Distribution of the presynaptic calcium sensors, synaptotagmin I/II and synaptotagmin III, in the goldfish and rodent retinas

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.