ACTIVATION OF POSTSYNAPTICALLY SILENT SYNAPSES DURING PAIRING-INDUCEDLTP IN CA1 REGION OF HIPPOCAMPAL SLICE

LONG-TERM potentiation (LTP) is an enhancement of synaptic strength th at can be produced by pairing of presynaptic activity with postsynapti c depolarization(1). LTP in the hippocampus has been extensively studi ed as a cellular model of learning and memory, but the nature of the u nderlying synaptic modification remains elusive, partly because our kn owledge of central synapses is still limited(2,3). One proposal(4,5) i s that the modification is postsynaptic, and that synapses expressing only NMDA (N-methyl-D-aspartate) receptors before potentiation are ind uced by LTP to express functional AMPA a-amino-3-hydroxy-5-methyl-4-is oxazoleproprionate) receptors. Here we report that a high proportion o f synapses in hippocampal area CAI transmit with NMDA receptors but no t AMPA receptors, making these synapses effectively non-functional at normal resting potentials. These silent synapses acquire AMPA-type res ponses following LTP induction, Our findings challenge the view(6-10) that LTP in CA1 involves a presynaptic modification, and suggest inste ad a simple postsynaptic mechanism for both induction and expression o f LTP.