Fig. 3.

PP stimulation producing population spike activity elicits an initial NMDA receptor-dependent potentiation preceding the depression. A–D, Summary plots as described in Results and in Figure 1. Insets are averages of three sweeps taken from representative experiments, at the time indicated by the characters. Calibration: 0.2 mV, 10 msec. A, PP stimulation (interstimulus interval 10–50 msec) associated with evident population spikes leading to an initial potentiation (n = 9). B, Data from experiments in which PP stimulation did (a;n = 5), and did not (b;n = 5) give rise to population spike activity are shown superimposed. Only nine consecutive PP stimulations (interstimulus interval 10 msec) were given.C, Nine consecutive PP stimulations (interstimulus interval 10 msec) associated with population spike activity were given in the presence of 50 μmd-AP-5 (n = 7). Note the absence of an initial potentiation. D, The same but in the presence of 100 μm Ni²⁺ (n = 2). Note the large initial potentiation.