Abstracts

Rationale: Nerve agents are amongst the most lethal chemical weapons developed. Organophosphate (OP) compounds such as DFP and paraoxon are pesticides that act similar to nerve agents. These compounds are readily available and therefore are attractive targets for being weaponized and used against civilian population by terrorists. OP compounds are irreversible inhibitors of acetylcholinesterase. Exposure to these compounds produces severe cholinergic symptoms and status epilepticus (SE) that if left untreated are fatal. Survivors of OP poisoning often exhibit chronic neurological disorders including acquired epilepsy and cognitive deficits. In this study we describe the development of a rat survival model of OP induced SE using paraoxon (POX) in order to realistically mimic both acute and long-term effects of nerve agent intoxication. Methods: Male Sprague-Dawley rats (250-300 g) were injected with POX (4 mg/kg, ice-cold PBS, s.c.). The ensuing cholinergic crisis was behaviorally observed and SE was recorded on EEG using surface electrodes implanted in brain. Treatment of OP poisoning included interventions with atropine bromide (10 mg/kg, saline, i.p.), 2-PAM (25 mg/kg, saline, i.m.) and diazepam (5 mg/kg, i.p.). Results: OP poisoning with POX (4 mg/kg) produced a rapid cholinergic crisis characterized by increased salivation, bradypnea, bradycardia followed by tonic-clonic jerks that evolved into unremitting seizures (SE) and a rapid death. Time to death following POX exposure was 6-8 min. Mortality was 100% when no therapeutic interventions were done. Treatment with atropine alone reduced acute mortality (30%) but the chronic mortality was high (90% at 72-h post POX exposure). Addition of 2-PAM to atropine further reduced both acute mortality (15%) but chronic mortality was still very high (80%). Addition of diazepam to this drug combination promptly terminated SE and markedly alleviated mortality. Acute mortality was 12% and chronic mortality was reduced to 18%. The EEG profile for POX induced SE was marked similar to those reported for other OP's and nerve agents. EEG alterations following POX administration were rapid and correlated with behavioral seizure manifestations. Low-voltage fast activity started to appear with 2-3 min of POX administration. High-voltage slow activity appeared within the next minute and rapidly progressed into high-voltage spiking correlating with behavioral seizure activity by approximately 5-6 minutes. By this time animals were in electrographic SE with fully developed seizures. The seizures continue unabated for one hour at the end of which they are terminated using diazepam. Conclusions: We have developed a rat survival model of OP intoxication using POX. Markedly improved survival rates were obtained by optimizing the three drug regimen for alleviating symptoms of POX poisoning. This model is ideally suited to test effective countermeasures for OP exposure and study molecular mechanisms underlying neurological disorders following OP intoxication.