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 synapt ic vesicles with the presynaptic plasma membrane. Recent work indicates tha t the precise complement of presynaptic proteins associated with transmitte r release and their isoforms vary among synapses, presumably conferring spe cific functional release properties. The retina contains two types of vesic ular synapses with distinct morphologic, functional, and biochemical charac teristics: ribbon and conventional synapses. Although the precise complemen t of presynaptic proteins is known to differ between conventional and ribbo n synapses and among conventional synapses, the distribution of VAMP isofor ms among retinal synapses has not been determined. The expression and local ization 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 isof orms were expressed in salamander retina. One isoform, recognized by an imm unoglobulin M antibody that recognizes both mammalian VAMP-1 and VAMP-2, wa s associated with photoreceptor and bipolar cell terminals as well as many conventional synapses, and probably corresponds to mammalian VAMP-2. A diff erent VAMP isoform associated with a subset of amacrine cells, was recogniz ed 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 sp ecifically recognize any salamander VAMPs in either immunocytochemical or i mmunoblotting experiments. Heterogeneous distribution of VAMP isoforms amon g conventional retinal synapses was confirmed by double labeling for synaps in I, a marker for conventional synapses. These studies indicate that VAMP isoforms are expressed heterogeneously among retinal synapses but cannot ac count 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 dif fers from their mammalian counterparts. J. Comp. Neurol. 431:424-436, 2001. (C) 2001 Wiley-Liss, Inc.