Calcium-activated potassium channels in insect pacemaker neurons as unexpected target site for the novel fumigant dimethyl disulfide

Dimethyl disulfide (DMDS), a plant-derived insecticide, is a promising fumigant as a substitute for methyl bromide. To further understand the mode of action of DMDS, we examined its effect on cockroach octopaminergic neurosecretory cells, called dorsal unpaired median (DUM) neurons, using whole-cell patch-clamp technique, calcium imaging and antisense oligonucleotide strategy. At low concentration (1 mu M), DMDS modified spontaneous regular spike discharge into clear bursting activity associated with a decrease of the amplitude of the afterhyperpolarization. This effect led us to suspect alterations of calcium-activated potassium currents (IKCa) and [Ca2+](i) changes. We showed that DMDS reduced amplitudes of both peak transient and sustained components of the total potassium current. IKCa was confirmed as a target of DMDS by using iberiotoxin, cadmium chloride, and pSlo antisense oligonucleotide. In addition, we showed that DMDS induced [Ca2+](i) rise in Fura-2-loaded DUM neurons. Using calcium-free solution, and (R, S)-(3,4-dihydro-6,7-dimethoxy-isoquinoline-1-yl)-2-phenylN, N-di-[ 2-(2,3,4-trimethoxy-phenyl) ethyl]-acetamide (LOE 908) [ an inhibitor of transient receptor potential (TRP)gamma], we demonstrated that TRP gamma initiated calcium influx. By contrast, omega-conotoxin GVIA (an inhibitor of N-type high-voltage-activated calcium channels), did not affect the DMDS-induced [Ca2+](i) rise. Finally, the participation of the calcium-induced calcium release mechanism was investigated using thapsigargin, caffeine, and ryanodine. Our study revealed that DMDS-induced elevation in [Ca2+](i) modulated IKCa in an unexpected bell-shaped manner via intracellular calcium. In conclusion, DMDS affects multiple targets, which could be an effective way to improve pest control efficacy of fumigation.