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Justin J. Grassmeyer, Matthew J Van Hook, Wallace B. Thoreson; Functional independence of synaptic ribbons in cone photoreceptors. Invest. Ophthalmol. Vis. Sci. 2017;58(8):5604.
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© ARVO (1962-2015); The Authors (2016-present)
Cone photoreceptors can have up to 50 presynaptic ribbons. We asked whether Ca2+ responses vary among individual ribbons in cones, as ribbon-to-ribbon differences could potentially shape contrast responses of second-order neurons.
In retinal slices of Ambystoma tigrinum, we combined whole-cell recordings and confocal Ca2+ imaging with low affinity indicator Oregon Green 488 BAPTA-5N to visualize ribbon-localized Ca2+ entry. Ca2+ responses to depolarizing stimuli were measured, normalized, and fit with Boltzmann functions adjusted for Ca2+ driving force to determine the voltage of half activation (V50) at each ribbon site. We assessed measurement sensitivity by shifting superfusate pH by 0.2 units, which caused readily detectable 3.3 mV shifts in both ICa and Ca2+ signal V50. Trial-to-trial differences in positioning the focal plane or region of interest for Ca2+ signal measurements did not significantly increase V50 variability.
Ca2+ signals varied in spatial extent, but not amplitude, with ribbon size, suggesting similar Ca2+ channel density among ribbons. V50 of whole-cell Ca2+ currents (ICa) varied modestly among cones (SD=2.5 mV). V50 of Ca2+ signals at cone ribbons showed greater variability (SD=3.5 mV). Some of this variability reflects cell-to-cell differences in ICa, but also reflects uncertainties in determining V50 from optical Ca2+ measurements (SD=1.7 mV). After subtracting variance from both sources, the remaining genuine ribbon-to-ribbon variability in Ca2+ signal V50 was small (SD=1.4 mV). For these experiments, we had blocked inhibitory synaptic feedback from horizontal cells (HCs) to cones. With feedback active, depolarizing a simultaneously voltage-clamped HC from -60 to 0 mV significantly inhibited ribbon Ca2+ signal amplitude and shifted V50 values by +2.2 ± 0.5 mV (n=15).
We found modest V50 variability among ribbons in the absence of HC to cone feedback. HC feedback at individual ribbons can produce additional V50 changes. Small differences can have significant effects: at a cone’s membrane potential in darkness of -40 mV, a V50 shift of 2.8 mV can alter Ca2+ influx by >30%. The balance of inputs from heterogeneously-activating ribbons could shape bipolar cell contrast sensitivity. For example, a bipolar cell receiving input from multiple ribbons with different V50 values will be sensitive to a wider range of intensities than one receiving inputs from identical ribbons.
This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.
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