Transmission at voltage-clamped giant synapse of the squid: evidence for cooperativity of presynaptic calcium action.

Synaptic transmission was studied at the squid giant synapse with voltage clamp control of both presynaptic and postsynaptic cells. Because presynaptic voltage gradients can complicate interpretation of electrophysiological data obtained from this preparation, we used local Ca application to restrict Ca influx and transmitter release to a short and relatively isopotential portion of the elongated presynaptic terminal. Under these conditions, we found that postsynaptic current varies approximately as the third power of presynaptic Ca current. This finding is consistent with the hypothesis that several Ca ions cooperate in triggering secretion of a single transmitter quantum.