Distributions of two homologous synaptic vesicle proteins, synaptoporin and synaptophysin, in the mammalian retina

Synaptophysin and synaptoporin are homologous proteins that are among the most abundant synaptic vesicle proteins. Despite their high degree of sequence similarity, they are differentially distributed in the brain. The distribution of synaptophysin and synaptoporin was examined in the adult rat and rabbit retina by using single- and double-labeling immunocytochemistry with conventional light microscopy and confocal laser scanning microscopy. In the rat retina, synaptophysin immunoreactivity was found in the outer plexiform layer in terminals of photoreceptors and was homogeneously distributed throughout the inner plexiform layer. Synaptoporin immunoreactivity, however, was restricted to the inner plexiform layer. Labeling was most prominent in three distinct bands of the inner plexiform layer separated by two bands of very low synaptoporin immunoreactivity. In the rabbit retina, synaptophysin and synaptoporin immunoreactivity were found in the inner and outer plexiform layers. In the inner plexiform layer, labeling for both vesicle proteins was homogeneous, with no detectable stratification. In the outer plexiform layer, synaptophysin was present in photoreceptor terminals, and synaptoporin was present in horizontal cells. Staining of isolated rabbit retinal cells confirmed that both the axonless A type and the axon-bearing B type horizontal cells are immunoreactive for synaptoporin. In addition, electron microscopy of synaptoporin-immunostained rabbit retinas revealed no labeling of photoreceptor terminals but of putative synaptic sites in horizontal cells in the outer plexiform layer. No functional correlation was found in the expression of either synaptic vesicle protein with the type of neuron or synapse (ribbon or conventional).