Vesicle-associated membrane protein isoforms in the tiger salamander retina

Vesicle associated membrane protein (VAMP; also known as synaptobrevin) is a key component of the core complex needed for docking and fusion of synaptic vesicles with the presynaptic plasma membrane. Recent work indicates that the precise complement of presynaptic proteins associated with transmitter release and their isoforms vary among synapses, presumably conferring specific functional release properties. The retina contains two types of vesicular synapses with distinct morphologic, functional, and biochemical characteristics: ribbon and conventional synapses. Although the precise complement of presynaptic proteins is known to differ between conventional and ribbon synapses and among conventional synapses, the distribution of VAMP isoforms among retinal synapses has not been determined. The expression and localization of VAMP isoforms in the salamander retina, a major model system for studies of retinal circuitry, was examined by using immunocytochemical and immunoblotting methods. Both methods indicated that at least two VAMP isoforms were expressed in salamander retina. One isoform, recognized by an immunoglobulin M antibody that recognizes both mammalian VAMP-1 and VAMP-2, was associated with photoreceptor and bipolar cell terminals as well as many conventional synapses, and probably corresponds to mammalian VAMP-2. A different VAMP isoform associated with a subset of amacrine cells, was recognized only by antibodies directed against the N-terminus of mammalian VAMP-2. An antiserum directed against the N-terminus of mammalian VAMP-1 did not specifically recognize any salamander VAMPs in either immunocytochemical or immunoblotting experiments. Heterogeneous distribution of VAMP isoforms among conventional retinal synapses was confirmed by double labeling for synapsin I, a marker for conventional synapses. These studies indicate that VAMP isoforms are expressed heterogeneously among retinal synapses but cannot account for the differences in transmitter release characteristics at ribbon and conventional synapses. These results also corroborate previous studies in Xenopus indicating that the N-terminus of nonmammalian VAMP isoforms differs from their mammalian counterparts.