Point mutations in domain III of a Drosophila neuronal Na channel confer resistance to allethrin

Voltage-gated sodium channels are the presumed site of action of pyrethroid insecticides and DDT. We screened several mutant sodium channel Drosophila lines for resistance to type I pyrethroids. In insecticidal bioassays the para(74) and para(DN7) fly lines showed greater than 4-fold resistance to a llethrin relative to the allethrin sensitive Canton-S control line. The ami no acid substitutions of both mutants are in domain III. The point mutation associated with para(74) lies within the S6 transmembrane region and the a mino acid substitution associated with para(DN7) lies within the S4-S5 link er region. These sites are analogous to the mutations in domain II underlyi ng knockdown resistance (kdr) and super-kdr, naturally occurring forms of p yrethroid resistance found in houseflies and other insects. Electrophysiolo gical studies were performed on isolated Drosophila neurons from wild type and para(74) embryos placed in primary culture for three days to two weeks. The mutant para(74) sodium currents were kinetically similar to wild type currents, in activation, inactivation and time to peak. The only observed d ifference between para(74) and wild-type neurons was in the affinity of the type I pyrethroid, allethrin. Application of 500 nM allethrin caused remov al of inactivation and prolonged tail currents in wild type sodium channels but had little or no effect on para(74) mutant sodium channels. (C) 2000 E lsevier Science Ltd. All rights reserved.