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Dendritic and Synaptic Effects in the Hippocampus of Rats Acutely Intoxicated with Diisopropylfluorophosphate


Individuals who survive the cholinergic crisis triggered by acute intoxication with organophosphate (OP) cholinesterase inhibitors often experience chronic morbidity, including cognitive deficits, depression and anxiety, and spontaneous recurrent seizures. The pathogenic mechanisms that link the acutely toxic effects of OPs to chronic neurological outcomes remain speculative. One potential mechanism is altered synaptic connectivity as a result of changes in dendritic morphology. While acute OP intoxication has been reported to alter dendritic morphology in the hours after exposure, whether these effects persist at more delayed times post-exposure is not known. Here, we address this data gap by testing the hypothesis that acute OP intoxication causes persistent changes in the cytoarchitecture of dendrites and synapses. To test our hypothesis, adult male Sprague-Dawley rats (200-250 g) were administered a single dose of diisopropylfluorophosphate (DFP; 4 mg/kg s.c.) or vehicle (VEH; saline) followed 1 min later by atropine sulfate (2 mg/kg i.m.) and 2-pralidoxime (25 mg/kg i.m.). All DFP-intoxicated rats (n = 28) exhibited characteristic signs of cholinergic crisis (i.e., SLUD: salivation, lacrimation, urination, defecation) and seizure behavior within minutes after exposure. Analysis of Golgi-stained pyramidal neurons and quantitative immunohistochemistry (IHC) of synaptic vesicle glycoprotein 2A (SV2A), postsynaptic density protein 95 (PSD95), and microtubule associated protein 2 (MAP2), were used to characterize the effects of acute OP intoxication on dendrites and synapses at 10 and 28 days post exposure (DPE). Golgi-stained pyramidal neurons in the CA1 hippocampus of DFP animals displayed increased arborization of basilar dendrites at 10 DPE but no changes relative to vehicle controls were seen at 28 DPE. DFP animals showed no changes in dendritic spine density at either time post-exposure, but at both times, the percentage of spines with immature morphology (filopodia) was increased. Quantitative IHC revealed decreased MAP2 immunoreactivity (IR) but no significant differences in synapses in the CA1 hippocampus of DFP animals at 10 and 28 DPE. Since histologic techniques do not allow for longitudinal measurements and cannot be translated to clinical evaluation of human survivors, we also evaluated in vivo positron emission tomography (PET) for quantifying SV2A binding as an approach for assessing synaptic density. DFP animals displayed no significant difference in the SV2A binding in the whole hippocampus relative to vehicle controls as assessed by PET standardized uptake value (SUV), but there was a significant correlation with histological assessments of synapses. These findings support the hypothesis that altered dendritic morphology, notably increased dendritic complexity of individual neurons but decreased regional MAP2 IR at 10 DPE, contributes to the chronic morbidity associated with acute OP intoxication. This study also revealed that global assessments of synaptic density by PET are positively correlated with histologic measures, suggesting that further exploration of SV2A PET as an approach for assessing synaptic density in human survivors of acute OP intoxication is warranted.

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