Label-free Imaging of the Reversible Rhodopsin Dynamics in a Living Eye

Abstract

Vision begins with photoisomerization of a retinal, triggering further conformational changes, followed by a phototransduction cascade in photoreceptors. Microelectrode recordings revealed an early receptor potential (ERP) signal accompanying these conformational transitions. Such techniques are invasive, obscured by multiple electrical processes in the retina, and in rods, they are limited to the contribution of a small fraction of rhodopsin embedded in the nascent disc membrane and plasma membrane. Recent advances in phase-sensitive OCT (Optoretinography, ORG) enable detection of nanoscale deformations of retinal cells associated with physiological processes. However, in previous ORG studies, focused primarily on cones, cell deformation related to ERP was largely obscured by osmotic swelling and long stimuli did not resolve the isomerization dynamics of photopigments. Here, we demonstrate a very robust electromechanical signature of photoisomerization in rods at microsecond-scale temporal resolution. Green flash induces sub-millisecond-fast contraction of the outer segments by hundreds of nanometers, while subsequent UV flash reverts the activated molecules, producing an opposite response of a similar magnitude. This approach surpasses the sensitivity of electrical methods by integrating the response across all the discs in rod outer segments. Noninvasive and label-free imaging of the rhodopsin dynamics in a living eye opens the door to fundamental studies of visual transduction and to differential diagnosis of the photoreceptors dysfunction.