Histomorphogenesis of soman-induced encephalocardiomyopathy in Sprague-Dawley rats

Long-term changes in neuroimaging markers, cognitive function and psychiatric symptoms in an experimental model of Gulf War Illness

AIMS

Gulf War Illness (GWI) is a multi-symptom disease with debilitating cognitive and emotional impairments in veterans. GWI, like epilepsy, is caused by chemical neurotoxicity and manifests from disturbances in neuronal excitability. However, the mechanisms underlying such devastating neurological and psychiatric symptoms remain unclear. Here we investigated the long-term changes in neural behavior and brain structural abnormalities in a rat model of GWI. GWI is linked to exposure to GWI-related organophosphate chemicals (pyridostigmine bromide or PB and insecticide DEET, permethrin) during the stressful Gulf war.

METHODS

To mimic GWI, we generated an experimental GWI prototype in rats by daily exposure to GWI-related chemicals with restraint stress (GWIR-CS) for 4 weeks. Changes in MRI scan and cognitive function were assessed at 5- and 10- months post-exposure.

KEY FINDINGS

In MRI scans, rats displayed significant increases in lateral ventricle T2 relaxation times at both 5- and 10-months after GWIR-CS, indicating alterations in the cerebrospinal fluid (CSF) density. Furthermore, at 10 months, there were significant decreases in the volumes of the hippocampus and thalamus and an increase in the lateral ventricle volume. At both time points, they exhibited impairments in multiple neurobehavioral tests, confirming substantial deficits in memory and mood function. GWI-CS rats also displayed aggressive behavior and a marked decrease in social interaction and forced swimming, indicating depression.

CONCLUSIONS

These results confirm that chronic GWIR-CS exposure led to cognitive and psychiatric symptoms with concurrent neuroimaging abnormalities in CSF, with morphological neural lesions, demonstrating the role of divergent etiological mechanisms in GWI and its comorbidities.

Organophosphorus diisopropylfluorophosphate (DFP) intoxication in zebrafish larvae causes behavioral defects, neuronal hyperexcitation and neuronal death

With millions of intoxications each year and over 200,000 deaths, organophosphorus (OP) compounds are an important public health issue worldwide. OP poisoning induces cholinergic syndrome, with respiratory distress, hypertension, and neuron damage that may lead to epileptic seizures and permanent cognitive deficits. Existing countermeasures are lifesaving but do not prevent long-lasting neuronal comorbidities, emphasizing the urgent need for animal models to better understand OP neurotoxicity and identify novel antidotes. Here, using diisopropylfluorophosphate (DFP), a prototypic and moderately toxic OP, combined with zebrafish larvae, we first showed that DFP poisoning caused major acetylcholinesterase inhibition, resulting in paralysis and CNS neuron hyperactivation, as indicated by increased neuronal calcium transients and overexpression of the immediate early genes fosab, junBa, npas4b, and atf3. In addition to these epileptiform seizure-like events, DFP-exposed larvae showed increased neuronal apoptosis, which were both partially alleviated by diazepam treatment, suggesting a causal link between neuronal hyperexcitation and cell death. Last, DFP poisoning induced an altered balance of glutamatergic/GABAergic synaptic activity with increased NR2B-NMDA receptor accumulation combined with decreased GAD65/67 and gephyrin protein accumulation. The zebrafish DFP model presented here thus provides important novel insights into the pathophysiology of OP intoxication, making it a promising model to identify novel antidotes.

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 Influence of Oxime Selection on the Efficacy of Antidotal Treatment of Soman-Poisoned Rats

1. The influence of some acetylcholinesterase reactivators (HI-6, obidoxime, pralidoxime) on the efficacy of antidotal treatment to eliminate soman-induced disturbance of respiration and circulation and to protect experimental animals poisoned with supralethal dose of soman (1.5 × LD50) was investigated in a rat model with on-line monitoring of respiratory and circulatory parameters. 2. Obidoxime or pralidoxime in combination with atropine were insufficient to enable soman-poisoned rats to survive for 2 hours when given 1 minute after the administration of soman. 3. On the other hand, the ability of the oxime HI-6 in combination with atropine to prevent soman-induced alteration of respiration and circulation was significantly higher. Some rats treated with HI-6 in combination with atropine were fully protected against the lethal toxic effects of soman within 2 hours following soman administration. 4. Our findings confirm that the oxime HI-6 seems to be a much more suitable and efficacious acetylcholinesterase reactivator for the antidotal treatment of severe acute soman-induced poisoning than currently used obidoxime or pralidoxime.

Assessment of brain oxygenation imbalance following soman exposure in rats

Nerve agents (NAs) are potent organophosphorus (OP) compounds with applications in chemical warfare. OP compounds act by inhibiting acetylcholinesterase (AChE). Soman (O-pinacolyl methylphosphonofluoridate) is one of the most potent NAs. It is well known that small doses of NAs can be lethal, and that even non-lethal exposure leads to long-term mental debilitation/neurological damage. However, the neuropathology following exposure to sub-lethal nerve agents is not well understood. In this study, we examined changes in tissue oxygenation (pO₂) in the cortex and hippocampus after a sub-lethal dose of soman [80-90 μg/kg; subcutaneous]. pO₂ changes can provide information regarding oxygen delivery and utilization and may be indicative of a disruption in cerebral blood flow and/or metabolism. Changes in oxygenation were measured with chronically implanted oxygen sensors in awake and freely moving rats. Measurements were taken before, during, and after soman-induced convulsive seizures. Soman exposure resulted in an immediate increase in pO₂ in the cortex, followed by an even greater increase that precedes the onset of soman-induced convulsive seizures. The rise in hippocampus pO₂ was delayed relative to the cortex, although the general pattern of brain oxygenation between these two regions was similar. After convulsive seizures began, pO₂ levels declined but usually remained hyperoxygenated. Following the decline in pO₂, low frequency cycles of large amplitude changes were observed in both the cortex and hippocampus. This pattern is consistent with recurring seizures. Measuring real-time changes in brain pO₂ provides new information on the physiological status of the brain following soman exposure. These results highlight that the measurement of brain oxygenation could provide a sensitive marker of nerve agent exposure and serve as a biomarker for treatment studies.

Zebrafish is a predictive model for identifying compounds that protect against brain toxicity in severe acute organophosphorus intoxication

Acute organophosphorus (OP) intoxication is a worldwide clinical and public health problem. In addition to cholinergic crisis, neurodegeneration and brain damage are hallmarks of the severe form of this toxidrome. Recently, we generated a chemical model of severe acute OP intoxication in zebrafish that is characterized by altered head morphology and brain degeneration. The pathophysiological pathways resulting in brain toxicity in this model are similar to those described in humans. The aim of this study was to assess the predictive power of this zebrafish model by testing the effect of a panel of drugs that provide protection in mammalian models. The selected drugs included “standard therapy” drugs (atropine and pralidoxime), reversible acetylcholinesterase inhibitors (huperzine A, galantamine, physostigmine and pyridostigmine), N-methyl-d-aspartate (NMDA) receptor antagonists (MK-801 and memantine), dual-function NMDA receptor and acetylcholine receptor antagonists (caramiphen and benactyzine) and anti-inflammatory drugs (dexamethasone and ibuprofen). The effects of these drugs on zebrafish survival and the prevalence of abnormal head morphology in the larvae exposed to 4 µM chlorpyrifos oxon [1 × median lethal concentration (LC₅₀)] were determined. Moreover, the neuroprotective effects of pralidoxime, memantine, caramiphen and dexamethasone at the gross morphological level were confirmed by histopathological and transcriptional analyses. Our results demonstrated that the zebrafish model for severe acute OP intoxication has a high predictive value and can be used to identify new compounds that provide neuroprotection against severe acute OP intoxication.

Zebrafish Models for Human Acute Organophosphorus Poisoning

Terrorist use of organophosphorus-based nerve agents and toxic industrial chemicals against civilian populations constitutes a real threat, as demonstrated by the terrorist attacks in Japan in the 1990 s or, even more recently, in the Syrian civil war. Thus, development of more effective countermeasures against acute organophosphorus poisoning is urgently needed. Here, we have generated and validated zebrafish models for mild, moderate and severe acute organophosphorus poisoning by exposing zebrafish larvae to different concentrations of the prototypic organophosphorus compound chlorpyrifos-oxon. Our results show that zebrafish models mimic most of the pathophysiological mechanisms behind this toxidrome in humans, including acetylcholinesterase inhibition, N-methyl-D-aspartate receptor activation, and calcium dysregulation as well as inflammatory and immune responses. The suitability of the zebrafish larvae to in vivo high-throughput screenings of small molecule libraries makes these models a valuable tool for identifying new drugs for multifunctional drug therapy against acute organophosphorus poisoning.

Protection by a transdermal patch containing eserine and pralidoxime chloride for prophylaxis against (±)-Anatoxin A poisoning in rats

The prophylactic and neuroprotective impact of a transdermal patch containing eserine and pralidoxime chloride (2-PAM) against (±)-Anatoxin A poisoning was investigated using Wistar strain albino rats. Rats were smooth-shaved on the dorsal side, attached with a drug-in-adhesive matrix type prophylactic transdermal patch for 72 h and challenged with subcutaneous injection of three doses (1.0, 1.5 and 2.0×LD50) of (±)-Anatoxin A. The LD50 value of (±)-Anatoxin A was determined to be 1.25mg/kg, and at this particular dose (1.0×LD50) of toxin induced severe clinical symptom including extreme seizures in rats, resulting acute brain injuries in discrete brain regions, leading to 100% mortality within 5 min. The anticonvulsant effect, antiarrythmic effect, nerve conduction study, clinical observations and mortality, neuroprotective effect as well as skin histopathology of the prophylactic transdermal patch against (±)-Anatoxin A poisoning were investigated systematically. It was found that seizures, tachycardia, nerve damage, clinical symptoms, brain injuries and mortality induced by such lethal toxin were effectively prevented by the prophylactic patch treatment up to certain LD50 level. Hence, it could be a choice of potential therapeutic regimen against such lethal poisoning.

Exposure to nerve agents: from status epilepticus to neuroinflammation, brain damage, neurogenesis and epilepsy

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

Characterizing the behavioral effects of nerve agent-induced seizure activity in rats: increased startle reactivity and perseverative behavior

The development and deployment of next-generation therapeutics to protect military and civilian personnel against chemical warfare nerve agent threats require the establishment and validation of animal models. The purpose of the present investigation was to characterize the behavioral consequences of soman (GD)-induced seizure activity using a series of behavioral assessments. Male Sprague-Dawley rats (n=24), implanted with a transmitter for telemetric recording of encephalographic signals, were administered either saline or 1.0 LD₅₀ GD (110 μg/kg, sc) followed by treatment with a combination of atropine sulfate (2 mg/kg, im) and the oxime HI-6 (93.6 mg/kg, im) at 1 min post-exposure. Seizure activity was allowed to continue for 30 min before administration of the anticonvulsant diazepam (10 mg/kg, sc). The animals that received GD and experienced seizure activity had elevated startle responses to both 100- and 120-dB startle stimuli compared to control animals. The GD-exposed animals that had seizure activity also exhibited diminished prepulse inhibition in response to 120-dB startle stimuli, indicating altered sensorimotor gating. The animals were subsequently evaluated for the acquisition of lever pressing using an autoshaping procedure. Animals that experienced seizure activity engaged in more goal-directed (i.e., head entries into the food trough) behavior than did control animals. There were, however, no differences between groups in the number of lever presses made during 15 sessions of autoshaping. Finally, the animals were evaluated for the development of fixed-ratio (FR) schedule performance. Animals that experienced GD-induced seizure activity engaged in perseverative food trough-directed behaviors. There were few differences between groups on other measures of FR schedule-controlled behavior. It is concluded that the GD-induced seizure activity increased startle reactivity and engendered perseverative responding and that these measures are useful for assessing the long-term effects of GD exposure in rats.

Hypertonic mannitol in mice poisoned by a convulsive dose of soman: antilethal activity without neuroprotection

A convulsive dose of soman induces seizure-related brain damage (SRBD), including cerebral edema (CE) and cell death. In 1993, an American study demonstrated that hypertonic mannitol (Mann) intravenously (i.v.) administered 1 min and 5h post-soman was an effective neuroprotectant in intoxicated rats. Using a similar protocol, we recently failed to reproduce this success in intoxicated mice. In the present study, also performed in mice, the persistence or the amplitude of the osmotic load was increased by reducing the time interval between two injections of Mann or by augmenting the number of injections. Mice were pre-treated with the oxime HI-6 and then intoxicated with a convulsive dose of soman (172 microg/kg). Afterward, they were administered a first i.v. bolus of Mann 20% 1 min post-challenge and a second one either 5, or 2, or 1h after. Additional animals were given either one (1 min post-soman), or two (1 min and 1h post-soman), or three (1 min, 1 and 2h post-soman) series of three injections of Mann at 5 min intervals. Non-intoxicated mice treated with Mann (same protocols as above) and intoxicated mice treated with Mann vehicle (saline) served as controls. At 24h post-intoxication, the survivors were sacrificed and their brains prepared for quantitative histological assessment of cell damage, CE, and ventricle size. Whatever the protocol, Mann had no effect on soman-induced convulsions but did provide considerable antilethal activity. Histologically, Mann did not reduce the cell damage or CE. It even showed a dose-dependent trend toward aggravation of SRBD in some regions and promoted subarachnoid hemorrhages. Conversely, in one of the treatment protocol, it reduced soman-induced enlargement of ventricle size. Although treatment with hypertonic Mann showed some benefit on mortality and ventricle size, it failed to be an effective neuroprotector in soman-intoxicated mice and even increased the detrimental impact of soman at the cerebral level. Therefore, no clear recommendation could be drawn from the present study in view of a possible clinical use of hyperosmolar treatment in the medical management of soman poisoning.

Debilitating stresses do not increase blood-brain barrier permeability: Lack of the involvement of corticosteroids

The involvement of corticosteroids in stress-induced change in blood-brain barrier (BBB) permeability was investigated. Mice were adrenalectomized and administered with pyridostigmine bromide (PB) or Evan's blue, markers of BBB penetration, followed by 18-h cold-restraint stress (CRS). Rats were administered with mifepristone, a corticosteroid receptor blocker, and the markers, followed by 4-h water immersion-restraint stress (WIRS). Separately, soman was administered to induce seizures-mediated BBB opening. CRS did not induce PB and Evan's blue penetration, which were not affected by adrenalectomy. Also, the markers were not detected in the brain of rats subjected to WIRS, regardless of the treatment of mifepristone. In comparison, 1-h epileptic seizures increased the penetration of Evan's blue by 875%. The results suggest that in contrast to seizure-related BBB opening, profound stresses do not practically increase the BBB permeability, and that corticosteroids are not involved in the stress-induced BBB penetration of charged chemicals and albumin-dye complex.

Cerebral edema induced in mice by a convulsive dose of soman. Evaluation through diffusion-weighted magnetic resonance imaging and histology

PURPOSE

In the present study, diffusion-weighted magnetic resonance imaging (DW-MRI) and histology were used to assess cerebral edema and lesions in mice intoxicated by a convulsive dose of soman, an organophosphate compound acting as an irreversible cholinesterase inhibitor.

METHODS

Three hours and 24 h after the intoxication with soman (172 microg/kg), the mice were anesthetized with an isoflurane/N(2)O mixture and their brain examined with DW-MRI. After the imaging sessions, the mice were sacrificed for histological analysis of their brain.

RESULTS

A decrease in the apparent diffusion coefficient (ADC) was detected as soon as 3 h after the intoxication and was found strongly enhanced at 24 h. A correlation was obtained between the ADC change and the severity of the overall brain damage (edema and cellular degeneration): the more severe the damage, the stronger the ADC drop. Anesthesia was shown to interrupt soman-induced seizures and to attenuate edema and cell change in certain sensitive brain areas. Finally, brain water content was assessed using the traditional dry/wet weight method. A significant increase of brain water was observed following the intoxication.

CONCLUSIONS

The ADC decrease observed in the present study suggests that brain edema in soman poisoning is mainly intracellular and cytotoxic. Since entry of water into the brain was also evidenced, this type of edema is certainly mixed with others (vasogenic, hydrostatic, osmotic). The present study confirms the potential of DW-MRI as a non-invasive tool for monitoring the acute neuropathological consequences (edema and neurodegeneration) of soman-induced seizures.

Neuronal regeneration partially compensates the delayed neuronal cell death observed in the hippocampal CA1 field of soman-poisoned mice

Soman poisoning induces long-term neuropathology characterized by the presence of damaged neurons up to 2 months after exposure in various central brain areas, especially the hippocampal CA1 layer. Rapid depletion of this layer could therefore be expected. Surprisingly, the CA1 layer remained consistently visible, suggesting delayed death of these damaged neurons, potentially accompanied by neuronal regeneration. To address this issue, mice were exposed to a convulsive dose of soman (110 microg/kg followed by 5.0mg/kg of atropine methyl nitrate (MNA) 1 min later) and brains were collected from day 1 to day 90 post-exposure. Damaged and residual healthy neurons were quantified on brain sections using hemalun-phloxin and fluorojade staining or neuronal nuclei antigen (NeuN) immunohistochemistry. On post-soman day 1, a moderate neuronal cell death was noticed in the hippocampal CA1 layer. In this area, an important and steady quantity of damaged neurons (about 48% of the whole pyramidal neurons) was detected from post-soman day 1 to day 30. Thus, throughout this period, damaged neurons seemed to survive, as confirmed by the unmodified depth of the hippocampal CA1 layer. The dramatic disappearance of the damaged neurons occurred only later during the experiment and was almost complete at day 90 after soman exposure. Interestingly, between day 30 and day 90 following poisoning, an increase in the number of residual healthy pyramidal neurons was observed. These different kinetic patterns related to the density of total, damaged and residual healthy neurons after soman poisoning demonstrate that neuronal regeneration is delayed in the hippocampal CA1 layer and is concomitant to the death of damaged neurons.

Anticonvulsant treatment of sarin-induced seizures with nasal midazolam: An electrographic, behavioral, and histological study in freely moving rats

Centrally mediated seizures and convulsions are common consequences of exposure to organophosphates (OPs). These seizures rapidly progress to status epilepticus (SE) and contribute to profound brain injury. Effective management of these seizures is critical for minimization of brain damage. Nasal application of midazolam (1.5 mg/kg) after 5 min of sarin-induced electrographic seizure activity (EGSA) ameliorated EGSA and convulsive behavior (238 +/- 90 s). Identical treatment after 30 min was not sufficient to ameliorate ECoG paradoxical activity and convulsive behavior. Nasal midazolam (1.5 mg/kg), together with scopolamine (1 mg/kg, im) after 5 min of EGSA, exerted a powerful and rapid anticonvulsant effect (53 +/- 10 s). Delaying the same treatment to 30 min of EGSA leads to attenuation of paroxysmal ECoG activity in all cases but total cessation of paroxysmal activity was not observed in most animals tested. Cognitive tests utilizing the Morris Water Maze demonstrated that nasal midazolam alone or together with scopolamine (im), administered after 5 min of convulsions, abolished the effect of sarin on learning. Both these treatments, when given after 30 min of convulsions, only decreased the sarin-induced learning impairments. Whereas rats which were not subject to the anticonvulsant agents did not show any memory for the platform location, both treatments (at 5 min as well as at 30 min) completely abolished the memory deficits. Both treatments equally blocked the impairment of reversal learning when given at 5 min. However, when administered after 30 min, midazolam alone reversed the impairments in reversal learning, while midazolam with scopolamine did not. Rats exposed to sarin and treated with the therapeutic regimen with the exclusion of midazolam exhibited severe brain lesions that encountered the hippocampus, pyriform cortex, and thalamus. Nasal midazolam at 5 min prevented brain damage, while delaying the midazolam treatment to 30 min of EGSA resulted in brain damage. The addition of scopolamine to midazolam did not alter the above observation. In summary, nasal midazolam treatment briefly after initiation of OP-induced seizure leads to cessation of EGSA and prevented brain lesions and behavioral deficiencies in the rat model.

Protection by a transdermal patch containing physostigmine and procyclidine of soman poisoning in dogs

The prophylactic efficacy of a combinational patch system containing physostigmine and procyclidine against soman intoxication was evaluated using dogs. Female beagle dogs (body weights 9-10 kg) were shaved on the abdominal side, attached with a matrix-type patch (7x7 cm) containing 1.5% of physostigmine plus 6% procyclidine for 2 days, and challenged with subcutaneous injection of serial doses (2-10 LD50) of soman. Separately, in combination with the patch attachment, atropine (2 mg/dog) plus 2-pralidoxime (600 mg/dog) or atropine plus 1-[([4-(aminocarbonyl)pyridinio]methoxy)methyl]-2-[(hydroxyimino)methyl]pyridinium (HI-6, 500 mg/dog) were injected intramuscularly 1 min after soman poisoning. The LD50 value of soman was determined to be 9.1 microg/kg, and high doses (> or = 1.4 LD50) of soman induced salivation, emesis, defecation and diarrhea, tremors and seizures, and recumbency of dogs, leading to 100% mortality in 24 h. The prophylactic patch, which led to mean 18.5-18.8% inhibition of blood cholinesterase activity by physostigmine and mean 7.9-8.3 ng/ml of blood concentration of procyclidine, exerted a high protection ratio (4.7 LD50), in comparison with relatively-low effects of traditional antidotes, atropine plus 2-pralidoxime (2.5 LD50) and atropine plus HI-6 (2.7 LD50). Noteworthy, a synergistic increase in the protection ratio was achieved by the combination of the patch with atropine plus HI-6 (9 LD50), but not with atropine plus 2-pralidoxime (5 LD50). In addition, the patch system markedly attenuated the cholinergic signs and seizures induced by soman, especially when combined with atropine plus HI-6, leading to elimination of brain injuries and physical incapacitation up to 6 LD50 of soman poisoning. Taken together, it is suggested that the patch system containing physostigmine and procyclidine, especially in combination with atropine and HI-6, could be a choice for the quality survival from nerve-agent poisoning.

Protection by sustained release of physostigmine and procyclidine of soman poisoning in rats

The efficacy of a combinational prophylactic regimen on the lethality, convulsions, and loss of morphological and functional integrities of the brain induced by an organophosphate soman was investigated in rats. The rats were implanted subcutaneously with osmotic minipumps containing the combinational prophylactic regimen composed of physostigmine, a reversible cholinesterase inhibitor, and procyclidine, an N-methyl-D-aspartate antagonist possessing anticholinergic action, for 3 days, and intoxicated subcutaneously with soman (160 microg/kg, 1.3 LD50). The doses of combinational regimen in minipumps were optimized to achieve 30-35% inhibition of blood cholinesterase activity by physostigmine and 50-100 ng/ml of blood concentrations of procyclidine as clinically available doses, respectively. In comparison, 1-[([4-(aminocarbonyl)pyridinio]methoxy)methyl]-2-[(hydroxyimino)methyl]pyridinium (HI-6, 125 mg/kg) was administered intraperitoneally 30 min prior to the soman challenge in control groups to reduce mortality of rats without affecting convulsions. Soman induced profound limbic convulsions and 30% mortality, leading to increased blood-brain barrier permeability, neural injuries, learning and memory impairments, and physical incapacitation of survived rats pretreated with HI-6. The combinational regimen, at optimal doses without adverse effects on passive avoidance performances (72 microg/kg/h of physostigmine plus 432 microg/kg/h of procyclidine), exerted full protective effects against lethality, convulsions, blood-brain barrier opening, brain injuries, learning and memory impairments, and physical incapacitation induced by soman. Taken together, it is suggested that the combination of physostigmine and procyclidine, at adequate doses, could be a choice to provide the victims of organophosphate poisoning with chance of intensive care for survival and neuroprotection.

Nasal midazolam as a novel anticonvulsive treatment against organophosphate-induced seizure activity in the guinea pig

Seizures and status epilepticus, which may contribute to brain injury, are common consequences of exposure to organophosphorus (OP) cholinesterase inhibitors. Effective management of these seizures is critical. To investigate the efficacy of nasal midazolam as an anticonvulsive treatment for OP exposure, as compared to intramuscular midazolam, guinea pigs were connected to a recording swivel for electrocorticograph (ECoG) monitoring and clinical observation. The experimental paradigm consisted of pyridostigmine pretreatment (0.1 mg/kg i.m.) 20 min prior to sarin exposure (1.2x LD(50,) 56 micro g/kg i.m.). One minute post-exposure, atropine (3 mg/kg i.m.) and TMB-4 (1 mg/kg im) were administered. Within 3-8 min after sarin exposure all animals developed electrographic seizure activity (EGSA), with convulsive behavior. Treatment with midazolam (1 mg/kg i.m.) 10 min after the onset of EGSA abolished EGSA within 389+/-181 s. The same dose was not effective, in most cases, when given 30 min after onset. However, a higher dose (2 mg/kg) was found efficacious after 30 min (949+/-466 s). In contrast, nasal application of midazolam (1 mg/kg) was found most effective, with significant advantages, in amelioration of EGSA and convulsive behavior, when given 10 min (216+/-185 s) or 30 min (308+/-122 s) following the onset of EGSA ( P<0.001). Thus, nasal midazolam could be used as a novel, rapid and convenient route of application against seizure activity induced by nerve agent poisoning.

Effects of combinational prophylactics composed of physostigmine and procyclidine on soman-induced lethality, seizures and brain injuries

The antidotal, anticonvulsant and neuroprotective effects of physostigmine (PhS) and procyclidine (PC), the combinational prophylactics for organophosphate poisoning, were evaluated. For the investigation of dose-response relationship in rats and guinea pigs, various doses (0-6 mg/kg) of PC in combination with a fixed dose (0.1 mg/kg) of PhS were pretreated subcutaneously 30 min prior to subcutaneous poisoning with soman. Procyclidine in combination with PhS exhibited remarkable synergistic effects in a dose-dependent manner, leading to 1.92-5.07 folds of protection ratio in rats and 3.00-4.70 folds in guinea pigs. On the other hand, a low effect (1.65 fold) was achieved with the traditional antidotes atropine (17.4 mg/kg) plus 2-pralidoxime (30 mg/kg) treated immediately after soman poisoning, compared with a marked protection (5.50 fold) with atropine (17.4 mg/kg) plus HI-6 (125 mg/kg) in unpretreated rats. Noteworthy, the combinational prophylactics greatly potentiated the effect of atropine plus 2-pralidoxime to 6.13 or 12.27 folds and that of atropine plus HI-6 to 12.00 or 21.50 folds with 1.0 or 3.0 mg/kg of PC, respectively. A high dose (100 μg/kg, 1.3×LD(50)) of soman induced severe epileptiform seizures in rats pretreated with HI-6 (125 mg/kg), resulting in brain injuries in discrete brain regions under histopathological examination in 24 h. Interestingly, such seizures and excitotoxic brain injuries were fully prevented by pretreatment with PhS (0.1 mg/kg) and PC (1 mg/kg). Taken together, it is proposed that the prophylactics composed of PhS and PC could be a promising regimen for the prevention of lethality, seizures and brain injuries induced by soman poisoning.

Neuroprotective effects of currently used antidotes in soman-poisoned rats

The neuroprotective effects of antidotes (atropine, obidoxime, obidoxime/atropine mixture) on rats poisoned with soman at a sublethal dose (54 microg/kg, im, 80% of LD(50) value) were studied. The soman-induced neurotoxicity was monitored using a functional observational battery (FOB) and an automatic measurement of motor activity. The neurotoxicity of soman was monitored at 24 h and 7 days following soman challenge. The results indicate that obidoxime alone is not able to protect the rats from the lethal effects of soman. Three soman-poisoned rats treated with obidoxime alone died within 24 h. On the other hand, atropine alone or combined with obidoxime allows all soman-poisoned rats to survive within 7 days following soman challenge. Atropine alone and combined with obidoxime seems to be relatively effective antidotal treatment for the elimination of soman-induced neurotoxicity in the case of sublethal poisonings, although the antidotal mixture is significantly less effective than atropine alone because obidoxime can counteract the beneficial effects of atropine. Obidoxime appears to be practically ineffective to diminish soman-induced neurotoxicity. The neuroprotective effects of antidotal mixture consisting of atropine and obidoxime depend on the antimuscarinic effects of atropine only. Thus, the replacement of obidoxime by more effective acetylcholinesterase (AChE) reactivators is necessary to increase the neuroprotective efficacy of antidotal treatment in the case of soman poisonings.

Soman-induced hypertension in conscious rats is mediated by prolonged central muscarinic stimulation

The acetylcholinesterase inhibitor, soman, induces marked and sustained hypertension and tachycardia associated with a convulsive syndrome in rats. The aims of the present study were to distinguish between the cardiovascular and convulsant effects of soman and to determine whether the maintenance of the soman-induced hypertension and tachycardia depends solely on a central muscarinic effect. To this end, using a computerised analysis of blood pressure (BP) in conscious freely moving rats, we examined the consequences on the increase in mean BP (MBP) and heart rate (HR) induced by soman (60 micrograms/kg, i.v.) of 1) a pre-treatment with the anticonvulsant drug diazepam (3 mg/kg, i.v.) and 2) atropine sulphate (10 mg/kg, i.v.) administered 10 or 60 min after the intoxication. Pretreatment with diazepam prevented the convulsions, assessed by electroencephalogram (EEG) recording, but modified neither the magnitude nor the kinetics of the pressor and tachycardic effects of soman (delta MBP = 74 +/- 2 and 73 +/- 5 mmHg, delta HR = 69 +/- 10 and 79 +/- 7 bpm, maximum MBP = 186 +/- 3 and 182 +/- 6 mmHg, maximum HR = 545 +/- 9 and 522 +/- 16 bpm in solvent- (n = 8) and diazepam- (n = 8) pre-treated rats, respectively). Whatever its time of administration, atropine sulphate fully and immediately reversed the rise in BP induced by soman. The soman-induced tachycardia was also suppressed by atropine administered 10 min after soman whereas it persisted when atropine was injected 60 min after the intoxication. These results show that the cardiovascular effects of soman can occur independently of the convulsive syndrome and that the maintenance of the soman-induced hypertension depends entirely on a permanent central muscarinic stimulation.

Organophosphate-induced brain injuries: delayed apoptosis mediated by nitric oxide

The features of organophosphate-induced brain injuries were investigated. Rats were poisoned intraperitoneally with 9 mg/kg (1.8 LD(50)) of diisopropylfluorophosphate. Pyridostigmine bromide (0.1 mg/kg) and atropine methylnitrate (20 mg/kg), which are centrally inactive, were pre-treated intramuscularly to reduce the mortality and eliminate peripheral signs. Diisopropylfluorophosphate induced severe limbic seizures, and early necrotic and delayed apoptotic brain injuries. The necrotic brain injury was observed to be maximal as early as 1 h after diisopropylfluorophosphate treatment predominently in hippocampus and piriform/entorhinal cortices, showing a spongiform change (malacia) of neuropils in severe cases. In contrast, typical apoptotic (TUNEL-positive) cells started to appear at 12 h in thalamus, and a mixed type in amygdala. Separately, nitrite/nitrate content in cerebrospinal fluid was found to significantly increase after 2 h, reaching a maximal level at 6 h. Pre-treatment with l-N(G)-nitroarginine, an inhibitor of nitric oxide synthase, reduced nitrite/nitrate content and, noteworthy, attenuated only apoptotic brain injury in all four brain regions without affecting seizure intensity and necrotic injury. Taken together, the delayed apoptotic injury of brain induced by diisopropylfluorophosphate poisoning in rats might be mediated in part through nitric oxide production.

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.

A role of nitric oxide in organophosphate-induced convulsions

The effects of nitric oxide-regulating compounds on convulsions and mortality of rats administered i.p. with diisopropylfluorophosphate was investigated. l-N(G)-nitroarginine methyl ester, a nitric oxide synthase inhibitor possessing an anticholinergic action, markedly attenuated the intensity of convulsions and significantly reduced the mortality rate. A similar result was obtained with anticholinergic procyclidine, an N-methyl-d-aspartate receptor antagonist. Noteworthy, l-N(G)-nitroarginine, another inhibitor of nitric oxide synthase, significantly attenuated the seizure intensity when administered in combination with atropine sulfate (5 mg/kg), though either l-N(G)-nitroarginine or atropine sulfate was inactive alone. It is suggested that nitric oxide may be a proconvulsant or a convulsion-promoting factor in anticholinesterase poisoning, and both the reduction of nitric oxide level and blockade of cholinergic systems may be required for more effective protection of seizures.

Early histopathologic and ultrastructural changes in the heart of Sprague-Dawley rats following administration of soman

Male Sprague-Dawley rats were given atropine methylnitrate (20 mg/kg) and HI-6 (125 mg/kg) ip 10 min before a single injection of 130 micrograms soman/kg sc, and the heart was examined by light and electron microscopy 10, 25, 45, 90, and 180 min after the onset of seizures. Seizures appeared within 6-11 min after treatment. Control rats were given saline sc in place of soman. Early myocardial lesions consisting of hypercontraction and hyperextension of sarcomeres, focal myocytolysis, and contraction bands were detected in individual or groups of myocardial fibers. Hypercontraction was characterized by shortening of the sarcomere length, disappearance of the I and H bands, and thickening of the Z line. In contrast, hyperextended sarcomeres had thickened I and H bands. Myocytolysis was characterized by a progressively severe focal dissolution of myofilaments and edema of the affected sarcoplasmic area. Contraction bands appeared to result from the breakdown of markedly hypercontracted myofibril bundles. Due to the presence of a number of surviving myofilaments and the preservation of the sarcolemmal tube, distortion of the overall myocytic structure was minimal. Changes in the mitochondria and other intracellular organelles were also minimal and nonspecific. The close resemblance of morphologic findings to those induced by catecholamines supports the view that soman-induced myocardial damage is secondary to a treatment-related release of unphysiologic amounts of endogenous catecholamines.