Differential Labeling of Endocytotic Proteins Among Synapses in the Goldfish and Mouse Retina

Abstract

Abstract: : Purpose: Current models of vesicular synaptic transmission require recovery of synaptic vesicles from the presynaptic membrane after fusion and exocytosis of neurotransmitter. The GTPase dynamin has been suggested to be critical to synaptic vesicle endocytosis. Recent studies in goldfish bipolar cell terminals indicate that rapid endocytosis of synaptic vesicles is ATP-, rather than GTP-dependent, suggesting that dynamin may not be required for rapid vesicular recycling at all synapses. To better understand synaptic vesicle recycling at retinal synapses, we examined the distribution of dynamin, and two other proteins associated with endocytosis, clathrin and amphiphysin, in the retinas of goldfish and mouse. Methods: The distribution of dynamin, amphiphysin and clathrin was examined immunocytochemically in the goldfish and mouse retina. Double-labeling with known cell-specific and synaptic markers was used to assess localization of these proteins at identified synapses. Results: Dynamin, clathrin, and amphiphysin all were localized to both ribbon and conventional synapses in mouse and goldfish retinas, however, labeling for these proteins differed appreciably among synapses indicating that these proteins are not expressed uniformly. Conventional terminals in the IPL showed strong dynamin labeling, but the ribbon synapses of bipolar cells and photoreceptors showed only weak labeling. Amphiphysin also showed strong labeling in conventional terminals but prominently labeled ribbon terminals. In the mouse retina, rod terminals labeled much more strongly than cone terminals. Clathrin labeling in conventional synapses was very strong, but ribbon synapses showed considerably weaker labeling. In the mouse retina, rod terminals labeled much more strongly than cone terminals. Conclusion: All three endocytosis proteins are present at both conventional and ribbon synapses, however, these proteins are not uniformly distributed. The differences in distribution suggest that there may be functional heterogeneity in synaptic vesicle endocytosis at retinal synapses. The labeling differences between rods and cones also suggest that endocytosis can differ even at similar types of synapses.

Keywords: 330 bipolar cells • 594 synapse