Differential in vivo inhibition of the foetal and maternal brain acetylcholinesterase by bromophos in the rat.

Bromophos, an organophosphorus compound, is known to cross the placental barrier. The response of the foetal brain acetylcholinesterase (AChE) to in vivo Bromophos exposure is not known. This study measured the in vivo time-course of cholinesterase (ChE) inhibition and recovery in rat maternal serum, brain, and foetal brain after administration of a single acute oral dose of Bromophos (500 mg/kg b.w.) on Day 18 of pregnancy. ChE inhibition in all tissues started as early as 2 h and reached a maximum at 16 h post-exposure. Foetal brain AChE was inhibited least and the first to recover followed by maternal brain and serum. The in vivo ID50 values for the ChE inhibition by Bromophos were 2.02, 205.0, 952.92 mg/kg b.w. and the in vitro IC50 values were 119.12, 115.17, 112.14 microM for the maternal serum, brain, and foetal brain, respectively. The IC50 values show that maternal serum, brain, and foetal brain are equisensitive to Bromophos. However, the ID50 values suggest that they have differential in vivo sensitivity to Bromophos. The foetal brain seems to be protected against the AChE inhibition by Bromophos, probably by detoxication in the maternal, placental, and foetal compartments.