholinesterase levels returned to baseline values (Dabisch et al., 2007). In the respiratory system, the excessive stimulation of muscarinic receptors results in contraction of smooth muscles in the airways, causing asthma-like symptoms and tightness of the chest (Dabisch and Taylor, 2010). Its peripheral effects can be blocked through the use of anticholinergics such as atropine and oxime scavengers, such as HI-6 (Capacio and Shih, 1991, Philippens et al., 1992, Shih and McDonough, 1999).

Overstimulation of muscarinic receptors also affects the heart and leads to desensitization and more variability in heart rate (Dabisch and Taylor, 2010). Tryphonas and Clement (1995) suggested that myocardial damage is neurogenic and associated with prolonged seizure activity that causes lesions in the central nervous system. The same study showed that the damage involved myocytolysis, fibrosis and inflammatory processes consistently occurring in the left ventricle. Tryphonas and co-workers (1996) also detected several histological changes including hypercontraction and hyperextension of sarcomeres and myocytolysis as early as 10 min after soman exposure in rats. McDonough and co-workers (1995) observed cardiac damage occurring at a much higher percentage than brain damage in animals treated with anticonvulsants such as diazepam (DZP), indicating no correlation between the heart and brain damage. Interestingly, Britt and co-workers (2000) observed a weak correlation between brain damage and heart damage in rhesus macaques 10 days after exposure to soman. In the same study, no significant difference was found when comparing groups of animals with and without behavioral seizures, although it is speculated that the latter may have presented subclinical epileptiform activity (EEG was not recorded).

NA can rapidly cross the blood brain barrier (BBB) and induce severe seizures, initially through overstimulation of cholinergic pathways (Shih and McDonough, 1997). Seizures are clinical manifestations of exacerbated synchronicity and excitation in specific neuronal networks and can result in severe brain damage if prolonged (Shorvon, 2000). Seizures can reversibly open the BBB, with permeability reaching a peak at between 30 and 60 min of EEGraphic seizure activity (Carpentier et al., 1990). Usually, the most severe brain damage can be avoided if DZP, a GABAergic agonist, is injected early enough after the onset of status epilepticus (SE), which is defined as prolonged and sustained seizures (McDonough and Shih, 1997, Sloviter, 1999). However, delayed treatment with DZP can only momentarily block the SE (McDonough and Shih, 1993, Harris et al., 1994, de Araujo Furtado et al., 2010). As the SE progress, glutamatergic networks are recruited (Wasterlain and Shirasaka, 1994, McDonough and Shih, 1997) and several other neurochemical changes may occur. These changes increase the level of excitability in the brain drastically and are difficult to reverse. The secondary events of SE, which are called recurrent seizures (RS), begin a few hours later and, if left untreated, can potentially result in more brain damage.

As a result of brain damage, several neuroplastic changes take place. Among these changes, axonal sprouting (Mello et al., 1993) and neurogenesis (Parent et al., 1997) have been seen in several chemically induced seizure models. These attempts at brain rewiring may be compensatory, but could actually be detrimental, since axonal sprouting and neurogenesis appear temporally with spontaneous recurrent seizures (SRS) in animal models of epilepsy (Leite et al., 1990, Mello et al., 1993). According to Dudek and Staley (2012), the axonal sprouting and formation of new synaptic circuits that could be either excitatory or inhibitory has acquired attention because of its time-dependence that may play a role on the latent period and epileptogenesis. This review assesses how an initial acute exposure to NA can induce epilepsy and considers the role played by inflammatory markers, neurogenesis and axonal sprouting in the hippocampus. The main focus of this review is on the soman model, due to the resistance of this type of NA to standard therapy, since soman-inhibited acetylcholinesterase rapidly goes through permanent chemical changes or “aging” making the reactivation of acetylcholinesterase impossible (Shih and McDonough, 1999).

Section snippets

Animal models of seizures

Several animal models have been used to study the consequences of NA exposure because of similar symptomatology following NA exposure. For example, the guinea pig model is widely used since these animals display a better response to pre-treatment with reversible acetylcholinesterase inhibitors and atropine post-treatment than mice or rats (Albuquerque et al., 2006). Interestingly, the s.c. LD50 of soman is lower in guinea pigs than in rats (Maxwell et al., 1988), due to the fact that the former

Recommendation for exposed human populations

According to Marrs and co-workers (2006), the maintenance of vital body functions, heart rate, blood pressure, ECG and arterial blood gases monitoring and avoidance of further NA absorption is crucial. Skin decontamination should be performed only when the exposure is dermal. When ocular exposure occurs, the eyes should be irrigated with water or 0.9% sodium chloride solution. The combination of atropine and oxime is standard, but there are controversies with respect to the type and dose of

Conclusions

Consequences of exposure to soman are severe and irreparable. SE induced through NA exposure leads to huge and widespread neuronal damage in the entire brain, affecting important areas such as the hippocampus, amygdala, piriform cortex, cortex and thalamus. Sustained SE and downstream sequelae can cause prolonged physical incapacitation in rodent and non-human primate species. Studies to reduce the consequences of soman exposure, mainly to treat soldiers potentially exposed to battlefield NA,

Conflicts of interest

None of the authors has any conflict of interest to disclose.

Acknowledgements

Dr. Marcio de Araujo Furtado is currently employed by Clinical RM, Inc., and under contract to the Walter Reed Army Institute of Research. Dr. Franco Rossetti is supported administratively by a National Research Council fellowship. The present study was supported financially by the Defense Threat Reduction Agency (Grant I.E0042-08-WR-C, Principal Investigator: Dr. Debra Yourick).

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    Material has been reviewed by the Walter Reed Army Institute of Research. There is no objection to its presentation and/or publication. The opinions or assertions contained herein are the private views of the author, and are not to be construed as official, or as reflecting true views of the Department of the Army or the Department of Defense. Research was conducted in compliance with the Animal Welfare Act and other federal statutes and regulations relating to animals and experiments involving animals and adheres to principles stated in the Guide for the Care and Use of Laboratory Animals, NRC Publication, 1996 edition.

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