Figure 2.
Reduced fast release but increased asynchronous release upon substitution of extracellular Ca2+ with Sr2+. A, Examples of afferent fiber stimulation-evoked train of EPSCs (A1; 50 APs, 100 Hz) recorded initially in the presence of 2 mm external Ca2+ (black trace), and after replacing Ca2+ with Sr2+ (red trace). Stimulation artifacts were blanked. Current traces on the right (A2) show the late release activity during a 3 s interval, and corresponding mEPSC frequency plots. mEPSC frequency analyses before and after 100 Hz trains (A2, bottom), show that Sr2+ markedly increased the asynchronous release rate after the train (n = 4 cells). The baseline mEPSC frequency before the train was not significantly changed in the presence of Sr2+ (p = 0.34). In this and all subsequent figures, black and red traces and data symbols represents data obtained in the presence of Ca2+ and Sr2+, respectively. *p < 0.05. **p < 0.01. B, Plots of peak EPSC amplitudes (top) and cumulative EPSCs (bottom) as a function of stimulus number during 100 Hz trains, both in the presence of 2 mm Ca2+ and after switching the extracellular solution to 2 mm Sr2+. Back-extrapolation of line fits to the late part of the cumulative EPSC plot indicates approximately similar pool size, despite a different late slope. Dividing the first EPSC amplitude by the pool size provides an estimate for initial release probability (prel). C, Individual and average values for the first EPSC amplitude; pool size; prel; and the late slope, each in the presence of 2 mm Ca2+ or Sr2+. Note that the decreased first EPSC amplitude (by ∼75%) is, to a large part, explained by the decreased prel (∼60% relative decrease; p < 0.001). *p < 0.05. ***p < 0.001.