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Cycloxaprid, an oxabridged cis-nitromethylene neonicotinoid, showed high insecticidal activity in Hemipteran insect pests. In this study, the action of cycloxaprid was characterized by recombinant receptor Nlα1/rβ2 and cockroach neurons. On Nlα1/β2 in Xenopus oocytes, cycloxaprid acted as a full agonist. The imidacloprid resistance-associated mutation Y151S reduced the Imax of cycloxaprid by 37.0% and increased EC50 values by 1.9-fold, while the Imax of imidacloprid was reduced by 72.0%, and EC50 values increased by 2.3-fold. On cockroach neurons, the maximum currents elicited by cycloxaprid were only 55% of that of acetylcholine, a full agonist, but with close EC50 values of that of trans-neonicotinoids. In addition, cycloxaprid inhibited acetylcholine-evoked currents on insect neurons in a concentration-dependent manner when co-applied with acetylcholine. Cycloxaprid at low concentrations significantly inhibited the activation of nAChRs by acetylcholine, and its inhibition potency at 1 µM was higher than its activation potency on insect neurons. Two action potencies, activation, and inhibition, by cycloxaprid on insect neurons provided an explanation for its high toxicity to insect pests. In summary, as a cis-nitromethylene neonicotinoid, cycloxaprid showed high potency on both recombinant nAChR Nlα1/β2 and cockroach neurons, which guaranteed its high control effects on a variety of insect pests.
Figure 1. Cycloxaprid acted on recombinant receptor Nlα1/β2 in Xenopus oocytes as a full agonist. (A) Chemical structures of cycloxaprid and imidacloprid. (B) Representative currents elicited by acetylcholine, imidacloprid, and cycloxaprid on recombinant receptor Nlα1/β2 expressed in Xenopus oocytes. The concentrations (µM) were provided above the currents following drug name, acetylcholine (ACh), imidacloprid (Imi), and cycloxaprid (Cyc). (C) Concentration-response curve for inward currents induced by acetylcholine (n = 7), imidacloprid (n = 6), and cycloxaprid (n = 12). The currents were normalized to the maximum currents elicited by 1000 µM acetylcholine and presented as mean ± SEM. (D) The dose-inhibition relationship for hybrid receptor Nlα1/β2 (n = 11). The curve was obtained by perfusing oocytes with increasing concentrations of the antagonist DHβE for 30 s prior to application of 1000 µM cycloxaprid in the continued presence of the antagonist for 10 s. The data were normalized to the response of each oocyte to 1000 µM cycloxaprid and presented as mean ± SEM.
Figure 2. Effects of Y151S mutation on agonist potency on the receptor Nlα1Y151S/β2. (A) Representative currents elicited by imidacloprid and cycloxaprid on wildtype receptor Nlα1/β2 (left) and mutant receptor Nlα1Y151S/β2 (right). The concentrations (µM) were provided following drug name, imidacloprid (Imi) and cycloxaprid (Cyc). (B) Concentration-response curve for inward currents induced by imidacloprid and cycloxaprid (n = 6–12). The currents were normalized to the maximum currents elicited by 1000 µM imidacloprid on the wildtype receptor Nlα1/β2. Data were presented as mean ± SEM.
Figure 3. Cycloxaprid acted on cockroach DUM neurons as a partial agonist. (A) Representative currents elicited by acetylcholine and cycloxaprid on cockroach DUM neurons. The concentrations (µM) were provided above the currents following drug name, acetylcholine (ACh), and cycloxaprid (Cyc). (B) Concentration-response curve for inward currents induced by cycloxaprid (n = 12). The currents were normalized to the maximum currents elicited by 1000 µM ACh, as indicated by the blank diamond (◊). A separate filled diamond (♦) showed the normalized currents from 1000 µM Cyc. (C) Representative currents elicited by the application of ACh and co-application of ACh and Cyc on cockroach neurons (n = 15). Concentrations were provided following drug names. + meant ACh plus Cyc. (D) Cyc concentration-dependent inhibition on inward currents elicited by 1000 µM ACh on cockroach neurons (n = 14–20). The currents were normalized to the maximum currents elicited by 1000 µM ACh. Data were presented as mean ± SEM. We performed one-way ANOVA with Tukey’s multiple comparisons. Different letters indicated significant differences at 0.05 level.
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