The exposure to toxic organophosphate (OP) insecticides and chemical warfare agents continues to endanger many of the world's population. One potentially dire consequence of such exposure is the prolonged impairment of cognitive function. Mechanistic studies of OPs to date have focused primarily the effects of overtly toxic doses, however, little is known about the cellular and behavioral consequences of repeated exposure to doses of these agents that produce no overt signs of acute toxicity (i.e., subthreshold doses). This issue is very important since detectible levels of OPs can remain in the environment for extended periods. Accordingly, our long-term goal is further elucidate OP mechanisms such that more effective therapeutic strategies can be developed for patients suffering from exposure. The objective of this application is to identify specific relationships between cellular and biochemical manifestations of repeated, subthreshold exposures to OPs and cognitive function in an experimental animal model. We have compelling preliminary evidence from rat studies suggesting that one mechanism underlying memory dysfunction associated with repeated, subthreshold OP exposure is the impairment of fast axonal transport resulting from interactions with the motor protein, kinesin. Since axonal transport plays such a fundamental role in neuronal function, and since the cholinergic system in the brain is so important for cognitive processes, we have developed the hypothesis that the compromise of fast axonal transport by OPs, over time, leads to deficiencies in the expression of cholinergic macromolecules in sensitive regions of the brain that result in the impairment of memory function. The rationale for the proposed research is that a clear understanding of the mechanisms underlying the cognitive effects of subthreshold exposures to OPs will help us in designing treatments to reverse the effects of these agents. To test the hypothesis we propose two specific aims: 1): To evaluate the residual behavioral manifestations (especially cognitive effects) of repeated, subthreshold, exposures to both neurotoxic and non-neurotoxic OPs in an experimental animal model. 2): To identify specific relationships between OP-induced cognitive changes and impairments in fast axonal transport. We will use a water maze task to measure spatial learning, an 8-arm radial arm maze task to assess working memory, and a microtubule motility assay and video enhanced-differential interference contrast microscopy to study OP effects on kinesin and axonal transport, respectively. Immunoblotting methods and receptor autoradiography will be used to measure OP effects on the expression of key cholinergic markers in the brain, and organotypic culture methods will be employed to measure toxicity of OPs to the hippocampus. At the completion of this research we expect to identify temporal cellular changes resulting from repeated, subthreshold exposures to two representative OPs (DFP and chlorpyrifos) and their correlation with cognitive changes. These studies are significant because they will contribute to a better understanding of the toxicity associated with a class of agents that continues to pose a significant environmental risk to millions of people worldwide. ? ?
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