A Pediatric Rat Model of Organophosphate-Induced Refractory Status Epilepticus: Characterization of Long-Term Epileptic Seizure Activity, Neurologic Dysfunction and Neurodegeneration

Abstract

Children are highly vulnerable to the neurotoxic effects of organophosphates (OPs), which can cause neuronal developmental defects, including intellectual disability, autism, epilepsy, and related comorbidities. Unfortunately, no specific pediatric OP neurotoxicity model currently exists. In this study, we developed and characterized a pediatric rat model of status epilepticus (SE) induced by the OP diisopropylfluorophosphate (DFP) and examined its impact on long-term neurological outcomes. Postnatal day 21 rats were exposed to a DFP regimen with standard antidotes. Progressive behavioral deteriorations were assessed over a three-month period. Development of epileptic seizures, ictal discharges, high-frequency oscillations (HFOs), and interictal spikes were monitored by video-electroencephalography recordings. Histology−stereology analysis was performed to assess neurodegeneration, neuroinflammation, and morphologic abnormalities. DFP-exposed, post-SE animals exhibited significantly elevated levels of anxiety and depression than age-matched controls at 1, 2, and 3 months post-exposure. DFP-exposed animals displayed aggressive behavior and a marked decline in object recognition memory, as well as prominent impairment in spatial learning and memory. DFP-exposed animals had striking electrographic abnormalities with the occurrence of displayed epileptic seizures, ictal discharges, HFOs, and interictal spikes, suggesting chronic epilepsy. Neuropathological analysis showed substantially fewer principal neurons and inhibitory interneurons with a marked increase in reactive microglia and neuroinflammation in the hippocampus and other brain regions. DFP-exposed animals also exhibited mossy fiber sprouting indicating impaired network formations. Long-term epileptic seizures and neuropsychiatric functional deficits induced by DFP were consistent with neuropathological defects. Collectively, this pediatric model displays many hallmarks of chronic sequelae reminiscent of children exposed to OPs, suggesting that it will be a valuable tool for investigating pathologic mechanisms and potential treatment strategies to attenuate long-term OP neurotoxicity. SIGNIFICANCE STATEMENT Millions of children are exposed to organophosphates (OPs) used in agriculture or chemical incidents. This study investigated the long-term impact of neonatal exposure to the OP chemical diisopropylfluorophosphate (DFP) on neurobehavioral and neurodevelopmental outcomes in adulthood. DFP exposure caused long-lasting behavioral abnormalities, epileptic seizures, and bilateral brain defects with an array of neurological sequelae seen in children’s OP neurotoxicity. Thus, this model provides a novel tool to explore therapeutic interventions that mitigate long-term neurotoxic effects of children exposed to OP-induced seizures and status epilepticus.

Neurosteroids as Novel Anticonvulsants for Refractory Status Epilepticus and Medical Countermeasures for Nerve Agents: A 15-Year Journey to Bring Ganaxolone from Bench to Clinic

“…Untreated rats had epileptic seizures 30 days after soman exposure, which were significantly reduced in GX-treated animals. We also investigated the ability of GX to mitigate long-term (10-month) neuropsychiatric impairments, chronic neurodegeneration, and neuroinflammation in a pediatric model of acute DFP exposure ( Singh et al, 2024b ). GX has neuroprotective effects against long-term memory dysfunction, neurodegeneration, and neuroinflammation in this OPNA model ( Neff and Reddy, 2024 ), underscorinag the potential use of neurosteroid therapy to mitigate chronic neuropsychiatric sequelae following acute OPNA exposure.…”

Progress and Challenges in Developing Medical Countermeasures for Chemical, Biological, Radiological, and Nuclear Threat Agents

“…This journal issue presents three studies that used pediatric models to test potential MCs and the long-term impact of neonatal exposure to OP agents. Reddy and colleagues established a pediatric rat model of SE induced by the OP compound DFP ( Singh et al, 2024b ). The observed long-lasting behavioral abnormalities, epileptic seizures, and bilateral brain defects mimic the neurologic sequelae seen in children exposed to OPs.…”

Special Section on Medical Countermeasures, From Bench to Battlefield: Translating Experimental Therapies for Effective Combat Against Chemical Threats–Editorial

“…In addition, there are three papers on pediatric models of OP neurotoxicity. The first paper describes establishing a pediatric rat model for OP-induced status epilepticus, reflecting longlasting behavioral abnormalities seen in exposed children (Singh et al, 2024b). The second paper investigated tezampanel and caramiphen as effective MCs for soman-induced status epilepticus in infant rats (Furtado et al, 2024).…”