Exocytosis and calcium channel properties in the goldfish bipolar neuron

In neurons, cell-to-cell signaling through chemical transmission requires exocytosis of synaptic vesicles subsequent to a rise in cytosolic calcium concentration ([Ca2+]). This rise in calcium is typically mediated by ions entering the cell through plasma membrane voltage-gated calcium channels (VGCCs). Bipolar cells are neurons located in the retina and signal to amacrine and ganglion cells by release of small (∼30-50 nm) synaptic vesicles. Previous research into properties of exocytosis in the bipolar neuron has utilized strong stimulation unlikely to be experienced in vivo. This work focuses on bipolar neuron characteristics using stimulation similar to that experienced by these cells in the living organism. Bipolar neurons contain vesicles that are able to exocytose quickly upon electrical stimulation---termed the immediately releasable pool (IRP)---and these vesicles are thought to be attached to proteinaceous structures termed synaptic ribbons. The IRP is sensitive to 10mM [EGTA] and responds to Ca2+ entry through many VGCCs enabling high rates of exocytosis. Each bipolar neuron contains ∼8,000-24,000 channels demonstrating a small single channel conductance (∼0.3-0.8 pS). Calcium channels are clustered at active zones and ∼8,000 channels corresponds to ∼270 channels per active zone. These neurons might utilize such large numbers of low conductance calcium channels to (a) reduce voltage noise from stochastic channel opening, and/or (b) allow for transient and sustained exocytosis for prolonged periods.