Tolerance to chlorphenvinphos in rats assessed on the basis of changes in locomotor behavior in rotating wheels

Spontaneous locomotor activity in rotating wheels was investigated in rats exposed repeatedly (i.p. daily injections, five days a week for two weeks) to an agricultural organophosphorus pesticide, chlorphenvinphos (CVP) at doses of 1.0 and 3.0 mg/kg. After a seven day interval each rat was injected with a single 3.0 mg/kg test dose of CVP in order to assess the stability of tolerance. Concomitant changes in blood and the brain ChE activity were also investigated. It was found that exposure to CVP at a low dose (1.0 mg/kg), resulting in less than 50% reduction of ChE activity in blood and in the brain, did not produce changes in spontaneous locomotion in rotating wheels in the rat. Higher doses (3.0 mg/kg) inhibited blood and the brain ChE by more than 50% and reduced locomotion. Under conditions of repeated exposure to CVP at the symptomatic (3.0 mg/kg) dose ChE activity remained low throughout the exposure period, however, locomotor activity returned to a normal level, i.e. tolerance developed, within less than five days. Seven days after termination of the repeated exposure, the behavioral subsensitivity to CVP still remained. The biochemical data suggest that it may be related, at least partially, to a diminished vulnerability of ChE in some parts of the brain to CVP induced inhibition.