Abstracts

Rationale: The discovery of possible therapeutic agents for the treatment of consequences of chemical warfare nerve agent exposure relies on the use of animal models that closely model human organophosphate (OP) toxicity. The carboxylesterase knockout (CaE-KO) mouse specifically lacks plasma CaE activity, similar to humans, which eliminates the confounding effect of an enzyme whose activity is a key determinant of the severity of OP toxicity. We investigate the toxic effects of various seizure-inducing doses of soman (GD) in CaE-KO mice treated with atropine sulfate and the oxime HI-6 1 min after exposure and with midazolam 15 min after onset of status epilepticus (SE). Methods: Mice were implanted with telemetry transmitters to record EEG, temperature and activity. On day of nerve agent exposure, mice received 46 ug/kg GD or 98 ug/kg GD (sc), followed by an admix administration (ip) of atropine sulfate (4 mg/kg) and the oxime HI-6 (50 mg/kg), or saline. Rodents were treated with the anticonvulsant midazolam (5 mg/kg, ip) 15 min after SE. Fourteen (14) days after GD exposure, mice were euthanized and perfused with 4% paraformaldehyde for the collection of brains for immunohistochemistry processing. Brains were sectioned coronally, stained with cresyl violet for structural identification, and labelled with NeuN (to identify neurons), GAD67 (to identify GABA interneurons) and Iba1 (to identify microglia). Results: CaE-KO mice exposed to GD showed a dose-dependent response in the severity of behavioral seizures (Racine score) and seizure duration. Mice exposed to 98 ug/kg GD developed SE and displayed an increase in the EEG power in the delta and theta band. The power in delta, theta and alpha band remained chronically increased, while the power in the beta band decreased. The CaE-KO mice that developed SE displayed spontaneous recurrent seizures (SRS; latency 5-8 days), increased neuroinflammation, loss of neurons, and loss of GABA interneurons in brain regions sensitive to prolonged seizure. Conclusions: The CaE-KO mouse model can be used to screen compounds against both the acute effects (e.g., SE) and the chronic consequences (e.g., SRS and neuropathology) of exposure to GD, especially since when treatment with midazolam is delayed, SE that is refractory to benzodiazepines develops. Experiments are set to determine whether or not the extensive brain damage is correlated with a persistent increase in the EEG power (delta band) and its correlation with SRS. Funding: Funding was provided by DTRA/JSTO.