Muscarinic acetylcholine receptor activation synergizes the knockdown and toxicity of GABA‐gated chloride channel insecticides

BACKGROUND: Pest management requires continual identification of new physiological targets and strategies to control pests affecting agriculture and public/animal health. We propose the muscarinic system as a target for agrochemicals because of its physiological importance. Unlike the muscarinic system, gamma‐amino butyric acid (GABA) receptors are an established insecticide target. Here, we investigated target‐site synergism using small molecule probes (agonist and antagonist) against the muscarinic system and their ability to enhance the toxicity of GABAergic insecticides in Drosophila melanogaster (Meigen). RESULTS: Oral delivery of pilocarpine (muscarinic agonist) enhanced the toxicity of dieldrin, fipronil, and lindane, resulting in synergist ratios (SRs) between 4–32‐fold (orally delivered) or between 2–67‐fold when insecticides were topically applied. The synergism between pilocarpine and the GABA‐insecticides was greater than the synergism observed with atropine (muscarinic antagonist), and was greater, or comparable, to the synergism observed with the metabolic inhibitor piperonyl butoxide. In addition to lethality, pilocarpine increased the knockdown of lindane. The mechanism of synergism was also investigated in the central nervous system using extracellular electrophysiology, where pilocarpine (3 μmo/L) lowered the half‐maximal inhibitory concentration (IC(50)) of lindane from 1.3 (0.86–1.98) μmol/L to 0.17 (0.14–0.21) μmol/L and fipronil's IC(50) from 2.2 (1.54–3.29) μmol/L to 0.56 (0.40–0.77) μmol/L. CONCLUSION: Convergence of the cellular function between the muscarinic and GABAergic systems enhanced the insecticidal activity of GABA receptor blocking insecticides through the modulation of the central nervous system (CNS). The future impact of the findings could be the reduction of the active ingredient needed in a formulation with the development of muscarinic synergists. © 2022 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.