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K. Rabl, L. Cadetti, W.B. Thoreson; Kinetics of vesicular release and replenishment in rods and cones. . Invest. Ophthalmol. Vis. Sci. 2004;45(13):1319.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: To examine kinetics of exocytosis and recovery from paired pulse depression in synaptic terminals of rods and cones. Methods: Exocytosis triggered by membrane depolarization in rods and cones was monitored by increases in capacitance or post–synaptic currents (PSCs) during simultaneous pre– and post–synaptic recordings in salamander retinal slice. Results:The amplitude of the exocytotic capacitance jump in rods and cones increased with the duration (10 ms to 2 s) of a step depolarization (–70 to –10 mV). Capacitance/duration relations were fit by dual exponential functions with time constants of 45.6 ms and 4.5 s for rods and 5.6 ms and 5.0 s for cones. The integrated charge transfer of PSCs was used as another index of glutamate release. This approach also revealed two kinetic components of exocytosis although there was a smaller difference between fast components of rod– vs. cone–driven cells. Exocytosis exhibited paired pulse depression in which the capacitance increase evoked by a 100 ms step (–70 to –10 mV) was depressed by prior application of an identical step. To compare the rate of recovery from this paired pulse depression, we varied the interval between the first and second pulse from 100 ms – 30 s. The amplitude of the capacitance jump evoked by the second pulse recovered with a single exponential time course in rods (τ = 142 ms). Recovery of ON bipolar cell PSCs evoked by depolarizing steps applied to presynaptic rods showed a similar time constant (158 ms). In contrast, the recovery of cone capacitance responses exhibited a double exponential time course (τ = 53 & 715 ms). The calcium current recorded during the second pulse was not significantly depressed by the prior pulse suggesting that paired pulse depression is largely due to depletion of the pool of releasable vesicles in the synaptic terminal. Recovery from paired pulse depression with shorter 20 ms pulses also showed a monoexponential time course in rods and bi–exponential time course in cones. Conclusions: Rods and cones exhibit two kinetically distinct phases of release. Both cell types show a similar slow component (τ ∼ 5 s) but the fast component of release from cones is ∼10x faster than the fast component in rods. Recovery from paired pulse depression appears to reflect replenishment of the pool of releasable vesicles and followed a monoexponential time course in rods but bi–exponential time course in cones. The kinetics of vesicular release and replenishment may contribute to a decrease in synaptic transmission from photoreceptors to second order cells at high temporal frequencies, although the faster release kinetics of cones may allow them to maintain release at higher frequencies than rods.
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