Age differences in organophosphorus nerve agent-induced seizure, blood brain barrier integrity, and neurodegeneration in midazolam-treated rats

Abstract

Exposure to organophosphorus nerve agents irreversibly inhibits acetylcholinesterase and may lead to cholinergic crisis and seizures. Although benzodiazepines are the standard of care after nerve agent-induced status epilepticus, when treatment is delayed for up to 30 min or more, refractory status epilepticus can develop. Adult male rodents are often utilized for evaluation of therapeutic efficacy against nerve agent exposure. However, there may be age and sex differences in toxicity and in therapeutic response. We previously reported that juvenile rats are less susceptible to the lethal effects of soman compared to adults, while pups are the most susceptible. Here, we report on age and sex differences in delayed midazolam treatment efficacy on survival, seizures and brain pathology. Male and female pups, juvenile and adult rats were exposed to an equitoxic dose of soman and treated with atropine sulfate and the oxime asoxime chloride (HI-6 dimethanesulphonate) 1 min after exposure and with midazolam 40 min after seizure onset, determined by EEG in juvenile and adult rats, and by behavior in pups. Survival, seizure data, and spontaneous recurrent seizures were evaluated. Brains were processed to assess neurodegeneration, neuroinflammation, and blood brain barrier (BBB) integrity. Juvenile and adult rats exposed to soman and treated with midazolam had BBB disruption, epileptogenesis, neurodegeneration, microglial activation, and astrogliosis; adult rats had poorer outcomes. Pups and juvenile rats exposed to soman had poor survival prior to midazolam treatment but most survived once treated; overall, neurodegeneration or disrupted BBB integrity was not detected in midazolam-treated pups. We found that age is a determinant factor in soman-induced toxicity and response to standard medical countermeasures. In addition, we observed sex differences in response to soman in juveniles and males with respect to body weight growth curves and in neuronal loss in juveniles and adults. Adjunct therapies to midazolam are warranted and it is important to evaluate both age and sex as factors in therapeutic response.