Central Nervous System Endogenous Phosphorylation in Organophosphorus Delayed Neurotoxicity.

Abstract

Some organophosphorus compounds (OPs) produce a characteristic axonopathy approximately two weeks after a single dose. The relationship among biochemical, morphological and clinical events after exposure to these compounds is not well defined. The earliest biochemical event known to occur after exposure is irreversible phosphorylation of brain proteins. This occurs within four hours of dosing. Clinical signs of neurotoxicity become apparent 7-10 days after exposure and progress from ataxia to complete paralysis of limbs by days 14 or 15. Neuropathological lesions are evident in both central and peripheral regions of the nervous system during the latter portion of this two-week period; clinical manifestations occurring prior to pathological alterations in most cases. It is possible that exogenous OP phosphorylation of proteins may disrupt existing cellular systems of phosphorylation mediated by protein kinases. The purpose of this study was to assess the integrity of these systems after exposure to neurotoxic organophosphorus compounds and evaluate their relationship to development of delayed neurotoxicity. Endogenous phosophorylation was studied in subcellular fractions from brainstem of control and OP-treated White Leghorn hens. Proteins phosphorylated after incubation with (gamma)-{('32)P}-ATP were separated and visualized with sodium dodecyl sulfate polyacrylamide gel electrophoresis and autoradiography. Densitometry was used to quantitate the results obtained. Phosphorylation was decreased in a single phosphoprotein (MW = 5.0 x 10('4)) from hens sacrificed 15 days after dosing with neurotoxic doses of dibutyl dichlorvos (1.5 and 2.8 mg/kg producing 33.3% and 37.0% decreases, respectively). A subneurotoxic dose of this compound (0.28 mg/kg) did not produce this decreased phosphorylation. The effect was correlated with the onset of clinical signs of neurotoxicity. At time points prior to 10 days after dosing (1, 3, and 7 days) no significant decrease in phosphorylation was noted nor were clinical signs evident. Histopathological staining of tissue sections from brainstem of animals sacrified 15 days after dosing with 2.8 mg/kg dibutyl dichlorvos revealed lesions typical of OP neuropathy. These results suggest that endogenous phosphorylation in the central nervous system is disrupted in a specific way by dibutyl dichlorvos in vivo and that the effect correlates temporally with the degenerative phase of the neuropathy in evidence both clinically and histopathologically.