Figure 2. The effect of DSI stimulation on VSI-B-evoked EPSPs was dependent on the time between the DSI spike train and the VSI-B spike. A, A schematic diagram showing the experimental configuration. Intracellular microelectrodes were placed in the somata of DSI, VSI-B, and VFN. DSI and VSI-B were stimulated by injecting brief current pulses (7-10 nA, 20 msec). The membrane potential of VFN was set at approximately -70 mV by constantly injecting a hyperpolarizing current through another electrode to prevent action potential firing triggered by the EPSP. B1, Simultaneous intracellular recordings from VFN, VSI-B, and DSI show that DSI stimulation increased the amplitude of VSI-B-evoked EPSPs recorded in VFN when VSI-B was stimulated shortly after DSI. VSI-B was stimulated repeatedly to fire single action potentials at a 60 sec interval. After the third VSI-B action potential (i), DSI was stimulated to fire a train of action potentials (10 Hz, 5 sec) that ended 7 sec before the next VSI-B spike (ii). The amplitude of the EPSP in VFN was enhanced after DSI stimulation. In this recording, VFN showed a slow depolarization in response to a DSI spike train, which appeared to be the summated unitary EPSPs. The depolarized membrane potential in VFN recovered to the original potential within 5 sec. Action potentials in the VSI-B and DSI traces are truncated. Vertical calibration bars indicate 5 mV. B2, Expanded, superimposed waveforms of VSI-B-evoked EPSPs recorded in VFN (top trace) and VSI-B action potentials (bottom trace) before (i, solid trace) and 7 sec after (ii, dotted trace) the DSI stimulation show the synaptic enhancement. The action potentials in VSI-B are truncated to show the resting potential and spike afterhyperpolarization. C1, When the DSI spike train preceded VSI-B by 37 sec, then the amplitude of the VSI-B-evoked EPSP in VFN decreased. The stimulus protocol was the same as in B except for the timing of the DSI spike train. Vertical calibration bars indicate 5 mV. C2, The decreased amplitude of the EPSPs can be seen in superimposed waveforms evoked before (iii, solid trace) and 37 sec after (iv, dashed trace) the DSI stimulation. Resting potentials of VSI-B and DSI were -47 and -44 mV, respectively.