Effect of pyrethroid (allethrin and fenvalerate) on excitability changes following nerve impulse

Pyrethroid insecticides induce two different clinical symptoms. The difference has been reported to depend on the effect on the sodium channel opening time. We performed the present study to confirm this report by a double stimulation method. Allethrin (type I) 5 mg/kg and fenvalerate (type II) 1.5 kg/mg were injected into pentobarbital anesthetized rats. Tail nerve excitability was examined by a double stimulation method at from 4 msec to 40 msec intervals. The excitability was expressed as a percentage of the response to conditioned stimuli following supernormal stimuli to response to unconditioned stimuli. In control rats, the largest excitability was 94.6 +/- 19.2% at an 8 msec stimulus interval. After allethrin injection, we obtained two types of excitability changes. In some rats, supernormal excitability was present from 4 msec to 15 msec up to 120%. In other rats, the excitability was lower than that in the control from 4 msec to 15 msec. In fenvalerate treated rats, there was supernormal excitability from 4 msec to 40 msec. The peak of excitability was 122.4 +/- 35.3% at 10 msec. By double stimulation methods, we obtained characteristic nerve excitability changes in allethrin and fenvalerate treated rats after symptoms disappeared. These results showed that it should be possible to detect human nerve subclinical changes following lower doses or during chronic exposure.