Abstracts

Rationale: Organophosphate (OP) chemicals are highly toxic compounds and include commonly used household, industrial and agricultural chemicals and chemical warfare nerve agents (CWNA). Given the similarity of OP compounds to the CWNAs, the weaponization of OP agents represents a significant security concern. OPs inhibit the enzyme acetylcholinesterase, causing cholinergic crises that can ultimately evolve into status epilepticus (SE). Lethal OP exposure produces high mortality, while survival is associated with debilitating neurological morbidities such as cognitive deficits and spontaneous recurrent seizures (SRS). Rodent models have utilized various OP agents and demonstrated induction of SE, acute neuropathology, and behavioral morbidities. Here we compared characteristics of SRSs following SE induced by OP agents paraoxon (a metabolite of pesticide parathion), DFP (a CWNA surrogate), and CWNA sarin (GB) in rats.

Methods: Male Sprague-Dawley rats (~300g) were injected with POX (2 mg/kg, s.c) or DFP (4 mg/kg, s.c.). One minute later, they received atropine sulfate (0.5 mg/kg, i.m.) and 2-PAM (25 mg/kg, i.m). Rats quickly developed SE, which we stopped with midazolam (2 mg/kg, i.m.) at 1-h post-SE. For sarin studies, rats were injected with GB (132 mg/kg, s.c.) followed one minute later by atropine methyl nitrate (2 mg/kg, i.m.) and 2-PAM (25 mg/kg, i.m.). We stopped SE with midazolam (5 mg/kg, i.m.) at 1-h and 3-h post SE. Approximately 6-months following OP-SE, rats were fitted with surface electrodes and screened for SRS using continuous video-EEG monitoring for ten days.

Results: In the POX group, video-EEG monitoring revealed the presence of SRS in 64% of rats (n= 16 out of 25). The average seizure duration was 57.7 ± 6.5 s with a seizure frequency of 22.3 ± 2.7 seizures/day. The behavioral correlates of these seizures consisted of a sudden cessation of activity, associated with vacuous chewing, facial twitching, head-jerks, and forelimb clonus. Most of the observed seizures were generalized convulsive (stage 4) with a severity score of 4.09 ± 0.36 as noted using the Racine scale. In the DFP group, video-EEG monitoring revealed the presence of SRS in 57% of rats (n= 8 out of 14). DFP rats exhibited average seizure duration of 60.83 ± 12.8 s at a frequency of 19.63 ± 6.8 seizures/day. Seizure severity was 3.07 ± 0.325. In the sarin group, video-EEG monitoring revealed the presence of SRS in 58.3% of rats (n= 21 out of 36). Sarin rats exhibited a seizure frequency of 7.4 ± 1.6 seizures/day, while the seizure severity was 3.2 ± 0.28. Timm staining revealed the presence of a dense, continuous band of mossy fiber sprouting within the intermolecular layer in the hilar region of the dentate gyrus of POX and DFP rats.

Conclusions: This study demonstrates that SE induced by various OP agents leads to SRS development, a hallmark of chronic epilepsy. These OP SE-induced SRS models offer a unique resource in identifying molecular mechanisms and evaluating effective pharmacological countermeasures for the treatment of OP comorbidities.

Funding: NIH CounterACT Program, Office of the Director, NINDS Grant No. U01NS105058