A rodent model of human organophosphate exposure producing status epilepticus and neuropathology

Abstract

Exposure to organophosphates (OPs) often results in seizures and/or status epilepticus (SE) that produce neural damage within the central nervous system (CNS). Early control of SE is imperative for minimizing seizure-related CNS neuropathology. Although standard therapies exist, more effective agents are needed to reduce OP-induced SE and neuronal loss, particularly therapies with efficacy when administered 10's of minutes after the onset of SE. To evaluate novel antiseizure compounds, animal models should simulate the CNS effects of OP exposure observed in humans. We characterized in rats the effects of the OP, diisopropyl flourophosphate (DFP) as a function of dose and route of administration of supporting agents (pyridostigmine, 2-PAM, atropine); outcome measures were mortality, electrographic seizure activity during SE, and subsequent CNS neuropathology. Doses of DFP between 3 and 7mg/kg consistently caused SE, and the latency to behavioral tremors and to subsequent initiation of SE were dose related. In distinction, all doses of DFP that resulted in electrographic SE (3-7mg/kg) produced seizures of similar intensity and duration, and similar CNS neuropathology (i.e., the effects were all-or-none). Although SE was similar across doses, mortality progressively increased with higher doses of DFP. Mortality was significantly lower when the route of administration of therapeutic agents was intramuscular compared to intraperitoneal. This rodent model of OP poisoning demonstrates pathological characteristics similar to those observed in humans, and thus begins to validate this model for investigating potential new therapeutic approaches.

Keywords: Antiseizure; EEG; Neuroprotection; Seizures; Status epilepticus.

Figure 1

Schematic summary of protocol.

Figure 2

Latency from DFP to first tremor (Panel A) and latency from first tremor to the initial electrographic seizure (Panel B) in animals administered 2–7 mg/kg DFP. p<0.05 re: lowest dose in panel. (Ns= 2 mg/kg, 12; 3 mg/kg, 10; 4 mg/kg, 10; 4.5 mg/kg, 15; 6.5 mg/kg, 14; 7.0 mg/kg, 12), Panel A, *p<0.05 re: 2.0 mg/kg; Panel B *p<0.05 re: 3.0 mg/kg, †p<0.05 re: 4 mg/kg.

Figure 3

Representative EEG recordings from rats administered either 7 mg/kg (Panel A) or 2 mg/kg (Panel B) DFP. Recordings from within the dashed boxes are shown below each EEG with an expanded time scale. The arrows denote DFP administration.

Fig. 4

SE analysis using power (bandwidth filtered 20–70 Hz) (Panels A and B) and spike rate (Panels C and D) following administration of 2.0, 3.0, 4.0, 4.5, 6.5, or 7.0 mg/kg DFP. Traces represent means + the 95% confidence intervals. *p<0.05, 2 mg/kg DFP compared to all other groups.

Fig. 5

Top Panels - Representative sections stained with Flouro-Jade B from the amygdalae of animals given 2 mg/kg (left panel) or 7 mg/kg (right panel) DFP. Middle and Lower Panels - Summary data showing the numbers of FJB stained cells in representative brain regions from animals given 3–7 mg/kg DFP. (N’s= 3.0 mg/kg, 10; 4.0 mg/kg, 9; 4.5 mg/kg, 10; 6.5 mg/kg, 9; 7.0 mg/kg, 10)

Fig. 6

Seizure intensity, measured by EEG power in the gamma band (20–70 Hz)(top panel), and mean spike frequency (bottom panel) in rats given therapeutic agents IM vs. IP with DFP (4.5 mg/kg).