Long-term potentiation is associated with increases in quantal content and quantal amplitude

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Abstract
LONG-TERM potentiation (LTP) of synaptic transmission in CA1 neurons of the hippocampus, elicited by the conjunction of presynaptic firing and postsynaptic depolarization, is an important model of plasticity, which may underlie memory storage1–3. Although induction of LTP takes place in the postsynaptic cell4–7, it is not clear whether it is expressed through an enhancement of transmitter release8–12 or through an increased postsynaptic response to the same amount of transmitter13–6. Analysis of the trial-to-trial amplitude fluctuations of synaptic signals, that is quantal analysis, gives an important insight into the probabilistic mechanisms of transmission, although attempts to apply it to the mode of expression of LTP have so far yielded inconsistent results9–12,15, at least in part because they have relied on models of transmitter release that have not been confirmed experimentally17–19. Here we report clear evidence for quanta! fluctuation in a subset of cells. Induction of LTP in these cells causes abrupt increases in either quantal content or quantal amplitude, or both. This shows that two different mechanisms can underlie the maintenance of LTP.