| Amphibians are generally less sensitive than mammals to the acute toxicity of organophosphorus (OP) insecticides, but the basis for these differences is unclear. This study compared in vivo sensitivity of a mammal (Mus musculus, CD-1 outbred) and an amphibian (the Great Plains toad, Anaxyrus cognatus) to chlorpyrifos oxon (CPO), the active metabolite of the organophosphorus pesticide chlorpyrifos. Following intraperitoneal dosing, adult male toads were about 13-fold less sensitive to CPO than adult male mice based on acute maximum tolerated doses (MTDs). At equi-toxic doses (0.6, 0.8 or 1 x MTD), brain acetylcholinesterase (AChE) inhibition was noted in mice but not in toads. In contrast, fatty acid amide hydrolase (FAAH) in liver was inhibited by CPO in both species. Carboxylesterase, a common enzyme involved with OP detoxification, was undetectable in toad brain. Toad brain AChE was about 15-fold less sensitive in vitro to inhibition by CPO than mouse enzyme (IC50, 20 min at 37 ºC: 136 vs 9 nM), roughly agreeing with in vivo differences in sensitivity. We used an indirect inhibition assay to evaluate possible species differences in target-site detoxification of CPO. Toad brain homogenate was markedly more effective at inactivating CPO in vitro than an equivalent amount of mouse brain. These data suggest that the relative insensitivity of toad brain AChE to CPO contributes to the resistance to acute toxicity and may be due to more effective target-site detoxification. |
