Rab3 proteins and SNAP-25, essential components of the exocytosis machinery in conventional synapses, are absent from ribbon synapses of the mouse retina.

GTP-binding rab proteins, present in synaptic vesicles and endocrine secretory granules, have been shown to be involved in the control of regulated exocytosis. We found rab3 proteins in immunoblots of diverse areas of the mouse central nervous system (spinal cord, olfactory bulb, hippocampus, cerebellum and neocortex). Immunohistochemical observations at light- and electron-microscopical levels in the hippocampus and other areas revealed rab3 proteins in virtually all synaptic fields and terminals of the areas investigated. In the retina, rab3A immunoreactivity was confined to the inner and outer plexiform layers. Ultrastructural examination revealed that rab3A was present in conventional terminals in the inner plexiform layer and in horizontal cell processes of the outer plexiform layer. In contrast ribbon synapses, which play a key role in transferring information from the photoreceptor cells to the central nervous system, were immunonegative. We also tested whether other proteins of the rab3 family are present in ribbon synapses. However, using an antibody recognizing rab3B and rab3C in addition to rab3A, we found no immunoreactivity in these synapses. Interestingly, we observed also no immunoreactivity for synaptosomal-associated protein 25 (SNAP-25) in ribbon synapses, but conventional synapses and horizontal cell processes were heavily stained. Our data show that the known rab3 and SNAP-25 isoforms, which are components of the secretory apparatus of conventional synapses, are absent from ribbon synapses of the retina. Our observations suggest different mechanisms of transmitter exocytosis in conventional and ribbon terminals.