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Gabriel Peinado, Kaitryn Ronning, Wolfgang Baehr, Marie E. Burns, Edward N. Pugh; Mouse cones expressing only S-opsin exhibit rod-like saturation. Invest. Ophthalmol. Vis. Sci. 2018;59(9):1978. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
Cone opsins vary in biophysical properties that contribute to their functional differences, such as wavelength selectivity and spontaneous activation rate. We tested whether mouse M- and S-opsins exhibit functional differences in intensity regimes that bleach substantial fractions of their pigments.
To probe cone function at high bleach levels, we used in vivo ERG recording. Mice were maintained on a 12D:12L light cycle, dark-adapted overnight and anesthetized with 1.5% isofluorane. Full-field cone-driven ERGs of mice expressing only S-opsin (S-only) or only M-opsin (M-only) were measured in the presence of 360 nm or 510 nm backgrounds spanning 7 log10 units, whose maximal intensities were estimated to produce >99% bleaching. Flash sensitivity and the fraction of the cone circulating current suppressed were measured for each background intensity. Results were obtained with and without pharmacologically blocking the contributions to ERGs from ON-bipolar cells, OFF-bipolar cells and Müller cells. The time course of dark-adaptation was measured by delivering sub-saturating flashes after extinction of a strongly bleaching background.
The flash sensitivity of ERGs driven by M-only cones followed Weber-Fechner behavior, even in the presence of the most intense 360 nm or 510 nm backgrounds (2 x 107 and 4 x 107 hv µm-2 s-1, respectively). The circulating current of M-only cones did not decline below ~50% of maximum with the most intense backgrounds. In contrast, the flash sensitivity of ERGs driven by S-only cones deviated from Weber-Fechner behavior in the presence of 360 nm backgrounds exceeding 1 x 106 hv µm-2 s-1, and the circulating current level decreased monotonically to below 20% of maximum with the most intense background. The dark-adaptation of ERGs of S-only animals was very greatly slowed relative to that of M-only animals.
Although a principal function of cones is to operate in intense daytime illumination, mouse S- and M-opsins cannot equally support signaling at high light intensities: unlike cones expressing only M-opsin, under such experimental conditions cones expressing S-opsin show impaired flash sensitivity, near total loss of circulating current and impaired dark-adaptation, behaviors similar to those of rods.
This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.
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