In the retina, adenosine is released in the dark and has been shown to inhibit Ca influx through voltage-gated Ca channels in cones. Therefore, we tested whether adenosine can inhibit exocytosis from isolated cone photoreceptors. Simultaneous measurements of membrane exocytosis and Ca were made from cones using the activity-dependent dye, Synaptored-C2, and the Ca indicator dye, Fluo-4. Adenosine suppressed exocytosis in cones, indicating that transmitter release is also reduced from cone terminals, and further supports an inhibitory mechanism for modulating transmitter release onto second-order neurons. Furthermore, this raises the possibility that adenosine might be neuroprotective for photoreceptors and second-order neurons by suppressing Ca levels in cones and reducing exocytosis of L-glutamate, respectively.
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