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Trends Neurosci. 2010 Jun;33(6):259-66. doi: 10.1016/j.tins.2010.03.003. Epub 2010 Apr 3.

The low synaptic release probability in vivo.

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Department of Neuroscience, Erasmus MC, University Medical Center Rotterdam, 3015 GE Rotterdam, The Netherlands.


The release probability, the average probability that an active zone of a presynaptic terminal releases one or more vesicles following an action potential, is tightly regulated. Measurements in cultured neurons or in slices indicate that this probability can vary greatly between synapses, but on average it is estimated to be as high as 0.5. In vivo, however, the size of synaptic potentials is relatively independent of recent history, suggesting that release probability is much lower. Possible causes for this discrepancy include maturational differences, a higher spontaneous activity, a lower extracellular calcium concentration and more prominent tonic inhibition by ambient neurotransmitters during in vivo recordings. Existing evidence thus suggests that under physiological conditions in vivo, presynaptic action potentials trigger the release of neurotransmitter much less frequently than what is observed in in vitro preparations.

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