Figure 5. Conditions determining the observation of the persistent PTD.

A, The persistent PTD is measured in slices from mice of postnatal ages 12–14 (average = 13.1±0.2, n = 26), 29–35 (average = 33.3±0.4, n = 27) and 93–103 (average = 98.4±0.9, n = 13). The persistent PTD is significant at all ages (p<0.02), but the amplitude is doubled for ages in the range of 29–35 (p<0.005). B, Persistent PTD amplitude plotted as a function of the frequency of 10-pulse bursts. 20, 50, 100 and 200 Hz are compared (n = 3, 5, 5 and 8 respectively). On average, the persistent PTD is significant for all frequencies above 50 Hz (p<0.004), but not for 20 Hz (p = 0.11). C, Persistent PTD amplitude plotted for different conditions. The persistent PTD is not dependent on the synaptic transmission monitoring rate, as there is no significant difference between rates 0.25 Hz (n = 34) and 0.0625 Hz (n = 8; p = 0.32), indicating that the persistent PTD is not due to a fatigue of neurotransmission. Initial experiments were done in 3 mM extracellular [Ca²⁺]. The persistent PTD has identical amplitude in 2 mM extracellular [Ca²⁺] (n = 27; p = 0.62). The persistent PTD is identical in wild type animals (n = 7; p = 0.69). The persistent PTD is similar in absence of GABAzine (n = 7; p = 0.73) and at a near-physiological temperature (n = 14; p = 0.33). D, Persistent PTD amplitude for different temporal patterns of 200 Hz trains. A 4 pulse train evokes a persistent PTD barely significant (p = 0.05; n = 35) and 6 times smaller than the persistent PTD evoked by a 10-pulse train (p = 3·10⁻⁶; n = 34). In comparison, the SSE evoked by a 4 pulse train is only 1.5 times smaller than the SSE evoked by a 10 pulse train (see Fig. 7A). Repeating the 4 pulse-trains 3 times with 100 ms intervals rescues the persistent PTD, which is not significantly different from the one induced by a 10 pulses train (p = 0.27; n = 11). A 30 pulse train led to high variability in the persistent PTD amplitude, which on average was not significant (not different from baseline; p = 0.11, n = 11). E-F, Following a ≥30 min stable baseline, the 10-pulse 200 Hz burst evokes a persistent PTD which lasts for at least 30 min. Averaged fEPSP/FV amplitude (E) and PPF (C) are normalized to the 10 min baseline prior burst stimulation. The change in PPF also appears to last for 30 min (p<0.01; two asterisks).