How Does Calcium-Dependent Intracellular Regulation of Voltage-Dependent Sodium Current Increase the Sensitivity to the Oxadiazine Insecticide Indoxacarb Metabolite Decarbomethoxylated JW062 (DCJW) in Insect Pacemaker Neurons?

Abstract

Decarbomethoxylated JW062 (DCJW), the active component of the oxadiazine insecticide (S)-methyl 7-chloro-2,5-dihydro-2-[[(methoxycarbonyl)[4-(trifluoromethoxy)phenyl] amino]carbonyl] indeno[1,2-e][1,3,4]oxadiazine-4a(3H)-carboxylate (DPX-JW062)(indoxacarb), was tested on 2 inward voltage-dependent sodium currents (named INa1 and INa2) expressed in shortterm cultured dorsal unpaired median neurons of the cockroach Periplaneta americana. Under whole-cell voltageclamp conditions, application of DCJW resulted in a biphasic dose-dependent inhibition of the global sodium current amplitude illustrating the differing sensitivity of sodium channels to DCJW. INa2 was less sensitive to DCJW [half-maximal inhibitory concentration (IC 50 ) ϭ 1.6 M] compared with INa1 (IC 50 ϭ 1.7 nM). Although a previous study demonstrated that INa1 was regulated by the cAMP/protein kinase A cascade, we showed that INa2 was mainly regulated in an opposite way by the activation of calcium-calmodulin-dependent protein phosphatase 2B (PP2B) and calcium-calmodulin-dependent protein kinase II (CaM-kinase II). Furthermore, we demonstrated that activation of CaM-kinase II by intracellular calcium via the calcium-calmodulin complex affected the sensitivity of INa2 channels to DCJW. By increasing the intracellular calcium concentration and/or using 1,2-bis(oaminophenoxy)ethane-N,N,NЈ,NЈ-tetraacetic acid (BAPTA) (a calcium chelator), N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide hydrochloride (W7) (a calmodulin inhibitor), cyclosporine A (a PP2B inhibitor), and 1-[N,O-bis(5-isoquinolinesulfonyl)-N-methyl-L-tyrosyl]-4-phenylpiperazine (KN-62) (a CaM-kinase II inhibitor), we revealed that activation of CaMkinase II was involved in the modulation of the voltage dependence of steady-state inactivation and that the CaMkinase II pathway activated by elevation of the intracellular calcium concentration might render INa2 channels approximately 3000-fold more sensitive to DCJW. These results indicated that manipulating specific intracellular signaling pathways involved in the regulation of sodium channels might have fundamental consequences for the sensitivity of insects to insecticides. This finding reveals an exciting research area that could lead to improvement in the efficiency of insecticides.Voltage-dependent sodium channels are critical elements of electrical activity in excitable cells and thus are important targets of diverse neuroactive compounds, including pyrethroids and pyrazoline-type insecticides (Dong, 2007;Soderlund, 2008;Silver et al., 2009). Synthetic pyrethroid insecticides, which are derived from natural compounds (pyrethrins), have been used for more than 40 years and account for 25% of the worldwide insecticide market, because of their potent and rapid insect intoxication, low toxicity for mammals, and lim-B.M. was supported by a doctoral fellowship from the Région Pays de la Loire.Article, publication date, and citation information can be found at http://jpet.aspetjournals.org.

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