The amperometric traces were processed by a macro of Igor Pro 6 (Version 6.3.7.2; Page 16 of 24 WaveMetrics, Lake Oswego, OR) designed for analysis of quantal release by the group of David Sulzer at Columbia University. Peaks were detected if they exceeded a threshold of 3 times the RMS noise. Statistical analysis for amperometric measurements data were performed in GraphPad Prism 7 (GraphPad Software, Inc., San Diego, CA). The average of all parameters are calculated as the mean of medians of all recorded traces.