Figure 2. Recordings in hippocampal slices show evidence of trisynaptic and backprojecting pathways.

A. Extracellular recordings of evoked responses to 5 stimulation sites (1–5) and 4 recording sites (A–D). Recordings were made sequentially in the same slice in response to a fixed stimulus for each stimulus site.

B. A schematic is used to allow better comparisons of fimbria-evoked and outer molecular layer-evoked responses recorded at four sites in the same slices: the outer molecular layer, granule cell layer, CA3c cell layer, and CA3b cell layer. X marks recording site locations.

C. Comparison of evoked responses to fimbria stimulation in the same slice demonstrate the responses in the granule cell layer following pyramidal cell activation by the fimbria, but do so with a delay. Superimposition of responses illustrates that the onset of the granule cell layer field potential begins approximately 1–2 ms after the antidromic population spike recorded in area CA3b. This delay suggests an intermediary synapse, probably in CA3c or the hilus, between CA3b and granule cells. Indeed, the granule cell field potential begins immediately after the CA3c orthodromic population spike, which probably is due to CA3b recurrent excitation of CA3c pyramidal cells. At the same time, hilar cells are also activated and innervate granule cells (see later figures), although CA3c pyramidal cells could innervate granule cells also (Li et al., 1994). The bottom trace is an IPSP recorded intracellularly from a granule cell in the same slice in response to the same stimulus. It shows that the onset of the IPSP is similar to the onset of the granule cell layer field potential, which likely reflects the average of many IPSPs in granule cells situated around the extracellular electrode.