An electrophysiological study of susceptible (cooper) and resistant (kdr; super‐kdr) strains of the adult housefly (Musca domestica L.) using an isolated mesothoracic leg preparation

AbstractSensory fibres in the isolated metathoracic leg of houseflies were used as a neurophysiological assay to investigate nerve insensitivity factors in knockdown‐resistant strains (kdr, super‐kdr) against DDT, the pyrethroids deltamethrin and RU15525, and the N‐alkyl amide BTG502. Several neurophysiological effects were induced by these compounds, but the only consistent one was an elevation of the spontaneously occurring firing rate. Whilst both resistant strains possess a nerve insensitivity factor, that of the more resistant super‐kdr flies appeared to be weaker than that of kdr. When probed with BTG502, super‐kdr nerves were no more resistant to this compound than those of the susceptible Cooper strain. Kdr nerves were considerably less sensitive (at least 100‐fold) than either of these other two strains. These results indicate that the voltage‐dependent sodium channels are different in all three strains and that the higher resistance shown by super‐kdr over kdr flies cannot be accounted for by simple enhancement of the kdr nerve‐insensitivity factor. Therefore, target sites other than the voltage‐dependent sodium channel have to be considered to account, fully for super‐kdr resistance. Such sites could occur at synaptic contacts, possibly involving phosphorylation of proteins and/or calcium‐ion regulation.