Suppression of pyrethroid-dependent neurotransmitter release from synaptosomes of knockdown-resistant house flies under pulsed-depolarization conditions during continuous perfusion

The uptake and release of 3H-labeled neurotransmitters were used as functional assays to assess the actions of selected neurotoxins on the synaptosomal membranes prepared from susceptible and knockdown-resistant house flies. The uptake of both [3H]choline and [3H]norepinephrine was found to be temperature sensitive. Only [3H]choline uptake was selectively inhibited by hemicholinium-3, whereas [3H]norepinephrine was inhibited by fluoxetine. Comparatively, the uptake of neurotransmitters into synaptosomes prepared from susceptible flies was more sensitive to inhibition at lower temperatures than that elicited by synaptosomes prepared from knockdown-resistant flies. A reproducible release of [3H]neurotransmitter was evoked by pulsed-depolarization in the presence of elevated K+ or 100 μM veratridine. Pretreatment with deltamethrin resulted in a substantial enhancement of [3H]neurotransmitter release during pulsed-depolarization of synaptosomes prepared from susceptible flies. This enhanced neurotransmitter release was greatly reduced or absent when synaptosomes of knockdown-resistant flies were examined. Synaptosomes of knockdown-resistant flies were particularly recalcitrant to deltamethrin-enhanced release when pulse-depolarized by veratridine. No enhanced neurotransmitter release due to deltamethrin pretreatment was apparent from either synaptosomal preparation under nondepolarizing conditions. These results are consistent with the hypothesis that knockdown resistance is most likely due to a decreased affinity of the voltage-gated sodium channel for pyrethroids.