Exposure to Chlorphenvinphos, an Organophosphate Insecticide, Prevents from Behavioral Sensitization to Amphetamine

Abstract

Exposure to organophosphorus (OP) pesticides, irreversible inhibitors of acetylcholinesterase (AChE), may result in long-lasting alterations in the functional state of the central nervous system. In earlier studies, we found that a single exposure of the rat to chlorphenvinphos (CVP), an OP pesticide, made the animal hyposensitive to amphetamine (AMPH) three weeks posttreatment. A repeated administration of AMPH is known to result in a progressive increase in the behavioral sensitivity to the psychostimulant. It makes it likely that treatment with AMPH after the CVP exposure may result in amelioration of the CVP-induced hyposensitivity to the psychostymulant. The purpose of the present experiment was to check out this supposition. At the first stage, the relationship between the CVP dose and the effect on sensitivity to AMPH was tested. The rats were given CVP once intraperitoneally (i.p.) at a dose of 0.0, 1.0 or 3.0 mg/kg. Three weeks later their open field behavior was assessed before and after i.p. administration of 0.25, 0.5 or 1.0 mg/kg of AMPH. At the subsequent stage, the susceptibility of the CVP-treated rats to AMPH sensitization by repeated AMPH treatment was investigated. For this purpose each of the rats was repeatedly treated with AMPH in its home cage (one injection/day for five days). At stage two, the daily AMPH dose received by each animal was of the same magnitude as that received at stage one. Two weeks after the last AMPH treatment dose, the motor response to a test AMPH dose (0.5 mg/kg) was measured in all rats. The results of stage one confirmed a significant reduction of behavioral sensitivity to AMPH in the CVP-treated rats. The results of stage two indicated that the CVP-induced decrease in sensitivity to AMPH was not ameliorated by a repeated treatment with AMPH at any of the used doses. In fact, in the rats exposed to the high CVP dose, repeated treatment with AMPH resulted, dose dependently, in augmenting of the depressive effect of the pesticide. It appears then that treatment to an OP pesticide reduces the rat's sensitivity to AMPH and makes the animal resistant to sensitization by repeated treatment with the psychostimulant.