The mechanisms by which long-term potentiation (LTP) is expressed are controversial, with evidence for both presynaptic and postsynaptic involvement. We have used confocal microscopy and Ca2+-sensitive dyes to study LTP at individual visualized synapses. Synaptically evoked Ca2+ transients were imaged in distal dendritic spines of pyramidal cells in cultured hippocampal slices, before and after the induction of LTP. At most synapses, from as early as 10 min to at least 60 min after induction, LTP was associated with an increase in the probability of a single stimulus evoking a postsynaptic Ca2+ response. These observations provide compelling evidence of a presynaptic component to the expression of early LTP at Schaffer-associational synapses. In most cases, the store-dependent evoked Ca2+ transient in the spine was also increased after induction, a novel postsynaptic aspect of LTP.
