Figure 2. Dkk1 rapidly and reversibly reduces the number of synaptic sites in hippocampal neurons. A, B, Immunofluorescence microscopy (A) and quantification (B) of the number of several presynaptic and postsynaptic protein clusters in hippocampal neurons treated with vehicle or Dkk1 for 2 h. Scale bar, 5 μm. Syn-1, Synapsin-1. C, Hippocampal cells were treated with Dkk1 for 2 or 24 h, then TUNEL assays were performed, counterstained with Hoechst. C, Control. D, Hippocampal cells were treated with Dkk1 or control media for 2 h, then Dkk1 was removed and the cells incubated for another 2 d in the absence of Dkk1. Quantification shows that the effect of Dkk1 is reversible. C, Control. E, F, FM1-43 uptake shows that Dkk1 decreases the number of synaptic vesicle recycling sites (puncta). Scale bars: left, 20 μm; right, 10 μm. *p < 0.05, **p < 0.005, ***p < 0.001. Error bars represent SEM.