Large [Ca2+]i transients are associated with LTP induction in cholinergic interneurons. (A) Each train of the HFS (100 Hz frequency, 3-sec duration, 20-sec interval) induced a large [Ca2+]i transient (gray line), closely paralleling the membrane depolarization of the recorded neuron, as shown at higher magnitude in the inset. Note that spikes induced by HFS stimulation are truncated. After HFS delivery, the amplitude of the DPSPs (upward deflections) was significantly potentiated (see also the two representative traces indicated by the arrows, shown at higher sweep speed). The potentiation lasted up to 60 min (dotted lines in A indicate 20 min interval). R.M.P., 61 mV. (R) Ratio values calculated from pairs of 340 and 380 nm images corrected for background fluorescence. (B) Amplitude of the DPSPs recorded from another neuron, plotted as a function of time.