Acetylcholinesterase and neuropathy target esterase in chickens treated with acephate

Reports that near-lethal doses of the pesticide methamidophos (O,S-dimethyl phosphoramidothioate) caused a delayed neurotoxicity (OPIDN) in humans and that another phosphoramidate, isofenphos, caused OPIDN in the hen at high doses, prompted a study of the abilities of acephate (O,S-dimethyl acetylphosphoramidothioate) to inhibit brain acetylcholinesterase (AChE) and neuropathy target esterase (NTE) in vivo. Hens were treated orally with 5-700 mg/kg of acephate, or im with 50-200 micrograms/kg of diisopropyl-fluorophosphate (DFP, positive control) and sacrificed 24 hr later. Brain homogenates were assayed for AChE as an estimate of acute toxicity, for NTE to indicate acephate's potential to cause OPIDN, and for residues of acephate and its metabolite methamidophos. A range finding study confirmed the LD50 level for acephate was approximately 800 mg/kg. Regression analyses indicated an ID50 (a dose that inhibits 50% of activity) for acephate inhibition of AChE of 10 mg/kg and an extrapolated ID50 for inhibition of NTE of 1300 mg/kg, almost twice the LD50. In contrast, ID50 values for DFP were similar for AChE (146 micrograms/kg) and NTE (132 micrograms/kg). Brain methamidophos levels were 10 to 16 percent of the total acephate plus methamidophos brain concentration. The lower the dose of acephate, the higher was the relative percentage of methamidophos. The results show acephate is a more potent inhibitor of AChE than it is of NTE in hens and suggest it would be difficult to administer a single dose of acephate sufficient to cause OPIDN without killing the animal.