Changes in brain bioelectrical activity (EEG) after repetitive exposure to an organo-phosphate anticholinesterase. II. Rat.

Effects of repetitive exposure (ten times in a period of two weeks) to chlorphenvinphos (CVP), at daily doses of 0.5 and 1.0 mg/kg, i.p., were studied in adult male Wistar rats of imp-DaK stock. It was found that 3 hrs after the last exposure, the cholinesterase (ChE) activity in the blood and brain was close to 50% of the control value in the 0.5 mg/kg group and less than 50% in the 1.0 mg/kg group. In both groups, normalization of ChE activity in plasma took less time than in erythrocytes, and the normalization of ChE activity in erythrocytes proceeded faster than in the majority of the brain areas studied. Electrophysiological investigations revealed a retardation of age-related epileptic-like cortical activity. This effect, however, was present only in the 1.0 mg/kg group, and only in the period of decreased ChE activity in the brain. Spectral analysis revealed an increase in 1-4 Hz activity in the cortical EEG of the 0.5 mg/kg group and heightened theta activity (4-7 and 7-9 Hz bands) in the hippocampal EEG of the 1.0 mg/kg group. The later effects were detected after a time sufficient for full normalization of ChE activity and manifested themselves most clearly in the presence of an acoustic stimulus associated with pain. The above results are in agreement with earlier observations on rabbits exposed repetitively to CVP. Data from both species suggest that, in the case of repetitive exposure to CVP, neither plasma nor erythrocyte ChE activity is a reliable indicator of toxicity, and that such exposure to this OP may lead to changes in EEG outlasting the period of lowered ChE activity in the blood and brain.