Figure 5.
LTP induction facilitated coincidence detection for distal inputs. A, Protocols for assessing spiking probability elicited by two synchronous or asynchronous (interval, 10 ms) EPSPs before and after LTP induction. Schematic diagram on the left depicts two independent distal SC inputs (200–300 μm from soma). Top traces, Sample EPSPs from SP, showing the increase in amplitude after LTP induction and subsequent adjustment back to the control level. Middle traces, Sample EPSPs from SC, showing no change in amplitude. Bottom traces, Samples of summed EPSPs elicited by synchronous and asynchronous stimulation of SP and SC before and after LTP induction, with spiking (arrowhead, truncated) induced by synchronous summation after LTP. B, Example experiment showing spike initiation by summed EPSPs elicited at two distal inputs at different interstimulus intervals before (left) and after (right) LTP induction. Traces above depict summed responses for two SC inputs stimulated at 13 different intervals (Δt = −18 to +18 ms), recorded from 15 consecutive trials before and after LTP induction. The traces with Δt = 18 at trials 5 and 9 of the control period are magnified. Dashed lines indicate synchronous summation (Δt = 0 ms). Histograms below show probability of spiking at different intervals for the 15 trails shown above. Note that the probability at Δt = 0 ms was set at ∼0.5 before LTP induction by adjusting the initial EPSP amplitudes. After LTP induction, the stimulus strength was reduced so that the EPSP amplitude of SP was the same as the previous LTP induction. C, The average spiking probability (±SEM) for summed EPSPs at Δt = 0, ±6, ±12, and ±18 ms before and after LTP induction. Solid lines represent the best first-order exponential fit. (**p = 0.0085; n = 4). D, Probability of spiking elicited by stimulating two SC inputs at distal dendrites plotted against the EPSP slope of SP (summation interval, 10 ms). The strength of SP was adjusted to elicit EPSPs of different amplitude, whereas that of SC was kept constant. The data were sorted into bins of EPSP slope of SP (bin size, 0.3 mV/ms). No significant difference was found between the data sets before and after LTP induction (p = 0.15; Mann–Whitney U test). prob., Probability.