Pyrethroid insecticides: Actions of deltamethrin and related compounds on insect axonal sodium channels

The actions of deltamethrin and eight other pyrethroids were tested on isolated giant axons of the cockroach Periplaneta americana, using microelectrode and oil-gap, single-fibre electrophysiological recording techniques. Deltamethrin at micromolar concentrations induced a slow progressive depolarization of the axon membrane accompanied by a gradual reduction in action potential amplitude. The deltamethrin-induced depolarization was enhanced by an increase in stimulation frequency and was reduced in the presence of the sodium channel blocking agent saxitoxin (1 × 10^?7 M).Other synthetic pyrethroids (biopermethrin and its 1S enantiomer, biotetramethrin, s-bioallethrin, bioresmethrin and its 1S enantiomer, cismethrin and kadethrin) were also studied. In contrast to the findings with deltamethrin all other compounds, apart from the 1S isomers which were inactive, induced prolonged negative (depolarizing) after-potentials. Deltamethrin appears to affect a small fraction of sodium channels which are held in a modified open-state, whereas the pyrethroids which generate large negative after-potentials appear to induce a brief alteration of the open-state sodium channels with a larger number of channels affected. Differences between the actions of pyrethroids on insect axonal sodium channels and whole insects are discussed.