Augmentation controls the fast rebound from depression at excitatory hippocampal synapses

Short-term plasticity occurs at most central chemical synapses and includes both positive and negative components, but the principles governing interaction between components are largely unknown. The residual Ca2+ that persists in presynaptic terminals for several seconds after repetitive use is known to enhance neurotransmitter release under artificial, low probability of release conditions where depression is absent; this is termed augmentation. However, the full impact of augmentation under standard conditions at synapses where depression dominates is not known because of possibly complicated convolution with a variety of potential depression mechanisms. This report shows that residual Ca2+ continues to have a large enhancing impact on release at excitatory hippocampal synapses recovering from depression, including when only recently recruited vesicles are available for release. No evidence was found for gradual vesicle priming or for fast refilling of a highly releasable subdivision of the readily releasable pool (RRP). And decay of enhancement matched the clearance of residual Ca2+, thus matching the behavior of augmentation when studied in isolation. Because of incomplete RRP replenishment, synaptic strength was not typically increased above baseline when residual Ca2+ levels were highest. Instead residual Ca2+ caused single pulse release probability to rebound quickly from depression and then depress quickly during subsequent bursts of activity. Together, these observations can help resolve discrepancies in recent timing estimates of recovery from depression. Additionally, in contrast to results obtained under reduced release conditions, augmentation could be driven to a maximal level, occluding paired-pulse facilitation and other mechanisms that increase release efficiency.