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Bipolar cells play a vital role in the transfer of visual information across the vertebrate retina. The synaptic output of these neurons is regulated by factors that are extrinsic and intrinsic. Relatively little is known about the intrinsic factors that regulate neurotransmitter exocytosis. Much of what we know about intrinsic presynaptic mechanisms that regulate glutamate release has come from the study of the unusually large and accessible synaptic terminal of the goldfish rod-dominant bipolar cell, the Mb1 bipolar cell. However, over the past several years, examination of presynaptic mechanisms governing neurotransmitter release has been extended to the mammalian rod bipolar cell. In this review, we discuss the recent advances in our understanding of synaptic vesicle dynamics and neurotransmitter release in rodent rod bipolar cells and consider how these properties help to shape the synaptic output of the mammalian retina.

To better understand synaptic signaling at the mammalian rod bipolar cell terminal and pave the way for applying genetic approaches to the study of visual information processing in the mammalian retina, synaptic vesicle dynamics and intraterminal calcium were monitored in terminals of acutely isolated mouse rod bipolar cells and the number of ribbon-style active zones quantified. We identified a releasable pool, corresponding to a maximum of 7 s. The presence of a smaller, rapidly releasing pool and a small, fast component of refilling was also suggested. Following calcium channel closure, membrane surface area was restored to baseline with a time constant that ranged from 2 to 21 s depending on the magnitude of the preceding Ca2+ transient. In addition, a brief, calcium-dependent delay often preceded the start of onset of membrane recovery. Thus, several aspects of synaptic vesicle dynamics appear to be conserved between rod-dominant bipolar cells of fish and mammalian rod bipolar cells. A major difference is that the number of vesicles available for release is significantly smaller in the mouse rod bipolar cell, both as a function of the total number per neuron and on a per active zone basis.

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.