A–C, Comparison of fluorescence intensity changes (ΔF/F) of Ca2+ responses in WT and OMP−/− knobs to a 1-s pulse of 100 µM IBMX (A, B) or 80 mM KCl (C) using 1 mM external Ca2+. Recovery time course of the signals was fitted with single exponential functions (dashed lines). The decay time constants (τ) of the fitted curves are indicated. These results demonstrate an apparent defect in the kinetics of removal of elevated Ca2+ i from the dendritic knobs of the OSNs of OMP−/− mice. D, Ca2+ response of a single WT knob stimulated with a 1-s pulse of IBMX (100 µM) followed by a 5-s pulse of caffeine (10 mM), both in low external Ca2+ solution (0.6 µM), confirming that the caffeine-induced Ca2+ transient depends on an intracellular source. E, Comparison of Ca2+ transients in WT and OMP−/− knobs evoked by a 5-s pulse of caffeine (10 mM) in low extracellular Ca2+ solution (0.6 µM). Recovery time course of the signals was fitted with single exponential functions (dashed lines). The decay time constants (τ) of the fitted curves are indicated. F, Bar graphs showing collected results from OSN knobs of WT and OMP−/− mice. For stimulation with IBMX, WT: τ = 8.5±1.3 s (n = 25, N = 8), OMP−/−: τ = 21.3±2.2 s (n = 13, N = 4). For stimulation with KCl, WT: τ = 20.1±1.7 s (n = 13, N = 4), OMP−/−: τ = 28.7±3.2 s (n = 13, N = 4). For stimulation with caffeine in 1 mM Ca2+ o, WT: τ = 13.9±1.6 s (n = 15, N = 5), OMP−/−: τ = 26.8±1.9 s (n = 15, N = 4), **p<0.0001 and in low external Ca2+ (0.6 µM), WT: τ = 13.1±1.8 s (n = 19, N = 6), OMP−/−: τ = 27.3±1.9 s (n = 15, N = 5), *p<0.001; **p<0.0001.