Two strategies for the efficient use of synaptic vesicles

The fidelity of neurotransmission at synapses depends on the maintained availability of transmitter-filled synaptic vesicles (SVs) in the presynaptic terminal. The availability of SVs is in turn governed by the number of releasable vesicles in the presynaptic terminal and the rate at which these vesicles are recycled within the terminal. Thus, elucidating these two rate-limiting determinants of the SV cycle is essential for understanding the constraints on information transfer at synapses. Recently, we developed a new functional method for counting releasable SVs in cultured hippocampal autaptic neurons. Extending this method, we show here that, in addition to a larger than previously-thought population of releasable vesicles, presynaptic terminals are endowed with the ability to utilize a rapid local recycling mechanism, thus boosting the efficiency of the SV cycle under high-frequency stimulus conditions.