Light Evokes Ca2+ Spikes in the Axon Terminal of a Retinal Bipolar Cell

Bipolar cells in the vertebrate retina have been characterized as nonspiking interneurons. Using patch-clamp recordings from goldfish retinal slices, we find, however, that the morphologically well-defined Mb1 bipolar cell is capable of generating spikes. Surprisingly, in dark-adapted retina, spikes were reliably evoked by light flashes and had a long (1-2 s) refractory period. In light-adapted retina, most Mb1 cells did not spike. However, an L-type Ca2+ channel agonist could induce periodic spiking in these cells. Spikes were determined to be Ca2+ action potentials triggered at the axon terminal and were abolished by 2-amino-4-phosphonobutyric acid (APB), an agonist that mimics glutamate. Signaling via spikes in a specific class of bipolar cells may serve to accelerate and amplify small photo-receptor signals, thereby securing the synaptic transmission of dim and rapidly changing visual input.