Some organophosphorus (OP) compounds produce delayed neurotoxicity (OPIDN). The adult chicken is the test animal for OPIDN. This effect is manifested as ataxia followed by paralysis resulting in Wallerian-type degeneration of the central and peripheral nervous systems. Early histopathologic changes are aggregation of neurotubules and neurofilaments followed by their condensation. Preliminary studies have suggested that Ca2+ is the initiating factor, subsequent to protein phosphorylation, in the development of OPIDN. It is planned to study the proposed mechanism set forth for OPIDN. It is proposed that neurotoxic OPs phosphorylate a neurotoxicity target protein whose normal function is unknown but could be related to calcium homeostasis. The subsequent increase in Ca2+ brings about depolymerization of microtubules to tubulins and an increased Ca2+-calmodulin-dependent phosphorylation of tubulin. As a result, tubulin is aggregated. Accumulation of such structures leads to the disruption of axoplasmic transport and the accumulation of mitochondria at the distal parts of the axons. Broken down mitochondria release Ca2+ into the axoplasm. This disrupts axonal membrane mechanisms for intracellular/extracellular ionic gradient, which leads to focal internodal swelling and degeneration that spreads stomatofugally to involve the entire distal axon. We propose to study the effect of the neurotoxic OP, DFP, in comparison with the non-neurotoxic parathion on the normal phosphorylating mechanisms of neuronal proteins and on Ca2+ concentration of the axoplasm of neurons. Similar experiments will be conducted on chicks and rats (insensitive to OPIDN). The effect of DFP or intracellular Ca++ concentration will be studied using biochemical and electrophysiological techniques. The effect of delayed neurotoxic versus non-delayed neurotoxic OPs administered in vivo or incubated with samples in vitro, on Ca++ uptake into synaptosomal, microsomal and mitochondrial fractions will be investigated using 45Ca++. Detection of free cytosolic Ca++ will be studied using Arsenazo III to monitor alterations in free cytosolic Ca++ after OP treatment. The kinetics of polymerization and depolymerization of microtubules in the presence of Ca++, OPs, calmodulin, and tubulin associated calmodulin kinase (TACK) will be studied. The isolation of TACK from treated and untreated animals will be used to investigate if this kinase is directly affected by OP treatment. Electrophysiological recordings of alterations in internal Ca++ concentration in Aplysia before and after OP treatment will be investigated.
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