Abstracts

Rationale: Organophosphate insecticides such as paraoxon and diisopropylfluorophosphate (DFP) are neurotoxic chemicals that cause lethal convulsions and injury. Like nerve agents, DFP is an irreversible cholinesterase inhibitor and thereby cause persistent seizures, status epilepticus (SE), and brain injury. Benzodiazepines are the first-line drugs for the treatment of such seizures, but there is evidence for partial or complete resistance, a condition known as refractory SE. In this study, we investigated the efficacy of the newer benzodiazepine midazolam in the DFP model of OP intoxication. Methods: Acute OP intoxication was induced in rats by exposure to DFP (1-4 mg/kg, SC). Animals were given standard antidotes atropine and pralidoxime to increase the survival. Midazolam was administered 10, 40, 60 or 120 min after DFP. The progression of seizures was monitored by video-EEG recordings for 24 h. The onset and termination of seizure activity was determined based on the video-EEG recordings. Animals were perfused for determination of neuronal damage. The extent of brain damage was assessed by neuropathology quantifications of cell loss, neurodegeneration and surviving cell numbers by unbiased stereology. Results: DFP caused seizures within ~10 min and progressed into SE lasting for several hours. It was associated with massive neuronal loss in many brain regions including hippocampus and amygdala. Midazolam (1-5 mg/kg) controlled seizures when given at 10 min, but it was completely ineffective when given at 40, 60 or 120 min after DFP--a profile indicative of refractoriness to benzodiazepines. Early therapy (10-min) or delayed therapy (40-min) was not associated with neuroprotection due to damage caused by seizure recurrence. Conclusions: DFP model replicates many features of OP nerve intoxication, including seizures, neuronal injury and neurodegeneration. The DFP-induced seizures are progressively resistant to delayed treatment with midazolam, confirming the benzodiazepine refractoriness of DFP-induced SE and brain injury. Supported by NIH CounterACT grants NS076426 & NS083460 (to DSR).