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W.B. Thoreson, L. Cadetti, K. Rabl; Properties of spontaneous miniature excitatory post–synaptic potentials (mEPSCs) in salamander OFF bipolar cells. . Invest. Ophthalmol. Vis. Sci. 2004;45(13):1144.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: To determine whether properties of mEPSCs in OFF bipolar cells are consistent with independent quantal release. Methods: Spontaneous mEPSCs and depolarization–evoked EPSCs were recorded from OFF bipolar cells of the tiger salamander retina in the presence of picrotoxin and strychnine. Results: The amplitude distributions of mEPSCs were skewed with an excess of large mEPSCs. This skewed distribution persisted when cells were held at –90 mV to increase mEPSC amplitude so it is not simply due to an under–sampling of small events. Electrotonic filtering does not significantly reduce mEPSC amplitude since the half–width of mEPSCs does not increase significantly as their amplitude decreases. Large mEPSCs, which can be fivefold larger than the average mEPSC, exhibited the same kinetics as small events. Multi–component Gaussian curves provided better fits to the amplitude histogram than a single Gaussian curve, significantly reducing the coefficient of variation from 0.34 to 0.3 (n=8). These results suggest that up to 1/3 of the large events may be due to the simultaneous release of multiple vesicles. The relationship between individual mEPSCs and depolarization–evoked EPSCs obtained with paired simultaneous pre– and post–synaptic recordings appears linear. 1) The number of vesicles released by depolarizing steps from rods in the retinal slice was determined using capacitance measurement techniques and found to be linearly correlated with the integrated EPSC charge transfer measured during paired recordings (in the presence of cyclothiazide to inhibit glutamate receptor desensitization). 2) Individual mEPSCs can be scaled linearly to match the amplitude of EPSCs evoked by brief 10 ms steps used to stimulate a nearly synchronous burst of release from rods. 3) After factoring in rates of vesicular release from rods or cones, mEPSCs can be summed linearly to reconstruct EPSC waveforms evoked by depolarizing steps (50–200 ms). Conclusions:The statistical characteristics of individual mEPSCs are consistent with the quantal hypothesis of release and furthermore suggest that a large fraction of events arise from the simultaneous release of multiple vesicles, as has been suggested for hair cells. The actions of individual mEPSCs sum linearly to generate the depolarization–evoked EPSC. Unlike other synapses where glutamate receptor saturation leads to a deviation from linearity with large EPSCs, each vesicle released from the photoreceptor terminal exerts an equivalent post–synaptic effect thus promoting the faithful transmission of light–evoked responses across the photoreceptor synapse.
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