Figure 7.

The effects of inhibition of inhibitory and excitatory synaptic transmission on SyGCaMP2 responses to SC-AC responses in CA1 and the measurement of long-term potentiation (LTP) following theta burst stimulation. (A) Pairs of stimuli were applied to SC-AC fibers every 10 s to record fEPSPs from the s. radiatum. At 5-min intervals, 10 stimuli were applied at 10 Hz and SyGCaMP2 fluorescence peak responses were measured. After 15 min, Picrotoxin (blue bars and traces), AP5 (green) and DNQX (red) were cumulatively applied, each for 15 min. After 45 min, all were washed off. fEPSP and SyGCaMP2 responses were expressed as a percentage of baseline F/F₀ responses and plotted against time. Data represent the means and SEMs from six separate slices from three different mice. Traces to the right show SyGCaMP2 responses that were sampled before, during and after drug applications at the times indicated by colors. (B) fEPSPs were recorded every 10 s and the peak amplitudes of averaged responses plotted against time each minute. At 5-min intervals, bursts of 10 stimuli were applied at a frequency of 10 Hz and SyGCaMP2 peak responses from the s. radiatum measured. After 20 min, theta burst stimulation was applied and measurements of fEPSPs and SyGCaMP2 responses continued as before. After approximately 2 h, DNQX was applied to examine the proportion of the potentiated response that was presynaptic in origin. Experiments were performed in six young (<2 months of age) and six old (>18 months of age) animals. To the right are illustrated examples of traces taken before and 60 min after theta burst stimulation. (C) The effects of DNQX were tested on SyGCaMP2 peak responses in naive slices that had not previously undergone theta burst stimulation. The means and SEM of six separate experiments are shown. Representative traces recorded before and during DNQX application are shown on the right. (D) The effects of DNQX on SyGCaMP2 responses in naïve slices were compared with those from slices that had undergone LTP. The effect of DNQX was expressed as a percentage of the potentiated response rather than the original baseline. The means and SEM of six separate experiments from animals that were <2 months old in each case are shown. A Mann-Whitney U-test was used to test for statistically significant differences between potentiated and naive responses (*P = 0.022).