Suppression of transmitter release at the neuromuscular junction

Tetrodotoxin and tetraethylammonium were used on frog and rat endplates, to inhibit voltage-activated sodium and potassium currents. Under these conditions, large endplate potentials can be evoked by applying super-threshold depolarizing current pulses (of 50-100 ms duration) to the motor nerve endings. When the current intensity is increased several­ fold above threshold, the response is suppressed during the current flow, and delayed until the pulse is terminated. This phenomenon is similar to and complements earlier observations on the squid giant synapse.