Soman-induced status epilepticus, epileptogenesis, and neuropathology in carboxylesterase knockout mice treated with midazolam.

OBJECTIVE

Exposure to chemical warfare nerve agents (CWNAs), such as soman (GD), can induce status epilepticus (SE) that becomes refractory to benzodiazepines when treatment is delayed, leading to increased risk of epileptogenesis, severe neuropathology, and long-term behavioral and cognitive deficits. Rodent models, widely used to evaluate novel medical countermeasures (MCMs) against CWNA exposure, normally express plasma carboxylesterase, an enzyme involved in the metabolism of certain organophosphorus compounds. To better predict the efficacy of novel MCMs against CWNA exposure in human casualties, it is crucial to use appropriate animal models that mirror the human condition. We present a comprehensive characterization of the seizurogenic, epileptogenic, and neuropathologic effects of GD exposure with delayed anticonvulsant treatment in the plasma carboxylesterase knockout (ES1-/-) mouse.

METHODS

Electroencephalography (EEG) electrode-implanted ES1-/- and wild-type (C57BL/6) mice were exposed to various seizure-inducing doses of GD, treated with atropine sulfate and the oxime HI-6 at 1 minute after exposure, and administered midazolam at 15-30 minutes following the onset of seizure activity. The latency of acute seizure onset and spontaneous recurrent seizures (SRS) was assessed, as were changes in EEG power spectra. At 2 weeks after GD exposure, neurodegeneration and neuroinflammation were assessed.

RESULTS

GD-exposed ES1-/- mice displayed a dose-dependent response in seizure severity. Only ES1-/- mice exposed to the highest tested dose of GD developed SE, subchronic alterations in EEG power spectra, and SRS. Degree of neuronal cell loss and neuroinflammation were dose-dependent; no significant neuropathology was observed in C57BL/6 mice or ES1-/- mice exposed to lower GD doses.

SIGNIFICANCE

The US Food and Drug Administration (FDA) animal rule requires the use of relevant animal models for the advancement of MCMs against CWNAs. We present evidence that argues for the use of the ES1-/- mouse model to screen anticonvulsant, antiepileptic, and/or neuroprotective drugs against GD-induced toxicity, as well as to identify mechanisms of GD-induced epileptogenesis.