Transmitter release by graded local depolarization of presynaptic nerve terminals at the crayfish neuromuscular junction.

Synaptic currents were recorded at single nerve terminals on the crayfish opener muscle by means of a patch-clamp electrode. Current pulses depolarizing the terminal were applied through the electrode which caused release of transmitter quanta. Such 'pulse-elicited excitatory postsynaptic currents' (pEPSCs) were not affected by the presence of tetrodotoxin, and no antidromic action potentials were detected in the motor nerve fiber after terminal depolarizations eliciting maximal pEPSCs. The amplitude of pEPSCs was graded in a wide range depending on amplitude and duration of the pulse, with different quantitative relationships for 'fast' and 'slow' synapses. It appears that these nerve terminals are inexcitable and are depolarized by the electronic spread of the motor nerve action potential.