Figure 5: Repetitive stimulation progressively unmutes PC–MC excitatory synapses.

(a) Thirty action potentials elicited at 10 Hz (left) and at 30 Hz (right) in a PC (blue traces). Voltage clamp traces from a connected MC at −65 mV. Three successive trials in green, average of eight trials in red. The inter-trial-interval here was 40 s. Below, raster plots of synaptic transfer for eight trains show presynaptic action potentials (blue bars), and EPSCs (red dots) triggered at monosynaptic latencies (0–3 ms). More EPSCs were elicited during the 30 Hz than the 10 Hz train and by late (last five) compared to early spikes (first five). (b) Detail of early and late PC spikes and MC-responses in 30 Hz trains. The transmission transfer rate was higher for late stimuli. (c) Poststimulus-histogram of EPSCs at monosynaptic latencies, of 0–3 ms, show peaks at 1.16 and 1.14 ms (median) for trains at 10 and 30 Hz. Total counts were higher for 30 than 10 Hz, due to the frequency dependence of release. (d) PC–MC synaptic efficacy (transfer rate × absolute potency) showed a strong dependence on spike number during a train and spike frequency (n=9, Friedman test, P=0.0002). Facilitation occurred at 30 Hz but not at 10 Hz (*Dunn’s multiple comparison test, P<0.05). (e) Late/early transfer rate and potency plotted against late/early efficacy (n=15 pairs, 30 Hz stimulation). Increased efficacy resulted from a higher transfer rate rather than changes in potency. (f) Non-linear cumulative efficacy (mean±s.e.m., summed efficacy over time) plotted against time shows facilitation dynamics and frequency dependence. (g) The synaptic frequency increased more than the 3-fold change in presynaptic spike frequency (red dashed line), for both early and late spikes, during 10 and 30 Hz trains. Stimulus artifacts blanked in (a,b). For detailed statistics, see Supplementary Table 3.