Fig. 3. Synaptic enhancement induced by 5 Hz stimulation during L-channel blockade is mechanistically similar to LTP induced by high-frequency stimulation. A1, Experiment conducted on an individual aged rat slice in which synaptic enhancement was first induced in one population of synapses (S1, filled arrow), using 5 Hz stimulation in the presence of nifedipine. After the 5 Hz episode, nifedipine was washed from the recording medium, and two bursts of 100 Hz stimulation were delivered to S1 (open arrow, S1). Approximately 30 min later, a second round of 100 Hz stimulation was delivered to naive synapses in a second input (open arrow, S2).A2, Average data from three aged rat slices in which induction of synaptic enhancement by 5 Hz stimulation preceded the induction of LTP by 100 Hz stimulation. Note that, although 100 Hz stimulation produced further potentiation in S1, the percentage of increase in the synaptic response was much less than the increase observed at naive synapses (i.e., S2) after 100 Hz stimulation. B1, For an individual aged rat slice, LTP was induced in one pathway (open arrow, S1), using 100 Hz stimulation. After induction of LTP, nifedipine was washed into the recording medium, and a round of 5 Hz stimulation was applied to the potentiated synapses (closed arrow, S1). At 30 min after the first round of 5 Hz stimulation, a second 5 Hz train was applied to nonpotentiated synapses (S2) in the continued presence of nifedipine.B2, Average data from five aged rat slices in which induction of LTP by 100 Hz stimulation preceded the induction of synaptic enhancement by 5 Hz stimulation. Note that, after the induction of LTP in S1, 5 Hz stimulation in the presence of nifedipine fails to induce synaptic enhancement. However, 5 Hz stimulation applied to nonpotentiated synapses in S2 produces robust enhancement. Error bars indicate SEM.