Syntaxin 4 is localized to the tips of horizontal cell processes at ribbon synapses, but not to dendrites of ON- or OFF-bipolar cells. A–C: Syntaxin 4 is present in the tips of horizontal cell processes in the ribbon synaptic complexes of rods and cones. Syntaxin 4 and calbindin, a horizontal cell marker, show double labeling in the individual puncta associated with rod terminals (arrowheads) and in the clusters of puncta associated with cone terminals (arrows) in the OPL, indicating that the syntaxin 4 is present at the tips of horizontal cell processes invading rod and cone terminals. Images shown from a projection of 37 optical sections with total thickness of 5.24 μm. D–F: Syntaxin 4 is not present in the dendrites of rod or ON-cone bipolar cells. Double labeling for syntaxin 4 and Go-alpha, a marker for rod and ON-cone bipolar cells, shows no colocalization. ON-bipolar cell dendrites reach up to the level of the syntaxin 4-positive puncta and terminate. In favorably oriented complexes in rod terminals (arrowheads), rod bipolar cell dendrites extending into the distal OPL terminate between the syntaxin 4-positive doublets corresponding to horizontal cell processes. No colocalization of syntaxin 4 and Go-alpha is present in ON-cone bipolar cell dendrites in cone terminals (ellipse). Image shown is a single optical section. G–I: Syntaxin 4 is not present at flat contacts between OFF-cone bipolar cells and cone terminals. Double labeling for syntaxin 4 and peanut agglutinin (PNA), a marker for flat contacts (arrowheads) between OFF-cone bipolar cells and cone terminals (CT) shows no colocalization. Syntaxin 4 labeling in processes directly proximal to the cone terminals is also visible (*). Images shown from a projection of 26 optical sections with total thickness of 6.71 μm. INL, inner nuclear layer. Scale bars = 10 μm for A–C; 5 μm for D–F; 2 μm for G–I.