Interactions between anticholinesterases in an in vitro central nervous system preparation

Abstract

Organophosphate compounds have been widely developed as pesticides (e.g. paraoxon) and also as chemical warfare agents (nerve agents, e.g. sarin). These compounds rapidly inhibit the enzyme acetylcholinesterase (AChE), causing overstimulation within the cholinergic nervous system. If left untreated, this can be fatal. Current medical countermeasures to nerve agent poisoning consist of pretreatment with pyridostigmine and an emergency therapy comprising atropine, diazepam and pralidoxime. As well as a replacement pretreatment for pyridostigmine, physostigmine has been proposed as a component of a next generation of therapy to nerve agent poisoning, along with scopolamine and HI6. In animal studies, this therapy has been shown to lessen the level of incapacitation and increase survivability post poisoning with nerve agent. The exact mechanism of action of physostigmine in this combination is as yet unclear.
The primary aim of this study was to test the hypothesis that the beneficial effect of physostigmine in the proposed therapy is due to reversible inhibition of AChE, thereby protecting it from irreversible inhibition by nerve agent. To test this, extracellular field potentials were recorded from the molecular layer of the dentate gyrus in an in vitro slice model developed from the guinea pig. This response was shown to be modulated by the application of physostigmine and the nerve agent sarin and interactions between the two inhibitors were characterised. The results provided evidence for protection of ChE by physostigmine. The functional response (field potential) was related to cholinesterase activity measured in slices exposed to sarin.
This is the first evidence of the mechanism of action of physostigmine protection against nerve agent in the CNS. Not only will these results support the use of physostigmine as a pre-treatment, it also supports its use as a possible immediate therapy.

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ORCID for Ashley Pringle: orcid.org/0000-0003-2421-4380

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