Sex as a biological variable in the rat model of diisopropylfluorophosphate‐induced long‐term neurotoxicity

Abstract

Sex differences in response to neurotoxicant exposure that initiates epileptogenesis are understudied. We used telemetry-implanted male and female adult rats exposed to an organophosphate (OP) neurotoxicant, diisopropylflourophosphate (DFP), to test sex differences in the severity of status epilepticus (SE) and the development of spontaneous recurrent seizures (SRS). Females had significantly less severe SE and decreased epileptiform spikes compared with males, although females received a higher dose of DFP than males. The estrous stages had no impact on seizure susceptibility, but rats with severe SE had a significantly prolonged diestrus. A previously demonstrated disease-modifying agent, an inducible nitric oxide synthase inhibitor, 1400W, was tested in both sexes. None of the eight males treated with 1400W developed convulsive SRS during 4 weeks post-DFP exposure, while two of seven females developed convulsive SRS. Concerning gliosis and neurodegeneration, there were region-specific differences in the interaction between sex and SE severity. As SE severity influences epileptogenesis, and as females had significantly less severe SE, sex as a biological variable should be factored into the design of future OP nerve agent experiments while evaluating neurotoxicity and optimizing potential disease-modifying agents.

Sex-specific acute and chronic neurotoxicity of acute diisopropylfluorophosphate (DFP)-intoxication in juvenile Sprague-Dawley rats

“…EEG recordings from a separate cohort of animals confirmed DFP-induced neurological hyperactivity and increased spiking events in each sex and supported the observations of more severe behavioral seizures in males relative to females. Notably, the behavioral seizure response observed in our juvenile rat DFP model is less severe than has been previously reported for adult rat DFP models using nearly identical seizure scales ( Gage et al, 2020 , Supasai et al, 2020 ), suggesting age-specific seizure responses to DFP. This interpretation is supported by a recent publication that found seizure susceptibility to DFP increased significantly with age, ranging from the early postnatal period into young adulthood ( Scholl et al, 2018 ).…”

“…This is consistent with literature demonstrating increased expression of cholinergic and glutamatergic receptors between the post-weaning period (∼PND 28) and early adulthood (∼PND 60) in rats ( Akazawa et al, 1994 , Monyer et al, 1994 , Jett, 1998 ). Although EEG activity was only recorded from a small cohort of animals that were not scored for seizure behavior, it has been demonstrated that seizure behavior is highly correlated with electrographic seizure activity in both male and female models of acute OP intoxication ( Tanaka et al, 1996 , McDonough and Shih, 1997 , Gage et al, 2020 ).…”

“…The observed sex difference in seizure activity in juvenile rats after acute OP intoxication is consistent with other models of chemical-induced seizures. For example, adult female rats have been reported to have less severe SE than male rats in response to pilocarpine ( Scharfman and MacLusky, 2014 ) or DFP ( Gage et al, 2020 ). While the mechanistic underpinnings of this sex difference are unknown, a few hypotheses have been proposed.…”

“…One hypothesis involves sex hormones ( Velíšková and Desantis, 2013 ); however, it is unlikely that sex hormones are driving the differential seizure behavior we observed in this study because PND 28 rats have yet to experience a hormonal surge ( Bell, 2018 ). Further, it was recently reported that estrous stage has no impact on seizure susceptibility in adult rats acutely intoxicated with DFP ( Gage et al, 2020 ). Rather, it is more likely that the divergent seizure activity between males and females reflects the sexually dimorphic expression of excitatory and inhibitory receptors in the brain.…”

“…The regional effects of DFP on neuroinflammation in the juvenile rat model mirrored the regional effects of DFP on neurodegeneration and were largely comparable to the adult DFP model ( Siso et al, 2017 ), with some of the most severe inflammation occurring in the piriform cortex, thalamus, and hippocampus. Two primary neuroimmune cell types, microglia and astrocytes, are known to be activated in the adult brain for months following acute DFP intoxication ( Gage et al, 2020 , Supasai et al, 2020 ). This response typically coincides with increased levels of pro-inflammatory cytokines, sustained neuronal injury, and an aberrant phagocytic response that may further damage healthy cells (reviewed in Guignet and Lein, 2018 ).…”