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Amy K. Berntson, Catherine W. Morgans; Distribution of the presynaptic calcium sensors, synaptotagmin I/II and synaptotagmin III, in the goldfish and rodent retinas. Journal of Vision 2003;3(4):3. doi: https://doi.org/10.1167/3.4.3.
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Synaptic vesicle exocytosis is triggered by rises in calcium up to 100 µM at the site of vesicle fusion. The synaptic vesicle proteins synaptotagmin 1 and 2 (Syt I and Syt II) bind calcium at similarly high concentrations and have been proposed as the calcium sensors for fast neurotransmitter release. However, 1 µM calcium produces tonic transmitter release at photoreceptor and bipolar cell synapses in the goldfish retina, suggesting that these synapses use a higher affinity calcium sensor. Immunofluorescent staining with a panel of Syt I/II antibodies detected Syt I/II in both photoreceptor and bipolar cell terminals of the rodent retina. By contrast, no staining of either photoreceptor or protein kinase C (PKC)-labeled bipolar cell terminals was detected in the goldfish retina with any of the Syt I/II antibodies. The high affinity calcium sensor synaptotagmin 3 (Syt III) was localized to the synaptic layers of both goldfish and rodent retinas; however, while Syt III was associated with PKC-labeled bipolar cell terminals in the goldfish retina, it did not co-localize with PKC in the mouse retina. These results suggest that, unlike in their mammalian counterparts, synaptic vesicle exocytosis in goldfish photoreceptor and bipolar cell terminals utilizes a calcium sensor other than Syt I/II, possibly Syt III.
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