Figure 3. Different calcium signals in small and large terminals of the same cell.

(A) A BC expressing GCaMP5. Large and small terminals are indicated by the blue and red arrows. Scalebar 10 µm. (B) Responses of the terminals in (A) to a stimulus modulated at 1 Hz. (C) Power spectrum of the responses from large and small terminals calculated for different stimulus frequencies. (D) Mixed BC filled with OGB-5N in a slice of goldfish retina. Scale bar 10 µm. (E) Spatially averaged Ca²⁺ signals in the small (red) and large (black) terminal during a 2 s depolarizing current step (10 pA). This BC did not generate spikes. (F) A comparison of Ca²⁺ signals in the small and large terminal of a “spiking” BC. Each spike caused a calcium transient that was larger and faster in the smaller terminal. (G–I) Time constants of the calcium signal rise (t_rise), decay (t_decay), and peak amplitude, evaluated in 16 pairs of small and large terminals. For small and large, τ_rise = 0.25±0.20 and 1.16±0.64 s; τ_decay = 1.42±0.82 and 2.91±2.05 s; amplitude = 1.29±0.38 and 0.68±0.21 µM. All these parameters were significantly different in small and large terminals (p<0.001; Wilcoxon ranked sum, n = 16 cells from nine adult retinae). The average radii of the small and large terminals were 1.1±0.3 and 5.2±1.2 µm, respectively. (J) Time constants of calcium decay were directly proportional to terminal radius determined using imaging (grey, n = 32 terminals) or capacitance measurements (black, n = 20 terminals). The linear fit was constrained to go through the origin. Error bars show 1 standard deviation (SD).