The histochemistry of the cholinesterases in the central nervous system of susceptible and resistant strains of the house-fly,Musca domesticaL., in relation to diazinon poisoning

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Abstract
The distribution of cholinesterases hydrolysing acetyl- and butyrylthiocholine in the brain and thoracic ganglion of Musca domestica L. was examined histochemically in untreated flies, and in flies poisoned with diazinon (O,O-diethyl 0–2–isopropyl–4–methyl–6–pyrimidinyl phosphorothioate) of both susceptible and resistant strains.The inhibition of cholinesterase after poisoning was, to a greater or lesser extent, confined to the peripheral region of the ganglia, and to other specific areas such as the suboesophageal ganglion and lamina ganglionaris. The extent of inhibition increased with increasing doses of poison, and increasing time after treatment. The degree of inhibition could be broadly correlated with the condition of the fly; badly affected or moribund flies having less active cholinesterase than living and unaffected flies. Together with these areas of more or less complete inhibition were other areas, especially in the neuropile or synaptic regions, in which there remained large amounts of active enzyme. Active enzyme was still present in these areas 24 hours after apparent death of the fly. Total inhibition of cholinesterase throughout the central nervous system was rarely seen, even after very high doses of diazinon causing over 99 per cent. kill.Inhibition did not occur in the ganglia of living flies of a diazinon-resistant strain after a dose which caused inhibition and death in the normal susceptible strain. With 25 times this dose, causing approximately 99 per cent, kill in the resistant flies, cholinesterase was inhibited in them also, although to a somewhat lesser extent than in the susceptible flies given a comparable lethal dose.The data provided strong evidence that if death is caused by inhibition of cholinesterase of the nervous system it is due to local inhibition and not to generalised inhibition.