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Michael A. Freed, Zhiyin Liang; The Bipolar To Ganglion Cell Ribbon Synapse Adds Noise To The Information It Transmits. Invest. Ophthalmol. Vis. Sci. 2012;53(14):1942.
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© ARVO (1962-2015); The Authors (2016-present)
Noise generated by retinal synapses interferes with the detection and discrimination of visual stimuli. Thus is it is important to estimate how much noise a synapse adds to the information it transmits.
Excitatory currents due to vesicular release from bipolar cells were recorded from small guinea pig ganglion cells (GC) (200-250 µm diameter dendritic arbor). A 6-sec white noise stimulus was repeated about 50 times. Noise was measured as the variance of currents over repeats. To derive the average instantaneous rate of quanta released onto the ganglion cell N(t), we deconvolved the average evoked current with the average time course of single quantal EPSCs recorded during intervals of constant illumination (minis). Quantal release at a bipolar synapse is a Poisson process and is well approximated as P(N(t)). Thus we generated the instantaneous quantal rate for a given trial i as P(N(t))i and convolved it with the mini to generate excitatory currents. Because these excitatory currents had only one source of noise -- quantal release at the synapse -- their variance across trials estimated synaptic noise.
Synaptic noise was about 41 ± 4% of the total recorded noise with no difference between ON and OFF cells (n = 18 cells). The instantaneous quantal rate had a clear baseline where it fell to zero rate interrupted by brief periods where it increased to 1.0 ± 0.3 quanta/s. The mGluR6 antagonist LY341495 suppressed light evoked currents in ON ganglion cells, increased the baseline quantal rate above zero, and increased the proportion of synaptic noise by 110 ± 29% (6 cells).
LY341495 opens cation channels on the ON bipolar cell's dendrites and depolarizes it, increasing its rate of spontaneous vesicular release onto the GC. This explains the observed increase in the baseline quantal rate and the subsequent increase in synaptic noise. Our general conclusion is that vesicular release at bipolar synapses on a GC contribute about 40% of the noise in its excitatory currents. The remaining ~60% of noise is presumably from the presynaptic circuit. Thus the synapse generates slightly less noise than it transmits from the presynaptic circuit, which indicates that the fidelity of the synapse is slightly better than the fidelity of the presynaptic circuit.
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