Figure 10.

Laser scanning photostimulation reveals that CA2 excitatory pyramidal neurons receive excitatory synaptic input from CA3 cells but not from dentate granule cells. A: The mouse hippocampal slice image with the superimposed photostimulation sites (16 × 16 cyan dots, spaced at 100 μm × 100 μm). The cell body locations of the sequentially recorded CA2 and distal CA3 cells are indicated by the red and yellow circles, respectively. The short black lines indicate the borders of CA2. B: The pyramidal cell morphology of the recorded cells. The yellow arrow points to the distal CA3 cell bordering CA2. C: Intrasomatic current injection responses of the two recorded cells. D: An array of photostimulation-evoked response traces from the corresponding sites in A, with the recorded CA2 cell held in voltage clamp mode to detect inward excitatory postsynaptic currents (EPSCs). The red circle indicates the recorded cell body location of the CA2 cell. Only the 250 ms of the recorded traces after the onset of laser photostimulation are shown. F: Different forms of photostimulation responses are illustrated by the traces, which are expanded and shown separately. Trace 1 is an example of a large direct response (excluded for further analysis) to glutamate uncaging on the cell body. Trace 2 shows an example of relatively small direct response, with over-riding synaptic responses (blue). Trace 3 is a typical example of synaptic input responses. E: The color-coded, averaged input map constructed from D (with each square corresponding to one stimulation site) superimposed with the hippocampal contour, illustrating the pattern and strength of synaptic input to the recorded neuron. The input amplitude from each stimulation site is a measurement of average integrated strength of individual EPSCs in the specified analysis window, with the baseline spontaneous response subtracted from the photostimulation response of the same site. The numbered sites correspond to the illustrated response traces. G–I: Photostimulation-based circuit mapping for the distal CA3 cell, as formatted in D–F. Scale bar = 50 μm in B; 200 μm in E (applies to E,H).