The role of voltage-gated chloride channels in type II pyrethroid insecticide poisoning.

Pyrethroids act on mammalian sodium channels, but we have previously shown that low concentrations of the type II pyrethroid deltamethrin also decrease the open channel probability (P(o)) of voltage-gated chloride channels. This effect would be expected to amplify the sodium channel-mediated signs of poisoning produced by pyrethroids. In the present study we evaluated potential chloride channel agonists in vitro, and then tested the most effective of these on pyrethroid-poisoned rats to determine the practical significance of chloride channel effects in vivo. Patch clamp experiments showed that, for voltage-gated maxi chloride channels in excised, inside-out patches from mouse N1E 115 neuroblastoma cells, ivermectin (10(-7) M) and pentobarbitone (10(-6) M) significantly increased open channel probability (p </= 0.01 and p </= 0.02, respectively), whereas phenobarbitone, hexobarbitone, mephobarbitone, thiopentone, and barbituric acid did not. This suggested that, if chloride channels were important in vivo, ivermectin and pentobarbitone should antagonize type II pyrethroid poisoning and phenobarbitone should not. Male F344 rats were then pretreated with ivermectin (4 mg/kg iv), equisedative doses of either pentobarbitone (15 mg/kg ip) or phenobarbitone (45 mg/kg ip), or solvent controls. This was followed by deltamethrin (1.5 or 2 mg/kg iv) or the type I pyrethroid cismethrin (4 mg/kg iv). Ivermectin produced a marked fall in deltamethrin-induced salivation (p </= 0.05) and also (in anesthetized rats) in repetitive electromyogram discharge and muscle twitch (p </= 0.01 and p </= 0.05, respectively). Pentobarbitone significantly reduced the motor signs score due to deltamethrin (p </= 0.01). Ivermectin therefore protected against the peripheral signs of deltamethrin poisoning and pentobarbitone protected against the central signs. As expected phenobarbitone had no protective effects. The motor signs produced by the type I pyrethroid cismethrin (which does not act on chloride channels) were not diminished by either barbiturate. The peripheral benzodiazepine receptor blocker PK11195 did not diminish the protective action of ivermectin on the muscle twitch (p </= 0.05), although it partially reversed the block of salivation (p </= 0.05). These results support the hypothesis that the voltage-dependent chloride channel is a toxicologically significant additional site of action for deltamethrin and that the use of chloride channel agonists can provide a rationale for a novel and effective therapy against type II pyrethroid poisoning.

Forshaw PJ ，Lister T ，Ray DE 《TOXICOLOGY AND APPLIED PHARMACOLOGY》

Structure-activity and interaction effects of 14 different pyrethroids on voltage-gated chloride ion channels.

Burr SA ，Ray DE 《-》

Actions of pyrethroid insecticides on voltage-gated chloride channels in neuroblastoma cells.

Ray DE ，Sutharsan S ，Forshaw PJ 《NEUROTOXICOLOGY》

Evidence for a separate mechanism of toxicity for the Type I and the Type II pyrethroid insecticides.

Breckenridge CB ，Holden L ，Sturgess N ，Weiner M ，Sheets L ，Sargent D ，Soderlund DM ，Choi JS ，Symington S ，Clark JM ，Burr S ，Ray D ... -  《-》

Effects of the β1 auxiliary subunit on modification of Rat Na(v)1.6 sodium channels expressed in HEK293 cells by the pyrethroid insecticides tefluthrin and deltamethrin.

We expressed rat Nav1.6 sodium channels with or without the rat β1 subunit in human embryonic kidney (HEK293) cells and evaluated the effects of the pyrethroid insecticides tefluthrin and deltamethrin on whole-cell sodium currents. In assays with the Nav1.6 α subunit alone, both pyrethroids prolonged channel inactivation and deactivation and shifted the voltage dependence of channel activation and steady-state inactivation toward hyperpolarization. Maximal shifts in activation were ~18 mV for tefluthrin and ~24 mV for deltamethrin. These compounds also caused hyperpolarizing shifts of ~10-14 mV in the voltage dependence of steady-state inactivation and increased in the fraction of sodium current that was resistant to inactivation. The effects of pyrethroids on the voltage-dependent gating greatly increased the size of sodium window currents compared to unmodified channels; modified channels exhibited increased probability of spontaneous opening at membrane potentials more negative than the normal threshold for channel activation and incomplete channel inactivation. Coexpression of Nav1.6 with the β1 subunit had no effect on the kinetic behavior of pyrethroid-modified channels but had divergent effects on the voltage-dependent gating of tefluthrin- or deltamethrin-modified channels, increasing the size of tefluthrin-induced window currents but decreasing the size of corresponding deltamethrin-induced currents. Unexpectedly, the β1 subunit did not confer sensitivity to use-dependent channel modification by either tefluthrin or deltamethrin. We conclude from these results that functional reconstitution of channels in vitro requires careful attention to the subunit composition of channel complexes to ensure that channels in vitro are faithful functional and pharmacological models of channels in neurons.

He B ，Soderlund DM 《-》