The electrode was kept at 700 mV vs. a Ag/AgCl reference/counter electrode (World Precision Instruments, Inc., USA) using an amplifier instrument (Axopatch 200B, Axon Instruments, USA). The signal was digitized at 10 kHz and filtered with an internal low pass Bessel filter at 2 kHz. The signal was displayed in real time (AxoScope 8.1, Axon Instruments, USA) and stored digitally.