Differential state-dependent modification of inactivation-deficient Nav1.6 sodium channels by the pyrethroid insecticides S-bioallethrin, tefluthrin and deltamethrin.

Pyrethroid insecticides disrupt nerve function by modifying the gating kinetics of transitions between the conducting and nonconducting states of voltage-gated sodium channels. Pyrethroids modify rat Na(v)1.6+β1+β2 channels expressed in Xenopus oocytes in both the resting state and in one or more states that require channel activation by repeated depolarization. The state dependence of modification depends on the pyrethroid examined: deltamethrin modification requires repeated channel activation, tefluthrin modification is significantly enhanced by repeated channel activation, and S-bioallethrin modification is unaffected by repeated activation. Use-dependent modification by deltamethrin and tefluthrin implies that these compounds bind preferentially to open channels. We constructed the rat Na(v)1.6Q3 cDNA, which contained the IFM/QQQ mutation in the inactivation gate domain that prevents fast inactivation and results in a persistently open channel. We expressed Na(v)1.6Q3+β1+β2 sodium channels in Xenopus oocytes and assessed the modification of open channels by pyrethroids by determining the effect of depolarizing pulse length on the normalized conductance of the pyrethroid-induced sodium tail current. Deltamethrin caused little modification of Na(v)1.6Q3 following short (10ms) depolarizations, but prolonged depolarizations (up to 150ms) caused a progressive increase in channel modification measured as an increase in the conductance of the pyrethroid-induced sodium tail current. Modification by tefluthrin was clearly detectable following short depolarizations and was increased by long depolarizations. By contrast modification by S-bioallethrin following short depolarizations was not altered by prolonged depolarization. These studies provide direct evidence for the preferential binding of deltamethrin and tefluthrin (but not S-bioallethrin) to Na(v)1.6Q3 channels in the open state and imply that the pyrethroid receptor of resting and open channels occupies different conformations that exhibit distinct structure-activity relationships.

McCavera SJ ，Soderlund DM 《-》

Divergent actions of the pyrethroid insecticides S-bioallethrin, tefluthrin, and deltamethrin on rat Na(v)1.6 sodium channels.

We expressed rat Na(v)1.6 sodium channels in combination with the rat beta(1) and beta(2) auxiliary subunits in Xenopus laevis oocytes and evaluated the effects of the pyrethroid insecticides S-bioallethrin, deltamethrin, and tefluthrin on expressed sodium currents using the two-electrode voltage clamp technique. S-Bioallethrin, a type I structure, produced transient modification evident in the induction of rapidly decaying sodium tail currents, weak resting modification (5.7% modification at 100 microM), and no further enhancement of modification upon repetitive activation by high-frequency trains of depolarizing pulses. By contrast deltamethrin, a type II structure, produced sodium tail currents that were ~9-fold more persistent than those caused by S-bioallethrin, barely detectable resting modification (2.5% modification at 100 microM), and 3.7-fold enhancement of modification upon repetitive activation. Tefluthrin, a type I structure with high mammalian toxicity, exhibited properties intermediate between S-bioallethrin and deltamethrin: intermediate tail current decay kinetics, much greater resting modification (14.1% at 100 microM), and 2.8-fold enhancement of resting modification upon repetitive activation. Comparison of concentration-effect data showed that repetitive depolarization increased the potency of tefluthrin approximately 15-fold and that tefluthrin was approximately 10-fold more potent than deltamethrin as a use-dependent modifier of Na(v)1.6 sodium channels. Concentration-effect data from parallel experiments with the rat Na(v)1.2 sodium channel coexpressed with the rat beta(1) and beta(2) subunits in oocytes showed that the Na(v)1.6 isoform was at least 15-fold more sensitive to tefluthrin and deltamethrin than the Na(v)1.2 isoform. These results implicate sodium channels containing the Na(v)1.6 isoform as potential targets for the central neurotoxic effects of pyrethroids.

Tan J ，Soderlund DM 《-》

Differential state-dependent modification of rat Na(v)1.6 sodium channels expressed in human embryonic kidney (HEK293) cells by the pyrethroid insecticides tefluthrin and deltamethrin.

We expressed rat Na(v)1.6 sodium channels in combination with the rat β1 and β2 auxiliary subunits in human embryonic kidney (HEK293) cells and evaluated the effects of the pyrethroid insecticides tefluthrin and deltamethrin on expressed sodium currents using the whole-cell patch clamp technique. Both pyrethroids produced concentration-dependent, resting modification of Na(v)1.6 channels, prolonging the kinetics of channel inactivation and deactivation to produce persistent "late" currents during depolarization and tail currents following repolarization. Both pyrethroids also produced concentration dependent hyperpolarizing shifts in the voltage dependence of channel activation and steady-state inactivation. Maximal shifts in activation, determined from the voltage dependence of the pyrethroid-induced late and tail currents, were ~25mV for tefluthrin and ~20mV for deltamethrin. The highest attainable concentrations of these compounds also caused shifts of ~5-10mV in the voltage dependence of steady-state inactivation. In addition to their effects on the voltage dependence of inactivation, both compounds caused concentration-dependent increases in the fraction of sodium current that was resistant to inactivation following strong depolarizing prepulses. We assessed the use-dependent effects of tefluthrin and deltamethrin on Na(v)1.6 channels by determining the effect of trains of 1 to 100 5-ms depolarizing prepulses at frequencies of 20 or 66.7Hz on the extent of channel modification. Repetitive depolarization at either frequency increased modification by deltamethrin by ~2.3-fold but had no effect on modification by tefluthrin. Tefluthrin and deltamethrin were equally potent as modifiers of Na(v)1.6 channels in HEK293 cells using the conditions producing maximal modification as the basis for comparison. These findings show that the actions of tefluthrin and deltamethrin of Na(v)1.6 channels in HEK293 cells differ from the effects of these compounds on Na(v)1.6 channels in Xenopus oocytes and more closely reflect the actions of pyrethroids on channels in their native neuronal environment.

He B ，Soderlund DM 《-》

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 《-》

Human and rat Nav1.3 voltage-gated sodium channels differ in inactivation properties and sensitivity to the pyrethroid insecticide tefluthrin.

Tan J ，Soderlund DM 《-》