Abstract: The release of ³H-labeled norepinephrin was used to assess the effect of selected neurotoxins on synaptosomes from brains of the susceptible (SP),deltamethrinresistant(Del-R) and permethrin-resistant(2Cl-R) houseflies. A reproducible release of ³H-neurotransmitter was evoked by pulsed-depolarization in the presence of elevated K⁺(55 mol/L). The enhanced ³H-norepinephrin release from housefly synaptosomes of Del-R and 2Cl-R reduced by 47.0% and 51.0% respectively in comparison with that from the susceptible strain. The synaptosomes pretreated with 10^-5 mol/L deltamethrin in SP, Del-R and 2Cl-R strains resulted in 80.3%, 26.5% and 70.5% enhancement of ³H-neurotransmitter release during pulseddepolarization, respectively. These results were consistent with the hypothesis that knocked down resistance is most likely due to a decreased affinity of the voltagegated sodium channel for deltamethrin. However, the resistance against permethrin showed little effect on synaptosomal efflux of ³H-norepinephrin^.

Key words: neurotransmitter release, housefly, synaptosome, pyrethroid, resistance