| Abstract: | Permethrin at concentrations as high as 10?6 had no detectable effect upon neuromuscular transmission at 20-21°C in the body-wall muscles of susceptible larvae of Lucilia sericata and susceptible and resistant (kdr) strains of larvae of Musca domestica. At the higher concentrations of 5 × 10?6M, muscle cells in all strains and species of larva were slowly depolarised, resulting in failure of neuromuscular transmission when the level of depolarisation corresponded with the peak height of the postsynaptic potentials. Multiple postsynaptic potentials were sometimes obtained in response to a single electrical stimulus to the motor axon. Neither of these effects is considered to result from interference with release of the neurotransmitter or its binding to the postsynaptic receptors. Spontaneous activity in sensory nerve fibres from susceptible larvae was increased followed by intermittent bursting and block at concentrations of permethrin as low as 10?9M at 20-21°C and 10?10 at 26°C. Resistant larvae of M. domestica were 1000 times less sensitive, needing concentrations of permethrin as high as 10?6M at 20-21°C and 10?7M at 26°C to affect sensory discharge. The results are discussed in relationship to the cause of knockdown by pyrethroid insecticides and to possible changes in the nervous system which may be brought about by the kdr resistance factor. |
