Efficacy comparison of scopolamine (SCP) and diazepam (DZ) against soman-induced lethality in guinea pigs

Sarin-Induced Neuroinflammation in Mouse Brain Is Attenuated by the Caspase Inhibitor Q-VD-OPh

Organophosphates cause hyperstimulation of the central nervous system, leading to extended seizures, convulsions, and brain damage. Sarin is a highly toxic organophosphate nerve agent that has been employed in several terrorist attacks. The prolonged toxicity of sarin may be enhanced by the neuroinflammatory response initiated by the inflammasome, caspase involvement, and generation/release of proinflammatory cytokines. Since neurodegeneration and neuroinflammation are prevalent in sarin-exposed animals, we were interested in evaluating the capacity of quinolyl-valyl-O-methylaspartyl-[-2,6-difluorophenoxy]-methyl ketone (Q-VD-OPh), a pan caspase inhibitor to attenuate neuroinflammation following sarin exposure. To test this hypothesis, sarin-exposed C57BL/6 mice were treated with Q-VD-OPh or negative control quinolyl-valyl-O-methylglutamyl-[-2,6-difluorophenoxy]-methyl ketone, sacrificed at 2- and 14-day time points, followed by removal of the amygdala and hippocampus. A Bio-Rad 23-Plex cytokine analysis was completed on each tissue. The results suggest that exposure to sarin induced a dramatic increase in interleukin-1β and 6 other cytokines and a decrease in 2 of the 23 cytokines at 2 days in the amygdala compared with controls. Q-VD-OPh attenuated these changes at the 2-day time point. At 14 days, six of these cytokines were still significantly different from controls. Hippocampus was less affected at both time points. Diazepam, a neuroprotective drug against nerve agents, caused an increase in several cytokines but did not have a synergistic effect with Q-VD-OPh. Treatment of sarin exposure with apoptosis inhibitors appears to be a worthwhile approach for further testing as a comprehensive counteragent against organophosphate exposure. SIGNIFICANCE STATEMENT: A pan inhibitor of caspases (Q-VD-OPh) was proposed as a potential antidote for sarin-induced neuroinflammation by reducing the level of inflammation via inflammasome caspase inhibition. Q-VD-OPh added at 30 minutes post-sarin exposure attenuated the inflammatory response of a number of cytokines and chemokines in the amygdala and hippocampus, two brain regions sensitive to organophosphate exposure. Apoptotic marker reduction at 2 and 14 days further supports further testing of inhibitors of apoptosis as a means to lessen extended organophosphate toxicity in the brain.

Efficacy of atropine and scopolamine on airway contractions following exposure to the nerve agent VX

Nerve agents are highly toxic organophosphorus compounds that inhibit acetylcholinesterase resulting in rapid accumulation of the neurotransmitter acetylcholine (ACh) causing a cholinergic syndrome including respiratory failure. In the present study, respiratory responses and antimuscarinic treatment efficacy was evaluated ex vivo using rat precision-cut lung slices (PCLS) exposed to the nerve agent VX. The respiratory effects were evaluated either by adding exogenous ACh directly to the culture medium or by applying electric-field stimulation (EFS) to the PCLS to achieve a release of endogenous ACh from neurons in the lung tissue. The airway contraction induced by both methods was enhanced by VX and resulted in lingering airway recovery, in particular when airways were exposed to a high VX-dose. Both contractions induced by EFS and exogenously added ACh were significantly reduced by administration of the antimuscarinic drugs atropine or scopolamine. Two additions of atropine or scopolamine after maximal ACh-induced airway response was demonstrated effective to reverse the contraction. By adding consecutive doubled doses of antimuscarinics, high efficiency to reduce the cholinergic airway response was observed. However, the airways were not completely recovered by atropine or scopolamine, indicating that non-muscarinic mechanisms were involved in the smooth muscle contractions. In conclusion, it was demonstrated that antimuscarinic treatment reversed airway contraction induced by VX but supplemental pharmacological interventions are needed to fully recover the airways. Further studies should therefore clarify the mechanisms of physiological responses in lung tissue following nerve agent exposures to improve the medical management of poisoned individuals.

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.

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

INTRODUCTION

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

METHODS

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

RESULTS

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

DISCUSSION

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

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.

Protective Effects of Aerosolized Scopolamine Against Soman-Induced Acute Respiratory Toxicity in Guinea Pigs

The protective efficacy of the antimuscarinic agent scopolamine was evaluated against soman (o-pinacolyl methylphosphonofluoridate [GD])-induced respiratory toxicity in guinea pigs. Anesthetized animals were exposed to GD (841 mg/m3) by microinstillation inhalation exposure and treated 30 seconds later with endotracheally aerosolized scopolamine (0.25 mg/kg) and allowed to recover for 24 hours. Treatment with scopolamine significantly increased survival and reduced clinical signs of toxicity and body weight loss in GD-exposed animals. Analysis of bronchoalveolar lavage (BAL) fluid showed normalization of GD-induced increased cell death, total cell count, and protein following scopolamine treatment. The BAL fluid acetylcholinesterase and butyrylcholinesterase levels were also increased by scopolamine treatment. Respiratory dynamics parameters were normalized at 4 and 24 hours post–GD exposure in scopolamine-treated animals. Lung histology showed that scopolamine treatment reduced bronchial epithelial and subepithelial inflammation and multifocal alveolar septal edema. These results suggest that aerosolized scopolamine considerably protects against GD-induced respiratory toxicity.

Scopolamine treatment for severe extra-pyramidal signs following organophosphate (chlorpyrifos) ingestion

BACKGROUND

The use of competitive inhibitors of acetylcholine other than atropine, for patients with organophosphate poisoning is controversial. Because scopolamine ability to cross the blood-brain barrier is better than that of atropine, it has been suggested that it should be used in patients with organophosphate poisoning who have central nervous system manifestations.

CASE DESCRIPTION

A 17-year-old girl was admitted to the pediatric ward after ingesting chlorpyrifos as a suicidal attempt. She reported vomiting three times. She had no other symptoms for 12 hours and then over the course of 36 hours gradually developed extrapyramidal signs and became comatose. She was treated with intravenous scopolamine. Within 3 minutes the patient started to respond to verbal commands and answered simple questions rigidity subsided, and she was able to sit in bed. She was discharged after 4 days with no neurological sequelae.

CONCLUSIONS

We suggest, that in patients with organophosphate poisoning who have mainly central nervous system toxicity scopolamine administration might be considered.

Anticonvulsant effects of phencynonate hydrochloride and other anticholinergic drugs in soman poisoning: neurochemical mechanisms

Previous studies have paid little attention to the anticonvulsant effect of anticholinergic drugs that act on both muscarinic (M) and nicotinic (N) receptors during soman-induced seizures. Therefore, with the establishment of a soman-induced seizures model in rats, this study evaluated the efficacy in preventing soman-induced convulsions of two antagonists of both the M and N receptors, phencynonate hydrochloride (PCH) and penehyclidine hydrochloride (8018), which were synthesized by our institute, and of other anticholinergic drugs, and investigated the mechanisms of their antiseizures responses. Male rats, previously prepared with electrodes to record electroencephalographic (EEG) activity, were pretreated with the oxime HI-6 (125 mg kg-1, i.p.) 30 min before they were administered soman (180 microg kg-1, s.c.). All animals developed seizures subsequent to this treatment. Different drugs were given at different times (5, 20 and 40 min after seizures onset) and their anticonvulsant effects were monitored and compared using the two variables, i.e. the dose that could totally control the ongoing seizures, as well as the speed of seizures control. The anticonvulsant effects of atropine, scopolamine and 8018 decreased with the progression of the seizures, and they eventually lost their anticonvulsant activity when the seizures had progressed for 40 min. In contrast, PCH showed good anticonvulsant effectiveness at 5 and 20 min, and especially at 40 min after seizures onset. Of the anticholinergic drugs tested, atropine, scopolamine, and 8018 showed no obvious protection against pentylenetetrazol (PTZ)-induced convulsions or N-methyl-D-aspartate (NMDA)-induced lethality in mice. However, PCH antagonized the PTZ-induced convulsions in a dose-dependant manner with an ED50 of 10.8 mg kg-1, i.p. (range of 7.1-15.2 mg kg-1) and partly blocked the lethal effects of NMDA in mice. PCH also dose-dependently inhibited NMDA-induced injury in rat primary hippocampal neuronal cultures, suggesting a possible neuroprotective action in vivo. In conclusion, our study suggests that the mechanisms of PCH action against soman-induced seizures might differ from those of the M receptor antagonists atropine and scopolamine, and that of the antagonist of both the M and N receptors, 8018. The pharmacological profile of PCH might include anticholinergic and anti-NMDA properties. Compared with the currently recommended anticonvulsant drug diazepam, with known NMDA receptor antagonists such as MK-801 and with conventional anticholinergics such as scopolamine and atropine, the potent anticonvulsant effects of PCH during the entire initial 40 min period of soman poisoning, and its fewer adverse effects, all suggest that PCH might serve as a new type of anticonvulsant for the treatment of seizures induced by soman.

An improved brain slice model of nerve agent-induced seizure activity

A brain slice model was developed to investigate the mechanisms of seizure activity induced by soman and the effectiveness of potential anticonvulsant drugs. Unlike previously reported slice studies with nerve agents, this model contains the entorhinal cortex as well as the hippocampus. This allows the study of the spread of seizure discharges within the limbic system and the development of prolonged, sustained discharges that are rarely seen in the simple hippocampal slice preparation. Soman (1 microM) induced a second population spike in the evoked field potential in the CA1 or CA3 region within 15-20 min. In almost all the slices tested, this developed into spontaneous seizure activity within 30-40 min. As well as interictal bursts, many slices also showed longer periods of high-frequency bursting analogous to ictal seizure activity that originated in the entorhinal cortex. This activity appeared similar to that induced by the muscarinic agonist pilocarpine. Both the second population spike and the spontaneous discharges could be blocked by diazepam and by AMPA/kainate antagonists, but not by the NMDA antagonists AP5 and MK-801. This study confirms that the combined hippocampal-entorhinal cortex slice preparation is a suitable model for investigating the origin and propagation of nerve-agent-induced seizures within the limbic system.

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

Anticonvulsants for soman-induced seizure activity

This report describes studies of anticonvulsants for the organophosphorus (OP) nerve agent soman: a basic research effort to understand how different pharmacological classes of compounds influence the expression of seizure produced by soman in rats, and a drug screening effort to determine whether clinically useful antiepileptics can modulate soman-induced seizures in rats. Electroencephalographic (EEG) recordings were used in these studies. Basic studies were conducted in rats pretreated with HI-6 and challenged with 1.6 x LD50 soman. Antimuscarinic compounds were extremely effective in blocking (pretreatment) or terminating soman seizures when given 5 min after seizure onset. However, significantly higher doses were required when treatment was delayed for more than 10 min, and some antimuscarinic compounds lost anticonvulsant efficacy when treatment was delayed for more than 40 min. Diazepam blocked seizure onset, yet seizures could recur after an initial period of anticonvulsant effect at doses </=2.5 mg/kg. Diazepam could terminate ongoing seizures when given 5 min after seizure onset, but doses up to 20 mg/kg were ineffective when treatment was delayed for 40 min. The GABA uptake inhibitor, tiagabine, was ineffective in blocking or terminating soman motor convulsions or seizures. The glutamate receptor antagonists, NBQX, GYKI 52466, and memantine, had weak or minimal antiseizure activity, even at doses that virtually eliminated signs of motor convulsions. The antinicotinic, mecamylamine, was ineffective in blocking or stopping seizure activity. Pretreatment with a narrow range of doses of alpha2-adrenergic agonist, clonidine, produced variable protection (40-60%) against seizure onset; treatment after seizure onset with clonidine was not effective. Screening studies in rats, using HI-6 pretreatment, showed that benzodiazepines (diazepam, midazolam and lorazepam) were quite effective when given 5 min after seizure onset, but lost their efficacy when given 40 min after onset. The barbiturate, pentobarbital, was modestly effective in terminating seizures when given 5 or 40 min after seizure onset, while other clinically effective antiepileptic drugs, trimethadione and valproic acid, were only slightly effective when given 5 min after onset. In contrast, phenytoin, carbamazepine, ethosuximide, magnesium sulfate, lamotrigine, primidone, felbamate, acetazolamide, and ketamine were ineffective.

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.

Antagonism of soman-induced convulsions by midazolam, diazepam and scopolamine

The effects of midazolam (MDZ), diazepam (DZ) and scopolamine (SCP) therapies on soman-induced electrocorticogram (ECoG) and biceps femoris electromyogram (EMG) activities and brain lesions were assessed in male rats. Animals received pyridostigmine (26 micrograms/kg, im) 30 min before soman (87.1 micrograms/kg, im) followed by therapy consisting of atropine (1.5 mg/kg) admixed with 2-PAM (25 mg/kg, im) 1 min later; MDZ (0.5 mg/kg), DZ (1.77 mg/kg) or SCP (0.43 mg/kg) was administered im at 1 min after the onset of convulsions (CVs). Typically, within 5 min after soman the ECoG profile changed to a full-blown, spike-and-dome epileptiform (SDE) pattern followed by CVs and increased amplitude of EMG activity. Treatment with SCP restored ECoG and EMG profiles by 30 min. At 2 hr after exposure only 1 animal demonstrated a slight abnormality in ECoG activity which was normal at 24 hr. Similarly, DZ and MDZ restored EcoG and EMG profiles by 30 min; however, in contrast to SCP, 83% of the animals demonstrated reappearance of SDE 2 hrs after soman. SCP therapy also enabled rats to move about in their cages by 30 min post treatment. In contrast, DZ- and MDZ-treated rats remained incapacitated as late as 2 hr post-exposure. Animals were euthanized at 24 hr, and the extent of soman-induced brain lesions was determined by light microscopic analysis. When present, brain lesions were minimal in SCP-treated rats. The mean brain lesion scores across all experimental conditions ranked as follows: soman control > MDZ > DZ > or = SCP = saline control. These observations suggest that SCP may be highly effective in severe soman intoxication.