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

Sensory 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 delamethrin 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 occuring 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.