Fig. 6. The C-terminus of NCS-1 peptide (CTN) affects the action potential-induced fura-2 Ca²⁺ signal in growth cones in an activity-dependent manner. (A) Representative epifluorescent images of Ca²⁺ signals in L. stagnalis neurites before (control) and after CTN treatment (50 µM). The Ca²⁺ ratiometric signals (F_340/380) were obtained at rest (basal) or elicited by one, four or six depolarization-induced action potentials. Action potentials were evoked by 1 second depolarization using an intracellular sharp electrode impaled into the soma. (B) Representative examples showing the Ca²⁺ signal (F_340/380) were as a linear function of the number of action potentials in the neurites under different conditions. The Ca²⁺ signal from a single growth cone region was plotted against the number of action potentials used to evoke the response. The line represents a linear best fit of the data, which shows no apparent saturation of the signal within the range of action potentials used. The slopes of the plots are 0.037±0.004 (control), 0.041±0.002 (srCTN) and 0.019±0.002 (CTN). (C) Summary of the slope, as described in B, from three to eight cells with treatments as indicated. The mean slope dependencies are 0.037±0.006 (control, n=4), 0.042±0.004 (srCTN, n=3) and 0.021±0.003 (CTN, n=8). The data are presented as mean±s.e.m. Asterisk indicates significant difference (P<0.05) from the controls. AP, action potentials.