Processes Underlying One Form of Synaptic Plasticity: Facilitation

Facilitation is one of the most prevalent forms of synaptic plasticity, and is often invoked as a quality which is important in the nervous system’s ability to generate adaptive behavior. The squid giant synapse provides an excellent opportunity to explore the biophysical mechanism of synaptic facilitation. Previous studies showed that facilitation is not due to changes in presynaptic action potentials or after-potentials. Evidence summarized here indicates that facilitation is also not a consequence of presynaptic calcium channel properties, nor is it a reflection of growing increments in presynaptic calcium concentration with repeated activity. Moreover, arsenazo III absorbance microspectrophotometry has revealed a residual calcium following presynaptic activity, and injection of calcium presynaptically facilitates spike-evoked transmitter release. A nonlinear relation between calcium and transmitter release is demonstrated, and this plus a mathematical model of diffusive calcium movements within the presynaptic terminal account for both the time course of transmitter release and the magnitude and decay of facilitation following an action potential.