Spontaneous serotonergic EPSCs. A, Spontaneous synaptic activity recorded from a layer 4 interneuron.B, Synaptic currents were blocked by bath application of the 5-HT₃ receptor antagonist ICS 205-930.C, Cumulative amplitude distribution of serotonergic EPSCs. Addition of the 5-HT₃ receptor antagonist ICS 205-930 (10 nm) shifted the distribution to the left (p < 0.001, one-way ANOVA; number of events analyzed = 85 and 31). D, Cumulative frequency distribution of serotonergic events. Addition of the 5-HT₃receptor antagonist ICS 205-395 (10 nm) shifted the amplitude distribution toward longer interevent intervals (p < 0.001, one-way ANOVA; number of events analyzed = 85 and 30). PTX, Picrotoxin.