Changes in Fluo-5N fluorescence induced by a train of action potentials, CmC, and depolarizing pulses. (A) Simultaneous recordings of a train of action potentials (bottom left trace) induced by a burst of supraliminal current pulses, 0.5 ms in duration at a frequency of 50 Hz during 1 s in current clamp conditions, and of the associated changes in Fluo-5N fluorescence (top trace) in the presence of an external Tyrode solution in a control fiber (C57BL6 mouse). The first action potential of the train is shown on an expanded time scale next to the train. (B) Change in Fluo-5N fluorescence induced by exposure of the same fiber as in A 2 min later to 1 mM CmC at a holding potential of −80 mV in current clamp conditions. Fluorescence images were captured at a frequency of 15 Hz in A and 1 Hz in B.