Exocytosis-mediated glutamate release from ribbon-type synaptic terminals of retinal bipolar cells was studied using AMPA receptors and simultaneous membrane capacitance measurements. Release onset (delay 2+ channel opening and closing. Asynchronous release was not copious and we estimate that there are ~5 Ca2+ channels per docked synaptic vesicle. Depending on Ca2+ current amplitude, release occurred in a single fast bout or in two successive bouts with fast and slow onset kinetics. The second, slower bout may reflect a mobilization rate of reserve vesicles toward fusion sites that is accelerated by increasing Ca2+ influx. Bipolar cell synaptic ribbons thus are remarkably versatile signal transducers, capable of transmitting rapidly changing sensory input, as well as sustained stimuli, due to their large pool of releasable vesicles.
ASJC Scopus subject areas