Comparison of inhibitory activity of various organophosphorus compounds against acetylcholinesterase and neurotoxic esterase of hens with respect to delayed neurotoxicity

A variety of organophosphorus (OP) compounds with and without delayed neurotoxicity were examined for inhibitory power against neurotoxic esterase (NTE) and acetylcholinesterase (AchE) of hen brain in vitro and in vitro. Generally, delayed neurotoxicity induced by OP compounds correlated with high inhibition of NTE in vitro, whereas in vitro studies comparing I₅₀S for both enzymes did not provide a guide to evaluation of delayed neurotoxicity. Single oral administration of delayed neurotoxic EPN, leptophos and TOCP resulted in more than 80 per cent inhibition of brain NTE at neurotoxic doses, whereas non-delayed neurotoxic methyl parathion, fenitrothion and cyanophos caused weak inhibition at near lethal doses which gave rise to severe inhibition of brain AchE. A delayed neurotoxic dose of (?)-EPN caused more severe inhibition of brain NTE as compared with the same dose of the non-delayed neurotoxic ( + )-isomer. However, a few compounds produced severe inhibition of NTE at non-delayed neurotoxic doses. Hens paralysed by repeated administration of a low level of leptophos showed significant decreases in NTE activity of the brain and spinal cord.