Effects of type I and type II pyrethroids on the startle response in rats.

The effects of type I and type II pyrethroids on the acoustic startle response of male Wistar rats were studied in three experiments. Pyrethroids were administered p.o. in 0.2 ml corn oil. In Expt. 1, techn. cis-permethrin (0, 30, 60, 90 mg/kg) and techn. cis, trans-cypermethrin (0, 60, 90, 120 mg/kg) increased startle in a dose-dependent way, whereas overt behavior and habituation of the startle response were not affected. In Expt 2, NAK 1901 (0, 1, 2.5, 4 mg/kg) enhanced startle amplitudes dose dependently and deltamethrin (0, 2, 4, 6 mg/kg) had no effect on startle. After both deltamethrin and NAK 1901 toxic signs increased dose dependently but an effect on potentiation of the startle reflex was not found. In Expt. 3, deltamethrin (0, 2, 4, 6 mg/kg) was administered and the highest dose attenuated startle amplitudes significantly, whereas toxic signs increased dose dependently. Deltamethrin had no effect on habituation of the startle response. It is concluded that type I pyrethroids increase the amplitude of the startle reflex; this effect does not depend on the presence of overt signs of toxicity. Type II pyrethroids have dissimilar effects on startle. Results are discussed suggesting that the effects of type II pyrethroids on startle amplitude are mediated by an indirect metabolic effect and a direct effect at the site of the muscle.