Function-specific blockage of M1 and M3 muscarinic acetylcholine receptors by VX and echothiophate

Certain organophosphate (OP) cholinesterase inhibitors (ChEIs) are also known to bind to the muscarinic acetylcholine receptor (mAChR). The functional consequences of such binding were investigated here using the following OP compounds: VX, echothiophate, sarin, and soman. VX (charged at physiological pH) and echothiophate (formally charged) inhibited a specific signal transduction pathway in CHO cells expressing either the M1 or M3 mAChR. Hence, they blocked carbamylcholine (CCh)-induced cyclic adenosine monophosphate (cAMP) synthesis (μM) and had almost no effect on CCh-induced phosphoinositide (PI) hydrolysis. These substances were inactive on forskolin-induced cAMP inhibition signaling in CHO cells expressing M2 mAChR. In binding studies, using [3H]-N-methyl scopolamine ([3H]NMS) as the competitor ligand, the ChEIs, VX and echothiophate exhibited binding to rat cortical mAChR with Ki values in the μM range. The non-charged compounds, sarin and soman, were inert in modulating both cAMP metabolism and PI hydrolysis in CHO cells expressing M1, M2, and M3 mAChRs, and no binding was observed in presence of [3H]NMS. These data suggest that VX and echothiophate act as function-specific blockers via a non-classical path of antagonistic activity, implying the involvement of allosteric/ectopic-binding site in M1 and M3 mAChRs. The functionally selective antagonistic behavior of echothiophate and VX makes them potential tools for dissecting the interactions of the mAChR with different G proteins.