Synaptic plasticity is thought to be a fundamental process of learning and memory. It is well known that presynaptic active zone size and postsynaptic PSD size relate to their functions, and the active zone size and the PSD size in a synapse is positively correlated. Thus, pre- and postsynaptic functions should be balanced after synaptic plasticity. Especially, after the postsynaptic plasticity, such as spine enlargement by repetitive two-photon glutamate uncaging, information should flow in a retrograde manner. However, how presynaptic and postsynaptic terminal communicate during the synaptic plasticity is still poorly understood even in hippocampal pyramidal neurons. It is partly because, fundamentally a measurement of release probability takes several tens of seconds or more, and a method for real time observation of presynaptic function had not been developed.
Here we report a fluorescent probe 'iSLIM' for immediate measurement of a presynaptic function based on FRET (Forster resonance energy transfer)-FLIM (fluorescence lifetime image microscopy) method. We fused mTurquoise and syntaxin genes as a FRET donor, and Venus and VAMP2 as an acceptor. We transfected the donor and acceptor cDNAs in a same neuron. The docking of synaptic vesicles to presynaptic active zones, which means maturation or increase of presynaptic functions, changed the fluorescence lifetime of the probe. We expressed the probe in hippocampus CA3 pyramidal neurons which send axons to CA1 region where we induced synaptic plasticity in a dendrite.
We will show the time course of presynaptic plasticity after the dendritic spine geometric plasticity induced by repetitive caged glutamate uncaging or by electric stimulations.