Refined electrophysiological analysis suggests that a depressant toxin is a sodium channel opener rather than a blocker

The effects of a recombinant depressant insect toxin from Leirurus quinquestriatus hebraeus, Lqh IT2-r, have been studied in current and voltage-clamp conditions on the isolated axonal and DUM neuron preparations of the cockroach Periplaneta americana. Lqh IT2-r depolarizes the axon, blocks the evoked action potentials, and modifies the amplitude and the kinetics of the sodium current. The inward transient peak current is greatly decreased and is followed by a maintained slow activating-deactivating sodium current. The slow component develops at membrane potentials more negative than the control, and has a time constant of activation of several tens of milliseconds. The flaccid properties of Lqh IT2-r do not correspond to a blockage of the Na⁺ channels, but may be attributed to modified Na⁺ channels which open at more negative potential, activate slowly and do not inactivate normally.