Placental transfer of methyl parathion (MP), an organophosphate pesticide, could involve effects on cholinergic system. To analyze whether placental cholinergic system is altered by prenatal exposure to MP, expression of muscarinic cholinergic receptors (M1 and M2 subtypes; mAChR) was determined in pregnant rats exposed to MP at 0.0, 1.0, 1.5, and 2.0 mg/kg. An immunohistochemical analysis for M1 and M2 mAChR was performed, and the density of the mAChR signal was measured by image analysis. M1 and M2 mAChR were found in the trophoblast present in the labyrinth, with an 18% predominance of M2 over M1 in the non-exposed group. The expression of M1 and M2 mAChR in placentas exposed to MP showed a decrease when compared with the non-exposed group (P < 0.05); a dose–response effect was not detected. These results demonstrate that prenatal exposure to MP causes changes in the placental expression of mAChR M1 and M2, suggesting that related placental cholinergic functions could be affected.
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