Effects of single or repeated dermal exposure to methyl parathion on behavior and blood cholinesterase activity in rats

The effects of a single or repeated dermal administration of methyl parathi on on motor function, learning and memory were investigated in adult female rats and correlated with blood cholinesterase activity. Exposure to a sing le dose of 50 mg/kg methyl parathion (75% of the dermal LD50) resulted in a n 88% inhibition of blood cholinesterase activity and was associated with s evere acute toxicity. Spontaneous locomotor activity and neuromuscular coor dination were also depressed. Rats treated with a lower dose of methyl para thion, Le. 6.25 or 12.5 mg/kg, displayed minimal signs of acute toxicity. B lood cholinesterase activity and motor function, however, were depressed in itially but recovered fully within 1-3 weeks. There were no delayed effects of a single dose of methyl parathion on learning acquisition or memory as assessed by a step-down inhibitory avoidance learning task. Repeated treatm ent with 1 mg/kg/day methyl parathion resulted in a 50% inhibition of blood cholinesterase activity. A decrease in locomotor activity and impairment o f memory were also observed after 28 days of repeated treatment. Thus, a si ngle dermal exposure of rats to doses of methyl parathion which are lower t han those that elicit acute toxicity can cause decrements in both cholinest erase activity and motor function which are reversible. In contrast, repeat ed low-dose dermal treatment results in a sustained inhibition of cholinest erase activity and impairment of both motor function and memory. Copyright (C) 2001 National Science Council, ROC and S. Karger AG, Basel.