Differences in the neuroexcitatory actions of pyrethroid insecticides and sodium channel-specific neurotoxins in rat and trout brain synaptosomes.

The effects of pyrethroid insecticides and other sodium channel-specific neurotoxins on synaptosomal membrane potential were investigated in rat and trout brain synaptosomes using the membrane-permeant lipophilic cation [3H]tetraphenylphosphonium (TPP+). Concentration-dependent and tetrodotoxin-sensitive decreases in TPP+ accumulation, indicative of membrane depolarization, were produced by veratridine, aconitine, scorpion (Leiurus quinquestriatus) venom, and type I and type II pyrethroids, in both species. Veratridine, aconitine, and Leiurus venom were more potent and efficacious membrane-depolarizing agents in rat synaptosomes than in trout synaptosomes. Type II (deltamethrin, cypermethrin) pyrethroids produced similar depolarizing responses in rat and trout synaptosomes; however, the 1R-cis-alpha R isomer of deltamethrin, which had no effect on membrane potential in rat synaptosomes, depolarized trout synaptosomes. This isomer of deltamethrin was also shown to produce toxicity in trout, but not in rats. The type I pyrethroids, permethrin and NRDC 157, exhibited significantly greater intrinsic activity in trout brain synaptosomes, producing maximal membrane depolarizations that were three times greater than those observed in rat brain synaptosomes. These results provide evidence of species-specific differences in the membrane-depolarizing properties of pyrethroid insecticides and sodium channel-specific neurotoxins. They also suggest that some of the neurotoxin binding domains of the voltage-sensitive sodium channel in trout brain differ from those in mammalian brain. The hypersensitivity of fish to the neurotoxic actions of pyrethroid insecticides may be related to these differences.