Function of the synaptic ribbon in rod and cone photoreceptors

• Synaptic ribbons are elelctron-dense structures in the presynaptic terminals of certain neurons that continuouosly release neurotransmitter, including rods, cones, bipolar cells, and hair cells.
• Vesicles are lined up adjacent to the ribbon. They release their content (glutamate) by fusing to the plasma membrane, but exactly what the ribbon does is not completely clear.
• Our studies suggest that the ribbon restrains vesicle fusion by controlling the rate of vesicle delivery to the plasma membrane.

Cell and molecular mechanisms of lateral inhibition

• The reciprocal synapse between horizontal cells (HCs) and cones is the site of origin of lateral inhibition, a fundamental mechanism for enhancing visual contrast.
• Cones communicate to HCs via an excitatory feed-forward synapse. HCs communicate back to cones via a negative feed-back synapse, which inhibits cone neurotransmitter release.
• To test the hypothesis that protons are the negative feedback transmitter, we developed genetically-encoded fluorescent pH indicators and installed them in HCs or cones in transgenic zebrafish.
• Imaging results allow us to visualize how light changes proton concentration, and show us how far the proton signals from the active zones of synaptic contact between HCs and cones. Ongoing studies are continuing to explore mechanisms of signal transmission at this unique and fascinating synapse.

Lateral inhibition is enhanced by a positive feedback synapse

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• Lateral inhibition is mediated by from HCs of the positive feedback signal from HCs, and investigating whether similar phenomena occur elsewhere in the nervous system.