Changes in rat brain cholinesterase activity and muscarinic receptor density during and after repeated oral exposure to chlorpyrifos in early postnatal development

The effects of repeated oral exposures to the organophosphorus insecticide chlorpyrifos (CPS) on brain muscarinic receptor densities, together with cholinesterase (ChE) activity, were studied in early postnatal rats. Initially, the effects on esterases from lactational exposure to CPS were investigated in young rats by administering CPS (100, 150, or 200 mg/kg subcutaneously in corn oil) to dams 1 day postpartum, yielding a significant body burden of CPS in the dams for possible excretion in the milk. Brain ChE inhibition in pups was less severe than in dams, whereas liver carboxylesterase (CbxE) inhibition in pups was at the same level as in dams. Because of the limited brain ChE inhibition obtained following lactation, pups were exposed to CPS directly by gavage, using 3 dosing regimens to yield a dose response. The rats were gavaged with CPS in corn oil on alternate days from postnatal day (PND) 1 through PND 21. Rats in the low-dosage group received 11 treatments at 3 mg/kg, those in the medium-dosage group received 3 treatments at 3 mg/kg and 8 at 6 mg/kg, and those in the high dosage group received 3 treatments at 3 mg/kg, 4 at 6 mg/kg, and 4 at 12 mg/kg. ChE activity in brain homogenates were inhibited significantly by 29% and 63% in the low- and high-dosage groups, respectively, on PND 22 and by 17% in the high dosage group on PND 40. Muscarinic receptor densities in brain synaptosomes were reduced using 3H-N-methylscopolamine (NMS) and 3H-quinuclidinyl benzilate (QNB) as ligands, with the effects more prominent from 3H-NMS. Densities of both ligands recovered to the control level several days after terminating treatment. The results indicate that pups are apparently exposed to only limited amounts of chlorpyrifos and/or its oxon through the milk when dams are exposed to extremely high chlorpyrifos levels. In addition, repeated direct oral exposures of early postnatal rats to CPS will result in persistent brain ChE inhibition and will transiently reduce muscarinic receptor density.