DISTRIBUTIONS OF 2 HOMOLOGOUS SYNAPTIC VESICLE PROTEINS, SYNAPTOPORINAND 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 br ain. The distribution of synaptophysin and synaptoporin was examined i n the adult rat and rabbit retina by using single- and double-labeling immunocytochemistry with conventional light microscopy and confocal l aser scanning microscopy. In the rat retina, synaptophysin immunoreact ivity was found in the outer plexiform layer in terminals of photorece ptors 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 b ands of the inner plexiform layer separated by two bands of very low s ynaptoporin immunoreactivity. In the rabbit retina, synaptophysin and synaptoporin immunoreactivity were found in the inner and outer plexif orm layers. In the inner plexiform layer, labeling for both vesicle pr oteins was homogeneous, with no detectable stratification. In the oute r plexiform layer, synaptophysin was present in photoreceptor terminal s, and synaptoporin was present in horizontal cells. Staining of isola ted rabbit retinal cells confirmed that both the axonless A type and t he axon-bearing B type horizontal cells are immunoreactive for synapto porin. 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 lay er. No functional correlation was found in the expression of either sy naptic vesicle protein with the type of neuron or synapse (ribbon or c onventional). (C) 1996 Wiley-Liss, Inc.