UC Davis

Organophosphate cholinesterase inhibitors (OPs) are a class of neurotoxic chemicals that have been used globally as both pesticides and chemical warfare agents. Most recently, OPs have been used during the Syrian civil war, a political assassination in Asia, and terrorist attacks in Europe. Survivors of high-dose acute OP intoxication can experience convulsions, prolonged seizures, or death if not treated within minutes of exposure. During the months or years that follow, humans who survive acute OP intoxication often experience significant changes in brain structure, general cognition, memory, or electrographic activity. Even those who received standard of care treatment with atropine, oximes, and benzodiazepines may experience chronic neurological deficits or electrographic abnormalities. For this reason, there is an urgent need for more efficacious therapeutics to combat acute OP poisoning. To develop more effective therapeutic strategies, it is critical to better understand how acute OP intoxication impacts the brain and the reasons for the limitations of the current standard of care. A critical question in the field is whether seizure-independent mechanisms contribute to long-term neurological damage following acute OP intoxication, and if so, which mechanisms play a role. Additionally, very few studies have examined the influence of sex and age on the chronic neurological impacts of acute OP intoxication. This dissertation seeks to address these data gaps using a rat model of acute intoxication with OP threat agent diisopropylfluorophosphate (DFP) as a model. Chapter 2 demonstrates that the current standard of care reduced seizure activity but offered only partial neuroprotection at 3 and 6 months post-intoxication, and failed to protect against chronic microglial activation and midbrain mineralization. Studies described in Chapter 3 leveraged a seizure-resistant subset of rats acutely intoxicated with DFP to determine whether DFP induced neurological damage independent of seizure activity. Seizure-resistant animals showed pronounced neuronal degeneration and mineralization, although these outcomes were not as severe or as persistent as in those animals who experienced seizures. Chapter 5 describes the development of a juvenile rat model of acute DFP intoxication that addressed potential sex differences and better reflected the diversity of individuals potentially affected by OP intoxication. Juvenile males had more severe seizures, astrogliosis, and aberrant neurogenesis compared to juvenile females, but both sexes experienced persistent microglial activation and cognitive deficits following DFP intoxication. This model also suggested that juveniles were generally more resistant to OP-induced seizures and experienced a later onset of neuroinflammation compared to their adult counterparts. These findings have important implications for the development of medical countermeasures against OP threat agents. The observation that both benzodiazepine-treated and seizure-resistant animals experienced chronic neuronal degeneration despite having little to no seizure activity suggests that seizure-independent mechanisms contribute to the long-term neurological effects of acute OP intoxication. My findings also suggest that both age and sex are biological variables that influenced OP-induced neurotoxicity, and should, therefore, be important considerations in preclinical assessment of toxicity and therapeutic efficacy. Lastly, the temporal patterns of microglial activation in juvenile animals, as well as the persistence of microglial activation despite antiseizure treatment, point to microglial-mediated neuroinflammation as a likely factor contributing to long-term neurological consequences following acute OP intoxication.