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Synaptic physiology

Getting up to speed on release

Nature Reviews Neuroscience volume 2, page 385 (2001)Cite this article

So, how fast can a conventional synapse release transmitter from vesicular stores? Previous estimates have been surprisingly low, at around 20 vesicles per second per release site; too slow, in fact, to follow the high-frequency firing observed at many central synapses. One possible explanation for this lies in the method used to determine the rate of transmitter release. By monitoring postsynaptic responses as a proxy for presynaptic exocytosis, a number of factors could have led to underestimates of the rate of release; for example, the saturation or desensitization of postsynaptic receptors. A more direct measure of the number of vesicles fusing with the presynaptic membrane can be made at synapses where the nerve terminal is large; in this case, individual fusion events can be detected as an increase in the presynaptic membrane capacitance. Would this approach lead to higher estimates of the speed of release at central synapses?

To address this question, Sun and Wu studied glutamate release at one of the largest synapses in the rat brainstem — the calyx of Held. By recording from the large presynaptic terminal at this synapse, which fires at high frequency in vivo, they were able to make presynaptic capacitance measurements, while simultaneously monitoring the excitatory postsynaptic currents (EPSCs) arising from glutamate release. As they report in Neuron, Sun and Wu found that EPSC amplitudes were saturated when capacitance measurements had only reached 35% of their maximum value, supporting the prediction that measuring presynaptic vesicular fusion does indeed lead to higher estimates of the amount of transmitter released — more than 300 vesicles per second per active zone in the case of this preparation.

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References

ORIGINAL RESEARCH PAPER

  1. Sun, J.-Y. & Wu, L.-G. Fast kinetics of exocytosis revealed by simultaneous measurements of presynaptic capacitance and postsynaptic currents at a central synapse. Neuron 30, 171–182 (2001)

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FURTHER READING

  1. Bollmann, J. H., Sakmann, B. & Borst, J. G. Calcium sensitivity of glutamate release in a calyx-type terminal. Science 289, 953–957 (2000)

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  2. Schneggenburger, R. & Neher, E. Intracellular calcium dependence of transmitter release rates at a fast central synapse. Nature 406, 889–893 (2000)

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Authors
  1. Rebecca Craven
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Craven, R. Getting up to speed on release. Nat Rev Neurosci 2, 385 (2001). https://doi.org/10.1038/35077539

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