Abstract
Prominent electron dense projections such as synaptic ribbons are observed at presynaptic terminals of sensory neurons that release transmitter tonically. These characteristic structures are thought to tether synaptic vesicles at the sites adjacent to active zones and to increase the size of the readily releasable pool. However, functional significance of prominent electron dense projections is not yet known. Applying total internal reflection fluorescence (TIRF) microscopy to goldfish retinal bipolar cell terminals, we visualized Ca2+ entry sites, synaptic ribbons, and vesicle fusion sites. Ca2+ entry sites were colocalized with ribbons. Depolarization-induced vesicle fusion occurred immediately around ribbons and then sustainedly at ribbon-free sites. Activation of protein kinase C (PKC), which is known to potentiate the delayed sustained component of transmitter release, specifically increased the number of vesicle fusion events at ribbon-free sites. Electron microscopy revealed that PKC activation selectively increased the number of docked vesicles at ribbon-free sites, which faced the neuronal processes with the postsynaptic density. Ribbon-associated and ribbon-free active zones were nearly equal in number in the Mb1 bipolar cell terminal. Our results suggest that the ribbon and ribbon-free synapses may transmit transient and sustained signals, respectively. [J Physiol Sci. 2007;57 Suppl:S10]
