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

Abstract

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.

Figure 1

Structures and isomeric compositions of S-bioallethrin, tefluthrin and deltamethrin.

Figure 2

Representative control and tefluthrin (100 μM)-modified sodium currents recorded from an oocyte expressing rat Na_v1.6 sodium channels using the indicated depolarization protocol. The dashed line indicates zero current. Values for the kinetic parameters τ_fast, τ_slow, and τ₁ are summarized in Table 1.

Figure 3

Late currents measured in the absence and presence of pyrethroids (100 μM). Currents were recorded at the end of a 40-ms step depolarization from −100 mV to 0 mV. Values are means ± SE of 37 (control), 3 (S-bioallethrin), 11 (tefluthrin) or 12 (deltamethrin) separate experiments with different oocytes. Asterisks indicate values significantly different from control (P < 0.01; one-way ANOVA with Dunnett’s post-hoc analysis).

Figure 4

Representative sodium tail currents recorded from oocyte expressing rat Na_v1.6 sodium channels following exposure to S-bioallethrin, tefluthrin or deltamethrin (100 μM). Traces show currents recorded beginning 0.5 ms after repolarization to −100 mV from a step depolarization to 0 mV. The dashed line indicates zero current.

Figure 5

Effects of S-bioallethrin, tefluthrin or deltamethrin (100 μM) on the voltage dependence of activation of rat Na_v1.6 sodium channels expressed in oocytes. Conductances of peak transient sodium currents measured upon depolarization from −100 mV to a range of test potentials are plotted as a function of test potential. Values are means; of 19 (control), 4 (S-bioallethrin), 6 (tefluthrin) or 9 (deltamethrin) separate experiments with different oocytes; bars show SE values larger than the data point symbols. Conductance – voltage curves were fitted to mean conductance values using the Boltzmann equation.

Figure 6

(A) Effects of S-bioallethrin, tefluthrin or deltamethrin (100 μM) on the voltage dependence of steady-state inactivation of rat Na_v1.6 sodium channels expressed in oocytes. Amplitudes of peak transient currents obtained during a 40-ms test depolarization to 0 mV following 100-msec conditioning prepulses from −140 mV to a range of conditioning potenticals are plotted as a function of prepulse potential. Values are means of 19 (control), 4 (S-bioallethrin), 6 (tefluthrin) or 9 (deltamethrin) separate experiments with different oocytes; bars show SE values larger than the data point symbols. The dashed line indicates zero current (complete inactivation). Current – voltage curves were fitted to mean current amplitude values using the Boltzmann equation. (B) Effects of S-bioallethrin, tefluthrin or deltamethrin (100 μM) on the amplitude of the inactivation-resistant component of sodium current during a 40-ms test depolarization to 0 mV following a 100-ms conditioning prepulse to 20 mV. Values are means ± SE as in Fig. 6A. Asterisks indicate values significantly different from control (P < 0.01; one-way ANOVA with Dunnett’s post-hoc analysis).

Figure 7

Representative traces recorded from oocytes expressing Na_v1.6 sodium channels exposed to tefluthrin (top) or deltamethrin (bottom) using the indicated pulse protocol. Control traces were recorded prior to insecticide exposure. Following equilibration with insecticide traces were recorded before or after the application of a high-frequency train of 100 depolarizing prepulses.

Figure 8

(A) Effect of repeated depolarizing prepulses on the extent of modification of rat Na_v1.6 sodium channels by S-bioallethrin, tefluthrin and deltamethrin. Values are means ± SE of 4 (S-bioallethrin), 15 (tefluthrin) or 12 (deltamethrin) separate experiments with different oocytes. (B) Comparison of the extent of resting (after 0 prepulses) and maximal use-dependent (after 100 prepulses) modification of rat Na_v1.6 sodium channels by S-bioallethrin, tefluthrin and deltamethrin. Values for use-dependent modification marked with asterisks are significantly different from values for the resting modification by the same compound (paired t-tests, P < 0.05).

Figure 9

Concentration dependence of resting (0 prepulses) and use-dependent (100 prepulses) modification of Na_v1.6 sodium channels by tefluthrin and deltamethrin. Values are means ± SE of 5 separate experiments with each compound. The dashed line indicates 10% channel modification.

Figure 10

(A) Effect of repeated depolarizing prepulses on the extent of modification of rat Na_v1.2 sodium channels by tefluthrin and deltamethrin. Values are means ± SE of 8 (tefluthrin) or 12 (deltamethrin) separate experiments with different oocytes. (B) Concentration dependence of resting (0 prepulses) and use-dependent (100 prepulses) modification of Na_v1.2 sodium channels by tefluthrin and deltamethrin. Values are means ± SE of 5 separate experiments with each compound. The dashed line indicates 10% channel modification.

Figure 11

Comparison of resting (0 prepulses) and use-dependent (100 prepulses) modification of four rat sodium channel isoforms by 100 μM tefluthrin. The Na_v1.2, Na_v1.3 and Na_v1.6 isoforms were expressed with the rat β1 and β2 subunits whereas the Na_v1.8 subunit was expressed without β subunits. Data for Na_v1.6 and Na_v1.2 channels are from this study (Figs. 8A and 10A); data for Na_v1.3 and Na_v1.8 channels are from previous studies in this laboratory (Choi and Soderlund, 2006; Tan and Soderlund, 2009) and are means ± SE of 6 (Na_v1.3) and 3 (Na_v1.8) separate experiments with different oocytes. Values for use-dependent modification marked with asterisks are significantly different from values for the resting modification of the same channel (paired t-tests, P < 0.05).