Purpose: Many proteins associated with synaptic vesicle exocytosis are differentially distributed among synapses in the retina and elsewhere in the central nervous system. The synapse-specific distribution of these proteins and their isoforms is thought to contribute to synapse-specific functional differences. Vesicle-associated membrane protein (VAMP, also known as synaptobrevin) is an integral synaptic vesicle membrane protein that is part of the fusion core complex needed for docking and fusing of synaptic vesicles at the synaptic active zone. Two VAMP isoforms have been identified that are considered to be synaptic, VAMP-1 and VAMP-2, however their distributions among the various synapses in the mammalian retina have not been characterized.
Methods: Single- and double-labeling immunocytochemistry was used to investigate the distribution of the synaptic VAMP isoforms, VAMP-1 and VAMP-2, in the mouse retina.
Results: VAMP-2 was the predominant isoform in both synaptic layers. Double-labeling studies using conventional and ribbon-synapse-specific markers showed that VAMP-2 was broadly distributed among conventional and ribbon synapses. In contrast, the distribution of VAMP-1 was very limited. In the outer retina, only weak labeling was present in photoreceptor terminals. In the inner retina, labeling for VAMP-1 was found in the dendrites, cell bodies, and axons of some ganglion cells, as demonstrated by double labeling with the ganglion cell markers, microtubule-associated protein-1 and Brn-3a. VAMP-1 labeling did not colocalize with amacrine or bipolar cell markers, nor did it colocalize with other pre-synaptic markers, suggesting that VAMP-1 is not associated directly with neurotransmitter release in the inner retina. Labeling for VAMP-1 identified a set of large ganglion cells that ramified in the mid-IPL (inner plexiform layer), suggesting that they may show ON-OFF responses. Some of these cells had cell bodies displaced to the inner nuclear layer. The dendrites of the large VAMP-1-immunoreactive ganglion cells did not co-stratify with the cholinergic plexuses of the starburst amacrine cells (labeled for choline acetyltransferase) and therefore are unlikely to show directional selectivity. However, these cells are likely to receive input from bipolar cells and a population of putative glutamatergic amacrine cells.
Conclusions: VAMP-1 and VAMP-2 are differentially distributed among the synapses of the mouse retina. VAMP-2 is the predominant isoform and is widely expressed at ribbon and conventional synapses in both plexiform layers. VAMP-1 expression in the mouse retina is much more limited and is not restricted to presynaptic terminals. In the OPL, VAMP-1 is co-expressed with VAMP-2 presynaptically in photoreceptor terminals. However, VAMP-1 expression in the IPL is associated with ganglion cells and does not appear to be localized to presynaptic terminals. VAMP-1 is a specific marker for a set of large ganglion cells and displaced ganglion cells that ramify in the mid-IPL and are likely to have ON-OFF physiology.