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Figure 5. Presynaptic and postsynaptic Ca2+ influx at single synaptic contacts recorded by using two-photon excitation fluorescence microscopy. A, top, Fluorescence image of a pyramid-to-bitufted cell pair filled with rhod-2 and OGB-1, respectively. The pyramidal neuron is shown in red, the interneuron in green. The region with an identified synaptic contact is marked by the rectangle and shown at higher magnification at the bottom. The arrowhead points to a synaptic contact. The presynaptic axon is hardly visible, but lightens up during Ca2+ influx (see B). Line scans shown in B were performed at the region indicated by the white vertical line. B, Fluorescence line scans recorded during unitary synaptic stimulation on the presynaptic bouton (red) and the postsynaptic dendrite (green). In the pyramidal neuron three APs were evoked at 10 Hz (see C). C, Each of the three APs evoked in the pyramidal neuron gave rise to a Ca2+ influx into the presynaptic terminal (top traces, same experiment shown in B). The postsynaptic neuron responded with two EPSPs, but only during one was a postsynaptic Ca2+ signal also measured (lower traces). Presumably, the second EPSP was evoked at another contact. Fluorescence data were not filtered. Note: No Ca2+ transient was recorded on the other dendrite also covered by the line scan (see B), indicating the synaptic origin of the Ca2+ signals.
