Enantiomer (+)physostigmine prevents organophosphate‐induced subjunctional damage at the neuromuscular synapse by a mechanism not related to cholinesterase carbamylation

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Abstract
The natural alkaloid (‐)PHY is a reversible anticholinesterase carbamate, but in contrast, its optical isomer (+)PHY, is a very weak anticholinesterase. We have shown that treatment of rats with atropine and (‐)PHY prior to injections of a lethal dose of the irreversible organophosphate sarin (0.13 mg/kg) not only protected 100% of the animals from lethality but also reduced the size of the subneural lesions of the nicotinic synapses of skeletal muscle. Similar protection against lethality is provided by pretreatment with (+)PHY. At the concentration used (0.3 mg/kg), there was no detectable inhibition of AChE activity. We have examined the protection afforded by (+)PHY or (‐)PHY against lethality and myopathy due to organophosphate agents such as sarin. The major alterations in the soleus motor endplates 1 hr after drug treatment were as follows: (1) A single sublethal dose of sarin (0.08 mg/kg) produced enlarged, blistered, and severely disrupted subjunctional regions, with muscle damage extendig beyond the endplate to include myofiber necrosis and subsequent phagocytosis; (2) (+)PHY (0.3 mg/kg) produced no obvious damage in the postjunctional region; (3) (‐)PHY (0.1 mg/kg) had a selective effect in inducing irregularities of the subjunctional sarcomere band patterns without any gross vacuolization; (4) light microscopic data indicated that the combination of atropine and (+)PHY, or of atropine and (‐)PHY (0.1 mg/kg), 30 min prior to a lethal dose of sarin, offered dramatic reduction in the average dimension of lesions. Lesions were detectable in most endplates but recognizable changes were markedly less severe than those seen in sarin myopathy. Few instances of extensive muscle damage and myofiber necrosis were visible.In each of the 3 experimental groups examined, the extrajunctional myopathy was reversible with substantial recovery being observed on days 5 and 10 after drug treatment.In addition to its anticholinesterase activity, (‐)PHY is also a powerful ion channel blocker of the nicotinic acetylcholine receptor. As for (+)PHY, its protective effect almost certainly does not depend at inhibition of acetylcholinesterase, but as with (‐)PHY, it my depend on a direct blockade at the nicotinic‐acetylcholine receptor and its ion channel.