Syntaxin 4 does not colocalize with syntaxin 1 or other presynaptic markers for conventional or ribbon synapses in the inner plexiform layer (IPL). A–C: Syntaxin 4-positive puncta (arrows) found in the inner nuclear layer (INL) and IPL do not show labeling for syntaxin 1, which is expressed in the processes and conventional synapses of amacrine cells. Image shown is a projection of 15 optical sections with total thickness of 2.19 μm. D–F: Syntaxin 4 positive puncta (arrows) only very rarely show any colocalization with synapsin I (circle), a marker for conventional presynaptic terminals. Image shown is a projection of 10 optical sections with total thickness of 1.31 μm. G–I: Syntaxin 4-labeling (arrows) is only rarely associated with presynaptic terminals identified by labeling for the ubiquitous presynaptic marker synaptic vesicle protein 2 (SV2; arrowheads). Terminals showing weak colocalization are indicated by the ellipse. Image shown is a projection of 9 optical sections with total thickness of 1.17 μm. J–L: Syntaxin 4-positive puncta in the IPL (arrows) do not label for VGLUT1, indicating that syntaxin 4 is not present in bipolar cell terminals. Occasionally syntaxin 4-positive puncta make contact with VGLUT1-positive bipolar cell terminals (arrowhead). Image shown is a projection of 13 optical sections with total thickness of 1.78 μm. GCL, ganglion cell layer. Scale bars = 5 μm for A–C and G–I; 10 μm for D–F and J–L.