Hippocampal long-term potentiation preserves the fidelity of postsynaptic responses to presynaptic bursts

Hippocampal cells often fire prolonged bursts of action potentials, resulti ng in dynamic modulation of postsynaptic responses; yet long-term potentiat ion (LTP) has routinely been studied using only single presynaptic stimuli given at low frequency. Recent work on neocortical synapses has suggested t hat LTP may cause a "redistribution of synaptic strength" in which synaptic responses to the first stimulus of a presynaptic burst of action potential s are potentiated with later responses depressed. We have examined whether this redistribution occurs at hippocampal synapses during LTP. Using prolon ged bursts that result in maximal short-term depression of later responses within the burst, we found that LTP resulted in a uniform potentiation of i ndividual responses throughout the burst rather than a redistribution of sy naptic strength. This occurred both at Schaffer collateral-CA1 synapses and at CA3-CA3 synapses, the latter being activated and monitored using paired recordings. Thus in the hippocampus, LTP preserves the fidelity of postsyn aptic responses to presynaptic bursts by a uniform increase rather than a r edistribution of synaptic strength. a finding that suggests there are impor tant differences between neocortex and hippocampus in how long-term changes in synaptic strength are used to encode new information.