Syntaxin 4 labeling in the outer plexiform layer (OPL) is associated with processes post-synaptic to the photoreceptor terminals, not the terminals themselves. A–C: Double labeling for syntaxin 4 and the photoreceptor terminal marker PSD-95 show that syntaxin 4-positive puncta (arrows) are spatially associated with rod terminals (RT) in the distal OPL and with cone terminals (CT) in the proximal OPL. Images shown from a projection of 27 optical sections with total thickness of 3.79 μm. D–F: At high magnification, syntaxin 4-positive puncta in the distal OPL appear as singlets or doublets (arrows) enveloped by the terminals of rods (RT) labeled for PSD-95. Image shown is a projection of 22 optical sections with total thickness of 3.2 μm. G–I: In the proximal OPL, syntaxin 4 labeled structures (arrows) are associated with cone terminals (CT). Several syntaxin-4 positive puncta are enveloped by the cone terminal. In addition, syntaxin 4 labeling is localized to a cluster of processes in the OPL just proximal to the cone terminal (*) that can be seen to connect to the puncta within the cone terminal. Numerous rod terminals (RT) and associated syntaxin 4 puncta are visible distal to the cone terminal. Image shown is a projection of 34 optical sections with total thickness of 7.09 μm. J–L: Syntaxin 4-labeled structures (arrowheads) in the OPL do not colocalize with synaptic vesicles labeled for VGLUT1 in photoreceptor terminals (PT). Images shown from a projection of 13 optical sections with total thickness of 1.75 μm. M–O: Synaptic ribbons in rod and cone terminals do not label for syntaxin 4. However, syntaxin 4-positive puncta in the OPL are spatially associated with synaptic ribbons and often can be seen to follow the contours of the ribbons (arrowheads) labeled for kinesin II in rod and cone terminals. Circle indicates location of a cone terminal. Images shown from a projection of 12 optical sections with total thickness of 1.60 μm. ONL, outer nuclear layer. Scale bars = 5 μm for A–C, and J–O; 2 μm for D–I.