‘Silent synapses’ show responses from high-affinity NMDA receptors (NMDARs) but not low-affinity AMPA receptors (AMPARs), but gain AMPAR responses upon long-term potentiation (LTP). Using the rapidly reversible NMDAR antagonist l-AP5 to assess cleft glutamate concentration ([glu]cleft), we found that it peaked at ≪170 μM at silent neonatal synapses, but greatly increased after potentiation. Cyclothiazide (CTZ), a potentiator of AMPAR, revealed slowly rising AMPA EPSCs at silent synapses; LTP shortened their rise times. Thus, LTP at silent synapses increased rate-of-rise and peak amplitude of [glu]cleft. Release probability reported by NMDARs remained unchanged during LTP, implying that [glu]cleft increases arose from immediately presynaptic terminals. Our data suggest that changes in the dynamics of fusion-pore opening contribute to LTP.
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This work was supported by the Silvio Conte-NIMH Center for Neuroscience Research at Stanford (R.W.T.), a Dean's Fellowship (S.C.) and a fellowship of the Boehringer Ingelheim Fonds (J.K.). We thank D.V. Madison and P. Mermelstein for comments on the manuscript, N. C. Harata and E. T. Kavalali for discussions and D. Wheeler and D. Profitt for technical support. We are grateful to other members of the Tsien lab for their support.
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