Effect of different administration paradigms on cholinesterase inhibition following repeated chlorpyrifos exposure in late preweanling rats.

Chlorpyrifos (CPS) is widely used in agricultural settings and residue analysis has suggested that children in agricultural communities are at risk of exposure. This has resulted in a large amount of literature investigating the potential for CPS-induced developmental neurotoxic effects. Two developmental routes of administration of CPS are orally in corn oil at a rate of 0.5 ml/kg and subcutaneously in dimethyl sulfoxide (DMSO) at a rate of 1.0 ml/kg. For comparison between these methods, rat pups were exposed daily from days 10 to 16 to CPS (5 mg/kg) either orally dissolved in corn oil or subcutaneously dissolved in DMSO, both at rates of either 0.5 or 1.0 ml/kg. A representative vehicle/route group was present for each treatment. Both the low and high volume CPS in DMSO subcutaneous groups were lower than that of the low and high volume CPS in oil oral groups. At 4 h following the final administration, serum carboxylesterase was inhibited > 90% with all treatments. For cholinesterase activity in the cerebellum, medulla-pons, forebrain, and hindbrain, and serum, inhibition in the CPS-oil groups was similar and inhibition in the CPS-DMSO groups was similar. However, significantly greater inhibition was present in the high volume CPS-DMSO group as compared to the CPS-oil groups. Inhibition in the low volume CPS-DMSO group was generally between that in the CPS-oil groups and the high volume CPS-DMSO group. These data suggest that using DMSO as a vehicle for CPS may alter the level of brain ChE inhibition.