Malaria mosquito (Anopheles gambiae) populations are commonly controlled with pyrethroid insecticides, but widespread resistance establishes a need for new insecticides with unique modes of action. Insects are known to possess muscarinic acetylcholine receptors (mAChRs), and muscarinic agonists are effective insecticides, but suffer from high mammalian toxicity. Of the three types of mAChRs in insects (A, B, and C), type-B activation is thought to decrease levels of cAMP and is pharmacologically distinct, having little sensitivity to the classical mAChR antagonist, atropine. Recently, we identified a novel pyrazole oxime (747) that is toxic to An. gambiae (WHO paper assay LC50 = 0.122 mg/mL), has low toxicity to mice (oral LD50 &gt; 2,000 mg/kg), and shares some structural similarity to pilocarpine, a non-selective mAChR agonist. Pilocarpine and 747 also shared similar signs of intoxication in mosquitoes, which include lethargic movement and loss of flight behavior. Pilocarpine toxicity was antagonized by atropine injection, whereas 747 toxicity was not. Consequently, we hypothesized that 747 affects the insect mAChR-B receptor. Exposure of a larval Drosophila melanogaster CNS preparation to 10 μM 747 resulted in a biphasic effect on the CNS firing rate; a slight increase followed by a nearly complete inhibition of nerve discharge. Neither effect was blocked by atropine. Similarly, pilocarpine (10 μM) had a biphasic effect on CNS nerve firing, but with a strong initial increase in the CNS firing rate, followed by a strong decrease; both are sensitive to atropine. Additionally, preincubation of the CNS with 747 (10 μM) completely blocked the neuroexcitatory effect of octopamine, an agonist known to increase nerve firing via increased cAMP levels. Future studies will involve biochemical analysis of receptor-mediated cAMP levels to further define the action of 747 on mAChR-B receptors.

Muscarinic acetylcholine receptors as a target for mosquitocide development

254th National Meeting (2017)

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