All or none block of single Na⁺ channels by tetrodotoxin

The effects of tetrodotoxin on single Na⁺-channel currents recorded from excised patches of neuroblastoma cells were examined. Tetrodotoxin was found to cause a dose-dependent reduction in the frequency at which Na⁺ channels conduct during a series of depolarizations. Surviving conducting states had normal open times and current amplitudes. These effects could be explained by a model which includes initial binding of tetrodotoxin to a closed state of the channel with stable, complete block during the time the channel would normally be gated open.