Differential expression of the presynaptic protein SNAP-25 in mammalian retina

We have studied the expression of the nerve terminal protein synaptosomal associated protein 25 (SNAP-25) in the retina of adult rat, mouse, and monkey, as well as in the developing mouse retina. To evaluate SNAP-25 expression, its distribution was compared to those of the synaptic vesicle-associated proteins synapsin I and synaptophysin. In situ hybridization in adult rat retinas suggested that SNAP-25 mRNA is mainly expressed by ganglion, amacrine, and horizontal cells, but not by photoreceptors and bipolar cells. In all species, the SNAP-25 polypeptide was most abundant in the inner part of the inner and outer plexiform layers and was also found in the ganglion cell axons. In adult retina, synapsin I and synaptophysin were also mainly localized in synaptic fields and processes but all three proteins showed a distinct pattern of distribution. Finally, in mouse retina, the three proteins were first detectable at embryonic day 16 and subsequently showed developmentally regulated changes in their cellular localization. These results suggest that SNAP-25 is predominantly expressed in specific subtypes of conventional synapses, but not ribbon synapses, and that it may also be involved in the physiology of nonvesicular terminals of horizontal cells. Our study also suggests that combinatorial expression of different components of the presynaptic specialization may contribute to synaptic functional diversity.