Synaptotagmin I is the leading candidate for the calcium sensor that
triggers exocytosis at conventional synapses. However, physiological
characterization of the calcium sensor for phasic release at the
ribbon-style synapses of the goldfish Mb1 bipolar cell demonstrates a
lower than predicted affinity for calcium, suggesting that a modified
or different sensor triggers exocytosis at this synapse. We examined
synaptotagmin immunolabeling in goldfish retina using two different
antibodies directed against synaptotagmin epitopes that specifically
labeled the expected 65-kDa protein on western blots of goldfish and
mouse retinal membranes. The first antiserum strongly labeled
conventional synapses in the inner plexiform layer (IPL), but did not
label the ribbon-style synapse-containing synaptic terminals of
goldfish Mb1 bipolar cells or photoreceptors. The second antibody also
specifically labeled the expected 65-kDa protein on western blots but
did not label any synapses in the goldfish retina. A third
synaptotagmin antibody that performed poorly on western blots
selectively labeled goldfish photoreceptor terminals. These results
suggest that synaptotagmin may exist in at least three distinct
“forms” in goldfish retinal synapses. These forms, which
are differentially localized to conventional synapses, bipolar cell,
and photoreceptor terminals, may represent differences in isoform,
posttranslational modifications, epitope availability, and
protein-binding partners. Labeling with these antibodies in the
salamander and mouse retina revealed species-specific differences,
indicating that synaptotagmin epitopes can vary across species as well
as among synapses.