Parathion administration in the monkey: Time course of inhibition and recovery of blood cholinesterases and visual discrimination performance

Abstract

The effects of parathion [ O,O-diethyl O- p-nitrophenyl phosphorothionate] administration on activities of acetyl-cholinesterase (AChE) in blood and pseudocholinesterase (ChE) in plasma were studied in bonnet ( Macaca radiata) and rhesus ( M. mulatta) monkeys. In addition, the effects of parathion administration on performance of learned visual discrimination tasks were studied in rhesus monkeys. Peak inhibition of AChE and ChE occurred about 6 hr after administration of 0.5, 1.0, and 2.0 mg/kg parathion po. The degree of peak inhibition was greater for ChE than for AChE, was dose-related for both enzymes, and was of about the same magnitude in both species, even though control values for AChE and ChE differed in the two species. Activities of both enzymes returned to control values within 2 wk at all dose levels. Administration of 2.0, 1.5, and 1.0 mg/kg parathion abolished performance of the learned tasks 5 hr later and for as long as 3–7 days. When performance of the tasks returned after the 2.0 mg/kg dose, the level of performance remained below pretreatment values for up to 3 wk. A dose of 0.5 mg/kg did not affect performance. Comparison of the present findings with other work showed that reversal of blood AChE and ChE inhibition after parathion administration occurred more rapidly in monkey than in man but required more time in monkey than in mouse. It was observed that scopolamine and methyl scopolamine also blocked visual discrimination performance.