Insect muscarinic acetylcholine receptor: Pharmacological and toxicological profiles of antagonists and agonists

The insect muscarinic acetylcholine receptor (mAChR) is evaluated as a potential target for insecticide action. The mammalian M2/M4-selective antagonist radioligand [H-3]AF-DX 384 (a pirenzepine analogue) binds to Drosophila mAChR at a single high-affinity site identical to that for the nonselective antagonist [H-3]quinuclidinyl benzilate (QNB) and with a pharmacological profile distinct from that of all mammalian mAChR subtypes. Three nonselective antagonists (QNB, scopolamine, and atropine) show the highest affinity (K-i = 0.5-2.4 nM) at the Drosophila target, and AF-DX 384 and M3-selective 4-DAMP (dimethyl-4-(diphenylacetoxy)piperidinium iodide) rank next in potency (K-i = 5-18 nM). Eleven muscarinic antagonists generally exhibit higher affinity than eight agonists. On injection into houseflies, the antagonists 4-DAMP and (S)-(+)-dimethindene produce suppressed movement, the agonist (methyloxadiazolyl)quinuclidine causes knockdown and tremors, and all of them inhibit [H-3]QNB binding ex vivo, indicating possible mAChR-mediated intoxication. The insect mAChR warrants continuing study in lead generation to discover novel insecticides.