Presynaptic potentials and facilitation of transmitter release in the squid giant synapse
Abstract
Presynaptic potentials were studied during facilitation of transmitter
release in the squid giant synapse. Changes in action potentials were found to
cause some, but not all, of the facilitation during twin-pulse stimulation. During
trains of action potentials, there were no progressive changes in presynaptic action
potentials which could account for the growth of facilitation. Facilitation could still
be detected in terminals which had undergone conditioning depolarization or
hyperpolarization. Facilitation could be produced by small action potentials in low
[Ca++]o and by small depolarizations in the presence of tetrodotoxin. Although the
production of facilitation varied somewhat with presynaptic depolarization, nevertheless,
approximately equal amounts of facilitation could be produced by depolarizations
which caused the release of very different amounts of transmitter.