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The Journal of Neuroscience, January 15, 2000, 20(2):568-578
Synaptic Depression and the Kinetics of Exocytosis in Retinal
Bipolar Cells
Juan
Burrone and
Leon
Lagnado
Medical Research Council Laboratory of Molecular Biology, Cambridge
CB2 2QH, United Kingdom
The capacitance technique was used to investigate exocytosis at the
ribbon synapse of depolarizing bipolar cells from the goldfish retina.
When the Ca2+ current was activated strongly, the
rapidly releasable pool of vesicles (RRP) was released with a single
rate-constant of ~300-500 sec 1. However, when
the Ca2+ current was activated weakly by
depolarization in the physiological range ( 45 to 25 mV), exocytosis
from the RRP occurred in two phases. After the release of 20% or more
of the RRP, the rate-constant of exocytosis fell by a factor of 4-10.
Thus, synaptic depression was caused by a reduced sensitivity to
Ca2+ influx, as well as simple depletion of the RRP.
In the resting state, the rate of exocytosis varied with the amplitude
of the Ca2+ current raised to the power of 2. In the
depressed state, the sensitivity to Ca2+ influx was
reduced approximately fourfold. The initial phase of exocytosis
accelerated e-fold for every 2.1 mV depolarization over the
physiological range and averaged 120 sec1 at 25 mV.
The synapse of depolarizing bipolar cells therefore responds to a step
depolarization in a manner similar to a high-pass filter. This
transformation appears to be determined by the presence of rapidly
releasable vesicles with differing sensitivities to
Ca2+ influx. This might occur if vesicles were
docked to the plasma membrane at different distances from
Ca2+ channels. These results suggest that the ribbon
synapse of depolarizing bipolar cells may be a site of adaptation in
the retina.
Key words:
synapse; vesicle; exocytosis; depression; retina; depolarizing bipolar cell
Copyright © 2000 Society for Neuroscience 0270-6474/00/202568-11$05.00/0
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