The effect of chlorpyrifos and chlorpyrifos-oxon on brain cholinesterase, muscarinic receptor binding, and neurotrophin levels in rats following early postnatal exposure

Chlorpyrifos (CPS) is a widely used diethyl organophosphorus insecticide in agricultural settings. Household and urinary residue analysis has suggested that children in agricultural communities are at risk of exposure to diethyl organophosphorus insecticides. The effects of repeated postnatal exposure to CPS and its metabolite chlorpyrifos-oxon (CPO) on total muscarinic acetylcholine receptor (mAChR) binding, nerve growth factor (NGF) levels, and brain derived neurotrophic factor (BDNF) levels in the forebrain of neonatal rats were investigated. Peak inhibition of brain cholinesterase (ChE) for CPS and CPO was determined after acute exposure to dosages of each compound (a low and a high for each), which produced similar degrees of initial ChE inhibition. Pups were administered CPS (1.5 or 3.0 mg/kg), CPO (0.25 or 0.35 mg/kg), or the corn oil vehicle by daily gavage from postnatal day 1 (PND 1) through PND 6. This exposure paradigm resulted in persistent ChE inhibition by CPS but only transient inhibition by CPO, suggesting that, even though the initial ChE inhibition is similar between compounds, the effects of repeated exposure differ significantly. Forebrain mAChR density, as measured by the binding of H-3-QNB, and NGF levels were significantly reduced on PND 4 and 7 after CPS but not on PND 12. No effects on mAChR density or NGF levels were observed with CPO. No effects on BDNF levels were observed with either compound. The data suggest that the persistent ChE inhibition and decreased mAChR binding may play a role in the decreased NGF levels following CPS exposure.