Abstract
Capsaicin inhibits the current flow in the voltage-gated sodium channels (VGSCs) of mammals and insects. The aim of the present study was to elucidate capsaicin toxicity in pyrethroid-susceptible and knockdown resistant (kdr; with reduced pyrethroid sensitivity) strains of the German cockroach (Blattella germanica) and its effects on VGSCs carrying the kdr mutation. Injection tests revealed that adult cockroaches from susceptible and kdr strains exhibited sluggish movement and paralysis upon abdominal capsaicin administration, consistent with its inhibitory effect on VGSC currents. The LD(50) values of capsaicin were 16 and 36 µg per insect for the susceptible and kdr strains, respectively, yielding a resistance ratio of 2.3. Two-electrode voltage clamp assays showed that the EC(50) values for the capsaicin-mediated inhibition of VGSC currents were 4.27 and 9.19 µM for susceptible and kdr mutant channels, respectively, yielding a resistance ratio of 2.2. The findings indicate that capsaicin retains inhibitory activity against insect VGSCs even in the presence of kdr mutations.