(A) Diagram illustrating the configuration for the experiment. SLM: stratum lacunosum-moleculare, SR: stratum radiatum, SP: stratum pyramidale. S: stimulating electrode, R: recording electrode. (B) Sample traces of EPSPs evoked by PP stimuli using an electrode placed in SLM of CA1. (C) Mean input–output curves of somatic EPSPs in CA1 (n = 5) and CA2 (n = 6-12) PNs. (D) Simultaneous whole-cell recording from a CA2 PN (top) and extracellular field potential recording from the CA2 cell body layer (bottom) in the absence (black traces) or the presence (red traces) of 20-μM NBQX and 50-μM D-APV. Top: somatic AP evoked by a single PP stimulus. Bottom: extracellular population spike (PS) in CA2 cell body layer. (E) Histogram (bars) and cumulative plot (circles) of PP stimulus threshold required to evoke APs in different CA2 PNs (n = 42 cells). (F) Mean input–output curves of PS in CA1 (n = 5), CA2 (n = 12), and CA3b (n = 6) cell body layers evoked by single PP stimuli. Inset: sample traces of PS in response to a single PP stimulus recorded in CA1, CA2, and CA3b cell body layers. The noise level was measured using a section of baseline that did not exhibit a PS. (G) Sample traces of sub-threshold and AP waveforms in response to somatic current injection (I) and PP stimulation (PP). The dashed lines indicate mean minimal somatic depolarization required to fire APs in response to somatic current injection (black) and PP stimulation (red), respectively. (H) The pooled data from CA2 PNs which fire APs in response to PP stimulation (n = 27). Peak somatic voltage amplitude plotted against stimulating intensity. (I) Expanded view of sub-threshold EPSP response in (H). Note, the dashed lines in (H) and (I) indicate mean minimal somatic depolarization required to fire APs in response to somatic current injection (black, 27.2 ± 1.2 mV, n = 5) and PP stimulation (red, 15.6 ± 0.6 mV, n = 27), respectively.