Presynaptic holding potential changes alter the pattern of PSC amplitudes at MLI–MLI synapses during train stimulation. (A) Scheme describing the experimental procedure. Synaptically connected MLIs were recorded under whole-cell configuration. The presynaptic cell was cyclically stimulated with a pattern given at −60 (black), −50 (red), or −80 mV (blue), consisting of a 9-s prepulse followed by a train of APs at 25 or 50 Hz, whereas the postsynaptic cell was kept at −60 mV. (B and C) Representative experiment from a presumed six–docking sites synapse. (B) Top: Consecutive postsynaptic responses to trains of presynaptic APs (25 Hz; timing indicated by vertical dotted gray lines) grouped according to the presynaptic holding potential. Bottom: Mean PSCs showing increased synaptic depression with depolarizing prepulse and decreased synaptic depression with hyperpolarizing prepulse. (C) Postsynaptic mean amplitudes as a function of stimulus time from the experiment shown in B. (D) Summary results from 12 synapses stimulated at 25 Hz (left) and 8 synapses stimulated at 50 Hz (right). Peak responses for different holding potentials are similar for i = 1 (first AP) and at the end of the train but differ for the second and third APs. (insets) Expanded view of PSC amplitudes starting from the second AP in the −50- (red) and −80-mV (blue) protocols. ±SEM intervals are indicated by shaded colored areas in C and D, main panels, and by error bars in D, insets. Note that individual entries represent trials from one experiment in C and means from various experiments in D. In C and D, statistically significant differences from the results at a holding potential of −60 mV are indicated as follows: *, P < 0.05; **, P < 0.01.