Synaptic vesicle pools: Classical and emerging roles

Abstract

Synaptic vesicles display a marked functional specialisation that warrants the definition of the so-called synaptic vesicle pools. In the classical three-pool model, the readily releasable pool (1–2 % of all vesicles) provides fast initial neurotransmitter release, the recycling pool (10–20 %) maintains release during physiological levels of stimulation, and the reserve pool (~80 %) is inert in terms of neurotransmitter release under physiological stimulation. But this classification according to release propensity does not convey the whole range of functional versatility of synaptic vesicles. Recent research has demonstrated that the release-incompetent reserve pool may act as a buffer for soluble proteins essential to synaptic vesicle recycling. Furthermore, new pool concepts have been proposed which illustrate several emerging aspects of synaptic vesicle pool physiology. A super-pool of vesicles is exchanged between synapses, with potential implications for synaptic plasticity. A surface or readily retrievable pool of synaptic vesicles decorates the membrane of synaptic boutons and may be essential for maintaining the recycling pool through rapid compensatory endocytosis. Recent data on the pool of spontaneously releasing vesicles suggests that there may be more functional and molecular heterogeneity among synaptic vesicles than anticipated. Finally, understanding the regulation of pool transitions remains a largely unresolved issue.