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Robert J Zawadzki, Pengfei Zhang, Mayank Goswami, Edward N Pugh; In vivo measurements and modeling of light-dependent lengthening and increased reflectivity of the mouse rod photoreceptor outer segments: G-protein activation-based optophysiology.. Invest. Ophthalmol. Vis. Sci. 2017;58(8):3586.
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© ARVO (1962-2015); The Authors (2016-present)
To present our in vivo observations of OCT-based measurements of increase in length and reflectivity of the mouse photoreceptors outer segments associated with G-protein activation, and to propose a model explaining the light-dependent kinetics of observed signals.
We imaged retinas of Balb/c, C57Bl/6J and Gnat1-/- mice with a custom SLO-OCT system that allowed precise control of rhodopsin bleaching (over a 200-fold range). Infrared OCT was used to measure, over 5 min, the time course of rod outer segment (ROS) elongation and increases in backscattering in response to stimuli delivered to dark adapted mice.
We observed no signals in response to light stimulation in Gnat1-/- mice (lacking transducing expression), and conclude that the optophysiologic effects are triggered by the G-protein cascade activated by photoisomerized rhodopsin. The maximal amplitude and rates of ROS swelling were 10.0 ± 2.1% and 0.11% s-1 of dark adapted length, respectively. The observed increase in backscattering from the base of the ROS is explained by a model of different cytoplasmic swelling rates of the nascent basal discs and the rest of the ROS.
The 10% elongation corresponds to an ~20% increase in cytoplasmic volume and interpreting the steady-state elongation as an osmotic equilibrium leads to the conclusion that total activation of phototransduction produces a remarkable 65 mOsM increase in osmolytes in the ROS, which constitutes a serious osmotic stressor. Translocation of transducin off the disc membranes and inter-discal spring-like links may serve to reduce this stress, which could however be more serious in photoreceptors lacking normal structural integrity.
This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.
Lengthening of ROS and increased backscattering from the OS base (IS/OS) and tips (ROST) in Balb/c mice in response to 10% bleach of the rhodopsin. A. Location of light exposure (blue rectangle) and OCT scans (4 red dashed lines). B, C. OCT B-scans taken before (B) and 2 min after (C) bleaching. D. The amplitude of the OCT backscattering from the IS/OS and from the ROST plotted as functions of time. E. The magnitude of the shift of the axial position of the ELM and IS/OS plotted as a function of time. F. The position shift of the IS/OS band plotted as a function of the amplitude of the OCT backscattering signal from the IS/OS and ROST.
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