Abstracts

Rationale: Organophosphate compounds, such as parathion and diisopropyl fluorophophate, are similar to the nerve agents such as sarin. These compounds are irreversible inhibitors of the enzyme acetylcholinesterase. They are extremely toxic and there is a growing concern that organophosphate agents could be weaponized to cause mass civilian causalities. We have developed a rodent model of organophosphate paraoxon (an active metabolite of parathion) toxicity that is being used to screen for medical countermeasures against an organophosphate attack. It is well known that the survivors of nerve gas exposure and chronic organophosphate exposure exhibit neurobehavioral deficits such as mood changes, depression, and memory impairment, among others. In this study, we investigated whether rats treated using the standard atropine, 2-PAM and diazepam therapy following exposure to high levels of organophosphate exhibit long-term neurological problems.Methods: Male Sprague-Dawley rats (250-300 g) were injected with paraoxon (4 mg/kg, ice-cold PBS, s.c.). One minute following paraoxon injection, rats were injected with atropine sulfate (2 mg/kg, saline, i.p.) and 2-PAM (25 mg/kg, saline, i.m.). Rats exhibited cholinergic crisis, including the occurrence of status epilepticus. Seizures were stopped with three sequential injections of diazepam (5 mg/kg, i.p.) along with 2-PAM at 1, 3 and 5-hr following the onset of status epilepticus. Surviving rats were maintained in the vivarium, and approximately 1-year following paraoxon exposure they were screened for depressive symptoms using forced swim test and cognitive deficits using the novel object recognition test.Results: One year post paraoxon exposure, rats subjected to forced swim test exhibited increased immobility time (78.5 8.5 s, n= 5) indicative of a despair-like state that was significantly higher (p= 0.04) compared to age matched control rats (44.5 3.8 s, n= 6). When tested in a novel object recognition paradigm, these paraoxon intoxication survivor rats exhibited a negative discrimination ratio (-0.06 0.15, n= 5) indicative of impaired recognition memory that was significantly lower (p= 0.007) compared to age matched control rats (0.79 0.05, n= 6).Conclusions: We have observed the development of neurological co-morbidities such as depression and cognitive deficits in rats surviving lethal paraoxon intoxication one year after the initial exposure. This is in agreement with human data that indicates chronic neurobehavioral impairments following high-dose acute or low-dose chronic exposure to organophosphates. We are currently validating the chronic neurological symptoms observed in paraoxon intoxicated rats using additional behavioral assays. This model is being used to identify some of the molecular bases for organophosphate- induced chronic morbidities and to develop novel pharmacological therapies for effective treatment of these disorders.