The limitations of diazepam as a treatment for nerve agent-induced seizures and neuropathology in rats: comparison with UBP302

A longitudinal MRI and TSPO PET-based investigation of brain region-specific neuroprotection by diazepam versus midazolam following organophosphate-induced seizures

Acute poisoning with organophosphorus cholinesterase inhibitors (OPs), such as OP nerve agents and pesticides, can cause life threatening cholinergic crisis and status epilepticus (SE). Survivors often experience significant morbidity, including brain injury, acquired epilepsy, and cognitive deficits. Current medical countermeasures for acute OP poisoning include a benzodiazepine to mitigate seizures. Diazepam was long the benzodiazepine included in autoinjectors used to treat OP-induced seizures, but it is now being replaced in many guidelines by midazolam, which terminates seizures more quickly, particularly when administered intramuscularly. While a direct correlation between seizure duration and the extent of brain injury has been widely reported, there are limited data comparing the neuroprotective efficacy of diazepam versus midazolam following acute OP intoxication. To address this data gap, we used non-invasive imaging techniques to longitudinally quantify neuropathology in a rat model of acute intoxication with the OP diisopropylfluorophosphate (DFP) with and without post-exposure intervention with diazepam or midazolam. Magnetic resonance imaging (MRI) was used to monitor neuropathology and brain atrophy, while positron emission tomography (PET) with a radiotracer targeting translocator protein (TSPO) was utilized to assess neuroinflammation. Animals were scanned at 3, 7, 28, 65, 91, and 168 days post-DFP and imaging metrics were quantitated for the hippocampus, amygdala, piriform cortex, thalamus, cerebral cortex and lateral ventricles. In the DFP-intoxicated rat, neuroinflammation persisted for the duration of the study coincident with progressive atrophy and ongoing tissue remodeling. Benzodiazepines attenuated neuropathology in a region-dependent manner, but neither benzodiazepine was effective in attenuating long-term neuroinflammation as detected by TSPO PET. Diffusion MRI and TSPO PET metrics were highly correlated with seizure severity, and early MRI and PET metrics were positively correlated with long-term brain atrophy. Collectively, these results suggest that anti-seizure therapy alone is insufficient to prevent long-lasting neuroinflammation and tissue remodeling.

Efficacy of a combined anti-seizure treatment against cholinergic established status epilepticus following a sarin nerve agent insult in rats

The development of refractory status epilepticus (SE) following sarin intoxication presents a therapeutic challenge. Here, we evaluated the efficacy of delayed combined double or triple treatment in reducing abnormal epileptiform seizure activity (ESA) and the ensuing long-term neuronal insult. SE was induced in rats by exposure to 1.2 LD50 sarin followed by treatment with atropine and TMB4 (TA) 1 min later. Double treatment with ketamine and midazolam or triple treatment with ketamine, midazolam and levetiracetam was administered 30 min post-exposure, and the results were compared to those of single treatment with midazolam alone or triple treatment with ketamine, midazolam, and valproate, which was previously shown to ameliorate this neurological insult. Toxicity and electrocorticogram activity were monitored during the first week, and behavioral evaluations were performed 2 weeks post-exposure, followed by biochemical and immunohistopathological analyses. Both double and triple treatment reduced mortality and enhanced weight recovery compared to TA-only treatment. Triple treatment and, to a lesser extent, double treatment significantly ameliorated the ESA duration. Compared to the TA-only or the TA+ midazolam treatment, both double and triple treatment reduced the sarin-induced increase in the neuroinflammatory marker PGE2 and the brain damage marker TSPO and decreased gliosis, astrocytosis and neuronal damage. Finally, both double and triple treatment prevented a change in behavior, as measured in the open field test. No significant difference was observed between the efficacies of the two triple treatments, and both triple combinations completely prevented brain injury (no differences from the naïve rats). Delayed double and, to a greater extent, triple treatment may serve as an efficacious delayed therapy, preventing brain insult propagation following sarin-induced refractory SE.

N-methyl-d-aspartate receptors: Structure, function, and role in organophosphorus compound poisoning

Acute organophosphorus compound (OP) poisoning induces symptoms of the cholinergic crises with the occurrence of severe epileptic seizures. Seizures are induced by hyperstimulation of the cholinergic system, but are enhanced by hyperactivation of the glutamatergic system. Overstimulation of muscarinic cholinergic receptors by the elevated acetylcholine causes glutamatergic hyperexcitation and an increased influx of Ca2+ into neurons through a type of ionotropic glutamate receptors, N-methyl-d-aspartate (NMDA) receptors (NMDAR). These excitotoxic signaling processes generate reactive oxygen species, oxidative stress, and activation of the neuroinflammatory response, which can lead to recurrent epileptic seizures, neuronal cell death, and long-term neurological damage. In this review, we illustrate the NMDAR structure, complexity of subunit composition, and the various receptor properties that change accordingly. Although NMDARs are in normal physiological conditions important for controlling synaptic plasticity and mediating learning and memory functions, we elaborate the detrimental role NMDARs play in neurotoxicity of OPs and focus on the central role NMDAR inhibition plays in suppressing neurotoxicity and modulating the inflammatory response. The limited efficacy of current medical therapies for OP poisoning concerning the development of pharmacoresistance and mitigating proinflammatory response highlights the importance of NMDAR inhibitors in preventing neurotoxic processes and points to new avenues for exploring therapeutics for OP poisoning.

Neurosteroids as Novel Anticonvulsants for Refractory Status Epilepticus and Medical Countermeasures for Nerve Agents: A 15-Year Journey to Bring Ganaxolone from Bench to Clinic

This article describes recent advances in the use of neurosteroids as novel anticonvulsants for refractory status epilepticus (RSE) and as medical countermeasures (MCs) for organophosphates and chemical nerve agents (OPNAs). We highlight a comprehensive 15-year journey to bring the synthetic neurosteroid ganaxolone (GX) from bench to clinic. RSE, including when caused by nerve agents, is associated with devastating morbidity and permanent long-term neurologic dysfunction. Although recent approval of benzodiazepines such as intranasal midazolam and intranasal midazolam offers improved control of acute seizures, novel anticonvulsants are needed to suppress RSE and improve neurologic function outcomes. Currently, few anticonvulsant MCs exist for victims of OPNA exposure and RSE. Standard-of-care MCs for postexposure treatment include benzodiazepines, which do not effectively prevent or mitigate seizures resulting from nerve agent intoxication, leaving an urgent unmet medical need for new anticonvulsants for RSE. Recently, we pioneered neurosteroids as next-generation anticonvulsants that are superior to benzodiazepines for treatment of OPNA intoxication and RSE. Because GX and related neurosteroids that activate extrasynaptic GABA-A receptors rapidly control seizures and offer robust neuroprotection by reducing neuronal damage and neuroinflammation, they effectively improve neurologic outcomes after acute OPNA exposure and RSE. GX has been selected for advanced, Biomedical Advanced Research and Development Authority-supported phase 3 trials of RSE and nerve agent seizures. In addition, in mechanistic studies of neurosteroids at extrasynaptic receptors, we identified novel synthetic analogs with features that are superior to GX for current medical needs. Development of new MCs for RSE is complex, tedious, and uncertain due to scientific and regulatory challenges. Thus, further research will be critical to fill key gaps in evaluating RSE and anticonvulsants in vulnerable (pediatric and geriatric) populations and military persons. SIGNIFICANCE STATEMENT: Following organophosphate and nerve agent intoxication, refractory status epilepticus (RSE) occurs despite benzodiazepine treatment. RSE occurs in 40% of status epilepticus patients, with a 35% mortality rate and significant neurological morbidity in survivors. To treat RSE, neurosteroids are better anticonvulsants than benzodiazepines. Our pioneering use of neurosteroids for RSE and nerve agents led us to develop ganaxolone as a novel anticonvulsant and neuroprotectant with significantly improved neurological outcomes. This article describes the bench-to-bedside journey of bringing neurosteroid therapy to patients, with ganaxolone leading the way.

Sex Differences in Organophosphate Model of Benzodiazepine-Refractory Status Epilepticus and Neuronal Damage

Sex differences are common in human epilepsy. Although men are more susceptible to seizure than women, the mechanisms underlying sex-specific vulnerabilities to seizure are unclear. The organophosphate (OP) diisopropylfluorophosphate (DFP) is known to cause neurotoxicity and status epilepticus (SE), a serious neurologic condition that causes prolonged seizures and brain damage. Current therapies for OP poisoning and SE do not consider neuronal variations between male and female brains. Therefore, we investigated sex-dependent differences in electrographic seizure activity and neuronal injury using the DFP model of refractory SE in rats. Electroencephalogram recordings were used to monitor DFP-induced SE, and the extent of brain injury was determined using fluoro-jade-B staining to detect cellular necrosis. After DFP exposure, we observed striking sex-dependent differences in SE and seizure activity patterns as well as protective responses to midazolam treatment. Following acute DFP exposure, male animals displayed more severe SE with intense epileptiform spiking and greater mortality than females. In contrast, we observed significantly more injured cells and cellular necrosis in the hippocampus and other brain regions in females than in males. We also observed extensive neuronal injury in the somatosensory cortex of males. The anticonvulsant effect of midazolam against SE was limited in this model and found to be similar in males and females. However, unlike males, females exhibited substantially more protection against neuronal damage after midazolam treatment. Overall, these results demonstrate significant sex-dependent differences in DFP-induced refractory SE and neuronal damage patterns, suggesting that it may be possible to develop sex-specific neuroprotective strategies for OP intoxication and refractory SE. SIGNIFICANCE STATEMENT: Sex-dependent differences in neurotoxicity and status epilepticus (SE) are key biological variables after organophosphate (OP) exposure. Here, we investigated sex-dependent differences in SE and brain injury after acute diisopropylfluorophosphate exposure. Male rats had more severe SE and less survival than females, while females had more neuronal damage. Females had more neuroprotection to midazolam than males, while both sexes had similar but partial anticonvulsant effects. These findings suggest that a sex-specific therapeutic approach may prevent neurological complications of OP-induced SE.

Protective Activity of Novel Hydrophilic Synthetic Neurosteroids on Organophosphate Status Epilepticus-induced Chronic Epileptic Seizures, Non-Convulsive Discharges, High-Frequency Oscillations, and Electrographic Ictal Biomarkers

Nerve agents and organophosphates (OP) are neurotoxic chemicals that induce acute seizures, status epilepticus (SE), and mortality. Long-term neurologic and neurodegenerative effects manifest months to years after OP exposure. Current benzodiazepine anticonvulsants are ineffective in preventing such long-term neurobehavioral and neuropathological changes. New and effective anticonvulsants are needed for OP intoxication, especially for mitigating the long-term sequelae after acute exposure. We developed neurosteroids as novel anticonvulsants and neuroprotectants in OP exposure models. In this study, we evaluated the long-term efficacy of novel synthetic neurosteroids in preventing the development of chronic epilepsy and hyperexcitable ictal events in a rat OP model of SE. Rats were exposed to the OP nerve agent surrogate diisopropylfluorophosphate (DFP), and the experimental groups were treated with the synthetic neurosteroid valaxanolone (VX) or lysaxanolone (LX) 40 minutes post-exposure in conjunction with midazolam. Video-electroencephalography was monitored for two months to assess spontaneous recurrent seizures (SRS), epileptiform discharges, interictal spikes, and high-frequency oscillations (HFOs). Within 60 days of DFP exposure, rats developed chronic epilepsy characterized by frequent SRS, epileptiform discharges, and HFOs. LX treatment was associated with a dose-dependent reduction of epilepsy occurrence and overall seizure burden with a significant decrease in SRS and epileptiform discharges. It also significantly reduced the occurrence of epileptic biomarkers of HFOs and interictal spikes, indicating potential disease-modifying activity. Similarly, the neurosteroid analog VX also significantly attenuated SRS, discharges, HFOs, and ictal events. These results demonstrate the long-term protective effects of synthetic neurosteroids in the OP-exposed post-SE model, indicating their disease-modifying potential to prevent epilepsy and ictal abnormalities. SIGNIFICANCE STATEMENT: The effects of nerve agents and organophosphate (OP) exposure are persistent, and survivors suffer from a number of devastating, chronic neurological dysfunctions. Currently, there is no specific therapy for preventing this disastrous impact of OP exposure. We propose synthetic neurosteroids that activate tonic inhibition provide viable options for preventing the long-term neurological effects of OP intoxication. The results from this study reveal the disease-modifying potential of two novel synthetic neurosteroids in preventing epileptogenesis and chronic epileptic seizures after OP-induced SE.

Long-Term Neuropsychiatric Developmental Defects after Neonatal Organophosphate Exposure: Mitigation by Synthetic Neurosteroids

Children are much more susceptible to the neurotoxic effects of organophosphate (OP) pesticides and nerve agents than adults. OP poisoning in children leads to acute seizures and neuropsychiatric sequela, including the development of long-term disabilities and cognitive impairments. Despite these risks, there are few chronic rodent models that use pediatric OP exposure for studying neurodevelopmental consequences and interventions. Here, we investigated the protective effect of the neurosteroid ganaxolone (GX) on the long-term developmental impact of neonatal exposure to the OP compound, diisopropyl-fluorophosphate (DFP). Pediatric postnatal day-28 rats were acutely exposed to DFP, and at 3 and 10 months after exposure, they were evaluated using a series of cognitive and behavioral tests with or without the postexposure treatment of GX. Analysis of the neuropathology was performed after 10 months. DFP-exposed animals displayed significant long-term deficits in mood, anxiety, depression, and aggressive traits. In spatial and nonspatial cognitive tests, they displayed striking impairments in learning and memory. Analysis of brain sections showed significant loss of neuronal nuclei antigen(+) principal neurons, parvalbumin(+) inhibitory interneurons, and neurogenesis, along with increased astrogliosis, microglial neuroinflammation, and mossy fiber sprouting. These detrimental neuropathological changes are consistent with behavioral dysfunctions. In the neurosteroid GX-treated cohort, behavioral and cognitive deficits were significantly reduced and were associated with strong protection against long-term neuroinflammation and neurodegeneration. In conclusion, this pediatric model replicates the salient features of children exposed to OPs, and the protective outcomes from neurosteroid intervention support the viability of developing this strategy for mitigating the long-term effects of acute OP exposure in children. SIGNIFICANCE STATEMENT: An estimated 3 million organophosphate exposures occur annually worldwide, with children comprising over 30% of all victims. Our understanding of the neurodevelopmental consequences in children exposed to organophosphates is limited. Here, we investigated the long-term impact of neonatal exposure to diisopropyl-fluorophosphate in pediatric rats. Neurosteroid treatment protected against major deficits in behavior and memory and was well correlated with neuropathological changes. Overall, this pediatric model is helpful to screen novel therapies to mitigate long-term developmental deficits of organophosphate exposure.

Preventing Long-Term Brain Damage by Nerve Agent-Induced Status Epilepticus in Rat Models Applicable to Infants: Significant Neuroprotection by Tezampanel Combined with Caramiphen but Not by Midazolam Treatment

Acute exposure to nerve agents induces a peripheral cholinergic crisis and prolonged status epilepticus (SE), causing death or long-term brain damage. To provide preclinical data pertinent to the protection of infants and newborns, we compared the antiseizure and neuroprotective effects of treating soman-induced SE with midazolam (MDZ) versus tezampanel (LY293558) in combination with caramiphen (CRM) in 12- and 7-day-old rats. The anticonvulsants were administered 1 hour after soman exposure; neuropathology data were collected up to 6 months postexposure. In both ages, the total duration of SE within 24 hours after soman exposure was significantly shorter in the LY293558 plus CRM groups compared with the MDZ groups. Neuronal degeneration was substantial in the MDZ-treated groups but absent or minimal in the groups treated with LY293558 plus CRM. Loss of neurons and interneurons in the basolateral amygdala and CA1 hippocampal area was significant in the MDZ-treated groups but virtually absent in the LY293558 plus CRM groups. Atrophy of the amygdala and hippocampus occurred only in MDZ-treated groups. Neuronal/interneuronal loss and atrophy of the amygdala and hippocampus deteriorated over time. Reduction of inhibitory activity in the basolateral amygdala and increased anxiety were found only in MDZ groups. Spontaneous recurrent seizures developed in the MDZ groups, deteriorating over time; a small percentage of rats from the LY293558 plus CRM groups also developed seizures. These results suggest that brain damage can be long lasting or permanent if nerve agent-induced SE in infant victims is treated with midazolam at a delayed timepoint after SE onset, whereas antiglutamatergic treatment with tezampanel and caramiphen provides significant neuroprotection. SIGNIFICANCE STATEMENT: To protect the brain and the lives of infants in a mass exposure to nerve agents, an anticonvulsant treatment must be administered that will effectively stop seizures and prevent neuropathology, even if offered with a relative delay after seizure onset. The present study shows that midazolam, which was recently approved by the Food and Drug Administration for the treatment of nerve agent-induced status epilepticus, is not an effective neuroprotectant, whereas brain damage can be prevented by targeting glutamate receptors.

A Pediatric Rat Model of Organophosphate-Induced Refractory Status Epilepticus: Characterization of Long-Term Epileptic Seizure Activity, Neurologic Dysfunction and Neurodegeneration

Children are highly vulnerable to the neurotoxic effects of organophosphates (OPs), which can cause neuronal developmental defects, including intellectual disability, autism, epilepsy, and related comorbidities. Unfortunately, no specific pediatric OP neurotoxicity model currently exists. In this study, we developed and characterized a pediatric rat model of status epilepticus (SE) induced by the OP diisopropylfluorophosphate (DFP) and examined its impact on long-term neurological outcomes. Postnatal day 21 rats were exposed to a DFP regimen with standard antidotes. Progressive behavioral deteriorations were assessed over a three-month period. Development of epileptic seizures, ictal discharges, high-frequency oscillations (HFOs), and interictal spikes were monitored by video-electroencephalography recordings. Histology-stereology analysis was performed to assess neurodegeneration, neuroinflammation, and morphologic abnormalities. DFP-exposed, post-SE animals exhibited significantly elevated levels of anxiety and depression than age-matched controls at 1, 2, and 3 months post-exposure. DFP-exposed animals displayed aggressive behavior and a marked decline in object recognition memory, as well as prominent impairment in spatial learning and memory. DFP-exposed animals had striking electrographic abnormalities with the occurrence of displayed epileptic seizures, ictal discharges, HFOs, and interictal spikes, suggesting chronic epilepsy. Neuropathological analysis showed substantially fewer principal neurons and inhibitory interneurons with a marked increase in reactive microglia and neuroinflammation in the hippocampus and other brain regions. DFP-exposed animals also exhibited mossy fiber sprouting indicating impaired network formations. Long-term epileptic seizures and neuropsychiatric functional deficits induced by DFP were consistent with neuropathological defects. Collectively, this pediatric model displays many hallmarks of chronic sequelae reminiscent of children exposed to OPs, suggesting that it will be a valuable tool for investigating pathologic mechanisms and potential treatment strategies to attenuate long-term OP neurotoxicity. SIGNIFICANCE STATEMENT: Millions of children are exposed to organophosphates (OPs) used in agriculture or chemical incidents. This study investigated the long-term impact of neonatal exposure to the OP chemical diisopropylfluorophosphate (DFP) on neurobehavioral and neurodevelopmental outcomes in adulthood. DFP exposure caused long-lasting behavioral abnormalities, epileptic seizures, and bilateral brain defects with an array of neurological sequelae seen in children's OP neurotoxicity. Thus, this model provides a novel tool to explore therapeutic interventions that mitigate long-term neurotoxic effects of children exposed to OP-induced seizures and status epilepticus.

Brief isoflurane administration as an adjunct treatment to control organophosphate-induced convulsions and neuropathology

Organophosphate-based chemical agents (OP), including nerve agents and certain pesticides such as paraoxon, are potent acetylcholinesterase inhibitors that cause severe convulsions and seizures, leading to permanent central nervous system (CNS) damage if not treated promptly. The current treatment regimen for OP poisoning is intramuscular injection of atropine sulfate with an oxime such as pralidoxime (2-PAM) to mitigate cholinergic over-activation of the somatic musculature and autonomic nervous system. This treatment does not provide protection against CNS cholinergic overactivation and therefore convulsions require additional medication. Benzodiazepines are the currently accepted treatment for OP-induced convulsions, but the convulsions become refractory to these GABAA agonists and repeated dosing has diminishing effectiveness. As such, adjunct anticonvulsant treatments are needed to provide improved protection against recurrent and prolonged convulsions and the associated excitotoxic CNS damage that results from them. Previously we have shown that brief, 4-min administration of 3%-5% isoflurane in 100% oxygen has profound anticonvulsant and CNS protective effects when administered 30 min after a lethal dose of paraoxon. In this report we provide an extended time course of the effectiveness of 5% isoflurane delivered for 5 min, ranging from 60 to 180 min after a lethal dose of paraoxon in rats. We observed substantial effectiveness in preventing neuronal loss as shown by Fluoro-Jade B staining when isoflurane was administered 1 h after paraoxon, with diminishing effectiveness at 90, 120 and 180 min. In vivo magnetic resonance imaging (MRI) derived T2 and mean diffusivity (MD) values showed that 5-min isoflurane administration at a concentration of 5% prevents brain edema and tissue damage when administered 1 h after a lethal dose of paraoxon. We also observed reduced astrogliosis as shown by GFAP immunohistochemistry. Studies with continuous EEG monitoring are ongoing to demonstrate effectiveness in animal models of soman poisoning.

Disease-modifying effects of a glial-targeted inducible nitric oxide synthase inhibitor (1400W) in mixed-sex cohorts of a rat soman (GD) model of epilepsy

BACKGROUND

Acute exposure to seizurogenic organophosphate (OP) nerve agents (OPNA) such as diisopropylfluorophosphate (DFP) or soman (GD), at high concentrations, induce immediate status epilepticus (SE), reactive gliosis, neurodegeneration, and epileptogenesis as a consequence. Medical countermeasures (MCMs-atropine, oximes, benzodiazepines), if administered in < 20 min of OPNA exposure, can control acute symptoms and mortality. However, MCMs alone are inadequate to prevent OPNA-induced brain injury and behavioral dysfunction in survivors. We have previously shown that OPNA exposure-induced SE increases the production of inducible nitric oxide synthase (iNOS) in glial cells in both short- and long- terms. Treating with a water soluble and highly selective iNOS inhibitor, 1400W, for 3 days significantly reduced OPNA-induced brain changes in those animals that had mild-moderate SE in the rat DFP model. However, such mitigating effects and the mechanisms of 1400W are unknown in a highly volatile nerve agent GD exposure.

METHODS

Mixed-sex cohort of adult Sprague Dawley rats were exposed to GD (132 μg/kg, s.c.) and immediately treated with atropine (2 mg/kg, i.m) and HI-6 (125 mg/kg, i.m.). Severity of seizures were quantified for an hour and treated with midazolam (3 mg/kg, i.m.). An hour post-midazolam, 1400W (20 mg/kg, i.m.) or vehicle was administered daily for 2 weeks. After behavioral testing and EEG acquisition, animals were euthanized at 3.5 months post-GD. Brains were processed for neuroinflammatory and neurodegeneration markers. Serum and CSF were used for nitrooxidative and proinflammatory cytokines assays.

RESULTS

We demonstrate a significant long-term (3.5 months post-soman) disease-modifying effect of 1400W in animals that had severe SE for > 20 min of continuous convulsive seizures. 1400W significantly reduced GD-induced motor and cognitive dysfunction; nitrooxidative stress (nitrite, ROS; increased GSH: GSSG); proinflammatory cytokines in the serum and some in the cerebrospinal fluid (CSF); epileptiform spikes and spontaneously recurring seizures (SRS) in males; reactive gliosis (GFAP + C3 and IBA1 + CD68-positive glia) as a measure of neuroinflammation, and neurodegeneration (especially parvalbumin-positive neurons) in some brain regions.

CONCLUSION

These findings demonstrate the long-term disease-modifying effects of a glial-targeted iNOS inhibitor, 1400W, in a rat GD model by modulating reactive gliosis, neurodegeneration (parvalbumin-positive neurons), and neuronal hyperexcitability.

Disease-Modifying Effects of a Glial-targeted Inducible Nitric Oxide Synthase Inhibitor (1400W) in Mixed-sex Cohorts of a Rat Soman (GD) Model of Epilepsy

Background Acute exposure to seizurogenic organophosphate (OP) nerve agents (OPNA) such as diisopropylfluorophosphate (DFP) or soman (GD), at high concentrations, induce immediate status epilepticus (SE), reactive gliosis, neurodegeneration, and epileptogenesis as a consequence. Medical countermeasures (MCMs- atropine, oximes, benzodiazepines), if administered in < 20 minutes of OPNA exposure, can control acute symptoms and mortality. However, MCMs alone are inadequate to prevent OPNA-induced brain injury and behavioral dysfunction in survivors. We have previously shown that OPNA exposure-induced SE increases the production of inducible nitric oxide synthase (iNOS) in glial cells in both short- and long- terms. Treating with a water soluble and highly selective iNOS inhibitor, 1400W, for three days significantly reduced OPNA-induced brain changes in those animals that had mild-moderate SE in the rat DFP model. However, such mitigating effects and the mechanisms of 1400W are unknown in a highly volatile nerve agent GD exposure. Methods Mixed-sex cohort of adult Sprague Dawley rats were exposed to GD (132µg/kg, s.c.) and immediately treated with atropine (2mg/kg, i.m) and HI-6 (125mg/kg, i.m.). Severity of seizures were quantified for an hour and treated with midazolam (3mg/kg, i.m.). An hour post-midazolam, 1400W (20mg/kg, i.m.) or vehicle was administered daily for two weeks. After behavioral testing and EEG acquisition, animals were euthanized at 3.5 months post-GD. Brains were processed for neuroinflammatory and neurodegeneration markers. Serum and CSF were used for nitrooxidative and proinflammatory cytokines assays. Results We demonstrate a significant long-term (3.5 months post-soman) disease-modifying effect of 1400W in animals that had severe SE for > 20min of continuous convulsive seizures. 1400W significantly reduced GD-induced motor and cognitive dysfunction; nitrooxidative stress (nitrite, ROS; increased GSH: GSSG); proinflammatory cytokines in the serum and some in the cerebrospinal fluid (CSF); epileptiform spikes and spontaneously recurring seizures (SRS) in males; reactive gliosis (GFAP + C3 and IBA1 + CD68 positive glia) as a measure of neuroinflammation, and neurodegeneration (including parvalbumin positive neurons) in some brain regions. Conclusion These findings demonstrate the long-term disease-modifying effects of a glial-targeted iNOS inhibitor, 1400W, in a rat GD model by modulating reactive gliosis, neurodegeneration, and neuronal hyperexcitability.

Delayed tezampanel and caramiphen treatment but not midazolam protects against long-term neuropathology after soman exposure

Prolonged status epilepticus (SE) can cause brain damage; therefore, treatment must be administered promptly after seizure onset to limit SE duration and prevent neuropathology. Timely treatment of SE is not always feasible; this would be particularly true in a mass exposure to an SE-inducing agent such as a nerve agent. Therefore, the availability of anticonvulsant treatments that have neuroprotective efficacy even if administered with a delay after SE onset is an imperative. Here, we compared the long-term neuropathology resulting from acutely exposing 21-day-old male and female rats to the nerve agent soman, and treating them with midazolam (3 mg/kg) or co-administration of tezampanel (10 mg/kg) and caramiphen (50 mg/kg), at 1 h postexposure (~50 min after SE onset). Midazolam-treated rats had significant neuronal degeneration in limbic structures, mainly at one month postexposure, followed by neuronal loss in the basolateral amygdala and the CA1 hippocampal area. Neuronal loss resulted in significant amygdala and hippocampal atrophy, deteriorating from one to six months postexposure. Rats treated with tezampanel-caramiphen had no evidence of neuropathology, except for neuronal loss in the basolateral amygdala at the six-month timepoint. Anxiety was increased only in the midazolam-treated rats, at one, three, and six months postexposure. Spontaneous recurrent seizures appeared only in midazolam-treated rats, at three and six months postexposure in males and only at six months in females. These findings suggest that delayed treatment of nerve agent-induced SE with midazolam may result in long-lasting or permanent brain damage, while antiglutamatergic anticonvulsant treatment consisting of tezampanel and caramiphen may provide full neuroprotection.

Antiseizure and Neuroprotective Efficacy of Midazolam in Comparison with Tezampanel (LY293558) against Soman-Induced Status Epilepticus

Acute exposure to nerve agents induces status epilepticus (SE), which can cause death or long-term brain damage. Diazepam is approved by the FDA for the treatment of nerve agent-induced SE, and midazolam (MDZ) is currently under consideration to replace diazepam. However, animal studies have raised questions about the neuroprotective efficacy of benzodiazepines. Here, we compared the antiseizure and neuroprotective efficacy of MDZ (5 mg/kg) with that of tezampanel (LY293558; 10 mg/kg), an AMPA/GluK1 receptor antagonist, administered 1 h after injection of the nerve agent, soman (1.2 × LD50), in adult male rats. Both of the anticonvulsants promptly stopped SE, with MDZ having a more rapid effect. However, SE reoccurred to a greater extent in the MDZ-treated group, resulting in a significantly longer total duration of SE within 24 h post-exposure compared with the LY293558-treated group. The neuroprotective efficacy of the two drugs was studied in the basolateral amygdala, 30 days post-exposure. Significant neuronal and inter-neuronal loss, reduced ratio of interneurons to the total number of neurons, and reduction in spontaneous inhibitory postsynaptic currents accompanied by increased anxiety were found in the MDZ-treated group. The rats treated with LY293558 did not differ from the control rats (not exposed to soman) in any of these measurements. Thus, LY293558 has significantly greater efficacy than midazolam in protecting against prolonged seizures and brain damage caused by acute nerve agent exposure.

DFP-Induced Status Epilepticus Severity in Mixed-Sex Cohorts of Adult Rats Housed in the Same Room: Behavioral and EEG Comparisons

Sex is a biological variable in experimental models. In our previous diisopropylfluorophosphate (DFP) studies, female rats required a higher dose of DFP to achieve a somewhat similar severity of status epilepticus (SE) as males. In those studies, male and female rats were bought separately from the same vendor, housed in different rooms, and the DFP used was from different batches. We had also shown that surgery for epidural electrodes implantation reduces the threshold for SE. Our recent study in the soman (GD) model using a mixed-sex cohort of rats housed individually but in the same room showed that females achieved significantly higher SE severity than males for the same dose of GD. In this study, we demonstrate that housing the mixed-sex cohorts in the same room and treating them with DFP (4 mg/kg, s.c.) from the same pool, though from different batches, yielded reproducible SE severity in both sexes and both telemetry (surgery) and non-telemetry (non-surgery) groups. We conducted experiments in four mixed-sex cohorts of adult Sprague-Dawley rats. In females, the surgery for implanting the telemetry devices reduced the latency to convulsive seizure (CS) and increased SE severity compared to non-telemetry females. However, there were no sex differences in latency or SE severity within telemetry or non-telemetry groups. Once animals reached CS stage ≥3, they remained in CS stage in both sexes until midazolam was administered. Midazolam (3 mg/kg, i.m.) treatment 1-one-hour post-DFP significantly reduced epileptiform spikes in both sexes. The mortality was only 2% in 24 h. Irrespective of sex or stage of estrous cycle or surgery, the animals had continuous convulsive SE for ∼40 min. In telemetry rats, electrographic changes correlated with behavioral seizures. However, there was a significant difference in SE severity and the latency between directly-observed behavioral CS and EEG-based CS quantification in both sexes. Overall, these results suggest that housing both sexes in the same room and treating with DFP in a mixed-sex cohort from the same pool of reagents will minimize variability in SE severity. Such rigorous experiments will yield better outcomes while testing disease-modifying agents in epilepsy models.

Soman (GD) Rat Model to Mimic Civilian Exposure to Nerve Agent: Mortality, Video-EEG Based Status Epilepticus Severity, Sex Differences, Spontaneously Recurring Seizures, and Brain Pathology

Modeling a real-world scenario of organophosphate nerve agent (OPNA) exposure is challenging. Military personnel are premedicated with pyridostigmine, which led to the development of OPNA models with pyridostigmine/oxime pretreatment to investigate novel therapeutics for acute and chronic effects. However, civilians are not premedicated with pyridostigmine/oxime. Therefore, experimental models without pyridostigmine were developed by other laboratories though often only in males. Following OPNA exposure, prolonged convulsive seizures (CS) or status epilepticus (SE) are concerning. The duration and severity of CS/SE determine the extent of brain injury in survivors even after treating with medical countermeasures (MCM)/antidotes such as atropine, an oxime, and an anticonvulsant such as diazepam/midazolam. In this study, using a large mixed sex cohort of adult male and female rats, without pretreatment, we demonstrate severe SE lasting for >20 min in 82% of the animals in response to soman (GD,132 μg/kg, s.c.). Atropine sulfate (2 mg/kg, i.m.) and HI-6 (125 mg/kg, i.m.) were administered immediately following soman, and midazolam (3 mg/kg, i.m.) 1 h post-exposure. Immediate MCM treatment is impractical in civilian exposure to civilians, but this approach reduces mortality in experimental models. Interestingly, female rats, irrespective of estrous stages, had an average of 44 min CS (stage ≥ 3), while males had an average of 32 min CS during SE, starting from soman exposure to midazolam treatment. However, in telemetry device implanted groups, there were no significant sex differences in SE severity; males had 40 min and females 43 min of continuous CS until midazolam was administered. No animals died prior to midazolam administration and less than 5% died in the first week after soman intoxication. In telemetered animals, there was a direct correlation between EEG changes and behavioral seizures in real-time. In the long-term, convulsive spontaneously recurring seizures (SRS) were observed in 85% of randomly chosen animals. At 4-months post-soman, the brain histology confirmed reactive gliosis and neurodegeneration. The novel findings of this study are that, in non-telemetered animals, the SE severity following soman intoxication was significantly greater in females compared to males and that the estrous cycle did not influence the response.

Long-term effect of neonatal antagonism of ionotropic glutamate receptors on dendritic spines and cognitive function in rats

Glutamate is the most abundant excitatory neurotransmitter in the hippocampus where mediates its actions by activating glutamate receptors. The activation of these receptors is essential for the maintenance and dynamics of dendritic spines and plasticity that correlate with learning and memory processes during neurodevelopment and adulthood. We studied in adults the effect of blocking ionotropic glutamate receptors (NMDAR, AMPAR, and KAR) functions at neonatal age (PD1-PD15) with their respective antagonists D-AP5, GYKI-53655 and UBP-302. We first evaluated memory using a new object recognition test in adults. Second, we evaluated the levels of glial fibrillary acidic protein, synaptophysin and actin with immunohistochemistry in the CA1, CA3, and dentate gyrus regions of the hippocampus and, finally, the number of dendritic spines and their dynamics using Golgi-Cox staining. We found that ionotropic glutamate receptor function blockade at neonatal age causes a reduction in short and long-term memory in adulthood and a reduction in the expression of synaptophysin and actin protein levels in the hippocampus regions studied. This blockade also reduced the number of dendritic spines and modified dendritic dynamics in the CA1 region. The antagonism of the three types of ionotropic glutamate receptors reduced the mushrooms and bifurcated types of spines and increased the thin spines. The number of stubby spines was reduced by D-AP5, increased by UPB-302, and not affected by GYKI-53655. Our results indicate that the blockade of neonatal ionotropic glutamate receptors produces alterations that persist until adulthood.

Neuroprotection by delayed triple therapy following sarin nerve agent insult in the rat

The development of refractory status epilepticus (SE) induced by sarin intoxication presents a therapeutic challenge. In our current research we evaluate the efficacy of a delayed combined triple treatment in ending the abnormal epileptiform seizure activity (ESA) and the ensuing of long-term neuronal insult. SE was induced in male Sprague-Dawley rats by exposure to 1.2LD50 sarin insufficiently treated by atropine and TMB4 (TA) 1 min later. Triple treatment of ketamine, midazolam and valproic acid was administered 30 min or 1 h post exposure and was compared to a delayed single treatment with midazolam alone. Toxicity and electrocorticogram activity were monitored during the first week and behavioral evaluation performed 3 weeks post exposure followed by brain biochemical and immunohistopathological analyses. The addition of both single and triple treatments reduced mortality and enhanced weight recovery compared to the TA-only treated group. The triple treatment also significantly minimized the duration of the ESA, reduced the sarin-induced increase in the neuroinflammatory marker PGE2, the brain damage marker TSPO, decreased the gliosis, astrocytosis and neuronal damage compared to the TA+ midazolam or only TA treated groups. Finally, the triple treatment eliminated the sarin exposed increased open field activity, as well as impairing recognition memory as seen in the other experimental groups. The delayed triple treatment may serve as an efficient therapy, which prevents brain insult propagation following sarin-induced refractory SE, even if treatment is postponed for up to 1 h.

Comparative profile of refractory status epilepticus models following exposure of cholinergic agents pilocarpine, DFP, and soman

Status epilepticus (SE) is a medical emergency with continuous seizure activity that causes profound neuronal damage, morbidity, or death. SE incidents can arise spontaneously but mostly are elicited by seizurogenic triggers. Chemoconvulsants such as the muscarinic agonist pilocarpine and, organophosphates (OP) such as the pesticide diisopropylfluorophosphate (DFP) and, the nerve agent soman, can induce SE. Pilocarpine, DFP, and soman share a common feature of cholinergic crisis that transitions into a state of refractory SE, but their comparative profiles remain unclear. Here, we evaluated the comparative convulsant profile of pilocarpine, DFP, and soman to produce refractory SE and brain damage in rats. Behavioral and electrographic seizures were monitored for 24 h after exposure, and the extent of brain injury was determined by histological markers of neuronal injury and degeneration. Seizures were elicited rather slowly after pilocarpine as compared to DFP or soman, which caused rapid onset of spiking that swiftly developed into persistent SE. Time-course of SE activity after DFP was comparable to that after soman, a potent nerve agent. Diazepam controlled pilocarpine-induced SE, but it was ineffective in reducing OP-induced SE. All three agents produced modestly different degrees of neuronal injury and neurodegeneration in the brain. These results reveal distinct convulsant and neuronal injury patterns following exposure to cholinergic agonists, OP pesticides, and nerve agents. A battery of SE models, especially SE induced by cholinergic agents and other etiologies including epilepsy and brain tumors, is essential to identify novel anticonvulsant therapies for the management of refractory SE.

Comparison of neuropathology in rats following status epilepticus induced by diisopropylfluorophosphate and soman

The increasing number of cases involving the use of nerve agents as deadly weapons has spurred investigation into the molecular mechanisms underlying nerve agent-induced pathology. The highly toxic nature of nerve agents restrict their use in academic research laboratories. Less toxic organophosphorus (OP) based agents including diisopropylfluorophosphate (DFP) are used as surrogates in academic research laboratories to mimic nerve agent poisoning. However, neuropathology resulting from DFP-induced status epilepticus (SE) has not been compared directly to neuropathology observed following nerve agent poisoning in the same study. Here, the hypothesis that neuropathology measured four days after SE is the same for rats exposed to DFP and soman was tested. Adult Sprague-Dawley rats were injected with soman or DFP to induce SE. Cortical electroencephalography (EEG) was recorded prior to and during soman-induced SE. EEG power analysis of rats administered soman revealed prolonged electrographic SE similar to that of rats that endure uninterrupted SE following injection of DFP. Rats that experienced soman-induced SE displayed less hippocampal neuroinflammation and gliosis compared to rats administered DFP. Seizure-induced weight change, blood-brain barrier (BBB) leakiness and neurodegeneration in most seizure sensitive limbic brain regions were similar for rats that endured SE following soman or DFP. The amalgamated pathology score calculated by combining pathological measures (weight loss, hippocampal neuroinflammation, gliosis, BBB integrity and neurodegeneration) was similar in rats administered the OP agents. These findings support use of the rat DFP model of SE as a suitable surrogate for investigating some, but not all delayed consequences produced by nerve agents.

Novel Genetically Modified Mouse Model to Assess Soman-Induced Toxicity and Medical Countermeasure Efficacy: Human Acetylcholinesterase Knock-in Serum Carboxylesterase Knockout Mice

The identification of improved medical countermeasures against exposure to chemical warfare nerve agents (CWNAs), a class of organophosphorus compounds, is dependent on the choice of animal model used in preclinical studies. CWNAs bind to acetylcholinesterase and prevent the catalysis of acetylcholine, causing a plethora of peripheral and central physiologic manifestations, including seizure. Rodents are widely used to elucidate the effects of CWNA-induced seizure, albeit with a caveat: they express carboxylesterase activity in plasma. Carboxylesterase, an enzyme involved in the detoxification of some organophosphorus compounds, plays a scavenging role and decreases CWNA availability, thus exerting a protective effect. Furthermore, species-specific amino acid differences in acetylcholinesterase confound studies that use oximes or other compounds to restore its function after inhibition by CWNA. The creation of a human acetylcholinesterase knock-in/serum carboxylesterase knockout (C57BL/6-Ces1ctm1.1LocAChEtm1.1Loc/J; a.k.a KIKO) mouse may facilitate better modeling of CWNA toxicity in a small rodent species. The current studies characterize the effects of exposure to soman, a highly toxic CWNA, and evaluate the efficacy of anti-seizure drugs in this newly developed KIKO mouse model. Data demonstrate that a combination of midazolam and ketamine reduces seizure duration and severity, eliminates the development of spontaneous recurrent seizures, and protects certain brain regions from neuronal damage in a genetically modified model with human relevance to organophosphorus compound toxicity. This new animal model and the results of this study and future studies using it will enhance medical countermeasures development for both defense and homeland security purposes.

Magnetic resonance imaging analysis of long-term neuropathology after exposure to the nerve agent soman: correlation with histopathology and neurological dysfunction

Nerve agents (NAs) produce acute and long-term brain injury and dysfunction, as evident from the Japan and Syria incidents. Magnetic resonance imaging (MRI) is a versatile technique to examine such chronic anatomical, functional, and neuronal damage in the brain. The objective of this study was to investigate long-term structural and neuronal lesion abnormalities in rats exposed to acute soman intoxication. T2-weighted MRI images of 10 control and 17 soman-exposed rats were acquired using a Siemens MRI system at 90 days after soman exposure. Quantification of brain tissue volumes and T2 signal intensity was conducted using the Inveon Research Workplace software and the extent of damage was correlated with histopathology and cognitive function. Soman-exposed rats showed drastic hippocampal atrophy with neuronal loss and reduced hippocampal volume (HV), indicating severe damage, but had similar T2 relaxation times to the control group, suggesting limited scarring and fluid density changes despite the volume decrease. Conversely, soman-exposed rats displayed significant increases in lateral ventricle volumes and T2 times, signifying strong cerebrospinal fluid expansion in compensation for tissue atrophy. The total brain volume, thalamic volume, and thalamic T2 time were similar in both groups, however, suggesting that some brain regions remained more intact long-term after soman intoxication. The MRI neuronal lesions were positively correlated with the histological markers of neurodegeneration and neuroinflammation 90 days after soman exposure. The predominant MRI hippocampal atrophy (25%) was highly consistent with massive reduction (35%) of neuronal nuclear antigen-positive (NeuN+ ) principal neurons and parvalbumin-positive (PV+ ) inhibitory interneurons within this brain region. The HV was significantly correlated with both inflammatory markers of GFAP+ astrogliosis and IBA1+ microgliosis. The reduced HV was also directly correlated with significant memory deficits in the soman-exposed cohort, confirming a possible neurobiological basis for neurological dysfunction. Together, these findings provide powerful insight on long-term region-specific neurodegenerative patterns after soman exposure and demonstrate the feasibility of in vivo neuroimaging to monitor neuropathology, predict the risk of neurological deficits, and evaluate response to medical countermeasures for NAs.

The evolution of the pilocarpine animal model of status epilepticus

The pilocarpine animal model of status epilepticus is a well-established, clinically translatable model that satisfies all of the criteria essential for an animal model of status epilepticus: a latency period followed by spontaneous recurrent seizures, replication of behavioural, electrographic, metabolic, and neuropathological changes, as well as, pharmacoresistance to anti-epileptic drugs similar to that observed in human status epilepticus. However, this model is also characterized by high mortality rates and studies in recent years have also seen difficulties in seizure induction due to pilocarpine resistant animals. This can be attributed to differences in rodent strains, species, gender, and the presence of the multi-transporter, P-glycoprotein at the blood brain barrier. The current paper highlights the various alterations made to the original pilocarpine model over the years to combat both the high mortality and low induction rates. These range from the initial lithium-pilocarpine model to the more recent Reduced Intensity Status Epilepticus (RISE) model, which finally brought the mortality rates down to 1%. These modifications are essential to improve animal welfare and future experimental outcomes.

Phenobarbital as alternate anticonvulsant for organophosphate-induced benzodiazepine-refractory status epilepticus and neuronal injury

Objective

Organophosphates (OPs) such as diisopropylfluorophosphate (DFP) and soman are lethal chemical agents that can produce seizures, refractory status epilepticus (SE), and brain damage. There are few optimal treatments for late or refractory SE. Phenobarbital is a second‐line drug for SE, usually after lorazepam, diazepam, or midazolam have failed to stop SE. Practically, 40 minutes or less is often necessary for first responders to arrive and assist in a chemical incident. However, it remains unclear whether administration of phenobarbital 40 minutes after OP intoxication is still effective. Here, we investigated the efficacy of phenobarbital treatment at 40 minutes postexposure to OP intoxication.

Methods

Acute refractory SE was induced in rats by DFP injection as per a standard paradigm. After 40 minutes, subjects were given phenobarbital intramuscularly (30‐100 mg/kg) and progression of seizure activity was monitored by video‐EEG recording. The extent of brain damage was assessed 3 days after DFP injections by neuropathology analysis of neurodegeneration and neuronal injury by unbiased stereology.

Results

Phenobarbital produced a dose‐dependent seizure protection. A substantial decrease in SE was evident at 30 and 60 mg/kg, and a complete seizure termination was noted at 100 mg/kg within 40 minutes after treatment. Neuropathology findings showed significant neuroprotection in 100 mg/kg cohorts in brain regions associated with SE. Although higher doses resulted in greater protection against refractory SE and neuronal damage, they did not positively correlate with improved survival rate. Moreover, phenobarbital caused serious adverse effects including anesthetic or comatose state and even death.

Significance

Phenobarbital appears as an alternate anticonvulsant for OP‐induced refractive SE in hospital settings. A careful risk‐benefit analysis is required because of negative outcomes on survival and cardio‐respiratory function. However, the need for sophisticated support and critical monitoring in hospital may preclude its use as medical countermeasure in mass casualty situations.

Electroencephalographic analysis in soman-exposed 21-day-old rats and the effects of midazolam or LY293558 with caramiphen

Acute nerve agent exposure induces status epilepticus (SE), which can cause brain damage or death. Research aiming at developing effective therapies for controlling nerve agent-induced SE is commonly performed in adult rats. The characteristics of nerve agent-induced SE in young rats are less clear; relevant knowledge is necessary for developing effective pediatric therapies. Here, we have used electroencephalographic (EEG) recordings and analysis to study seizures in postnatal day 21 rats exposed to 1.2 × LD₅₀ of soman, and compared the antiseizure efficacy of midazolam (MDZ)-currently considered by the Food and Drug Administration to replace diazepam for treating SE in victims of nerve agent exposure-with that of LY293558, an AMPA/GluK1 receptor antagonist, administered in combination with caramiphen, an antimuscarinic with N-methyl-d-aspartate receptor antagonistic properties. Prolonged SE developed in 80% of the rats and was reflected in behavioral seizures/convulsions. Both MDZ and LY293558 + caramiphen stopped the SE induced by soman, but there was a significant recurrence of seizures within 24 h postexposure only in the MDZ-treated group, as revealed in the raw EEG data and their representation in the frequency domain using a fast Fourier transform and in spectral analysis over 24 hours. In contrast to the high efficacy of LY293558 + caramiphen, MDZ is not an effective treatment for SE induced by soman in young animals.

A rat model of organophosphate-induced status epilepticus and the beneficial effects of EP2 receptor inhibition

This review describes an adult rat model of status epilepticus (SE) induced by diisopropyl fluorophosphate (DFP), and the beneficial outcomes of transient inhibition of the prostaglandin-E2 receptor EP2 with a small molecule antagonist, delayed by 2-4 h after SE onset. Administration of six doses of the selective EP2 antagonist TG6-10-1 over a 2-3 day period accelerates functional recovery, attenuates hippocampal neurodegeneration, neuroinflammation, gliosis and blood-brain barrier leakage, and prevents long-term cognitive deficits without blocking SE itself or altering acute seizure characteristics. This work has provided important information regarding organophosphate-induced seizure related pathologies in adults and revealed the effectiveness of delayed EP2 inhibition to combat these pathologies.

Neurosteroid and benzodiazepine combination therapy reduces status epilepticus and long-term effects of whole-body sarin exposure in rats

OBJECTIVE

Our objective was to evaluate the protective efficacy of the neurosteroid pregnanolone (3α-hydroxy-5β pregnan-20-one), a GABAA receptor-positive allosteric modulator, as an adjunct to benzodiazepine therapy against the chemical warfare nerve agent (CWNA) sarin (GB), using whole-body exposure, an operationally relevant route of exposure to volatile GB.

METHODS

Rats implanted with telemetry transmitters for the continuous measurement of cortical electroencephalographic (EEG) activity were exposed for 60 minutes to 3.0 LCt50 of GB via whole-body exposure. At the onset of toxic signs, rats were administered an intramuscular injection of atropine sulfate (2 mg/kg) and the oxime HI-6 (93.6 mg/kg) to increase survival rate and, 30 minutes after seizure onset, treated subcutaneously with diazepam (10 mg/kg) and intravenously with pregnanolone (4 mg/kg) or vehicle. Animals were evaluated for GB-induced status epilepticus (SE), spontaneous recurrent seizures (SRS), impairment in spatial memory acquisition, and brain pathology, and treatment groups were compared.

RESULTS

Delayed dual therapy with pregnanolone and diazepam reduced time in SE in GB-exposed rats compared to those treated with delayed diazepam monotherapy. The combination therapy of pregnanolone with diazepam also prevented impairment in the Morris water maze and reduced the neuronal loss and neuronal degeneration, evaluated at one and three months after exposure.

SIGNIFICANCE

Neurosteroid administration as an adjunct to benzodiazepine therapy offers an effective means to treat benzodiazepine-refractory SE, such as occurs following delayed treatment of GB exposure. This study is the first to present data on the efficacy of delayed pregnanolone and diazepam dual therapy in reducing seizure activity, performance deficits and brain pathology following an operationally relevant route of exposure to GB and supports the use of a neurosteroid as an adjunct to standard anticonvulsant therapy for the treatment of CWNA-induced SE.

Mechanism-based novel antidotes for organophosphate neurotoxicity

This article describes current pursuits for developing novel antidotes for organophosphate (OP) intoxication. Recent mechanistic studies of benzodiazepine-resistant seizures have key consequences for victims of OP pesticide and nerve agent attacks. We uncovered why current therapies are not able to stop the OP-induced seizures and brain cell death and what type of drug might be better. OP exposure down regulates critical inhibitory GABA-A receptors, kills neurons, and causes massive neuroinflammation that will cause more neuronal death, which causes the problem of too few benzodiazepine receptors. The loss of inhibitory interneurons creates a self-sustaining seizure circuit and refractory status epilepticus. Thus, there is an urgent need for mechanism-based, new antidotes for OP intoxication. We have discovered neurosteroids as next-generation anticonvulsants superior to midazolam for the treatment of OP poisoning. Neurosteroids that activate both extrasynaptic and synaptic GABA-A receptors have the potential to stop seizures more effectively and safely than benzodiazepines. In addition, neurosteroids confers robust neuroprotection by reducing neuronal injury and neuroinflammation. The synthetic neurosteroid ganaxolone is being considered for advanced development as a future anticonvulsant for nerve agents. Experimental studies shows striking efficacy of ganaxolone and its analogs in OP exposure models. They are also effective in attenuating long-term neuropsychiatric deficits caused by OP exposure. Overall, neurosteroids represent rational anticonvulsants for OP intoxication, even when given late after exposure.

Antiseizure and neuroprotective effects of delayed treatment with midazolam in a rodent model of organophosphate exposure

OBJECTIVE

Exposure to organophosphates (OPs) and OP nerve agents (NAs) causes status epilepticus (SE) and irreversible brain damage. Rapid control of seizure activity is important to minimize neuronal injury and the resulting neurological and behavioral disorders; however, early treatment will not be possible after mass release of OPs or NAs.

METHODS

We utilized a delayed-treatment model of OP exposure in adult rats by administration of diisopropyl fluorophosphate (DFP) to study the relationship between the antiseizure and neuroprotective effects of the "standard-of-care" benzodiazepine, midazolam (MDZ), when given at 30, 60, and 120 minutes after SE onset. After electroencephalography (EEG) recordings, neural damage in serial brain sections was studied with Fluoro-Jade B staining.

RESULTS

MDZ-induced seizure suppression was equivalent in magnitude regardless of treatment delay (ie, seizure duration). When assessed globally (ie, normalized across 10 different brain regions) for each treatment delay, MDZ administration resulted in only nonsignificant reductions in neuronal death. However, when data for MDZ treatment were combined from all three delay times, a small but significant reduction in global neuronal death was detected when compared to vehicle treatment, which indicated that the substantive MDZ-induced seizure suppression led to only a small reduction in neuronal death.

SIGNIFICANCE

In conclusion, MDZ significantly reduced DFP-induced SE intensity when treatment was delayed 30, 60, and even up to 120 minutes; however, this reduction in seizure intensity had no detectable effect on neuronal death at each individual delay time. These data show that although MDZ suppressed seizures, additional neuroprotective therapies are needed to mitigate the effects of OP exposure.

Inducible nitric oxide synthase inhibitor, 1400W, mitigates DFP-induced long-term neurotoxicity in the rat model

Chemical nerve agents (CNA) are increasingly becoming a threat to both civilians and military personnel. CNA-induced acute effects on the nervous system have been known for some time and the long-term consequences are beginning to emerge. In this study, we used diisopropylfluorophosphate (DFP), a seizurogenic CNA to investigate the long-term impact of its acute exposure on the brain and its mitigation by an inducible nitric oxide synthase (iNOS) inhibitor, 1400W as a neuroprotectant in the rat model. Several experimental studies have demonstrated that DFP-induced seizures and/or status epilepticus (SE) causes permanent brain injury, even after the countermeasure medication (atropine, oxime, and diazepam). In the present study, DFP-induced SE caused a significant increase in iNOS and 3-nitrotyrosine (3-NT) at 24 h, 48 h, 7d, and persisted for a long-term (12 weeks post-exposure), which led to the hypothesis that iNOS is a potential therapeutic target in DFP-induced brain injury. To test the hypothesis, we administered 1400W (20 mg/kg, i.m.) or the vehicle twice daily for the first three days of post-exposure. 1400W significantly reduced DFP-induced iNOS and 3-NT upregulation in the hippocampus and piriform cortex, and the serum nitrite levels at 24 h post-exposure. 1400W also prevented DFP-induced mortality in <24 h. The brain immunohistochemistry (IHC) at 7d post-exposure revealed a significant reduction in gliosis and neurodegeneration (NeuN+ FJB positive cells) in the 1400W-treated group. 1400W, in contrast to the vehicle, caused a significant reduction in the epileptiform spiking and spontaneous recurrent seizures (SRS) during 12 weeks of continuous video-EEG study. IHC of brain sections from the same animals revealed a significant reduction in reactive gliosis (both microgliosis and astrogliosis) and neurodegeneration across various brain regions in the 1400W-treated group when compared to the vehicle-treated group. A multiplex assay from hippocampal lysates at 6 weeks post-exposure showed a significant increase in several key pro-inflammatory cytokines/chemokines such as IL-1α, TNFα, IL-1β, IL-2, IL-6, IL-12, IL-17a, MCP-1, LIX, and Eotaxin, and a growth factor, VEGF in the vehicle-treated animals. 1400W significantly suppressed IL-1α, TNFα, IL-2, IL-12, and MCP-1 levels. It also suppressed DFP-induced serum nitrite levels at 6 weeks post-exposure. In the Morris water maze, the vehicle-treated animals spent significantly less time in the target quadrant in a probe trial at 9d post-exposure compared to their time spent in the same quadrant 11 days previously (i.e., 2 days prior to DFP exposure). Such a difference was not observed in the 1400W and control groups. However, learning and short-term memory were unaffected when tested at 10-16d and 28-34d post-exposure. Accelerated rotarod, horizontal bar test, and the forced swim test revealed no significant changes between groups. Overall, the findings from this study suggest that 1400W may be considered as a potential therapeutic agent as a follow-on therapy for CNA exposure, after controlling the acute symptoms, to prevent mortality and some of the long-term neurotoxicity parameters such as epileptiform spiking, SRS, neurodegeneration, reactive gliosis in some brain regions, and certain key proinflammatory cytokines and chemokine.

Targeting the glutamatergic system to counteract organophosphate poisoning: A novel therapeutic strategy

One of the devastating effects of acute exposure to organophosphates, like nerve agents, is the induction of severe and prolonged status epilepticus (SE), which can cause death, or brain damage if death is prevented. Seizures after exposure are initiated by muscarinic receptor hyperstimulation-after inhibition of acetylcholinesterase by the organophosphorus agent and subsequent elevation of acetylcholine-but they are reinforced and sustained by glutamatergic hyperexcitation, which is the primary cause of brain damage. Diazepam is the FDA-approved anticonvulsant for the treatment of nerve agent-induced SE, and its replacement by midazolam is currently under consideration. However, clinical data derived from the treatment of SE of any etiology, as well as studies on the control of nerve agent-induced SE in animal models, have indicated that diazepam and midazolam control seizures only temporarily, their antiseizure efficacy is reduced as the latency of treatment from the onset of SE increases, and their neuroprotective efficacy is limited or absent. Here, we review data on the discovery of a novel anticonvulsant and neuroprotectant, LY293558, an AMPA/GluK1 receptor antagonist. Treatment of soman-exposed immature, young-adult, and aged rats with LY293558, terminates SE with limited recurrence of seizures, significantly protects from brain damage, and prevents long-term behavioral deficits, even when LY293558 is administered 1 h post-exposure. More beneficial effects and complete neuroprotection is obtained when LY293558 administration is combined with caramiphen, which antagonizes NMDA receptors. Further efficacy studies may bring the LY293558 + caramiphen combination therapy on the pathway to approval for human use.

Benzodiazepine-refractory status epilepticus, neuroinflammation, and interneuron neurodegeneration after acute organophosphate intoxication

Nerve agents and some pesticides such as diisopropylfluorophosphate (DFP) cause neurotoxic manifestations that include seizures and status epilepticus (SE), which are potentially lethal and carry long-term neurological morbidity. Current antidotes for organophosphate (OP) intoxication include atropine, 2-PAM and diazepam (a benzodiazepine for treating seizures and SE). There is some evidence for partial or complete loss of diazepam anticonvulsant efficacy when given 30 min or later after exposure to an OP; this condition is known as refractory SE. Effective therapies for OP-induced SE are lacking and it is unclear why current therapies do not work. In this study, we investigated the time-dependent efficacy of diazepam in the nerve agent surrogate DFP model of OP intoxication on seizure suppression and neuroprotection in rats, following an early and late therapy. Diazepam (5 mg/kg, IM) controlled seizures when given 10 min after DFP exposure ("early"), but it was completely ineffective at 60 or 120 min ("late") after DFP. DFP-induced neuronal injury, neuroinflammation, and neurodegeneration of principal cells and GABAergic interneurons were significantly reduced by early but not late therapy. These findings demonstrate that diazepam failed to control seizures, SE and neuronal injury when given 60 min or later after DFP exposure, confirming the benzodiazepine-refractory SE and brain damage after OP intoxication. In addition, this study indicates that degeneration of inhibitory interneurons and inflammatory glial activation are potential mechanisms underlying these morbid outcomes of OP intoxication. Therefore, novel anticonvulsant and neuroprotectant antidotes, superior to benzodiazepines, are desperately needed for controlling nerve agent-induced SE and brain injury.

Acute and long-term consequences of exposure to organophosphate nerve agents in humans

Nerve agents are organophosphate (OP) compounds and among the most powerful poisons known to man. A terrorist attack on civilian or military populations causing mass casualties is a real threat. The OP nerve agents include soman, sarin, cyclosarin, tabun, and VX. The major mechanism of acute toxicity is the irreversible inhibition of acetylcholinesterase. Acetylcholinesterase inhibition results in the accumulation of excessive acetylcholine levels in synapses, leading to progression of toxic signs including hypersecretions, tremors, status epilepticus, respiratory distress, and death. Miosis and rhinorrhea are the most common clinical findings in those individuals acutely exposed to OP nerve agents. Prolonged seizures are responsible for the neuropathology. The brain region that shows the most severe damage is the amygdala, followed by the piriform cortex, hippocampus, cortex, thalamus, and caudate/putamen. Current medical countermeasures are only modestly effective in attenuating the seizures and neuropathology. Anticonvulsants such as benzodiazepines decrease seizure activity and improve outcome, but their efficacy depends upon the administration time after exposure to the nerve agent. Administration of benzodiazepines may increase the risk for seizure recurrence. Recent studies document long-term neurologic and behavior deficits, and technological advances demonstrate structural brain changes on magnetic resonance imaging.

Midazolam-Resistant Seizures and Brain Injury after Acute Intoxication of Diisopropylfluorophosphate, an Organophosphate Pesticide and Surrogate for Nerve Agents

Organophosphates (OP) such as the pesticide diisopropylfluorophosphate (DFP) and the nerve agent sarin are lethal chemicals that induce seizures, status epilepticus (SE), and brain damage. Midazolam, a benzodiazepine modulator of synaptic GABA-A receptors, is currently considered as a new anticonvulsant for nerve agents. Here, we characterized the time course of protective efficacy of midazolam (0.2-5 mg/kg, i.m.) in rats exposed to DFP, a chemical threat agent and surrogate for nerve agents. Behavioral and electroencephalogram (EEG) seizures were monitored for 24 hours after DFP exposure. The extent of brain injury was determined 3 days after DFP exposure by unbiased stereologic analyses of valid markers of neurodegeneration and neuroinflammation. Seizures were elicited within ∼8 minutes after DFP exposure that progressively developed into persistent SE lasting for hours. DFP exposure resulted in massive neuronal injury or necrosis, neurodegeneration of principal cells and interneurons, and neuroinflammation as evident by extensive activation of microglia and astrocytes in the hippocampus, amygdala, and other brain regions. Midazolam controlled seizures, neurodegeneration, and neuroinflammation when given early (10 minutes) after DFP exposure, but it was less effective when given at 40 minutes or later. Delayed therapy (≥40 minutes), a simulation of the practical therapeutic window for first responders or hospital admission, was associated with reduced seizure protection and neuroprotection. These results strongly reaffirm that the DFP-induced seizures and brain damage are progressively resistant to delayed treatment with midazolam, confirming the benzodiazepine refractory SE after OP intoxication. Thus, novel anticonvulsants superior to midazolam or adjunct therapies that enhance its efficacy are needed for effective treatment of refractory SE.

Full Protection Against Soman-Induced Seizures and Brain Damage by LY293558 and Caramiphen Combination Treatment in Adult Rats

Acute exposure to nerve agents induces status epilepticus (SE), which causes brain damage or death. LY293558, an antagonist of AMPA and GluK1 kainate receptors is a very effective anticonvulsant and neuroprotectant against soman; however, some neuronal damage is still present after treatment of soman-exposed rats with LY293558. Here, we have tested whether combining LY293558 with an NMDA receptor antagonist can eliminate the residual damage. For this purpose, we chose caramiphen (CRM), an antimuscarinic compound with NMDA receptor antagonistic properties. Adult male rats were exposed to 1.2 × LD₅₀ soman, and at 20 min after soman exposure, were injected with atropine + HI-6, or atropine + HI-6 + LY293558 (15 mg/kg), or atropine + HI-6 + LY293558 + CRM (50 mg/kg). We found that (1) the LY293558 + CRM treatment terminated SE significantly faster than LY293558 alone; (2) after cessation of the initial SE, seizures did not return in the LY293558 + CRM-treated group, during 72 h of monitoring; (3) power spectrum analysis of continuous EEG recordings for 7 days post-exposure showed increased delta and decreased gamma power that lasted beyond 24 h post-exposure only in the rats who did not receive anticonvulsant treatment; (4) spontaneous recurrent seizures appeared on day 7 only in the group that did not receive anticonvulsant treatment; (5) significant neuroprotection was achieved by LY293558 administration, while the rats who received LY293558 + CRM displayed no neurodegeneration; (6) body weight loss and recovery in the LY293558 + CRM-treated rats did not differ from those in control rats who were not exposed to soman. The data show that treatment with LY293558 + CRM provides full antiseizure and neuroprotective efficacy against soman.

Beneficial Outcome of Urethane Treatment Following Status Epilepticus in a Rat Organophosphorus Toxicity Model

The efficacy of benzodiazepines to terminate electrographic status epilepticus (SE) declines the longer a patient is in SE. Therefore, alternative methods for ensuring complete block of SE and refractory SE are necessary. We compared the ability of diazepam and a subanesthetic dose of urethane to terminate prolonged SE and mitigate subsequent pathologies. Adult Sprague Dawley rats were injected with diisopropylfluorophosphate (DFP) to induce SE. Rats were administered diazepam (10 mg/kg, ip) or urethane (0.8 g/kg, s.c.) 1 h after DFP-induced SE and compared to rats that experienced uninterrupted SE. Large-amplitude and high-frequency spikes induced by DFP administration were quenched for at least 46 h in rats administered urethane 1 h after SE onset as demonstrated by cortical electroencephalography (EEG). By contrast, diazepam interrupted SE but seizures with high power in the 20- to 70-Hz band returned 6–10 h later. Urethane was more effective than diazepam at reducing hippocampal neurodegeneration, brain inflammation, gliosis and weight loss as measured on day 4 after SE. Furthermore, rats administered urethane displayed a 73% reduction in the incidence of spontaneous recurrent seizures after four to eight weeks and a 90% reduction in frequency of seizures in epileptic rats. By contrast, behavioral changes in the light/dark box, open field and a novel object recognition task were not improved by urethane. These findings indicate that in typical rodent SE models, it is the return of SE overnight, and not the initially intense 1–2 h of SE experience, that is largely responsible for neurodegeneration, accompanying inflammation, and the subsequent development of epilepsy.

Comparing the Antiseizure and Neuroprotective Efficacy of LY293558, Diazepam, Caramiphen, and LY293558-Caramiphen Combination against Soman in a Rat Model Relevant to the Pediatric Population

The currently Food and Drug Administration-approved anticonvulsant for the treatment of status epilepticus (SE) induced by nerve agents is the benzodiazepine diazepam; however, diazepam does not appear to offer neuroprotective benefits. This is of particular concern with respect to the protection of children because, in the developing brain, synaptic transmission mediated via GABA_A receptors, the target of diazepam, is weak. In the present study, we exposed 21-day-old male rats to 1.2 × LD₅₀ soman and compared the antiseizure, antilethality, and neuroprotective efficacy of diazepam (10 mg/kg), LY293558 (an AMPA/GluK1 receptor antagonist; 15 mg/kg), caramiphen (CRM, an antimuscarinic with NMDA receptor-antagonistic properties; 50 mg/kg), and LY293558 (15 mg/kg) + CRM (50 mg/kg), administered 1 hour after exposure. Diazepam, LY293558, and LY293558 + CRM, but not CRM alone, terminated SE; LY293558 + CRM treatment acted significantly faster and produced a survival rate greater than 85%. Thirty days after soman exposure, neurodegeneration in limbic regions was most severe in the CRM-treated group, minimal to severe-depending on the region-in the diazepam group, absent to moderate in the LY293558-treated group, and totally absent in the LY293558 + CRM group. Amygdala and hippocampal atrophy, a severe reduction in spontaneous inhibitory activity in the basolateral amygdala, and increased anxiety-like behavior in the open-field and acoustic startle response tests were present in the diazepam and CRM groups, whereas the LY293558 and LY293558 + CRM groups did not differ from controls. The combined administration of LY293558 and CRM, by blocking mainly AMPA, GluK1, and NMDA receptors, is a very effective anticonvulsant and neuroprotective therapy against soman in young rats.

Status Epilepticus: Behavioral and Electroencephalography Seizure Correlates in Kainate Experimental Models

Various etiological factors, such as head injury, chemical intoxication, tumors, and gene mutation, can induce epileptogenesis. In animal models, status epilepticus (SE) triggers epileptogenesis. In humans, convulsive SE for >30 min can be a life-threatening medical emergency. The duration and severity of convulsive SE are highly variable in chemoconvulsant animal models. A continuous video-electroencephalography (EEG) recording, and/or diligent direct observation, facilitates quantification of exact duration of different stages of convulsive seizures (Racine stages 3–5) to determine the severity of SE. A continuous convulsive SE for >30 min usually causes high mortality in some rodents and results in widespread brain damage in the surviving animals, in spite of treating with antiepileptic drugs (AEDs). AEDs control behavioral seizures but not EEG seizures. The severity of initial SE impacts epileptogenesis and cognitive function; therefore, quantitative assessment of behavioral SE and EEG in animal models will help to understand the impact of SE severity on epileptogenesis. There are several excellent reviews on experimental models of seizure/SE/epilepsy. This review focusses on the comparison of induction and characterization of behavioral SE and EEG correlates in mice and rats induced by kainate. We also discuss the advantages of repeated low dose of kainate (i.p. route), which minimizes variability in the initial SE severity between animals and reduces mortality rate. A refined approach to induce SE with kainate also addresses the two of the 3Rs (i.e., refinement and reduction), the guiding principles for ethical and scientific standpoint of animal research.

Glial source of nitric oxide in epileptogenesis: A target for disease modification in epilepsy

Epileptogenesis is the process of developing an epileptic condition and/or its progression once it is established. The molecules that initiate, promote, and propagate remarkable changes in the brain during epileptogenesis are emerging as targets for prevention/treatment of epilepsy. Epileptogenesis is a continuous process that follows immediately after status epilepticus (SE) in animal models of acquired temporal lobe epilepsy (TLE). Both SE and epileptogenesis are potential therapeutic targets for the discovery of anticonvulsants and antiepileptogenic or disease-modifying agents. For translational studies, SE targets are appropriate for screening anticonvulsive drugs prior to their advancement as therapeutic agents, while targets of epileptogenesis are relevant for identification and development of therapeutic agents that can either prevent or modify the disease or its onset. The acute seizure models do not reveal antiepileptogenic properties of anticonvulsive drugs. This review highlights the important components of epileptogenesis and the long-term impact of intervening one of these components, nitric oxide (NO), in rat and mouse kainate models of TLE. NO is a putative pleotropic gaseous neurotransmitter and an important contributor of nitro-oxidative stress that coexists with neuroinflammation and epileptogenesis. The long-term impact of inhibiting the glial source of NO during early epileptogenesis in the rat model of TLE is reviewed. The importance of sex as a biological variable in disease modification strategies in epilepsy is also briefly discussed.

Susceptibility to Soman Toxicity and Efficacy of LY293558 Against Soman-Induced Seizures and Neuropathology in 10-Month-Old Male Rats

Acute nerve agent exposure causes prolonged status epilepticus (SE), leading to death or long-term brain damage. We have previously demonstrated that LY293558, an AMPA/GluK1 kainate receptor antagonist, terminates SE induced by the nerve agent soman and protects from long-term brain damage, in immature rats and young-adult rats, even if administered with a relatively long latency from the time of exposure. However, susceptibility to the lethal consequences of SE increases with age, and mortality by SE induced by soman is substantially greater in older animals. Therefore, in the present study, we compared the susceptibility to soman toxicity of 10-month-old male rats with that of young-adult male rats (42 to 50 days old) and examined the protective efficacy of LY293558 in the older group. A lower percentage of the 10-month-old rats developed SE after injection of 1.2 × LD₅₀ soman, compared to the young adults, the latency to seizure onset was longer in the older rats, and seizure intensity did not differ between the two age groups. However, mortality rate in the older rats who developed SE was higher than in the young adults. Acetylcholinesterase activity in the amygdala, hippocampus, and piriform cortex did not differ between the two age groups. Administration of LY293558 at 20 or 60 min post-exposure suppressed SE, increased 24-h survival rate, decreased the long-term risk of death, reduced neuronal degeneration in the amygdala, hippocampus, piriform, and entorhinal cortices, and facilitated recovery from body weight loss. Thus, LY293558 is an effective countermeasure against soman toxicity also in older animals.

The M1 Muscarinic Receptor Antagonist VU0255035 Delays the Development of Status Epilepticus after Organophosphate Exposure and Prevents Hyperexcitability in the Basolateral Amygdala

Exposure to organophosphorus toxins induces seizures that progress to status epilepticus (SE), which can cause brain damage or death. Seizures are generated by hyperstimulation of muscarinic receptors, subsequent to inhibition of acetylcholinesterase; this is followed by glutamatergic hyperactivity, which sustains and reinforces seizure activity. It has been unclear which muscarinic receptor subtypes are involved in seizure initiation and the development of SE in the early phases after exposure. Here, we show that pretreatment of rats with the selective M₁ receptor antagonist, VU0255035 [N-(3-oxo-3-(4-(pyridine-4-yl)piperazin-1-yl)propyl)-benzo[c][1,2,5]thiadiazole-4 sulfonamide], significantly suppressed seizure severity and prevented the development of SE for about 40 minutes after exposure to paraoxon or soman, suggesting an important role of the M₁ receptor in the early phases of seizure generation. In addition, in in vitro brain slices of the basolateral amygdala (a brain region that plays a key role in seizure initiation after nerve agent exposure), VU0255035 blocked the effects produced by bath application of paraoxon-namely, a brief barrage of spontaneous inhibitory postsynaptic currents, followed by a significant increase in the ratio of the total charge transferred by spontaneous excitatory postsynaptic currents over that of the inhibitory postsynaptic currents. Furthermore, paraoxon enhanced the hyperpolarization-activated cation current I_h in basolateral amygdala principal cells, which could be one of the mechanisms underlying the increased glutamatergic activity, an effect that was also blocked in the presence of VU0255035. Thus, selective M₁ antagonists may be an efficacious pretreatment in contexts in which there is risk for exposure to organophosphates, as these antagonists will delay the development of SE long enough for medical assistance to arrive.

A rodent model of human organophosphate exposure producing status epilepticus and neuropathology

Exposure to organophosphates (OPs) often results in seizures and/or status epilepticus (SE) that produce neural damage within the central nervous system (CNS). Early control of SE is imperative for minimizing seizure-related CNS neuropathology. Although standard therapies exist, more effective agents are needed to reduce OP-induced SE and neuronal loss, particularly therapies with efficacy when administered 10's of minutes after the onset of SE. To evaluate novel antiseizure compounds, animal models should simulate the CNS effects of OP exposure observed in humans. We characterized in rats the effects of the OP, diisopropyl flourophosphate (DFP) as a function of dose and route of administration of supporting agents (pyridostigmine, 2-PAM, atropine); outcome measures were mortality, electrographic seizure activity during SE, and subsequent CNS neuropathology. Doses of DFP between 3 and 7mg/kg consistently caused SE, and the latency to behavioral tremors and to subsequent initiation of SE were dose related. In distinction, all doses of DFP that resulted in electrographic SE (3-7mg/kg) produced seizures of similar intensity and duration, and similar CNS neuropathology (i.e., the effects were all-or-none). Although SE was similar across doses, mortality progressively increased with higher doses of DFP. Mortality was significantly lower when the route of administration of therapeutic agents was intramuscular compared to intraperitoneal. This rodent model of OP poisoning demonstrates pathological characteristics similar to those observed in humans, and thus begins to validate this model for investigating potential new therapeutic approaches.

Neurosteroids for the potential protection of humans against organophosphate toxicity

This article describes the therapeutic potential of neurosteroids as anticonvulsant antidotes for chemical intoxication caused by organophosphate pesticides and nerve agents or gases like sarin and soman. Toxic manifestations following nerve agent exposure, as evident in chemical attacks in Japan and Syria, include hypersecretion, respiratory distress, tremors, convulsions leading to status epilepticus (SE), and death. Benzodiazepines, such as diazepam, are the current anticonvulsants of choice for controlling nerve agent-induced life-threatening seizures, SE, and brain injury. Benzodiazepines can control acute seizures when given early, but they are less effective for delayed treatment of SE, which is characterized by rapid desensitization of synaptic GABA_A receptors, benzodiazepine resistance, and brain injury. Neurosteroid-sensitive extrasynaptic GABA_A receptors, however, remain unaffected by such events. Thus, anticonvulsant neurosteroids may produce more effective protection than benzodiazepines against a broad spectrum of chemical agents, even when given late after nerve agent exposure.

Evaluation of acetylcholine, seizure activity and neuropathology following high-dose nerve agent exposure and delayed neuroprotective treatment drugs in freely moving rats

Organophosphorus nerve agents such as soman (GD) inhibit acetylcholinesterase, producing an excess of acetylcholine (ACh), which results in respiratory distress, convulsions and status epilepticus that leads to neuropathology. Several drugs (topiramate, clobazam, pregnanolone, allopregnanolone, UBP 302, cyclopentyladenosine [CPA], ketamine, midazolam and scopolamine) have been identified as potential neuroprotectants that may terminate seizures and reduce brain damage. To systematically evaluate their efficacy, this study employed in vivo striatal microdialysis and liquid chromatography to respectively collect and analyze extracellular ACh in freely moving rats treated with these drugs 20 min after seizure onset induced by a high dose of GD. Along with microdialysis, EEG activity was recorded and neuropathology assessed at 24 h. GD induced a marked increase of ACh, which peaked at 30 min post-exposure to 800% of control levels and then steadily decreased toward baseline levels. Approximately 40 min after treatment, only midazolam (10 mg/kg) and CPA (60 mg/kg) caused a significant reduction of ACh levels, with CPA reducing ACh levels more rapidly than midazolam. Both drugs facilitated a return to baseline levels at least 55 min after treatment. At 24 h, only animals treated with CPA (67%), midazolam (18%) and scopolamine (27%) exhibited seizure termination. While all treatments except for topiramate reduced neuropathology, CPA, midazolam and scopolamine showed the greatest reduction in pathology. Our results suggest that delayed treatment with CPA, midazolam, or scopolamine is effective at reducing GD-induced seizure activity and neuropathology, with CPA and midazolam capable of facilitating a reduction in GD-induced ACh elevation.

Long-term neuropathological and behavioral impairments after exposure to nerve agents

One of the deleterious effects of acute nerve agent exposure is the induction of status epilepticus (SE). If SE is not controlled effectively, it causes extensive brain damage. Here, we review the neuropathology observed after nerve agent-induced SE, as well as the ensuing pathophysiological, neurological, and behavioral alterations, with an emphasis on their time course and longevity. Limbic structures are particularly vulnerable to damage by nerve agent exposure. The basolateral amygdala (BLA), which appears to be a key site for seizure initiation upon exposure, suffers severe neuronal loss; however, GABAergic BLA interneurons display a delayed death, perhaps providing a window of opportunity for rescuing intervention. The end result is a long-term reduction of GABAergic activity in the BLA, with a concomitant increase in spontaneous excitatory activity; such pathophysiological alterations are not observed in the CA1 hippocampal area, despite the extensive neuronal loss. Hyperexcitability in the BLA may be at least in part responsible for the development of recurrent seizures and increased anxiety, while hippocampal damage may underlie the long-term memory impairments. Effective control of SE after nerve agent exposure, such that brain damage is also minimized, is paramount for preventing lasting neurological and behavioral deficits.

The Glutamate-Glutamine Cycle in Epilepsy

Epilepsy is a complex, multifactorial disease characterized by spontaneous recurrent seizures and an increased incidence of comorbid conditions such as anxiety, depression, cognitive dysfunction, and sudden unexpected death. About 70 million people worldwide are estimated to suffer from epilepsy, and up to one-third of all people with epilepsy are expected to be refractory to current medications. Development of more effective and specific antiepileptic interventions is therefore requisite. Perturbations in the brain's glutamate-glutamine cycle, such as increased extracellular levels of glutamate, loss of astroglial glutamine synthetase, and changes in glutaminase and glutamate dehydrogenase, are frequently encountered in patients with epilepsy. Hence, manipulations of discrete glutamate-glutamine cycle components may represent novel approaches to treat the disease. The goal of his review is to discuss some of the glutamate-glutamine cycle components that are altered in epilepsy, particularly neurotransmitters and metabolites, enzymes, amino acid transporters, and glutamate receptors. We will also review approaches that potentially could be used in humans to target the glutamate-glutamine cycle. Examples of such approaches are treatment with glutamate receptor blockers, glutamate scavenging, dietary intervention, and hypothermia.

The basolateral amygdala γ-aminobutyric acidergic system in health and disease

The brain comprises an excitatory/inhibitory neuronal network that maintains a finely tuned balance of activity critical for normal functioning. Excitatory activity in the basolateral amygdala (BLA), a brain region that plays a central role in emotion and motivational processing, is tightly regulated by a relatively small population of γ-aminobutyric acid (GABA) inhibitory neurons. Disruption in GABAergic inhibition in the BLA can occur when there is a loss of local GABAergic interneurons, an alteration in GABAA receptor activation, or a dysregulation of mechanisms that modulate BLA GABAergic inhibition. Disruptions in GABAergic control of the BLA emerge during development, in aging populations, or after trauma, ultimately resulting in hyperexcitability. BLA hyperexcitability manifests behaviorally as an increase in anxiety, emotional dysregulation, or development of seizure activity. This Review discusses the anatomy, development, and physiology of the GABAergic system in the BLA and circuits that modulate GABAergic inhibition, including the dopaminergic, serotonergic, noradrenergic, and cholinergic systems. We highlight how alterations in various neurotransmitter receptors, including the acid-sensing ion channel 1a, cannabinoid receptor 1, and glutamate receptor subtypes, expressed on BLA interneurons, modulate GABAergic transmission and how defects of these systems affect inhibitory tonus within the BLA. Finally, we discuss alterations in the BLA GABAergic system in neurodevelopmental (autism/fragile X syndrome) and neurodegenerative (Alzheimer's disease) diseases and after the development of epilepsy, anxiety, and traumatic brain injury. A more complete understanding of the intrinsic excitatory/inhibitory circuit balance of the amygdala and how imbalances in inhibitory control contribute to excessive BLA excitability will guide the development of novel therapeutic approaches in neuropsychiatric diseases.

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.