Ian Mellor
Single-channel studies of voltage-gated sodium channels and their interaction with pyrethroid insecticides
Pyrethroids are synthetic analogues of natural insecticides known as pyrethrins that are found in some species of chrysanthemum plant. Pyrethrins and pyrethroids target voltage-gated sodium channels causing a slowing of their inactivation and deactivation processes and are selective for insect over mammalian channels. We have studied the effects of the pyrethroid deltamethrin on the single channel parameters of wild-type and I874M mutant rat NaV1.2 sodium channels expressed in Xenopus oocytes using patch-clamping. The I874M mutant is more susceptible to pyrethroids as it simulates an insect sodium channel at this position. The conductances of wild-type and mutant channels were 19.8pS and 19.6pS, respectively, and were unaffected by deltamethrin. However, the reversal potentials for both channels were increased by the pyrethroid. Histograms of open times were best-fit by single exponentials. The open time constants of wild-type and mutant channels were similarly voltage-dependent (0.01ms mV-1). For openings at -40mV, the mean open time constant of the wild-type channel was significantly shorter (0.48 ± 0.05ms) than that of the mutant channel (0.65 ± 0.08 ms). Deltamethrin (≥ 0.1μM wild-type; ≥ 1nM mutant) induced a second population of openings that were prolonged in duration. This population, and its open time constant, was increased when the concentration of deltamethrin was raised. When the proportion of prolonged channel openings was used as a measure of the potency of deltamethrin, the pyrethroid was found to be ~100 times more potent on the mutant than on the wild-type channel. Low concentrations of deltamethrin increased the frequency of opening (fo) of wild-type and mutant channels during a depolarisation, but fo was reduced by 1 μM (wild-type) and 0.01μM (mutant) deltamethrin. Following application of deltamethrin, both wild-type and mutant channels often remained open following membrane repolarisation to -100mV, with fo being dependent on the deltamethrin concentration.