Hypothermia as potential therapeutic approach to attenuating soman-induced seizure, neuropathology, and mortality with an adenosine A1 receptor agonist and body cooling

Organophosphorus nerve agents, such as soman (GD), produce excitotoxic effects resulting in sustained status epilepticus (SSE) and brain damage. Previous work shows that neuronal inhibitory effects of A1 adenosine receptor (A1AR) agonists, such as N6- Bicyclo (2.2.1)-hept-2-yl-5'-chloro-5'-deoxyadenosine (Cl-ENBA), suppresses GD-induced SSE and improves neuropathology. Some other physiologic effects of these agonists are hypothermia, hypotension, and sedation. Hypothermia may also shield the brain from injury by slowing down chemical insults, lessening inflammation, and contributing to improved neurological outcomes. Therefore, we attempted to isolate the hypothermic effect from ENBA by assessing the neuroprotective efficacy of direct surface body cooling in a rat GD-induced SSE model, and comparing the effects on seizure termination, neuropathology, and survival. Male rats implanted with a body temperature (Tb) transponder and electroencephalographic (EEG) electrodes were primed with asoxime (HI-6), exposed to GD 30 min later, and then treated with Cl-ENBA or had Tb lowered directly via body cooling at 30 min after the onset of seizure activity. Afterwards, they were either allowed to develop hypothermia as expected, or received thermal support to maintain normothermic Tb for a period of 6-h. Neuropathology was assessed at 24 h. Regardless of Cl-ENBA or surface cooling, all hypothermic GD-exposed groups had significantly improved 24-h survival compared to rats with normothermic Tb (81% vs. 39%, p < 0.001). Cl-ENBA offered neuroprotection independently of hypothermic Tb. While hypothermia enhanced the overall efficacy of Cl-ENBA by improving survival outcomes, body cooling didn't reduce seizure activity or neuropathology following GD-induced SSE.

Learning and memory function preserved by delayed A1 adenosine receptor agonist treatment following soman intoxication in rats and a humanized esterase mouse model

Exposure to organophosphorus compounds, such as soman (GD), cause widespread toxic effects, sustained status epilepticus, neuropathology, and death. The A1 adenosine receptor agonist N-bicyclo-(2.2.1)-hept-2-yl-5'-chloro-5'-deoxyadenosine (ENBA), when given 1 min after GD exposure, provides neuroprotection and prevents behavioral impairments. Here, we tested the ability of ENBA at delayed treatment times to improve behavioral outcomes via a two-way active avoidance task in two male animal models, each consisting of saline and GD exposure groups. In a rat model, animals received medical treatments (atropine sulfate [A], 2-PAM [P], and midazolam [MDZ]) or AP + MDZ + ENBA at 15 or 30 min after seizure onset and were subjected to behavioral testing for up to 14 days. In a human acetylcholinesterase knock-in serum carboxylesterase knock-out mouse model, animals received AP, AP + MDZ, AP + ENBA, or AP + MDZ + ENBA at 15 min post seizure onset and were subjected to the behavioral task on days 7 and 14. In rats, the GD/AP + MDZ + ENBA group recovered to saline-exposed avoidance levels while the GD/AP + MDZ group did not. In mice, in comparison with GD/AP + MDZ group, the GD/AP + MDZ + ENBA showed decreases in escape latency, response latency, and pre-session crossings, as well as increases in avoidances. In both models, only ENBA-treated groups showed control level inter-trial interval crossings by day 14. Our findings suggest that ENBA, alone and as an adjunct to medical treatments, can improve behavioral and cognitive outcomes when given at delayed time points after GD intoxication.

Seizure suppression and neuroprotection in soman-exposed rats following delayed intramuscular treatment of adenosine A1 receptor agonist as an adjunct to standard medical treatment

Soman produces excitotoxic effects by inhibiting acetylcholinesterase in the cholinergic synapses and neuromuscular junctions and resulting in soman-induced sustained status epilepticus (SSE). Our previous work showed delayed intramuscular (i.m.) treatment with A1 adenosine receptor agonist N-bicyclo-[2.2.1]-hept-2-yl-5'-chloro-5'-deoxyadenosine (ENBA) alone suppressed soman-induced SSE and prevented neuropathology. Using this same rat soman seizure model, we tested if delayed therapy with ENBA (60 mg/kg, i.m.) would terminate seizure, protect neuropathology, and aid in survival when given in conjunction with current standard medical countermeasures (MCMs): atropine sulfate, 2-PAM, and midazolam (MDZ). Either 15- or 30-min following soman-induced SSE onset, male rats received atropine and 2-PAM plus either MDZ or MDZ + ENBA. Electroencephalographic (EEG) activity, physiologic parameters, and motor function were recorded. Either 2- or 14-days following exposure surviving rats were euthanized and perfused for histology. All animals treated with MDZ + ENBA at both time points had 100% EEG seizure termination and reduced total neuropathology compared to animals treated with MDZ (2-day, p = 0.015 for 15-min, p = 0.002 for 30-min; 14-day, p < 0.001 for 15-min, p = 0.006 for 30-min), showing ENBA enhanced MDZ's anticonvulsant and neuroprotectant efficacy. However, combined MDZ + ENBA treatment, when compared to MDZ treatment groups, had a reduction in the 14-day survival rate regardless of treatment time, indicating possible enhancement of MDZ's neuronal inhibitory effects by ENBA. Based on our findings ENBA shows promise as an anticonvulsant and neuroprotectant in a combined treatment regimen following soman exposure; when given as an adjunct to standard MCMs, the dose of ENBA needs to be adjusted.

A novel genetically modified mouse seizure model for evaluating anticonvulsive and neuroprotective efficacy of an A1 adenosine receptor agonist following soman intoxication

Recently a novel humanized mouse strain has been successfully generated, in which serum carboxylesterase (CES) knock out (KO) mice (Es1-/-) were further genetically modified by knocking in (KI), or adding, the gene that encodes the human form of acetylcholinesterase (AChE). The resulting human AChE KI and serum CES KO (or KIKO) mouse strain should not only exhibit organophosphorus nerve agent (NA) intoxication in a manner more similar to humans, but also display AChE-specific treatment responses more closely mimicking those of humans to facilitate data translation to pre-clinic trials. In this study, we utilized the KIKO mouse to develop a seizure model for NA medical countermeasure investigation, and then applied it to evaluate the anticonvulsant and neuroprotectant (A/N) efficacy of a specific A1 adenosine receptor (A1AR) agonist, N-bicyclo-(2.2.1)hept-2-yl-5'-chloro-5'-deoxyadenosine (ENBA), which has been shown in a rat seizure model to be a potent A/N compound. Male mice surgically implanted with cortical electroencephalographic (EEG) electrodes a week earlier were pretreated with HI-6 and challenged with various doses (26 to 47 μg/kg, SC) of soman (GD) to determine a minimum effective dose (MED) that induced sustained status epilepticus (SSE) activity in 100% of animals while causing minimum lethality at 24 h. The GD dose selected was then used to investigate the MED doses of ENBA when given either immediately following SSE initiation (similar to wartime military first aid application) or at 15 min after ongoing SSE seizure activity (applicable to civilian chemical attack emergency triage). The selected GD dose of 33 μg/kg (1.4 x LD50) generated SSE in 100% of KIKO mice and produced only 30% mortality. ENBA at a dose as little as 10 mg/kg, IP, caused isoelectric EEG activity within minutes after administration in naïve un-exposed KIKO mice. The MED doses of ENBA to terminate GD-induced SSE activity were determined to be 10 and 15 mg/kg when treatment was given at the time of SSE onset and when seizure activity was ongoing for 15 min, respectively. These doses were much lower than in the non-genetically modified rat model, which required an ENBA dose of 60 mg/kg to terminate SSE in 100% GD-exposed rats. At MED doses, all mice survived for 24 h, and no neuropathology was observed when the SSE was stopped. The findings confirmed that ENBA is a potent A/N for both immediate and delayed (i.e., dual purposed) therapy to victims of NA exposure and serves as a promising neuroprotective antidotal and adjunctive medical countermeasure candidate for pre-clinical research and development for human application.

Intramuscularly administered A1 adenosine receptor agonists as delayed treatment for organophosphorus nerve agent-induced Status Epilepticus

Exposure to organophosphorus nerve agents (NAs) like sarin (GB) and soman (GD) can lead to sustained seizure activity, or status epilepticus (SE). Previous research has shown that activation of A1 adenosine receptors (A1ARs) can inhibit neuronal excitability, which could aid in SE termination. Two A1AR agonists, 2-Chloro-N6-cyclopentyladenosine (CCPA) and N-Bicyclo(2.2.1)hept-2-yl-5'-chloro-5'-deoxyadenosine (ENBA), were effective in terminating GD-induced SE in rats when administered via intraperitoneal (IP) injection. However, IP injection is not a clinically relevant route of administration. This study evaluated the efficacy of these agonists in terminating NA-induced SE when administered via intramuscular (IM) route. Adult male rats were exposed subcutaneously (SC) to either GB (150 μg/kg) or GD (90 μg/kg) and were treated with ENBA or CCPA at 15, 30, or 60 min after seizure onset or left untreated. Up to 7 days after exposure, deeply anesthetized rats were euthanized and perfused brains were removed for histologic assessment of neuropathology (i.e., neuronal damage) in six brain regions (amygdala, cerebral cortex, piriform cortex, thalamus, dorsal hippocampus, and ventral hippocampus). A total neuropathy score (0-24) was determined for each rat by adding the scores from each of the six regions. The higher the total score the more severe the neuropathology. With the GB model and 60 min treatment delay, ENBA-treated rats experienced 78.6% seizure termination (N = 14) and reduced neuropathology (11.6 ± 2.6, N = 5), CCPA-treated rats experienced 85.7% seizure termination (N = 14) and slightly reduced neuropathology (20.7 ± 1.8, N = 6), and untreated rats experienced no seizure termination (N = 13) and severe neuropathology (22.3 ± 1.0, N = 4). With the GD model and 60 min treatment delay, ENBA-treated rats experienced 92.9% seizure termination (N = 14) and reduced neuropathology (13.96 ± 1.8, N = 9), CCPA-treated rats experienced 78.6% seizure termination (N = 14) and slightly reduced neuropathology (22.0 ± 0.9, N = 10); and untreated rats experienced 16.7% seizure termination (N = 12) and severe neuropathology (22.0 ± 1.8, N = 5). While ENBA and CCPA both demonstrate a clear ability to terminate SE when administered up to 60 min after seizure onset, ENBA offers more neuroprotection, making it a promising candidate for NA-induced SE.

Soman-induced toxicity, cholinesterase inhibition and neuropathology in adult male Göttingen minipigs

Animal models are essential for evaluating the toxicity of chemical warfare nerve agents (CWNAs) to extrapolate to human risk and are necessary to evaluate the efficacy of medical countermeasures. The Göttingen minipig is increasingly used for toxicological studies because it has anatomical and physiological characteristics that are similar to those of humans. Our objective was to determine whether the minipig would be a useful large animal model to evaluate the toxic effects of soman (GD). We determined the intramuscular (IM) median lethal dose (LD50) of GD in adult male Göttingen minipigs using an up-and-down dosing method. In addition to lethality estimates, we characterized the observable signs of toxicity, blood and tissue cholinesterase (ChE) activity and brain pathology following GD exposure. The 24 h LD50 of GD was estimated to be 4.7 μg/kg, with 95 % confidence limits of 3.6 and 6.3 μg/kg. As anticipated, GD inhibited ChE activity in blood and several tissues. Neurohistopathological analysis showed neurodegeneration and neuroinflammation in survivors exposed to 4.7 μg/kg of GD, including in the primary visual cortex and various thalamic nuclei. These findings suggest that the minipig will be a useful large animal model for assessing drugs to mitigate neuropathological effects of exposure to CWNAs.

Evaluation of adenosine A1 receptor agonists as neuroprotective countermeasures against Soman intoxication in rats

Soman, an organophosphorus (OP) compound, disrupts nervous system function through inactivation of acetylcholinesterase (AChE), the enzyme that breaks down acetylcholine at synapses. Left untreated, a state of prolonged seizure activity (status epilepticus, SE) is induced, causing widespread neuronal damage and associated cognitive and behavioral impairments. Previous research demonstrated that therapeutic stimulation of A1 adenosine receptors (A1ARs) can prevent or terminate soman-induced seizure. This study examined the ability of three potent A1AR agonists to provide neuroprotection and, ultimately, prevent observable cognitive and behavioral deficits following exposure to soman. Sprague Dawley rats were challenged with a seizure-inducing dose of soman (1.2 x LD50) and treated 1 min later with one of the following A1AR agonists: (6)-Cyclopentyladenosine (CPA), 2-Chloro-N6-cyclopentyladenosine (CCPA) or N-bicyclo(2.2.1)hept-2-yl-5'-chloro-5'-deoxyadenosine (cdENBA). An active avoidance shuttle box task was used to evaluate locomotor responses to aversive stimuli at 3, 7 and 14 days post-exposure. Animals treated with CPA, CCPA or cdENBA demonstrated a higher number of avoidance responses and a faster reaction to the aversive stimulus than the soman/saline control group across all three sessions. Findings suggest that A1AR agonism is a promising neuroprotective countermeasure, capable of preventing the long-term deficits in learning and memory that are characteristic of soman intoxication.

The tertiary oxime monoisonitrosoacetone penetrates the brain, reactivates inhibited acetylcholinesterase, and reduces mortality and morbidity following lethal sarin intoxication in guinea pigs

The brain is a critical target for the toxic action of organophosphorus (OP) inhibitors of acetylcholinesterase (AChE) such as the nerve agent sarin. However, the available oxime antidote 2-PAM only reactivates OP-inhibited AChE in peripheral tissues. Monoisonitrosoacetone (MINA), a tertiary oxime, reportedly reactivates AChE in the central nervous system (CNS). The current study investigated whether MINA would be beneficial as a supplemental oxime treatment in preventing lethality and reducing morbidity following lethal sarin exposure, MINA supplement would improve AChE recovery in the body, and MINA would be detectable in the CNS. Guinea pigs were exposed to sarin and treated with atropine sulfate and 2-PAM at one minute. Additional 2-PAM or MINA was administered at 3, 5, 15, or 30 min after sarin exposure. Survival and morbidity were assessed at 2 and 24 h. AChE activity in brain and peripheral tissues was evaluated one hour after MINA and 2-PAM treatment. An in vivo microdialysis technique was used to determine partitioning of MINA into the brain. A liquid chromatography-tandem mass spectrometry method was developed for the analysis of MINA in microdialysates. MINA-treated animals exhibited significantly higher survival and lower morbidity compared to 2-PAM-treated animals. 2-PAM was significantly more effective in reactivating AChE in peripheral tissues, but only MINA reactivated AChE in the CNS. MINA was found in guinea pig brain microdialysate samples beginning at ~10 min after administration in a dose-related manner. The data strongly suggest that a centrally penetrating oxime could provide significant benefit as an adjunct to atropine and 2-PAM therapy for OP intoxication.

Development of a Larval Zebrafish Model for Acute Organophosphorus Nerve Agent and Pesticide Exposure and Therapeutic Evaluation

Organophosphorus compound exposure remains a present threat through agricultural accidents, warfare, or terrorist activity. The primary mechanism of organophosphorus toxicity is through inhibition of the enzyme acetylcholinesterase, with current emergency treatment including anticholinergics, benzodiazepines, and oxime reactivators. However, a need for more effective and broadly acting countermeasures remains. This study aimed to develop larval zebrafish as a high-throughput model for evaluating novel therapeutics against acute organophosphorus exposure. Larval zebrafish at six days post-fertilization were exposed to acute concentrations of seven organophosphorus compounds and treated with one of three oximes. Lethality studies indicated similar relative toxicity to that seen in the established rodent model, with chemical warfare agents proving more lethal than organophosphorus pesticides. Additionally, the organophosphorus-specific response for oxime reactivation of acetylcholinesterase was comparable to what has been previously reported. Behavioral studies measuring the visual motor response demonstrated greater efficacy for centrally acting oxime compounds than for those that are contained to the peripheral tissue. Overall, these results support the use of this larval zebrafish model as a high-throughput screening platform for evaluating novel treatments following acute organophosphorus exposure.

Comparative effects of scopolamine and phencynonate on organophosphorus nerve agent-induced seizure activity, neuropathology and lethality

The efficacy of anticonvulsant therapies to stop seizure activities following organophosphorus nerve agents (NAs) has been documented as being time-dependent. We utilized the guinea pig NA-seizure model to compare the effectiveness of phencynonate (PCH) and scopolamine (SCP) when given at the early (at time of seizure onset) or late (40 min after seizure onset) phase of seizure progression. PCH possesses both anticholinergic and anti-NMDA activities, while SCP is a purely anti-muscarinic compound. Animals with cortical electrodes were pretreated with pyridostigmine bromide 30 min prior to exposure to a 2.0 x LD₅₀ subcutaneous dose of a NA (GA, GB, GD, GF, VR, or VX), followed one min later with atropine sulfate and 2-PAM. At either early or late phase, animals were treated with either PCH or SCP and the 24-h anticonvulsant ED₅₀ doses were determined. When administered at seizure onset, PCH, and SCP were both effective at terminating seizure activity against all NAs, with ED₅₀ values for SCP generally being lower. At the 40 min time, ED₅₀ values were obtained following GA, GD, GF, and VR challenges for SCP, but ED₅₀ value was obtained only following GD for PCH, indicating a superior efficacy of SCP. When seizure activity was controlled, a significant improvement in weight loss, neuropathology, and survival was observed, regardless of treatment or NA. Overall, these results demonstrate the differing efficacies of these two similarly structured anticholinergic compounds with delayed administration and warrant further investigation into the timing and mechanisms of the seizure maintenance phase in different animal models.

Mechanisms of acetylcholinesterase protection against sarin and soman by adenosine A1 receptor agonist N6-cyclopentyladenosine

Organophosphorus nerve agents (NAs) irreversibly inhibit acetylcholinesterase (AChE), the enzyme responsible for breaking down the neurotransmitter acetylcholine (ACh). The over accumulation of ACh after NA exposure leads to cholinergic toxicity, seizure, and death. Current medical countermeasures effectively mitigate peripheral symptoms, however; the brain is often unprotected. Alternative acute treatment with the adenosine A₁ receptor agonist N⁶-cyclopentyladensosine (CPA) has previously been demonstrated to prevent AChE inhibition as well as to suppress neuronal activity. The mechanism of AChE protection is unknown. To elucidate the feasibility of potential CPA-AChE interaction mechanisms, we applied a truncated molecular model approach and density functional theory. The candidate mechanisms studied are reversible enzyme inhibition, enzyme reactivation, and NA blocking prior to enzyme conjugation. Our thermodynamic data suggest that CPA can compete with the NAs sarin and soman for the active site of AChE, but may, in contrast to NAs, undergo back-reaction. We found a strong interaction between CPA and NA conjugated AChE, making enzyme reactivation unlikely but possibly allowing for CPA protection through the prevention of NA aging. The data also indicates that there is an affinity between CPA and unbound NAs. The results from this study support the hypothesis that CPA counters NA toxicity via multiple mechanisms and is a promising therapeutic strategy that warrants further development.

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.

Zebrafish as a model for acetylcholinesterase-inhibiting organophosphorus agent exposure and oxime reactivation

The current research progression efforts for investigating novel treatments for exposure to organophosphorus (OP) compounds that inhibit acetylcholinesterase (AChE), including pesticides and chemical warfare nerve agents (CWNAs), rely solely on in vitro cell assays and in vivo rodent models. The zebrafish (Danio rerio) is a popular, well-established vertebrate model in biomedical research that offers high-throughput capabilities and genetic manipulation not readily available with rodents. A number of research studies have investigated the effects of subacute developmental exposure to OP pesticides in zebrafish, observing detrimental effects on gross morphology, neuronal development, and behavior. Few studies, however, have utilized this model to evaluate treatments, such as oxime reactivators, anticholinergics, or anticonvulsants, following acute exposure. Preliminary work has investigated the effects of CWNA exposure. The results clearly demonstrated relative toxicity and oxime efficacy similar to that reported for the rodent model. This review surveys the current literature utilizing zebrafish as a model for OP exposure and highlights its potential use as a high-throughput system for evaluating AChE reactivator antidotal treatments to acute pesticide and CWNA exposure.

Probing the activity of a non-oxime reactivator for acetylcholinesterase inhibited by organophosphorus nerve agents

Currently fielded treatments for nerve agent intoxication include atropine, an acetylcholine receptor antagonist, and pralidoxime (2PAM), a small molecule reactivator of acetylcholinesterase (AChE). 2PAM reactivates nerve agent-inhibited AChE via direct nucleophilic attack by the oxime moiety on the phosphorus center of the bound nerve agent. Due to a permanently charged pyridinium motif, 2PAM is not thought to cross the blood brain barrier and therefore cannot act directly in the neuronal junctions of the brain. In this study, ADOC, a non-permanently charged, non-oxime molecule initially identified using pesticide-inhibited AChE, was characterized in vitro against nerve agent-inhibited recombinant human AChE. The inhibitory and reactivation potentials of ADOC were determined with native AChE and AChE inhibited with tabun, sarin, soman, cyclosarin, VX, or VR and then compared to those of 2PAM. Several structural analogs of ADOC were used to probe the reactivation mechanism of the molecule. Finally, guinea pigs were used to examine the protective efficacy of the compound after exposure to sarin. The results of both in vitro and in vivo testing will be useful in the design of future small molecule reactivators.

In vivo oxime administration does not influence Ellman acetylcholinesterase assay results

Organophosphorus compounds (OPs) are potent inhibitors of acetylcholinesterase (AChE). Treatment for OP poisoning is by administration of atropine sulfate, an oxime, and diazepam. Oximes such as 2-PAM are used to reactivate OP-inhibited AChE so as to restore normal enzymatic function and serve as a true antidote. There are reports of non-enzymatic hydrolysis by oximes of acetylthiocholine in in vitro preparations in the widely used Ellman assay for AChE activity, which may confound the interpretation of AChE activity by producing elevated results. The purpose of this experiment was to determine if there is appreciable interference by therapeutic levels of oximes on the results of the Ellman assay in assessing AChE reactivation by oxime compounds in vivo. When therapeutic doses of oximes (2-PAM, HI-6, MMB-4, or MINA) were administered intramuscularly to guinea pigs and samples collected 60 min later, there was no statistical difference between oxime and saline control groups in measured AChE activity in various tissue samples, including blood. With appropriate dilution of samples prior to spectrophotometric assay, the Ellman assay is an acceptable method to measure in vivo oxime reactivation of inhibited AChE. Inclusion of an oxime control group to insure that this particular type of interference is not causing false readings in the assay is a prudent step.

Evaluation of nine oximes on in vivo reactivation of blood, brain, and tissue cholinesterase activity inhibited by organophosphorus nerve agents at lethal dose

The capability of several oximes (HI-6, HLö7, MMB-4, TMB-4, carboxime, ICD 585, ICD 692, ICD 3805, and 2-PAM) to reactivate in vivo AChE inhibited by the nerve agents sarin, cyclosarin, VX, or VR in blood, brain regions, and peripheral tissues in guinea pigs was examined and compared. Animals were injected subcutaneously with 1.0 LD(50) of sarin, cyclosarin, VR, or VX, and treated intramuscularly 5 min later with one of these compounds. Toxic signs and lethality were monitored, and tissue AChE activities were determined at 60 min after nerve agent. The animals exposed to sarin or cyclosarin, alone or with non-oxime treatment, some died within 60 min; however, when treated with an oxime, no animal died. For VR or VX, all animals survived for 60 min after exposure, with or without non-oxime or oxime therapy. These nerve agents caused differential degrees of inhibition: in whole blood sarin = cyclosarin > VR = VX; in brain regions sarin > cyclosarin > VX > VR; and in peripheral tissues sarin > VX > cyclosarin > VR. These oximes exhibited differential potency in reactivating nerve agent-inhibited AChE in various peripheral tissues, but not AChE activity in the brain regions. There was no difference in the AChE reactivating potency between the dichloride and dimethanesulfonate salts of HI-6. AChE inhibited by sarin was the most and cyclosarin the least susceptible to oxime reactivation. Overall, MMB-4 appeared to be, among all oximes tested, the most effective in vivo AChE reactivator against the broadest spectrum of nerve agents.

Effects of 4-pyridine aldoxime on nerve agent-inhibited acetylcholinesterase activity in guinea pigs

Methoxime (MMB-4) is a leading candidate oxime acetylcholinesterase (AChE) reactivator to replace pralidoxime (2-PAM) for therapeutic treatment of nerve agent intoxication. 4-Pyridine aldoxime (4-PA) is a synthetic starting material, a breakdown product, and a probable metabolite of MMB-4. There is a possibility that 4-PA may adversely interact with the nerve agent, thereby affecting nerve agent toxicity and biological AChE activity. This study evaluated the effects of 4-PA on sarin (GB)-, cyclosarin (GF)-, and VX-induced toxicity and AChE activity in blood, brain, and peripheral tissues of guinea pigs. Animals were pretreated with atropine methyl nitrate (1.0 mg/kg, im) 15 min prior to subcutaneous administration with 1.0 x LD(50) of GB, GF, or VX and then treated 15 min after the administration of nerve agents with 4-PA (3.5, 7.0, or 14.0 mg/kg, im). The dose-response effects of 4-PA alone were also examined. Toxic signs and lethality were monitored, blood and tissues were collected, and AChE activities were determined at 60 min after nerve agent administration. Under the condition of this study, all animals exposed to nerve agents exhibited some degree of toxic signs such as salivation, lacrimation, rhinorrhea, and convulsions. 4-PA at the three doses tested neither induced toxic signs nor altered the toxicity of GB, GF, or VX at the 1.0 x LD(50) exposure dose. Additionally, it did not modify the AChE activity in blood, brain, and peripheral tissues by itself or affect the AChE activity inhibited by a 1.0 x LD(50) dose of these three nerve agents in guinea pigs.

Protection against sarin-induced seizures in rats by direct brain microinjection of scopolamine, midazolam or MK-801

Control of seizure activity is critical to survival and neuroprotection following nerve agent exposure. Extensive research has shown that three classes of drugs, muscarinic antagonists, benzodiazepines, and N-methyl-D: -aspartate antagonists, are capable of moderating these seizures. This study began to map the neural areas in rat brain that respond to these three drug classes resulting in anticonvulsant effects. Drugs of each class (scopolamine, midazolam, MK-801) were evaluated for their ability to prevent sarin-induced seizures when injected into specific brain areas (lateral ventricle, anterior piriform cortex, basolateral amygdala, area tempestas). Animals were pretreated by microinjection with saline or a dose of drug from one of the three classes 30 min prior to receiving 150 microg/kg sarin, subcutaneously, followed by 2.0 mg/kg atropine methylnitrate, intramuscularly. Animals were then returned to their cages, where electroencephalographic activity was monitored for seizures. Anticonvulsant effective doses (ED(50)) were determined using an up-down dosing procedure over successive animals. Scopolamine provided anticonvulsant effects in each area tested, while midazolam was effective in each area except the lateral ventricle. MK-801 was only effective at preventing seizures when injected into the basolateral amygdala or area tempestas. The results show a unique neuroanatomical and pharmacological specificity for control of nerve agent-induced seizures.

Anticonvulsants for nerve agent-induced seizures: The influence of the therapeutic dose of atropine

Two guinea pig models were used to study the anticonvulsant potency of diazepam, midazolam, and scopolamine against seizures induced by the nerve agents tabun, sarin, soman, cyclosarin, O-ethyl S-(2-(diisopropylamino)ethyl)methylphosphonothioate (VX), and O-isobutyl S-(2-diethylamino)ethyl)-methyl phosphonothioate (VR). Animals instrumented for electroencephalogram recording were pretreated with pyridostigmine bromide (0.026 mg/kg i.m.) 30 min before challenge with 2 x LD50 (s.c.) of a nerve agent. In model A, atropine sulfate (2.0 mg/kg i.m.) and pyridine-2-aldoxime methylchloride (2-PAM; 25.0 mg/kg i.m.) were given 1 min after nerve agent challenge, and the tested anticonvulsant was given (i.m.) 5 min after seizure onset. In model B, a lower dose of atropine sulfate (0.1 mg/kg i.m.) was given along with 2-PAM 1 min after nerve agent challenge, and the anticonvulsant was given at seizure onset. With the lower dose of atropine, seizure occurrence increased to virtually 100% for all agents; the time to seizure onset decreased for sarin, cyclosarin, and VX; the signs of nerve agent intoxication were more severe; and coma resulted frequently with cyclosarin. The anticonvulsant ED50 doses for scopolamine or diazepam were, in general, not different between the two models, whereas the anticonvulsant ED50 values of midazolam increased 3- to 17-fold with the lower atropine dose. Seizure termination times were not systematically effected by the different doses of atropine. The order of anticonvulsant effectiveness within each model was scopolamine > or = midazolam > diazepam. The findings indicate that the dose of atropine given as antidotal therapy can significantly influence measures of nerve agent toxicity and responsiveness to anticonvulsant therapy.

Circadian rhythms of heart rate and locomotion after treatment with low-dose acetylcholinesterase inhibitors

This study tested the hypothesis that repeated exposure to low levels of sarin, pyridostigmine bromide (PB) or their combination, at doses equivalent to those possibly experienced by veterans of the 1991 Persian Gulf War, could lead to persistent or delayed autonomic effects and thus help to explain the cause of clinical findings in this population. Male Sprague-Dawley rats were treated for 3 weeks with: saline injection (0.5 ml kg(-1), s.c., 3 times weekly) with tap drinking water (control); saline injection with PB (80 mg l(-1) in drinking water); sarin injection (62.5 microg kg(-1), s.c., 0.5 x LD(50), 3 times weekly) with tap drinking water (sarin); or sarin injection with PB in drinking water (sarin + PB). At 2, 4 or 16 weeks post-treatment, heart rate (HR) and locomotor activity (LA) were studied by radiotelemetry. Two weeks posttreatment, HR in drug-treated animals was significantly lower than in controls. A decrease in low-frequency HR power spectrum (PS) was found at 00:00 h and 08:00 h with sarin + PB and at 00:00 h with sarin, while total power was enhanced with sarin + PB at 22:00 h. Minimal effects of drug treatments on HR and HR PS were detected at 4 and 16 weeks post-treatment. No significant differences in LA between control and other groups were found. Since no consistent long-term effects were found in any of the variables studied, these experiments do not support the hypothesis that repeated administration of low doses of PB and the nerve agent sarin can induce persistent or delayed alterations in autonomic function.

Cerebral acetylcholine and choline contents and turnover following low-dose acetylcholinesterase inhibitors treatment in rats

Male Sprague-Dawley rats were treated for 3 weeks with (1) regular tap drinking water plus subcutaneous (s.c.) saline (0.5 ml/kg) injections three times/week, (2) pyridostigmine bromide (PB) in drinking water (80 mg/L) plus s.c. saline injections three times/week, (3) regular tap drinking water plus s.c. sarin (0.5 x LD(50)) injections three times/week, or (4) PB in drinking water plus s.c. sarin injections three times/week. Repeated doses of sarin, in the presence or absence of PB, were devoid of acute toxicity during the three-week treatment period. Two, 4, and 16 weeks post-treatment, animals were given an intravenous pulse injection of choline labeled with 4 deuterium atoms (D4Ch) followed, after 1 min, by microwave fixation of the brain in vivo. Tissue levels of endogenous acetylcholine (D0ACh), endogenous choline (D0Ch), D4Ch, and ACh synthesized from D4Ch (D4ACh) were measured by gas-chromatography mass-spectrometry in hippocampus, infundibulum, mesencephalon, neocortex, piriform cortex, and striatum. Ch uptake from blood and ACh turnover were estimated from D4Ch and D4ACh concentrations in brain tissue, respectively. Statistically significant differences among brain regions were found for D0Ch, D4Ch, D0ACh and D4ACh at 2, 4 and 16 weeks post-treatment. However, differences in the values of these parameters between control and drug treatments were found only for D0ACh and D0Ch at 2 and 4 weeks, but not at 16 weeks post-treatment. In conclusion, the results from these experiments do not support a delayed or persistent alteration in cholinergic function after exposure to low doses of PB and/or sarin.

In vivo cholinesterase inhibitory specificity of organophosphorus nerve agents

The purpose of this project was to determine and compare the time-related changes in blood, brain, and tissue acetylcholinesterase (AChE) activity during the first hour after exposure to six organophosphorus nerve agents (GA, GB, GD, GF, VR, and VX) in Hartley guinea pigs. Animals were pretreated with atropine methyl nitrate (1.0mg/kg, i.m.) to minimize peripheral toxic effects 15 min before they were given a 1.0 x LD50 subcutaneous dose of a nerve agent. At 0, 5, 10, 15, 30, and 60 min after nerve agent, animals were humanely euthanized. Blood was collected and brain regions (brainstem, cortex, hippocampus, midbrain, cerebellum, striatum, and spinal cord) and peripheral tissues (diaphragm, skeletal muscle, and heart) were dissected and processed for AChE activity. All six nerve agents produced maximum inhibition of AChE in red blood cells between 5 and 10% of the control within 10 min after exposure. In whole blood, differential effects were observed among the agents: GB, GD, and GF produced more rapid and greater inhibition than did GA, VR, and VX. GF was the most rapid, producing a maximum inhibition to 5% of the control in 5 min, while VR and VX were slower reaching maximum inhibition to 30% of the control at 15 min. The enzyme activity in the majority of the brain regions was more markedly inhibited by the G-agents than by the V-agents. The G-agents caused rapid AChE inhibition, reaching maximum levels (20-30% of control) at 15 min and GA produced the most rapid effects. V-agents produced much slower and less AChE inhibition, reaching maximum (35-60% of control) at 30 min. In the diaphragm, VR, VX, and GD produced more rapid and greater AChE inhibition than other G-agents; GA produced the slowest and least inhibition. In the skeletal muscle, VX induced the most rapid and severe inhibition, while GA the least inhibition. In the heart, all agents produced very rapid inhibition, and GD produced the most severe inhibition of AChE activity. These observations suggest that G-agents and V-agents are tissue compartment specific in their ability to inhibit AChE activity.

Low-dose cholinesterase inhibitors do not induce delayed effects on cerebral blood flow and metabolism

The acetylcholinesterase (AChE) inhibitors sarin and pyridostigmine bromide (PB) have been proposed as causes of neurobehavioral dysfunction in Persian Gulf War veterans. To test possible delayed effects of these agents, we exposed rats to low (subsymptomatic) levels of sarin (0.5 LD50 s.c. 3 times weekly) and/or PB (80 mg/L in drinking water) for 3 weeks. Controls received saline s.c. and tap water. At 2, 4 and 16 weeks after exposure, regional cerebral blood flow (rCBF) and glucose utilization (rCGU) were measured in conscious animals with the Iodo-14C-antipyrine and 14C-2 deoxyglucose methods, respectively. Two weeks after exposure, PB+sarin caused significant rCBF elevations, but no changes in rCGU, in neocortex, with lesser effects on allocortex. Four weeks after exposure, the same general pattern was found with sarin. Only a few changes were found at 16 weeks post-treatment. The predominant effects of sarin or PB+sarin on rCBF at earlier times after treatment are consistent with the well known direct cerebral vascular effect of cholinergic agonists. The lack of changes in rCBF and rCGU observed at 16 weeks after treatment does not support the hypothesis that repeat exposure to low-dose cholinesterase inhibitors can generate permanent alterations in cerebral activity.

Effects of Fosphenytoin on Nerve Agent‐InducedStatus epilepticus

This study evaluated the effectiveness of fosphenytoin as a single or adjunctive anticonvulsant treatment for nerve agent-induced status epilepticus. Guinea pigs, implanted with cortical electroencephalographic (EEG) recording electrodes, were pretreated with pyridostigmine bromide (0.026 mg/kg, intramuscular (i.m.)) 30 min before challenge with 56 micrograms/kg, subcutaneous (s.c.), (2 x LD50) of the nerve agent soman. One min after soman, the animals were treated (i.m.) with 2 mg/kg atropine sulfate admixed with 25 mg/kg of the oxime 2-pralidoxime chloride, and the EEG was observed for seizure onset. When administered (intraperitoneal, i.p.) therapeutically 5 min after seizure onset, only the highest fosphenytoin dose (180 mg/kg) was capable of terminating seizure activity in 50% of the animals tested (3 of 6). When fosphenytoin (18-180 mg/kg) was administered as a pretreatment, i.p., 30 min before soman challenge, seizures were blocked or terminated in a dose-dependent fashion (ED50 = 61.8 mg/kg; 40.5-94.7 mg/kg = 95% confidence limits). Combinations of diazepam and fosphenytoin were also tested for effectiveness. No dose of fosphenytoin (18-56 mg/kg), given in conjunction with a fixed dose of diazepam (4.8 mg/kg, i.m.) 5 min after seizure onset, enhanced the anticonvulsant effect of diazepam. When fosphenytoin (18 or 32 mg/kg, i.p.) was given as a pretreatment and diazepam was given 5 min after seizure onset, the 32 mg/kg dose of fosphenytoin significantly reduced the time for seizure control. These studies show that fosphenytoin, either alone or in combination with diazepam, has little or no therapeutic anticonvulsant effectiveness for nerve agent-induced status epilepticus.

Control of nerve agent-induced seizures is critical for neuroprotection and survival

This study evaluated the potency and rapidity of some anticholinergics (atropine, biperiden, and trihexyphenidyl) and benzodiazepines (diazepam and midazolam) as an anticonvulsant treatment against seizures induced by six nerve agents (tabun, sarin, soman, cyclosarin, VR, and VX) and summarized the relationship between anticonvulsant activity and nerve agent-induced lethality and neuropathology. Guinea pigs, previously implanted with cortical electrodes for EEG recording, were pretreated with pyridostigmine bromide (0.026 mg/kg im) 30 min prior to challenge with 2x LD50 dose (sc) of a given nerve agent; in a separate experiment, animals were challenged with 5x LD50 sc of soman. One minute after agent challenge the animals were treated im with 2 mg/kg atropine SO(4) admixed with 25 mg/kg 2-PAM Cl. Five minutes after the start of EEG seizures, animals were treated im with different doses of anticholinergics or benzodiazepines and observed for seizure termination. The time to seizure onset, the time to seizure termination, and 24-h lethality were recorded. The anticonvulsant ED50 of each drug for termination of seizures induced by each agent was calculated and compared. Brain tissue from animals that survived 24 h was examined for pathology. All drugs were capable of terminating seizure activity, with midazolam and trihexyphenidyl being significantly more potent than the other drugs, and midazolam being more rapid in controlling seizure than atropine, trihexyphenidyl, or diazepam against each agent. Seizures induced by sarin or VX required lower doses of all the test anticonvulsants. The dose of a given drug that was an effective anticonvulsant against a 2x LD50 challenge of soman was equally effective against seizures induced by a 5x LD50 challenge. All nerve agents were capable of producing neuropathology. Seizure control was strongly associated with protection against acute lethality and brain pathology.

Delayed neurologic and behavioral effects of subtoxic doses of cholinesterase inhibitors

We tested the hypothesis that pyridostigmine bromide (PB) intake and/or low-level sarin exposure, suggested by some as causes of the symptoms experienced by Persian Gulf War veterans, induce neurobehavioral dysfunction that outlasts their effects on cholinesterase. Adult male Sprague-Dawley rats were treated during 3 weeks with s.c. saline, PB in drinking water (80 mg/l), sarin (62.5 microg/kg; 0.5x LD(50), three times/week s.c.), or PB in drinking water + sarin. Animals were tested for passive avoidance, nociceptive threshold, acoustic startle, and open field activity 2, 4, or 16 weeks after treatment. Two weeks after sarin, acoustic startle was enhanced, whereas distance explored in the open field decreased. These effects were absent with PB + sarin or PB by itself. No effect on any variable was found at 4 weeks, whereas at 16 weeks sarin induced a decrease and PB + sarin induced an increase in habituation in the open field test. Nociceptive threshold was elevated in the PB + sarin group at 16 weeks. No effect of treatment on passive avoidance was noted in any group. Brain regional acetylcholinesterase and cholineacetyltransferase activities were not affected at any time after treatment, but muscarinic receptors were down-regulated in hippocampus, caudate putamen, and mesencephalon in the sarin group at 2 weeks. In conclusion, this study gives further support to the use of PB against nerve agent poisoning and does not support the hypothesis that delayed symptoms experienced by Persian Gulf War veterans could be due to PB, alone or in association with low-level sarin exposure.

Protective action of the serine protease inhibitor N-tosyl-L-lysine chloromethyl ketone (TLCK) against acute soman poisoning

Soman-poisoned rats display cholinergic crisis, a systemic mast cell degranulation characteristic of anaphylactic reactions and an excitotoxin-like sequential seizure and neuronal degeneration. The protection of guinea pigs from soman lethality by prophylactic administration of the serine protease inhibitor suramin suggests a possible proteolytic component in soman poisoning. The present study tested the effect of N-tosyl-L-lysine chloromethyl ketone (TLCK), an inhibitor of trypsin-like serine proteases, on soman-induced toxic signs (convulsions, righting reflex) and survival time. Nine control guinea pigs receiving 2 x LD(50) (56 microg kg(-1), s.c.) of soman immediately followed by a therapeutic dose of atropine sulfate (17.4 mg kg(-1) i.m.) experienced severe convulsions, and 8/9 lost the righting reflex. Six of these nine animals expired within 65 min; the three remaining animals survived 24 h to termination of the experiment. When a second group of animals were given TLCK (12 mg kg(-1), i.p.) 30 min prior to a 2 x LD(50) soman challenge and atropine-sulfate therapy, 5/9 experienced convulsions and only 3/9 lost the righting reflex. All nine animals survived beyond 4 h, with six surviving to 24 h. Compared with soman controls, prophylaxis with TLCK significantly prevented the loss of righting reflex (P = 0.05) and enhanced 4-h survival (P = 0.005). Although, convulsions were reduced and 24-h survival was improved in TLCK-treated animals, these results were not statistically significant. The protection from soman toxicity by chemically distinct protease inhibitors such as suramin and TLCK suggests a role for pathological proteolytic pathways in soman intoxication.

Immunity to K1 killer toxin: internal TOK1 blockade

K1 killer strains of Saccharomyces cerevisiae harbor RNA viruses that mediate secretion of K1, a protein toxin that kills virus-free cells. Recently, external K1 toxin was shown to directly activate TOK1 channels in the plasma membranes of sensitive yeast cells, leading to excess potassium flux and cell death. Here, a mechanism by which killer cells resist their own toxin is shown: internal toxin inhibits TOK1 channels and suppresses activation by external toxin.

Efficacy of biperiden and atropine as anticonvulsant treatment for organophosphorus nerve agent intoxication

The ability of the nerve agents tabun, sarin, soman, GF, VR, and VX to produce brain seizures and the effectiveness of the anticholinergics biperiden HCl or atropine SO4 as an anticonvulsant treatment were studied in a guinea-pig model. All animals were implanted a week prior to the experiment with cortical electrodes for electroencephalogram (EEG) recordings. On the day of exposure, the animals were pretreated with pyridostigmine (0.026 mg/kg, i.m.) 30 min prior to challenge with a 2 x LD50 dose (s.c.) of a given agent. In separate experiments, animals were challenged with 5 x LD50 (s.c.) of soman. One minute after agent challenge, the animals were treated intramuscularly (i.m.) with 2 mg/kg atropine SO4 admixed with 25 mg/kg 2-PAM Cl and then observed for the onset of seizure activity. Five minutes after the start of nerve agent-induced EEG seizures, animals were treated i.m. with different doses of biperiden HCl or atropine SO4 and observed for seizure termination. The anticonvulsant ED50 of biperiden HCl and atropine SO4 for termination of seizures induced by each nerve agent was calculated and compared. With equally toxic doses (2 x LD50) of these agents, continuous EEG seizures (status epilepticus) developed in all animals challenged with soman, tabun, or VR, and in more than 90% of the animals challenged with GF or sarin. In contrast, only 50% of the animals developed seizures when challenged with VX. The times to onset of seizures for soman, tabun, GF, and sarin were very similar (5-8 min) while for VR, it was about 10 min. In the case of VX, not only was the time to seizure development longer (20.7 min), but the seizure activity in 19% of the animals terminated spontaneously within 5 min after onset and did not return. Under these conditions, the anticonvulsant ED50s of biperiden HCl for soman, GF, VR, tabun, sarin, and VX were 0.57, 0.51, 0.41, 0.2, 0.1, and 0.09 mg/kg, respectively, while those of atropine SO4 for soman, VR, tabun, GF, sarin, and VX were 12.2, 11.9, 10.4, 10.3, 5.1, and 4.1 mg/kg, respectively. In separate experiments, the anticonvulsant ED50 doses of biperiden for animals challenged with 2 or 5 x LD50 of soman were 0.48 (95% confidence limits 0.25-0.73) or 0.57 (95% CI 0.38-0.84) mg/kg, respectively, while the anticonvulsant ED50s for atropine (12.2 mg/kg, i.m.) were identical under these same two challenge conditions. The present study demonstrates that all nerve agents can produce status epilepticus and that the therapeutic effectiveness of atropine and biperiden roughly paralleled the seizurogenic potential of these agents.

Anticonvulsant treatment of nerve agent seizures: anticholinergics versus diazepam in soman-intoxicated guinea pigs

A total of eight anticholinergic drugs (aprophen, atropine, azaprophen, benactyzine, biperiden, procyclidine, scopolamine, trihexyphenidyl) were tested in parallel with diazepam for the ability to terminate seizure activity induced by the nerve agent soman. Guinea pigs, implanted with electrodes to record cortical electroencephalographic (EEG) activity, were pretreated with pyridostigmine Br (0.026 mg/kg, i.m.) and 30 min later challenged with 2 x LD50 soman (56 microg/kg, s.c.) followed 1 min later by treatment with atropine SO4 (2 mg/kg, i.m.) and pralidoxime chloride (2-PAM Cl; 25 mg/kg, i.m.). All guinea pigs developed sustained seizure activity following this treatment. Dose-effect curves were determined for the ability of each drug to terminate seizure activity when anticonvulsant treatment was given either 5 or 40 min after seizure onset. Body weight gain and recovery of behavioral performance of a previously trained one-way avoidance task were measured after exposure. With the exception of atropine, all anticholinergic drugs were effective at lower doses than diazepam in terminating seizures when given 5 min after seizure onset; benactyzine, procyclidine and aprophen terminated seizures most rapidly while scopolamine, trihexyphenidyl, biperiden, and diazepam were significantly slower. When given 40 min after seizure onset, diazepam was the most potent compound tested, followed by scopolamine, benactyzine and biperiden; atropine was not effective when tested 40 min after seizure onset. For diazepam, the time to terminate the seizure was the same whether it was given at the 5- or 40-min delay. In contrast, most anticholinergics were significantly slower in terminating seizure activity when

Comparative evaluation of benzodiazepines for control of soman-induced seizures

This study evaluated the ability of six benzodiazepines to stop seizures produced by exposure to the nerve agent soman. Guinea pigs, previously prepared with electrodes to record electroencephalographic (EEG) activity, were pretreated with pyridostigmine (0.026 mg/kg, i.m.) 30 min before challenge with soman (56 microg/kg, s.c.) and then treated 1 min after soman exposure with atropine (2.0 mg/kg, i.m.) and pralidoxime chloride (2-PAM Cl; 25 mg/kg, i.m.). All animals developed seizures following this treatment. Benzodiazepines (avizafone, clonazepam, diazepam, loprazolam, lorazepam, and midazolam) were given i.m. 5 or 40 min after seizure onset. All benzodiazepines were effective in stopping soman-induced seizures, but there were marked differences between drugs in the rapidity of seizure control. The 50% effective dose (ED50) values and latencies for anticonvulsant effect for a given benzodiazepine were the same at the two times of treatment delay. Midazolam was the most potent and rapidly acting compound at both treatment times. Since rapid seizure control minimizes the chance of brain damage, use of midazolam as an anticonvulsant may lead to improved clinical outcome in the treatment of nerve agent seizures.

A molecular target for viral killer toxin: TOK1 potassium channels

Killer strains of S. cerevisiae harbor double-stranded RNA viruses and secrete protein toxins that kill virus-free cells. The K1 killer toxin acts on sensitive yeast cells to perturb potassium homeostasis and cause cell death. Here, the toxin is shown to activate the plasma membrane potassium channel of S. cerevisiae, TOK1. Genetic deletion of TOK1 confers toxin resistance; overexpression increases susceptibility. Cells expressing TOK1 exhibit toxin-induced potassium flux; those without the gene do not. K1 toxin acts in the absence of other viral or yeast products: toxin synthesized from a cDNA increases open probability of single TOK1 channels (via reversible destabilization of closed states) whether channels are studied in yeast cells or X. laevis oocytes.

Organophosphorus nerve agents-induced seizures and efficacy of atropine sulfate as anticonvulsant treatment

The ability of five organophosphorus nerve agents (tabun, sarin, soman, GF, and VX) to produce brain seizures and the effectiveness of atropine as an anticonvulsant treatment against these nerve agents were studied in two different animal models--the rat and guinea pig. All animals were implanted with cortical electrodes for EEG recordings. Five minutes after the start of nerve agent-induced EEG seizures, animals were treated intramuscularly (IM) with different doses of atropine sulfate and observed for seizure termination. The anticonvulsant ED50 of atropine sulfate for termination of seizures induced by each nerve agent was calculated and compared. In the rat model, selected oximes were administered either before, concurrent with, or following challenge with a 1.6 x LD50 dose of a given nerve agent to maximize seizure development with certain agent/oxime combinations. The choice and the timing of oxime administration significantly effected the incidence of seizure development by different nerve agents. When oxime administration did not effect seizure development (tabun, soman) the anticonvulsant ED50 for atropine sulfate was the same, regardless of the nerve agent used to elicit the seizure. When oxime administration reduced the incidence of seizure occurrence (sarin, GF, VX), the anticonvulsant ED50 dose of atropine sulfate for a nerve agent was lower. In the guinea pig model, animals were pretreated with pyridostigmine prior to challenge with 2 x LD50 of a given agent, and treated 1 min later with atropine sulfate (2 mg/kg) and 2-PAM (25 mg/kg). Under these conditions, the incidence, latency of seizure development, and anticonvulsant ED50s of atropine for soman-, tabun-, and GF-elicited seizures were virtually identical. With sarin, although the latency of seizure development was the same as with soman, tabun, and GF, seizures occurred with a lower incidence, and the anticonvulsant ED50 of atropine was lower. With VX, the latency of seizure development was notably longer, while the incidence of seizure development and anticonvulsant ED50 of atropine were significantly lower than with soman, tabun, or GF. In both models, a lower incidence of seizure development predicted a lower anticonvulsant dose of atropine. In the rat, the incidence of seizure development and the anticonvulsant effectiveness of atropine was highly dependent on the oxime used. In the guinea pig, higher doses of atropine sulfate were required to control soman-, tabun-, or GF-induced seizures, perhaps reflecting the lower cholinesterase reactivating ability of 2-PAM against these agents.

Characterization of anion exchangers in an inner medullary collecting duct cell line

Although the inner medullary collecting duct (IMCD) plays a major role in urinary acidification, the molecular identification of many of the specific components of the transport system in this nephron segment are lacking. A cultured line of rat IMCD cells was used to characterize the mediators of cellular HCO3 exit. This cell line functionally resembles alpha-intercalated cells. Physiologic experiments document that HCO3- transport is a reversible, electroneutral, Cl dependent, Na+-independent process. It can be driven by Cl-gradients and inhibited by stilbenes such as 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid. Immunohistochemical analysis, using a rabbit polyclonal antibody against the carboxy-terminal 12 amino acids of anion exchanger 1 (AE1), revealed a distribution of immunoreactive protein that is consistent with a basolateral localization of AE in cultured cells and in alpha-intercalated cells identified in sections of rat kidney cortex. Immunoblot revealed two immunoreactive bands (approximately 100 and 180 kD in size) in membranes from cultured IMCD cells, rat renal medulla, and freshly isolated IMCD cells. The mobility of the lower molecular weight band was similar to that of AE1 in red blood cell ghosts and kidney homogenate and therefore probably represents AE1. The mobility of the 180-kD band is similar to that for rat stomach and kidney AE2 and therefore probably represents AE2. Selective biotinylation of the apical or basolateral membrane proteins in cultured IMCD cells revealed that both AE1 and AE2 are polarized to the basolateral membrane. Northern blot analysis documented the expression of mRNA for AE1 and AE2 but not AE3. Furthermore, the cDNA sequence of AE1 and AE2 expressed by these cells was found to be virtually identical to that reported for kidney AE1 and rat stomach AE2. It is concluded that this cultured line of rat IMCD cells expresses two members of the anion exchanger gene family, AE1 and AE2, and both of these exchangers probably mediate the electroneutral Cl--dependent HCO3-transport observed in this cell line.

Mapping of cerebral metabolic activation in three models of cholinergic convulsions

Glucose utilization of four cerebral cortex and 35 subcortical regions (CGU) was analyzed in three models of cholinergic seizures induced by the following compounds: 1) soman (pinacolylmethylphosphonofluoridate) an organophosphorus cholinesterase inhibitor, 100 microg/kg SC after pretreatment with pyridostigmine 26 microg/kg IM (n = 6); 2) physostigmine, a carbamate cholinesterase inhibitor, 1.31 mg/kg infused IV over 75 min (n = 6); and 3) pilocarpine, a direct cholinergic agonist, 30 mg/kg SC (n = 6). Physostigmine and pilocarpine were preceded by 3 mmol/kg LiCl IP 20 hrs earlier. Animals injected with saline SC (n = 6) were used as controls. Step-wise discriminant analysis successfully classified 100% of the cases into the four experimental groups with data from only six regions. Pyridostigmine-soman induced the most widespread and greatest increases in CGU. More restricted and lower levels of activation were observed with Li-pilocarpine while Li-physostigmine induced significant increases in CGU only in globus pallidus, entopeduncular nucleus, and substantia nigra. These three regions, which are functionally related, were also activated in the other two models of cholinergic convulsions and may represent the initial step in cholinergic activation of the CNS. Li-pilocarpine failed to activate most of the brainstem and the superior colliculus. All cortical regions were activated by Li-pilocarpine and pyridostigmine-soman, while they were inhibited by Li-physostigmine. This phenomenon may be due in part to the lack of activation with physostigmine of the basal forebrain nuclei (lateral septum, medial septum, vertical and horizontal limbs of the diagonal band, and substantia innominata) resulting in a decreased drive of cortical metabolism.

Neuropharmacological mechanisms of nerve agent-induced seizure and neuropathology

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

Neurochemical mechanisms in soman-induced seizures

This study examined brain regional neurotransmitter level changes as a function of seizure duration following soman intoxication. Rats, implanted with cortical electrodes and pretreated with HI-6, received a convulsant dose of soman. At selected times after seizure onset the EEG recording electrodes were removed and the animal was killed. Spinal cord cholinesterase (ChE) activity was rapidly and maximally depressed, while brain acetylcholine (ACh) levels showed elevations as early as 3 min after soman treatment and reached significantly high levels at time of seizure onset. Norepinephrine (NE) levels decreased starting 5 min after seizure onset and continued to decline. Levels of dopamine (DA) and of its metabolites 3,4-dihydroxyphenylacetic acid and homovanillic acid were elevated as early as 5 min after seizure onset and thereafter. The brain levels of aspartate were markedly decreased at and after 20 min of seizures; levels of glutamate were depressed at 80 min in the cortex. Levels of gamma-aminobutyric acid (GABA) were significantly increased in the cortex starting at 20 min after seizure onset, and in the striatum and hippocampus at 80 min after onset. The levels of glutamine, glycine and taurine were not changed at any time studied. These findings are consistent with the notion that inhibition of ChE and elevation of ACh initiate the seizure process, resulting in secondary changes in DA turnover and release of NE, and later changes in excitatory (aspartate, glutamate) and inhibitory (GABA) amino acid transmitters.