Spontaneous eIPSPs are rare events in CA1.

A. Simultaneous recording of spontaneous activity in the CA3 and CA1 regions of a mouse in control (top traces, K⁺3 mM) and high K⁺ (lower traces, K⁺5 mM). The upper CA1 trace was specifically selected to illustrate a CA1 eIPSP, which were very rare events in CA1, even in presence of high-K⁺ induced intense multi-unit activity (lower CA1 trace). B–C. Relationship between multi-unit activity and eIPSP frequency (B, mice, n = 6; C, rats, n = 3), measured in CA3 (blue diamonds) and CA1 (white diamonds) simultaneously, in presence of various extracellular K⁺ concentrations (range 3–8 mM). Note that eIPSP frequency in CA1 remains low whatever the frequency of multi-unit activity. D. Superimposed traces of rat CA1 eIPSPs that either occured spontaneously (CA1-spontaneous, upper traces) or were evoked by minimal extracellular electrical stimulation close to the CA1 pyramidal layer (CA1-evoked, lower traces). E. Crosscorrelogram between CA1-MUA and CA3-eIPSPs (rats, n = 3).