Postfusional regulation of cleft glutamate concentration during LTP at 'silent synapses'.

'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 microM 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.