Virginia Polytechnic Institute and State University Blacksburg, Virginia
The rapid evolution of insecticide resistance results in the need to develop novel strategies to control pests. Insecticide mixtures are utilized to enhance the toxicity of insecticides. Muscarinic acetylcholine receptors (mAChRs) have been investigated as an insecticidal target, but no insecticides with this mode of action are currently on the market. We utilized the non-selective mAChR agonist, pilocarpine, and antagonist (atropine) to investigate the target-site synergism of lindane. Lindane is an organochloride acting on gamma-aminobutyric acid chloride channel (GABAA). Using extracellular electrophysiology of third-instar Drosophila melanogaster larvae, 10 µM pilocarpine significantly increased neuroexcitation of the central nervous system (CNS) in both CS-OR (wild-type strain) and 1675 (resistant strain, rdl). This neuroexcitation was completely blocked by 10 µM atropine. Pilocarpine also affected the neural inhibition of lindane. Specifically, pilocarpine (3 µM) lowered lindane’s half-maximal inhibitory concentration (IC50) from 1.3 µM to 0.20 µM in CSOR. In the rdl strain, pilocarpine’s effect was also observed, where the IC50 decreased from 3.9 µM (lindane alone) to 1.3 µM (lindane + pilocarpine). The electrophysiological results were consistent with toxicological observation in CSOR adult flies. That is, a mixture of 6000 ppm pilocarpine increased the oral toxicity of lindane from 0.97 ppm to 0.03 ppm. However, the same level of synergism was not observed in the rdl strain. Interestingly, atropine also synergized the toxicity of lindane. This work shows that perturbation of the muscarinic system can enhance the toxicity of GABAergic insecticides, but likely will not break resistance.
