Role of nitric oxide in ATP/UTP-induced exocytosis. (A–F) Confocal images of FM1-43–loaded cultures (A, C, and E) before and (B, D, and F) 150 s after depolarization. 40 s after the start of imaging, cells were depolarized by perfusing high K⁺ (60 mM) HEPES buffer. (A and B) control, (C and D) in presence of 30 μM UTP, (E and F) in presence of 30 μM UTP and 100 μM c-PTIO. (G) KCl-induced reduction in FM1-43 fluorescence was inhibited by ATP and UTP. (H) Inhibitory effect of UTP was prevented by pretreatment of cells with L-NAME. (I) Inhibitory effect of UTP was prevented by bath-applied c-PTIO. Application of 100 μM quenched the FM1-43 fluorescence even in the absence of depolarization but it was fully recovered after removal of c-PTIO. (J) Relative fluorescence intensities at 150 s after KCl depolarization along with given treatments. Fluorescence in 5 mM KCl buffer is also included for comparison. The data shown are mean ± SEM for 10–20 synapses from at least three experiments for each treatment (*, P < 0.0001; #, P > 0.1).