Abstract: Effects of the optically-active pyrethroids, alpha-cypermethrin and thetacypermethrin upon the voltage dependent sodium channels of central neurons of larval Helicoverpa armigera (Hübner) were studied using the whole-cell patch clamp technique. Results show that the sodium currents (I_Na) of central neurons increased quickly after the application of alpha-cypermethrin. At the same time, the activation voltage of the sodium channel shifted about 10 mV in the direction of hyperpolarization, which indicated that alpha-cypermethrin caused sodium channels of larval H. armigera to be more easily activated. After treatment with alphacypermethrin for 10－15 min, I_Na decreased quickly, which revealed that alphacypermethrin also had an effect on opening sodium channels. The latency to peak current was shortened after the application of alpha-cypermethrin, while the time constants of decay did not change. The activation voltage of the sodium channel also shifted towards more negative potentials (around 10 mV) after the application of theta-cypermethrin. I_Na decreased slowly during the action of thetacypermethrin and more time was required to depress it. In conclusion, these data indicate that alpha cypermethrin and theta-cypermethrin have effects on both closed and open states of sodium channels in the central neurons of larval H. armigera and that alpha-cypermethrin is the more effective of these two pyrethroids.

Key words: Helicoverpa armigera, neuron, sodium channel, opt