Function-specific blockage of M(1) and M(3) muscarinic acetylcholine receptors by VX and echothiophate

Brain Res. 2006 Apr 26;1085(1):102-10. doi: 10.1016/j.brainres.2006.02.070. Epub 2006 Apr 3.

Abstract

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 M(1) or M(3) mAChR. Hence, they blocked carbamylcholine (CCh)-induced cyclic adenosine monophosphate (cAMP) synthesis (muM) 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 M(2) mAChR. In binding studies, using [(3)H]-N-methyl scopolamine ([(3)H]NMS) as the competitor ligand, the ChEIs, VX and echothiophate exhibited binding to rat cortical mAChR with K(i) values in the muM range. The non-charged compounds, sarin and soman, were inert in modulating both cAMP metabolism and PI hydrolysis in CHO cells expressing M(1), M(2), and M(3) mAChRs, and no binding was observed in presence of [(3)H]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 M(1) and M(3) 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.

Publication types

  • Comparative Study

MeSH terms

  • Animals
  • Atropine / pharmacology
  • CHO Cells
  • Cholinesterase Inhibitors / chemistry
  • Cholinesterase Inhibitors / pharmacology*
  • Colforsin / pharmacology
  • Cricetinae
  • Cyclic AMP / metabolism
  • Dose-Response Relationship, Drug
  • Drug Interactions
  • Echothiophate Iodide / chemistry
  • Echothiophate Iodide / pharmacology*
  • Muscarinic Agonists / pharmacokinetics
  • Muscarinic Antagonists / pharmacology
  • N-Methylscopolamine / pharmacokinetics
  • Organothiophosphorus Compounds / chemistry
  • Organothiophosphorus Compounds / pharmacology*
  • Oxotremorine / pharmacokinetics
  • Phosphatidylinositols / metabolism
  • Protein Binding / drug effects
  • Receptor, Muscarinic M1 / drug effects*
  • Receptor, Muscarinic M1 / physiology
  • Receptor, Muscarinic M3 / drug effects*
  • Receptor, Muscarinic M3 / physiology
  • Transfection / methods
  • Tritium / pharmacokinetics

Substances

  • Cholinesterase Inhibitors
  • Muscarinic Agonists
  • Muscarinic Antagonists
  • Organothiophosphorus Compounds
  • Phosphatidylinositols
  • Receptor, Muscarinic M1
  • Receptor, Muscarinic M3
  • Tritium
  • Colforsin
  • Oxotremorine
  • Atropine
  • VX
  • Echothiophate Iodide
  • Cyclic AMP
  • N-Methylscopolamine