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A rise in [Ca(2+)](i) provides the trigger for neurotransmitter release at neuronal boutons. We have used confocal microscopy and Ca(2+) sensitive dyes to directly measure the action potential-evoked [Ca(2+)](i) in the boutons of Schaffer collaterals. This reveals that the trial-by-trial amplitude of the evoked Ca(2+) transient is bimodally distributed. We demonstrate that "large" Ca(2+) transients occur when presynaptic NMDA receptors are activated following transmitter release. Presynaptic NMDA receptor activation proves critical in producing facilitation of transmission at theta frequencies. Because large Ca(2+) transients "report" transmitter release, their frequency on a trial-by-trial basis can be used to estimate the probability of release, p(r). We use this novel estimator to show that p(r) increases following the induction of long-term potentiation.

Original publication




Journal article



Publication Date





1109 - 1127


Department of Pharmacology, University of Oxford, Mansfield Road, Oxford OX1 3QT, UK.


Hippocampus, Presynaptic Terminals, Animals, Animals, Newborn, Rats, Rats, Wistar, Receptors, N-Methyl-D-Aspartate, Neurotransmitter Agents, Theta Rhythm, Organ Culture Techniques, Male