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Kristopher Sheets, Cataldo Schietroma, Guido Zampighi, Nicholas Brecha; Light Modulates Omega Figure Density in Cone Photoreceptor Synaptic Terminals at Non-Ribbon, but not Ribbon, Locations. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):1715.
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© ARVO (1962-2015); The Authors (2016-present)
The classic location of synaptic activity in a photoreceptor is at the base of the ribbon near the arciform density. We have reported the presence of omega figures, indicative of synaptic vesicle exocytosis, at ribbon-related and non-ribbon locations in mouse rod photoreceptor terminals (Zampighi et al., 2011). Subsequently, non-ribbon release was reported in salamander rods by TIRF microscopy (Chen et al., 2013). Here we investigate ribbon- and non-ribbon-related omega figures in mouse cone photoreceptor synaptic terminals.
Conical tomography tilt series were collected (11,000x, tilt 55°, 5° in-plane rotation increment) from 90 nm ultrathin sections of light- and dark-adapted eyes from C57Bl/6J mice. Three, randomly selected, ribbon synapses in cone pedicles per condition were reconstructed with in-house software (mean volume: 0.254 ±0.056 um3). Analysis was performed using Fiji/ImageJ software. Density is reported as omega figures per 106 nm2 of plasma membrane. Statistics were assessed by one-way ANOVA using R.
In addition to the lateral elements of the classic ribbon structure, photoreceptors encapsulated circular projections of horizontal cell dendrites having an appearance similar to spinules reported in goldfish retina. The number of these pseudo-spinules observed per reconstruction ranged from 3 to 7; no difference was noted between dark and light conditions. Omega figure density at the base of the ribbon exhibited no difference between conditions, nor did clathrin-coated budding endocytotic vesicle density. Overall, the density of non-ribbon omega figures in dark conditions was significantly higher than in the light (199.8 vs 18.9, p=0.001). This was true for both lateral elements (p=0.011) and pseudo-spinules (p=0.035).
Omega figure density at non-ribbon locations showed an expected decrease with light at both lateral elements and psuedo-spinules. Interestingly, omega figure density near ribbons did not exhibit any difference between conditions. One possible explanation is that synaptic cycling near ribbons is faster than at non-ribbon locations. Alternatively, non-ribbon omega figures may represent a significant portion of exocytosis in the cone pedicle.
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